Snubber Circuits

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. Robbins Professor Dept. of Electrical Engineering University of Minnesota 200 Union St. SE. Minneapolis, MN 555455

Copyright 1997

Snubbers - 1 W.P. Robbins © 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 (

•

Limiting the rate-of-rise (

•

Shaping the switching trajectory of the device as it turns on/off

di ) of currents through the dt semiconductor device at device turn-on

dv ) of voltages across the dt semiconductor device at device turn-off

Snubbers - 2 W.P. Robbins © 1997

Types of Snubber Circuits

1.

Unpolarized series R-C snubbers

•

2.

3.

Used to protect diodes and thyristors

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 during device turn-off dt

Polarized L-R snubbers •

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

•

Limit

di during device turn-on dt

Snubbers - 3 W.P. Robbins © 1997

Need for Diode Snubber Circuit

+ V d -

Lσ D

Rs

Io

i

Df

v

(t)

Df

f

0

• R s - Cs = snubber circuit

Sw

Df = d t

V d Lσ t I rr

t

(t)

V d

Lσ

•

• S w closes at t =

Cs

di Io

• L σ = stray inductance

d i Lσ d t

di Lσ

Diode breakdown if V d + Lσ dt

> BVBD

Snubbers - 4 W.P. Robbins © 1997

• Diode voltage without snubber

Equivalent Circuits for Diode Snubber Lσ + V d

cathode

Diode snap-off

anode

-

i Df

Rs

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

t

• Simplified snubber - the capacitive snubber Lσ + V d -

• Rs = 0 + v Cs

•v Cs

Cs

= -v

Df

-

d2vCs

vCs

Governing equation -

•

Boundary conditions - v Cs (0+) = 0 and i Lσ(0+) = I r r

LσCs

=

Vd

•

dt 2

+

Cs

L σCs

Snubbers - 5 W.P. Robbins © 1997

Performance of Capacitive Snubber

•

vCs (t) = Vd - V d cos(ωot) + Vd

•

•

1

ωo =

LσCs

 Vcs,max = Vd 1 + 

;

Cbase Cs

sin(ωot)

I r r 2 C base = Lσ   V  d Cbase   1 + Cs 

5

4

V Cs,max

3

Vd 2

1

0 0

0.2

0.4

0.6

0.8

1

1.2 C base Cs

Snubbers - 6 W.P. Robbins © 1997

1.4

1.6

1.8

2

Effect of Adding Snubber Resistance •

Equivalent circuit with snubber resistance R s

Lσ -

+ v

V d

Df

(t) Cs

+

-

•

Governing equation

•

Boundary conditions vDf (0+) = - I r r R s

•

Rs

LσCs

d2vDf

+ Rs Cs

dt 2

dvDf (0+)

and

=-

dt

dvDf dt

I rr

-

Cs

+ vDf = -Vd

R s Vd Lσ

+

I r r Rs2 Lσ

Solution for v Df (t)

vDf (t) = - Vd -

ωa = ωo

tan(φ) = -

α2 1 ωo2 Rb ωa Lσ

Cbase

V d e- αt

-

Cs

;

ωo =

sin(ωa t - φ − ξ)

1 LσCs

;

α=

Rs 2Lσ

Lσ Vd α α ; tan(ξ) = ; R base = , C base = ωa ωa I rr (Vd/ I r r )2 Snubbers - 7 W.P. Robbins © 1997

Performance of R-C Snubber •

At t = tm vDf (t) = Vmax

•

tm =

tan- 1 (ωa /α)

