Monday, November 1, 2010

What is a switching type power supply ...

Q Keith, KF4BXT, asks, “What is a switching type power supply and what type is typically used and/or recommended for running a station? I think most of us, by now, understand that our power supplies should be regulated and filtered, but are there reasonable ways to add regulation and filtering to those that don’t already have it?”

A In a linear power supply, the line voltage goes directly into a low-frequency transformer where it is stepped down to the appropriate low voltage before rectification, filtering and regulation. In a switching power supply, the line voltage is directly rectified and filtered to produce a high dc voltage. This voltage is then “switched” at a high frequency rate (not RF, but certainly higher than audio—perhaps 50 kHz, for example) by switching transistors. It is then fed into a high-frequency transformer and the output is rectified and filtered. Regulation can be done in the output stage, but more typically, the regulation is done at the switching transistor to allow the amount of energy fed to the transformer to be adjusted as needed.

One advantage of the switching technique is that higher frequency components are much smaller and lighter weight for the same power capability than their low frequency counterparts. Another advantage is that, since the transformer is the least efficient part of the supply, controlling its input power (as is done in a switching power supply) can provide much better efficiency. The power lost as heat in a linear supply is typically 40-60% of the output power. In a switching supply, that typically drops to 10-20%. 

The disadvantages of a switching supply are the increased complexity (more likelihood of a component failure), increased cost (many more parts) and tendency to create radiated RF (the switching waveform is usually pretty close to a square wave, so it contains a lot of harmonics). This last item has been the main one that has kept switching supplies out of the ham market until recently. Current designs use an extensive amount of filtering and radiation suppression techniques to greatly reduce unwanted RF.

Adding filtering and regulation to a linear supply is a simple
matter. Information on calculating filter component values for a particular desired ripple can be found in The ARRL Handbook chapter on power supplies. However, regulation will come at a cost in reduced output capacity—if you have an unregulated supply that puts out 15 V, you probably won’t be able to get more than 13 V from a regulator system attached to it.

All switching supplies have some kind of regulation, although some designs are quite crude and could use improvement. I don’t suggest trying to modify a switching supply unless you have studied switching power supply design extensively.

From February 2001