INEU-2013-05-slva061 [149,89 KiB]

INEU-2013-05-slva061 [149,89 KiB]
Understanding Boost Power
Stages in Switchmode
Power Supplies
Application
Report
March 1999
Mixed Signal Products
SLVA061
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Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Boost Power Stage Steady-State Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1 Boost Steady-State Continuous Conduction Mode Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Boost Steady-State Discontinuous Conduction Mode Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Critical Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Boost Power Stage Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1 Boost Continuous Conduction Mode Small-Signal Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 Boost Discontinuous Conduction Mode Small-Signal Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 Component Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Output Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Output Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Power Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Output Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
19
20
21
22
5 Example Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Understanding Boost Power Stages
iii
Figures
List of Figures
1 Boost Power Stage Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Boost Power Stage States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 Continuous Mode Boost Power Stage Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4 Boundary Between Continuous and Discontinuous Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5 Discontinuous Current Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6 Discontinuous Mode Boost Power Stage Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7 Power Supply Control Loop Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8 Boost Nonlinear Power Stage Gain vs. Duty Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9 DC and Small Signal CCM PWM Switch Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10 CCM Bost Power Stage Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
11 Averaged (Nonlinear) DCM PWM Switch Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12 DCM Boost Power Stage DC Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
13 Small Signal DCM Boost Power Stage Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
iv
SLVA061
Understanding Boost 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 boost power stage
only and does not cover control circuits. The report includes detailed steady-state and
small-signal analysis of the boost power stage operating in continuous and discontinuous
modes. A discussion of power stage component requirements is included.
1 Introduction
The three basic switching power supply topologies in common use are the buck,
boost, and buck-boost. These topologies are non-isolated, i.e., the input and
output voltages share a common ground. There are however, isolated derivations
of these non-isolated topologies. The power supply topology refers to how the
switches, output inductor, and output capacitor are connected. Each topology
has unique properties, including 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 boost is a popular non-isolated power stage topology, sometimes called a
step-up power stage. Power supply designers choose the boost power stage
because the required output voltage is always higher than the input voltage, is
the same polarity, and is not isolated from the input. The input current for a boost
power stage is continuous, or non-pulsating, because the input current is the
same as the inductor current. The output current for a boost power stage is
discontinuous, or pulsating, because the output diode conducts only during a
portion of the switching cycle. The output capacitor supplies the entire load
current for the rest of the switching cycle.
This application report describes steady-state operation and gives ideal
waveforms for the boost converter in continuous and discontinuous modes. The
duty-cycle-to-output-voltage transfer function is given using the PWM switch
model.
Figure 1 shows a simplified schematic of the boost power stage with a drive circuit
block included. Power switch Q1 is an n-channel MOSFET. The output diode is
CR1. Inductor L and capacitor C make up the effective output filter. The capacitor
equivalent series resistance (ESR), RC , and the inductor dc resistance, RL , are
included in the analysis. Resistor R represents the load seen by the power supply
output.
1
Boost Power Stage Steady-State Analysis
L
RL
c
p
IL = ic
g
+
VI
VO
d
CR1
Q1
C
R
s
Drive
Circuit
RC
a
ia
Figure 1. Boost Power Stage Schematic
During normal operation of the boost power stage, Q1 is repeatedly switched on
and off with the on and off times governed by the control circuit. This switching
action creates a train of pulses at the junction of Q1, CR1, and L. Although
inductor L is connected to output capacitor C only when CR1 conducts, an
effective L/C output filter is formed. It filters the train of pulses to produce a dc
output voltage, VO. The following sections give a more detailed quantitative
analysis.
2 Boost Power Stage Steady-State Analysis
A power stage can operate in continuous or discontinuous inductor current mode.
In continuous inductor current mode, current flows continuously in the inductor
during the entire switching cycle in steady-state operation. In discontinuous
inductor current mode, inductor current is 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 modes are discussed in greater detail later, and design guidelines
are given for the inductor value to maintain a chosen mode of operation as a
function of rated load. It is 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 MOSFET. The advantages of using an n-channel MOSFET are its
lower RDS(on) (compared to a p-channel MOSFET), and the ease of driving it in
a boost power stage configuration.
