The buck converter steps down the input voltage and hence is also known as the step-down converter. Figure 13.1a is the most widely used buck converter circuit. The switching device used in this converter may be the bipolar junction transistor (BJT), metal-oxide semiconducting field-effect transistor (MOSFET), or insulated gate bipolar transistor (IGBT). The switch is turned on and off periodically at high frequency, typically in tens of kilohertz as shown in Figure 13.1b. The duty ratio D of the switch is defined as
duty ratio time on period
switching fon onD T
T T= = = ⋅ requency. (13.1)
Since the buck converter is widely used to charge the battery, it has yet another name, the battery charge converter. It is required to buck (step down) the dc bus voltage to the battery voltage during charging. Its operation during one on and off period triggering signal is shown in Figure 13.2. During the on-time, the switch is closed and the circuit operates as in Figure 13.2a. The dc source charges the inductor and capacitor, in addition to supplying power to the load. During the off-time, the switch is open, and the circuit operates as in Figure 13.2b. The power drawn from the dc source is zero. However, full load power is supplied by the energy stored in the inductor and the capacitor, with the diode carrying the return current. Thus, the inductor and the capacitor provide short-time energy storage to ride through the off period of the switch. The load current during this period is known as the freewheeling current, and the diode is known as a freewheeling diode. The voltage and current waveforms over one complete cycle are displayed in Figure 13.3. The line voltage is either fully turned on or off. The inductor current decays during off-time and rises during on-time, and so do the switch current and the diode current. Either the switch or the diode carries the inductor current as shown in the last two waves. A suitable
bleeding (dummy) resistor at the load terminals is sometimes incorporated in the design to keep the converter working without the load.