Page 309 - Rashid, Power Electronics Handbook

15 Resonant and Soft-Switching Converters 299

15.14.4 Quasi-Resonant Soft-Switched Inverter 300 V
[47] cr1
200
15.14.4.1 Circuit Operation
Consider an inverter fed by a dc voltage source V . A front- Voltage(V) 100 V cr2
s
stage interface circuit shown in Fig. 15.53 can be added 0
between the dc voltage source and the inverter. The front
-100
stage circuit consists of a quasi-resonant circuit in which the
t t 1 t 2 t 3 t 4 t 5
®rst half of the resonance cycle is set to occur to create the -200 0
zero-voltage condition whenever inverter switching is needed. Time
After inverter switching has been completed, the second half of FIGURE 15.54 Typical waveforms for V cr1 and V cr2 .
the resonance cycle takes place so that the dc link voltage is set
back to its normal level. To avoid excessive losses in the
this current then ¯ows in the opposite direction. Then C will
r1
resonant circuit, a small capacitor C is normally used to
r1 be recharged via L , C , C and D . Diode D turns off when
provide the dc link voltage while the large smoothing dc link r r2 r1 r r
the current in L becomes zero. Here V cr1 will not go beyond 1
r
capacitor C is isolated from the resonant circuit just before
1 per unit because C is clamped to supply voltage by diode D .
the zero-voltage duration. This method avoids the require- r1 b
Switch S can be turned on again at zero voltage condition
b
ment for pulling the dc voltage of the bulk capacitor to zero.
when V cr1 returns to normal dc supply voltage. After Dr turns
The period for this mode is from t to t in Fig. 15.54. In
1
0
this mode, switch S is turned on and switches S and S are
r2
b
r1
turned off. The inverter in Fig. 15.53 works like a conventional
L
dc link inverter and V cr1 ¼ V c1 in this mode. The voltage s
across switch S is zero. Before an inverter switching takes C r2
b
place, switch S r1 is triggered at t to discharge C . This L r
r1
1
operating mode ends at t when V cr1 approaches zero. The Vs C r1 I O
2
equivalent circuit in this mode is shown in Fig. 15.55a. The
switch S must be turned off at zero voltage when switch S is S r1 D r
b
r1
triggered. After S r1 is triggered, C r1 will be discharged via the
loop C , C , L and S . Under conditions of V cr2 0 and
r
r1
r1
r2
C C , energy stored in C will be transferred to C r2 and (a) Mode 2
r1
r1
r2
V cr1 falls to zero in the ®rst half of the resonant cycle in the
equivalent circuit of Fig. 15.55a. V cr1 will be clamped to zero
by the freewheel diodes in the inverter bridge and will not L s
become negative. Thus, V can be pulled down to zero for
cr1 C r2
zero voltage switching. When the current in inductor L
r L r
becomes zero, switch S can be turned off at zero current.
r1 Vs C I
Inverter switching can take place in the period from t to t r1 O
2 3
in which V remains zero. This period must be longer than
cr1 S D
the turn-on and turn-off times of the switches. When inverter r1 r
switching has been completed, it is necessary to reset the
voltage of capacitor C . The equivalent circuit in this mode is (b) Mode 4
r1
shown in Fig. 15.55b. The current in inductor L reaches zero
r
at t . Due to the voltage in V cr2 and the presence of diode D ,
r
3
L
s
S r2 C r2
L
s
C
r2 Vs I O
D b L r L r
DC S S r2 C r1
source b to
inverter
C
1
S (c) Mode 5
r1
FIGURE 15.55 Equivalent circuits of the quasi-resonant circuit for
FIGURE 15.53 Quasi-resonant circuit for soft-switched inverter. different modes.