bretth wrote:Any thing I should be aware of?
Yes, many issues to be aware of when usings MOSFETS in automotive applications.
Consider what happens when Vgs drops below Vgs(th). The low RdsOn goes away and the magic smoke that powers all semiconductors may be released. VgsMax must also not be exceeded to ensure reliablility.
Automotive circuits must be able to tolerate supply voltages that drop below 5 Volts for several seconds and rise to 40 volts for several milliseconds.
The high voltage can be solved by a Zener diode from gate to source. That will keep Vgs below VgsMax.
The low voltage is a more difficult problem to solve. The logic level device you have chosen makes this dangerous condition much less likely, but for a conventional MOSFET, Vgs needed to insure low RdsOn may be 8 volts or more. If the microcontroller has an ADC, it could be used to monitor battery voltage and turn off the MOSFET if a safe Vgs can not be met. This is not a comprehensive solution, but it is a practical one for a DIY project.
The current required to keep a MOSFET on is very small, but the current required to turn it on (and off) is much higher due to the gate capacitance. Slow turn on puts the MOSFET in the "danger zone" where Rds not at it's minimum. For a circuit that does not turn on/off rapidly this may not be a problem. Reducing R1 and R2 will promote faster turn off/on.
Derating components is desireable if practical. Using a part rated at 1.5 A max to switch 1 A is more likely to fail than one rated at 5 A. Does the load have significant capacitance? Inductance? Design for worst case.
Low side switching (N channel to ground) is often prefered for many reasons. For examplle, it is easier to ensure Vgs is within range and has fast rise and fall times. Also, no current will flow if the output is shorted to the vehicle chasis. N channel devices are usually less expensive than P channel.
MOSFETs can self-oscillate for many reasons. Low value gate resistors (47 to 470 ohm) and ferrite beads are the usual fix. PCB layout may be critical.
When driving motors it is important to size the MOSFET for stall current, not run current. This can be 5 to 10 times higher.
Driving incandescent bulbs is quite difficult due to the very high inrush current. A cold filliment has very low resistance. When a bulb burns out, it may short out and destroy the MOSFET.
There are many MOSFET drivers and integrated driver/MOSFET chips available that attempt to provide protection. Great on paper, but far from perfect in the real world.
A quality automotive relay (Bosch, P&B) is a *very* rugged and reliable device. Don't assume something mechaincal must be inferior to something solid state. Cars still use many relays for many good reasons. Some applications such as fuel injectors, ABS and HID lighting require the fast switching of semiconductors, but the basics still are best served by relays. For simple high current (1A +) switching needs a $3 Bosch relay will likely last much longer than any $3 electronic circuit.