Genius ISO XFR-75W, 2093, 1493 FET drain voltage, Voltage across, Current in diode, Flyback pulse

SIMPLIFIED POWER SUPPLY CIRCUIT, FUNCTION, DESCRIPTION.

		+52V to +127V	V+					LOAD
					FLYBACK		+
								H Dy & EHT
	+				DIODE
AC		Res.			142
		GND						VIDEO
line					GND
User supplied		Error Amp.	C5184	FET				SECONDARIES
		Comp.
Isolation	+
			DRIVER			166
Transformer
				136
		VREF. OSC.	ENABLE
	V-	(-200V)	115	V-	137

The switching regulator includes the power FET 136 which passes current from V- to GND through the inductor 166 . During the time the FET is on, the current in the inductor is increasing and the inductor is storing energy.

When the FET is turned off, the stored energy in the inductor continues supplying current to GND. But in this case, the current path is from V+ to GND, instead of V-to GND. During this part of the cycle, the current in the inductor is decreasing.

Under normal conditions, the current will decrease to zero and the voltage will ring.

FET drain voltage

Current in inductor Current supplying GND

Voltage across

137

Current from V-

Current in diode

142

Current added to the +127V line

Flyback pulse

As can be seen from the waveforms, the largest number of changes occur when the FET is turned off. Also, the FET drain voltage switches fast due to the high inductor current. To minimize video interference from the power supply, the power supply is synchronized to the horizontal oscillator such that horizontal blanking is coincident with the FET turn off time.

The C5184 115 is the series regulator IC. All of the control circuits that are built into this IC work together to produce one output signal, which is the FET drive signal. This signal can take on many shapes depending on the load conditions of the power supply. The waveforms for normal operation are shown above.

For the shorted +127V to GND condition, which also occur right on power up,

the waveforms are:

FET Gate Drive

FET Drain Voltage

Inductor Current

The first FET pulse is a full on pulse which causes current to flow in the inductor. After the FET is turned off the current in the inductor drops much more slowly than normal since the inductor is discharging into a much lower than normal voltage. If the FET were turned on for full power in the

next cycle with current still flowing in the flyback diode, a current spike of 6A would occur, which is a power spike of 2,000W.

The reason for this is that the diode stores charge when current flows which turns into reverse current for a short time when the voltage is reversed across the diode.

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