The Principles of Switching Power Conversion--Introduction

• Introduction (below)
• Overview and Basic Terminology
• Understanding the Inductor
• Evolution of Switching Topologies

Introduction

Imagine we are at some busy "metro" terminus one evening at peak hour. Almost instantly, thousands of commuters swarm the station trying to make their way home. Of course there is no train big enough to carry all of them simultaneously. So, what do we do? Simple! We split this sea of humanity into several trainloads - and move them out in rapid succession.

Many of these outbound passengers will later transfer to alternative forms of transport. So For example, trainloads may turn into bus-loads or taxi-loads, and so on. But eventually, all these "packets" will merge once again, and a throng will be seen, exiting at the destination.

Switching power conversion is remarkably similar to a mass transit system. The difference is that instead of people, it’s energy that gets transferred from one level to another. So we draw energy continuously from an "input source," chop this incoming stream into packets by means of a 'switch' (a transistor), and then transfer it with the help of components (inductors and capacitors), that are able to accommodate these energy packets and exchange them among themselves as required. Finally, we make all these packets merge again, and thereby get a smooth and steady flow of energy into the output.

===

Primary Side Secondary Side isolation boundary Primary Side Control Board Input EMI Filter Bridge Rectifier Input (Bulk) Capacitor Switch (on Heatsink) Transformer Secondary Side Diodes Output Choke Output Capacitors Secondary Side Control Board Optocouplers

Fgr. 1: Typical Off-line Power Supply

===

So, in either of the cases above (energy or people), from the viewpoint of an observer, a stream will be seen entering, and a similar one exiting. But at an intermediate stage, the transference is accomplished by breaking up this stream into more manageable packets.

Looking more closely at the train station analogy, we also realize that to be able to transfer a given number of passengers in a given time (note that in electrical engineering, energy transferred in unit time is 'power') - either we need bigger trains with departure times spaced relatively far apart OR several smaller trains leaving in rapid succession. Therefore, it should come as no surprise, that in switching power conversion, we always try to switch at high frequencies. The primary purpose for that is to reduce the size of the energy packets, and thereby also the size of the components required to store and transport them.

Power supplies that use this principle are called 'switching power supplies' or 'switching power converters.'

'Dc-dc converters' are the basic building blocks of modern high-frequency switching power supplies. As their name suggests, they 'convert' an available dc (direct current) input voltage rail 'VIN,' to another more desirable or usable dc output voltage level 'VO.' 'Ac-dc converters' (see Fgr. 1), also called 'off-line power supplies,' typically run off the mains input (or 'line input'). But they first rectify the incoming sinusoidal ac (alternating current) voltage 'VAC' to a dc voltage level (often called the 'HVDC' rail, or 'high voltage dc rail') -- which then gets applied at the input of what is essentially just another dc-dc converter stage (or derivative thereof). We thus see that power conversion is, in essence, almost always a dc-dc voltage conversion process.

But it’s also equally important to create a steady dc output voltage level, from what can often be a widely varying and different dc input voltage level. Therefore, a 'control circuit' is used in all power converters to constantly monitor and compare the output voltage against an internal 'reference voltage.' Corrective action is taken if the output drifts from its set value.

This process is called 'output regulation' or simply 'regulation.' Hence the generic term 'voltage regulator' for supplies which can achieve this function, switching or otherwise.

In a practical implementation, 'application conditions' are considered to be the applied input voltage VIN (also called the 'line voltage'), the current being drawn from the output, that is, IO (the 'load current') and the set output voltage VO. Temperature is also an application condition, but we will ignore it for now, since its effect on the system is usually not so dramatic. Therefore, for a given output voltage, there are two specific application conditions whose variations can cause the output voltage to be immediately impacted (were it not for the control circuit). Maintaining the output voltage steady when VIN varies over its stated operating range VINMIN to VINMAX (minimum input to maximum input), is called 'line regulation. 'Whereas maintaining regulation when IO varies over its operating range IOMIN to IOMAX (minimum to maximum load), is referred to as 'load regulation.' Of course, nothing is ever "perfect," so nor is the regulation. Therefore, despite the correction, there is a small but measurable change in the output voltage, which we call "?VO" here. Note that mathematically, line regulation is expressed as "?VO/VO × 100% (implicitly implying it’s over VINMIN to VINMAX)." Load regulation is similarly "?VO/VO × 100%" (from IOMIN to IOMAX).

However, the rate at which the output can be corrected by the power supply (under sudden changes in line and load) is also important - since no physical process is "instantaneous" either. So the property of any converter to provide quick regulation (correction) under external disturbances is referred to as its 'loop response.' Clearly, the loop response is as before, a combination of its 'step-load response' and its 'line transient response.' As we move on, we will first introduce the reader to some of the most basic terminology of power conversion and its key concerns. Later, we will progress toward understanding the behavior of the most vital component of power conversion - the inductor. It’s this component that even some relatively experienced power designers still have trouble with! Clearly, real progress in any area cannot occur without a clear understanding of the components and basic concepts involved. Therefore, only after understanding the inductor well, will we go on to demonstrate that switching converters themselves are not all that mysterious either - in fact they evolve quite naturally out of our newly acquired understanding of the inductor.