mosfet relay control board

[bart70]

I am developing a PCB to control turning on various fans and electric motors. The control circuitry uses an ARM processor and via a CAN bus is sent signals from a main controller to turn devices on and off. Power for the processor and control circuit is fed from a 5 volt supply from the main controller via a cable. The 5 volt and ground are 26 gauge.

The PCB uses P-channel mosfet to switch the devices. The fets are fed from a 6 gauge battery cable connected to the PCB.

I assume I will need some Transient Voltage Suppressors on the mosfet drains to handle back EMF when the mosfet is turned off and if I want to be able to soft start the motors via PWM.

Question is will I need a separate ground to allow Back EMF directly to chassis ground instead of back through 26 gauge ground wire? I assume it would be best to have a separate ground on the PCB that only the TVS is attached to so that it does not cause a ground loop with the rest of the system?

 

[MikeHalloran]

Go to a junkyard and get a dead ECU.
Open it up and look at the multiple layers of defense re transient voltages.
Look up 'load dump transient' for some scary numbers.
See if you can find a wire smaller than 18 gage anywhere on a car.



Mike Halloran
Pembroke Pines, FL, USA

 

[bart70]

Mike,

thanks for your insightful response!

 

[Comcokid]

The transients ring both positive and negative. Fan motors can be especially hard to drive with a MOSFET as when you turn-off, not only is the inductive issues, but the rotating mass in the motor until it spins down is a generator. Most automotive fan motors still use brushes which are not 100% conducting all-of-the-time on the usec scale. Fast dV/dt transients can turn-on the substrate in the MOSFET if not dealt with carefully.

In industrial applications where loads like these are used the might be driven with a half-bridge controlling the lower MOSFET and timed carefully to deal with the transients. In cars, some smaller motors are controlled with MOSFETs, but in situations where it was designed and designed-verified together with the motor used.

I've not looked at a real recent car, but even my 10-year old cars with electronics use relays to control larger power items.

 

[LionelHutz]

You should look at PWM switching the negative lead. That seems much more common than PWM switching the positive lead. N channel mosfets typically have a lower on resistance as well.

 

[MacGyverS2000]

See if you can find a wire smaller than 18 gage anywhere on a car.

Stock speaker wires ;-) Sometimes I think they sprinkle copper dust into the plastic and call it a day...

Dan - Owner

http://www.Hi-TecDesigns.com