snubber circuits - ken-gilbert.comken-gilbert.com/images/pdf/snubber.pdf · · 2016-07-08types of...

Snubbers - 1W.P. Robbins © 1997

Snubber Circuits

A. Overview of Snubber Circuits

B. Diode Snubbers

C. Turn-off Snubbers

D. Overvoltage Snubbers

E. Turn-on Snubbers

F. Thyristor Snubbers

William P. RobbinsProfessor

Dept. of Electrical EngineeringUniversity of Minnesota

200 Union St. SE.Minneapolis, MN 555455

Copyright 1997


Function of Snubber Circuits

• Protect semiconductor devices by:

• Limiting device voltages during turn-off transients

• Limiting device currents during turn-on transients

• Limiting the rate-of-rise (didt

) of currents through the

semiconductor device at device turn-on

• Limiting the rate-of-rise (dvdt

) of voltages across the

semiconductor device at device turn-off

• Shaping the switching trajectory of the device as itturns on/off


Types of Snubber Circuits

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 dvdt

during device turn-off

3. Polarized L-R snubbers

• Used as turn-on snubbers to shapte the turn-off switching trajectory of controlled switches.

• Limit didt

during device turn-on


Need for Diode Snubber Circuit

-

+

Df

R s

Cs

Lσ

Vd

I o

Sw

I rr

i Dfd

d t

VdLσ

=Io

t

tVd

i (t)Df

v (t)Df

• Diode voltage without snubber

d

d tLσ

i Lσ

L = stray inductanceσ

S closes at t = 0w

R - C = snubber circuits s

•

•

•

• Diode breakdown if Vd + Lσ

diLσdt

> BVBD


Equivalent Circuits for Diode Snubber

Cs

R s

Lσ

+

-

Vdanode

cathode Diodesnap-off

• Worst case assumption- diode snaps off instantaneously at end of diode recovery

i Df

Cs

+

-

Vd

Lσ• R = 0s

v Cs

+

-

• v = -v Cs Df

• Simplified snubber - the capacitive snubber

t

• Governing equation - d2vCs

dt2 +

vCsLσCs

= Vd

L σCs

• Boundary conditions - vCs(0+) = 0 and iLσ(0+) = Ir r


Performance of Capacitive Snubber

• vCs(t) = Vd - Vd cos(ωot) + Vd Cbase

Cs sin(ωot)

• ωo = 1

LσCs ; Cbase = Lσ

I r r

Vd

2

• Vcs,max = Vd

1 + 1 + Cbase

Cs

0

1

2

3

4

5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

d

VCs,max

V

C

Cs

base


Effect of Adding Snubber Resistance

• Equivalent circuit with snubber resistance Rs

-

Vd

Cs

R s

Lσ

+

+

-

v (t)Df

• Governing equation LσCs d2vDf

dt2 + RsCs

dvDfdt

+ vDf = -Vd

• Boundary conditions

vDf(0+) = - Ir rRs and

dvDf(0+)

dt = -

I r rCs

- RsVd

Lσ +

I r rRs2

Lσ

• Solution for vDf(t)

vDf(t) = - Vd - Vd e- αt

CbaseCs

sin(ωat - φ − ξ)

ωa = ωo 1 - α2

ωo2

; ωo = 1

LσCs ; α =

Rs2Lσ

tan(φ) = - Rb

ωaLσ -

αωa

; tan(ξ) = αωa

; Rbase = VdI rr

, Cbase = Lσ

(Vd/Irr)2


Performance of R-C Snubber

• At t = tm vDf(t) = Vmax

• tm = tan-1(ωa/α)

ωa +

φ - ξωa

≥ 0

•Vmax

Vd = 1 + 1 + CN

- 1 - R N exp(-αtm)

• CN = Cs

Cbase and RN =

RsRbase

• Cbase = Ls I r r

2

Vd2

and Rbase = VdI rr

0

1

2

3

0 1 2

C = Cs base

R Is rr

Vd

V

V

max

d

R sR base

R s,opt

R base= 1.3


Diode Snubber Design Nomogram

0

0

0

00000000000000000 0 0 00

0

1

2

3

0 1 2 3

R

R s,op

base

V

V

max

ds,opt

for R = Rs

L I /2s rr2

WR

Wtot

L I /2s rr2

base/ CCs


Need for Snubbers with Controlled Switches

t o t1

t3 t4

t5 t6

L didt

L didt

VdIoIo

I o

Vd

L 1 L 2

L 3Sw

vsw

+

-

isw

vsw

isw

turn-on

turn-off

idealized switching loci

t ot1

t3t4

t 5t6

Vd

isw

vsw

• Overvoltage at turn-off due to stray inductance

• Overcurrent at turn-on due to diode reverse recovery

L1 L2 L3• , , = stray inductances

L1 L2 L3L = ++•


Turn-on Snubber for Controlled Switches

• Circuit configuration

Df

Ds

Cs

Rs

Vd

Io+

-

iDF

iCs

Turn-off snubber

Sw

• Equivalent circuit during switch turn-off

Cs

Io - iV

d

i sw

DfI o

sw

• Assumptions

1. No stray inductance.

2. i sw(t) = Io(1 - t/tf i )

