Impedance Measurement of a DC-DC power supply
Impedance Measurement of a DC-DC power supply
(OP)
How do you measure the input and output impedance of a dc-dc power supply? I need to characterize it from DC to 1MHz and I am wondering if an external circuitry has to be setup to measure it? I have tried the website and the FAQ and some suggested that input impedance is as simple as Z=V/I and the output impedance is the internal voltage drop of the supply between no load and full load, divide that by the full load current ((V1-V2)/Io). Could someone clarify to what seems to be a basic and fundamental problem?






RE: Impedance Measurement of a DC-DC power supply
Gunnar Englund
www.gke.org
RE: Impedance Measurement of a DC-DC power supply
Yeah, impedance analyzers are really out of the question.
RE: Impedance Measurement of a DC-DC power supply
You may need some kind of current transducer. A clamp-on current probe usually has a limited frequency range, but there are Fluke DC - several hundred kHz clamps. And Chauvin-Arnoux and Tektronix going well beyond 1 MHz.
An ordinary signal generator doesn't produce more than about 20 mA (10 V and 50 ohms internal impedance) so it may be insufficient if you are checking power supplies with more than about twenty times that current (bad accuracy).
There is an entirely different way of doing it. Just connect your generator via a blocking capacitor and see how much you can influence the output voltage. Measure AC voltage and calculate internal impedance from the measured AC component. Example: You have set the generator to 5 V RMS at no load. Connect to the PSU and measure AC signal. Say that is 0.2 V (in this example). You then know that the impedance reduces your original 5 V to 1/25th or 0.2 V. This means that the PSU has roughly 1/25 of the generator's 50 ohms or 2 ohms. You should set up the proper equation for the voltage divider created by generator 50 ohms and PSU internal impedance to get the correct values.
Gunnar Englund
www.gke.org
RE: Impedance Measurement of a DC-DC power supply
I would suggest using shunts, and being careful about
scope grounding.
If you use Gunnar's second method, calibrate the output
of the cap using some low-value resistors; the output
impedance will no longer be 50 ohms. If the generator
output jack has a maximum voltage rating, chances are
that it is capacitively coupled internally, and can be
directly connected (in which case it IS 50 ohms, if so
labeled).
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RE: Impedance Measurement of a DC-DC power supply
RE: Impedance Measurement of a DC-DC power supply
There is yet another way of doing it. Use a switch transistor and feed a square wave to the DUT. Take FFT of current and voltage. Then calculate impedance by dividing U and I at different frequencies in the spectra. With a switch transistor, you can easiy have frequency components up to 1 MHz.
If you do not need to go all the way to 1 MHz, you can use an audio amplifier to have almost any signal power you want. At least up to hundreds of watts.
Gunnar Englund
www.gke.org
RE: Impedance Measurement of a DC-DC power supply