DC-Link Caps Maximum Pulse Rise Time Rating

[schoolie]

Hello,

I'm interested in using some WIMA 600V DC-LINK (MKP4) film capacitors for an amp project. I noticed that the model that I bought has a "maximum pulse rise time" rating of 14 V/us. If I were using this cap as a reservoir cap, it would be seeing a 120hz full wave rectified signal with a peak voltage of around 460V. Would the maximum DV/dt be 2*pi*120*460 = 0.347 V/us? I'm not sure if I calculated that correctly, but is there any reason to doubt that the maximum rise time would be anywhere close to the rated maximum?

Thanks!

Rob

[Steve Conner]

The equation describing a capacitor is I=C*dV/dt, or in non-geek-speak: "The instantaneous current through the capacitor equals the capacitance, times the rate of change of voltage."

So, if a capacitor datasheet gives a maximum dv/dt rating, it is usually a peak current rating in disguise. In your case, say your capacitor was 100uF:

I=100e-6 * 14/1e-6 = 1400A which is far more than any tube amp PT I've ever seen is capable of providing under any circumstances, except maybe a direct lightning strike

You may want to redo this calculation with the actual capacitance, but the conclusion shouldn't change: these capacitors are designed for heavy-duty use in switchmode power supplies that subject them to far more abuse than a tube amplifier could.

[schoolie]

Thank you, Steve! That makes sense, V=q/c and dV/dt= 1/C* dq/dt. Looks like these caps will be perfect for the amp. Sure hope I don't see 350A in my build

[trobbins]

Rob, your equation was correct for 240Vrms sine across the cap, ie. 680Vp-p, whereas in a DC supply it may have much much lower AC ripple on it depending on where you are using it. When used for DC supply, the pertinent specs are DC voltage and AC voltage. Some caps give the AC spec at the rated max DC voltage level. I was looking at those specs a few weeks ago as I wanted to raise the DC supply level above what I was getting from a choke input supply - and adding 2-3uF before the choke gave me the boost I needed, but the cap has to cope with very large AC voltage level (not something for an electrolytic).

[R.G.]

One of my favorite semi-serious topics is the Immortal Amp. I was quite taken with the idea of replacing electros which will certainly rot away over time with film types that have no definite internal decay process, at least on a human lifetime range. I thought that film motor run caps would do it, and experimentation has showed that they do. DC link caps were not available as such, at least not easily, back when I was speculating on this. The film DC link caps would do the job nicely, perhaps better than motor run caps.

One thing this opens the door to is to use additional resistors and inductors in series with the high-quality film capacitance to dirty them down and emulate old, dying, grotty electros. Switchably.