TI-Understanding_BUCK_power_stages [173,68 KiB]

TI-Understanding_BUCK_power_stages [173,68 KiB]
Understanding Buck Power
Stages in Switchmode
Power Supplies
Application
Report
March 1999
Mixed Signal Products
SLVA057
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Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Buck Power Stage Steady-State Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Buck Steady-State Continuous Conduction Mode Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Buck Steady-State Discontinuous Conduction Mode Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Critical Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3 Buck Power Stage Small Signal Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1 Buck Continuous Conduction Mode Small Signal Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 Buck Discontinuous Conduction Mode Small-Signal Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4 Variations of the Buck Power Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1 Synchronous-Buck Power Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2 Forward Converter Power Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5 Component Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Output Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Output Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Power Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Catch Rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
23
24
25
26
6 Example Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Understanding Buck Power Stages in Switchmode Power Supplies
iii
Figures
List of Figures
1 Buck Power Stage Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Buck Power Stage States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Continuous-Mode Buck Power Stage Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4 Boundary Between Continuous and Discontinuous Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5 Discontinuous Current Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6 Discontinuous-Mode Buck Power Stage Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7 Power Supply Control Loop Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8 Boost Nonlinear Power Stage Gain vs Duty Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9 Averaged (Nonlinear) CCM PWM Switch Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10 DC and Small Signal CCM PWM Switch Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11 CCM Buck Power Stage Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12 Averaged (Nonlinear) DCM PWM Switch Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
13 DCM Buck Power Stage DC Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
14 Small Signal DCM PWM Switch Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
15 Synchronous Buck Power Stage Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
16 Forward Converter Power Stage Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
iv
SLVA057
Understanding Buck Power Stages in Switchmode Power
Supplies
Everett Rogers
ABSTRACT
A switching power supply consists of the power stage and the control circuit. The power
stage performs the basic power conversion from the input voltage to the output voltage
and includes switches and the output filter. This report addresses the buck power stage
only and does not cover control circuits. Detailed steady-state and small-signal analysis
of the buck power stage operating in continuous and discontinuous mode are presented.
Variations in the standard buck power stage and a discussion of power stage component
requirements are included.
1 Introduction
The three basic switching power supply topologies in common use are the buck,
boost, and buck-boost. These topologies are nonisolated, that is, the input and
output voltages share a common ground. There are, however, isolated
derivations of these nonisolated topologies. The power supply topology refers to
how the switches, output inductor, and output capacitor are connected. Each
topology has unique properties. These properties include the steady-state
voltage conversion ratios, the nature of the input and output currents, and the
character of the output voltage ripple. Another important property is the frequency
response of the duty-cycle-to-output-voltage transfer function.
The most common and probably the simplest power stage topology is the buck
power stage, sometimes called a step-down power stage. Power supply
designers choose the buck power stage because the output voltage is always
less than the input voltage in the same polarity and is not isolated from the input.
The input current for a buck power stage is discontinuous or pulsating due to the
power switch (Q1) current that pulses from zero to IO every switching cycle. The
output current for a buck power stage is continuous or nonpulsating because the
output current is supplied by the output inductor/capacitor combination; the
output capacitor never supplies the entire load current (for continuous inductor
current mode operation, one of the two operating modes to be discussed in the
next section).
This report describes the steady state operation of the buck power stage in
continuous-mode and discontinuous-mode operation with ideal waveforms
given. The duty-cycle-to-output-voltage transfer function is given after an
introduction of the PWM switch model.
Figure 1 shows a simplified schematic of the buck power stage with a drive circuit
block included. The power switch, Q1, is an n-channel MOSFET. The diode, CR1,
is usually called the catch diode, or freewheeling diode. The inductor, L, and
capacitor, C, make up the output filter. The capacitor ESR, RC , (equivalent series
resistance) and the inductor DC resistance, RL , are included in the analysis. The
resistor, R, represents the load seen by the power stage output.
1
Introduction
a
d
Q1
ia
+
g
L
c
s
VO
RL
CR1
IL = ic
C
VI
Drive
Circuit
p
R
RC
Figure 1. Buck Power Stage Schematic
During normal operation of the buck power stage, Q1 is repeatedly switched on
and off with the on and off times governed by the control circuit. This switching
action causes a train of pulses at the junction of Q1, CR1, and L which is filtered
by the L/C output filter to produce a dc output voltage, VO. A more detailed
quantitative analysis is given in the following sections.
2
SLVA057
Buck Power Stage Steady-State Analysis
2 Buck Power Stage Steady-State Analysis
A power stage can operate in continuous or discontinuous inductor current mode.
Continuous inductor current mode is characterized by current flowing
continuously in the inductor during the entire switching cycle in steady state
operation. Discontinuous inductor current mode is characterized by the inductor
current being zero for a portion of the switching cycle. It starts at zero, reaches
a peak value, and returns to zero during each switching cycle. The two different
modes are discussed in greater detail later and design guidelines for the inductor
value to maintain a chosen mode of operation as a function of rated load is given.
It is very desirable for a power stage to stay in only one mode over its expected
operating conditions, because the power stage frequency response changes
significantly between the two modes of operation.
For this analysis, an n-channel power MOSFET is used and a positive voltage,
VGS(ON) , is applied from the Gate to the Source terminals of Q1 by the drive circuit
to turn ON the FET. The advantage of using an n-channel FET is its lower RDS(on)
but the drive circuit is more complicated because a floating drive is required. For
the same die size, a p-channel FET has a higher RDS(on) but usually does not
require a floating drive circuit.
The transistor Q1 and diode CR1 are drawn inside a dashed-line box with
terminals labeled a, p, and c. The inductor current IL is also labeled iC and refers
to current flowing out of terminal c. These items are explained fully in the Buck
Power Stage Modeling section.
2.1
Buck Steady-State Continuous Conduction Mode Analysis
The following is a description of steady-state operation in continuous conduction
mode. The main result of this section is a derivation of the voltage conversion
relationship for the continuous conduction mode buck power stage. This result
is important because it shows how the output voltage depends on duty cycle and
input voltage or, conversely, how the duty cycle can be calculated based on input
voltage and output voltage. Steady-state implies that the input voltage, output
voltage, output load current, and duty-cycle are fixed and not varying. Capital
letters are generally given to variable names to indicate a steady-state quantity.
In continuous conduction mode, the Buck power stage assumes two states per
switching cycle. The ON state is when Q1 is ON and CR1 is OFF. The OFF state
is when Q1 is OFF and CR1 is ON. A simple linear circuit can represent each of
the two states where the switches in the circuit are replaced by their equivalent
circuits during each state. The circuit diagram for each of the two states is shown
in Figure 2.