Transistor Q1 and diode CR1 are drawn inside a dashed-line box with terminals
labeled a, p, and c. This is explained in the Boost Power Stage Modeling section.
2.1
Boost 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 boost power stage. This result
is important because it shows how the output voltage depends on duty cycle and
input voltage, or how, conversely, the duty cycle can be calculated based on input
and output voltages. 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.
2
SLVA061
Boost Power Stage Steady-State Analysis
In continuous conduction mode, the boost power stage assumes two states per
switching cycle. In the on state, Q1 is on and CR1 is off. In the off state, 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 circuit during each
state. Figure 2 shows the linear circuit diagram for each of the two states.
RL
L
p
c
VO
IL = ic
+
ON
State
C
RDS(on)
VI
RC
a
R
ia
RL
L
Vd
c
p
VO
IL = ic
+
OFF
State
C
VI
RC
ia
R
a
Figure 2. Boost 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 TOFF. Since there are
only two states per switching cycle for continuous conduction 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.
Understanding Boost Power Stages
3
Boost Power Stage Steady-State Analysis
IQ1
ICR1
IL Solid
IO Dashed
VDS–Q1 Solid
VO Dashed
TON
TOFF
TS
Figure 3. Continuous Mode Boost Power Stage Waveforms
Refer to Figures 1 and 2. During the on state, Q1, which presents a low
drain-to-source resistance, RDS(on) , has a small voltage drop of VDS . 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 across inductor L.
Diode CR1 is off during this time because it is reverse biased. The voltage applied
to the right side of L is the MOSFET on voltage, VDS . The inductor current, IL , flows
from the input source, VI , through Q1 to ground. During the on state, the voltage
across the inductor is constant and equal to VI – (VDS + IL × RL ). Adopting the
polarity convention for 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.
The inductor-current increase can be calculated by using a version of the familiar
relationship:
VL
+L
di L
dt
å DIL + VLL
DT
The inductor current increase during the on state is given by:
DI L
(
)) + VI * (VDSL) IL
R L)
T ON
The quantity ∆IL (+) is the inductor ripple current. During this period, all of the
output load current is supplied by output capacitor C.
4
SLVA061
Boost Power Stage Steady-State Analysis
Refer to Figure 1 and Figure 2. When Q1 is off, it presents a high drain-to-source
impedance. Therefore, since the current flowing in 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 applied to the left side
of L remains the same as before at VI – IL × RL . The voltage applied to the right
side of L is now the output voltage, VO , plus the diode forward voltage, Vd . The
inductor current, IL , now flows from the input source, VI , through CR1 to the output
capacitor and load resistor combination. During the off state, the voltage across
the inductor is constant and equal to (VO + Vd + IL × RL ) – VI . Maintaining the 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.
ǒ
Ǔ
The inductor current decrease during the off state is given by:
DI L
( –)
+
VO
) Vd ) IL
RL – VI
L
T OFF
The quantity ∆IL (–) is also the inductor ripple current.
In steady-state conditions, the current increase, ∆IL (+), during the on time and the
current decrease, ∆IL (–), during the off time are 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 boost voltage
conversion relationship:
ǒ Ǔ
The steady-state equation for VO is:
VO
+ ǒVI – IL
Ǔ
RL
1
) TTON
OFF
– V d – V DS
ǒ Ǔ
T ON
T OFF
And, using TS for TON + TOFF, and using D = TON / TS and (1–D) = TOFF / TS , the
steady-state equation for VO is:
VI – IL R L
D
VO
– V d – V DS
1–D
1–D
+
Notice that in simplifying the above, TON +TOFF is assumed to be equal to TS . This
is true only for continuous conduction mode, as the discontinuous conduction
mode analysis will show.
NOTE: An important observation: 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 it 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 used
repeatedly in this report. Volt-second balance on the inductor is a
physical necessity, and should be understood at least as well as
Ohms Law.
Understanding Boost Power Stages
5
Boost Power Stage Steady-State Analysis
In the above equations for ∆IL (+) and ∆IL (–), the output voltage was implicitly
assumed to be constant with no ac ripple voltage during the on and off times. This
is a common simplification and involves two separate effects. First, the output
capacitor is assumed to be large enough so that its voltage change is negligible.
