Page 7 of 26 Rev. A
3 Control power circuit
The control power (+12Vdc and +5Vdc) comes from the following sources (Figure S-2).
3.1 Start without input AC power (Cold start):
A “Cold start “is described as follows:
3.1.1 When “ON/OFF” switch (SW1) is pressed, a positive battery current flows
through SW1 to charge C15.
3.1.2 Q12 base receives a HI pulse and turn on (signal bypass C15 at t=0, and
charge C15 at t>0).
3.1.3 When Q12 turns on, the Q12 collector will drop to LOW and turn on Q11
(MPS2907A).
3.1.4 When Q11 turns on, the positive battery voltage via Q12 collector and
establishes a +12Vdc power supply. The +12Vdc power supply passes
through U03 (78L05) generating a +5Vdc logic power supply. And CPU
send HI signal (shutdown) to sustain Q12 on.
3.1.5 Figure W-1 shows the pulse on the C15, i.e. the base signal of Q12 during
the cold start.
3.2 Start with input AC power (AC start):
3.2.1 When we connect UPS to the utility, half-wave rectifier will activate Q12
through D2, ZD1, and Q16.
3.2.2 If “ON/OFF” switch is pressed, Q11 turns on and establishes a +12Vdc and
+5Vdc power supply similar to “Cold start”.
The SHUTDOWN network, triggered by pin 15 of the CPU, is used to shutdown the UPS on
battery mode. When a battery is in low battery voltage status, the CPU sends a “Low”
signal to turn off Q12. This causes Q11 to be turned off and isolates the control
power from batteries.
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