Peak-n-hold failure!

[rr]

Actually, no. I'll throw some math out if you don't mind...

With a saturated driver and injector, the injector resistance is
12 or 16 Ohm and the driver saturates at about 1.2Vmax. (Driver
saturation is the voltage drop across the driver when turned on).
I'll also use 13.8V as nominal vehicle volts.

So, (13.8V - 1.2V) / 12 Ohm = 1.05Amps (total current)

1.05A * 1.2V = 1.26 Watts (dissipated in driver transistor).


Now for the peak-n-hold's (we will ignore the ramp-up time for
the moment):

The injector resistance is under 2 Ohms, but we will use that. When
the driver goes into hold mode (1 amp), there will be a greater
voltage drop across the driver. Lets calc that first:

2 Ohm * 1A = 2V (drop across injector)

13.8V - 2V = 11.8V (drop across driver transistor)

11.8V * 1A = 11.8 Watts (dissipated in driver transistor)

Now, ramp-up (I'm chickening out on the math for this, been too long
since calculus...), the duration is about a milli-sec, while the
entire open time is 2.5 to 8 or more millisconds. So, the 'hold'
period is the major portion of the open time. Hence, lots of heat
buildup. (Even if you take the avg of the peak, 2A's it's still
more than the sat driver).


The reason for a peak and hold driver and injector combination, is
dynamic range of the injector.

There is a larger span between how little fuel can be metered,
versus, how large an amount of fuel can be metered. By using a
low impedence injector coil, a magnetic field can build faster,
yanking the injector open quicker, hence, better response time.

But, we can't be dumping 6+ Amps through the injector during the
whole open time, hence the hold function.

BobR.

I'm open to corrections, additions, and questions...


Walter Sherwin wrote:
> 
>BobR wrote:
> >The PnH driver has to dissapate more heat than a saturated
> >driver, because of the hold current/voltage. Your heat sink
> >may not be large enough: 12.6W vs. ~1.2W for sat inject.
> 
> If you look at the "average" power dissipation, over the entire commanded
> injector pulsewidth, would the average power dissipation of a P&H driver not
> be lower than that of it's saturated counterpart?  Kinda thought that was
> their reason for existing?
> 
> Walt.