Second, the voltage due to the capacitor ESR is 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 that VO can be
adjusted by adjusting the duty cycle, D, and is always greater than the input
because D is a number between 0 and 1. This relationship approaches one as
D approaches zero and increases without bound as D approaches one. 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
+ 1V–D
I
A simplified, qualitative way to visualize the circuit operation is to consider the
inductor as an energy storage element. When Q1 is on, energy is added to the
inductor. When Q1 is off, the inductor and the input voltage source deliver energy
to the output capacitor and load. The output voltage is controlled by setting the
on time of Q1. For example, by increasing the on time of Q1, the amount of energy
delivered to the inductor is increased. More energy is then delivered to the output
during the off time of Q1 resulting in an increase in the output voltage.
Unlike the buck power stage, the average of the inductor current is not equal to
the output current. To relate the inductor current to the output current, refer to
Figure 2 and Figure 3. Note that the inductor delivers current to the output only
during the off state of the power stage. This current averaged over a complete
switching cycle is equal to the output current because the average current in the
output capacitor must be equal to zero.
The relationship between the average inductor current and the output current for
the continuous mode boost power stage is given by:
I L(Avg)
T OFF
TS
or,
I L(Avg)
+
+I
L(Avg)
(1–D)
+I
O
ǒǓ
IO
1–D
Another important observation is that the average inductor current is proportional
to the output current, and since the inductor ripple current, ∆IL , is independent of
output load current, the minimum and the maximum values of the inductor current
track the average inductor current exactly. For example, if the average inductor
current decreases by 2 A due to a load current decrease, then the minimum and
maximum values of the inductor current decrease by 2 A (assuming continuous
conduction mode is maintained).
6
SLVA061
Boost Power Stage Steady-State Analysis
The foregoing analysis was for the boost 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 boost power
stage.
2.2
Boost Steady-State Discontinuous Conduction Mode Analysis
Now consider what happens when the load current is decreased and the
conduction mode changes from continuous to discontinuous. Recall that for
continuous conduction mode, the average inductor current tracks the output
current, i.e., if the output current decreases, then so does the average inductor
current. In addition, the minimum and maximum peaks of the inductor current
follow the average inductor current exactly.
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 is 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 boost 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 discontinuous current mode. A
power stage operating in discontinuous mode has three unique states during
each switching cycle as opposed to two states for continuous mode. Figure 4
shows the inductor current condition where the power stage is at the boundary
between continuous and discontinuous mode. This is where the inductor current
just 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
current conduction mode. Figure 5 shows this condition. The discontinuous mode
power stage frequency response is quite different from the continuous mode
frequency response and is given in the Boost Power Stage Modeling section.
Also, the input-to-output relationship is quite different, as the following derivation
shows.
Understanding Boost Power Stages
7
Boost Power Stage Steady-State Analysis
IL Solid
IO Dashed
∆IL
0
DTs
D3Ts
D2Ts
TS
Figure 5. Discontinuous Current Mode
To begin the derivation of the discontinuous current mode boost power stage
voltage conversion ratio, recall 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
the continuous mode case, and the circuits of Figure 9 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:
DI L
(
)) + VL
I
T ON
+ VL
I
D
TS
+I
PK
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:
DI L
( –)
+V
O
– VI
L
T OFF
+V
O
– VI
L
D2
TS
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:
8
SLVA061
Boost Power Stage Steady-State Analysis
VO
+V
)
T ON T OFF
T OFF
I
+V
D
I
) D2
D2
Now calculate the output current (the output voltage VO divided by the output load
R). It is the average over the complete switching cycle of the inductor current
during the D2 interval.
IO
ƪ
+ VR + T1
1
2
O
I PK
D2
ƫ
TS
ƪǒ Ǔ ƫ
S
Now, substitute the relationship for IPK (∆IL (+)) into the above equation to obtain:
IO
+ VR + T1
IO
+ VR + V
S
O
VI
L
1
2
O
D
2
I
D2
L
D
TS
D2
TS
TS
We now have two equations, one for the output current just derived, and one for
the output voltage, both in terms of VI , D, and D2. Now solve each equation for
D2 and set the two equations equal to each other. The resulting equation can be
used to derive an expression for the output voltage, VO .
