The right way to measure power supply efficiency must consider four important factors such as:
(1) Use the right measuring tools
(2) Sample the parameters correctly
(3) Use short connections
(4) Give some time before taking measurements
Use the Right Measuring Tools
Efficiency is the ratio of output power to input power. The output power is composed of DC voltage and current. On the other hand, the input power is composed of the voltage, current and power factor.
For the output measurement, a handheld DMM with high resolution set to voltmeter mode will work. There are a lot of options available. Ensure the voltage meter has zero offset voltage.
To accurately measure the output current, use a shunt resistor. The shunt resistor current rating must not be very big compared to the maximum current of the power supply because this will cause a measurement error at light loads. To measure the voltage drop of the shunt, use a high resolution DMM (6.5 digit or higher will give very good result).
Below are some shunt options from OHMITE. These are high accuracy shunts. If the maximum current of your power supply is 80A, then use the 100A shunt. Using 250A and higher will give measurement error.
Below illustration shows how the shunt is connected and how the high-resolution meter is hooked up.
To measure the input power, use a power meter. A power meter will give directly the real power. There is no need to measure the voltage, current and the power factor individually. This will avoid measurement errors. There are options of power meter like Fluke, Hioki and Yokogawa.
Sample the Parameters Correctly
Another key to measure efficiency accurately is to sample the parameters correctly. This means that the voltage measurements must be taken very near to the power supply input and output connectors.
If the input voltage is measured away from the power supply inlet, the sampled voltage is higher than the voltage that is feed to the power supply. This is because from the point where the voltage is measured down to the location of the power supply inlet has voltage drop.
This is also valid for the output voltage. When it is measured away from the output port, the measured value is lower than the actual power supply output due to the voltage drop.
Use Short Connections
One common mistake is to use long probes and connecting wires. These will introduce measurement errors. The connections must be short as possible.
Give Some Time Before Taking Measurements
Let the power supply run at least 30 minutes before taking measurements. Semiconductor devices like MOSFET, BJT, IGBT or diode have parameters that are dependent on temperature. At the time the power supply is run, these devices are not in their settling state yet. After some time, their settling state will be reached, and the measurements will be accurate.
This must be the complete setup.
