When Selecting Capacitors, Think About Resistance!

December 15, 2016

After our recent webinar, Maximum Load: The Wrong Specification for Pulsed Power, several attendees asked interesting questions about capacitor selection. In a pulsed application, the capacitor is a critical part of the system, and therefore selecting the right component is essential.

In practice the capacitor is also a critical factor in determining whether power averaging is the best approach. In most cases the additional cost of the capacitor is much less than the savings realized by sizing the power system to the average, rather than peak power. In addition to the cost, the choice of capacitor can also have an impact on losses and thermal design.

The presenter, Dave Berry, explained, “When selecting the capacitor, you have to take into account the ripple current, voltage and capacitance density of the capacitor type.  The lower the ESR and ESL the lower the losses.”

As there are high pulsed currents flowing into and out of the capacitor as it is charged and then delivers power to the load, any Equivalent Series Resistance (ESR) will dissipate power, creating heat within the capacitor and reducing the reliability of the component and the power system. Sometimes this means that the capacitors might need cooling, so the lower the ESR, the better.

Selecting the right capacitor will require a trade-off between size, cost and ESR. Supercapacitors (also called ultracapacitors), for example, might appear to be a good solution because of their ability to store much more energy per unit volume than conventional capacitors. These components, however, typically have higher ESR, which will cause more heat to be generated and energy to be wasted.

Connecting multiple capacitors in parallel was proposed in the Q&A after the webinar. Although this will give you a lower effective ESR, it can take up more space.

In general, the higher the voltage that is being supplied, the fewer options there will be. As Dave Berry commented, “Having a lower voltage will open the possibility of capacitor types.”

To learn more about power averaging, watch the on-demand recording of Maximum Load: The Wrong Specification for Pulsed Power.

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