3. i sw(t) uneffected by

snubber circuit.


Turn-off Snubber Operation

• Capacitor voltage and current for 0 < t < tf i

• i Cs(t) = I ot

tf i and vCs(t) =

I ot2

2Cstf i

• For Cs = Cs1, vCs = Vd at t = tf i yielding Cs1 = I otf i2Vd

• Circuit waveforms for varying values of Cs

iDf

iCs

Vd

vCs

C = Cs s1

t f i

isw

iDf

iDf

Io

t f it f i

iswi sw

C < Cs s1 C > Cs s


Benefits of Snubber Resistance at Sw Turn-on

IoDf

Cs

Io

RsVd

DsSw

t rrt ri +

i sw

t rr

0

discharge of C s

t ri t2

VdIo

vsw

vsw

Irr

Io

I rr

iDf

t rr

VdRsi sw

• Ds shorts out Rs during Swturn-off.

• During Sw turn-on, Dsreverse-biased and Csdischarges thru Rs.

• Turn-on with Rs = 0

• Energy stored on Csdissipated in Sw.

• Extra energy dissipation in Swbecause of lengthenedvoltage fall time.

• Turn-on with Rs > 0

• Energy stored on Csdissipated in Rs

rather than in Sw.

• Voltage fall time keptquite short.


Effect of Snubber Capacitance

• Switching trajectory

Cs < Cs1

Cs = Cs1

Cs > Cs1

Io

Vdvsw

i sw

RBSOA

• Energy dissipation

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4

WT / base

WR / Wbase

Wtotal / base

Cs / Cs1

W

W

WR = dissipation in

resistor

WT = dissipation in

switch Sw

Cs1 = I otf i2Vd

Wtotal = WR + WT

Wbase = 0.5 VdI otf i


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

• Reasonable value is Cs = Cs1

• Selection of Rs

• Limit icap(0+) = VdRs

< Ir r

• Usually designer specifies Ir r < 0.2 Io so

VdRs

= 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


Overvoltage Snubber for Controlled Switches

• Circuit configuration - Dov, Rov, and Cov form overvoltage snubber

+

-

Vd

L σ

Df I oRov

CovDovSw

• Overvoltage snubber limits magnitude of voltage developed across Sw as it turns off.

• Switch Sw waveforms without overvoltage snubber

• tf i = switch current fall time ; kVd = overvoltage on Sw

o tfi

VdI o

kVdi sw

vsw

• kVd = Lσ

diLσdt

= LσI otf i

• Lσ = kVdtf i

I o


Operation of Overvoltage Snubber

• Dov,Cov provide alternate path for inductor current as Sw turns off.

• Switch current can fall to zero much faster than Lσ current.

• Df forced to be on (approximating a short ckt) by Io after Sw is off.

• Equivalent circuit after turn-off of Sw.

+

-

Vd

L σRov

Cov

Dov

iLσ

vCov

+

-

vCov(0+) = Vd

• Equivalent circuit while inductor current decays to zero

+

-

Vd

L σCov

i Lσ

+

-

iLσ(0+) = Io

0π Lσ Cov

4

iLσ

vsw

vCov

I o Vd

Charge-up of Cov from Lσ

Cov (∆Vsw,max)2

2 = Lσ ( Io)2

2

∆Vsw,max

• Energy transfer from Lσ to Cov

Discharge of Cov thru Rovwith time constant R C ovov

iLσ

(t) = I cos[ ] Lσ Cov

to

.

