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Subject: Re: MOSFET overvoltage protection
Date: Tue, 08 Oct 2002 04:29:00 +0100
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"Bill Bowden" <**email@example.com*> wrote:
>Bob Wilson wrote in message
>> In article ,
>> **firstname.lastname@example.org* says...
>> >to drive an automotive ignition coil (about 6 millihenrys,
>> >1.5 ohms, and charged to 50 mJ).
>Ok, I added a 12 volt zener across the gate (NTE5127A) and a 200 volt,
>1 watt zener across the drain/source (NTE5105A) but I'm not sure of the
>power rating for the drain zener. The peak coil current is 4 amps, and when
>MOSFET turns off, the same current will be diverted into the zener with
>about 4*200 = 800 watts peak dissipation. The rep rate is 200 per second
>and DC input power is about 10 watts but I don't know what the efficiency
>Do you have an idea how much power the zener will need to dissipate?
You said it yourself - 50mJ at 200Hz = 10W.
If you are really getting 50mJ at 200Hz into the coil and the secondary is
unloaded then it all goes into the zener (less a bit into the MOSFET during
the turn off transient - dependant mostly on turn off speed).
Presumably you are trying to transfer all the energy into whatever is
loading the secondary. Anything going into the zener is a waste so you may
as well reduce the stored energy to whatever the secondary load can take.
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