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Classic Mac Tech Info ©2000 Thomas H. Lee, rev. April 17, 2002; All rights reserved Page 7 of 18
The switching power supply proper has three different subparts. One is a feedback con-
troller, another is the actual switching converter, and the final is an array of output rectifi-
ers and filters. The latter two subcircuits are where the most frequent failures occur.
First, here’s the control circuit:
FIGURE 5. Feedback control circuit
The feedback control circuit monitors the +12V supply voltage through a voltage divider
comprising R35, R56 and R38. This divided-down voltage is compared with a 6.2V zener
diode-derived reference voltage (from R34 and CR19). The system tries to maintain equal-
ity of the voltages presented to the input terminals of the op-amp by driving the switching
converter more or less hard as necessary to maintain a constant output voltage. An
optoisolator (U3) couples the control circuitry, which is all referenced to digital ground, to
the core switching converter, which is referenced to primary ground. Resistor R56 (located
just above the speaker) allows fine adjustment of the output voltage. Stability of the feed-
back system is assured by C21, R37, C27 and C28, which together constitute a combined
lead-lag compensator.
If the +12V supply voltage is too low, the LED inside the optoisolator is driven with less
than normal current. This, in turn, reduces the current through the optoisolator’s transistor,
which ultimately commands the switching core to increase its output. The opposite hap-
pens if the supply voltage is too high. This inverse relationship between LED drive current
and power supply output can be understood studying the schematic of the switching con-
verter proper:
+12V
R34
1k
CR19
6.2V
R35
C21
0.1µF
R56
500
R381.2k 1%
R37 10k
C28 0.001
C27
0.022
R36
470
U3
5
6
7
1
2
5
4
U1
+12V
–12V
4
11
To switching
converter core
U1 is an LM324 quad op-amp.
logic ground
1N5234B
1k 1%
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