TM 5-6675-323-14
(b) Filtered dc is applied to the series switch pass transistor A9-Q1.
Drive circuitry for this transistor and the chopper stage that follows it is
developed in PCA A10. T h e s e d r i v e p u l s e s i g n a l s a r e p r e s e n t i n t h e s w i t c h i n g
section of the power supply assembly which is described next.
(2) Square-wave
conversion
circuit.
C o n v e r t s the input dc to a square-wave
type of waveform.
( a ) Applied dc is changed to a pulsed dc by pulsing the base of a
s e r i e s s w i t c h p a s s t r a n s i s t o r a t a k H z r a t e . A driving pulse train from PCA A10
( U 3 - 6 ) t u r n s o n a n d o f f t r a n s i s t o r A 9 - Q 2 . W h i l e A9-Q2 is turned on, series switch
pass transistor A9-Q1 is reverse biased and, therefore, turned off. When A9-Q2 is
t u r n e d o f f , s e r i e s switch pass transistor A9-Q1 is turned on and provides dc voltage
to energy storage inductor L2 which is connected to the chopper circuit.
(b) A chopper circuit is used to convert the pulsed dc to a square-wave
type of waveform that is suitable as a transformer input to transformer T4. An 18
kHz chopper driving square-wave with a 50-percent duty cycle is supplied from PCA
A 1 0 - Q 6 and A10-Q7, which alternately turns on either chopper transistors Q5 and Q4
or A6 and Q3.
The chopper driving square signal drives the primary windings of
transformer T3 whose secondary windings cause proper switching of the two pairs of
c h o p p e r transistors Q5 and Q4 or Q6 and Q3.
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