缓冲电路参数选取

-
van
+
Ls Ls Ls
1 A
3
5
- vbn + vcn +
B
id C
Cs
4 6 2 Rs
1 The cause of overvoltages
The overvoltages occur due to the series inductance Lc (line reactances+transformer leakage inductance) and the snap-off of the thyristor reverse recovery current.
Phase-to-neutral waveforms
van vbn
vLL=vbn-van=vba
wt1
Equivalent Circuit for SCR Snubber Calculations +
Vba(ωt1) =1.44VLL -
iLc T3 on T1 after Recovery
diDf iDf(t) Io
dt
=
Vd
Ls
t Irr t
+ Vd -
Ls
Rs
Cs Sw
Io vDf(t) Diode voltage without snubber
Df
Vd diL s Ls dt
27-2 Need for Diode Snubber Circuit 2 overvoltage protection
2VLL
=
(27-20) (27-21)
• Snubber resistance Rs = 1.3 Rbase = 1.3
Irr • Energy dissipated per cycle in snubber resistance = W R LsIrr2 CsVd2 • WR = + = 18 w Ia1 VLL(trr)2 2 2
Ls + Vs -
Lf
D1
D3
+ Vd
Lf
is
D4 D2
-
Lsmooth Ls + Vs D1 snubber D3
+ Vd
is
D4 D2
Need for diode snubbers
-
27-3 Snubber Circuit for Thyristors
P
3-phase thyristor circuit with snubbers
27-2 Need for Diode Snubber Circuit 1 The cause of overvoltages
The overvoltages occur due to the stray or leakage inductance Lσ in series with the diode and the snapoff of the diode reverse recovery current.
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-off 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 snubbers • Used as turn-on snubbers to shape the turn-on switching trajectory of controlled switches. • Limit di/dt during device turn-on
Step-down converter
L1 L2 Io
Switch current and voltage waveforms
iSW
Ls dic dt Ls dic dt
V d
i
L3
sw
Sw
+ vsw -
I0 vSW t0 t1 t3
Vd t4 t5 t6
I0
L1, L2, L3= stray inductance Lσ=L1+L2+L3
P
Cs
iT1 Rs
2Lc A
Assumptions
Equivalent circuit after T1 reverse recovery
• Trigger angle = 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 vba(wt1) = 2 VLL • Worst case stray inductance Lc gives rise to reactance equal to or less than 5% of line impedance. VLL wLc = 0.05 (27-16) 3Ia1
Introducing a Rs-Cs snubber circuit Introducing a baseline capacitance Cbase and baseline resistance Rbase,
Cbase Ls ( Rbase V d I rr I rr 2 ) Vd (27 3)
Component Values for Thyristor Snubber
• Snubber capacitor Cs = Cbase = Lσ • From snubber equivalent circuit 2 Lσ • Irr = t = dt rr 2Ls • Vd = 2 VLL • Cs = Cbase = 0.05 VLL 25 wIa1trr 2
At t=t0, the switch voltage begins to rise, At t=t1, the diode begins to conduct, At t=t3, the voltage comes down to Vd, At t=t4, the switch current begins to rise, At t=t5, the freewheeling diode recovers, At t=t6, the switch voltage decreases to zero.
3 Ia1w 2VLL Irr 2 V d
(27-3) = 2 VLL (27-17) (27-18)
diLs dt
diLs
2VLL
trr = 2
t = 25 wIa1trr 0.05 VLL rr 3 Ia1w 8.7 wIa1trr VLL Vd
4 Implementation
(1) The converters those consist of an isolation transformer that introduced a substantial leakage inductance need diode snubber circuits. (2) Step-down, half-bridge and full-bridge converters may not require a diode snubber if Lσ is minimized by proper circuit layout.
(3) Sinle-phase line-frequency diode converters.
Ls + Vs is
D4 D2 D1 D3
+ Vd
-
No need for diode snubbers
If the reverse-recovery current of D1 snaps off, then D4 provides a path for the inductive current, and reverse-voltage across D1 is clamped to the Vd.
Switching trajectory of switch
isw t6 t0 turn-on t4 t3 Vd vsw t5 idealized switching loci turn-off t1
Overvoltage at turnoff due to stray inductance Overcurrent at tuBaidu Nhomakorabeanon due to diode reverse recovery
27-1 Overview of Snubber Circuits for HardSwitched Converters Function: 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/off
27-4 Need for Snubbers with Controlled Switches
Snubber circuits are used to protect the transistors by improving their switching trajectory. Three basic types of snubbers: 1 Turn-off snubbers 2 Turn-on snubbers 3 Overvoltage snubbers
Chapter 27：Snubber Circuits
27-1 Function and Types of snubber Circuits 27-2 Diodes Snubber 27-3 Snubber Circuits for Thyristors 27-4 Need for Snubbers with Transistors 27-5 Turn-off Snubber 27-6 Overvoltage snubber 27-7 Turn-on Snubber 27-8 Snubbers for Bridge Circuit Configurations 27-9 GTO Snubber Considerations
2
Rs,opt =1.3 Rbase
Vmax=the maximum reverse voltage across the diode.
Vmax
Vd 1
There is an optimum value of Rs=Ropt=1.3Rbase that minimizes Vmax.
RsIrr 0 0 1 Vd Rs Rbase 2
Fig. 27-11 (b)
Both voltage and current of the switch are high simultaneously at turn-on and turn-off, thus causing a high instantaneous power dissipation. The stray inductances result in overvoltage beyond Vd. The diode reverse-recovery current causes overcurrent beyond Io.
+
Ls Df
Rs
Io Cs
(27 11)
Vd -
Sw
3 Performance of R-C Snubber
Vmax Cbase Rs Rs 2 ( tmm ) 1 1 0.75( ) e Vd Cs Rbase Rbase
3
（27 10）
Cs=Cbase