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.

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

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

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

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

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

~ 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

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)

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

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