Help selecting a MOSFET, please

guyfrom7up sent me this schematic for an ignition coil driver. I'm trying to determine if the MOSFET, IRFP450, could be substituted with any of the MOSFETs from Electronics Goldmine. The search doesn't create a link, search "mosfet" to get the list.

I know I could get it from somewhere else, but EGM has parts for a few other projects, and I like to combine shipping.

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NachoMahma9 years ago
. Don't fudge too much on the diodes/cap/resistor in parallel with the MOSFET. That's to absorb/shunt the primary spike when the field collapses. Same function as the condensor on a points-type ignition or the clamping diode on a relay.
CameronSS (author)  NachoMahma9 years ago
So would this work as a substitute for the BYQ28E? The datasheet seems to give similar specs, although it's not specifically designed for this purpose.

Also, would one of these work for the .01 μF 2kV capacitor in that little protection circuit? I'm assuming that a disc capacitor] would be too wimpy there.

Sorry to sound so clueless, I'm learning!
. I'd be a worried about the lower PIV handling of the 24xx's. 2402 = 100V, 2404 = 200V - well below the 500V of a 450. There's gonna be a heck of a spike when the field collapses in the coil. If this is a short-term project, it may not be a problem, but I'd go with something that has a PIV closer to 500V (or higher).
. As far as freq, at 6000RPM a V8 fires 400 times/sec. A 4-cyl @ 6000RPM = 200. &c. Any MOSFET should be able to handle that.
. As long as the working voltage is 2kV or above, you should be OK. Can't see a voltage rating for the "wimpy" cap.
. Keep in mind that's it's been at least ten years since I did any parts subbing. Verify all this.
CameronSS (author)  NachoMahma9 years ago
I found an old microwave capacitor-.65μF, rated 2100V. It tests about .01μF from the capacitance meter, but it has an internal resistor of unknown value, and a search for the model number turned up only a Chinese porn site. Hopefully it'll work. Rather than messing with a TO-220-type rectifier, I think I'll just use two of the largest diodes I have.
The internal resistor has resistance up to 10M. The resistor is used to bleed the dangerous charge from the capacitor.
. Not sure what to tell you about the cap. For intermittent use, it will probably work. IIRC, the resistor is just to discharge to cap for safety purposes - should be OK to remove it. . I don't think "standard" diodes will give enough protection. . . What are you doing with this circuit? If it will only be used intermittently for a short-term project, protection may not be a big deal. For a car ignition, it will be much more important.
CameronSS (author)  NachoMahma9 years ago
. Ah! I'd try to keep the switch off as much as possible and keep a few spare MOSFETs on the shelf. If you start burning out 'FETs, you know where to look. ;)
Plasmana9 years ago
IRFP450 are quite expensive as well... I would use an IRF540 for this circuit, those types of MOSFET's are easy you get and not too expensive. I always use an IRF540 MOSFET for all of my high voltage generators.
CameronSS (author)  Plasmana9 years ago
How about the STW18NB40? The datasheet seems to show it having better specs than the IRFP450, but available and 79 cents a pop.
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