Power Electronics Ned Mohan Slides Ch27

Copyright by John Wiley & Sons 2003 Snubbers - *Snubber Circuits

A.Overview of Snubber Circuits

B.Diode Snubbers

C.Turn-off Snubbers

D.Overvoltage Snubbers

E.Turn-on Snubbers

F.Thyristor Snubbers

For clarifications: [email protected] Mathew

HoD- EEE

Copyright by John Wiley & Sons 2003*

Copyright by John Wiley & Sons 2003 Snubbers - *Overview of Snubber Circuits for Hard-Switched ConvertersFunction: Protect semiconductor devices by:

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/offTypes of Snubber Circuits

1.Unpolarized series R-C snubbersUsed to protect diodes and thyristors

2.Polarized R-C snubbersUsed 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 snubbersUsed as turn-on snubbers to shape the turn-off switching trajectory of controlled switches.Limit di/dt during device turn-onFor clarifications: [email protected]

Copyright by John Wiley & Sons 2003

Copyright by John Wiley & Sons 2003 Snubbers - *Need for Diode Snubber CircuitFor clarifications: [email protected]


Diode breakdown if Vd + Leq \F(diLs,dt) > BVBD

Copyright by John Wiley & Sons 2003 Snubbers - *Equivalent Circuits for Diode SnubberFor clarifications: [email protected]


Governing equation - eq \F(d2vCs,dt2) + eq \F(vCs,LsCs) = eq \F(Vd, LsCs)

Boundary conditions - vCs(0+) = 0 and iL(0+) = Irr

Copyright by John Wiley & Sons 2003 Snubbers - *Performance of Capacitive SnubberFor clarifications: [email protected]


vCs(t) = Vd - Vd cos(ot) + Vd eq \R(,\F(Cbase,Cs)) sin(ot)

o = eq \F(1,\R(,LsCs)) ; Cbase = L eq \B\BC\[(\F(Irr,Vd))2

Vcs,max = Vd eq \B\BC\{(1 + \R(,1 + \F(Cbase,Cs)) )

Copyright by John Wiley & Sons 2003 Snubbers - *Effect of Adding Snubber ResistanceSnubber Equivalent CircuitDiode voltage as a function of timeFor clarifications: [email protected]


eq \f(Vdf,Vd) (t) = - 1 - eq \f(e-t,\r() cos()) sin(at - + ) ; Rs 2 Rb

a = o eq \r(1- (/ o)2) ; = eq \f(Rs,2 L) ; o = eq \f(1,\r(LCs)) ; = tan-1 eq \b\bc\[(\f((2-x)\r(),\r(4 - x2)))

= eq \f(Cs,Cb) ; x = eq \f(Rs,Rb) ; Rb = eq \f(Vd,Irr) ; Cb = eq \f(L [Irr]2,Vd2) ; = tan-1(/a)

Governing equation L eq \F(d2i,dt2) + Rs eq \F(di,dt) + eq \f(i,Cs) = 0

Boundary conditions

i(0+) = Irr and eq \F(di(0+),dt) = eq \f(Vd - IrrRs,L)

Copyright by John Wiley & Sons 2003 Snubbers - *Performance of R-C SnubberFor clarifications: [email protected]


At t = tm vDf(t) = Vmax

tm = eq \F(tan-1(a/),a) + eq \f( - ,a) 0

EQ \F(Vmax,Vd) = 1 + EQ \R(1 + -1 - x) exp(-tm)

= EQ \F(Cs,Cbase) and x = EQ \F(Rs,Rbase)

Cbase = EQ \F(Ls Irr2,Vd2) and Rbase = EQ \F(Vd,Irr)

Copyright by John Wiley & Sons 2003 Snubbers - *Diode Snubber DesignFor clarifications: [email protected]


Copyright by John Wiley & Sons 2003 Snubbers - *Need for Snubbers with Controlled SwitchesStep-down converterSwitch current and voltage waveformsSwitching trajectory of switchFor clarifications: [email protected]


