Page 293 - Rashid, Power Electronics Handbook

15 Resonant and Soft-Switching Converters 283

i L
Lleak leak D1
N:1
D +
S +
-
V
C C V V
D S clamp pri out
+
i
Cclamp - i
sec
-
(a) Buck (b) Boost
S2
V in S1
C
DS
D
S
S D
(a)
(c) Buck-boost (d) Cuk
S1
0
S2
D 0
S
V +NV
in O
V
S1
S D 0
i
Lm
0
V
(e) Sepic (f) Zeta in
V
pri
FIGURE 15.21 Conventional ZVT-PWM converters. 0
-
NV
O
PWM converter and Figure 15.20b shows the associated wave- i Lleak
forms. The converter consists of a main switch S and an 0
auxiliary switch S . It can be seen that the voltage and current
1
waveforms of the switches are like squarewave type except i S1,peak
during turn-on and turn-off switching intervals, where ZVT i Cclamp
takes place. The main switch and the output diode are under 0
ZVS and are subjected to low voltage and current stresses. The
-i
auxiliary switch is under ZCS, resulting in low switching loss. S1,peak
The concept of ZVT can be extended to other PWM circuits
by adding the resonant circuit. Some basic ZVT-PWM conver- V C
ters are shown in Fig. 15.21. NV O
0

i
D1 0
15.8 Nondissipative Active Clamp t t t t t t t t
Network 0 1 2 3 (b) 4 5 6 7

The active-clamp circuit can utilize the transformer leakage FIGURE 15.22 Active clamp ¯yback converter: (a) Circuit schematics;
inductance energy and can minimize the turn-off voltage stress and (b) circuit waveforms.
in the isolated converters. The active clamp circuit provides a
means of achieving ZVS for the power switch and reducing the
rate of change of the diode's reverse recovery current. An voltage across the main switch (i.e., S ) is clamped to a
1
example of a ¯yback converter with active clamp is shown in minimum value. Then S is turned on with ZVS. However,
2
Fig. 15.22a and the circuit waveforms are shown in Fig. 15.22b. S is turned off with ®nite voltage and current, and has turn-
2
Clamping action is obtained by using a series combination of off switching loss. The clamp-mode ZVS-MRCs is discussed in
an active switch (i.e., S ) and a large capacitor so that the References [24 – 26].
2