1 SCR & Triac SnubbingSCR & Triac Snubbing -...

1SCR & Triac SnubbingSCR & Triac Snubbing

anode

cathode

Purdue University ECET 257 Power & RF Electronics

2OverviewOverview

High side vs low side switchHigh side vs low side switchCritical Rate of Rise of Current

problem– problem– SCR schematic solution

C iti l R t f Ri f V ltCritical Rate of Rise of VoltageTriac


3Low Side SwitchLow Side Switch

Switch closedSwitch closedVacross Rload =

120V120V

Switch openedVacross Rload = 0Vacross Rload

Von bottom of load = 120V !

even though load is off


4High Side SwitchHigh Side Switch

Switch closedSwitch closedVacross Rload =

120V120V

Switch openedVacross Rload = 0Vacross Rload

Von load = 0V

Do it this way!Purdue University ECET 257 Power & RF Electronics

y

5OverviewOverview





6Critical Rate of Rise of CurrentCritical Rate of Rise of Current

dianode

di/dtI > T > R dI => T => R down=> I up => T upp p=> R down => I up

b t h lburns out a channelcathode


7C iti l R t f Ri f C t FixCritical Rate of Rise of Current - Fix

Inductors oppose Δ IInductors oppose Δ I




L div LdtL =




L div LdtL =

Lvline peak

di=

didt SCRspec




L div LdtL =

Lvline peak

di=

didt SCRspec

If load is motor separate L not neededPurdue University ECET 257 Power & RF Electronics

If load is motor, separate L not needed.

11OverviewOverview





12Critical Rate of Rise of VoltageCritical Rate of Rise of Voltage

Parasitic CParasitic Canode - gate

noise on anode coupled to gatep g

+ noise => + vgate =>

SCR ONparasitic

crane fatality


13C iti l R t f Ri f V lt fixCritical Rate of Rise of Voltage - fix

Capacitors oppose ΔvCapacitors oppose Δv



Capacitors oppose ΔvCapacitors oppose ΔvIgnoring L, given a step at Vanode

( )v V ec Pt

RC= − −1




( )v V ec Pt

RC= − −1

dvdt

VRC

ec P tRC= −

dt RC




( )v V ec Pt

RC= − −1

dvdt

VRC

ec P tRC= −

dt RC

dv Vcworst

Pt= =0

Purdue University ECET 257 Power & RF Electronicsdt RCworst t=0


d Vdvdt

VRC

cworst

Pt= =0

C VDRM=CRload

dvdt SCRspec

=×


18Cap DischargeCap Discharge

During charge, VC may = Vline pk

h SC fi dWhen SCR firedRSCR 0.x ΩC idl di h th h SCRC rapidly discharges through SCRIC discharge ~ Vline pk/RSCR on

!= 160V / 0.2Ω = 320A !ISCR = Iload + IC discharge

R li i I IRS limits ISCR < ITSM

RV

I Isline pk> ≈

10VI I

line pk


I IsTSM load pk− −I ITSM load pk

19R bypass during chargeRS bypass during charge

C must be directly across SCRC must be directly across SCR during charge to short out spikesp

R limits I during discharge

Bypass R with diode

Idiode pk = VDRM / XC


20OverviewOverview





21Triac SnubbingTriac Snubbing

Use ifRS << Rload


22Triac SnubbingTriac Snubbing

Other wiseOther wise use two snubbers.

Use ifRS << Rload


1 SCR & Triac SnubbingSCR & Triac Snubbing -...

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