9-10
(2) Drive circuit of chopper regulator
Operation of drive circuit is explained here, though PWM
circuit including error amp. is explained in the next section.
Drive circuit uses drive transformer TK01 and push-pull
output as shown in Fig. 9.
If connect the secondary pulse of drive transformer directly
to gate and source of FET changes gate voltage, due to duty
ratio the driving can not be done.
This is the reason why the transformer can not transmit d.c.
voltage to the secondary. That is; the secondary pulse becomes
waveform of which average is zero. accordingly, the
secondary pulse varies with duty ratio as in Fig.12 c and c’,
and in case of c gate voltage is lacking and in case of c’ gate
voltage is excessive.
But in this model, this problem is solved by utilizing voltage
charged in CK11.
In period that the secondary pulse is negative, through DK01,
CK11 is rapidly charged to the peak value of pulse by
supplying current as solid line in Fig. 9. In the next, when
positive pulse comes, pulse generated across the secondary
of transformer and voltage stored in CK11 (Fig. 12 d d’) are
superimposed, and are added to gate of FET. By this way, the
gate voltage does not depend on duty ratio, and becomes
constant voltage approximately corresponding to amplitude
of secondary pulse (Fig. 12 e e’). As a result, stable driving
can be done. Into gate, only because current charging the gate
capacitance flows for a moment, voltage does not change
even in small capacitance of CK11.
RK09 has roles which control current flowing into gate,
delay a little switching speed of FET and suppress switching
noise. DK03, CK10 and RK81 suppress the ringing of pulse.
00
00
a
Emitter voltage of QK03
a'
bb'
Primary pulse of TK01
c
c'
Secondary pulse of TK01
dd'
Voltage across CK11
e
e'
V
G
-
S
of QH25
00
00
Fig. 12
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