Frequency Response Part 3: Important Performance Measurements
Important measurements to make for a power supply design and analysis.
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 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 .
In the aerospace industry, it is usually a requirement to measure the input bus to output voltage transfer function, also referred to as audiosusceptibility. This involves the inconvenient process of modulating the input voltage with the frequency response analyzer. More elaborate electronic power sources may have the capability of producing this perturbation, but we usually have to build a circuit to do customized injection, tailored to the specific input voltage and current levels of the converter.
Figure 3 shows a method to inject into the input rail of a power supply. A series-pass FET is used, and its gate is modulated via an isolation transformer. In setting up this test, make sure the FET is rated for the full input voltage, and properly heatsinked for the full input current.
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