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schematic

simulate this circuit – Schematic created using CircuitLab

I have been tasked to verify the input impedance of a medium powered DC/DC converter (<1kW), and no experienced colleagues to consult on the matter. I have quite a good description of the required test setup, including the choice of isolation transformer for coupling in the AC pertubation, current and voltage probes. I'm intentionally holding back on the details here for anonymity reasons, so apologies for that.

It is clear, that I need to exercise caution not to saturate the transformer core at low frequencies and keep a close eye on the primary and secondary levels.

What is missing, however, is the description of the power source, and this is where the uncertainty starts. Looking at the setup, I assume there's no way the source output capacitance isn't going to influence the measurements of the input impedance at the input of the converter.

The question is what calibration method to use for the VNA in order to compensate for the excess capacitance of the source. The VNA/FRA available is generic, and will dump the voltage magnitude/phase it sees at its inputs, and is not guiding the user through any calibration procedure.

How, oh wise masters, would you proceed in order to get a trustworthy measurement?

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    \$\begingroup\$ You can have a look at this paper published by Delta Electronics folks from Raleigh (NC) where they explore different approaches. You certainly should not insert a transformer in series with the input line but resort to a solid-state injector with a series-pass MOSFET for instance. For more about the interaction between the filter and the dc-dc converter, have a look at my APEC 2017. \$\endgroup\$ Jan 26, 2023 at 11:48
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    \$\begingroup\$ You don't need to care about the capacitance cos' you measure V and I at the converter directly. This AN from Omicron might be worth reading too. I would recommend you to simulate the \$Z_{in}\$ of your converter using SIMPLIS for instance (you use one my free 80+ ready-made template). Having an idea of what you should expect from the bench is good for engineering judgement. \$\endgroup\$ Jan 26, 2023 at 11:56
  • \$\begingroup\$ What an honour to get answers from Mr. Kint himself! I have two of your books on my bookshelf next to the one of Erickson and Maksimovic. But coming back to your comments, the use of this specific transformer (Solar) is prescribed in the measurement requirements. About the source capacitance, I did find the Delta paper, some Omicron and Venable ANs, which all ignore the influence of the source impedance. I can't quite get it into my head. The only thing which is different for the source and load measurements is the placement of the voltage probe behind or in front of the transformer secondary. \$\endgroup\$ Jan 26, 2023 at 12:25
  • \$\begingroup\$ I suppose another way to look at it is, that since it is a one-port, only the impedance of the port will relate the voltage and current phasors measured directly across it. A bit like the current and voltage relationship of a simple resistor won't change, even if a more complex circuit is connected to it. \$\endgroup\$ Jan 26, 2023 at 12:33
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    \$\begingroup\$ Oui, because you measure the input variables at the dc-dc input port, in principle, you do not care about the source impedance I guess. Also, I don't foresee an interaction with the dc source as you have to limit the perturbation level anyway to maintain a small-signal condition. \$\endgroup\$ Jan 26, 2023 at 13:00

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A short follow-up to share how the measurement went. Verbal Kint was right, it was just that simple. One learning that I did have, was that using a DMM for an RMS voltage measurement on the injection side is not a good idea above 1 kHz, as this is where the "True RMS" apparently ends. No harm done, but certainly educational. Better to use an oscilloscope in the future.

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