On 2/15/11 7:35 AM, Bayley Wang wrote:
2KVA is still well within the realm of an MMC, though. The Geek Group chart recommends 4x11-cap strings of CDE942's, which is quite a reasonable number. An MMC *will* work, whereas the pulse cap you linked to may have the wrong sort of dielectric.
Yes, 2kVA is in the range for an MMC, but when you start getting up towards 7-10 kVA it starts to shift towards getting a pulse cap.
but run the numbers.. 44 caps at $3.29 each is getting up around $150, plus the time to assemble it.
Double the size of your coil to 4kVA, and now you're talking $300 worth of capacitors... That starts to be in the range for surplus (unused) Maxwell capacitors, and if you actually had to pay someone to assemble the MMC, you might be up where new Maxwell caps are... (I haven't gotten a quote from Maxwell for over 10 years, so I don't know what prices are running)
The link I gave isn't for a particular capacitor, but is for derating equations, so you can take a capacitor designed for 1pps duty with no VR at, say, 30kV, and figure out whether it can take 20kV at 120pps with 50% reversal. Since life goes as the 7.5 power of the voltage, going from 30 to 20kV is a factor of almost 21.
The life isn't as strongly dependent on the VR as on voltage.--- I should note that to make a fair comparison, you need to recognize that the Geek Group MMC tables are running the caps somewhat over their "nameplate" ratings (a string of 11 2kV 0.15uF caps is 22kV DC rating, but way less than that AC rating, and particularly for RF), so when you're looking at pulse caps, you also need to figure out how much you want to push it by.
The other approach to derating is to look at the power dissipation in the capacitor.
On Feb 15, 2011 10:22 AM, "jimlux"<jimlux@xxxxxxxxxxxxx> wrote:On 2/15/11 6:21 AM, Bayley Wang wrote:It's sort of been agreed on that an MMC is the way to go. Not all pulse capacitors are rated for high-frequency, high-reprate, high-reversal operation.kind of depends on the size of the power supply. At a certain point, the pulse cap is cheaper than the MMC, especially if you are counting the value of time to assemble dozens of caps into an MMC. There are standard derating equations that Maxwell Labs (among others) publish to help you figure out what the revised ratings are for a cap being used in a manner other than it was designed. http://home.earthlink.net/~jimlux/hv/caplife.htm has some examples
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