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Theory of Operation
7942 and 7946
3-105. SWITCH MODE SUPPLY
The switch mode supply consists basically of an ac-
dc converter, aback-boost regulator, and a dc-dc
converter. The ac-dc converter rectifies and filters
the ac line voltage. This f il tered dc operating volt-
age is supplied to the back -boost regulator. The
regulated dc, approximately 150 Vdc, is then con-
verted to + 5, + 12, and -12 V dc by the dc-dc con-
verter. Included in the ac-dc converter are two
thermistors that limit the initial power-on surge
current to approximately 25 amperes peak at 115
Vac and 230 Vac.
3-22
3-106. POWER -ON RESET
The power-on reset circuit is activated by the
switch mode supply and produces Power Valid sig-
nal PVAL-H. At power on, PVAL-H remains low
for at least 100 milliseconds after the +5V output
reaches 4.7 5V or higher.
Signal PV AL-H then
goes to a high level. Signal PV AL-H will also go
low for at least 500 microseconds prior to the +5V
going below 4.75V.
Signal PVAL-H can be
monitored at a test point on the front of power
supply PCA-A4. See figure 4-7. PVAL-H is con-
nected to device dependent controller PCA - A 2.
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