Bypass capacitors

Question

I'm working on developing a bidirectional DC-DC converter between 30V (low side) and 65V (high side) with a current rating of 10A. I have little knowledge about bypassing the high and low power rails, it seems like a large electrolyte with a couple of parallel small ceramic capacitors is a common configuration, but I read that Film capacitors are more efficient for bypassing. My questions are rather general and I hope I can get an answer:

1. what is the proper type of the capacitors for bypassing the power rails?
2. How to decide the number and values of capacitors to put in parallel?

Answer 1

Typically ESR and ESL are important in selecting bypass capacitors as well as the total amount of capacitance on the input or the output when designing DC to DC converters.

If you are using and an IC most manufacturers will provide information on what capacitance is needed for input or output capacitors. In addition many manufacturers provide tools that can calculate the ripple on the output with different capacitors and include the parasitics.

Answer 2

it seems like a large electrolyte with a couple of parallel small ceramic capacitors is a common configuration,

What you are mentioning seems to be talking about Filtering and bypass capacitors. The usually larger filtering capacitors exist to remove low-frequency power supply noise, while the smaller valued bypass capacitors exist to remove high-frequency noise and provide a low impedance path for current surges.

To answer your two questions:

what is the proper type of the capacitors for bypassing the power rails?

The most important factor when choosing a capacitor for this purpose would be its frequency response, which is determined by equivalent series resistance (ESR) and equivalent series inductance (ESL) of the capacitor, as these will affect the capacitors performance in filtering and bypassing noise. In general, monolithic and ceramic type capacitors are used, as they have both low ESR and ESL.

How to decide the number and values of capacitors to put in parallel?

In general, when filtering and bypassing power supply signals, capacitors of multiple values are used to target different noise frequencies. This might include using 3 or more capacitors, one for low frequency noise (large capacitance value), one for mid frequency noise, and one for high frequency noise (low capacitance value). More capacitors may be used to target a wider noise bandwidth. To calculate the capacitor value needed for a specific frequency, I will reference a Texas Instruments application report reproduced below:

For further reading, I suggest exploring the application report as that gives example calculations and goes in depth about uses cases and other filtering methods.

Answer 3

I read that Film capacitors are more efficient for bypassing

Metalized film capacitors are usually good for supplying impulses of hundreds (if not thousands) of amps for a very short time. In other words they can "save the day" compared to using an electrolytic capacitor (too much ESR and ESL) or, using a ceramic (not capable of supplying hundreds of amps as an impulse without serious detrimental effects).

what is the proper type of the capacitors for bypassing the power rails?

Nearly always you need a bulk electrolytic capacitor to handle to slow moving but large average energy transactions. But, you should also use ceramic/film capacitors on power supply rails adjacent to devices that take high (and short-lived) surge currents.

How to decide the number and values of capacitors to put in parallel?

Well, you can try and work it out mathematically but, I've found the best and quickest and least-error-prone method is to use a simulator and mimic the characteristics of the capacitors used (ESR and ESL) and look how the voltage rail is perturbed when your target circuit is operational.

I'm working on developing a bidirectional DC-DC converter between 30V (low side) and 65V (high side) with a current rating of 10A.

Peak short-term currents may be several tens of amps and, it's likely that film capacitors may be useful. Anecdotally, on a big system I designed I had many dozens of 600 volt rated metalized film capacitors to soak up the transient currents in a switching converter.