Page 299 - Rashid, Power Electronics Handbook

15 Resonant and Soft-Switching Converters 289

DELCD. Separate delays are provided for the two half-bridges L D
F F
to accommodate differences in resonant capacitor charging
currents. The delay in each set is approximated by 1-phase C r L r C
AC supply Q2 Ds2 SW1 F
25 10 ÿ12 R DEL
t DELAY ¼ þ 25 ns ð15:5Þ
0:75ðV CS ÿ V ADS Þþ 0:5
FIGURE 15.34 The EP-QR boost-type ac-dc power factor correction
where R DEL is the resistor value connected between DELAB or circuit.
DELCD to ground.
The oscillator period is determined by R and C .It is
T T
de®ned as used to store energy in order to create the condition for soft
switching. The Q is used to control the resonance during the
2
5R C T main switch transition. It should be noted that all power
T
t OSC ¼ þ 120 ns ð15:6Þ devices including SW1, Q , and main power diode D are
48 1 F
turned on and off under ZV and=or ZC conditions. Therefore,
The maximum operating frequency is 1 MHz. The phase shift the large di=dt problem due to the reverse recovery of the
between the two sets of signals is controlled by the ramp power diode can be eliminated. The soft-switching method is
voltage and an error ampli®er output with a 7 MHz band- an effective technique for EMI suppression.
width. Together with the power factor correction technique, soft-
switching converters offer a complete solution to meet EMI
regulations for both conducted and radiated EMI. The opera-
15.11 Extended-Period Quasi-Resonant tion of the EP-QR boost PFC circuit [34, 35] can be described
(EP-QR) Converters in six modes (a-f) as shown in Fig. 15.35. The corresponding
idealized waveforms are included in Fig. 15.36.
Generally, resonant and quasi-resonant converters operate
with frequency control. The extended-period quasi-resonant
converters proposed by Barbi [33] offer a simple solution to 15.11.1 Circuit Operation
modify existing hard-switched converters into soft-switched
Interval I: (t 0 ± t 1 ). Due to the resonant inductor L , which
r
ones with constant frequency operation. This makes both
limits the di=dt of the switch current, switch SW is turned on
1
output ®lter design and control simple. Figure 15.33 shows a at zero-current condition with a positive gating signal V to
GS1
standard hard-switched boost-type PFC converter. In this start a switching cycle at t ¼ t . Current in D is diverted to
F
o
hard-switched circuit, the main switch SW1 could be subject inductor L . Because D is still conducting during this short
to signi®cant switching stress because the reverse recovery r F
current of the diode D could be excessive when SW1 is turned
F D
on. In practice, a small saturable inductor may be added in F
series with the power diode D in order to reduce the di=dt of L L
F r V r V
the reverse-recovery current. In addition, an optional R-C I C O I C O
S r i S r i
snubber may be added across SW1 to reduce the dv=dt of V Lr V Lr
Cr i Cr Cr i Cr
SW1. These extra reactance components can, in fact, be used
in the EP-QR circuit to achieve soft switching, as shown in Fig. (a) (b)
15.34. The resonant components L and C have small values
r
r
and can come from the snubber circuits of a standard hard- V G2 L L
r V O r V O
switched converter. Thus, the only additional component is I C I C
S r i S r i
the auxiliary switch Q . The small resonant inductor is put in V Cr i Lr V Cr i Lr
2
series with the main switch SW1 so that SW1 can be switched Cr Cr
on under ZC condition and the di=dt problem of the reverse- (c) (d)
D F
recovery current be eliminated. The resonant capacitor C is
r
V O
L D I C I
F F S r S
V Cr
1-phase i Cr
AC supply SW C F
(e) (f)
FIGURE 15.35 Operating modes of EP-QR boost-type ac-dc power
FIGURE 15.33 Boost-type ac-dc power factor correction circuit. factor correction circuit.