Hitachi CG2026S, Service Manual, Page: 15 / 42

14
CIRCUIT DESCIRIPTION
POWER SUPPLY
The ZX series of receivers incorporate a Motorola switch mode power supply using a MC44608 regulator controller IC.
The circuit provides power to the receiver in both standby and normal operation modes.
START UP
The switch on the mains supply is fed through the mains filter network TR801, the surge limiter resistor R828, the bridge
rectifier diodes D811/13/37/38, and reservoir capacitor producing approx. 320volts D.C to feed the switching MOSFET
Q801 via the primary winding of TR802 pins 6 and 7.
Start up resistor R801 feeds from a 500V coming from the mains through the adder diodes D809, D890 to pin 8 of IC800, the
IC uses 9mA current source and connects it internally to VCC at pin6 allowing a rapid charge enough for start up. Then IC800
responds with the oscillator starting to oscillate at a 40khz frequency fixed by the IC manufacturer.
The IC then produces, pulse width modulation pulses, at this frequency on pin 5 to drive the base of the switching FET
Q801, that will then switch current on and off through the primary of TR802, which will in turn provides voltages in the
secondary windings. The secondary winding voltages being proportional to the length of time that Q801 is turned on in each
cycle. The voltage produced between pins 4 and 3 of TR802 is rectified by D804 developing aprox. 12 volts on C810, which
takes over from the start up resistor to supply pin 8 of IC800.
The Demag pin at pin 1 offers 3 different functions: Zero voltage crossing detection (50mV), 24mA current detection and
120mA current detection. The 24mA level is used to detect the secondary reconfiguration status and the 120mA level to
detect an Over Voltage status called Quick OVP.
The VCC at pin 6 operates between 6 . 6V and 13V in normal operation, when this voltage exceeds 15V then the IC output is
disabled.
VOLTAGE REGULATION
After initial start up the secondary voltages of TR802 are established. These voltages then need to be regulated to the
required levels. In a switch mode power supply such as this, it is the ON time of the switching FET Q801 that determines
the output voltages produced. To provide regulation of the supply there is a feedback loop via an adjustable zener IC118 and
an OPTO- coupler connected to pin 3 of IC800. The reference voltage of IC118 is set to 2 . 5V to supply a B+ voltage of 115V.
Any fluctuation at this pin will cause IC800 to compensate it either by increasing or decreasing the voltage at the secondary
outputs.
VOLTAGE PROTECTION
The MC44608 offers two OVP functions:
1- A fixed function that detects when V CC is higher than 15.4V
2- A programmable function that uses the demag pin. The current flowing into the demag pin is mirrored and compared to
the reference current Iovp (120mA). Thus this OVP is quicker than normal number one as it directly sense the change in
current rather than waiting for a specific voltage value, and is called QOVP. In both cases, once an OVP condition is
detected, the output is latched off until a new circuit START–UP.
3- A software controlled function acts on pin 52 of IC501. This pin monitors feedback from both 8V and 5V via D512, then
compares these to a reference value Vref pre-set by the hardware through resistors R545, R546, R548. In normal mode
operation 1.2V < Vref < 2.4V. Any voltage outside this window will cause the micro controller to force the TV to stand by
mode by lowering the standby port. Refer to standby mode.
CURRENT PROTECTION
To monitor the current drawn by the receiver the source of Q801 is returned to the bridge rectifier through a low value
resistor R807. All the current drawn by the receiver will flow through that resistor each time Q801 conducts, this will produce
a voltage across the resistors proportional to the current drawn by the receiver. This voltage is fed to pin 2 of IC800 via
R806.When the receiver is working normally the voltage across R807 is only a fraction of a volt and is not large enough to
have any effect on IC800. Under fault conditions, if the receiver draws excessive current the voltage across R807 will rise.
This voltage is monitored by the current sense input pin 2.
This Current Sense pin senses the voltage developed on the series resistor R806 inserted in the source of the power
MOSFET. When I sense reaches 1V, the Driver output (pin 5) is disabled. This is known as the Over Current Protection
function. A 200mA current source is flowing out of the pin 3 during the start–up phase and during the switching phase in
case of the Pulsed Mode of operation. A resistor can be inserted between the sense resistor and the pin 3, thus a
programmable peak current detection can be performed during the SMPS stand–by mode.
SAFETY PRECAUTIONS
Remember that all the primary side components of the power supply shown to the left of TR8O2 on the diagram are live
to earth. It is recommended that a mains isolation transformer is used when servicing the receiver.
Many of the components in the power supply are safety critical. (R828, R809) is a surge-Iimiting resistor, limiting the
surge through the degauss coils when the reservoir capacitor is empty. These are marked with an exclamation mark in
a triangle on the circuit diagram. These components MUST be replaced only with parts of identical value and safety
characteristics. For reliability, it is recommended that only genuine parts are used for service replacements.
Always check the main supply voltage feeding the line output stage after replacing parts in the power supply or line
output circuit. The correct voltage is important for safety and reliability, the correct voltage should be 115 V ±2 V.