50V Discharge Magnetizer issues

[The Electrician]

I wish he could post a decent schematic with P/N's and photos.

You are correct "decent" caps are rated in ESR and will be below 20 mOhms for this size. These cost > $40. The ones must be the cheapest types eg $25 which have no ESR rating but have a rated ripple current at 85'C. Which ought to give you some insight in future.

If they rate only ripple current, the ESR is not the best.


My guess is these are what was used



470000 uF
100 × 250 mm
33.6 Amps rms
0.60 tan delta
5.00 mA leakage
LNR1J474MSE part number

In any case the high ESR will save his FETs from burnout after Vgs is fixed.

Note 190A Pk is much lower RMS , but there will be some max current for any capacitor for safety reasons,and they should be in a vented strong enclosure if you don't like surprises. Boom.

Since the Caps ESR and L ESR are dominant, the only way to increase current is Lower ESR caps with a controlled external R to limit current.

Get a few of these.

https://www.digikey.com/product-detail/en/ALS30A473MF063/399-11419-ND/4833250


peak current will be based on Ohms Law for ESR.

The ESR of the Nichicons you referenced can be calculated from the tanδ. The result I get is 17 milliohms. Not as good as the 8 milliohms of the Kemet, but a lot better than the 168 milliohms and 175 milliohms he measured.

Note that the ripple current rating of the Kemet is less than the Nichicon. The Nichicons aren't all that bad.

Either his caps have gone bad, or he hasn't measured ESR correctly.

 

[D.A.(Tony)Stewart]

ESR from 120Hz is usually lower than at 10KHz from ESL.

If he's getting 300A accurately, big assumption, 50V would indicate total ESR of 167 mOhms which if I correct my previous assumptions are Lesr + Cesr = 91 + 171/2= 176 which is pretty close now. Neglect FETs which are ~2 or less.

I guess they got caps cheaper than Nichicons..

Go for 0.2Farads 30 milliOhms

 

[The Electrician]

The ESL of electrolytics like that is around ten to twenty nanohenries, if measured right at the terminals.

He's got a 10 uH load, that plus wiring inductance totally swamps out the ESL of the caps.

I suspect that the resistance of the inductors for fast pulses is greatly increased by skin effect. I also wonder how he measured the ESR of the inductor.

It would be nice to see a photo of the layout.