Articles: Cases/PSU
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Especially valuable is the automatic construction of cross-load diagrams. To draw such a diagram, you need to measure the PSU’s output voltages at all possible combinations of loads, which involves a very large number of measurements. Performing such a test manually would require great assiduity and a lot of time from the human tester. Our program takes in the specified PSU parameters, builds a chart of allowable loads and goes along this chart, measuring the output voltages at each step. The results of the measurements are presented as a diagram. The whole process takes 15 to 30 minutes depending on the PSU wattage and measurement step, but does not require man’s intervention.

Efficiency and power factor measurements

We use additional equipment to measure the efficiency and power factor of a PSU: the tested PSU is connected to the 220V mains via a shunt. A Velleman PCSU1000 oscilloscope is connected to the shunt as well. Its screen shows an oscillogram of the current consumed by the PSU, so we can calculate the amount of power consumed from the mains. Knowing also the load on the PSU we set by ourselves, we can calculate the efficiency. The measurements are performed automatically: the above-described PSUCheck program can take all the necessary data from the oscilloscope’s software that is connected to the PC via USB.

To ensure maximum precision of the result, we count in the output voltage deflection when measuring the output power of the PSU. For example, if at a load of 10A the output voltage of the +12V rail sags to 11.7V, the corresponding item in the efficiency calculation will be 10A * 11.7V = 117W.

Velleman PCSU1000 oscilloscope

This very oscilloscope is also used to measure the output voltage ripple. The measurements are performed for the +5V, +12V and +3.3V rails at the maximum permissible load. The oscilloscope is connected via a differential setup with two shunting capacitors (this connection is recommended in ATX Power Supply Design Guide):

Output voltage ripple measurement

We’ve got a dual-channel oscilloscope, so we can measure the ripple on one rail at a time. We repeat the measurement three times and the three resulting oscillograms, one for each tracked rail, are combined into a single picture:

The oscilloscope’s settings are indicated in the bottom left corner of the picture: the vertical scale is 50 millivolts/division, and the horizontal scale, 10 microseconds/division. The vertical scale usually remains the same in our tests but the horizontal one can change: some PSUs have low-frequency pulsation at the output and we provide another oscillogram, with a horizontal scale of 2 milliseconds/division, for them.

The speed of the PSU fans, depending on its load, is measured in semi-automatic mode. Our optical tachometer Velleman DTO2234 doesn’t have a PC interface, so its readings have to be entered into the PC manually. During this process the load on the PSU is changing in steps from 50W to the permissible maximum. The fan speed is recorded after the PSU has worked for 20 minutes or more at each step.

We also measure the growth of temperature that is passing through the PSU using a dual-channel thermal-couple thermometer Fluke 54 II. One of its sensors measures the air temperature in the room and the other sensor measures the air temperature at the output of the PSU. To increase the repeatability of the results, we fasten the second sensor on a special fixed-height stand placing it at a definite distance from the PSU. So, the sensor is in the same position relative the PSU in every test, which ensures identical conditions for each tested product.

The resulting diagram shows both the fan speed and the difference in the air temperature. This helps evaluate the specifics of the particular cooling system better.

When it is necessary to check out the measurement precision of the testbed or calibrate it, we use a digital multimeter Uni-Trend UT70D. The testbed is calibrated using an arbitrary number of measurement points placed at different parts of the available range. In other words, to calibrate by voltage we connect a regulated PSU to the testbed. The output voltage of the PSU is changed in steps from 1-2V to the maximum level measured by the testbed on the appropriate channel. At each step we enter the precise voltage value, as reported by the multimeter, into the testbed control program and the program fills in the correction table. This method of calibration allows achieving a high precision of measurements through the entire range of values.

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