• Dov on for 0 < t < π LσCov

2

• tf i << π LσCov

2


Overvoltage Snubber Design

• Cov = Ls I o

2

(∆vsw,max)2

• Limit ∆vsw,max to 0.1Vd

• Using Ls = kVd tfi

I o in equation for Cov yields

• Cov = kVdtfiIo

2

I o(0.1Vd)2 =

100k tf i Io

V d2

• Cov = 200 Cs1 where Cs1 = 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 ≈ toff

2.3 Cov


Turn-on Snubber Circuit

• Circuit topology

Vd

+

-

LsD

Ls

Df

R Ls

Io

Sw

Vd

-

LsD

Ls

Df RLs Io

Sw

Df

+

Snubber circuit

• Circuit reduces Vsw as switch Sw turns on. Voltage drop Ls

diswdt

provides the voltage reduction.

• Switching trajectories with and without turn-on snubber.

swi

vswVd

I o

L sdiswdt

Without snubber

With snubber


Turn-on Snubber Operating Waveforms

• Small values of snubber inductance (Ls < Ls1)

tr i t rr

rrI

I oVd

vsw

i sw

• Large values of snubber inductance (Ls > Ls1).

I o

Vd

vsw

i sw

rrI

reduced

t ≈ > t + t IL os

Vd

on r i rr

• I rr reduced when Ls > Ls1 because Ir r proportional to disw

dt

•disw

dt controlled by

switch Sw and

drive circuit.

• ∆vsw = LsI otr i

•disw

dt limited by circuit

to VdLs

< I otr i

• Ls1 = Vdtr i

I o


Turn-on Snubber Recovery at Switch Turn-off

Vd

+

-

LsD

Ls

Df

RLs

Io

Sw

vsw

i sw V

d

I o R Ls

I o

t rv

IoRLsexp(-RLst/Ls)

• Overvoltage smaller if tf i 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

• Assume switch current fall timetri = 0.

• Inductor current must discharge thru DLs- RLs series segment.

• Switch waveformsat turn-off with turn-on snubber in circuit.


Turn-on Snubber Design Trade-offs

• Selection of inductor Ls

• Larger Ls decreases energy dissipation in switch at turn-on

• Wsw = WB (1 + Ir r/ Io)2 [1 - Ls/Ls1]

• WB = VdI otf i /2 and Ls1 = Vdtf i / Io

• Ls > Ls1 Wsw = 0

• Larger Ls increases energy dissipation in RLs

• WR = WB Ls / Ls1

• Ls > Ls1 reduces magnitude of reverse recovery current Ir r

• 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


Thyristor Snubber Circuit

id

1 3 5

24 6

+

+

+-

-

-

van

vbn

Rs

Cs

A

B

C

P

L

L

L

vcn

• van(t) = Vssin(ωt), vbn(t) = Vssin(ωt - 120°), vcn(t) = Vssin(ωt - 240°)

• Phase-to-neutral waveforms

vanvbn = v

ba

vanvbn

t1=vLL

• vLL(t) = 3 Vssin(ωt - 60°)

• Maximum rms line-to-line voltage VLL = 32

Vs


Equivalent Circuit for SCR Snubber Calculations

• Equivalent circuit after T1 reverse recovery

Cs

Rs

Vbc( )

t1T (on)3

T afterrecovery1

iT1

P

A

2 L

iL

+

-

• Assumptions

• Trigger angle α = 90° so that vLL(t) = maximum = 2 VLL

• Reverse recovery time trr << period of ac waveform so that

vLL(t) equals a constant value of vbc(ωt1) = 2 VLL

• Worst case stray inductance Lσ gives rise to reactance equal

to or less than 5% of line impedance.

• Line impedance = Vs

2Ia1 =

2VLL

6Ia1 =

VLL

3Ia1

where Ia1 = rms value of fundamental component of the

line current.

• ωLσ = 0.05 VLL

3Ia1


Component Values for Thyristor Snubber

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

• Snubber capacitor Cs = Cbase = Lσ

I r r

Vd

2

• From snubber equivalent circuit 2 Lσ diLσdt

= 2 VLL

• I rr = diLσdt

trr = 2VLL2Lσ

trr = 2VLL

2 0.05 VLL

3 I a1ω

trr = 25 ωI a1trr

• Vd = 2 VLL

• Cs = Cbase = 0.05 VLL

3 I a1ω

25 ωI a1trr

2VLL 2

= 8.7 ωI a1trr

VLL

• Snubber resistance Rs = 1.3 Rbase = 1.3 VdI rr

• Rs = 1.3 2VLL

25ωI a1trr =

0.07 VLL ωI a1trr

• Energy dissipated per cycle in snubber resistance = WR

• WR = LσI r r

2

2 +

CsVd2

2 = 18 ω Ia1 VLL(trr)2

snubber circuits - ken-gilbert.comken-gilbert.com/images/pdf/snubber.pdf · · 2016-07-08types of...

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