Vmax

•

Vd

+

ωa =1+

φ - ξ ≥0 ωa

1 + C N- 1 - R N exp(-αt m) Cs

•

CN =

•

Ls I r r 2 Cbase = Vd2

Cbase

and

RN =

and

Rs R base

R base =

Vd I rr

3 R s,opt

C s = C base

R

= 1.3

base

2 V max V d 1 Rs I rr Vd

0 0

1

Snubbers - 8 W.P. Robbins © 1997

R s R base

2

Diode Snubber Design Nomogram Wtot 2 L s I r r /2

3

WR 2 L s I r r /2 0

2 0 V max

0 0

0

for R = R s s,opt

Vd 00

00

0 00

0 00 0

00 0 0

1

0

0

00

R s,op R base

0 0

1

C s / Cbase

2

Snubbers - 9 W.P. Robbins © 1997

3

Need for Snubbers with Controlled Switches

L1

V d

i L3

•

L2

S

Io

L1 , L 2 , L 3 = stray inductances

• L

sw

+ vsw

w

-

L

i

= L + L + L 1 2 3

di dt

sw L

Io

di dt

Io

V d

vsw t 3

to t 1

i

sw t

6

t 4

idealized switching loci

t5

turn-off

to t1

t3 Vd

t

6

• Overvoltage at turn-off due to stray inductance • Overcurrent at turn-on due to diode reverse recovery

turn-on

t 4

t5

vsw

Snubbers - 10 W.P. Robbins © 1997

Turn-on Snubber for Controlled Switches

•

Circuit configuration

+

V

i

Io

d

Ds S

-

•

DF

D f

Turn-off snubber

Rs

w

Cs

i

Cs

Equivalent circuit during switch turn-off

• Assumptions

Io

Df

1. No stray inductance. 2. i s w (t) = Io(1 - t/t f i )

V

d

Io - i i sw

sw

Cs

Snubbers - 11 W.P. Robbins © 1997

3. i s w (t) uneffected by snubber circuit.

Turn-off Snubber Operation

•

Capacitor voltage and current for 0 < t < t f i I ot I ot 2 • i Cs (t) = and v Cs (t) = tf i 2Cs t f i

•

I ot f i For Cs = Cs1 , v Cs = Vd at t = tf i yielding C s1 = 2Vd

•

Circuit waveforms for varying values of C s

i

i

sw

i

sw

sw Io

i

i

Df t fi

i

i

Df t

Df

t fi

fi

Cs

V d v Cs Cs < Cs1

Cs = Cs1

Snubbers - 12 W.P. Robbins © 1997

Cs > C s

Benefits of Snubber Resistance at Sw Turn-on

D

Io

f

Io

• Ds shorts out R s during S w turn-off.

Rs

• During S w turn-on, D s reverse-biased and C s

V d Sw

Ds

discharges thru R s .

Cs

• Turn-on with R s = 0 discharge of C s

t rr vsw

i sw

t ri 0

• Extra energy dissipation in S w because of lengthened voltage fall time.

Io

V d t

• Energy stored on Cs dissipated in S w .

2

t ri + t rr

v

• Turn-on with R s > 0 • Energy stored on Cs dissipated in R s

i

sw

D

I

f

rather than in S w . • Voltage fall time kept quite short.

t rr

i sw

Io

Vd Rs I rr

Snubbers - 13 W.P. Robbins © 1997

rr

Effect of Snubber Capacitance •

Switching trajectory

i sw Io Cs < Cs1

RBSOA

Cs = Cs1 Cs > Cs1 V

•

vsw

d

Energy dissipation 1

WR = dissipation in resistor

W / total Wbase 0.8

WT = dissipation in switch S w

0.6 W / Wbase R

0.4

WT / W base

2Vd

Wtotal = WR + WT

0.2 0

Cs1 =

I ot f i

Wbase = 0.5 VdI ot f i 0

0.2

0.4

0.6

0.8

1

1.2

1.4

C /C s s1

Snubbers - 14 W.P. Robbins © 1997

Turn-off Snubber Design Procedure

• Selection of Cs • Minimize energy dissipation (W T) in BJT at turn-on • Minimize WR + WT • Keep switching locus within RBSOA • Reasonable value is C s = Cs1 • Selection of R s • Limit icap(0+) =

Vd Rs

< Ir r

• Usually designer specifies I r r < 0.2 Io so Vd = 0.2 Io Rs • Snubber recovery time (BJT in on-state) • Capacitor voltage = V d exp(-t/R s Cs ) • Time for vCs to drop to 0.1Vd is 2.3 Rs Cs • BJT must remain on for a time of 2.3 Rs Cs Snubbers - 15 W.P. Robbins © 1997

Overvoltage Snubber for Controlled Switches •

Circuit configuration - D ov, R ov, and C ov form overvoltage snubber

Lσ

+

D

f

Io

R

ov

Vd Sw

-

Dov

C ov

•

Overvoltage snubber limits magnitude of voltage developed across S w as it turns off.

