Page 168 - Rashid, Power Electronics Handbook

10 Diode Recti®ers 157

The voltage across L is zero forms shown in Fig. 10.29 cannot actually be achieved in the
P
real world. In the following, the effects of nonideal diodes and
Vð3Þ¼ V IN ð10:90Þ transformers will be examined.
The voltage across L is also zero
S
10.6.1.2 Circuit Using Ultra-Fast Diodes
Vð6Þ¼ 0 ð10:91Þ
Fig. 10.30 shows the waveforms of the forward converter
(circuit given in Fig. 10.28) when ultra-fast diodes are used
Inductive current IðL Þ continues to fall at the rate of
1
as D , D , and D . (Note that ultra-fast diodes are actually
R
M
F
dIðL Þ ¼ ÿV o ð10:92Þ much slower than Schottky diodes.) The waveforms are
1
dt L 1 obtained by PSpice simulations, based on the following
assumptions:
The switching cycle restarts when the switch M is turned on
1
again at t ¼ T. D is an MUR460 ultra-fast diode, and D and D are
M R F
From the waveforms shown in Fig. 10.29, the following MUR1560 ultra-fast diodes;
useful information (for continuous-mode operation) can be M is an IRF640 MOS transistor;
1
found: transformer T has a coupling coef®cient of 0.99999999
1
(which may be assumed to be 1); and
The output voltage V is equal to the average value of
o the switching operation of the converter has reached a
Vð9Þ,
steady state.
N S
V ¼ D V IN ð10:93Þ It is observed that a large spike appears in the current wave-
o
N P forms of diodes D and D (denoted as IðDRÞ and IðDFÞ in
F
R
The maximum current in the forward rectifying diode Fig. 10.30) whenever the MOS transistor M is turned on. This
1
D and ¯ywheel diode D is
R F is due to the relatively slow reverse recovery of the ¯ywheel
diode D . During the reverse recovery time, the positive
F
IðDRÞ ¼ IðDEÞ
max max voltage suddenly appearing across L S (which is equal to
1 V o V ðN =N Þ) drives a large transient current through D
¼ I þ ð1 ÿ DÞT ð10:94Þ IN S P R
o
2 L 1 and D . This current spike results in large current stress and
F
where V ¼ DV ðN =N Þ and I is the output load- power dissipation in D , D , and M .
F
1
R
o IN S P o
ing current. A method of reducing the current spikes is to use Schottky
The maximum reverse voltage of D and D is diodes as D and D , as will be described in what follows.
F
R
R F
VðDRÞ ¼ VðDFÞ
max max 10.6.1.3 Circuit Using Schottky Diodes
N S In order to reduce the current spikes caused by the slow
¼ Vð6; 9Þ max ¼ V IN ð10:95Þ
N P reverse recovery of recti®ers, Schottky diodes are now used as
D and D . The assumptions made here are (referring to the
The maximum reverse voltage of D M is R F
circuit shown in Fig. 10.28):
VðDMÞ max ¼ V IN ð10:96Þ D and D are MBR2540 Schottky diodes;
R F
The maximum current in D M is D M is an MUR460 ultra-fast diode;
M is an IRF640 MOS transistor;
1
V IN transformer T has a coupling coef®cient of 0.99999999;
1
IðDMÞ ¼ DT ð10:97Þ
max and
L
P
the switching operation of the converter has reached a
The maximum current in the switch M , denoted as steady state.
1
IDðM Þ,is
1
The new simulated waveforms are given in Fig. 10.31. It is
N S found that when Schottky diodes are used as D and D , the
F
R
IDðM Þ ¼ IðDRÞ þ IðDMÞ
1 max max max
N P amplitudes of the current spikes in IDðM1Þ,IðDRÞ, and IðDFÞ
can be reduced to practically zero. This solves the slow-speed

N S 1 V 0 V IN
¼ I þ ð1 ÿ DÞT þ DT problem of ultra-fast diodes.
o
N P 2 L 1 L P
ð10:98Þ 10.6.1.4 Circuit with Practical Transformer
However, it should be understood that, due to the nonideal The simulation results given in Figs. 10.29–10.31 (for the
characteristics of practical components, the idealized wave- forward converter circuit shown in Fig. 10.28) are based on