Measuring low ESL and low ESR of a DC-Link Capacitor with the MFIA Impedance Analyzer

Introduction:

This blog post describes how the MFIA (and MFLI with MF-IA option), can characterize the equivalent series inductance (ESL) and equivalent series resistance (ESR) of a DC-Link capacitor typically used in power inverters for electric vehicles. The ESR and ESL were measured over a frequency range between 1 kHz and 5 MHz revealing the ESL to be 9.5 nH (at 735 kHz) and the ESR to be 0.7 mOhm (at 12 kHz).

Background:

When DC-link capacitors are coupled to IGBT switch modules, for example in electric vehicle inverters, the ESL and ESR of the DC-link capacitor and its connectors have a significant influence on the overall design. Due to the energy stored in the parasitic inductance, voltage overshoot can occur when the IGBT is switched off. These unwanted voltage spikes during switch-off can be minimised or even eliminated by designing the DC-link capacitor and its connectors so they present a very low ESL to the IGBT.

Recent improvements of DC-LINK capacitors have lead to very low ESL and ESR, and here we use the MFIA and a low-ESL fixture to confirm the values stated by the manufacturer. We selected the TDK EPCOS B25655PXXX DC-Link XXX μF capacitor for this test. The stated values of ESR are 0.8 mOhm and the ESL of 15 nH.

 

 

Figure 1: Photograph showing the DC-Link capacitor of EPCOS TDK connected to the MFIA using a custom low-ESL fixture. The DC-Link capacitor has three sets of electrodes labelled U, V, & W, and each set were measured sequentially with the same fixture.

 

For the measurements taken in this blog post, we use a low-ESR fixture with flexible connectors which allow for the vertical offset of the DC-Link busbar connectors (which have been designed by the manufacturer to match the IGBT module). The fixture has four BNC connectors with the standard 22 mm spacing. It was connected between the front panel of the MFIA and the DC-Link capacitor, as shown in figure 1.

 

First, check the baseline

For accurate impedance measurements it is necessary to first run a compensation routine so that the impedance of the fixture can be neglected in the actual measured data. To do this compensation, we use the LabOne Compensation Advisor to run a Load-short routine from 1 kHz to 5 MHz.

This procedure allows us to define the measurement plane to be at the connectors of the fixture which contact the capacitor. Directly after running the procedure, we ran a short measurement to get an idea of the measurement baseline. Figure 2 shows a screenshot of the LabOne interface software, in which two sweeper windows are open to measure the absolute impedance from 1 kHz to 100 kHz and the inductance from 100 kHz to 5 MHz. The sweeps show a low baseline of 15.7 µOhm and 1.7 pH for the real part of impedance and inductance respectively. These low measurements give us the confidence that we can, subsequently, reliably measure the ESR of ESL of our capacitor.

 

 

Figure 2: Screenshot of LabOne showing two measurement sweeps of the short after running the fixture compensation routine. The frequency range was split into two parts; 1 kHz to 100 kHz in the upper sweep (red trace, Real(Z)), and 100 kHz to 5 MHz below (green trace; series inductance). It confirms the low baseline for both the real part impedance, Real(Z), and series inductance for this measurement setup. Click on the image to enlarge.

 

Now, get an overview

The next step is to measure the the DC-Link capacitor over the full frequency range of interest in order to look at the salient features of the impedance. Figure 3 shows a multi-trace sweep from 1 kHz to 5 MHz. The green trace shows the real part of the impedance, which can be considered the ESR. Below 90.8 kHz, the self resonance frequency (SRF) of the capacitor, the capacitance can be seen as the blue trace. At the lowest frequency of 1 kHz, the capacitance can be read as 121.999 µF, which is consistent with the given value of 120 uF +/- 10%. Above the SRF, the ESL can be seen as the light-blue trace. It shows three peaks annotated with black arrows at 175.9 kHz, 284.2 kHz and 749.7 kHz. In addition to the ESR and ESL, the green trace shows the absolute impedance, and the purple trace the phase.

 

 

Figure 3: Screenshot of LabOne showing a wide frequency sweep from 1 kHz to 5 MHz of the DC-link capacitor. The five traces are: Capacitance (blue), Real(Z) (green), Absolute Z (red), Series Inductance (light blue) and Phase (purple). The self resonance frequency of this capacitor is 90.8 kHz. Click on the image to enlarge.

Finally, take the data

It is clear from Figure 3 that the ESR should be measured at low frequency, whereas the ESL should be measured at higher frequency. Therefore two different sweeper windows were opened to cover two ranges; 1kHz to 100 kHz and 100 kHz to 5 MHz. As the capacitor has three different sets of electrodes (labelled U (red trace),  V (Green trace) and W (Blue trace)), each electrode set was measured sequentially and repeated five times to demonstrate the repeatability of the measurements. Figure 4 shows a LabOne screenshot of the measurement, with two sweeper windows covering the aforementioned two frequency ranges. The test signal amplitude is 900 mV, and we use the standard measurement settings to make each sweep of 200 points in 12 seconds.

The upper window shows a sweep of the real part of impedance, Real(Z), corresponding to the ESR. In total, there are fifteen traces on the sweeper, color coded to the electrode set. The traces overlap to a high degree due to the excellent repeatability of the measurement even after disconnecting and reconnecting. The ESR measured using the electrodes W (blue traces) can be read from the black arrow showing 718 µOhm at 11.35 kHz. This nicely agrees and confirms the stated value of ESR of 0.8 mOhm. The yellow trace in the sweeper corresponds to a short measurement.

The lower sweeper window in Figure 4 shows the ESL from 100 kHz to 5 MHz. Again, the traces are color-coded to match the three sets of electrodes, and are repeatable to a very high degree so that the traces overlap. Electrodes U and W show similar behaviour, showing three peaks at approximately 176 kHz, 283 kHz and 742 kHz. These electrodes are mechanically symmetric, so the behaviour should be similar. The center set of electrodes, U, in contrast shows just two peaks. The light-green trace corresponds to a short measurement.

Taking the value of ESL from the blue traces, at 742 kHz, we get a value of 9.49 nH. This is again in agreement of the stated value of <15 nH.

Figure 4: Screenshot of LabOne showing two sweeper windows each displaying fifteen traces, corresponding to five measurements of each set of three electrodes. The upper sweeper window displays the ESR and has a frequency range from 1 kHz to 100 kHz, the lower sweeper window displays the series inductance and covers the range from 100 kHz to 5 MHz. The traces are color-coded to group the three sets of electrodes (U red, V green and W blue). Click on the image to enlarge.

 

Conclusion
This blog shows how the MFIA impedance analyzer can be used to measure the ESR and ESL of DC-Link capacitors over a wide frequency range, not just the standard, fixed frequencies. It enables the manurfacturer values to be confirmed and shows their variation as a function of frequency and between the different electrodes. The low baseline of both ESR and ESL leaves a lot of headroom for future measurements, as manufacturers improve both ESR and ESL.

For a demo, or for further information on using the MFIA to measure low ESL and ESR, get in touch.