Understanding Buck Power Stages in Switchmode Power Supplies
3
Buck Power Stage Steady-State Analysis
L
a
c
VO
RL
RDS(on)
ia
IL = ic
+
ON
State
C
VI
R
RC
p
L
a
c
+
OFF
State
VO
RL
ia
IL = ic
C
VI
Vd
R
RC
p
Figure 2. Buck Power Stage States
The duration of the ON state is D × TS = TON where D is the duty cycle, set by the
control circuit, expressed as a ratio of the switch ON time to the time of one
complete switching cycle, Ts . The duration of the OFF state is called TOFF. Since
there are only two states per switching cycle for continuous mode, TOFF is equal
to (1–D) × TS . The quantity (1–D) is sometimes called D ′. These times are shown
along with the waveforms in Figure 3.
4
SLVA057
Buck Power Stage Steady-State Analysis
IQ1 = ia
ICR1 = ip
IL Solid
IO Dashed
∆IL
VC-P Solid
VO Dashed
TON
TOFF
TS
Figure 3. Continuous-Mode Buck Power Stage Waveforms
Referring to Figure 2, during the ON state, Q1 presents a low resistance, RDS(on) ,
from its drain to source and has a small voltage drop of VDS = IL × RDS(on) . There
is also a small voltage drop across the dc resistance of the inductor equal to
IL × RL . Thus, the input voltage, VI , minus losses, (VDS + IL × RL ), is applied to the
left-hand side of inductor, L. CR1 is OFF during this time because it is reverse
biased. The voltage applied to the right hand side of L is simply the output voltage,
VO . The inductor current, IL , flows from the input source, VI , through Q1 and to
the output capacitor and load resistor combination. During the ON state, the
voltage applied across the inductor is constant and equal to VI – VDS – IL × RL –
Vo. Adopting the polarity convention for the current IL shown in Figure 2, the
inductor current increases as a result of the applied voltage. Also, since the
applied voltage is essentially constant, the inductor current increases linearly.
This increase in inductor current during TON is illustrated in Figure 3.
The amount that the inductor current increases can be calculated by using a
version of the familiar relationship:
VL
+L
di L
dt
å DIL + VLL
ǒ
DT
Ǔ
The inductor current increase during the ON state is given by:
DIL()) +
V I–V DS–I L
L
R L –V O
T ON
This quantity, ∆ IL (+), is referred to as the inductor ripple current.
Understanding Buck Power Stages in Switchmode Power Supplies
5
Buck Power Stage Steady-State Analysis
Referring to Figure 2, when Q1 is OFF, it presents a high impedance from its drain
to source. Therefore, since the current flowing in the inductor L cannot change
instantaneously, the current shifts from Q1 to CR1. Due to the decreasing
inductor current, the voltage across the inductor reverses polarity until rectifier
CR1 becomes forward biased and turns ON. The voltage on the left-hand side
of L becomes –(Vd + IL × RL ) where the quantity, Vd , is the forward voltage drop
of CR1. The voltage applied to the right hand side of L is still the output voltage,
VO . The inductor current, IL , now flows from ground through CR1 and to the output
capacitor and load resistor combination. During the OFF state, the magnitude of
the voltage applied across the inductor is constant and equal to (VO + Vd + IL ×
RL ). Maintaining our same polarity convention, this applied voltage is negative (or
opposite in polarity from the applied voltage during the ON time). Hence, the
inductor current decreases during the OFF time. Also, since the applied voltage
is essentially constant, the inductor current decreases linearly. This decrease in
inductor current during TOFF is illustrated in Figure 3.
) ǒVd ) IL
Ǔ
The inductor current decrease during the OFF state is given by:
DIL(–) +
VO
RL
L
T OFF
This quantity, ∆ IL (–), is also referred to as the inductor ripple current.
In steady state conditions, the current increase, ∆ IL (+), during the ON time and
the current decrease during the OFF time, ∆ IL (–), must be equal. Otherwise, the
inductor current would have a net increase or decrease from cycle to cycle which
would not be a steady state condition. Therefore, these two equations can be
equated and solved for VO to obtain the continuous conduction mode buck
voltage conversion relationship.
Solving for VO :
VO
+ ǒVI–VDSǓ
T ON
T ON
) TOFF
–V d
T OFF
T ON
) TOFF –IL
RL
And, substituting TS for TON + TOFF, and using D = TON /TS and (1–D) = TOFF /TS ,
the steady-state equation for VO is:
VO
+ ǒVI–VDSǓ
D–V d
(1–D)–I L
RL
Notice that in simplifying the above, TON + TOFF is assumed to be equal to TS . This
is true only for continuous conduction mode as we will see in the discontinuous
conduction mode analysis.
NOTE: An important observation should be made here: Setting
the two values of ∆ IL equal to each other is equivalent to balancing
the volt-seconds on the inductor. The volt-seconds applied to the
inductor is the product of the voltage applied and the time that the
voltage is applied. This is the best way to calculate unknown
values such as VO or D in terms of known circuit parameters and
this method will be applied repeatedly in this paper. Volt-second
balance on the inductor is a physical necessity and should be
comprehended at least as well as Ohms Law.
6
SLVA057
Buck Power Stage Steady-State Analysis
In the above equations for ∆ IL (+) and ∆ IL (–), the dc output voltage was implicitly
assumed to be constant with no AC ripple voltage during the ON time and the OFF
time. This is a common simplification and involves two separate effects. First, the
output capacitor is assumed to be large enough that its voltage change is
negligible. Second, the voltage across the capacitor ESR is also assumed to be
negligible. These assumptions are valid because the ac ripple voltage is
designed to be much less than the dc part of the output voltage.
The above voltage conversion relationship for VO illustrates the fact that VO can
be adjusted by adjusting the duty cycle, D, and is always less than the input
because D is a number between 0 and 1. A common simplification is to assume
VDS , Vd , and RL are small enough to ignore. Setting VDS , Vd , and RL to zero, the
above equation simplifies considerably to:
VO
+ VI
D
Another simplified way to visualize the circuit operation is to consider the output
filter as an averaging network. This is a valid simplification because the filter cutoff
frequency (usually between 500 Hz and 5 kHz) is always much less than the
power supply switching frequency (usually between 100 kHz and 500 kHz). The
input voltage applied to the filter is the voltage at the junction of Q1, CR1, and L,
labeled as Vc–p . The filter passes the dc component (or average) of Vc–p and
greatly attenuates all frequencies above the output filter cutoff frequency. Thus,
the output voltage is simply the average of the Vc–p voltage.
To relate the inductor current to the output current, referring to Figures 2 and 3,
note that the inductor delivers current to the output capacitor and load resistor
combination during the whole switching cycle. The inductor current averaged
over the switching cycle is equal to the output current. This is true because the
average current in the output capacitor must be zero. In equation form, we have:
I L(Avg)
+ IO
This analysis was for the buck power stage operation in continuous inductor
current mode. The next section is a description of steady-state operation in
discontinuous conduction mode. The main result is a derivation of the voltage
conversion relationship for the discontinuous conduction mode buck power
stage.
