Making an 80 ampere (continuous) 12 volt MOSFET relay (?)

I will offer these 2 sites for some info. The second is more theory oreinted.

http://reibot.org/2011/09/06/a-beginners-guide-to-the-mosfet/

http://www.electronics-tutorials.ws/transistor/tran_6.html

Will the relay current be directional or bidirectional?

What is the maximum voltage that will be seen across the open terminals?

To get the resistance-drain-source (rds) down in the 20 milli-ohm range will require a few of these in parallel or rather expensive individual devices.

Are you switching resistive or inductive loads?

Sorry for all of the questions, but it makes it easier to walk through when the operating space is defined.

Parameters

Control voltage same as switched voltage (more or less).

Uni-directional, perhaps an opto-coupler would be best trigger? That an your run of the mill 555 for timed events.

Ambient temp of heatsink before powering up 30C

Switched amperage maximum 80. Will use freewheeling hyperfast diode, TVS, caps, to attenuate transients, and surges with inductive loads.

How large a PCB board and heatsinks is another issue. I do not trust convection so I will be digging up some 5vdc ball bearing fans and powering them with 7805's.

Pfft. Hairpin alternators, and lithium Ion obsoleted me in one respect. I'll scuttle sideways and punt.

Thanks for your help gentlemen.

Here is an 80 amp unit for $84.50 http://www.powerstream.com/battery-isolator-solid-state.htm ... but that is not what you asked for.

Here is some more reading titled "MOSFET as a Switch"
http://www.electronics-tutorials.ws/transistor/tran_7.html

Mex, here is a good device to start with, probably 2 in parallel.

N-Channel Enhancement MOSFET

The voltage gate-source (Vgs) controls the current in the drain-source (Ids). In an N-Channel Enhancement FET, the more positive the Vgs goes the more Ids will flow.

If the Vgs drive is not high enough for a desired Ids, the device will act as a constant current series load and will limit current through Ids. So typically Vgs is over driven (made greater) so a much higher current is able to flow when used as a switch.

The drive circuit for Vgs may be a little bit of a challenge depending how you connect everything.

Let me know how things progress and I will help where I can.

Ken

EDIT: Mex, PM me your e-mail address and I will send you some educational files on FET's.

High side switching primer...

App Note

MEXICOWANDERER wrote:
It's going to need a large heat sink and multiple MOSFETS. Control voltage might as well be close to supply voltage.

Why the large heat sink? Turn them on hard and fast and you will be dissipating very little power. Likewise when turning them off.