Power supply design

[vertcnc]

I am in thr processs od putting together a power supply to drive servos.
I need 60v at a max. of about 10 amps. How is the size of the ripple capacitor calculated? Any could sources for these caps?

Thanks Tim

[eman5oh]

Check this link http://www.geckodrive.com/ycom/docum...r_supplies.pdf

[vertcnc]

Thanks. Just what I was looking for.

[ViperTX]

Well I suspect that formula on the Gecko site is slightly wrong...I didn't read it all, but here is what I use:
C(in microfarads) = ((I * (1/f)) / Vripple ) x 1000000

I is in amperes and is the maximum DC load,
Vripple is the allowable ripple that you want on the output and
f is the frequency of the input line in Hertz.

Of course this is for a bulk DC supply (basically unregulated).

[ViperTX]

For the 60 VDC supply I would most likely select at the minimum a 100 VDC rated capacitor. Of course with that current you are looking for a bridge rectifier of around 20 amperes (capacitive load) with a PIV of 100 volts.

[ViperTX]

Darn, I just searched high and low for gigantic capacitors....as you will also...tried ebay, all the surplus outlets and ended up with mouser.com having new capacitors for much lower then all those outlets. You have to watch out with the date codes on electrolytic capacitors...they do have a shelf life....and some will swear that their capacitors have been in use for 20 years...which is possible...but why gamble.

[ViperTX]

*chuckle*....I made a mistake in my formula...it's 2f....or twice the frequency.

[Mechanoid]

Originally Posted by ViperTX Well I suspect that formula on the Gecko site is slightly wrong...I didn't read it all, but here is what I use: C(in microfarads) = ((I * (1/f)) / Vripple ) x 1000000

Yup. I agree. Essentially, their formula calculates C for Vripple/Vdc=(1000000/f)/80000=21% ripple at 60Hz or 25% at 50Hz mains frequency, but they do not mention that. Basically, the formula will work for your average motor drive, but they should have mentioned that.

While I am here and typing that, when selectiing a cap, take heed of its ESR, equivalent series resistance. A capacitor essentially "shorts" the AC component of rectified signal, and the average current through the cap will be Ic=(1-Vripple/Vdc) * 0.71 * Iload, the first term is for the part of the ripple current not going into the load but passing through the cap, and 0.71 is for the AC character of the ripple - it is at max at times but zero at other times; it is 1/√2. I usually omit the first term ans use Ic=0.71 * Iload to be on the safe side of tolerance errors.

Power dissipated by the cap is P=ESR*Ic², and can easily surpass 10W in a high current high voltage supply (ESR is usually higher in higher voltage-rated caps). You must assess this power, because a large cap explosion is loud enough to damage hearing and throws caustic debris at high speed. Try to find a "high ripple, low ESR" cap with higher temperature rating (125°C is the winner, 105°C good, 85°C rather not). ESR is specified in every capacitor's data sheet.

Modern caps have special crossing grooves in the case ("X marks the spot") to weaken it in a specific direction, and this is where cap's guts flow out if something goes wrong. In larger cans, the plastic bottom with contacts separates. Mount the caps so that they do not shoot through ventilation grilles of the PS case if possible.