2.2
Buck Steady-State Discontinuous Conduction Mode Analysis
We now investigate what happens when the load current is decreased. First,
observe that the power stage output current is the average of the inductor current.
This should be obvious since the inductor current flows into the output capacitor
and load resistor combination and the average current flowing in the output
capacitor is always zero.
Understanding Buck Power Stages in Switchmode Power Supplies
7
Buck Power Stage Steady-State Analysis
If the output load current is reduced below the critical current level, the inductor
current will be zero for a portion of the switching cycle. This should be evident
from the waveforms shown in Figure 3 since the peak to peak amplitude of the
ripple current does not change with output load current. In a (nonsynchronous)
buck power stage, if the inductor current attempts to fall below zero, it just stops
at zero (due to the unidirectional current flow in CR1) and remains there until the
beginning of the next switching cycle. This operating mode is called
discontinuous conduction mode. A power stage operating in discontinuous
conduction mode has three unique states during each switching cycle as
opposed to two states for continuous conduction mode. The load current
condition where the power stage is at the boundary between continuous and
discontinuous mode is shown in Figure 4. This is where the inductor current falls
to zero and the next switching cycle begins immediately after the current reaches
zero.
IL Solid
IO Dashed = IO(Crit)
∆IL
0
TON
TOFF
TS
Figure 4. Boundary Between Continuous and Discontinuous Mode
Further reduction in output load current puts the power stage into discontinuous
conduction mode. This condition is illustrated in Figure 5. The discontinuous
mode power stage frequency response is quite different from the continuous
mode frequency response and is shown in the Buck Power Stage Modeling
section. Also, the input to output relationship is quite different as shown in the
following derivation.
IL Solid
IO Dashed
∆IL
0
DTs
D3Ts
D2Ts
TS
Figure 5. Discontinuous Current Mode
8
SLVA057
Buck Power Stage Steady-State Analysis
To begin the derivation of the discontinuous conduction mode buck power stage
voltage conversion ratio, observe that there are three unique states that the
power stage assumes during discontinuous current mode operation. The ON
state is when Q1 is ON and CR1 is OFF. The OFF state is when Q1 is OFF and
CR1 is ON. The IDLE state is when both Q1 and CR1 are OFF. The first two states
are identical to those of the continuous mode case and the circuits of Figure 2 are
applicable except that TOFF ≠ (1–D) × TS . The remainder of the switching cycle
is the IDLE state. In addition, the dc resistance of the output inductor, the output
diode forward voltage drop, and the power MOSFET ON-state voltage drop are
all assumed to be small enough to omit.
The duration of the ON state is TON = D × TS where D is the duty cycle, set by the
control circuit, expressed as a ratio of the switch ON time to the time of one
complete switching cycle, Ts . The duration of the OFF state is TOFF = D2 × TS .
The IDLE time is the remainder of the switching cycle and is given as TS – TON
– TOFF = D3 × TS . These times are shown with the waveforms in Figure 6.
Without going through the detailed explanation as before, the equations for the
inductor current increase and decrease are given below.
The inductor current increase during the ON state is given by:
DIL()) +
VI
* VO
L
T ON
+ VI *L VO
D
TS
+ IPK
The ripple current magnitude, ∆ IL (+), is also the peak inductor current, Ipk ,
because in discontinuous mode, the current starts at zero each cycle.
The inductor current decrease during the OFF state is given by:
DIL(–) +
VO
T OFF
L
As in the continuous conduction mode case, the current increase, ∆ IL (+), during
the ON time and the current decrease during the OFF time, ∆ IL (–), are equal.
Therefore, these two equations can be equated and solved for VO to obtain the
first of two equations to be used to solve for the voltage conversion ratio:
VO
+ VI
) TOFF + VI
T ON
T ON
) D2
D
D
Now we calculate the output current (the output voltage VO divided by the output
load R). It is the average of the inductor current.
IO
+ IL(avg) + VRO + IPK
2
D
TS
) D2
TS
Ts
Now, substitute the relationship for IPK into the above equation to obtain:
IO
+ VRO + ǒV * VOǓ
I
D
2
TS
L
(D
) D2)
We now have two equations, the one for the output current just derived and the
one for the output voltage (above), both in terms of VI , D, and D2. We now solve
each equation for D2 and set the two equations equal to each other. Using the
resulting equation, an expression for the output voltage, VO , can be derived.
Understanding Buck Power Stages in Switchmode Power Supplies
9
Buck Power Stage Steady-State Analysis
The discontinuous conduction mode buck voltage conversion relationship is
given by:
VO
+ VI
1
)
Ǹ)
2
1
4 K
D2
Where K is defined as:
K
+ R2
L
TS
The above relationship shows one of the major differences between the two
conduction modes. For discontinuous conduction mode, the voltage conversion
relationship is a function of the input voltage, duty cycle, power stage inductance,
the switching frequency and the output load resistance while for continuous
conduction mode, the voltage conversion relationship is only dependent on the
input voltage and duty cycle.
IQ1
IPK
ICR1
IPK
IL Solid
IO Dashed
∆IL
VC-P Solid
VO Dashed
D3*TS
D*TS
D2*TS
TS
Figure 6. Discontinuous-Mode Buck Power Stage Waveforms
It should be noted that the buck power stage is rarely operated in discontinuous
conduction mode in normal situations, but discontinuous conduction mode will
occur anytime the load current is below the critical level.
10
SLVA057
Buck Power Stage Steady-State Analysis
2.3
Critical Inductance
The previous analyses for the buck power stage have been for continuous and
discontinuous conduction modes of steady-state operation. The conduction
mode of a power stage is a function of input voltage, output voltage, output
current, and the value of the inductor. A buck power stage can be designed to
operate in continuous mode for load currents above a certain level usually 5% to
10% of full load. Usually, the input voltage range, the output voltage and load
current are defined by the power stage specification. This leaves the inductor
value as the design parameter to maintain continuous conduction mode.
The minimum value of inductor to maintain continuous conduction mode can be
determined by the following procedure.
First, define IO(crit) as the minimum current to maintain continuous conduction
mode, normally referred to as the critical current. This value is shown in Figure 4
and is calculated as:
I O(crit)
+ D2IL
Second, calculate L such that the above relationship is satisfied. To solve the
above equation, either relationship, ∆IL (+) or ∆IL (–) may be used for ∆IL . Note also
that either relationship for ∆IL is independent of the output current level. Here,
∆IL (–) is used. The worst case condition (giving the largest Lmin ) is at maximum
input voltage because this gives the maximum ∆IL .
