MOSFET as a power supply output switch

[tszaboo]

Once again, if you ever plan to charge batteries with your bench power supply, use two FET, source and gate connected together. But in any case, it will be safer that way. Also I would toss the discrete transistors in favor to a FET driver, because they are designed to do this.

[KL27x]

The problem with a PFET is his power supply goes down to 1.5V. To drive the PFET gate fully at that voltage, he'd need to add a negative rail.

i have a secondary voltage of max 6-7volts.

The easiest thing to do is to use this secondary voltage to drive an NFET to switch the low side. That might not be desirable for various issues of convention/expectation, especially if your power supply is grounded. This would be a problem if you have your circuit plugged into a something else that is grounded.

And if you ever plan to, say, charge batteries with this power supply, might I suggest you ALSO put a relay on the high side output. In fact, in both of my DIY power supplies, I use two relays. I put one between the ADC and the output and one between the ADC and the load. So I can use my PSUs as voltage meters to read the float voltage of the load with the power supply output disconnected and conversely I can change and verify the no load voltage of the power supply without physically disconnecting the load. This is not as handy as having current limiting, but it adds a lot of utility for charging batteries, safely. And the audible click of the relays is oh so satisfying.

[illusive]

I needed those 6-7 volts to power up the microcontroller and stuff. Anyway, i decided to get a small secondary transformer with two output wingdings, one for the  low power stuff and the other to drive the MOSFET with a separate ground.

[EPTech]

Hi there,

You could put the feedback of your voltage regulation behind the high side FET switch, so as close as possible to the load, as it is supposed to be. That will get rif of the voltage drop of any FET you choose. The problem then is when you switch off the FET, you get a sort of open loop and the voltage before the switch will ramp up.
It should be possible to switch the feedback loop together with the FET. I feed back loop will be slow enough not to even notice the switch-over.

[illusive]

Yes that crossed my mind. But then really the feedback loop will get messy. I can turn off the regulator (with it's on/off pin) and the mosfet at the same time tho. I got a 3.1mohm mosfet and i'm not sure if it's really to compensate for its voltage drop.

[illusive]

I was looking into these photovoltaic mosfet drivers and they seem like the easiest way to drive the mosfet. But i can't find many of them, the only accessible to me is VOM1271. Can you recommend more of these IC's?