[Diy_efi] peak-and-hold injector driver ic's

[Klaus Allmendinger]

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Hi,

I looked at all the discussions in this list about p/h drivers. What would
be wrong in something as simple as the attached design?
Here's how it works:
U1 is 1/2 of a CD40106 CMOS schmitt-trigger/inverter IC (contains 6
inverters). C1/R1 determine peak time. Q1/Q2 are simple small signal
transistors just to switch on Q3 faster (gate charge). Q3 and Q4 are logic
level Mosfets. Q3 has to switch the peak current and must withstand 2 times
battery voltage. Q4 only switches the hold current and needs to withstand
only battery voltage. For a short period of time it also conducts the peak
current. Neither Q3 nor Q4 need heat-sinks. All hold power is dissipated in
R-Limit, a power resistor. Because resistors are much more rugged than
semiconductors and can operate at much higher temperatures, no heatsink is
required for R-limit as long as it is mounted away from the heat sensitive
devices. Switch-off time is faster because the flyback energy of the
injector inductance is dissipated via R-Limit. Switchoff time is determined
by L/R, where L is the inductance of the injector and R is the resistor in
series with the flyback current. Putting a shottky directly over the
injector allows as R only the internal resistance of the injector and is
thus much slower.

Example: 1.2 Ohm Injector, 12V battery voltage, 1A hold current. R-Limit =
10.8 Ohm. 
Assumption: Inductance of Injector 1mH (just a number).
		Injector closes at 37% I-hold.
Turnoff time ~ 0.833 ms for shottky directly over injector.
Turnoff time ~ 0.0833ms for design shown.


Comments please,

Regards,
Klaus

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