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CIRCUIT DESCRIPTION
Sub-Power Supply Circuit Description
Figure 1 is a simplified diagram of the main Power Supply used in the ZP94/95 series Projection Television
Chassis. The primary control element of the power supply is I901 (the Switching Regulator IC), in conjunc-
tion with switching transformer T901. These two components, along with the supporting circuitry, comprise
a closed loop regulation system. Unlike previous Pulse Width Modulated (PWM) Switch Mode Zenith
power supplies, the regulation system in the this chassis utilizes Frequency Control Modulation with an
operational frequency of 85KHZ to 100KHZ, corresponding to full load and no load conditions, respec-
tively. Primary regulation is provided by Q902, I902 and Q910, regulating the switching frequency at pin (3)
of I901 via pin 1, the regulation input to the IC.
B+ Generation for the Sub Power Supply Driver IC
(see figure2)
Vcc for the Driver IC is first generated by the AC input. This voltage is called Start-Up Voltage. I901
requires 21V DC to operate normal. However, it will begin operation at 14.5V DC on pin (4) of I901.
When AC is applied, AC is routed through the main fuse F901 (a 5 Amp fuse), then through the Line filters
L901, 902, 903 and 904 to prevent any internal high frequency radiation for radiating back into the AC
power line. After passing the line filters it arrives at the bridge rectifier D901 where it is converted to Raw
150V DC voltage to be supplied to the power supply switching transformer T901 pin (1). However, one leg
of the AC is routed to a half wave rectifier D902 where it is rectified, routed through R905 and R906 (both
a 5.6K ohm resistor), filtered by C907, clamped by a 30V Zener D904 and made available to pin (4) of
I901 as start up voltage. The Red LED D903 is illuminated by this power supply. When this voltage reaches
14.4Vdc, the internal Regulator of I901 is turned on and begins the operation of I901. When the power
supply begins to operate by turning on and off the internal Switch MOSFET, the Raw 150V DC routed
through T901 (in on pin 1 and out on pin 2), is connected to pin (3) of I901 which is the Drain. The Source
of the internal Switch MOSFET is routed out of pin (2) through three low ohm resistors to hot ground. This
on and off action causes the transformer to saturate building up the magnet field. When the internal Switch
MOSFET turns off, the magnet field collapses and the EMF is coupled over to the secondary windings, as
well as the drive windings. The drive windings at pin (8 and 9) produce a run voltage pulse which is rectified
by D905, filtered by C908 then routed through R908, clamped by D904 and now becomes run voltage
(22V) for I901.
<Figure 1>
T901
Switch Mode
Transformer
I902
Opti-Coupler
I903
Opti-Coupler
Q905
SCR
28V
Shutdown
Input
AC
1
3
Drain
I901
Switch
Mode
IC
Reg
Q914
SCR
4
Q910
Buffer
Q902
Buffer
Raw 150V