OK, here is a circuit I call the "electronic relay". It allows the PSU to slowly come up, charging the capacitor as it goes. No current is flowing through the 3Ω simulated load. When the capacitor voltage reaches ~10V, the bistable switch turns on with a nice snap, dumping the capacitor into the load. From that moment on, the PSU delivers the running current. When the PSU turns off, the bistable switch turns back off as the voltage decays below ~8V.
You can use this a place to start. Look at your local suppliers to see what PFETs you can buy there. Write back with the number of something you can find. You are looking for one that has a Vds of >25V, an Id >10A, Vth of -3 to -5V with an On resistance of a few tens of milliOhms.