Abstract: For overcurrent protection of a switch-mode power supply in a high power audio amplifier, an active feedback overload shut-down circuit acts though a control input of a PWM (pulse-width modulator) in the switch-mode power supply. In a multi-output switch-mode power supply, e.g. a dual-rectifier supply providing .+-.53 volts and .+-.100 volts suitable for a class H amplifier, instantaneous currents in each of the two rectifier circuits are monitored by primary windings of a current-sensing transformer, providing from its secondary winding a signal that is rectified and monitored by a comparator-timer for current overload at a predetermined threshold. The primary windings are made to have a particular turns ratio so as to proportion the protection sensitivity for each dc output voltage. Upon sensing overload from either rectifier circuit, the comparator-timer applies a shut-down signal to the PWM, shutting down the power supply safely for a predetermined time period, e.g. 17 seconds.

Abstract: The output of an audio power amplifier fed to a loudspeaker driver is sensed and when this output exceeds a predetermined level, a switch control circuit operates to place a current limiting device, such as a power resistor, in series with one of the driver feed lines, thereby reducing the power fed thereto. An indicator may also be provided to indicate when the overload condition exists. Operation is made more accurate and distortion minimized by employing separate sensing and power channels in the circuit, the first of these channels operating to sense the overload while the second of these channels utilizes a portion of the amplifier output for driving the switch control circuit for placing the power limiter in the speaker line. The sensing channel is designed to sense the average voltage of the amplifier output, thus providing control more directly responsive to power. The power channel is designed to use audio power from the amplifier output only occasionally, thus producing a minimum of distortion.