Switch mode power supplies are used in most electronic systems. Especially in applications where linear voltage regulators were used, switching power supplies offer the advantage of higher power efficiency. Evaluating a switch mode power supply circuit in the lab requires some special knowledge. Though measuring output ripple voltage, conversion efficiency, inductor current and loop stability seem straight forward, not applying the correct procedures will yield useless measurement results. In this article some of the most important procedures are introduced.
A good setup
The most important part of the evaluation is a decent test setup. Figure 1 shows a poor setup and Figure 2 shows an optimized setup. The difference is the length and the geometry of the cables. In Figure 3 we see the device under test in the middle and on the left some power source, usually a lab power supply and on the right we see the load. This load can be a passive resistive load, a reactive load such as a capacitor or Inductor or it can be an electronic load.
Since our power supply evaluation is targeted at testing the device under test and not the setup, we need to make sure that parasitic influences such as the cable inductances from the power source to the device under test as well as the cable inductances from the device under test to the load are as small as possible. Long cables with a large area in the current flow loop, indicated with red arrows in Figure 3, will increase the impedance of the cable. Load transients or line transients can easily be influenced by a setup with long cables. As a conclusion we use short cables and we reduce the area the current flow is surrounding by twisting the cable together as shown in Figure 2.
In the photographs from the bench,