Transistor Recommendation Please

Hah, forget NTE, go with the big boys and save money. You can get samples in many cases to experiment with. As for the FDH44N50, yes that's a BIG part, with a BIG die, as evidenced by its massive 750W power rating. This also means it has high capacitance (i.e. Ciss = 5335pF), which creates special problems, not to be taken lightly! It requires a high 108nC of gate charge. Anyway, if you're still interested, Mouser has them in stock, $16.62 each.

Now, the FDH44N50 is spec'd as a 44A 500V part. I'd guess that your application does not require faster than 50 - 100ns switching speeds. For voltages above 200V, you'd be better off with IGBTs. I've found that the IGBT-manufacturer crowd prefers to make 600V rather than 500V parts. You can get *really* big ones, like the IXYS IXGN200N60, a 200-amp part. Anyway, consider Fairchild's FGH50N6S2. Even though it's rated higher at 50A, it only requires 85nC of gate charge, and it's cheaper, $12.36 at Mouser. (The bad news is Mouser doesn't have the "D" version in stock. When switching high currents it's useful to have a stealth diode onboard.) I suggest you learn about IGBTs.

Is this a coil gun application as it sounds like one? If it is (or someting similar) round metal objects make extremely poor projectiles, basically due to minimum amount of flux lines thru their short length. Much better to go with long skinny metal objects (dry wall screws, etc) that maximise the flux).

Have you considered an SCR. The drive circuitry is a bit more involved but they are more readily available in the voltage and current ratings you require.

Craig

-- One of the few former EE students that actually could solder

Ouch.

Thank you very much. I've been looking thru the datasheet for the Fairchild FGH50N6S2, and it appears to be designed for this very type of application. But, being a self-taught hobbyist, I'm pretty weak when it comes to some (allot/all) of this stuff, so I have some stupid questions for you.

Thanks again for all your help.

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Pretty much. I actually have an application in mind for a form of kinetic art where ball bearings will be required since they will need to be rolling. That is if I don't kill myself first. ;-) I won't need enough propulsion to kill anyone, just enough to kick the ball into the air a few feet.

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OK, that pretty much makes sense. Thanks for the tip.

involved

I guess the problem with that would be in getting it turned back off after switching it on. My circuit works allot like a boosting SMPS in that an inductor is rapidly switched to ground as the voltage across a capacitor is built up. The only form of regulation though is by feedback from a voltage divider to tell the PIC to stop the PWM signal. I will need an SCR to dump the charge in the capacitor to the coil though.

What, that's a good price for something with so much silicon. The question is if it's overkill for your needs?

Whoa!! The threshold spec is where the current just starts to flow, did you note the IC = 250µA parameter? You'll need a serious gate voltage, did you see that VGE = 15V is used in most of the specs.

Furthermore, in general, you should rapidly switch the gate voltage, so the IGBT is either on, or off, spending a minimum time in between. Note the IGBT's 85nC gate charge, which comes from its nearly 3nF of gate capacitance. It takes I = Q/t = 1.7A of gate current to switch the gate in say 50ns. You can't get 15V from a PIC, nor 1.7A either, so you'll need a gate driver, like Microchip's TC4423 dual 3A part.

The latter is what you get with the "D" IGBT versions.

The goal is to protect the IGBT from short transient spikes. You certainly can use an external diode if you wish for convenience.

Find a local pinball machine shop - some tech there will know about slinging steel balls around with electromagnets. :-)

Good Luck! Rich

In that case, here are a few links (plenty more on the internet) I have found that give some good theory and practical application covering small to large devices, as well as experimental data, circuits, components, etc.

May give you some ideas or point you in a slightly different direction (and save you some time), or at the very least a good learning experience.

Exercise extreme caution when working with your 820uF cap charged up to

Also when you talk about wanting a more locally obtainable MOSFET, what country are you in (as it is not clear)?

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milliseconds

It's way overkill I do believe. ;-)

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speeds.

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IXGN200N60,

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IGBTs.

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That's what I was afraid of.

OK, thanks for the pointer to the gate driver. I'm considering going this route since they're cheap. As you can clearly tell, I'm not much of an analog designer especially where high power/voltage are involved, but I am learning (slowly ;-). Micros and particulary software are much more my forte.

After spending some time digging around Fairchild's site, I've come accross a couple of "logic level" IGBTs. The ISL9V3040P3 (who makes these crazy part numbers up) is rated for 17A continuous and 430V in a TO-220 package. With a 5V gate drive, it only needs 17nC of charge. But I guess that means that I still need a gate driver since the PIC can only supply about 25mA, and I still need around 340mA or so. Is that right? That 430V voltage rating should be fine and I'm only planning on having > 2) As for a protection diode should I put one between the inductor

Ok thanks again, but which is the better way to do it? I'm thinking that paralleling with the transistor probably takes a less hardy/speedy diode. Is that correct?

Before you say it too, I know that I could "potentially" kill myself with this so I will be very careful. I intend to start out with much smaller voltages and work my way up until the smoke comes out of something. Hopefully that won't be me. ;-)

Now here's a really stupid question. Will I have problems soldering lead-free parts using "regular" solder?

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Cool, I'll check those out.

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using!

I know, but we only live once anyway. ;-)

I've been tinkering for a long time, but not with high voltage AND big caps. But I do know the danger.

It wouldn't be exactly the same thing, though it would be the same amount of energy AIUI. However, my experiments are going to be with the innards of a standard Ford fender mounted starter solenoid. They're strong and cheap, but they're not real long so I'm thinking that some voltage will be required to build the biggest magnet field in the shortest amount of time.

what

Texas ;-)