On Power Supplies

Are you starting to learn electronics? A power supply is an excellent starter project: the basics are very straightforward, and a power supply is reasonable to built and troubleshoot. Even better, as your skills advance, there are some interesting – and not so obvious subtleties.

One of these subtleties is heatsinking. Designing a regulated power supply is not so much a matter of circuit design as it is in thermal management – getting rid of unwanted heat, or better still, avoiding the generation of heat altogether. Experienced EEs all have war stories about project crises that involve cooling problems. My first regulated power supply ended up under fire extinguisher foam because I undersized the heatsink. Another project (not mine) had to be sealed against the environment and surprise(!) this led to problems in removing heat. (One evidence of a project death spiral is the suggestion to use a Peltier Cooler to solve thermal management issues.)

There are a number of ‘regulated power supply’ kits available on the market. They generally leave out the big bits: the power supply transformer, the filter caps, and the heatsink. These are usually based on the LM317 adjustable regulator: you can spot this by the fact that the minimum output voltage is 1.5 volts or so. One example, on a tiny circuit board, claims at output voltage adjustable from 35 to 1.5 volts at an output current of 1 amp. The heatsink isn’t provided with the kit, and a ‘back of napkin’ calculation indicates why. At an output voltage of 1.5 volts and an input voltage of 37 volts the power dissipation is north of 35 watts. To keep the junction temperature under 100C at a worst case ambient of 40C would require a thermal resistance junction to ambient of about 1.7 degrees C per watt.
The junction resistance of an LM317 is 5 degrees C per watt. So even an infinite heatsink will not provide sufficient cooling. (Shameless plug: for the details of these calculations see Chapter 26 of Analog Circuit Design. For bonus marks, calculate the maximum output current for an infinite heatsink. It will be less than an amp.)

Or if you need a power supply and don’t want to build one there are many bench power supplies available at a reasonable price. They tend to be big and heavy because, unlike the kit described above, they include the line transformer and series regulator heatsink.

So, is there any way to get a physically small power supply with a useful output voltage and current? That was the challenge facing Syscomp when we began to design the PSM-101. The design has to use *switching*, in which the series pass element is either ON or OFF, and thereby dissapates zero power, with the duty cycle controlling the average output voltage. Switchers have low power dissipation, but aren’t easily adjustable down to zero output volts and they tend to be electrically (and sometimes acoustically!) noisy. So the PSM-101 uses a hybrid approach, with a switching regulator to minimize power dissipation and a series analog regulator to reduce noise and give accurate output control.

This supply circuitry was married with a microprocessor display and touch controls, which should be far more reliable than the usual mechanical rotary potentiometers.

As far as we know, the PSM-101 is the only 14 volt, 1 amp, USB controllable power supply that can fit in a trouser pocket. That makes it attractive to anyone that needs a *really* portable power supply.

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