1. 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.

2. Switched-Mode Power Supply (SMPS) with forward converter
primary
switching
circuit
UIN D1 C R0 U0
Ucont
Pulse Width Modulator (PWM)
Error amplifier - + e e’ -
Isolation
barrier
Uref + UO
U0 = Uref e’
CAUTION!
For forward converter
g < 0,5
Ucont

3. Isolation barrier e e’ 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

4. e’ ‘ IC1 IC2 2. Small pulse transformer providing isolation Uref Ucont
unpopular, complicated and expensive
specialised sets of IC are manufactured to ensure proper operation of this circuit

5. 3. Gate-drive (base-drive) pulse transformer performing isolation in feedback loop circuit
UIN U0
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

6. Stability criteria applied to switching power suppliesTw
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
-360
-450
-10
gain margin
f [kHz] 1 10
100
The definitions of stability terms - Bode plots

7. ~ 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

8. Error amplifier - e + C1 U0 R1 C2 C3 R2 R3 Uodn (Vr)
Typical diagram of error amplifier with input voltage divider and compensation circuits (gain and phase compensation)

9. - e’ + TL431 IC as a most popular error amplifier U0 Rx USS R1 TL431Ux -
Vr=2,5V R2
TL431 is often called a „programmable Zener Diode”

10. Self-oscillating flyback converter – simple SMPS solutionID 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