At 05:20 PM 7/24/01 -0700, you wrote:
>
>Why not a TIP121 or TIP132 darlington with Min. hfe 1000? Same package and
>pinout so you won't have to change the PCB. If you have extra CPU overhead,
>turn on the darlington and switch over to a 500Hz 1/3 or more duty cycle to
>minimize power dissipation.

The MOSFET I suggested looking at:
http://www.irf.com/product-info/datasheets/data/irliz44n.pdf
has compatible pinout to the TIP42C, and he can eliminate the heatsink.
Just change the resistor values eg. to 100R series and 10K "base to emitter"
(gate to source). It is a 55V/30A with 0.025 Ohm Rds(on) at 5V gate drive
and 0.035 Ohm Rds(on) at 4V gate drive (maximum). R-theta-JA is 65'C/W
for the fullpack device, so even at 0.035 Ohm (less than 200mV drop),
the 175'C maximum Tj won't be exceeded at 5A until the ambient temperature
is almost 120'C. (with no heatsink) That will be reliable. ;-)


Using a darlington (eg.
http://www.freetradezone.com/dc_images/85/16/D0258516.pdf
will work fine, but note the maximum Vce(sat) at 4A Ic
and 16mA base current is 2V, and at 6A Ic and a whopping 30mA of base
current it is only guaranteed to be < 4V. The SOA looks OK. But, even if
the output voltage is "only" 2V, that is 10W of power to dissipate, a
fairly large heatsink, which works against the lower price of the
darlington (about 60-70 cents US less in 100 qty). A cheaper MOSFET
with higher Rds(on) could be used, but a heat sink would likely be
required for reasonable Ta range.

The PWM idea is a good one, and if the chip has a built-in PWM module,
it could be put to use for this, provided the electrical and acoustic
noise was not troublesome. Just turn it on for 100msec or whatever, then
cut back for hold. The power dissipation in the coil will be greatly
reduced (but increased in the catch diode).

BTW, the catch diode will slow down the release of the solenoid, if
this is an issue.

Best regards,

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