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PowerViews
October 1, 2012
Why There's More to Power Supply Design than an IC Reference Circuit
Don Knowles
VP Engineering, N2Power
Power Channels: Packaging and Cooling, Power Components, Switch-Mode Power
To read the data sheets from some semiconductor companies, you could be forgiven for thinking that designing and building an AC-DC power supply is simply a matter of following the reference design and laying out a printed circuit board. When it’s this easy, why would anyone ever buy an off-the-shelf power supply?
Today, there’s a big selection of power supply ICs to choose from, many of which are very sophisticated, and some of which even support digitally-controlled PWM modules. But even when using proven ICs, there’s a host of traps into which the unwary power supply designer might fall – not least because of unpredictability in the performance of other components, the effects of different board layouts on system performance and EMI, and the need to meet international regulatory standards with respect to safety, EMI and power factor correction.
Take capacitors, for example. One 100µF/25Vdc capacitor may look much like another, but in power supply applications, particularly where high frequency switching is used, it is the capacitor’s equivalent series resistance (ESR) that is often the most important characteristic after ensuring that a component of adequate voltage rating is selected. It’s this that will determine its current handling capability, how hot it gets, and how long it will last in the application.
And what about cooling? A rule of thumb says that mean time between failure (MTBF) halves with every 10 degree C rise in the temperature of an electronic system, so you need to ensure that your power supply runs as cool as possible in order to maximize its service life. Every power supply has its own unique air flow characteristics. How do you choose between conduction and convection cooling and, if you go for the latter, how much air flow do you need to achieve? Even when that’s decided, you still have to find the right fan and ensure that you have a way to power it, and somewhere that it will fit.
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