From: Boris Mohar
Subject: Re: Current source circuit to drive high speed solenoid
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Date: Mon, 02 Sep 2002 07:30:04 -0400
NNTP-Posting-Date: Mon, 02 Sep 2002 07:31:28 EDT
Organization: Bell Sympatico
On Sun, 01 Sep 2002 11:31:01 GMT, "Spehro Pefhany"
>The renowned Frank Bemelman wrote:
>> For me, this is rather surprising. I would have thought the BFC
>> would swallow the flyback, which it seems to do with a transistor,
>> but not with the Fet.
>There's no clamp on the node with the inductor connected to the MOSFET
>drain. When the MOSFET turns off, that voltage goes up .. and up.. either
>until the parasitic capacitance in the solenoid takes care of the energy
>in the inductance (modified by whatever stuff happens when the core moves)
>or until the transistor breaks down. If the energy in the solenoid is low
>enough, this may be acceptable, if not, you have to put a clamp of some
>kind on there.
>For maximum speed of operation of the solenoid, the voltage should be
>allowed to go as high as possible.
If it is for a fuel injector than allowing the flyback to fly high is
mandatory, else th injectors will stay open for way too long. Messes up
high rpm operation as I discovered to my embarrassment.