•

Switch S w waveforms without overvoltage snubber •

i

t f i = switch current fall time ; kV d = overvoltage on Sw

kV d

di Lσ

Io • kV d = Lσ = Lσ dt tf i

s w Io

v

s w

V d

o

t

fi

Snubbers - 16 W.P. Robbins © 1997

• Lσ =

kV dt f i Io

Operation of Overvoltage Snubber •

Dov,C ov provide alternate path for inductor current as S w turns off. • Switch current can fall to zero much faster than L σ current.

•

Df forced to be on (approximating a short ckt) by I o after S w is off.

•

Equivalent circuit after turn-off of S w . i

Lσ Lσ

+

R

D ov

Vd

i

+

v

-

-

σ

+ C

ov

v

-

Cov

v

2

LσCov 2

v Cov (0+) = Vd i

(t) = I o cos[ Lσ

Vd

o

s w 0

π

i Lσ(0+) = I o t Lσ C ov

]

Discharge of Cov thru R ov with time constant R ov C ov ∆V sw,max

Lσ I

π

LσCov

• Equivalent circuit while inductor current decays to zero

Charge-up of Cov from Lσ i

• t f i <<

Cov

Lσ L

V d

ov

+

C ov

-

• Dov on for 0 < t <

π

• Energy transfer from Lσ to Cov Cov ( ∆V sw,max )2 Lσ ( I o )2 = 2 2

Lσ C ov 4

. Snubbers - 17 W.P. Robbins © 1997

Overvoltage Snubber Design

• Cov =

Ls I o 2 (∆v sw,max )2

• Limit ∆v sw,max to 0.1Vd

kV d t fi • Using Ls = Io

• Cov =

in equation for Cov yields

kV d t fi I o 2 I o (0.1Vd )2

100k t f i I o = V d2

• Cov = 200 Cs1 where Cs1 =

t fi I o 2Vd

which is used

in turn-off snubber

• Recovery time of C ov (2.3Rov Cov ) must be less than off-time duration, toff , of the switch Sw. • R ov ≈

t off 2.3 C ov

Snubbers - 18 W.P. Robbins © 1997

Turn-on Snubber Circuit •

Circuit topology

+ D

I

f

V d

R

Ls

+

D f

o

Snubber circuit Ls D

Ls

D V d

Ls

Sw

-

R

D

Ls

I

Ls

f

-

Sw

di s w

•

Circuit reduces V s w as switch S w turns on. Voltage drop Ls dt provides the voltage reduction.

•

Switching trajectories with and without turn-on snubber.

Io

i sw With snubber

Without snubber

Ls

di sw dt v V d

Snubbers - 19 W.P. Robbins © 1997

sw

o

Turn-on Snubber Operating Waveforms •

Small values of snubber inductance (L s < Ls1 )

v

I rr

s w

V

•

d

di s w

controlled by dt switch S w and drive circuit.

Io

Ls I o

• ∆vs w = tr i

i

•

s w

I rr

s w

reduced V

d

•

Io

di s w

limited by circuit dt Vd Io to < Ls tr i

• Ls1 =

s w t

•

trr

Large values of snubber inductance (L s > Ls1 ).

v

i

tr i

on

≈

Ls Io V

Vdt r i Io

> tr i + t r r

d

I r r reduced when Ls > Ls1 because I r r proportional to

Snubbers - 20 W.P. Robbins © 1997

di s w dt

Turn-on Snubber Recovery at Switch Turn-off

+ D

Io

f

V d

R

Ls

• Assume switch current fall time t ri = 0. • Inductor current must discharge thru DLs - R Ls series segment.

