[061] Input Impedance Measurements and Filter Interactions Part II

Measuring input filter output impedance.

Introduction

In this article, Dr. Ridley continues the discussion of power supplies with input filters. He shows how the output impedance of the input filter is measured, and demonstrates the importance of input filter damping.

Input Filter Measurements

As discussed in the last article of this series [1], an input impedance measurement gives information about the characteristics of the power supply's input terminals. We use this information in conjunction with measurements of the output impedance of the input filter to assess whether a system interaction is likely to occur. This is important since it can lead to instability of the power supply control loop.

Figure 1 shows a block diagram of a switching power supply connected to an input filter. In order to assess filter interactions correctly, all filter components should be considered to be part of the input filter. This includes any bypass capacitors at the input of the switching power supply, and any other filter components that may be included at the front end of the power supply. In the last article of this series [1], the measurement of the input impedance of the power supply was demonstrated, and the effect of a small bypass capacitor was shown.

fig 1

Fig. 1: Power supply with input filter module. All filter components, including input bypass capacitors, should be considered part of the input filter.

In this article, the techniques for measuring the input filter itself are presented. The output impedance of the filter can be measured as shown in Figure 2. The source from the frequency response analyzer is isolated through a wide-band transformer [2], and connected to the input filter in series with a current-sense resistor. If a 1-ohm resistor is used, no scaling is needed on the gain-phase measurement from the analyzer to convert to impedance values.

fig 2

Fig. 2: How to measure the output impedance of the input filter. Notice the input of the filter must be shorted for a proper measurement.

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