Transcript Lecture8b
Power Electronics Lecture(9) Prof. Mohammed Zeki Khedher Department of Electrical Engineering University of Jordan 1 Short pulse Long pulse Pulse train generator Pulse train with timer and AND gate Gate Triggering Methods - Efficient & reliable method for turning on SCR. • Types R - Triggering. RC - Triggering. UJT - Triggering. 6 R-Triggering vO a b LOAD i R1 R2 vS=Vmsint VT D R Vg Resistance firing circuit 7 RC Triggering vO LOAD + R D2 vS=Vmsint VC C D1 VT - RC half-wave trigger circuit 8 Gate triggering characteristics Gate input characteristics • • • • • • • • • • • • • • • • • IGM w IG rp o n s x q y z 1 3 4Load line m VG VGD FIGURE 18.2 Typical gate characteristics of an SCR. Gate protection circuit High temperature due to: Lecture Notes Snubber Circuits Outline Copyright © by John Wiley & Sons 2003 A. Overview of Snubber Circuits B. Diode Snubbers C. Turn-off Snubbers D. Overvoltage Snubbers E. Turn-on Snubbers F. Thyristor Snubbers Snubbers - 18 Overview of Snubber Circuits for Hard-Switched Converters Function: Protect semiconductor devices by: Types of Snubber Circuits • Limiting device voltages during turn-off transients • Limiting device currents during turn-on transients • Limiting the rate-of-rise (di/dt) of currents through the semiconductor device at device turn-on • Limiting the rate-of-rise (dv/dt) of voltages across the semiconductor device at device turn-off • Shaping the switching trajectory of the device as it turns on/off Copyright © by John Wiley & Sons 2003 1. Unpolarized series R-C snubbers • Used to protect diodes and thyristors 2. Polarized R-C snubbers • Used as turn-off snubbers to shape the turn-on switching trajectory of controlled switches. • Used as overvoltage snubbers to clamp voltages applied to controlled switches to safe values. • Limit dv/dt during device turn-off 3. Polarized L-R snubbers • Used as turn-on snubbers to shape the turn-off switching trajectory of controlled switches. • Limit di/dt during device turn-on Snubbers - 19 Need for Diode Snubber Circuit di + V d - L Rs Io Df Cs Io Df = d t V d L t i Df(t) I rr Sw t v (t) Df Vd • L = stray inductance • Sw closes at t = 0 • Rs - C s = snubber circuit • Copyright © by John Wiley & Sons 2003 • Diode voltage without snubber diL Diode breakdown if V d + L dt > BVBD Snubbers - 20 L di L d t Effect of Adding Snubber Resistance Snubber Equivalent Circuit • Governing equat ion L • Boundary con dition s i( 0 + ) = I rr and d2 i di i + Rs + = 0 2 dt C dt s V d - I rr Rs di(0 + ) = dt L Diode voltage as a function of time Vdf Vd (t) = - 1 a = o e-t sin(at - + ) ; Rs Š 2 R b cos() Rs 2 1- (/ o) ; = 2 L ; o = 1 (2-x) -1 ; = tan LCs 4 - x2 L [Irr]2 Cs Rs Vd = C ; x = R ; Rb = I ; Cb = ; = tan-1(/a) 2 b b rr Vd Copyright © by John Wiley & Sons 2003 Snubbers - 21 Need for Snubbers with Controlled Switches Step-down converter L1 V d i L3 • L2 sw Io + vsw - Sw L1 , L 2 , L 3 = str ay i nductances Switching trajectory of switch • L = L1 + L 2 + L 3 ideal ized s wi tchi ng lo ci i sw t6 t t 5 turn-of f o t1 turn-on • Over cur rent at tur n-on due to diode rever se recovery t 4 t3 V Copyright © by John Wiley & Sons 2003 • Over voltage at tur n-off due to str ay inductance d Snubbers - 22 vsw Turn-off Snubber for Controlled Switches Step-down converter with turn-off snubber Equivalent circuit during switch turn-off. • Simplifying a ssumptions 1. No stray inductance. 2. isw(t) = Io(1 - t/tfi) 3. isw(t) uneffected by snubber circuit. Copyright © by John Wiley & Sons 2003 Snubbers - 23 Turn-off Snubber Operation • Capacito r v olt age and current for 0 < t < • Iot t f i i Cs(t ) = t fi and v (t) = Cs Iot f i For Cs = Cs1 , v Cs = V d at t = t f i yiel ding C s1 = 2V d Circuit wavef orms for var yin g va lues of Cs Copyright © by John Wiley & Sons 2003 Snubbers - 24 Iot 2 2C t s fi Turn-on Snubber Step-down converter with turn-on snubber • Snubber reduces Vsw at switch turn-on due drop across inductor Ls. • Will limit rate-of-rise of switch current if Ls is sufficiently large. Switching trajectory with and without turn-on snubber. Copyright © by John Wiley & Sons 2003 Snubbers - 25 Thyristor Snubber Circuit 3-phase thyristor circuit with snubbers • v an( t ) = V ssin( t ) , v bn ( t ) = V ssin( t - 120 ) , v cn(t ) = V ssin( t - 240 ) Phase-to-neutral waveforms v an v 3 V ssin( t - 60 ) • v LL ( t ) = • Maximum rms lin e-t o-lin e volt age V LL = Copyright © by John Wiley & Sons 2003 3 Vs 2 v LL = v Snubbers - 26 bn v v an = ba t1 bn Trajectory comparision with and without capacitor Turn off snubber circuit