How to Measure DC/DC Converter Impedance in PLECS

[Munadir_Ahmed]

Output Impedance Measurement of a DC/DC Converter

The output impedance of a DC/DC converter can be measured by injecting a small current perturbation at a known frequency across the converter’s output terminals and observing the resulting change in output voltage. By analyzing the voltage response relative to the injected current over a range of frequencies, the converter’s output impedance can be plotted using the available analysis tools.

Figure 1: Measuring the Output Impedance of a Buck Converter

The figure above shows the PLECS demo model “Buck Converter with Analysis Tools”, illustrating how the output impedance measurement is performed.

Note: The perturbation signal alone cannot be used to calculate the output impedance, as the converter output current also includes the effect of the output resistor.

To use a different reference signal in PLECS Standalone:

1. Double-click the Small Signal Response block.
2. Enable Show reference input.
3. Feed the measured converter output current into the reference input port.

Input Impedance Measurement of a DC/DC Converter

Often, the input to a DC/DC converter is modeled as an ideal voltage source. The methodology used to measure output impedance cannot be directly applied to measure the input impedance because the ideal voltage source provides a zero-impedance path for the injected current, which interferes with the measurement.

Figure 2: Incorrect measurement of converter input impedance using a current-source based perturbation

Fig. 2 shows an attempted measurement of the input impedance by applying a current perturbation across the input, in the same way the output impedance was measured. The results of this setup are shown in Fig. 3 below.

Figure 3: Incorrect input impedance results using current-source based perturbation

An alternate way to introduce the disturbance is via a voltage perturbation superimposed on the ideal input voltage source. Fig. 4 below shows a setup that applies a voltage perturbation at the input.

Figure 4: Recommended voltage-source based perturbation to measure input impedance

The results from the setup are shown in Fig. 5 below:

Figure 5: Converter input impedance using a voltage perturbation based approach

[AsimIbrahim]

Hello,

Thank you for your recommendation. However, I have a few queries.

1- When you inject a voltage perturbation and measure the corresponding current, isn’t the resulting transfer function an admittance, not an impedance one?

2- As I can see it, your response current measurement to the injected voltage perturbation is measured at the control loop, which, for me, is more like loop gain rather than input measurement, nor open loop input impedance, nor closed loop input impedance. Or what do I miss here?