Mook said:
Try the IRF2204PB from digikey for ~$3.50 each.
16 will cost you about $50.
There was another device suggested earlier in the thread too, a BSC020N025S claiming
2 mOhms on its data sheet - but it has a package type I'm not confident I can heat
sink (never saw it before). The one you list is easier to mount ... thank you.
this is an H bridge circuit. Since your supply voltage is so small you
don't need T5, T6 R1, or R2 to drive the high side gates. You can drive them
directly.
Thanks to the tutorial below on these devices (I didn't know about the high input
capacitance) said:
A good quad mosfet driver for this would be the TC4467COE also from digikey.
http://www.coledd.com/electronics/robot/h-bridge_large.png
Very convenient, sounds easier than designing my own, but for this application I may
not have to be concerned with switching speed - and - if all devices are on during
transition, well, it might actually work better that way (especially if there is a
delay in the cycle for the p/s to achieve regulation or if it has overshoot issues).
I guess I can experiment if need be, but it is also cheap insurance to have the
suggested driver IC handy... .
To operate you turn on T4 then T1 for positive voltage across the motor.
Then turn them both off. Then turn on T3 then T4.
If you turn on the top and bottom fet at the same time you effectively short
your supply t ground through the mosfets. Usually the would blow the FETs or
the supply but in your case it might not be so critical.
In any event the p/s circuitry should limit current to 100 amps. Minimizing heat is
an issue more to do with both controlling thermal voltages and insuring room
temperature remains stable as possible during the measurement cycle(s).
Each of the 4 mosfets in the H bridge should actually be a parelleled est of
4 mosfets to reduct teh RDS to below 1mohm.
Yes, I do understand that, but until I read from your tutorial link I was unaware
there was no need for the equivalent of an emitter resistor like BJT's use. (Even)
if one device could handle the power, the leads need to be able to handle it too -
and of course the lower the on-resistance, the less heat - absolutely concur on the
parallel devices (thanks)!
http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm
when dealing with 100 amps, you had better have very heavy wires and PC
board traces.
Wiring is planned to be 2 ga. copper with crimp terminals. I guess I will have to
have multiple 12 or 10 Ga. copper wire runs - one from each FET to a common block for
cable assembly hook up.
You also need a decent heat sink to mount them too. 10-20 watts will be
dissipated in the H-bridge mosfets.
I was hoping the on-resistance looked low enough to use chassis mounting or small
heat sinks and a fan (was going to test with a thermocouple on one FET). Oh well,
too much is better than starting over due to burned up parts, but to satisfy
curiosity, I could always set the current limit low and monitor device temperature
rise? If so, I suppose the data sheets tell me where I've gone too far, but do you
recommend a certain temperature to run these at (ambient will be a controlled 74.3
degrees)?
Thanks to yours and others good advice on this 'project', hopefully my task won't
totally be dependent upon just luck.
I appreciate your input and all the references, recommendations and pointers!
Thanks again.
Best,
Stephen Kurzban