A6 Power Supplies and AC/DC Converters

Topic

Every power supply shares the same mission: to convert available electrical energy—typically alternating current from the mains—into a stable, regulated direct-current (DC) voltage. Linear regulators offer the simplest solution: they absorb excess voltage by dissipating it as heat through a transistor placed in series with the load—an application of the Joule effect described by the resistor model and Ohm's Law. While they offer low noise and very fast response times, their energy efficiency is poor precisely because they waste the entire difference between the input and output voltages as heat.

In contrast, switching converters transfer energy much more efficiently by using a high-speed switch alongside an inductor and a capacitor that store and release energy during each switching cycle—utilizing the energy-storage capabilities described by the reactive component model. The Buck converter (step-down), the most common type, delivers an output voltage directly proportional to the fraction of the cycle during which the switch remains closed; the Boost converter (step-up) does the opposite, delivering an output voltage higher than the input, with the voltage increasing as that fraction of time approaches the full duration of the cycle.

Increasing the switching frequency allows for the use of smaller, more compact inductors and capacitors, but it also raises the losses associated with each individual switching event, creating a design trade-off between size and efficiency. More sophisticated resonant converters achieve switching with virtually no loss—leveraging the resonant frequency principles found in the reactive component model—and attain the highest efficiency levels currently available in modern chargers and power supplies. That is why this module—which opens with two different ways of converting alternating current to direct current—concludes by showing that the efficiency advantage of a switching converter over a linear one comes at a price: it entails greater design complexity and a specific trade-off between component size and switching losses—a challenge that a linear regulator simply does not face.

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