[Diy_efi] MC3334

[Walter Javier Markiw]

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.



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