Hi Rowland yep that would work. keep up the good work.
The reason a Gate source zener is used is to ensure the Vgs does not exceed the FET parameters. Many TO220 type FETs have a +/-15V maximum Vgs...some higher some lower. So if your PSU is 14V and your FET can take 15V Vgs or higher your really don't need this zener and resistor. That said it won't hurt either.
The other thing is the FET on Rds (on resistance). This changes with current, heat and other things and OEMs give a graph of on Rds and a multiplication factor Vs gate junction temperature you must use to calculate actual on Rds. Choosing as low as you can get Rds on as you can source is the best option but do the sums.
Lets say you use an IRF4905, TO220 P channel mosfet. This has Vgs +/-20V so you don't need zener and resistor, Rds on @20 degrees C (junction temp) = .02ohms.
So if you pass 5A, power dissipated by the FET = I^2R = 5x5x.02 = .5watts.
Now most these TO220 FETS packages can dissipate 2.5W max with good heat-sink. So it looks ok, a touch warm but ok...but read on.
The junction temp will rise with current and ambient. Taking a WAG lets say junction temp = 80C (you can find this by de-rating and taking measurements of the case temp, look at the pdf specs). Thus we multiply the new on Rds by a factor (see spec sheets on device) by 1.25@80C. Rds on actual @ 80C = .025ohms.
Thus @5 Amps, pwr dissipation = 5 x 5x x .025 = .625watts. Your getting hotter you may need a heat-sink on the FET.
I'm just trying to give you an outline on FETs and some simple hurdles you need to watch for. A lot depends on Rds and how it behaves with temperature. Your max I draw is around 6A at present do you'll need a good low on Rds FET...I suspect lower than .02ohms...lets say look for one around .015ohms or less.
Of course you can parallel 2 of the P channel FETs to share power dissipation & I flow.
Now here's a trick...most P channel mosfet have higher on Rds than their N channel cousins, the P FET I quoted above as very good on Rds characteristics but you can go lower with an N channel FET. It is much easier to obtain a low on Rds N channel FET.
This explains it (opens/downloads an on semiconductor pdf on the subject). Basically use a N channel FET on the -ve rail with the gate connected to the +ve rail. Acts just like the P channel except stops reverse current flow thru -ve rail.
Hope
this helps and I haven't put you off at all.
