Need help with MOSFET interfacing

Hi folks. Kindof newbie with electronics at the component level, def
not an engineer.

I have a 5-volt pulse, from an Atmel 2313, running about 5KHz at
around a 60 percent duty cycle, that I am using to drive a heavy duty
auto ignition coil, currently (no pun intended) at about 28 Volts DC.
I have good sucess obtaining the quality of Plasma discharge cloud
that I need, albeit short sucess, using a Darlington NTE262 PNP
transistor. Short sucess, because it can't handle the amperage for
any real length of time. So I want to move to MOSFETs, but directly
driving the MSOFET from the 5-volt signal doesn't seem to drive it
into full conduction. Do I need to add a intermediate stage to drive
the MOSFET at a higher voltage than 5 volts? A simple transistor or
opto isolator perhaps? Thanks for any input.

Keith

 

Hi Keith,

Do a search for "logic level mosfet". I got 725 matches.

Jay

 

Hey! That's what I need. Thanks for the tip Jay. I also
found a super device from Phillips which is a logic level mosfet with
protection and feedback outputs in a 5-pin unit. Nobody seems to have
them though, and Digikey says they'd be $35 if they did.

Keith

 

These documents may be of some use:

http://focus.ti.com/lit/ml/slup169/slup169.pdf

http://www.irf.com/technical-info/appnotes/an-937.pdf

I've never played with automotive ignition coils but my understanding of
them is they are essentially flyback transformers. The primary current
ramps up to several amps (often limited with an external resistor for
running off of a 12V automotive battery) and then the primary switch turns
off. The stored energy then produces a very large voltage since it has
nowhere to go. This appears as both high voltage on the primary and high
voltage on the secondary. Eventually the voltage gets so high it normally
is supposed to arc over on the secondary, thus discharging the stored
energy.

My understanding of electronic automotive ignition systems is they usually
use either a high voltage bipolar junction transistor, a relatively high
voltage MOSFET, or sometimes an IGBT.

For IGBTs the IRGB14C40L is specifically designed for this kind of use:

http://www.irf.com/product-info/datasheets/data/irgs14c40l.pdf

For MOSFETs devices like the IRF740 (this is a relatively obsolete part, use
the IRF740A instead) are sometimes used:

http://www.irf.com/product-info/datasheets/data/irf740a.pdf

This is a 400V device. If you use a lower voltage rated part the device
will likely avalanche at a low voltage and the net result is your maximum
attainable output voltage will be substantially less than possible. Some
MOSFETs are sufficiently avalanche rugged they do not absolutely require
external voltage clamping circuitry.

 

Oh yeah. Some practical notes for this device. Digikey has them in stock
for $3.75 for a single unit or $2.63 each for ten. I believe in normal use
this device normally expects to see a gate resistor of around 1k. This is a
logic level device, so it is quite likely all you need to interface this
device with your microcontroller is a 1k resistor.

I recommend using this device. A relatively small heatsink is probably
required depending upon your circuit's usage requirements.

 

Hi Fritz,

Automotive ignition coils are 6 volt for starting. The ballast resister
drops the 12 volts from the battery to 6 volts for running. The resister is
jumped out for starting because the motor starter pulls the battery voltage
down so much

Jay.

 

Thanks folks. Good info all around.

Keith