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thumb-rule to design filter capacitors for audio power amps?

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Advanced Member level 1
Jul 13, 2001
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Filter Capacitors

Is there any thumb-rule to design filter capacitors for audio power amps?

Something like 1000UF for 1A of output current......


As a rule of thomb, I use 2000uF in my PS design.It always worked for me.Get good quality cap because they handle lots of current.

Regards, jaco

There is a simple equation about this problem.

dU= I/2fC
where dU is peak to peak ripple, I is load current, f is operating frequency and C is capacitor volume,F. In this way you can choose ripple amplitude and from equation estimate capacitator volume. For example if you have 1A current and capacitor 4700uF ripple will be
dU= 1/(2*50*0.0047)= 2.12V.

There are two more parameters in addition to capacitance (as referred by Moss in his equation) and voltage (big enough working voltage specification so we do not blow the caps).

These parameters are somewhat related and are:
1) ESR. This is the resistive component in the capacitor. It adds to the ripple voltage by ohm's law, propotional to the ripple current. Select capacitors with low ESR.
2) Maximum ripple current a capacitor can carry without overheating and with acceptable lifetime. It is specified on data sheets of capacitors, but be aware that it is specified usually at the cpapacitor's maximum temperature and with a certain (often bit short, like 1000 or 2000 hours) life time for the capacitor. Usually you should keep well below that value to achieve descent life time for the cap's

Full theory and practice is too much to put in one post, it is better to study capacitor producers' data sheets and application notes. Note that ESR and ripple current are also frequency dependent, and for capacitors intended for rectified mains frequency it is often given at 100 Hz, and for switchmode power supply parts on for instance 100 kHz. Generally, one can have greater (1.5 ... 2x, see capacitor data sheets) >100kHz HF ripple than 100 Hz mains frequency ripple to a given capacitor. It is also not recommended to use a "general purpose" capacitor on high frequency switch mode supplies.

Specially on switchmode supply ESR and ripple current parameters are very important, because on higher frequences capacitive reactance is easily small enough, and ESR becomes the real issue.

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