Post on 13-Oct-2018
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
Purdue University ECET 257 Power & RF Electronics
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
Purdue University ECET 257 Power & RF Electronics
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
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
Purdue University ECET 257 Power & RF Electronics
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
Purdue University ECET 257 Power & RF Electronics
7C iti l R t f Ri f C t FixCritical Rate of Rise of Current - Fix
Inductors oppose Δ IInductors oppose Δ I
Purdue University ECET 257 Power & RF Electronics
8C 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 =
Purdue University ECET 257 Power & RF Electronics
9C 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 =
Lvline peak
di=
didt SCRspec
Purdue University ECET 257 Power & RF Electronics
10C 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 =
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
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
Purdue University ECET 257 Power & RF Electronics
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
Purdue University ECET 257 Power & RF Electronics
13C iti l R t f Ri f V lt fixCritical Rate of Rise of Voltage - fix
Capacitors oppose ΔvCapacitors oppose Δv
Purdue University ECET 257 Power & RF Electronics
14C iti l R t f Ri f V lt fixCritical Rate of Rise of Voltage - fix
Capacitors oppose ΔvCapacitors oppose ΔvIgnoring L, given a step at Vanode
( )v V ec Pt
RC= − −1
Purdue University ECET 257 Power & RF Electronics
15C iti l R t f Ri f V lt fixCritical Rate of Rise of Voltage - fix
Capacitors oppose ΔvCapacitors oppose ΔvIgnoring L, given a step at Vanode
( )v V ec Pt
RC= − −1
dvdt
VRC
ec P tRC= −
dt RC
Purdue University ECET 257 Power & RF Electronics
16C iti l R t f Ri f V lt fixCritical Rate of Rise of Voltage - fix
Capacitors oppose ΔvCapacitors oppose ΔvIgnoring L, given a step at Vanode
( )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
17C iti l R t f Ri f V lt fixCritical Rate of Rise of Voltage - fix
d Vdvdt
VRC
cworst
Pt= =0
C VDRM=CRload
dvdt SCRspec
=×
Purdue University ECET 257 Power & RF Electronics
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
Purdue University ECET 257 Power & RF Electronics
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
Purdue University ECET 257 Power & RF Electronics
20OverviewOverview
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
Purdue University ECET 257 Power & RF Electronics
21Triac SnubbingTriac Snubbing
Use ifRS << Rload
Purdue University ECET 257 Power & RF Electronics
22Triac SnubbingTriac Snubbing
Other wiseOther wise use two snubbers.
Use ifRS << Rload
Purdue University ECET 257 Power & RF Electronics