In this article, Dr. Ridley continues the topic of frequency response of switching power supplies. Previous articles showed how a frequency response analyzer pulls out a single test frequency from a broad range of noise and signal. This third article shows how the AP300 analyzer can be connected to measure all of the essential transfer functions of a power supply and its components.

Measurement of Passive Components

As mentioned in previous articles in this magazine [1-3], all passive power components should be characterized across the frequency range over which they are required to function. This includes dc measurements for magnetic components, out to 30 MHz, the limit of conducted EMI measurements.

Figure 1 shows the test setup for high impedance measurements (greater than 1 ohm), typically used to characterize magnetics. This is used to measure magnetizing inductances, leakage inductance, resonances, and winding capacitances. Details of such measurements are given in [1-2]. Power magnetics are usually custom-designed, and should be measured to confirm performance. Off-the-shelf parts should also be measured since they are usually inadequately specified by vendors.

Figure 1: AP300 Setup for high impedance measurements, usually used to characterize magnetics and small capacitances

Figure 2 shows the test setup for low impedance measurements, down to as low as 1 mOhm. This setup is used to characterize power capacitors, and will show capacitor values, resonant frequencies, and equivalent series resistances. All of these quantities can have significant variation depending on the type of capacitor used, temperature, bias point, and batch sample. Most manufacturers do not provide enough accurate data to properly design a power supply, making this an essential measurement for any power capacitor [3].

Figure 2: AP300 Setup for low impedance measurements, usually used to characterize power capacitors.