There is a trick to measure power supply efficiency

This article will introduce two different methods for measuring the conversion efficiency of switching power supplies. The first method uses a wattmeter and two multimeters; the second method describes how to measure without a wattmeter, but is not as accurate.

Required Equipment

1. A programmable AC power supply or an autotransformer

2. An electronic load

3. A wattmeter and two DMMs (preferably one with high precision to measure current) or four DMMs (one with true RMS precision to measure input current and one with high precision to measure output current)

NOTE: When using a multimeter, it is very important that you set the multimeter to the appropriate range for the voltage and current values ​​you want to measure.

DC output power is simply the product of voltage and current, and can be measured with just two multimeters. We will use a high-precision multimeter to measure the current delivered to the load, and a standard multimeter to measure the output voltage of the power supply. Because there is a phase angle between voltage and current in an AC system, the input power cannot be simply calculated by multiplying the RMS input voltage by the RMS input current. Only the active power (P) consumed by the power supply must be considered. The reactive power Q returned to the power supply should not be considered.

The advantage of a wattmeter is that it accurately measures input power because it automatically corrects for power factor. If a wattmeter is not available, two multimeters can be used to measure the input voltage and current. However, this alternative method does not provide the same accuracy as using a wattmeter and also requires disconnecting the power supply under test.

Connect the voltmeter directly across the board output and to the electronic load. The voltage drop in the cable to the load is not taken into account when measuring the output voltage. In some applications, such as mobile phone chargers or laptop adapters, it is necessary to account for losses in the cable and measure the output voltage from the load. Then connect a high-precision ammeter in series with the load to measure the output current.

AC Connection Notes

If the device being used uses an on/off control scheme, the worst-case efficiency can be measured by quickly applying the power supply at the sense input voltage to fully load the output. However, applying the power supply will generate a very large inrush current while the bulk capacitor is charging. This can cause the fuse in the input ammeter to blow if it is set to a low range.

Recommended AC connection procedures for different SMPS control schemes

If the four multimeter method is used, the inrush current of the power supply should be measured first after quickly turning it on at low input voltage and highest load. Then consult the multimeter's data sheet to confirm that it can carry such a high peak current at high input voltage. For all other control schemes, the turn-on method will not affect the efficiency measurement, and it is recommended to slowly increase the AC voltage during the test to limit the inrush current.

Wattmeter Method

Connect a wattmeter to the power supply input and set the display to average mode to get a more stable reading. Turn on the AC input voltage and slowly increase it to the desired sense voltage. Load your power supply to full load. Then turn the power supply off, quickly turn it back on, and continue to complete the measurement. In this demonstration, the meter measured 4.97 volts and 4.005 amps at the power supply output. The voltage reading of the electronic load was 4.48 volts. This is due to the 490 mV voltage drop across the output cable and the multimeter voltage sense element, which highlights the importance of measuring the voltage at the power supply output. Therefore, the output power = 4.97 V 4.005 A = 19.90 watts. The wattmeter reading shows that the input power is 25.76 watts. Therefore, the power supply efficiency = 19.90 watts / 25.76 watts = 77.3%.

Multimeter Method

When using a multimeter, the input power can be measured after the diode rectifier stage converts the AC power to DC, thus avoiding the effects of the power factor. To improve the measurement accuracy, the losses in the components before the DC bus stage must be included. The diode rectifier bridge is usually the most lossy component in the input stage because the voltage drop across each diode can be as high as 0.9 volts in the worst case. Other components with very large impedance or voltage drop that can be measured can also be used to calculate their losses using this method.

Connecting a multimeter

Disconnect the DC bus between the rectifier bridge and the bulk capacitor C2. After disconnecting the DC bus behind the bulk capacitor, a multimeter is needed to measure the high-frequency switching current of the power supply, which a multimeter cannot accurately measure. Then, solder two wires that can be used to connect the multimeter to the circuit. Connect a true-rms, high-precision multimeter set to measure the current in the open circuit. Use another multimeter set to measure the voltage, connecting it to the positive DC terminal and the negative terminal of the bulk capacitor.

Testing Procedure

Turn on the AC power supply and slowly increase the voltage to the desired detection voltage. Increase the load of the power supply to full load. Set the input ammeter to the highest current range. Then cut off the AC input voltage and quickly re-install the power supply. In this demonstration, the power supply still provides 4.97 volts, 4.008 amps, and 19.92 watts of output power. At the input, the DC bus voltage is 151.6 volts and the input current is 0.166 amps. The input power is calculated as follows: AC input loss

Now, the power losses in the rectifier bridge must be taken into account:

Power loss estimate = Worst case total diode voltage drop Input current

= 1.8 V 0.166 A

= 0.299 W

Therefore, total input power = 25.1656 W + 0.299 W

= 25.46 W

Using this measurement method, the power supply efficiency can be calculated as: = 78.2%

Compared to the 77.3% calculated using the wattmeter measurement, we can see that the final error from the four multimeters is 0.9%.

Improve accuracy

The accuracy of this measurement can be improved by adjusting the input power to include the losses of other input stage components such as surge limiters, common mode chokes, and the DMM's current sensing element in addition to the losses of the diode bridge. To calculate these losses, measure the voltage drop across each component under normal operating conditions and multiply that voltage drop by the measured input current. Including these losses will increase the total input power and reduce the calculated efficiency.

However, this method will never give you as accurate results as measuring input power with a wattmeter. Measure a range of input and output values ​​to determine the cause of losses. Power supply efficiency is a function of input voltage and output load. When evaluating a power supply, it is often necessary to measure efficiency at several different input voltage levels to better determine where the losses are in the circuit. Plot the results on a graph showing the relationship between efficiency and input voltage at full load.

Effect of conduction loss on efficiency Effect of switching loss on efficiency

Efficiency drops at low input voltages, usually due to conduction losses caused by resistive elements in the circuit. These losses increase at low input voltages because higher currents are required to maintain the same output power. Efficiency drops at high input voltages, usually due to switching losses. These losses come from parasitic capacitance. Losses increase at high input voltages because parasitic capacitance charges and discharges at higher voltages. Once the causes of the losses have been identified and corrective action has been taken, the result is the following graph. Efficiency vs. input voltage for a well-designed power supply.