Electronics > Projects, Designs, and Technical Stuff
Overloading Film capacitors with ripple current, how well they tolerate it?
<< < (2/2)
Weston:
When using a film capacitor as a filter capacitor the reactive power is going to be low. In that situation most of the losses will occur in the electrodes, which is a function of the construction of the capacitor, and not (tan(d)) of the dielectric.

I spent a while a few months back looking at the ripple current rating of capacitors to minimize the cost of a resonant capacitor bank and got as far as testing some capacitors under a thermal camera with a big RF PA. The current ratings manufactures provide are typically an incoherent mess. The datasheet current ratings are typically given for a 10C temperature rise of the case over ambient for all case sizes. Given that you care about the internal hotspot temperature and larger capacitors have a lower surface area to volume ratio and a larger distance from the internal hotspot to the case, this implies that larger capacitors will have a higher internal temperature for the same 10C temperature rise of the case. At some high dv/dv you can destroy the electrodes, but for DC link applications you are basically going to be entirely limited by thermal dissipation which is a function of I^2.

Manufacturers also sometimes provide thermal derating curves which you can use to back out the thermal resistance from the hotspot to the case. If the derating curve stops at an ambient temperature above what you will operate at you can extrapolate it out and use it as a "over rating" curve.

I tested the B32642 series capacitors, which have lower capacitance than you want for DC link decoupling and are much smaller parts (so have a lower thermal resistance from the hotspot to the case) and found that I could run them at over 2x the rated current with no measurable loss in capacitance.

Most of the provided capacitor ratings also do not assume forced air cooling, you could also probably push the capacitors harder with forced air cooling.
Miyuki:
Outptu have about 30A ripple at resulting 150kHz at low voltages
At high input voltage it can have duty cycle shorter than 1/3 and input caps will see full ramps 0-70A what will get again 30A rms

I dont want to use electrolytic at least at input side to avoid huge input capacity to have possibility to measure dynamical load
Power stages are peak current controlled by MCU directly and I want to have just two stage LC filter at input and as small as possible to keep load appear resistive and not to affect measured source behavior

I also choose this topology to have simple control over it from software, I am not able to do this with resonant topology
Conrad Hoffman:
Find out what induction heater people use. I built a very small one and was amazed at how easily it heated film capacitors. Polyester was useless, only those Illinois parts did well.
T3sl4co1l:
Incidentally, induction heating caps are almost all metallized film, AFAIK.  Seems to be not so much the material (give or take how much metallization is applied, or whatever), but how it's put together (a floating electrode design is common IIRC).  Stonking great copper plates to sink the heat out of them is an obvious advantage.

The ratings aren't usually very useful for filtering, though.  2.4uF is a common "large" value.  You want 10-100 times that for filtering DC, even more at these low voltages.

You can get industrial caps in such values, with conduction or water cooling, but they're even more specialty, and you'll want to be sitting down before you see the price and availability / lead times...


--- Quote from: Miyuki on February 07, 2020, 05:47:11 pm ---Outptu have about 30A ripple at resulting 150kHz at low voltages
At high input voltage it can have duty cycle shorter than 1/3 and input caps will see full ramps 0-70A what will get again 30A rms

I dont want to use electrolytic at least at input side to avoid huge input capacity to have possibility to measure dynamical load
Power stages are peak current controlled by MCU directly and I want to have just two stage LC filter at input and as small as possible to keep load appear resistive and not to affect measured source behavior

--- End quote ---

What is this, DCM?  Yikes and a half!  At least do CCM.  For which, average current mode is easier: the filter takes up control loop bandwidth so your digital control is less strict.

Best case, the analog side is passively stable and safe: even if the MCU crashes, it's just stuck at whatever setpoint it was last given.  Can even add a missing pulse detector as a watchdog.

I don't know how "small as possible" you're expecting, as it sounds like you're planning on making it pretty big.  You need some pretty huge, ferrite cored inductors to handle that much current ripple, and both chokes and caps are huge at this frequency.  Small would be 200-1000kHz, SiC MOSFETs and schottkys, resonant as an option, phase interleave is still good, uhh ferrite cored chokes of course, maybe some beefy ceramic caps if cost is no object, but otherwise film caps will be fine.



--- Quote ---I also choose this topology to have simple control over it from software, I am not able to do this with resonant topology

--- End quote ---

Resonant LLC is typically controlled by frequency modulation, and on/off keying at light loads or low output voltages.  Protection/limiting is often similar, i.e. peak current mode.  Even easier from an MCU, I suppose...

It's a rather mysterious and opaque topology on first look, but it turns out a surprisingly crude control works out pretty well.  The real magic seems to be correctly choosing L, L and C with respect to Fmin and Fmax, and source and load ranges; the source range at least is pretty narrow.  Or, I think, one or the other is narrow, since the output range is actually quite flexible, and reciprocity is a thing.  But you can't have both flexible, because the impedance of at least one side (and therefore the available current flow at a given voltage) is set by the tank impedance.

Tim
Miyuki:
I want it as universal as possible to have input voltage from basically zero to 400V
And small inductor about 10uH are still reasonably small at currents up to 100A
And also it can keep switching losses low even with moderate speed Si diode with low Uf

"Just" that capacitors issue
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod