Considering the typical saturation voltages of >1V it doesn't look attractive yet they are popular:
Why?
There are plenty of cheap MOSFETs that are in the low tens of milliohms and thus show only a fraction of the losses while the coil current ramps up. You'd just have to add a zener from drain to gate in case the spark plug goes bad, the wire comes off or the controller lets the dwell time run out of hand. Plus some other gate protection but that's no problem.
-- Regards, Joerg http://www.analogconsultants.com/
4V COLD Start requirement I'd guess. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Thinking outside the box... producing elegant solutions,
by understanding what nature is hiding.
"It is not in doing what you like, but in liking what you do that
is the secret of happiness." -James Barrie
Probably small die = cheap.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
But losing >1V from 4V makes it proportionately even worse?
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
At what Vds(max)? ;-)
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: https://www.seventransistorlabs.com/
That could be the reason though a gate voltage booster isn't that tricky.
It does, that would be a wimpy spark or no spark.
-- Regards, Joerg http://www.analogconsultants.com/
From what I have seen IGBT in similar power classes aren't smaller in the die than FETs.
-- Regards, Joerg http://www.analogconsultants.com/
You don't need much above 400V for an ignition. Easy to do with either kind.
-- Regards, Joerg http://www.analogconsultants.com/
350V/5A is the general requirement, so I'd probably margin a wee-bit higher than 400V, maybe 600V.
I don't think you can get NMOSPower like that with logic level gate voltages.
Personally I've done it with Delco bipolar's where beta was only 3 at IC=5A ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Thinking outside the box... producing elegant solutions,
by understanding what nature is hiding.
"It is not in doing what you like, but in liking what you do that
is the secret of happiness." -James Barrie
ind.
most ignition igbt have an active clamp around 350-400V to protect the coil if there's a loose plug wire
I'd guess the answer is ruggedness. Automotive engineering is one of the most conservative around. Possibly once in the past someone did use a MOSFET and it failed and they've been averse ever since. Modern avalanche rated FETs may be getting tough enough but auto designers may feel comfortable with a solution they know and trust?
piglet
I don't do "most", I roll my own, based on "CD-Ignition-Basic.pdf" on the S.E.D/Schematics Page of my website. I was running experimental Ford Thunderbirds with "cruise control" back in the late '60's using that ignition (at Philco-Ford, Santa Clara; later embellished with multi-firing while I was at Dickson Electronics, Scottsdale). Be a piece-a-cake now with an appropriate NMOS. And it doesn't need a plug to survive... might toast the coil, though >:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Thinking outside the box... producing elegant solutions,
by understanding what nature is hiding.
"It is not in doing what you like, but in liking what you do that
is the secret of happiness." -James Barrie
il
r kind.
oil
protecting the coil is the point of the clamp
Aggressive marketing, obviously. Take the typical IROC (right-out-of-college) engineer, assign it to make an ignition, and Google will pull him into a maze of IGBT datasheets.
I'm fond of the old Mark10 design (Heathkit, too) with capacitive discharge, which works fine with forty-year-old SCRs.
My car has a potted mystery module and was designed with a LONG harness connection to the coil and reluctor sensor.
There was no active intelligence used in the parts of the design that I can see.
Actually I was thinking dissipation in the coil primary with no load. My scheme will apply 350V to the primary, period, secondary loaded or not; and the switch device only sees 350V, irrespective of load. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Thinking outside the box... producing elegant solutions,
by understanding what nature is hiding.
"It is not in doing what you like, but in liking what you do that
is the secret of happiness." -James Barrie
I'd do that as well although the usual zener from collector to base or drain to gate will rein in anything that wants to go past zener voltage plus Vbe or gate threshold.
There also needs to be a current limit for coil protection against melt-down.
That is what IIRC most capacitive discharge ignition circuits do. AFAIK that comes at the price of a shorter spark versus inductive.
-- Regards, Joerg http://www.analogconsultants.com/
Thanks, I have read papers like that. On of the things that puzzles me is evident in figure 10, over 1V in Vcesat. A FET can run circles around that and run with much less cooling, meaning an overall smaller module, less weight.
-- Regards, Joerg http://www.analogconsultants.com/
I remember the days when IGBTs came out and lots of less experienced used them for just about anything. Their designs worked but were expensive in production.
My Citroen had a really cool system. The coil's secondary fed both plugs in its two-cylinder engine (wasted spark method) which meant no distributor. It also meant no distributor could go bad. However, it could make for interesting effects when hand-cranking it on a cold day. You had to make sure no white or particularly clean car war parked behind and nobody stood near the exhaust. Once the bang was so bad that the 2nd muffler pot fell off and started rolling down the street. Embarrassing.
-- Regards, Joerg http://www.analogconsultants.com/
Yep. That's why I couldn't sell it to Ford, even with my later embellishment, multi-fire. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
Thinking outside the box... producing elegant solutions, by understanding what nature is hiding.
"It is not in doing what you like, but in liking what you do that is the secret of happiness." -James Barrie
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