Thanks to all who replied for this timely information.
 
I'm currently researching suitable switching devices
and techniques for an electronic ignition system for
my old motorcycle, PIC driven of course :)
 
There are some interesting IGBTs available specifically
designed for ignition applications (Fairchild & IR).
 
Regards,
 
Nigel Duckworth
 
 
 
-------Original Message-------
 
From: Russell McMahon
Date: 03/28/05 04:09:39
To: Microcontroller discussion list - Public.
Subject: Re: [EE]: MOSFet Troubles
 
 
> I'm a little out of my depth here
 
But you are still completel;y correct :-).
 
 
> but isn't the FET
> body diode in the wrong place to suppress the
> back EMF from the coil?
 
Yes. Wrong polarity.
 
> I thought current needed to circulate within the coil
> at switch-off which is the reason for the reverse
> biased diode commonly seen across relay coils etc?
 
Yes, and ...
 
> And assuming the "automotive coil" referred to
> by the OP is an ignition circuit then is
> suppressing the back EMF something
> you want to be doing anyway?
 
No - not if the plug fires correctly.
But, YES if it fails to fire.
 
In an ignition circuit you are relying on the "flyback" EMF to
generate the spark.
The primary will rise to a high voltage, and the secondary to a
proportionally higher voltage.
 
IF the spark plug fires then it will clamp the primary to a lower
voltage based on plug voltage and transformer turns ratio (and other
2nd order factors). If the plug does NOT fire then primary bvoltage
may rise VERY high. In an automotove circuit you MUST design for
somethng to dissipate the energy when the plug mis-fires. This may be
an auxilliary sparkgap, a zener (as Dave mentions) or the FET can be a
type designed to take repetitive avalanche energy. The energy is
usually severe and few FETs would stand it indefinitely. The FET MUSt
be specifically avalanche rated if used this way - ones that aren't
will conduct unevenly across the die and fry parts of thermselves
which leads to total failure.
 
The failed FET has probably been destroyed by avalanche energy. What
precautions are taken in the circuit to prevent this?
 
 
 
RM
 
 
 
 
>
>
>
> -------Original Message-------
>
> From: Herbert Graf
> Date: 03/27/05 22:28:48
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE]: MOSFet Troubles
>
>
> On Sun, 2005-03-27 at 12:34 -0500, Peter Johansson wrote:
>> Milosz Kardasinski writes:
>>
>> > The fet has a built in diode, is this diode insufficient?
>>
>> From what I've been able to gather, the internal protection diodes
>> are
>> not sufficient for highly inductive loads.
>
> Something I'd like to add is that the "diode" you see when it comes
> to
> FETs isn't something the manufacturer ADDED to the device. The
> "diode" (often referred to as the body diode) is a byproduct of how
> a
> FET is structured (basically if you have a FET you have a reverse
> biased
> diode as well). This is similar to how most CMOS structures have an
> SCR
> buried in the structure, on that can be activated if certain bad
> things
> happen (called "latchup" by many people). As such it's sometimes
> just an
> afterthought and in many datasheets isn't even indicated as present.
>
> The fact that IR included it in the datasheet makes me think that
> they
> have taken some care into designing the structure of the diode so
> that
> it can substitute for the external diode normally present (for
> example
> when using BJTs with inductive loads) in SOME cases. I don't believe
> that IR has ensured that the body diode is structured in such a way
> that
> it should be able to substitute for ALL cases (especially highly
> inductive loads).
>
> Generally, unless I have a VERY good reason (cost, space, etc.) I
> ALWAYS
> include an external diode, it just makes me more comfortable that
> things
> will behave the way I expect them to.
>
> TTYL
>
>
> -----------------------------
> Herbert's PIC Stuff:
> http://repatch.dyndns.org:8383/pic_stuff/
>
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