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Snubber Circuits Lecture Notes Snubber Circuits William P. Robbins Dept. of Electrical and Computer Engineering University of Minnesota Outline A. Overview of Snubber Circuits B. Diode Snubbers C. Turn-off Snubbers D. Overvoltage Snubbers E. Turn-on Snubbers F. Thyristor Snubbers Snubbers -1 1 W.P. Robbins Overview of Snubber Circuits for Hard-Switched Converters Function: Protect semiconductor devices by: Types of Snubber Circuits • Limiting device voltages during turn-off transients 1. Unpolarized series R-C snubbers • Used to protect diodes and thyristors • Limiting device currents during turn-on transients 2. Polarized R-C snubbers • Limiting the rate-of-rise (di/dt) of currents through • Used as turn-off snubbers to shape the turn-on the semiconductor device at device turn-on switching trajectory of controlled switches. • Used as overvoltage snubbers to clamp voltages applied to controlled switches to safe values. • Limiting the rate-of-rise (dv/dt) of voltages across • Limit dv/dt during device turn-off the semiconductor device at device turn-off 3. Polarized L-R snubbers • Shaping the switching trajectory of the device as it • Used as turn-on snubbers to shape the turn-off turns on/off switching trajectory of controlled switches. • Limit di/dt during device turn-on Snubbers - 2 W.P. Robbins Need for Diode Snubber Circuit di Df Vd = d t L L σ + σ R Io s I t o i Df (t) D I Vd f Cs rr - Sw v (t) t Df Vd • Lσ = stray inductance • S w closes at t = 0 • Diode voltage di L σ without snubber Lσ • Rs - Cs = snubber circuit d t diLσ • Diode breakdown if V + L > BV d σ dt BD Snubbers - 3 W.P. Robbins Equivalent Circuits for Diode Snubber L σ • Simplified snubber - the capacitive snubber + R s L Diode cathode σ Vd snap-off anode - + + v Vd Cs Cs - - i Df t • Rs = 0 • Worst case assumption- • v = -v diode snaps off instantaneously Cs Df at end of diode recovery 2 d vCs vCs Vd • Governing equation - + = dt2 LσCs LσCs + + • Boundary conditions - vCs(0 ) = 0 and iLσ(0 ) = Irr Snubbers - 4 W.P. Robbins Performance of Capacitive Snubber Cbase • vCs(t) = Vd - Vd cos(ωot) + Vd sin(ωot) Cs 2 1 Irr • ω = ; C = L o base σ V LσCs d Cbase • Vcs,max = Vd 1 + 1 + Cs 5 4 V 3 Cs,max Vd 2 1 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 C base C Snubbers - 5 s 5 W.P. Robbins Effect of Adding Snubber Resistance Snubber Equivalent Circuit Lσ i(t) d2i di i • Governing equation L + R + = 0 + - σ 2 s dt C R s dt s V v (t) d Df • Boundary conditions + V - I R Cs + di(0 ) d rr s - + i(0 ) = Irr and = dt Lσ Diode voltage as a function of time Vdf e-αt (t) = - 1 - sin(ω t - φ + ζ) ; R ≤ 2 R Vd η cos(φ) a s b R 2 s 1 -1 (2-x) η ωa = ωo 1- (α/ ωo) ; α = 2 L ; ωo = ; φ = tan σ LσCs 4 - ηx2 C R V L [I ]2 s s d σ rr -1 η = C ; x = R ; Rb = I ; Cb = 2 ; ζ = tan (α/ωa) b b rr Vd Snubbers - 6 W.P. Robbins Performance of R-C Snubber 3 • At t = tm vDf(t) = Vmax R s,opt C s = Cbase = 1.3 -1 R tan (ωa/α) φ - ξ base • t = ω + ω ≥ 0 m a a 2 V Vmax max • = 1 + 1 + -1 - x exp(- t ) η α m V Vd d Cs Rs 1 • η = and x = Cbase Rbase R s I rr 2 V Ls Irr Vd d • C = and R = base 2 base I 0 Vd rr 0 1 R s 2 R base Snubbers - 7 W.P. Robbins Diode Snubber Design Nomogram Wtot 2 3 L s I rr /2 WR 2 L s I rr /2 0 2 0 V 0 max for R = R V s s,opt 0 d 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 R s,op R base 0 0 1 2 3 Snubbers - 8 C / C s base 8 W.P. Robbins Need for Snubbers with Controlled Switches Step-down converter Switch current and voltage waveforms di Lσ L dt 1 L 2 I rr I o i sw V d di i Lσ V I sw + I o dt d o vsw S w L 3 - vsw t t t t t 3 t 5 6 o 1 4 • = stray inductances L1 , L 2 , L 3 Switching trajectory of switch • = Lσ L1 + L 2 + L3 idealized i sw switching t loci t 