Input Impedance Measurements and Filter Interactions Part III
What happens when the input filter is poorly designed.
In this article, Dr. Ridley continues the discussion of power supplies with input filters. He shows how the presence of a poorly-designed filter has a drastic effect on the loop gain of a voltage-mode controlled system. He also shows that the loop gain of a current-mode system can be a poor indicator of stability when an input filter is connected to the system.
As discussed in the previous articles of this series , 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.
Fig. 1: Power supply with input filter module. A damping network is used to control the output impedance peaking.
Figure 1 shows a power supply with an input filter. As shown in the last article of this series  a damping network is often needed to control the peaking of the output impedance of the filter.
Middlebrook's important work on input filter interactions showed that if the output impedance of the input filter is always less than the input impedance of the power stage, there would not be any instability caused by the presence of the input filter. This is a very important result since it allows us to analyze the power stage completely separately from the input filter. Switching power supplies are difficult enough to analyze and control, and Middlebrook recognized that engineers really don't want to extend their analysis further to include multiple reactive components of the input filter. He wanted to keep the math simple.
Figure 2 shows measurements of a power supply input impedance, compared with the output impedance of two different filter designs.
Fig. 2: Comparison of measurements of power supply input impedance and input filter output impedance. With an undamped filter, the output impedance exceeds the input impedance of the power supply.
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