[Diy_efi] Inbuilt mosfet zener to clamp injectors

[Bevan Weiss]

What's the maximum power dissipation of the package??

It would be a better idea to use a seperate zener diode, it makes it easier
to repair if the zener gets destroyed (just replace the cheaper zener), also
easier to faultfind (the zener is discoloured as opposed to the FET which
may not show damage, yet)

I'm sure that if you look into some manufacturers websites you will be able
to find some other packaging than a TO220, I think that IRF (International
Rectifiers) may have some in their D-PAC (sp??) packaging, which is SMT.

In what way can the PCB not handle an additional TO220?

----- Original Message -----
From: "Tony Bryant" <brd at paradise.net.nz>
To: <diy_efi at diy-efi.org>
Sent: Monday, October 21, 2002 11:25 PM
Subject: [Diy_efi] Inbuilt mosfet zener to clamp injectors


> I'm having power dissapation problems with my LM1949-like injector driver
> circuit, notably in the zener.
>
> I'm driving 3 injectors per channel, which judging by the 'scope traces
are
> around 12mH each (Early '80s JECS/Nissan injectors).
>
> What this means is the clamp zener has to suck around 9W  @ 7000RPM, which
is
> getting to be quite a serious diode.
>
> My PCB has no means to take a TO-220 packaged zener that could take this
load,
> but the drivers are 80V mosfets, which means if I simply throw away the
zener
> and rely on the mosfet's inbuilt zener,  I transfer the 9W to the mosfet,
> which when added to it's 15W normal load, yields something it can deal
with.
>
> The question is:
>
> How dodgy is it relying on the MOSFET's inbuilt zener? Am I asking for
> trouble?
>
>
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