The discontinuous conduction mode boost voltage conversion relationship is
given by:
VO
+V
1
)
I
Ǹ)
1
4
D2
K
2
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,
switching frequency, and output load resistance; for continuous conduction
mode, the voltage conversion relationship is only dependent on the input voltage
and duty cycle.
Understanding Boost Power Stages
9
Boost Power Stage Steady-State Analysis
IQ1
ICR1
IL Solid
IO Dashed
∆IL
VDS–Q1 Solid
VO Dashed
D3*TS
D*TS
D2*TS
TS
Figure 6. Discontinuous Mode Boost Power Stage Waveforms
In typical applications, the boost power stage is operated in either continuous
conduction mode or discontinuous conduction mode. For a particular application,
one conduction mode is chosen and the power stage is designed to maintain the
same mode. The next section gives relationships for the power stage inductance
to allow it to operate in only one conduction mode, given ranges for input and
output voltage and output load current.
2.3
Critical Inductance
The previous analyses for the boost 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 boost 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, 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.
10
SLVA061
Boost Power Stage Steady-State Analysis
First, define IO(Crit) as the minimum output current to maintain continuous
conduction mode, normally referred to as the critical current. This value is shown
in Figure 4. Since we are working toward a minimum value for the inductor, it is
more straightforward to perform the derivation using the inductor current. The
minimum average inductor current to maintain continuous conduction mode is
given by:
I (min–avg)
+ D2I
L
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 for the boost power stage (giving
the largest Lmin ) is at an input voltage equal to one-half of the output voltage
because this gives the maximum ∆IL .
Now, substituting and solving for Lmin :
L min
ǒ
w 12
VI
)V
DS
– IL
RL
Ǔ
T ON
I L(min)
The above equation can be simplified and put in a form that is easier to apply as
shown:
L min
w 16V
O
TS
I O(crit)
Using the inductor value just calculated ensures continuous conduction mode
operation for output load currents above the critical current level, IO(crit) .
Understanding Boost Power Stages
11
Boost Power Stage Modeling
3 Boost Power Stage Modeling
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.
Most power stages have a nonlinear voltage conversion ratio versus duty cycle.
Figure 8 illustrates this nonlinearity with a graph of the steady-state voltage
conversion ratio for a boost power stage operating in continuous conduction
mode as a function of steady-state duty cycle, D.
The nonlinear characteristics are a result of the switching action of the power
stage switching components, Q1 and CR1. Vorperian [5] observed 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
SLVA061
Boost Power Stage 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 of modeling power stages is to represent the ac behavior at
a given operating point and to be linear around the operating point. Linearity
allows the use of the the many analysis tools available for linear systems.
Referring again to Figure 8, choosing the operating point at D = 0.7, allows a
straight line to 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, if the variations
in duty cycle are kept small, a linear model accurately represents the nonlinear
behavior of the power stage being analyzed.
Since power stages can operate in continuous conduction mode (CCM) or
discontinuous conduction mode (DCM), the PWM switch models for the two
conduction modes are explained below.
3.1
Boost Continuous Conduction Mode Small–Signal Analysis
To model the boost power stage operation in CCM, use the CCM PWM switch
model derived in the application report Understanding Buck Power Stages in
Switchmode Power Supplies, TI Literature Number SLVA057. The PWM switch
model is inserted into the power stage circuit by replacing the switching elements.
The CCM PWM switch model is shown in Figure 9. This model is useful for
determining the DC operating point of a power stage and for finding ac transfer
functions of a power stage.
In Figure 1, the power transistor, Q1, and the catch diode, CR1, are drawn inside
a dashed-line box. These are the components that are replaced by the PWM
switch equivalent circuit. Terminals a, p, and c are terminal labels of the PWM
switch model.
