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Theory of Operation
SPG 422 Service Manual (B034000 and above)
3–15
Power Supply
The switching power supply generates 5 V for TTL and ECL devices. A stable
linear supply of 12 V is provided for powering the analog circuitry.
This type of power supply is called a current-mode-controlled, discontinuous,
flyback, switching power supply. The current output is distributed between the
four supplies as follows:
+ 12 V
0.8 Amps max
+5 V
7 Amps max
-5 V
6 Amps max
-12 V
0.8 Amps max
The maximum power is limited by the maximum current in the primary of T1.
This is also the only current limit for the
±
5 V supplies, as they have no
secondary current limit. The
±
12 V supplies are current limited on the second-
aries by the
±
12 V linear regulators, U150 and U152, and the secondary
±
14.5
pulse width modulator, U200.
The power inductor, T1, is driven by switching the voltage to its primary
winding on and off at a rate of approximately 60 kHz. T1 is not used as a
transformer, but as an energy storage device; the energy is stored in the primary
during the first half of the switching cycle, while voltage is being applied. On the
second half of the switching cycle, voltage to the primary is switched off and the
energy stored in T1 is transferred to the secondaries. Regulation is accomplished
by feedback from the +5 V supply to the Pulse Width Modulator controlling
voltage to the primary. This varies the length of time that voltage is applied to
the primary, causing it to store either more or less energy.
There is also circuitry to provide for operation from both 110 and 220 VAC
supplies, over voltage protection (crowbar) on the +5 V supply, and shutdown
circuitry that forces a restart of the supply if it remains in current limit for more
than a short period of time < 200 ms).
WARNING.
All primary voltages are referenced to a floating ground, not chassis
ground. An isolation transformer or a differential amplifier is therefore required
for troubleshooting the circuitry in the primary and the Pulse Width Modulator,
and in the supporting circuits. Potentials to chassis ground can be much higher
and the possibility exists that you could electrocute yourself.
This circuitry filters and rectifies the input AC voltage, charging capacitors C845
and C865 to approximately 320 VDC.
The line current passes through line filter LF950, fuse F940, and power switch
S930, and is applied to rectifier CR820. At the input of CR820, P810 is used to
Input, AC to DC Converter,
and Voltage Double
r
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