Switching power supply design rules

High-density dc/dc converters come in numerous combinations of input voltage, output voltage, and power levels. Modules such as these small PC-board-mounted packages provide output power to 150W. Resonant and quasi-resonant dc/dc converters are becoming more widely used because they are less expensive and more efficient than equivalent linear supplies. Not long ago, efficiency hinged on the converter's switching devices that wasted most of the power budget during relatively low-frequency transitions — typically 20 to 30 kHz — between on and off states. Improving efficiency by increasing the speed of these converters while using the same type of switching devices, however, peaked out at about 66 kHz. But because newer high-speed switching devices operate at 100 kHz and more while dissipating much less power during transitions, converter efficiency has increased dramatically. In a simplified diagram showing a single-ended forward converter, for example, the energy transfers from source to load during the "on" time of a single solid-state switch cycle.The circuit shown is quasi-resonant because the energy stored in capacitor Cr does not return to the inductor Lr as is done in a full-resonant converter. When the main switch, Q1, turns on, about a half cycle of current flows through the switch, transferring the energy from the input source to an LC circuit composed of Lr and Cr. The transformer here provides voltage scaling, electrical isolation from primary to secondary, and magnetic storage in the leakage inductance. The leakage inductance of transformer T1, is modeled as the inductor Lr in series with the secondary, and the capacitor Cr is across D2. The inductor stores energy as current, ), and the capacitor stores energy as voltage, Both diodes, D1 and D2, transfer energy from the input to the output. Diode D1 conducts when the main switch conducts and transfers energy from the leakage inductance