Panasonic GPH5D Troubleshooting Manual

Also see for TH-50PHD5: Operating instructions Operating instructions

Contents

Panasonic GPH5D Troubleshooting Manual Manual pdf 32 page image

33Low Voltage Power supplyVCC and Start-up voltage for the low voltage power supply is provided to IC650of the P5 Board by the standby power supply circuit (not shown). Upon start-upof the switching control circuit, a pulse width modulated signal is output at pin 9of connector P18/P18A to drive the switching transistor Q416. When Q416 is on,current flows via the primary winding of transformer T401 and Q416. As currentflows through the transformer, energy is built up and stored in the transformer.When Q416 turns off, the energy within the transformer begins to collapse. Asthe field collapses, energy is released in the windings of the transformer toprovide the secondary voltages.The rectified AC output at pin 10 of transformer T401 is applied to IC401 where itis regulated to 18V. This voltage is used to power the Drive Oscillator circuitboard that will be discussed later.Voltage RegulationThe 13.5V source and the VDA source voltage levels are monitored by IC650 ofthe P5 Board. A voltage increase of the 13.5V or an increase in the current flowof the VDA source causes IC416 to conduct harder, allowing more current to flowthrough the LED inside opto-coupler D432. This increases the conduction of thetransistor inside D432. Conversely, a decrease in the 13.5V supply voltage or adecrease in the current flow of the Vda source decreases the conduction ofIC416. Any change in the conduction of IC416 is monitored by pin 10 of IC450.As a result, the pulse width modulated output at pin 10 of IC650 adjusts to keepthe output level of the power supply constant. R545 is used to adjust Vda to theproper voltage level.