Arrangement for reducing the effects of capacitive coupling in a control circuit for a switched-mode power supply

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 and withdraws inordinately large currents into and from the pins of the controller. In the case of the VCTRL pin to which is connected a discharge capacitor, the resulting under voltage across the capacitor may be used by circuitry connected to the VCTRL pin to control the switching of the switching device resulting in throwing the switched-mode power supply out of regulation. A track-and-hold circuit is now connected between the VCTRL pin and the circuitry to mirror the voltage across the discharge capacitor. During the times when current would be withdrawn from the discharge capacitor due to capacitance coupling, the track-and-hold circuit isolates the circuitry from the resulting under voltage.