Our Test Equipment
In our PSU tests our main task is to check it out at different loads, up to the maximum one. Many reviewers used to employ an ordinary PC for that purpose, installing the tested PSU into it. This method had two drawbacks: you had no control over the amount of power consumed by the PC, and it was hard to load a really high-wattage PSU. The second problem is especially crucial today when the PSU manufacturers have started a race for reaching as high a wattage rating as possible, and the capabilities of their products have by far exceeded the demands of a typical PC. Of course, one may argue that there’s no point in testing PSUs at higher loads if no real PC needs more than 500W, but it would be odd not to check out a high-wattage PSU through its entire load range if we begin to test it at all.
We use an adjustable load with programmable control to test PSUs in our labs. The testbed is based on the well-known feature of metal-oxide-semiconductor field-effect transistors (MOSFETs): a MOSFET limits the current flowing in the drain-source circuit depending on the voltage in the gate.
Above you can see a simple schematic of a MOSFET-based current regulator: connecting the circuit to a PSU with an output voltage of +V and turning the lever of the variable resistor R1 we are changing the voltage on the gate of the transistor VT1 thus changing the current I that flows through it from zero to maximum (determined by the characteristics of the transistor and/or tested PSU).
This circuit is not perfect, though. When the transistor heats up, its characteristics change somewhat and the current I changes, too, although the control voltage on the gate remains the same. To solve the problem, we need to add a second resistor R2 and an operation amplifier DA1 into the circuit:
When the transistor is open, the current I flows through its drain-source circuit and through the resistor R2. The voltage on the latter equals U = R2*I, according to Ohm’s law. This voltage goes to the inverting input of the operation amplifier DA1. The non-inverting input of the same opamp receives the control voltage U1 from the variable resistor R1. The opamp has such properties that it tries to maintain the same voltage on both its inputs by changing its output voltage, which in our circuit goes to the MOSFET’s gate and regulates the current flowing through it.
Suppose the resistance R2 equals 1 Ohm, and we’ve set a voltage of 1V on the resistor R1. Then the opamp will change its output voltage in such a way that the voltage drop on the R2 is 1V, too. The current I now equals 1V / 1Ohm = 1A. If we set a voltage of 2V on the R1, the opamp will react by setting the current I at 2A, etc. If the current I and, accordingly, the voltage on the resistor R2 changes due to the heating-up of the transistor, the opamp will immediately adjust its output voltage to bring them back.
Thus, we have a perfectly controllable load that allows to smoothly change the current from zero to maximum with a turn of a lever. It maintains the value you set automatically for as long as you wish. It is also compact, being much easier to handle than a clumsy set of low-resistance resistors connected in groups to the tested PSU.
The maximum amount of power dissipated on the transistor is determined by its thermal resistance, the maximum allowable die temperature, and the temperature of the heatsink it is installed on. Our testbed employs International Rectifier IRFP264N transistors (a 168KB .PDF file) with an allowable die temperature of 175°C and a die-heatsink thermal resistance of 0.63°C/W. The cooling system of the testbed can keep the temperature of the heatsink under the transistor below 80°C (yes, the fans that do it are very noisy). Thus, the maximum amount of power dissipated on one transistor is (175-80)/0.63 = 150W. To reach the desired total load we use several parallel-connected loads designed like described above – they all receive a control signal from the same DAC. A parallel connection of two transistors with one opamp could be used, too, and the maximum dissipated power would be half as high as with one transistor.
There is one step left to a fully automated testbed: we can replace the resistor with a PC-controlled DAC to regulate the load from the PC. Connecting several such loads to a multi-channel DAC and installing a multi-channel ADC to measure the output voltages of the tested PSU in real time, we get a full-featured testbed for testing PC power supplies in the entire load range and at any variants of load distribution.
The photo above shows our tested in its current form. The two top blocks of heatsinks cooled with powerful 120x120x38mm fans carry the load transistors of the +12V channels. The more modest heatsink cools the transistors of the +5V and +3.3V channels. The gray block connected to the LPT port of the controlling PC contains the mentioned DAC, ADC and accompanying electronics. Measuring 290x270x200mm, this testbed allows testing PSUs with a wattage rating up to 1350W (up to 1100W on the +12V rail and up to 250W on the +5V and +3.3V rails).
We wrote a special program to control the testbed and automate certain tests. You can see its screenshot above. It allows to:
- set the load on each of the available channels manually:
- from 0 to 44A on the first +12V channel
- from 0 to 48A on the second +12V channel
- from 0 to 35V on the +5V channel
- from 0 to 25A on the +3.3V channel
- track the voltages of the tested PSU on the mentioned rails in real time
- automatically measure and build cross-load diagrams for the tested PSU
- automatically measure and build graphs of the PSU’s efficiency and power factor depending on load
- build graphs that show the dependence of fan speed on load in semi-automated mode
- calibrate the testbed in semi-automated mode to obtain more precise results