Understanding Boost Power Stages
13
Boost Power Stage Modeling
∧
d
Vap
D
a
c
+
–
ia
ic
D
1
∧
Ic d
p
Figure 9. DC and Small Signal CCM PWM Switch Model
Terminal a (active) connects to the active switch. Similarly, terminal p (passive)
connects to the passive switch. Lastly, terminal c (common) is common to both
the active and passive switches. All three commonly used power stage topologies
contain active and passive switches and the above terminal definitions can be
applied. In addition, substituting the PWM switch model 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 in
Figure 9 into the power stage in the appropriate orientation.
In the PWM switch model of Figure 9 and subsequent occurrences of the model,
the capital letters indicate steady-state (or dc) quantities dependent on the
operating point of the circuit under study. The lowercase letters indicate
time-variable quantities that can have dc and ac components. The lowercase
letters with a caret (hat) indicate the small ac variations of that particular variable.
^
For example, D represents the steady-state duty cycle, d represents small ac
variations of the duty cycle, and d or d(t) represents the complete duty cycle
including any dc component and ac variations.
The PWM switch model of Figure 9 is inserted into the boost power stage
schematic of Figure 1 by replacing transistor Q1 and output diode CR1 with the
model. Figure 10 shows the resulting model for the boost power stage. Examples
of dc analysis and ac small-signal analysis are given.
L
RL
c
D
IL = ic
p
1
–
+
VI
Vap ∧
d
D
C
+
∧
Ic d
ia
R
RC
a
Figure 10. CCM Boost Power Stage Model
14
SLVA061
VO
Boost Power Stage Modeling
An example dc analysis follows to illustrate how simple power stage analysis
^
becomes with the PWM switch model. For dc analysis, d is zero, L is a short, and
C is open. Then a simple loop equation gives:
–V I
but
V cp
)I
RL
C
+V
ap
)V )V +0
D
cp
+ –V
O
D
O
and
+ VRO + Ic–Ia + Ic–Ic
IO
D
å Ic + VRO
1
1–D
Substituting the above relationships for Vcp and Ic into the first equation and
solving for VO , gives:
VO
+V
1
1–D
I
1
)
1
RL
@ǒ Ǔ2
R 1–D
The above equation is usually expressed as a ratio of the output voltage, VO , to
the input voltage, VI , and is usually called M as shown:
M
1
+ VV + 1–D
O
I
1
)
1
RL
@ǒ Ǔ 2
R 1–D
An ac analysis can not be performed until after the dc analysis is completed
because PWM switch parameters Vap and Ic must be determined from the dc
analysis. 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 dutycycle-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 determines the parameter values of
the dependent sources; for example:
V ap
+ –V
O
and
IC
V
+ I + 1I–D + R Vǒ1–D Ǔ + RM ǒ1–D
Ǔ
O
O
I
L
These two equations are then used with loop equations to derive the
duty-cycle-to-output-voltage transfer function from the circuit shown in
Figure 10. Then set the input voltage equal to zero to get only the ac component
of the transfer function. Without going through all the details, it can be shown that
the transfer function can be expressed as:
Understanding Boost Power Stages
15
ǒ) Ǔ ǒ Ǔ
Boost Power Stage Modeling
v^ O
^
(s)
d
+G
s
w z1
1
do
1
1– ws
) w os Q )
z2
s2
w 2o
where,
G do
V
[ (1–D
)
2
ω z1
+R
C
ω z2
[ (1–D)
I
1
C
2
Ǹ
R–R L
L
)
[ ǸL 1 C R ) (1–D
R
ω
Q[
RL
) C ǒR1)R Ǔ
L
ωo
L
2
R
O
C
3.2
Boost Discontinuous Conduction Mode Small–Signal Analysis
To model the boost power stage operation in discontinuous conduction mode
(DCM), follow a similar procedure as above for CCM. A PWM switch model is
inserted into the power stage circuit by replacing the switching elements. The
derivation for the DCM PWM switch model is given in the application report
Understanding Buck-Boost Power Stages in Switchmode Power Supplies, TI
Literature Number SLVA059. This derivation can also be found in Fundamentals
of Power Electronics.[4] Figure 11 shows the large signal nonlinear version of the
DCM PWM switch model. 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
+ D2
2
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 .
a
p
p(t)
Re
c
Figure 11. Averaged (Nonlinear) DCM PWM Switch Model
16
SLVA061
Boost Power Stage Modeling
To illustrate discontinuous conduction mode power supply steady-state analysis
using this model, examine the boost power stage. The analysis proceeds like the
CCM case. The equivalent circuit is substituted into the original circuit. The
inductor is treated as a short circuit and the capacitor is treated as an open circuit.
