Overload protection for a switch mode power supply

DETAILED DESCRIPTION FIG.
1 illustrates an zero voltage switching forward converter or power supply 300.
A protection circuit 400, embodying an inventive feature, provides protection to zero voltage switching power supply 300.
Zero voltage switching power, supply 300 operates similarly in many respects to that described in the Fitzgerald patent.
Power, for example, 200 watts, is supplied toa load 303 and to a load 302 coupled to a secondary winding T1W2 and a secondary winding T1W3, respectively, of a transformer T1, during conduction time of a switching transistor Q1.
Transistor Q1 that operates as a switch is coupled in series with a primary winding T1W1 of transformer T1 for conducting current from an input supply, direct current (DC) voltage RAW B+.
A current transformer T2, considered a drive transformer, supplies a base current iB to switching transistor Q1.
Voltage RAW B+ can be derived from a bridge rectifier (not shown) that rectifies a mains supply voltage, and is coupled toa filter capacitor (not shown).
Also coupled to the emitter of transistor Q1 in series with transistor Q1 is a current sensing resistor R7.
A resonant capacitor C8 is coupled to primary winding T1W1 and to the collector of transistor Q1.
A resonant circuit 301 includes capacitor C8, a reflected capacitance CSEC, a current limiting supply inductor Lres, primary winding T1W1 and a primary winding T2W1 of transformer T2.
Primary winding T1W1 is coupled in series with the primary winding T2W1 of current transformer T2.
Resonant circuit 301 produces a half cycle resonant voltage VQ1 each cycle when transistor switch Q1 is turned off.
A collector voltage VQ1 across transistor Q1 (and on capacitor C8) rises to a peak and then fall to approximately zero in a substantially sinusoidal half wave.
After resonant voltage VQ! becomes close to zero, a series arrangement of resistor R7, a diode D2, a zener diode D20, coupled in parallel with a capacitor C2, and the base-collector junction of transistor Q1 form a low impedance that clamps voltage VQ1 to a voltage close to ground potential