SWITCH-MODE POWER SUPPLIES AND SYSTEMS

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SWITCH-MODE POWER SUPPLIES AND SYSTEMS
Lecture No 11
Switching converters with regulation feedback loop

• Silesian University of Technology
• Faculty of Automatic Control, Electronics
• and Computer Sciences
• Ryszard Siurek Ph.D., El. Eng.

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Switched-Mode Power Supply (SMPS) with forward
converter
D2
L
Converter primary switching circuit
D1
C
R0
U0
UIN
Ucont
Pulse Width Modulator (PWM)
Error amplifier

-
e
e
-
Isolation barrier
Uref

UO
U0 Uref
e
CAUTION! For forward converter g lt 0,5
Ucont
3
Isolation barrier
1. Optocoupler providing adequate isolation
USS
TSO
e
Uref
e
Ucont

• Error amplifier isolated from PWM
• optocoupler is a nonlinear component,
  parameters depandant on temperature and
• time (aging), should be used always inside
  closed feedback loop
• low cross-over frequency resulting with slower
  dynamic response when used in
• closed feedback loop
• very popular due to low price and simple
  implementation

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2. Small pulse transformer providing isolation
e
Uref
Ucont
IC1
IC2
U0

U0

• unpopular, complicated and expensive
• specialised sets of IC are manufactured to
  ensure proper operation of this circuit

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3. Gate-drive (base-drive) pulse transformer
performing isolation in feedback loop circuit
D2
L
T
D1
C
R0
U0
UIN
Error amplifier
Ucont
e
-
Uref

• most popular (used in 90 of computer power
  supplies)
• all components forming control feedback loop
  may be located in a single IC
• placed at the socondary (low voltage) side
• the only disadvantage gate-drive transformer
  should comply with strict safety
• requirements

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Stability criteria applied to switching power
supplies
U0
U0
Tw
Proper output voltage
Output voltage oscillations
Ku dB
f deg
Ku feedback loop gain f - total phase shift in
the closed-loop system
10
-270
phase margin
0
-360
-450
-10
gain margin
f kHz
1
10
100
The definitions of stability terms - Bode plots
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Measuremet method used to obtain Bode diagrams
Practical method of SMPS stability evaluation
I0
100
UIN
U0
20
PPOWER SUPPLY
U0
V3
Feedback loop under-compensated phase and gain
margins are too low
FEED-BACK LOOP CIRCUIT

U0
GEN.
V1
V2
Feedback loop over-compensated phase and gain
margins are too big
U0
V3 - very small signal transformer impedance is
negligible
Feedback loop optimum compensation
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Error amplifier
C1
U0
C2
R1
C3
R2
-
e

R3
Uodn (Vr)
Typical diagram of error amplifier with input
voltage divider and compensation circuits (gain
and phase compensation)
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TL431 IC as a most popular error amplifier
U0
Rx
USS
R1
TL431
e

TL431
-
Ux
Vr2,5V
R2
TL431 is often called a programmable Zener Diode
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Self-oscillating flyback converter simple SMPS
solution
ID
U0
UIN
IT
Critical flux flow is kept in the whole load
range Frequency varies according to load changes
RS
high load (I0)
IT
low load (I0)
ID