Now, substituting and solving for Lmin :
L min
w 12
ǒ
VO
) Vd ) IL
Ǔ
RL
T OFF(max)
I O(crit)
The above equation can be simplified and put in a form that is easier to apply as
shown:
L min
w
VO
ȡȧ *
Ȣ
1
2
VO
ȣȧ
Ȥ
VI(max)
I O(crit)
TS
Using the inductor value just calculated will guarantee continuous conduction
mode operation for output load currents above the critical current level, IO(crit) .
Understanding Buck Power Stages in Switchmode Power Supplies
11
Buck Power Stage Small Signal Modeling
3 Buck Power Stage Small Signal Modeling
We now switch gears moving from a detailed circuit oriented analysis approach
to more of a system level investigation of the buck power stage. This section
presents techniques to assist the power supply designer in accurately modeling
the power stage as a component of the control loop of a buck power supply. The
three major components of the power supply control loop (i.e., the power stage,
the pulse width modulator and the error amplifier) are shown in block diagram
form in Figure 7.
VI
Power Stage
VO
Duty Cycle
d(t)
Pulse-Width
Modulator
Error
Amplifier
Error Voltage
VE
Reference Voltage
Vref
Figure 7. Power Supply Control Loop Components
Modeling the power stage presents one of the main challenges to the power
supply designer. A popular technique involves modeling only the switching
elements of the power stage. An equivalent circuit for these elements is derived
and is called the PWM Switch Model where PWM is the abbreviation for pulse
width modulated. This approach is presented here.
As shown in Figure 7, the power stage has two inputs: the input voltage and the
duty cycle. The duty cycle is the control input, i.e., this input is a logic signal which
controls the switching action of the power stage and hence the output voltage.
Even though the buck power stage has an essentially linear voltage conversion
ratio versus duty cycle, many other power stages have a nonlinear voltage
conversion ratio versus duty cycle. To illustrate this nonlinearity, a graph of the
steady-state voltage conversion ratio for a boost power stage as a function of
steady-state duty cycle, D is shown in Figure 8. The nonlinear boost power stage
is used here for illustration to stress the significance of deriving a linear model.
The nonlinear characteristics are a result of the switching action of the power
stage switching components, Q1 and CR1. It was observed in reference [5] that
the only nonlinear components in a power stage are the switching devices; the
remainder of the circuit consists of linear elements. It was also shown in reference
[5] that a linear model of only the nonlinear components could be derived by
averaging the voltages and currents associated with these nonlinear
components over one switching cycle. The model is then substituted into the
original circuit for analysis of the complete power stage. Thus, a model of the
switching devices is given and is called the PWM switch model.
12
SLVA057
Buck Power Stage Small Signal Modeling
10
9
Voltage Conversion Ratio
8
7
6
5
4
3
2
1
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.8
0.9
1
Duty Cycle
Figure 8. Boost Nonlinear Power Stage Gain vs Duty Cycle
The basic objective behind modelling power stages is to represent the ac
behavior at a given operating point and to be linear around the operating point.
We want linearity so that we can apply the many analysis tools available for linear
systems. Referring again to Figure 8, if we choose the operating point at D = 0.7,
a straight line can be constructed that is tangent to the original curve at the point
where D = 0.7. This is an illustration of linearization about an operating point, a
technique used in deriving the PWM switch model. Qualitatively, one can see that
if the variations in duty cycle are kept small, a linear model accurately represents
the nonlinear behavior of the power stage being analyzed.
Since a power stage can operate in one of two conduction modes, i.e., continuous
conduction mode (CCM) or discontinuous conduction mode (DCM), there is a
PWM switch model for the two conduction modes. The CCM PWM Switch model
is derived here. The DCM PWM switch model is derived in the Application Report
Understanding Buck–Boost Converter Power Stages, TI Literature Number
SLVA059.
3.1
Buck Continuous Conduction Mode Small Signal Analysis
To start modeling the buck power stage, we begin with the derivation of the PWM
Switch model in (CCM). We focus on the CCM Buck power stage shown in
Figure 1. The strategy is to average the switching waveforms over one switching
cycle and produce an equivalent circuit for substitution into the remainder of the
power stage. The waveforms that are averaged are the voltage across CR1, vc–p ,
and the current in Q1, ia . The waveforms are shown in Figure 3.
Referring again to Figure 1, the power transistor, Q1, and the catch diode, CR1,
are drawn inside a dashed-line box. These are the components that will be
replaced by the PWM switch equivalent circuit. The terminals labeled a, p, and
c will be used for terminal labels of the PWM switch model.
Understanding Buck Power Stages in Switchmode Power Supplies
13
Buck Power Stage Small Signal Modeling
Now, an explanation of the terminal naming convention is in order. The terminal
named a is for active; it is the terminal connected to the active switch. Similarly,
p is for passive and is the terminal of the passive switch. Lastly, c is for common
and is the terminal that is common to both the active and passive switches.
Interestingly enough, all three commonly used power stage topologies contain
active and passive switches and the above terminal definitions can be also
applied. In addition, it is true that substituting the PWM switch model that we will
derive into other power stage topologies also produces a valid model for that
particular power stage. To use the PWM switch model in other power stages, just
substitute the model shown below in Figure 10 into the power stage in the
appropriate orientation.
Referring to the waveforms in Figure 3, regarded as instantaneous functions of
time, the following relationships are true:
ȡȥ
Ȣ
+ ȡȥ
Ȣ
i a(t)
v cp(t)
+
i c(t) during d
Ȁ
TS
v ap(t) during d
TS
during d
TS
during d
0
0
Ȁ
TS
where: ia (t) and ic (t) are the instantaneous currents during a switching cycle and
vcp (t) and vap (t) are the instantaneous voltages between the indicated terminals.
If we take the average over one switching cycle of the above quantities, we get:
ǀiaǁ + d ǀicǁ
ǀvcpǁ + d ǀvapǁ
(1)
(2)
where the brackets indicate averaged quantities.
Now, we can implement the above averaged equations in a simple circuit using
dependent sources:
a
ia
d ×〈ic〉
ic
+
–
c
d ×〈Vap〉
p
Figure 9. Averaged (Nonlinear) CCM PWM Switch Model
The above model is one form of the PWM switch model. However, in this form it
is a large signal nonlinear model. We now need to perform perturbation and
linearization and then the PWM switch model will be in the desired form, i.e.,
linearized about a given operating point.
14
SLVA057
Buck Power Stage Small Signal Modeling
The main idea of perturbation and linearization is assuming an operating point
and introducing small variations about that operating point. For example, we
assume that the duty ratio is fixed at d = D (capital letters indicate steady-state,
or dc quantities while lower case letters are for time-varying quantities). Then a
^
small variation, d, is added to the duty cycle so that the complete expression for
the duty cycle becomes:
d ( t)
+ D ) d ( t)
^
Note that the ^ (hat) above the quantities represents perturbed or small ac
quantities. We change notation slightly replacing the averaged quantities such as
〈ia 〉 with capitol letters (indicating dc quantities) such as Ia . Now we apply the
above process to equations (1) and (2) to obtain:
ǒ Ǔ ǒ ) Ǔ+
ǒ Ǔ ǒ ) Ǔ+
) ia + D ) d
V cp ) v cp + D ) d
^
Ia
^
^
Ic
^
V ap
^
ic
D
v^ ap
D
) D ic ) d Ic ) d ic
V ap ) D v ) d V ap ) d
^
Ic
^
^
^
^
^
ap
^
v^ ap
Now, separate steady-state quantities from ac quantities and also drop products
of ac quantities because the variations are assumed to be small and products of
two small quantities are assumed to be negligible. We arrive at the steady-state
and ac relationships or, in other words, the dc and small signal model:
+ D Ic
ia + D ic ) d Ic
V cp + D V ap
v cp + D v ap ) d
Ia
^
^
^
Steady-state
^
^
^
AC
Steady-state
V ap
AC
In order to implement the above equations into a simple circuit, first notice that
the two steady-state relationships can be represented by an ideal (independent
of frequency) transformer with turns ratio equal to D. Including the ac quantities
is straightforward after reflecting all dependent sources to the primary side of the
ideal transformer. The dc and small-signal model of the PWM switch is shown in
Figure 10. It can easily be verified that the model below satisfies the above four
equations.
a
∧
d
Vap
D
c
–
+
ia
D
1
ic
∧
Ic d
p
Figure 10. DC and Small Signal CCM PWM Switch Model
Understanding Buck Power Stages in Switchmode Power Supplies
15
Buck Power Stage Small Signal Modeling
This model can now be substituted for Q1 and CR1 in the buck power stage to
obtain a model suitable for dc or ac analysis and is shown in Figure 11.
a
ZL
∧
d
Vap
D
+
ia
+
VO
ic
D
1
L
RL
c
–
C
∧
Ic d
VI
R
ZRC
RC
p
Figure 11. CCM Buck Power Stage Model
To illustrate how simple power stage analysis becomes with the PWM switch
^
model, consider the following. For dc analysis, d is zero, L is a short, and C is an
open. Then by inspection one can see VI × D = VO . We also see that Vap = VI .
Thus, knowing the input voltage and output voltage, D is easily calculated. For
ac analysis, the following transfer functions can be calculated: open-loop
line-to-output, open-loop input impedance, open-loop output impedance, and
open-loop control-to-output. The control-to-output, or duty-cycle-to-output, is the
transfer function most used for control loop analysis. To determine this transfer
function, first, use the results from the DC analysis for operating point information.
This information is used to determine the parameter values of the dependent
sources; for example, Vap = VI . Then set the input voltage equal to zero because
we only want the ac component of the transfer function. Now, writing a voltage
loop equation for the VI – dependent voltage source – transformer primary loop
gives the transfer function from duty–cycle to vcp as shown:
–
) vDcp + 0 å vcp + Vap
^
V ap
^
d
D
^
^
d
å vcp + VI
^
^
d
or
v^ cp
^
d
+ VI
The transfer function from vcp to the output voltage is:
v^ O
+Z
Z RC(s)
) ZL(s) by voltage division
Where
R ǒ1 ) s R C CǓ
(parallel combination of output R and output C)
Z RC(s) +
1)s C
R)R
C
Z L(s) + R L ) s L
v^ cp
RC(s)
ǒ
16
SLVA057
Ǔ
Buck Power Stage Small Signal Modeling
So, after simplifying, the duty-cycle-to-output transfer function is:
v^ O
^
(s)
d
+
v^ cp
v^ O
(s)
(s)
+ VI
) RL
R
ƪǒ Ǔ ƫ
^
v^ cp
d
R
) Rc C
R RL
Rc )
) R)LR ) s2
R)R L
L
1
1
)s
C
L
C
)
)
R RC
R RL
The above is exactly what is obtained by other modeling procedures.
3.2
Buck Discontinuous Conduction Mode Small-Signal Analysis
To model the buck power stage operation in discontinuous conduction mode
(DCM), we follow a similar path as above for CCM. A PWM switch model is
inserted into the power stage circuit by replacing the switching elements. As
mentioned above, the derivation for the DCM PWM switch model is given
elsewhere. More details can be found in Fundamentals of Power Electronics. The
large signal nonlinear version of the DCM PWM switch model is shown in
Figure 12. This model is useful for determining the dc operating point of a power
supply. The input port is simply modeled with a resistor, Re . The value of Re is
given by:
Re
+ D22
L
Ts
The output port is modeled as a dependent power source. This power source
delivers power equal to that dissipated by the input resistor, Re . This model is
analogous to the (nonlinear) CCM PWM switch model shown in Figure 9.
a
p
p(t)
Re
c
Figure 12. Averaged (Nonlinear) DCM PWM Switch Model
To illustrate discontinuous conduction mode power supply analysis using this
model, we examine the buck power stage. The analysis proceeds like the CCM
case. The equivalent circuit is substituted into the original circuit. The DCM buck
power stage model schematic is shown in the Figure 13.
Understanding Buck Power Stages in Switchmode Power Supplies
17
Buck Power Stage Small Signal Modeling
Re
a
RL
c
ia
L1
VO
ic
p(t)
+
C
R
VI
p
Ip
Figure 13. DCM Buck Power Stage DC Model
Notice that this model has the inductor dc resistance included. To illustrate using
the model to determine the dc operating point, simply write the equations for the
above circuit. This circuit can be described by the network equations shown. First,
set the power dissipated in Re equal to the power delivered by the dependent
power source:
ǒ
Ǔ+
V I–V cp
Re
2
V cp
Ip
Where the current Ip is the difference between IC and IA as follows:
Ip
cp
+ Ic–Ia + VRO – VI–V
Re
ǒ
Ǔ
Now, substitute the equation for Ip into the following equation:
ǒ
Ǔ+
V I–V cp
Re
2
V cp
ǒǓ
V O V I–V cp
–
R
Re
Now we relate VCP to VO as follows:
V cp
+ VO )
VO
R
RL
The two equations above can be solved to give VO in terms of VI and D by
eliminating Vcp from the two equations and using our previous relationships for
Re and K.
The voltage conversion relationship for the DCM buck is given by:
VO
+ VI
) RL
R
R
1
)
Ǹ)
1
2
4 K
D2
)
R
R RL
This is similar to our previous steady-state result but with the effects of the
inductor resistance included.
