[Diy_efi] Flyback Diode (was 555 timer...)

[Mike]

At 09:54 PM 7/11/05, you wrote:
>You can't drive it too hard unless you take the gate outside the supplied
>voltage of the source and drain.  To the contrary, the greater the gate
>voltage (N-chan) the more 'on' the device (lower resistance and voltage
>drop.)

The key issue here is the drive capacity in general, gate voltage is one
thing but if you expect to provide best operation then the gate drive should
be from a low impedance source so there is less chance of parasitic
oscillation and the gate drive is commensurate with EMC issues. ie
Not to fast to cause oscillation or EMC problems... So attend to gate
drive like resistors and ferrite beads etc

Over doing gate by even a few volts for a short period is enough to fully
destroy FETs and this means that given the almost infinite impedance
of gates, suggest the slightest overvoltage from handling or badly grounded
wrist straps is enough to destroy FETs some long time *before* you get them
into the circuit to provide some useful function - well other that make the
semi companies richer than they already are...


Regards from


Mike Massen
Perth, Western Australia
VL Commodore Fuse Rail that wont warp or melt !
Twin Tyres etc
http://niche.iinet.net.au







>dh
>
>----- Original Message ----- 
>From: "Paul Bidmead" <bidmeadp at iinet.net.au>
>To: <diy_efi at diy-efi.org>
>Sent: Monday, July 11, 2005 12:40 AM
>Subject: Re: [Diy_efi] Flyback Diode (was 555 timer...)
>
>
>> beg my pardons....... i am using a different FET after i cooked a few
>'640s
>> from a low R injector on my gen-set
>> i then went to ecuv2.0 with the surface mount FET rated at 60v 40amps and
>a
>> 2" sqare heatsink i havnt blown one up yet (touch wood) but it doesnt get
>> hot.
>> im not sure if im driving it too hard but it "seems" not to mind
>>
>> //BidSpec
>>
>> ----- Original Message ----- 
>> From: "mike turner" <yardcooliediy at charter.net>
>> To: <diy_efi at diy-efi.org>
>> Sent: Saturday, July 09, 2005 6:11 AM
>> Subject: Re: [Diy_efi] Flyback Diode (was 555 timer...)
>>
>>
>> > The integral diode is generally slower than the external diode. This
>> > raises its dissipation above that of a faster diode. Also, using an
>> > external diode moves that dissipation off the mosfet which might be
>needed
>> > if you have omitted the mosfet heat sink for the bench test. :)
>> >
>> > Mike Turner