Switching-Regulated Power Supplies

The most common type of power supply today is the switching supply. These units use pulse-width modulation (PWM) to regulate output. Supplies today employ several different PWM circuit configurations. In all cases, the PWM logic signal drives the switching power transistor, and the power transistor drives the load. The switching transistor turns on and off rapidly, producing a chopped dc voltage. The chopped dc voltage is fed to a transformer which converts the pulsating dc to high-frequency ac. This ac is then fed to a second bridge rectifier which produces the final dc output. A sensing circuit continuously monitors the output voltage, adjusting the switching duty cycle to maintain a constant voltage output. Switching power supplies are more efficient than series-regulated ones because little power is dissipated in a switching transistor. Switching supplies are physically smaller than series-regulated types because components operating at the switching frequency (typically 20 kHz) are much smaller than those used in a nonswitching supply operating at 50 to 60 Hz. These power supplies are well suited where compactness, efficiency, and moderately accurate regulation