[Diy_efi] MC3334

[Les Wieckowski]

Hi Walter !
200 ohms is too much. Unfortunately the chip is unlikely to be able to drive
anything lower.
  The problem is that the mosfet has a finite gate capacitance which is
taking some time to charge up, hence the slow switch on.
  The solution is to use a complementary emmitter follower to drive the
mosfet gate. Use an NPN & PNP transistor with their bases connected together
& to the chip output. Substitute the 200 ohm pullup resistor on the chip
output pin with one of 1K value. Connect both emitters to the mosfet gate.
Connect NPN & PNP collectors to +12V & 0V respectively.
  This should charge & discharge the mosfet gate capacitance almost
instantaneously by eliminating the resistor time constant. Use transistors
with 2A collector current & VCE of at least 30V & hfe of 200+.
Best regards
                       Les
----- Original Message -----
From: Walter Javier Markiw <wm_ignition at hotmail.com>
To: <diy_efi at diy-efi.org>
Sent: 12 September 2002 17:57
Subject: [Diy_efi] MC3334


>
>
> I am currently working on a very simple ignition module based on the
> MC3334 High Energy Ignition Coil Driver,from Motorola.This is a
> rather old chip,in fact,Motorola has taken the datasheet away from its
> intranet,but I know for sure that the chip is still in production.They
sent
> me the datasheet.
> I am driving a 4 mH primary ignition coil with a IRF740 mosfet on an
> aluminium heatsink big enough to fit on one side of the distributor,and
> 50% dwell time from an inductive 50 Hz signal (which represents 750 rpm
> in a 4 cylinder engine).
> The mosfet is getting too hot after 3 minutes of switching.
> Current peak : 5,3 A
> Reverse voltage peak : 320 V
> Dwell time : 51 %
> Captor inductive : 5o Hz sinusoidal signal.
> The IRF740 is set in conduction through a 200 ohms 5 watts resistor
> from a 12 Volt bat.,and the MC3334 pulls the gate down to shut it off
> at the end of the dwell time(roughly 10 ms at 50 Hz),to induce the
magnetic
> field on the secondary winding of the coil,connected to the plug which
will
> produce the spark,and keeps it shut off untill the next
> captor signall is received.
> But a IRF740 drain scope read shows that what should be a fairly flat line
> (or a mild upward slope to 0,2 Volts)at ground level for the first couple
of
> milliseconds of the dwell time,shows an upward slope from ground to 1,5
> volts,which I believe springs from the fact that the IRF740 is not in full
> conduction,therefore,getting hot.
> Has anyone built a project on this chip?
> I will appreciate any help I gan get.
> Thanks in advance.
>
>
>
> _________________________________________________________________
> Chat with friends online, try MSN Messenger: http://messenger.msn.com
>
>
> _______________________________________________
> Diy_efi mailing list
> Diy_efi at diy-efi.org
> http://www.diy-efi.org/mailman/listinfo/diy_efi


_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi