The basis of our testbed for checking out power supply units is a semi-automatic device capable of setting any required load on the +5v, +12v, +3.3v and +5v standby power rails of the tested PSU, measuring the corresponding output voltages.
The hardware part of the device is based on a 4-channel Maxim MX7226 DAC to whose outputs power sources are attached. The power sources are made on operational amplifiers LM324D and high-power field transistors IRFP064N installed on heatsinks with forced air cooling.
Each transistor has a peak dissipated power of 200W, and since we use three such transistors in each of the heaviest-load rails (+5v and 12v), the testbed allows testing any existing ATX-compliant PSU up to the most powerful ones. Even considering the reduction of the allowable dissipated power of the transistors at high temperatures, the allowable load power on each rail is no less than 400 watts.
To measure the load currents and the output voltages of the tested PSU we use two 4-channel Maxim MX7824 ADCs – one converter is responsible for the currents, another for the voltages.
The testbed is fully controlled from the computer through the LPT port, from its powering-up to performing various tests, registering and processing the results. We wrote a special utility that allows to manually select the load currents independently for each power rail and to perform some non-standard tests (for example, to build the cross-load characteristic as described below) – fully automatically.
Besides the main testbed we use two auxiliary test devices. The first of them is a generator of rectangular pulses with a frequency that can be discretely varied from 60Hz to 40kHz:
The generator is attached to the tested PSU as a load – a switch allows to select the rail (+12v or +3.3v). In any case, the generator creates a load current of about 1.3 amperes. This allows to estimate how well the tested PSU reacts to relatively powerful load pulses of rectangular shape that follow at frequencies from tens of hertz to tens of kilohertz.