Please forward this error screen to 69. Amateur radio has been somewhat slow to voltage professional the magic gold mask switching power supplies for powering communication equipment. This is a pity, because «switchers», as they are often called, offer very attractive features, like small size, low weight, high efficiency, and low heating.
In this article I will describe the construction of a switching power supply designed to power a complete ham station, with several radios and accessories. It has current limiting, making it appropriate for direct connection to a 12V backup battery. Heat up your soldering iron and read on! You all know how a typical linear power supply operates: A heavy transformer takes the line voltage and converts it into something slightly above the desired final voltage.
Some diodes rectify it, a big filter capacitor smoothes out the DC, and a series pass transistor burns up the excess voltage, so you get the desired output. A simple control circuit drives the pass transistor to hold the output voltage constant. This relatively high frequency allows the use of a small, lightweight and low cost transformer to reduce the voltage. The output is then rectified and filtered.
There are several different topologies for switchers in common use, and the first decision a designer must take is which of them to consider. Among the factors affecting the decision are the power level, the number of outputs needed, the range of input voltage to be accepted, the desired tradeoff between complexity, quality and cost, and many more. For this power supply I decided to use the half bridge forward converter design. The second basic decision is which switching frequency to use. The present trend is to use ever higher frequencies. But by doing so it becomes more difficult to filter out the RF noise inevitably generated by the switching. For the main switching elements, bipolar transistors or MOSFETs can be used.
Bipolars have lower conduction losses, while MOSFETs switch faster. As in this design I wanted to keep the RF noise at an absolute minimum, very fast switching was not desired, so I used bipolar transistors. But these tend to become too slow if the driving is heavier than necessary. So, if the transistors have to switch at varying current levels, the drive to them must also be varied. The half bridge converter is best controlled by pulse width modulation. There are several ICs available for this exact purpose. I chose the 3524, which is very simple to use and easy to find.