Efficiency is the most crucial criteria for some power supplies. With high-efficiency controllers like the UCC28780, it is important now more than ever to understand how to conduct a proper efficiency measurement that adheres to regulation standards. In this post, I’ll show you how to properly connect and measure AC/DC power supply efficiency.
Proper test conditions:
Efficiency measurements are only as good as the connections from which they’re made. To make efficiency measurements as accurate as possible the first step is to make sure that no unnecessary equipment/circuitry is connected to the board, such as an oscilloscope connector or other meters that are not needed in the efficiency measurement test itself. If there are connections that draw power that cannot be removed, their power consumption needs to be noted in the measurement or calculation and not neglected. This is critical in low-load and stand-by power, in which any external connections will have a larger impact in the efficiency measurement. The next step is to place the probes for measurement. One option is to put the probes as close to the output as possible, like on the output capacitor or at the pins of jumper J6 in the UCC28780 evaluation module (EVM), as shown in Figure 1. However, if the power supply has a fixed cable, you should measure from the end of the fixed cable rather than directly from the output. Connecting this way ensures a proper Kelvin connection and provides high-accuracy measurements.
Figure 1: UCC28780EVM connection diagram for efficiency measurements
Now let’s move to the input side of efficiency measurements for AC/DC converters. A dedicated power meter is required, as the two-multimeter setup used on the output side would not be accurate for the input because the multi-meters would not account for the harmonic content like a power meter would. Similar to the output, your goal is still to place the measuring probes as close as possible to the board’s input. Both input and output connections are shown in Figure 2, and match the color scheme of Figure 1.
Figure 2: Probe connections for efficiency measurements
Proper testing procedure:
Title 10 Electronic Code of Federal Regulations outlines the correct way to conduct power-supply efficiency measurements, and will be linked in the additional resources. Here are the steps for performing an accurate efficiency measurement:
A test procedure would then look like this:
Figure 3 outlines the test procedure in a flowchart diagram.
Figure 3 Efficiency measurement procedure
Engineers can test things many ways. As the industry continues to push for higher and higher efficiencies in power supplies, it is important to have a good grasp on what a consistent testing environment and procedure look like.
Instead of using a power meter on the input, you can use an oscilloscope measuring voltage on one channel and current on the other. Use the multiplication math function of the scope, and *average* the V x I waveform over N cycles. That will give you an accurate power measurement including harmonics up to the scope BW.
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