Copyright by John Wiley & Sons 2003 Snubbers - *Turn-off Snubber for Controlled SwitchesStep-down converter with turn-off snubberEquivalent circuit during switch turn-off.For clarifications: [email protected]


Simplifying assumptions

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 - *Turn-off Snubber OperationFor clarifications: [email protected]


Capacitor voltage and current for 0 < t < tfi iCs(t) = eq \F(Iot,tfi)

For Cs = Cs1, vCs = Vd at t = tfi yielding Cs1 = eq \F(Iotfi,2Vd)

Circuit waveforms for varying values of Cs

Copyright by John Wiley & Sons 2003 Snubbers - *Benefits of Snubber Resistance at Switch Turn-onDs shorts out Rs during Sw turn-off.

During Sw turn-on, Ds reverse-biased and Cs discharges thru Rs.Turn-on with Rs = 0

Energy stored on Cs dissipated in Sw.

Extra energy dissipation in Sw because of lengthened voltage fall time.Turn-on with Rs > 0

Energy stored on Cs dissipated in Rs rather than in Sw.

Voltage fall time kept quite short.For clarifications: [email protected]


Copyright by John Wiley & Sons 2003 Snubbers - *Effect of Turn-off Snubber CapacitanceSwitching trajectoryEnergy dissipationFor clarifications: [email protected]


WR = dissipation in

resistor

WT = dissipation in

switch Sw

Cs1 = eq \F(Iotfi,2Vd)

Wtotal = WR + WT

Wbase = 0.5 VdIotfi

Copyright by John Wiley & Sons 2003 Snubbers - *Turn-off Snubber Design ProcedureFor clarifications: [email protected]


Selection of Rs

Limit icap(0+) = EQ \F(Vd,Rs) < Irr

Usually designer specifies Irr < 0.2 Io so EQ \F(Vd,Rs) = 0.2 Io

Snubber recovery time (BJT in on-state)

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 RsCs

Selection of Cs

Minimize energy dissipation (WT) in BJT at turn-on

Minimize WR + WT

Keep switching locus within RBSOA

Reasonable value is Cs = Cs1

Copyright by John Wiley & Sons 2003 Snubbers - *Overvoltage SnubberStep-down converter with overvoltage snubber comprised of Dov, Cov, and Rov.

Overvoltage snubber limits overvoltage (due to stray Inductance) across Sw as it turns off.For clarifications: [email protected]


kVd = Leq \F(diLs,dt) = Leq \F(Io,tfi)

L = eq \F(kVdtfi,Io)

Switch Sw waveforms without overvoltage snubber

tfi = switch current fall time ; kVd = overvoltage on Sw

Copyright by John Wiley & Sons 2003 Snubbers - *Operation of Overvoltage SnubberDov,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.

Equivalent circuit after turn-off of Sw.For clarifications: [email protected]


Dov on for 0 < t < eq \F(\R(,LsCov),2)

tfi

Copyright by John Wiley & Sons 2003 Snubbers - *Overvoltage Snubber DesignFor clarifications: [email protected]


Cov = EQ \F(Ls Io2,(Dvsw,max)2)

Limit vsw,max to 0.1Vd

Using Ls = EQ \F(kVd tfi, Io) in equation for Cov yields

Cov = EQ \F(kVdtfiIo2, Io(0.1Vd)2) = EQ \F(100k tfi Io, Vd)

Cov = 200 k Cs1 where Cs1 = EQ \F(tfiIo, 2Vd) which is used

in turn-off snubber

Recovery time of Cov (2.3RovCov) must be less

than off-time duration, toff, of the switch Sw.