18
SLVA057
Buck Power Stage Small Signal Modeling
To derive the small signal model, the circuit of Figure 13 is perturbed and
linearized following a procedure similar to the CCM derivation. To see the details
of the derivation, the reader is directed to reference [4] for details. The resulting
small signal model for the buck power stage operating in DCM is shown in
Figure 14.
2 × (1 – M) × VI
∧
VI
D × Re
+
∧
×d
1
Re
∧
× vO
∧
× vI
M × Re
2–M
2 × (1 – M) × VI
D × M × Re
∧
×d
∧
VO
M2 × Re
C
R
Re
Figure 14. Small Signal DCM PWM Switch Model
The duty-cycle-to-output transfer function for the buck power stage operating in
DCM is given by:
v^ O
^
d
Where
+ Gdo
1
1
) wsp
+ 2 DVO 12 ** MM
K
D+M
1*M
Ǹ
G do
M
+ VVO
I
K
and
+ R2
L
Ts
M
w p + 21 *
*M
1
R
C
Understanding Buck Power Stages in Switchmode Power Supplies
19
Variations of the Buck Power Stage
4 Variations of the Buck Power Stage
4.1
Synchronous-Buck Power Stage
A variation of the traditional buck power stage is the synchronous buck power
stage. In this power stage, an active switch such as another power MOSFET, Q2
in this example, replaces the rectifier, CR1. The FET is then selected so that its
ON-voltage drop is less than the forward drop of the rectifier, thus increasing
efficiency. Although this complicates the drive circuit design, the gain in efficiency
often makes this an attractive option. Other considerations unique to the
synchronous buck power stage are preventing cross–conduction and reverse
recovery of the parasitic pn diode internal to a MOSFET. Either the drive circuit
or the controller used must insure that both FETs are not on simultaneously; this
would place a very low resistance path from the input to ground and destructive
currents could flow in the FETs. A small amount of deadtime is necessary.
To explain the reverse recovery problem, realize that in normal operation the
internal diode of Q2 conducts for a short period at the beginning of the OFF state
during the deadtime. MOSFET Q2 is then turned on at the end of the deadtime
and its internal diode turns off. But for duty cycles approaching 1 (and very short
conduction time for Q2) Q2 may not be turned on after the deadtime. In that case,
the internal diode of Q2 is still conducting at the beginning of a new ON state when
MOSFET Q1 is turned on. Increased power dissipation due to the diode reverse
recovery current can occur if this happens.
Another characteristic of the synchronous buck power stage is that it always
operates in continuous conduction mode (CCM) because current can reverse in
Q2. Thus the voltage conversion relationship and the duty-cycle-to-output
voltage transfer function for the synchronous buck power stage are the same as
for the CCM buck power stage.
A simplified schematic of the Synchronous Buck power stage with a drive circuit
block included is shown in Figure 7. Both power switches are n-channel
MOSFETs. Sometimes, a p-channel FET is used for Q1, but Q2 is almost always
an n-channel FET.
Q1
L1
Va
VO
RL
+
VI
Q2
Drive
Circuit
C
R
RC
Figure 15. Synchronous Buck Power Stage Schematic
An example design using a synchronous buck power stage and the TL5001
controller is given in SLVP089 Synchronous Buck Converter Evaluation Module
User’s Guide, Texas Instruments Literature Number SLVU001A
20
SLVA057
Variations of the Buck Power Stage
Another example design using a synchronous buck power stage and the
TPS5210 controller is given in the Application Report Designing Fast Response
Synchronous Buck Regulators Using the TPS5210, TI Literature Number
SLVA044.
A third example design using a synchronous buck power stage and the TPS5633
controller is given in Synchronous Buck Converter Design Using TPS56xx
Controllers in SLVP10x EVMs User’s Guide, Texas Instruments Literature
Number SLVU007.
4.2
Forward Converter Power Stage
A transformer-coupled variation of the traditional buck power stage is the forward
converter power stage. The power switch is on the primary side of an isolation
transformer and a forward rectifier and a catch rectifier are on the secondary side
of the isolation transformer. This power stage provides electrical isolation of the
input voltage from the output voltage. Besides providing electrical isolation, the
isolation transformer can perform step-down (or step-up) of the input voltage to
the secondary. The transformer turns ratio can be designed so that reasonable
duty cycles are obtained for almost any input voltage/output voltage combination
thus avoiding extremely small or extremely high duty cycle values.
The forward converter power stage is very popular in 48-V input telecom
applications and 110-VAC or 220-VAC off-line applications for output power
levels up to approximately 250 Watts. The exact power rating of the forward
converter power stage, of course, is dependent on the input voltage/output
voltage combination, its operating environment and many other factors. The
capability of obtaining multiple output voltages from a single power stage is
another advantage of the forward converter power stage.
A simplified schematic of the forward converter power stage is shown in
Figure 16. Not shown in the schematic but necessary for operation is a means
of resetting the transformer, T1. There are many ways to accomplish this but a
complete discussion is beyond the scope of this report.
L
VO
T1
CR1
Np
+
Ns
VI
C
Q1
R
CR2
Drive
Circuit
Figure 16. Forward Converter Power Stage Schematic
The simplified voltage conversion relationship for the forward converter power
stage operating in CCM is given by:
VO
+ VI
Ns
Np
D
Understanding Buck Power Stages in Switchmode Power Supplies
21
Variations of the Buck Power Stage
The simplified voltage conversion relationship for the forward converter power
stage operating in DCM is given by:
VO
+ VI
Ns
Np
1
)
Where K is defined as:
K
+ R2
Ǹ)
2
1
4 K
D2
L
Ts
The simplified duty-cycle-to-output transfer function for the forward converter
power stage operating in CCM is given by:
v^ O
^
(s)
d
+ VI
Ns
Np
) RL
R
ƪǒ Ǔ ƫ
R
) Rc C
R RL
Rc )
) R)LR ) s2
R)R L
L
1
1
)s
C
L
C
)
)
R RC
R RL
Other power stages which are also variations of the buck power stage include but
are not limited to the half-bridge, the full-bridge, and the push-pull power stages.
22
SLVA057
Component Selection
5 Component Selection
This section presents a discussion of the function of each of the main components
of the buck power stage. The electrical requirements and applied stresses are
given for each power stage component.
The completed power supply, made up of a power stage and a control circuit,
usually must meet a set of minimum performance requirements. This set of
requirements is usually referred to as the power supply specification. Many times,
the power supply specification determines individual component requirements.
5.1
Output Capacitance
In switching power supply power stages, the function of output capacitance is to
store energy. The energy is stored in the capacitor’s electric field due to the
voltage applied. Thus, qualitatively, the function of a capacitor is to attempt to
maintain a constant voltage.
