TOSHIBA 28N33B
Manual de Servicio

CIRCUIT DESCIRIPTION POWER SUPPLY
The ZX series of receivers incorporate a Motorola switch mode power supply using a MC 44608-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 R831, the bridge rectifier D891, and reservoir capacitor C809 producing approx. 320 volts D.C to feed the switching MOSFET Q801 via the primary winding of TR802 pins 6 and 8. Start up resistor R801 feeds from a 500V coming from the mains through the adder diode D809 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 approx. 12 volts on C810, which takes over from the start up resistor to supply pin 8 of IC800. The Demag pin at pin1 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 pin6 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 IC818 and an OPTO- coupler connected to pin3 of IC800. The reference voltage of IC818 is set to 2,5V to supply a B+ voltage of 150V. 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 pin52 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 < 2.2V. 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 low value resistors R807 and R834. All the current drawn by the receiver will flow through that parallel resistors 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 and R834 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 these resistors will rise. This voltage is monitored by the current sense input pin2. 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. (R831,R809) are surge-limiting resistors, 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 150 V ± 2 V.
15