Rov EQ \F(toff,2.3 Cov)

Copyright by John Wiley & Sons 2003 Snubbers - *Turn-on SnubberStep-down converter with turn-on snubberSnubber 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.For clarifications: [email protected]


Copyright by John Wiley & Sons 2003 Snubbers - *Turn-on Snubber Operating WaveformsFor clarifications: [email protected]


eq \F(disw,dt) controlled by switch Sw and drive circuit.

vsw = eq \F(LsIo,tri)

Large values of snubber inductance (Ls > Ls1)

Small values of snubber inductance (Ls < Ls1)

eq \F(disw,dt) limited by circuit to eq \F(Vd,Ls) < eq \F(Io,tri)

Ls1 = eq \F(Vdtri,Io)

Irr reduced when Ls > Ls1 because Irr proportional to eq \R(,\F(disw,dt))

Copyright by John Wiley & Sons 2003 Snubbers - *Turn-on Snubber Recovery at Switch Turn-offFor clarifications: [email protected]


Assume switch current fall time

tri = 0.

Inductor current must discharge thru DLs- RLs series segment.

Overvoltage smaller if tfi smaller.

Time of 2.3 Ls/RLs required for inductor current to decay to 0.1 Io

Off-time of switch must be > 2.3 Ls/RLs

Switch waveforms at turn-off with turn-on snubber in circuit.

Copyright by John Wiley & Sons 2003 Snubbers - *Turn-on Snubber Design Trade-offsFor clarifications: [email protected]


Selection of inductor

Larger Ls decreases energy dissipation in switch at turn-on

Wsw = WB (1 + Irr/Io)2 [1 - Ls/Ls1]

WB = VdIotfi/2 and Ls1 = Vdtfi/Io

Ls > Ls1 Wsw = 0

Larger Ls increases energy dissipation in RLs

WR = WB Ls / Ls1

Ls > Ls1 reduces magnitude of reverse recovery current Irr

Inductor must carry current Io when switch is on - makes

inductor expensive and hence turn-on snubber seldom used

Selection of resistor RLs

Smaller values of RLs reduce switch overvoltage Io RLs at turn-off

Limiting overvoltage to 0.1Vd yields RLs = 0.1 Vd/Io

Larger values of RLs shortens minimum switch off-time of 2.3 Ls/RLs

Copyright by John Wiley & Sons 2003 Snubbers - *Thyristor Snubber CircuitPhase-to-neutral waveforms3-phase thyristor circuit with snubbersFor clarifications: [email protected]


van(t) = Vssin(t), vbn(t) = Vssin(t - 120),

vcn(t) = Vssin(t - 240)

vLL(t) = eq \R(,3) Vssin(t - 60)

Maximum rms line-to-line voltage VLL = eq \R(,\F(3,2)) Vs

Copyright by John Wiley & Sons 2003 Snubbers - *Equivalent circuit after T1 reverse recoveryAssumptionsEquivalent Circuit for SCR Snubber CalculationsFor clarifications: [email protected]


Trigger angle = 90 so that vLL(t) = maximum = eq \R(,2) VLL

Reverse recovery time trr

Copyright by John Wiley & Sons 2003 Snubbers - *Component Values for Thyristor SnubberFor clarifications: [email protected]


Use same design as for diode snubber but adapt the formulas to the thyristor circuit notation

Snubber capacitor Cs = Cbase = L eq \B\BC\[(\F(Irr,Vd))2

From snubber equivalent circuit 2 L eq \F(diLs,dt) = eq \R(,2) VLL

Irr = eq \F(diLs,dt) trr = eq \F(\R(,2)VLL,2Ls) trr = eq \F(\R(,2)VLL,2 \F(0.05 VLL,\R(,3) Ia1w)) trr = 25 Ia1trr

Vd = eq \R(,2) VLL

Cs = Cbase = eq \F(0.05 VLL,\R(,3) Ia1w) eq \B\BC\[(\F(25 wIa1trr, \R(,2)VLL)) 2 = eq \F(8.7 wIa1trr,VLL)

Snubber resistance Rs = 1.3 Rbase = 1.3 eq \F(Vd,Irr)

Rs = 1.3 eq \F(\R(,2)VLL,25wIa1trr) = eq \F(0.07 VLL, wIa1trr)

Energy dissipated per cycle in snubber resistance = WR

WR = eq \F(LsIrr2,2) + eq \F(CsVd2,2) = 18 Ia1 VLL(trr)2

*

Power Electronics Ned Mohan Slides Ch27

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