The Answers to Your Questions About Power Averaging

March 14, 2017
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Pquestion markower averaging is an approach that uses a power chain designed to supply the average current required by a pulsed load, instead of designing the power system to meet the maximum demand. A large capacitor is used to store the energy and deliver it to the load in pulses. This technique requires a power supply that can supply a highly capacitive load: something that is not possible with most existing products.

For more information about the approach, our white paper How Power Averaging Cuts Size and Cost For Pulsed Loads, provides an introduction to the technique, and we also have an on-demand webinar available, “Maximum Load: The Wrong Specification for Pulsed Loads”.

To get a deeper insight into the technique, check out the best questions that attendees asked after the live webinars below, and presenter Dave Berry’s answers:

Does the capacitor place any limit on the duty cycle of the load in a power averaging system?

This will depend upon the ripple current that you’re drawing out of that capacitor.  Many capacitors have a maximum ripple current, although sometimes you’ll need to contact the manufacturer to get this information. This can be driven by the equivalent series resistance (ESR), which will heat up that capacitor. The best capacitor for power averaging is one that has a very low ESR; that will reduce the heat generated.

Maximum duty cycles can be 50%, depending upon the capacitor being used, so you do need to select the capacitor carefully. As the duty cycle goes high, the ripple current reduces, but the benefits of power averaging also decrease, and when systems approach that 100%, you might as well begin to size the power system to supply the 100%.  As the duty cycle gets lower, power averaging becomes much more attractive, but the demands on the capacitor increase.

Can an ultracapacitor be used in place of capacitor for more energy storage?

You can, but there are several caveats.  Ultracapacitors are typically available in lower voltage ratings, but they can be stacked to either increase capacitance or handle higher voltages.

You still need to be careful that the ripple current is within a safe region, and with the higher ESRs that are typically associated with ultracapacitors, this can be more restrictive than for conventional capacitors.

How can I understand the behavior of the DCM in an overcurrent situation?

There isn’t a specific applications note, but all the information is contained in the DCM datasheets. If you have any questions that aren’t answered by the data, you can also contact our applications engineering team.

Is it only the DCM that can be used for power averaging? What about the PRM?

The PRM can be used in a similar manner.  I’ve worked with applications where we are using the PRM in a power averaging application.  One of the benefits of using the PRM is you have control over the feedback circuit, so you can make sure that it is stable during operation.

The PRM, however, is a non-isolated device, unlike the DCM. Although it will deliver higher efficiency, you can only use it where you don’t require isolation.  The PRM can be paired with the BCM for isolation.

Learn More About Power Averaging

To learn more about power averaging, watch the webinar, “Maximum Load: The Wrong Specification for Pulsed Loads”.

 

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