Page 295 - Rashid, Power Electronics Handbook

15 Resonant and Soft-Switching Converters 285

voltage and zero current. From t to t , T is switched on with The SRC has the following advantages. Transformer satura-
2
3
2
®nite switch current and voltage. At t , T is turned off softly tion can be avoided because the series capacitor can block the
3
2
and D conducts until t . dc component. The light load ef®ciency is high because both
2
4
device current and conduction loss are low. However, the
15.9.1.3 Continuous Conduction Mode with v < v S major disadvantages are that there is dif®culty in regulating
r
the output voltage under light load and no load conditions.
Figure 15.24e shows the circuit waveforms. From 0 to t , i Lr
1
transfers from D to T . Thus, T is switched on with zero Moreover, the output dc ®lter capacitor has to carry high
1
1
1
current and zero voltage. At t , T is switched off with ®nite ripple current, which could be a major problem in low-output
1
1
voltage and high-output current applications [29].
voltage and current, resulting in turn-off switching loss. From
t to t , D conducts. From t to t , T is switched on with zero
2
3
2
2
2
1
current and zero voltage. At t , T is switched off and i Lr 15.9.2 Parallel Resonant Converters
2
3
transfers from T to D . As the switches are turned on with Parallel resonant converters (PRCs) have their load connected
2
1
ZVS, lossless snubber capacitors can be added across the
in parallel with the resonant tank capacitor C [27 – 30]. The
switches. r
half-bridge con®guration is shown in Fig. 15.26. The SRC
The following parameters are de®ned: voltage conversion
behaves as a current source, whereas the PRC acts as a voltage
ratio M, characteristic impedance Z ; resonant frequency f ;
r
r
normalized load resistance r; and normalized switching source. For voltage regulation, PRC requires a smaller operat-
frequency g. ing frequency range than the SRC to compensate for load
variation.
M ¼ nV =V in ð15:2aÞ 15.9.2.1 Discontinuous Conduction Mode
o
p  The steady-state waveforms of the resonant inductor current
Z ¼ L =C r ð15:2bÞ
r
r
i Lr and the resonant capacitor voltage v Cr are shown in Fig.
p  15.27a. Initially both i Lr and v Cr are zero. From 0 to t , T 1
2
f ¼ 1=ð2p L C Þ ð15:2cÞ conducts and is turned on with zero current. When i Lr is less
r
r
r
than the output current I , i Lr increases linearly from 0 to t 1
o
2
T
r ¼ n R =Z ð15:2dÞ
L r and the output current circulates through the diode bridge.
From t to t , L resonates with C . Starting from t , i Lr
r
2
1
3
r
g ¼ f =f ð15:2eÞ
s r reverses its direction and ¯ows through D . Then T is turned
1
1
off with zero current and zero voltage. From t to t , v
q  3 4 Cr
2 2
M ¼ 1= ðg ÿ 1=gÞ =r þ 1 ð15:2fÞ decreases linearly due to the relatively constant value of I .At
o
t , when v equals zero, the output current circulates through
4 Cr
the diode bridge again. Both i and v will stay at zero for an
The relationships between M and g for different value of r are Lr Cr
interval. From t to t , the preceding operations will be
shown in Fig. 15.25. The boundary between CCM and DCM is 5 9
repeated for T and D . The output voltage is controlled by
at r ¼ 1:27g. When the converter is operating in DCM and 2 2
adjusting the time interval of [t , t ].
0:2 < g < 0:5, M ¼ 1:27rg. 4 5
15.9.2.3 Continuous Conduction Mode v < v
1 S r
This mode is similar to the operation in the DCM, but with a
0.67
0.9
higher switching frequency. Both i and v become contin-
0.75 Lr Cr
0.8
1 uous. The waveforms are shown in Fig. 15.27b. The switches
0.7
1.73
0.6 1.27 i I
o o
M 0.5 L
2.2 + T D f +
0.4 + 1 1
3 V /2 I i
0.3 d Lr B' B'B
- A
0.2 L
r=5 V r C C R V
0.1 d B r f o
+ B
0
V /2
d
0.5 0.6 0.7 0.8 0.9 1 - T D
- 2 2 -
g
FIGURE 15.25 M vs g in SRC. FIGURE 15.26 The PRC half-bridge con®guration.