6 5 t turn-off o • Overvoltage at turn-off t1 due to stray inductance turn-on • Overcurrent at turn-on due to diode reverse recovery t 4 t 3 vsw Vd Snubbers - 9 W.P. Robbins Turn-off Snubber for Controlled Switches Turn-off D snubber i f I + D o F V d Ds Step-down converter with turn-off snubber S R s w i - C C s s Equivalent circuit during switch turn -off. I o D f • Simplifying assumptions 1. No stray inductance. V d I - i o sw 2. isw(t) = Io(1 - t/tfi) i sw C s 3. isw(t) uneffected by snubber circuit. Snubbers - 10 W.P. Robbins Turn-off Snubber Operation 2 Iot I o t • Capacitor voltage and current for 0 < t < t i (t) = and v (t) = fi Cs t Cs 2C t fi s fi Iotfi • For Cs = Cs1, vCs = Vd at t = tfi yielding Cs1 = 2Vd Circuit waveforms for varying values of Cs i i i sw sw sw I o i D i i f Df Df t t fi t fi fi i C s V d v Cs Snubbers - 11 Cs < C Cs = Cs1 C s > C s1 s W.P. Robbins 1 Benefits of Snubber Resistance at Switch Turn-on vsw t D I I rr f o o • D shorts out R R s s i I s D rr during Sw turn-off. f V d V • During Sw turn-on, d S I w Ds D reverse-biased and i o s sw R s C s Cs discharges thru Rs. I rr • Turn-on with Rs > 0 discharge t rr • Energy stored on Cs dissipated of C s in Rs rather than in Sw. • Turn-on with Rs = 0 vsw • Voltage fall time kept quite short. • Energy stored on Cs dissipated V I o d in Sw. i sw t t ri 2 • Extra energy dissipation in Sw 0 because of lengthened voltage t ri + t rr fall time. Snubbers - 12 W.P. Robbins Effect of Turn-off Snubber Capacitance 1 Energy dissipation W / total Wbase W 0.8 WR = dissipation in resistor 0.6 WT = dissipation in W / W R base 0.4 switch Sw WT / Wbase W 0.2 Iotfi Cs1 = 2Vd 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Wtotal = WR + WT Cs / Cs1 i sw Wbase = 0.5 VdIotfi I o Cs < Cs1 RBSOA Switching trajectory Cs = Cs1 Cs > Cs1 V v d sw Snubbers - 13 W.P. Robbins Turn-off Snubber Design Procedure Selection of Cs • Minimize energy dissipation (WT) in BJT at turn-on • Minimize WR + WT • Keep switching locus within RBSOA Snubber recovery time (BJT in on-state) • Reasonable value is Cs = Cs1 • Capacitor voltage = Vd exp(-t/RsCs) • Time for vCs to drop to 0.1Vd is 2.3 RsCs • BJT must remain on for a time of 2.3 R C Selection of Rs s s Vd • Limit i (0+) = < I cap Rs rr Vd • Usually designer specifies I < 0.2 I so = 0.2 I rr o Rs o Snubbers - 14 W.P. Robbins Overvoltage Snubber L σ kV d + i D R s f I o ov w V d I V o d Sw Dov - Cov v s w t o fi • Step-down converter with overvoltage snubber comprised of Dov, Cov, and Rov. • Switch Sw waveforms without overvoltage snubber • t = switch current fall time ; kV = overvoltage on S • Overvoltage snubber limits fi d w overvoltage (due to stray diLσ Io Inductance) across Sw as it • kVd = Lσ = Lσ turns off. dt tfi kVdtfi • Lσ = Io Snubbers - 15 W.P. Robbins Operation of Overvoltage Snubber π LσCov i • D on for 0 < t < Lσ ov 2 L + σ R π LσCov Dov ov • t << fi 2 V d + C v - ov Cov - • Dov,Cov provide alternate path for inductor current as Sw turns off. • Switch current can fall to zero much faster than Ls current. • Df forced to be on (approximating a short ckt) by Io after Swis off. Snubbers - 16 W.P. Robbins Overvoltage Snubber Design Lο • Limit v to 0.1V = Io sw,max d Cov kVd tfi • Using L = in above equation yields Io € 2 kVdtfiIo 100k tfi Io • C = = ov 2 V Io(0.1Vd) d tfiIo • Cov = 200 k Cs1 where Cs1 = which is used 2Vd in turn-off snubber • Choose Rov so that the transient recovery of Cov is critically damped so that ringing is minimized. For a critically damped ω L L 1 o σ = σ circuit Q = 1 = . Rov Cov Rov 0.1Vd • Rov = Io € • Check that the recovery time of Cov (2.3L /Rov) is less € than off-time duration, toff, of the switch Sw.
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