Injector Driver Module

[Don.F.Broadus at ucm.com]

I'm working on an  injector driving module also. If you are going to fire
the injectors sequentially, then one injector would be on at a time in the
firing order sequence (1-8-4-3-6-5-7-2 GM V-8)  If you are going to batch
fire the injectors
Then only 4 would be on at a time.  The factory heat sinks on the driver
transistors are not that big so I doubt that the power dissipated is very
high.  The duty cycle is a combination of on time and off time, so even at
85% the injector would
Only be on for less than half of the time. The frequency of the injector
pulse needs to be known, at 6000 rpm the ignition
 pulses are 400 HZ. I would guess that the injector pulses would be the same
frequency for sequential firing and half of that for batch firing.  So far
my bench setup is a batch fire with  2msec on time at idle  and 10 msec at
wot. Any advice would be appreciated.

 
Thanks Don


	-----Original Message-----
	From:	Jose  Rodriguez [SMTP:JRodriguez at impcotechnologies.com]
	Sent:	Thursday, August 06, 1998 11:12 AM
	To:	DIY_EFI at efi332.eng.ohio-state.edu
	Subject:	Injector Driver Module

	  I am designing an injector driver module. I would like your input,
to see if my calculations are correct, particularly in my assumption that
all eight injectors could be on at the same time, for up to 85% duty cycle.
Thanks in advance for your answers!

	  The current specs I have are:

	  Eight injectors.
	  Injector coil resistance: 1.5 ohms (it could also be 2.3 ohms)
	  Injector current: 4A peak, 1A hold.
	  Duty cycle: up to 80..85%
	  Operating ambient temp: -40C..125C
	  Operating voltage: 4.5V to 16V.

	  I did some calculations, to estimate how much power would I need
to dissipate, worst case, ASSUMING ALL EIGHT INJECTORS COULD BE ON, FOR 85%
OF THE TIME (is this assumption correct?)

	  First, I set Vbatt=14V. In order to have 4A circulating, the
driver's output voltage would be (14-4*1.5)=8V. Injector's coil voltage =
(14-8) = 6V.
	  Instantaneous power at the driver: (8V)*(4A) = 32W. Instantaneous
power through injector's coil: (6V)*(4A) = 24W.
	  The 4A will only circulate for a brief period of time, so average
power over one cycle will be much less.

	  One amp, sustaining, automatically means (14*1)=14W have to be
dissipated between the injector driver and the injector coil (100% duty
cycle).
	  In order to have the 1A sustaining, the driver's output voltage
would be (14-1*1.5)=12.5V. Injector's voltage =(14-12.5) = 1.5V.
	  Inst. power at driver: (12.5V)*(1A) = 12.5W. Inst. power at
injector: (1.5V)*(1A) = 1.5W. (total=14W, as predicted).

	  Thus, since the Duty Cycle can be up to 85%, and disregarding the
initial 4A power peak, and the switching power losses, then the average
power for each injector driver is (12.5W)*(0.85) = 10.625W. The average
power for each injector coil is (1.5W)*(0.85)=1.275W.

	  If all 8 injectors could be on 85% of the time, then the maximum
average power at Vbatt=14V (again, disregarding some power losses) would be
(8)*(10.625W) = 85W at the injector driver side. Obviously, at 16V the power
dissipation would be higher (98.6W!).

	  As you can imagine, dissipating 98.6W+ into an under-the-hood
ambient of 125C, with no expected air flow, is a formidable task. The heat
sink's surface temperature should not be allowed to go above 150C; that
would mean the thermal resistance from the heat sink to ambient would be
(150-125)/98.6 = 0.25C/W. I can not see any feasible heat sink doing that,
without the need for forced cooling.