Lab 3- Introduction

DC-DC converters are widely used in regulated switch-mode power supplies, DC motor drives, renewable energy converters and battery energy storage applications. With reference to Figure 1, the converter produces a regulated output voltage from a DC input voltage, by adjusting the duty cycle D, controlling a switch network. The output voltage waveform quality is improved by employing an output filter stage and tailoring the converter switching frequency to meet the design specifications. As shown in Figure 2, the DC-DC converter switch pulses are generated by a using pulse-width modulation (PWM) modulator which compares a control signal (i.e. the reference duty cycle) to a carrier waveform with a fixed period to achieve the desired output waveform.

Figure 1: Switch network for a buck converter (a) schematic (b) switch voltage waveform.

Figure 2: Pulse width modular signals.

The schematics of three different types of one quadrant DC-DC converters are shown below, accompanied by their transfer characteristics as a function of the duty cycle D. Figure 3 shows the buck DC-DC converter topology, which produces a lower average output voltage than its input voltage. Figure 4 shows a boost DC-DC converter, which outputs a voltage higher than the input. A buck/boost converter is shown in Figure 5, which produces an inverted output voltage that can be either greater or less than the input. It should be noted that all three of these converters only operate in one quadrant of the current-voltage I-V plane. Both buck and boost converters output unidirectional positive voltage and currents, and thus operate in quadrant one. A buck-boost converter inverts input voltage and current, and thus operates in quadrant three. It is often desirable to employ a DC converter that works in more than one quadrant. For instance, having bidirectional current flow is often required and used in electric vehicles, grid-scale battery energy storage systems, and UPS systems. A two quadrant, half-bridge converter, shown below in Figure 6(a), operates in quadrants one and four, as shown below in Figure 6(b). A half-bridge converter combines the performance characteristics of the buck and boost single quadrant converters.