Figure 12 shows the DCM boost power stage model schematic.
L
c
p
VO
p(t)
Re
+
C
R
VI
a
Figure 12. DCM Boost Power Stage DC Model
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: The power dissipated by Re is:
VI 2
Re
The power delivered by the dependent power source is:
V p(t)
I p(t )
+
ǒ Ǔ
VO
R
V O–V I
ǒǓ ǒ Ǔ ǒǓ ǒ Ǔ
Setting the two power expressions equal to each other, after rearranging, gives:
ǒǓ
1 –V
O
R
VO2
VI
R
+
VI 2
Re
åV
2
O
1 –V
O
VI
(1)– V I
R
Re
+0
Solving the quadratic for VO , taking the larger of the two roots and simplifying,
gives the voltage conversion relationship as before.
VO
+V
1
I
)
Ǹ)
1
4
D2
K
2
Where
K
+ R2
L
TS
To derive the small signal model, the circuit of Figure 12 is perturbed and
linearized following a procedure similar to the one in the CCM derivation. See
reference [4] for the details of the derivation. Figure 13 shows the resulting small
signal model for the boost power stage operating in DCM.
Understanding Boost Power Stages
17
Boost Power Stage Modeling
(M–1)2
M
∧
VI
× Re
+
2 × M × VI
1
∧
∧
× VO
×d
D × (M–1) Re
(M–1)2 × Re
2 × M–1
∧
× VI
(M–1)2 × Re
∧
VO
2 × VI
∧
⋅× d (M–1)2 × Re
D × (M–1) ⋅Re
C
Figure 13. Small Signal DCM Boost Power Stage Model
The duty-cycle-to-output transfer function for the buck-boost power stage
operating in DCM is given by:
+G
v^ O
^
d
1
do
) wsp
1
Where
G do
M
+2
+ VV
VO
D
2
M–1
M–1
O
I
and
ωp
18
+ (M–21)
SLVA061
M–1
R C
R
Component Selection
4 Component Selection
This section discusses the function of the main components of the buck-boost
power stage, and gives electrical requirements and applied stresses for each
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 the power supply specification. Many times, the power supply
specification determines individual component requirements.
4.1
Output Capacitance
In switching power supply power stages, the function of output capacitance is to
store energy. The energy is stored in its electric field due to the voltage applied.
Thus, qualitatively, the function of a capacitor is to attempt to maintain a constant
voltage.
The output capacitance for a boost power stage is generally selected to limit
output voltage ripple to the level required by the specification. The series
impedance of the capacitor and the power stage output current determine 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
discussion gives guidelines for output capacitor selection.
For continuous inductor current mode operation, to determine the amount of
capacitance needed as a function of output load current, IO , 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. This
is because the output capacitor supplies the entire output load current during the
power stage on-state.
C
wI
O(Max)
fs
D Max
DV O
where,
IO(Max) is the maximum output current
and
DMax is the maximum duty cycle.
For discontinuous inductor current mode operation, to determine the amount of
capacitance needed, the following equation is used assuming all the output
voltage ripple is due to the capacitor’s capacitance.
C
w
I O(Max)
ȡȧ Ǹ ȣȧ
ȢD Ȥ
1–
fS
2 L
R TS
VO
In many practical designs, to get the required ESR, a capacitor with much more
capacitance than is needed must be selected.
For continuous 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 Boost Power Stages
19
Component Selection
ESR
v
ǒ
DV O
I O(Max)
1–D Max
Ǔ
) D2IL
For 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 simply:
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 output diode current, ICR1, 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
+I
O
Ǹ
D
1–D
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.
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.
4.2
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 flow of current.
Thus, qualitatively, the function of an inductor is usually to attempt to maintain a
constant current or, equivalently, to limit the rate of change of current flow.
