Is there a specific design for filter capacitors other than selecting a voltage value higher than the supply value,and choosing an appropriate capacitance value??

  • \$\begingroup\$ There are numerous selection parameters for capacitors in filters. Could you be a bit more specific on what you are trying to achieve? \$\endgroup\$ – Peter Smith Aug 27 '15 at 7:38
  • \$\begingroup\$ This is the condition: I have a 32V supply to be connected to Vcc of a high side driver. \$\endgroup\$ – Ashok Aug 27 '15 at 8:34

From your comment, I will assume you are trying to decouple the 32V power rail.

Just what device you use will depend on a couple of factors in particular:

  1. What is the dynamic current you expect to see? This can drive the effective series resistance requirement. It also drives the capacitance you will need.

  2. What speed will your drivers be operating at? The important thing here is the transition rate of the driver, not the base frequency. This can drive the physical package you use due to effective series inductance. It also sets how far away from the driver the device can be and still provide decoupling.

If you use ceramic capacitors with a class 2 dielectric (such as X7R, X5R, Y5V and so forth - there is a nice series of tables at this Wikipedia page) then use a voltage rating for the part of at least twice the voltage you are decoupling due to DC bias effects.

There are numerous types of capacitor and each has its own advantages and disadvantages depending on what it is you are trying to achieve.

Most decoupling uses either ceramic or tantalum devices, but take care with tantalums as the act of soldering them down can cause internal damage to the dielectric that can cause a spectacularly pyrotechnic failure. Ceramics can likewise be damaged.

If you use a through hole component, then the above soldering precautions are greatly relaxed.

A note on ceramic devices: due to their natural Piezoelectric effect, they can 'sing' if the driver you are using draws current from them at a rate in the audible range, and this can be a bit disconcerting.

As with all device choices, there is always a trade-off of device performance based on what you are trying to achieve.

Note that most IC vendors provide decoupling guidance for their devices; if not, then a good start for most devices is 100nF, but power devices may need this and some bulk decoupling in the order of 10s or even 100s of microfarads or more.

I hope that can get you started on choosing the devices you need.

[Update] Added some specific guidance

When choosing a decoupling capacitor, the first thing is indeed the voltage rating.

For 32V (assuming that to be nominal and 10%) I would be looking at no less than 50V part in tantalum, or 100V in ceramic (for the simple reason that the ratings for ceramics step from 25V to 35V to 50V and then mostly to 100V and I want at least twice the maximum voltage rating).

This may limit what you can use to a particular technology and then I would choose the capacitance based on the requirements of the circuit. Without full details of the circuit and what it is driving, I cannot go that far as yet.

  • \$\begingroup\$ okk.thankuu Peter. the voltage rating of capacitor also matters rite?? since my supply is 32 prbably a higher rating should be selected?/ \$\endgroup\$ – Ashok Aug 27 '15 at 9:12

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