The value of output capacitance of a Buck power stage is generally selected to
limit output voltage ripple to the level required by the specification. Since the
ripple current in the output inductor is usually already determined, the series
impedance of the capacitor primarily determines the output voltage ripple. The
three elements of the capacitor that contribute to its impedance (and output
voltage ripple) are equivalent series resistance (ESR), equivalent series
inductance (ESL), and capacitance (C). The following gives guidelines for output
capacitor selection.
For continuous inductor current mode operation, to determine the amount of
capacitance needed as a function of inductor current ripple, ∆ IL , switching
frequency, fS , and desired output voltage ripple, ∆VO , the following equation is
used assuming all the output voltage ripple is due to the capacitor’s capacitance.
C
w8
DI L
fS
DV O
where ∆IL is the inductor ripple current defined in section 2.1.
For discontinuous inductor current mode operation, to determine the amount of
capacitance needed as a function of inductor current ripple, ∆ IL , output current
IO, switching frequency, fS , and output voltage ripple, ∆VO , the following equation
is used assuming all the output voltage ripple is due to the capacitor’s
capacitance.
C
w
I O(Max)
ǒ Ǔ
1
fS
*
I
2
O(Max)
DV O
DI L
where ∆IL is the inductor ripple current defined in section 2.2.
In many practical designs, to get the required ESR, a capacitor with much more
capacitance than is needed must be selected.
For both continuous or discontinuous inductor current mode operation and
assuming there is enough capacitance such that the ripple due to the capacitance
can be ignored, the ESR needed to limit the ripple to ∆VO V peak-to-peak is:
Understanding Buck Power Stages in Switchmode Power Supplies
23
Component Selection
ESR
v DDVI O
L
Ripple current flowing through a capacitor’s ESR causes power dissipation in the
capacitor. This power dissipation causes a temperature increase internal to the
capacitor. Excessive temperature can seriously shorten the expected life of a
capacitor. Capacitors have ripple current ratings that are dependent on ambient
temperature and should not be exceeded. Referring to Figure 3, the output
capacitor ripple current is the inductor current, IL, minus the output current, IO.
The RMS value of the ripple current flowing in the output capacitance (continuous
inductor current mode operation) is given by:
I C RMS
+ DIL Ǹ63 + DIL(0.289).
ESL can be a problem by causing ringing in the low megahertz region but can be
controlled by choosing low ESL capacitors, limiting lead length (PCB and
capacitor), and replacing one large device with several smaller ones connected
in parallel.
For some high-performance applications such as a synchronous buck hysteretic
regulator controlled by the TPS5210 from Texas Instruments, the output
capacitance is selected to provide satisfactory load transient response, because
the peak-to-peak output voltage ripple is determined by the TPS5210 controller.
For more information, see the Application Report Designing Fast Response
Synchronous Buck Regulators Using the TPS5210, TI Literature Number
SLVA044.
Three capacitor technologies—low-impedance aluminum, organic semiconductor, and solid tantalum—are suitable for low-cost commercial
applications. Low-impedance aluminum electrolytics are the lowest cost and
offer high capacitance in small packages, but ESR is higher than the other two.
Organic semiconductor electrolytics, such as the Sanyo OS-CON series, have
become very popular for the power-supply industry in recent years. These
capacitors offer the best of both worlds—a low ESR that is stable over the
temperature range and high capacitance in a small package. Most of the
OS-CON units are supplied in lead-mounted radial packages; surface-mount
devices are available but much of the size and performance advantage is
sacrificed. Solid-tantalum chip capacitors are probably the best choice if a
surface-mounted device is an absolute must. Products such as the AVX TPS
family and the Sprague 593D family were developed for power-supply
applications. These products offer a low ESR that is relatively stable over the
temperature range, high ripple-current capability, low ESL, surge-current testing,
and a high ratio of capacitance to volume.
5.2
Output Inductance
In switching power supply power stages, the function of inductors is to store
energy. The energy is stored in their magnetic field due to the current flowing.
Thus, qualitatively, the function of an inductor is usually to attempt to maintain a
constant current or sometimes to limit the rate of change of current flow.
24
SLVA057
Component Selection
The value of output inductance of a buck power stage is generally selected to limit
the peak-to-peak ripple current flowing in it. In doing so, the power stage’s mode
of operation, continuous or discontinuous, is determined. The inductor ripple
current is directly proportional to the applied voltage and the time that the voltage
is applied, and it is inversely proportional to its inductance. This was explained
in detail previously.
Many designers prefer to design the inductor themselves but that topic is beyond
the scope of this report. However, the following discusses the considerations
necessary for selecting the appropriate inductor.
In addition to the inductance, other important factors to be considered when
selecting the inductor are its maximum dc or peak current and maximum
operating frequency. Using the inductor within its dc current rating is important to
insure that it does not overheat or saturate. Operating the inductor at less than
its maximum frequency rating insures that the maximum core loss is not
exceeded, resulting in overheating or saturation.
Magnetic component manufacturers offer a wide range of off-the-shelf inductors
suitable for dc/dc converters, some of which are surface mountable. There are
many types of inductors available; the most popular core materials are ferrites
and powdered iron. Bobbin or rod-core inductors are readily available and
inexpensive, but care must be exercised in using them because they are more
likely to cause noise problems than are other shapes. Custom designs are also
feasible, provided the volumes are sufficiently high.
Current flowing through an inductor causes power dissipation due to the
inductor’s dc resistance; the power dissipation is easily calculated. Power is also
dissipated in the inductor’s core due to the flux swing caused by the ac voltage
applied across it but this information is rarely directly given in manufacturer’s data
sheets. Occasionally, the inductor’s maximum operating frequency and/or
applied volt-seconds ratings give the designer some guidance regarding core
loss. The power dissipation causes a temperature increase in the inductor.
Excessive temperature can cause degradation in the insulation of the winding
and also cause increased core loss. Care should be exercised to insure all the
inductor’s maximum ratings are not exceeded.
The loss in the inductor is given by:
P inductor
+ I 2Lrms
R Cu
) PCore [ I 2O
R Cu
) PCore
where, RCu is the winding resistance.
5.3
Power Switch
In switching power supply power stages, the function of the power switch is to
control the flow of energy from the input power source to the output voltage. In
a buck power stage, the power switch (Q1 in Figure 1) connects the input to the
output filter when the switch is turned on and disconnects when the switch is off.
The power switch must conduct the current in the output inductor while on and
block the full input voltage when off. Also, the power switch must change from one
state to the other quickly in order to avoid excessive power dissipation during the
switching transition.
Understanding Buck Power Stages in Switchmode Power Supplies
25
Component Selection
The type of power switch considered in this report is a power MOSFET. Other
power devices are available but in most instances, the MOSFET is the best
choice in terms of cost and performance (when the drive circuits are considered).
The two types of MOSFET available for use are the n-channel and the p-channel.
