405_Switch_Mode_Converters

405_Switch_Mode_Converters
In this Session
•
•
•
•
•
•
DC-DC Switch-Mode Converters
Applications:
• Regulated switch mode dc power supplies
• dc motor drives
DC-DC power conversion
Linear Vs Switch based
Control of DC-DC converters
Characteristics of Buck
Inductor effects
Design Considerations
dc-dc Converters:
• Step-down (buck) converter
• Step-up (boost) converter
• Step-down/step-up (buck-boost) converter
• Cuk converter
• Full-bridge converter
7-1
7-2
1
Power Conversion
Functional Block Diagram of DC-DC Converter System
• Analogous to Transformer:
– Linear System
– Switching System
• Linear systems can only step down
Controlled dc output at a desired
voltage level
Unregulated dc voltage obtained by
rectifying the line voltage, and therefore will
fluctuate with line voltage magnitude
7-3
– Bipolar transistor or MOSFET pass unit
operated in linear mode; i.e. Drive to pass unit
is proportionally changed to maintain the
output voltage
– Dissipates considerable power (Vdrop Iload)
7-4
2
Linear Power Supplies
Linear Regulator
• Headroom Loss causes linear regulator to be
35-65% efficient
• Has to consider a costly heat sink: eg. If a 5V
regulator has 12V input and supplying 100mA, it
must dissipate 700mW to deliver 500mW to the
load (42% efficient)
• But still cost effective in step down applications
for <10W applications (exception: low drop-out
regulator)
• Linear regulators are simple and cheap;
considerable quieter as no HF switching
7-5
7-6
3
7-7
7-8
4
Control of DC-DC Converters
In a dc-dc converter:
• Average output dc voltage must be controlled to equal a desired level.
• Utilizes one or more switches to transform dc from one level to another.
• The average output voltage is controlled by controlling the switch on and
off durations (ton and toff).
• Let’s consider the following switch-mode dc-dc converter:
•
•
•
7-9
Average output dc voltage Vo depends on ton and toff.
Switching is done at a constant frequency with switching time period Ts.
This method is called pulse-width modulation (PWM) in which the duty ratio
D is varied to control Vo, where D=ton/Ts
7-10
5
PWM
Control of DC-DC Converters (cont’d)
•
•
•
The switch control signal, which
controls the on and off states of the
switch, is generated by comparing a
signal level control voltage vcontrol with
a repetitive waveform.
The switching frequency is the
frequency of the sawtooth waveform
with a constant peak.
The duty ratio D can be expressed as
D
t on vcontrol
 ^
Ts
V st
7-11
7-12
6
Switching Power Supplies
Buck (Forward Mode) Converter
• Two basic types of PWM switching power
supplies:
– Forward Mode
– Boost Mode
• Forward mode converter is recognised by
the presence of an L-C filter on its output.
• The L-C filter creates a DC output voltage,
which is essentially the Volt-time average
of L-C input AC rectangular waveform
7-13
• Most applications need a reasonably
steady DC voltage
• Linear Regulators have large fluctuations
(0 - +Vin)
• So need a filter for energy recovery
• Buck DC-DC converter is a step-down
converter with a filter (forward mode)
7-14
7
• Assumptions:
– Inductor current is continuous
– Average inductor voltage is zero
– Average capacitor current is zero
– Ideal components
– Filter capacitor is large
7-15
7-16
8
Analysis
7-17
7-18
9
7-19
7-20
10
Maximum, Minimum and Average Inductor Current
7-21
7-22
11
Output Voltage Ripple
7-23
7-24
12
Step-Down (Buck) Converter - Summary
•
•
•
•
•
•
converts dc from one level to another
the average output voltage is controlled by the
ON-OFF switch
pulse-width modulation (PWM) switching is
employed
lower average output voltage than the dc input
voltage Vd depending on the duty ratio, D
D=ton/Ts
Average output:
V0 

Ts

1 Ts
1  ton
 v0 t dt    Vd dt   0 dt 
Ts 0
Ts  0
t on

ton
Vd  DVd
Ts
•
•
Applications:
•
• regulated switch mode dc power supplies
•
• dc motor drives
•
7-25
low-pass filter: to reduce output voltage fluctuations
diode is reversed biased during ON period, input
provides energy to the load and to the inductor
energy is transferred to the load from the inductor
during switch OFF period
in the steady-state, average inductor voltage is zero
in the steady-state, average capacitor current is zero
7-26
13
Concept Quiz
Step-Down (Buck) Converter: Continuous current conduction mode
•
•
Inductor current iL flows continuously
Average inductor voltage over a time period
must be zero
Ts
t on
Ts
 v dt   v dt   v dt  0
L
0
L
0
L
t on
Area A and B must be equal , therefore ,
Vd  V0 ton  V0 Ts  ton 
or
V0 ton

D
Vd Ts
duty ratio 
Assuming a lossless circuit
Vd I d  V0 I 0
and
I 0 Vd 1


I d V0 D
Buck converter is like a dc transformer where the turns ratio can be controlled
electronically in a range of 0-1 by controlling D of the switch
7-27
In a Buck converter operating in steady
state, the input voltage is 15 V, the output
voltage is 12 V and the output power is 60
W.
Calculate the average value of the current
from the input voltage source.
A. 5 A
B. 4 A (correct)
C. 3 A
7-28
14
7-29
7-30
15
PSpice Modeling:
Simulation Results
16
12
8
4
0
-4
-8
450us
I(C1)
455us
460us
I(L1)
V(L1:1,L1:2)
465us
470us
475us
480us
485us
490us
495us
500us
Time
7-31
7-32
16
• Following parameters are needed to select
an inductor:
An Example
– Max input voltage
– Output voltage
– Switching frequency
– Max ripple current
– Duty Cycle
7-33
7-34
17
Example…..
Solution….
For a buck converter, R=1 ohm, Vd=40 V, V0=5 V, fs=4 kHz. Find the duty ratio
and “on” time of the switch.
D = V0 /Vd = 5/40 = 0.125 = 12.5%
Ts = 1/fs = 0.25 ms = 250 ms
Ton = DTs = 31.25 ms
Toff = Ts – ton = 218.75 ms
When the switch is “on”: VL = Vd - V0 = 35 V
When the switch is “off”: VL = -V0 = - 5 V
I0 = IL = V0 / R = 5 A
Id = D I0 = 0.625 A
7-35
7-36
18
Example 2
Solution
7-37
7-38
19
Summary Buck (Step-Down DCDC) Converters
Buck Disadvantages
• Major disadvantage is its topology only
– Buck is not an isolated topology; hence for
safety reasons the forward converter (Buck)
cannot be used for voltages greater than 42.5
VDC
– If isolated BUCK – 60-200 VDC
7-39
• Buck (Step-Down DC-DC) Converters
– Applications
– Operation in Steady State
7-40
20
Quiz
In a Buck converter operating in steady state, the input
voltage is 20 V, the output voltage is 12 V and the
output power is 60 W. The switching frequency is 200
kHz. It is designed such that the peak-peak ripple in
the inductor current doesn’t exceed 2 A.
Calculate the minimum value of the inductance that must
be used.
A. 12.0 micro-henries (correct)
B. 6.0 micro-henries
C. 3.6 micro-henries
7-41
21
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement