Power Supply - Philips DVDR985 Technical Training Manual

Power Supply

This unit uses a stand alone Switch Mode Power
Supply, SMPS. A MOSFET transistor turns On
and Off in an oscillator fashion, driving a trans-
former. The primary half of the supply uses a Hot
Ground. The primary side of the circuit provides
drive and coarse control of the power supply.
The secondary side of the circuit rectifies and fil-
ters the output of the transformer to produce
many output voltages. It uses a cold ground, sig-
nal ground system. Two of the output voltages
are monitored for precise regulation. The 12Vdc
is supplied to the anode of the Optic Coupler's
diode, and the 5Vdc Standby is feed to the
Shunt Regulator. The regulation path includes an
Optic Coupler to accommodate the different
grounding systems.
Circuit Description
AC Input Circuit
Refer to Figure 16. The input circuit consists of
a lightning protection circuit and an EMI filter.
The lightning protection circuit consists of
R3120, spark gaps 1124 and 1125. L5110,
L5115, C2120 and L5121 form the EMI filter. It
prevents noise coming in or out through the
mains. The AC input is rectified by diodes 6151,
6152, 6153, 6154, and filtered by C2126. The
voltage on C2126 is approximately 155V. It can
vary from 150V to 160V, depending on the AC
input voltage.
Start Circuit
This circuit consists of R3125, 3126, R3139,
R3141, C2140, and R3132. When the power
plug is connected to AC, the MOSFET 7125 will
start conducting as soon as the gate voltage
reaches the threshold value. A current starts to
flow in the primary winding of 5125, Pins 2 and
4. The MOSFET will be fed forward via the wind-
ing connected to Pins 7 and 8 by R3150 and
C2146. While the MOSFET is conducting, ener-
gy is building up in the transformer. The current
flow through the MOSFET is sensed by R3133,
3134, and 3135. When the current level rises
high enough to provide a voltage drop on these
components and large enough to turn On 7140,
7125 is turned Off by 7140. Diodes 6130, 6131
and 6132 protect the control circuit in case of
failure of the MOSFET by providing an upperlimit
to the voltage that can remain on the source of
the MOSFET.
Coarse Regulation
The positive portion of the signal on Pins 7 and 8
will be rectified via R3150 and D6140, charging
C2140 via R3140. In time, the voltage on C2140
will reach 15 to 20Vdc. This value depends on
the value of the Mains voltage and the load. The
negative portion of the signal on Pins 7 and 8
will be rectified via R3150 and D6142. This will
charge C2151 to approximately -15Vdc. This is
used as a regulation supply.
The control circuit consists of T7140, D6141,
C2144, C2145, C2147, R3147, and 3148. This
circuit controls the conduction time and the
switching frequency of the MOSFET. It switches
Off the MOSFET as soon as the voltage on the
source of T7125 reaches a certain value. This
value depends on the error voltage at the emitter
of T7140, which can be a negative (+/- 0.6V).
The voltage fed back by the regulation circuit
defines this error voltage.
Precise Regulation
The regulation circuit consists of an Optic-
Coupler, 7200, 7251, and a voltage divider net-
work. The Optic-Coupler isolates the Hot Ground
referenced voltage on the emitter of 7140 from
the Cold Ground referenced voltage on 7251.
7251, a Shunt Regulator, has two component
characteristics. It is a very stable and accurate
reference diode and a high gain amplifier.
7251 will conduct from cathode to anode when
the reference is higher than the internal refer-
ence voltage of about 2.5Vdc. If the reference
voltage is lower, the cathode current is almost
zero. The cathode current flows through the LED
of the Optic-Coupler, controlling the current
through the transistor portion of the Optic-
Coupler. The collector current of 7200 will adjust
the feedback level of the error voltage at the
emitter of T7140.
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