How to Keep a FLYBACK Switch Mode Supply Stable with a Critical‐Mode Controller

INTRODUCTION Switch Mode Power Supplies (SMPS) can operate in two different conduction modes, each one depicting the level of the current circulating in the power choke when the power switch is turned on. As will be shown, the properties of two black boxes delivering the same power levels but working in different conduction modes, will change dramatically in DC and AC conditions. The stress upon the power elements they are made of will also be affected. This article explains why the vast majority of low-power FLYBACK SMPS (off-line cellular battery chargers, VCRs, etc.) operate in the discontinuous area and present a new integrated solution especially dedicated to these particular converters. Defining the Mode Figures 1(a) and 1(b) show the general shape of a current flowing through the converter’s coil during a few cycles. In the picture, the current ramps up when the switch is closed (ON time) building magnetic field in the inductor’s core. When the switch opens (OFF time), the magnetic field collapses and, according to LENZ’s law, the voltage across the inductance reverses. In that case, the current has to find some way to continue its flow and start its decrease (in the output network for a FLYBACK, through the freewheel diode in a BUCK etc.).