Abstract: In a switched-mode power supply, when the controller and the switching device are encapsulated together with a heat sink, in which the drain of the switching device is connected to the heat sink, the heat sink and the paths to the various pins of the controller forms various parasitic capacitances which, when the switching device switches injects inordinately large currents into the pins of the controller. In the case of the D.sub.MAG input, this may result in throwing the switched-mode power supply out of regulation. The sample-and-hold circuit connected to the D.sub.MAG input includes an additional comparator for comparing the current on the D.sub.MAG input to an extra large current. If the current on the D.sub.MAG input exceeds this extra large current, the sampling switch of the sample-and-hold circuit is held open while a clamp circuit is engaged on the D.sub.MAG input.

Abstract: A circuit arrangement for use in a controller circuit of a switched mode power supply for turning on a power device in the switched-mode power supply when a voltage across the power device is at a minimum. The circuit arrangement includes circuitry for detecting when a change in the voltage across the power device is substantially zero, and for detecting when the voltage across the power device is less than a predetermined level. When these two criteria are met, the circuit arrangement generates a turn-on signal for the power device.