In spice I have these waveforms across the capacitor:
Voltage: 5Vpp 100 Khz triangle wave on 270VDC Current: 2App (swings -1A to +1A.) square wave at 100Khz. Capacitance: 1uF Max. Ambient temp: 30C
Would a polypropylene film or electrolytic be better? There's lots of spare PCB space..I prefer picking the longer life component..
I've read that heat ages electrolytics and one way to heat up an electrolytic is with lots of ripple current to make I^2R heat.
When I looked at the polypropylene data sheets
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I found AC vs frequency derating graphs. What is this derating based on?? Heat? Material breakdown? My app only has 5Vpp of AC and so a 400VDC pp film cap falls into the safety zone at 100Khz.. But will it outlast an electrolytic? How do I figure out if this cap gets toasty from the pulse current? Do I look at the tangent of loss angle spec? D from BC
For relative low frequencies, the maximum AC voltage is limited because of internal corona discharge in air pockets, that may erode the dielectric. At high frequencies, heat generation inside the dielectric is the limiting factor. At even higher frequencies, the limiting factor is current carrying capability of the contacts. Mostly the graphs are based on 10 degrees internal temperature rise.
You also have to derate when the temperature will be higher (for example because of heat generated by nearby components.
Electrolytic capacitors. You need a big cap (several 100 uF to mF) to handle a ripple current of about 600mA rms. The larger the size, the higher the ripple current capability. Ripple current is mostly based on 5..10 degrees temperature rise with respect to ambient. Same current at lower temperature gives somewhat higher temperature rise because of higher ESR at low temperature. The life time is specified at given temperature, rated voltage and rated ripple current (I thought).
When you stay below the maximum ripple current, you gain a factor 2 for every 10 degrease lower temperature then specified in the data sheet. Like with the foil capacitor, check the actual temperature of the electrolytic capacitor. When you use them according to the specification, they have a predictable lifetime. Life time can be shortened significantly by wrong treatment (for example certain chemicals for cleaning). Most manufacturers have good information on how to calculate the life time based on ripple current, frequency, ambient temperature and voltage and how to treat their capacitors (for example ELNA, Nichicon, Panasonic).
Do you also check your design for inrush current and overload situations? Standard or SMPS capacitors are not made for fast discharging.
When the value of the capacitor is of importance, a foil capacitor is a good option.
Thanks.. Especially the bit about film cap derating graphs are based on a 10 degree temp rise...I didn't know that.... If needed, I'll check out the capacitor manufacture sites for additional reading on capacitor specifications.. D from BC
Polyethylene and Polypropylene have the lowest maximum operating temperature, but lowest dielectric losses and best capacitance versus temperature stability.
Polyester can be used up to 125 degr (derate), and have higher loss. Polyphenylene has highest operating temperature and moderate loss. Polycabonate is not that popular anymore (but it is not a bad dielectric).
You should check the datasheet. Also you should be aware of the construction. Wounded capacitors have some higher inductance then "stacked" capacitors. I had it once in a half bridged semi resonant converter. Most of the ringing was caused by the (wounded technology) capacitor.
Epcos has a general document on foil capacitors that may help you choose the best capacitor.
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