NotTaR of Television Sets : HV power supply fundamentals            
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High Voltage Power Supply Problems

HV power supply fundamentals

Most, if not all, TVs derive the high voltage for the CRT second anode, focus, and (sometimes) screen (G2) from the horizontal deflection system. This technique was developed quite early in the history of commercial TV and has stuck for a very simple reason - it is very cost effective. A side effect is that if the horizontal deflection fails and threatens to burn a (vertical) line into the CRT phosphors, the high voltage dies as well.

Most TV high voltage supplies operate as follows:

  1. Horizontal output transistor (HOT) turns on during scan. Current increases linearly in primary of flyback transformer since it appears as an inductor. Magnetic field also increases linearly. Note: flyback is constructed with air gap in core. This makes it behave more like an inductor as far as the primary drive is concerned.

  2. HOT shuts off at end of scan. Current decreases rapidly. Magnetic field collapses inductively coupling to secondary and generates HV pulse. Inductance and capacitance of flyback, snubber capacitors, and parasitic capacitance of circuitry and yoke form a resonant circuit. Ideally, voltage waveform across HOT during flyback (retrace) period will be a single half cycle and is clamped by damper diode across HOT to prevent undershoot.

  3. Secondary of flyback is either a single large HV winding with HV rectifiers built in (most often) or an intermediate voltage winding and a voltage multiplier built in or a separate unit (see the section: What is a tripler?. The output will be DC HV pulses.

  4. The capacitance of the CRT envelope provides the needed filtering to adequately smooth the HV pulses into a DC voltage.

  5. A high resistance voltage divider provides the several kV focus voltage and sometimes the several hundred volt screen (G2) voltage as well. Often, the adjustments for these voltages are built into the flyback. Sometimes they are mounted separately. The focus and screen are generally the top and bottom knobs, respectively.

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