[Diy_efi] Inbuilt mosfet zener to clamp injectors

[Mike]

Hi,

In my humble experience, a mosfet's internal flyback diode might look
like a zener but its not at all reliable, maybe you need a snubber
network, wy not just use a diode across the coil instead of a zener,

rgds

mike


At 11:25 PM 21/10/2002 +1300, you wrote:
>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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