Power Electronics Europe April/May Issue 2022

32 CONVERTER DESIGN www.murata.com Issue 2 2022 Power Electronics Europe www.power-mag.com DC/DC Converter Design Basics Most electronic equipment incorporates DC/DC conversion in some form. The switched-mode technique is an efficient solution which enables step up and step down in voltage as well as isolation, with small magnetics. This article gives a broad review of the technology and some commercial implementations. Ann-Marie Bayliss, Senior Product Marketing Manager, and John Quinlan, Strategic Technical Marketing Manager, both Murata Power Solutions, USA DC to DC conversion has been a challenge to system and product designers, starting in a major way when Edison lost out to Westinghouse in the ‘War of the currents DC versus AC’ in the late 19th century. Distributing AC at increasing power required ever-higher voltages to keep currents low and cables of reasonable size, but this could easily be achieved with transformers. DC could only be stepped up at the time with unwieldy motor-generator sets so the rest is history, Westinghouse won and AC distribution became the standard for the 20th century. The last century was also the beginning of the age of electronics, which overwhelmingly requires DC for its components, so conversion from the AC distribution voltage to DC became necessary. But what DC? Circuitry can require anything from sub 1 V for a processor to several kilovolts for the magnetron in a microwave oven, sometimes together. If the rails required need to be accurate with changes in load and AC line, active regulation circuitry is also needed. Early equipment used 50/60 Hz transformers to drop the AC to lower voltages, which could then be rectified, smoothed to DC and regulated down to a lower voltage by a ‘linear’ series transistor, but when load, line and tolerances are taken into account, the power taken from the AC line is around twice the load power, worst case. The transformer is also large, heavy and expensive so the arrangement is far from ideal. If a system DC rail needs to be stepped up in voltage, there is no ‘linear’ way to achieve this. Switched-mode conversion solves the problem The practical solution to efficient DC down- and up-conversion is the ‘switched-mode’ technique. When isolation is not needed, ‘buck’ and ‘boost’ converters or derived variants are used. The buck (Figure 1, left) effectively ‘chops’ the input DC at high frequency so that its average is lower and then smooths the resulting waveform with an LC filter. The ‘chopping’ transistor is either fully on or off, in both cases dissipating little power and the output voltage is set by the transistor switching duty cycle. The boost converter (Figure 1, right) operates a little differently – the chopping alternately stores energy in the inductor magnetic field, then releases it. Energy can be released at any chosen voltage, higher than the input. Other circuit arrangements such as the simple buck- boost and ?uk can produce voltage inversion, while the SEPIC, ZETA and others can produce positive output voltages lower or higher than the input. An example of Murata’s 78SR series [1] buck is shown in Figure 2. The module has an input range of 7.5 V to 36 V for an output of 3.3 V at 0.5 A. At full load and 12 V input it achieves 83 % efficiency, dissipating about 0.7 W. It is pin compatible with the popular ‘78xx’ series linear regulators which would dissipate a stressful 4.35 W under the same conditions requiring substantial heatsinking. While this through-hole part can change out an existing linear regulator for a boost in efficiency, better performance still is achieved by surface mount ‘Point-of-Load’ DC/DC modules with land-grid array footprints. Figure 1: The buck and boost DC-DC converter outlines LEFT Figure 2: 78SR series buck converter rated at 0.5A