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

Tue Mar 11 02:10:18 GMT 2003

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Hi,

If you really need high efficiency in the injector driver then PWM hold is
a good approach. The problem is that during hold time the series resistance
in the flyback path should be as low as possible to keep the injector
current as constant as possible. During switchoff you want a large series
resistance so that the injector's field collapses as fast as possible. The
attached design suggestion should do that.
R1/C1 determine the peak time. The flyback voltage from the peak pulse
charges C2 via diode D2. After a delay determined by R2 and the gate
capacitance of Q4, Q4 conducts and shorts out Rflyback. This keeps the
flyback path at low impedance during hold. At the end of the injection
pulse Q5 conducts and puts the gate of Q4 at 0V. Q4 is thus switched off
and the flyback energy dissipates through Rflyback, switching the injector
off fast. The design self oscillates during hold, creating the PWM control.
The PWM frequency is determined by the inductance of the Injector.
Unfortunately Rcurrent is neccessary (increasing the turn-on time) unless a
current sensing MosFET is used for Q3.

Regards,
Klaus

At 11:41 PM 3/10/03 +0100, you wrote:
>Hi, 
>
>Klaus Allmendinger writes:
>> be wrong in something as simple as the attached design?
>
>simplicity: exellent 
>
>I also thought about the emitter-follower design
>driven by a CMOS gate.
>But would CD40106 not burn at 14 V supply? Maybe a diode
>at GND and another at VCC can effectively decrease its
>supply without having much effect on FET operation.
>(if it worx, my circuit can use the same and get rid of a few
>components). 
>
>> Example: 1.2 Ohm Injector, 12V battery voltage, 1A hold current. R-Limit =
>> 10.8 Ohm. 
>
>So in hold mode efficiency is about 10%. Much worse than
>with pulse-mode flyback current limiting.
>Adding those 8 simple components (sum ~ $1) worth
>the efficiency, I think: 
>
>http://x-dsl.hu/genboard/injector/ 
>
>I uploaded injdrive2.pdf
>(thanx to Alexei Pavlov for his valuable comments) 
>
>injdrive and injdrive2 are very similar.
>There is a small difference around Q4.
>Some will like one, others the other. injdrive2 has a little higher 
>dissipation, but
>setting values might be simpler since margins are higher. The same PCB can 
>be designed that can be populated
>either way. Look at injdrive.txt for more info. 
>
>Marcell
>-- 
>+36 70 2 654 789 
>
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>
>
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