20
SLVA061
Component Selection
The value of output inductance of a boost 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 in the inductor due
to its 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 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
+
ǒǓ
IO
1–D
2
R Cu
)P
Core
where, RCu is the winding resistance.
4.3
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 boost 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 inductor while on and block the
full output 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 Boost Power Stages
21
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.
n-channel MOSFETs are popular for use in boost power stages because driving
the gate is simpler than the gate drive required for a p-channel MOSFET.
ǒǓ
ǒ Ǔǒ
The power dissipated by the power switch, Q1, is given by:
P D(Q1)
+
IO
1–D
where
2
R DS(on)
D
)
1
2
ǒV Ǔ
O
IO
1–D
2
tr
)tǓ
f
fs
)Q
Gate
V GS
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 versus 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
output 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 inductor current. However, many times the junction temperature is the
limiting factor, so the MOSFET junction temperature should also be calculated to
make sure that it is not exceeded. The junction temperature can be estimated as
follows:
TJ
+T )P
where
A
D
R ΘJA
TA is the ambient or heatsink temperature
RΘJA is the thermal resistance from the MOSFET chip to the ambient
air or heatsink.
4.4
Output Diode
The output diode 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 output 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).
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. The switching
losses which occur at the transitions from conducting to non conducting states
are very small compared to conduction losses and are usually ignored.
22
SLVA061
fs
Component Selection
The power dissipated by the output rectifier is given by:
P D(Diode)
+V
D
IO
where VD is the forward voltage drop of the output rectifier.
The junction temperature can be estimated as follows:
TJ
+T )P
A
D
R ΘJA
Understanding Boost Power Stages
23
Example Design
5 Example Design
An example design using a boost power stage and the TL5001 controller is given
in SLVP088 20 V to 40 V Adjustable Boost Converter Evaluation Module User’s
Guide, Texas Instruments Literature Number SLVU004.
24
SLVA061
Summary
6 Summary
This application report described and analyzed the operation of the boost power
stage. The two modes of operation, continuous conduction mode and
discontinuous conduction mode, were examined. Steady-state and small-signal
analyses were performed on the boost power stage.
The main results of the steady-state analyses are summarized below.
The voltage conversion relationship for CCM is:
VO
+ V –I1–D R
I
L
L
D
1–D
–V d–V DS
or, a slightly simpler version:
VO
+V
1
1–D
I
1
)
1
RL
R (1–D)
2
which can be simplified to:
VO
+V
1
1–D
I
The relationship between the average inductor current and the output current for
the continuous mode boost power stage is given by:
I L(Avg)
+ (1I–D)
O
The discontinuous conduction mode boost voltage conversion relationship is
given by:
VO
+V
1
I
Ǹ
) 1 ) 4 KD
2
2
Where K is defined as:
K
+ R2
L
TS
The major results of the small-signal analyses are summarized below.
The small-signal duty-cycle-to-output transfer function for the boost power stage
operating in CCM is given by:
v^ O
^
(s)
d
+G
ǒ) Ǔ ǒ Ǔ
s
w z1
1
do
1
1– ws
) w os Q )
z2
s2
w 2o
where,
Understanding Boost Power Stages
25
Summary
G do
V
[ (1–D
)
2
ω z1
+R
C
ω z2
[ (1–D)
I
1
C
2
Ǹ
R–R L
L
)
[ ǸL 1 C R ) (1–D
R
ω
Q[
RL
) C ǒR1)R Ǔ
L
ωo
L
2
R
O
C
The small-signal duty-cycle-to-output transfer function for the boost power stage
operating in DCM is given by:
v^ O
^
d
+G
1
do
1
) wsp
where
G do
M
+2
+ VV
VO
D
2
M–1
M–1
O
I
and
ωp
+ (M–21)
M–1
R C
Also presented are requirements for the boost power stage components based
on voltage and current stresses applied during the operation of the boost power
stage.
For further study, an example design and several references are given.
26
SLVA061
References
7 References
1. Application Report Designing With The TL5001 PWM Controller, TI Literature
Number SLVA034A.
2. Application Report Designing Fast Response Synchronous Buck Regulators
Using the TPS5210, 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.
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Understanding Boost Power Stages
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SLVA061
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