P-channel MOSFETs are popular for use in buck power stages because driving
the gate is simpler than the gate drive required for an n-channel MOSFET.
The power dissipated by the power switch is given by:
P D(MOSFET)
+ I 2O RDS(on) D ) 12
f s ) Q Gate V GS f s
VI
IO
ǒ)Ǔ
tr
tf
Where:
tr and tf are the MOSFET turn-on and turn-off switching times
QGate is the MOSFET gate-to-source capacitance
Other than selecting p-channel or n-channel, other parameters to consider while
selecting the appropriate MOSFET are the maximum drain-to-source breakdown
voltage, V(BR)DSS, and the maximum drain current, ID(Max).
The MOSFET selected should have a V(BR)DSS rating greater than the maximum
input voltage, and some margin should be added for transients and spikes. The
MOSFET selected should also have an ID(Max) rating of at least two times the
maximum power stage output current. However, many times this is not sufficient
margin and the MOSFET junction temperature should be calculated to make sure
that it is not exceeded. The junction temperature can be estimated as follows:
TJ
+ TA ) PD
R QJA
Where:
TA is the ambient or heatsink temperature
RΘJA is the thermal resistance from the MOSFET chip to the ambient
air or heatsink.
5.4
Catch Rectifier
The catch rectifier conducts when the power switch turns off and provides a path
for the inductor current. Important criteria for selecting the rectifier include: fast
switching, breakdown voltage, current rating, low–forward voltage drop to
minimize power dissipation, and appropriate packaging. Unless the application
justifies the expense and complexity of a synchronous rectifier, the best solution
for low–voltage outputs is usually a Schottky rectifier. The breakdown voltage
must be greater than the maximum input voltage, and some margin should be
added for transients and spikes. The current rating should be at least two times
the maximum power stage output current (normally the current rating will be much
higher than the output current because power and junction temperature
limitations dominate the device selection).
26
SLVA057
Component Selection
The voltage drop across the diode in a conducting state is primarily responsible
for the losses in the diode. The power dissipated by the diode can be calculated
as the product of the forward voltage and the output load current for the time that
the diode is conducting. The switching losses which occur at the transitions from
conducting to nonconducting states are very small compared to conduction
losses and are usually ignored.
The power dissipated by the catch rectifier is given by:
P D(Diode)
+ VD
IO
(1
* D)
where VD is the forward voltage drop of the catch rectifier.
The junction temperature can be estimated as follows:
TJ
+ TA ) PD
R QJA
Understanding Buck Power Stages in Switchmode Power Supplies
27
Example Designs
6 Example Designs
An example design using a buck power stage, the TPS2817 MOSFET driver, and
the TL5001 controller is given in SLVP097 Buck Converter Evaluation Module
User’s Guide, Texas Instruments Literature Number SLVU002A.
Another example design using a buck power stage and the TL5001 controller is
given in SLVP087 Buck Converter Evaluation Module User’s Guide, Texas
Instruments Literature Number SLVU003A.
A third example design using a buck power stage, the TPS2817 MOSFET driver,
and the TL5001 controller is given in SLVP101, SLVP102, and SLVP103 Buck
Converter Design Using the TL5001 User’s Guide, Texas Instruments Literature
Number SLVU005.
28
SLVA057
Summary
7 Summary
This application report described and analyzed the operation of the buck power
stage. The two modes of operation, continuous conduction mode and
discontinuous conduction mode, were examined. Steady-state and small-signal
were the two analyses performed on the buck power stage. The synchronous
buck power stage and the forward converter power stage were presented as
variations of the basic buck power stage and a few of the other possible variations
were listed.
The main results of the steady-state analyses are summarized below.
The voltage conversion relationship for CCM is:
VO
+ ǒV * V Ǔ
I
* Vd
D
DS
(1
* D) * IL
RL
which simplifies to:
VO
+V
D
I
Ǹ)
The voltage conversion relationship for DCM is:
VO
+V
) RL
R
I
R
1
where K is defined as:
+ R2
K
)
1
2
4 K
D2
)
R
R RL
L
TS
The DCM voltage conversion relationship can be simplified to:
VO
+V
I
1
)
Ǹ)
2
1
4 K
D2
The major results of the small-signal analyses are summarized below.
The small-signal duty-cycle-to-output transfer function for the buck power stage
operating in CCM is given by:
v^ O
^
d
(s)
+ VI
R
ƪǒ Ǔ ƫ
) Rc C
R RL
Rc )
) R)LR ) s2
R)R L
L
1
) RL
R
1
)s
C
L
C
Understanding Buck Power Stages in Switchmode Power Supplies
)
)
R RC
R RL
29
Summary
The small-signal duty-cycle-to-output transfer function for the buck power stage
operating in DCM is given by:
v^ O
^
d
+ Gdo
Where
G do
and
+2
1
1
VO
D
M
w p + 21 *
*M
) wsp
1
2
*M
*M
1
R
C
Also presented were requirements for the buck power stage components based
on voltage and current stresses applied during the operation of the buck power
stage.
For further study, several references are given in addition to example designs.
30
SLVA057
References
8 References
1. Application Report Designing With The TL5001 PWM Controller, TI Literature
Number SLVA034A.
2. Application Report Designing Fast Response Synchronous Buck Regulators
Using theTPS5210, TI Literature Number SLVA044.
3. V. Vorperian, R. Tymerski, and F. C. Lee, Equivalent Circuit Models for
Resonant and PWM Switches, IEEE Transactions on Power Electronics,
Vol. 4, No. 2, pp. 205–214, April 1989.
4. R. W. Erickson, Fundamentals of Power Electronics, New York: Chapman
and Hall, 1997.
5. V. Vorperian, Simplified Analysis of PWM Converters Using the Model of the
PWM Switch: Parts I and II, IEEE Transactions on Aerospace and Electronic
Systems, Vol. AES–26, pp. 490–505, May 1990.
6. E. van Dijk, et al., PWM-Switch Modeling of DC-DC Converters, IEEE
Transactions on Power Electronics, Vol. 10, No. 6, pp. 659–665, November
1995.
7. G. W. Wester and R. D. Middlebrook, Low-Frequency Characterization of
Switched Dc-Dc Converters, IEEE Transactions an Aerospace and
Electronic Systems, Vol. AES–9, pp. 376–385, May 1973.
8. R. D. Middlebrook and S. Cuk, A General Unified Approach to Modeling
Switching-Converter Power Stages, International Journal of Electronics,
Vol. 42, No. 6, pp. 521–550, June 1977.
9. E. Rogers, Control Loop Modeling of Switching Power Supplies, Proceedings
of EETimes Analog & Mixed-Signal Applications Conference, July 13–14,
1998, San Jose, CA.
Understanding Buck Power Stages in Switchmode Power Supplies
31
32
SLVA057
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