Ls D

Ls

Sw

-

I o R Ls exp(-R Ls t/Ls )

Io R

Ls

is w vs w

V d

Io

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

t rv

•

Overvoltage smaller if t f i smaller.

•

Time of 2.3 Ls /R Ls required for inductor current to decay to 0.1 I o

•

Off-time of switch must be > 2.3 L s /R Ls

Snubbers - 21 W.P. Robbins © 1997

Turn-on Snubber Design Trade-offs •

Selection of inductor L s •

•

Larger L s decreases energy dissipation in switch at turn-on •

Ws w = WB (1 + I r r / I o)2 [1 - Ls /L s1 ]

•

WB = VdI ot f i /2 and L s1 = Vdt f i / I o

•

Ls > Ls1 Ws w = 0

Larger L s increases energy dissipation in R Ls •

•

WR = WB Ls / L s1

•

Ls > Ls1 reduces magnitude of reverse recovery current I r r

•

Inductor must carry current I o when switch is on - makes inductor expensive and hence turn-on snubber seldom used

Selection of resistor R Ls •

Smaller values of R Ls reduce switch overvoltage I o R Ls at turn-off •

•

Limiting overvoltage to 0.1V d yields R Ls = 0.1 Vd/ I o

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

Snubbers - 22 W.P. Robbins © 1997

Thyristor Snubber Circuit P 1 van +

L

- v bn +

L

-

-

5

3

A B

i d

L

v cn +

C Cs 4

6

2

Rs

• van (t) = Vs sin(ωt), v bn(t) = Vs sin(ωt - 120°), vcn (t) = Vs sin(ωt - 240°)

• Phase-to-neutral waveforms

v

v bn

an

v LL= v v v bn an = ba •

•

vLL(t) =

t 1

3 Vs sin(ωt - 60°)

Maximum rms line-to-line voltage V LL =

Snubbers - 23 W.P. Robbins © 1997

3 V 2 s

Equivalent Circuit for SCR Snubber Calculations •

Equivalent circuit after T1 reverse recovery

i

L

2 L + V ( bc

T after 1 recovery

t ) 1

Cs

T3 (on)

-

•

P

i

T1

Rs

A

Assumptions •

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

•

Reverse recovery time t r r << period of ac waveform so that vLL(t) equals a constant value of v bc(ωt 1) =

•

2 VLL

2 VLL

Worst case stray inductance L σ gives rise to reactance equal to or less than 5% of line impedance.

•

Line impedance =

Vs 2I a1

=

2VLL 6I a1

=

VLL 3I a1

where I a1 = rms value of fundamental component of the line current.

•

ωLσ = 0.05

VLL 3I a1

Snubbers - 24 W.P. Robbins © 1997

Component Values for Thyristor Snubber

•

•

Use same design as for diode snubber but adapt the formulas to the thyristor circuit notation I r r 2 Snubber capacitor C s = Cbase = Lσ   V  d

•

•

di Lσ

From snubber equivalent circuit 2 L σ dt

I rr =

di Lσ dt

2VLL

tr r =

2Lσ

2VLL

tr r = 2

0.05 V LL

=

2 VLL

t rr = 25 ωI a1t r r

3 I a1ω

•

Vd =

•

0.05 V LL  2 5 ωI a1t r r  2 8.7 ωI a1t r r   = Cs = Cbase = VLL 3 I a1ω  2VLL 

•

Vd

Snubber resistance R s = 1.3 Rbase = 1.3 I

•

•

2 VLL

R s = 1.3

2VLL 25ωI a1t r r

=

rr

0.07 V LL ωI a1t r r

Energy dissipated per cycle in snubber resistance = W R

•

WR =

LσI r r 2 2

+

Cs Vd2 2

= 18 ω I a1 VLL(tr r )2

Snubbers - 25 W.P. Robbins © 1997