Resonant capacitors on Chinese ZVS drivers

[John123]

Sorry if this is a basic question but I'm a bit rusty after almost a year since messing with this stuff.

You know those ZVS drivers and induction heater kits you get on ebay, does anyone know what types of resonant capacitors they use on them? I know they're rated for high frequency and high RMS currents, but have forgotten the what they're called such as polypropylene and MKP etc. Having to trawl through datasheets and find out if pulse and resonant duty is mentioned is very slow.

The red resin dipped job I'm currently using is getting pretty warm even in a 30w switcher!

[Mads Barnkob]

You are correct. Polypropylene / MKP.

Many types of filtering capacitors are the same kind, but with reliable data on them. X1/X2 safety capacitors are most likely to be good enough, but properly higher quality pulse capacitor rejects or with higher losses.

Resonant capacitors for ZVS induction heater is close to identical to the ones used in Tesla coils, so you can use the Tesla coil MMC guide to find the correct ones.

[John123]

Thanks. Before putting the red resin dipped 400v film capacitor in I checked its ESR with my Chinese component tester and it came back as close to .00 ohms, it was just a surprise to see it heat up still. Average current from the 24v PSU is about 1.5 amps so still seems odd it heats at those power levels.

[davekni]

Capacitor heating is driven by RMS capacitor current.  For these ZVS circuits, capacitor current doesn't change with load.  It is determined by only voltage and frequency.  With a perfect high-Q coil, DC input current should be low (ideally zero except for gate resistor power).  If resonant Q is lowered mostly by the caps, then input power is mostly going to cap heating.  I've used this as a quick way to screen unknown caps - connect to a ZVS oscillator with a good high-Q coil and measure input current.

Most (but not all) of the red resin-dipped capacitors I have are mylar (polyester) dielectric, so high loss for ZVS use.

[dreamth]

The 100v cap , the bigger one at the right is the low loss one.It might be polypropylene foil or polystirene foil...i ave no idea but usually the low loss capacitors are the bigger ones.They are from a very high quality resonant Hitachi  power supply for high speed trains converters.

[John123]

Thanks Dave and Dreamth.

I think that "M" on those red caps are Panasonic "Matsushita" (yeah you read that right lol).

I came across a Kemet R75 which hasn't given up yet, my use is not quite as harsh as ZVS driver use.