Technical Article: How to Optimize Your Power Measurement Setup

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Technical Article: How to Optimize Your Power Measurement Setup [Copy link]

ask:

How can you ensure that your switching regulators are tested as efficiently as possible?

answer:

Before a circuit designer decides to use a particular power supply, he or she first tests it carefully. The data sheet of the switching regulator IC provides valuable information about how the entire power supply performs in the actual application and how to obtain the corresponding characteristics through laboratory testing. Circuit simulation, such as LTspice, is useful and can help optimize the circuit. However, simulation is not a substitute for hardware testing. For that matter, parasitic parameters are either difficult to estimate or difficult to simulate.

Therefore, the power supply is thoroughly tested in the lab, either on prototypes developed in-house or, in most cases, using existing evaluation boards from the respective power supply IC manufacturer.

Figure 1. Connections for power operation

When connecting the test circuit, there are several things to consider. Figure 1 shows a schematic diagram of the test setup. The input side of the circuit under test must be connected to the power supply and the output side to the load. This sounds trivial, but there are some important details that must be noted.

Minimize line inductance

Figure 1 is a schematic diagram of the setup used to evaluate a power converter. We want to test the behavior of the power circuit, not the effects of the connecting wires between the test board and the lab power supply or to the load at the output. To reduce the effects of these connecting wires, two important measures should be taken. First, the connecting wires should be as short as possible; short wires have lower inductance values than long wires. Second, the parasitic inductance can be further reduced by minimizing the area of the current path. An obvious way to achieve this is to use twisted wire. This makes the current path area dependent only on the length of the wire and the thickness of the wire sheath. Figure 2 shows the connections for testing the voltage converter, using twisted connecting wires to reduce the line parasitic inductance.

Figure 2. Practical operating setup using short stranded wires

In power supplies based on switching regulators, AC is present on both the input and output sides. Depending on the circuit topology, pulsed currents may occur on the input side, for example in a step-down converter (buck controller). Start-up behavior as well as load changes also need to be tested. Under these operating conditions, the connecting lines in the test setup also carry AC.

Add local energy storage devices at the input end

If you want to test how fast a power supply responds to a load transient, the DUT must provide enough energy. The source of energy on the input side of the DUT should not be the limiting factor. To ensure this is not the case, it is recommended to place a larger value capacitor at the input of the power supply, as shown in green in Figure 1. It ensures that the load transient test can be performed correctly.

However, it is important to ensure that the subsequent use of the power supply is subject to very clear conditions. The impact of energy storage devices on the input must be well understood so that the power supply input capacitor can be sized correctly.

Another aspect of the large capacitor in Figure 1 must also be considered. If a voltage transient needs to be applied to the power supply input to test the behavior, this capacitor will significantly slow down the voltage transient experienced by the circuit under test. Therefore, for these tests, this capacitor should be removed.

In summary, the tasks related to power supply design may seem simple, but there are many things that must be considered, such as connecting the circuit to the lab bench. The power lines of the circuit under test and the power lines away from the circuit under test need to be treated as AC circuits, so these cables must be short and twisted to reduce the parasitic inductance of these connecting cables. This is not redundant work for circuit designers, and doing so will make the test results close to the original intention of our test. If the influence of the test setup is reduced, the remaining results will be more valuable. Over time, experienced power engineers have developed methods to optimize circuit evaluation. If you follow all the tips in this article, you can complete the evaluation smoothly.