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ESR and ESL of DC-Link Capacitors

Related products: MFIA, MF-IA

Application Description

DC-link capacitors are widely used in inverter circuits to convert DC electricity back to AC. On electrical vehicles, for example, when the coupled insulated gate bipolar transistor (IGBT) switches off, a voltage spike proportional to the parasitic inductance is induced. Reducing the equivalent series inductance (ESL) of the capacitor is therefore important to prevent shock damage. Furthermore, it is desirable to have a low equivalent series resistance (ESR) to enhance the device efficiency and suppress heat dissipation. Characterization of low ESL and low ESR with an impedance analyzer is thus crucial to verify the capacitor performance over the frequency range of interest. In turn, such measurements set the benchmark for future component development.

Measurement Strategies

Impedance measurement of a DC-link capacitor with the Zurich Instruments MFIA Impedance Analyzer

Figure 1: Schematic showing the impedance measurement of a DC-link capacitor with the MFIA Impedance Analyzer through a custom low-ESL fixture.

A multi-instrument setup for the characterization of ESL and ESR comprises an LCR meter to measure the capacitance at low frequency, and a combination of oscilloscope and signal generator to derive the ESL and the ESR from the equation of self-resonant impedance. This technique is bound to a fixed frequency, and its accuracy is limited at low ESL values due to the high parasitic impedance present in the setup (originating from cables, connectors, shunt resistance, etc.).

Using an impedance analyzer with a suitable fixture simplifies the setup (see Figure 1) and enhances the measurement capabilities. The MFIA Impedance Analyzer includes a compensation advisor enabling the measurement plane to be set at the device connector based on a short-load compensation procedure. In combination with a suitable 4-terminal fixture that matches the connector layout of the device under test, the MFIA can reach a measurement baseline as low as 10 pH and 10 μOhm over a frequency range going from 100 Hz to 5 MHz (see Figure 2). Reaching such low baselines means that the much higher ESR and ESL of the component can be measured with confidence over the full operating frequency range. In addition to the ESR and the ESL, the frequency-dependent capacitance values are automatically determined from the built-in equivalent circuits and displayed with the LabOne® Sweeper tool. If the component exhibits multiple resonance peaks, these are clearly distinguished thanks to the high impedance and frequency resolution.

ESL and ESR measurements of DC-link capacitors

Figure 2: ESL and ESR measurements of low-cost (blue curve) and high-performance (red curve) DC-link capacitors compared with baselines (green curves).

The Benefits of Choosing Zurich Instruments

  • A single instrument allows you to carry out measurements of low ESL and ESR.
  • With the compensation advisor you can achieve reliable and repeatable measurements of low ESL and ESR.
  • Visualizing multiple parametric traces in the same plot is straightforward thanks to the time-domain Plotter tool and the frequency-domain Sweeper module.
  • The LabOne Sweeper tool enables you to compare your measurements with historical reference spectra.
  • You can improve traceability by using unique instrument and compensation setting files.

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