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PCBD-June2015

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34 The PCB Design Magazine • June 2015 in Figure 1), our application-dependent set of capacitors now becomes part of the control feedback loop. Unfortunately, certain com- bination of output capacitors may cause the converter to become unstable, something we want to avoid. This raises the need to test, measure and/or simulate the control-loop sta- bility. Figure 1 shows the block diagram of a switching-mode step-down DC-DC converter (also commonly called a buck converter) to- gether with the usual connections and setup for measuring the loop stability. This is also called the gain-phase measurement, because we are mostly interested in the phase of the loop gain as a function of frequency. The phase value where the gain magnitude drops to unity is called the phase margin. A typical measured data set is shown in Figure 2, where the labels identify the phase margin. The heavy blue line on the chart goes with the left vertical axis and it shows the loop gain magnitude in dB. The gain magnitude reach - es unity (zero dB) at slightly above 100 kHz. There is a large dot at this frequency on the thin green curve, indicating the crossover fre- quency on the horizontal axis and the phase margin on the right vertical axis. The phase margin in this case is around 45 degrees, which is usually considered as acceptable. To measure the phase margin, we need to inject a test signal into the control loop. A suitable location is the top of the feedback voltage divider, where it connects to the out - put voltage. We cut the loop open at this point and insert the test signal in series to the volt- age divider. The top side of the signal injec- tion faces the converter output, which is low impedance, whereas the low side of the injec- tion impedance faces the upper voltage divid- er resistor, which is usually in the hundreds of ohms range or higher. AVOIDING OVERLOAD IN GAIN-PHASE MEASuREMENTS continues Figure 2: Typical gain-phase plot with the phase margin identified. quiet power

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