Side pincushion modulator circuit with overstress protection

Abstract

A side pincushion corrected deflection circuit for a television receiver includes a deflection winding, a trace capacitor and a first switch for generating scanning current in the deflection winding. A modulator circuit includes a modulator inductor and capacitor and a second switch for generating a sawtooth modulator current in the inductor. The trace and modulator capacitors are charged from a B+ operating voltage source through the primary winding of a flyback transformer. A transistor stage in the modulator circuit shunts current away from the modulator capacitor to control the modulator and trace capacitor voltages. A vertical rate bias voltage is applied to the transistor stage and varies the shunt current at a vertical rate in a parabolic manner to provide side pincushion correction. Under fault conditions, such as an open-circuited deflection winding or trace capacitor, the modulator capacitor voltage tends to undesirably increase above the peak voltage developed under normal operating conditions. To prevent overstressing the modulator transistor stage, a diode clamp is coupled between the modulator capacitor and a source of reference voltage, thereby preventing the modulator capacitor voltage from substantially exceeding the reference voltage level.