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Topic: Does it look like I've wired this MOSFET up correctly? (Read 1 time) previous topic - next topic


The idea here is I've got an N-channel mosfet switching the power supply for my board. 5V and xGND is fed in through the power pins.  When the user flips the switch, 5V is fed to the MOSFET's gate, which is normally pulled down to xGND. This then allows everything else on the board that is connected to GND, like the 3V regulator, to be turned on by being grounded through the MOSFET to xGND.

I also have an Enable pin on my power connector.  This will be connected to +5V if either the on board switch of the external switch is on.  In this way, the external power source can be switched on and off by the board.  Or, the external power source can switch the board on and off if it applied 5V to that pin.

So what do you think?  Does this schematic look good?


Sep 03, 2012, 07:07 pm Last Edit: Sep 03, 2012, 07:26 pm by sdturner Reason: 1
Yes, it will do what you want, however...

Why don't you use the switch directly and avoid the transistor?

Why not use the Enable pin on the regulator?

It is typical to leave the grounds connected and switch the voltage supply(ies). This is important in higher voltage systems because if a person touches the circuit they are more likely to be touching / standing on ground.



The reason I'm doing things this way is because I found a problem at the last minute with my circuit and I can't see any other way to correct it.

I had a linear regulator in my circuit, which could be switched on and off by the switch on the board, or by an external switch.  This regulator was supposed to supply up to 3A.  But to switch that much current normally one would need a large bulky switch.  Since I wanted to be able to use any switch, I decided to use a regulator with an enable pin instead, and have the switch control that.

Problem is, I'd planned to send the boards out for manufacture tomorrow but on Friday I was doing some last minute calculations to make sure everything was good to go, and I discovered I'd made a horrible mistake in my power dissipation calculations.  Instead of being able to dissipate 20W, my linear regulator and 2x2" PCB were only going to be able to dissipate 2W.

The obvious solution to this problem was to use a switching regulator.  But:

Complexity and cost:
- Switching regulators are complex.  They require something like 15 external parts including an inductor.
- Because they're complex, it would take too long and be too risky or me to design one with so little time remaining to check everything.
- I considered using a drop-in solution like a Simple Switcher, and that I could fit on my board since it has the inductor built in, but those are expensive. Like $6 each.  It would cost me $1000 extra to put those on 250 boards.
- standalone switching regulator with inductor, resistors and caps, and the assembly costs associated with those would probably cost me that much as well.

Input voltage:
- Most of the switching regulator chips I could find listed 7V as their minimum input voltage.  The Simple Switcher was 7.5V
- I wanted to be able to install these boards in both small and large devices.  Small being handheld.
- I can barely fit 4 AAA batteries or a 7.2-7.4V rechargeable pack in these handheld devices.  So I can't fit enough batteries to get to 8V.
- I also want to be able to power the devices on 12V so people only need one battery pack for both microcontroller and audio amp.

All of these factors left me with two apparent options.  Keep the linear regulator, and limit the board to 1A at 7.4V or 2A at 6V, or use the Simple Switcher and require 8V input minimum, which would preclude installing the boards in any small devices. 

After going to all the trouble to make the board only 2x2" though, I didn't want to lose the ability to put the board in small devices.  Besides admitting defeat I'd lose sales for all those devices it could have been used in.  At the same time, I couldn't limit the board to 7.4V.  Most of my customers who have preordered want to run the board on 12V.  Yet I also really couldn't afford to spend another $1000 to make that happen.

And I couldn't make two different versions of the board with different regulators.  That too would add lots of expense.

After wracking my brain for a couple days though, I finally hit upon a solution.  The UBEC:

An UBEC is a Universal Battery Eliminator Circuit.  Or put more plainly, a buck voltage regulator.  Like the Simple Switcher.  Except these are cheap.  Roughly half the cost of the Simple Switcher.  Around $3-4 ea.  Not much more than my linear regulator.  And they seem to be able to work down to around 6V.  I don't know how they can accomplish this when the Simple Switcher can't because there's not a lot of info out there about them, but one fellow said they just stop regulating at around 5.5V and pass thru whatever the battery voltage is.  I guess that's better than just shutting down when the input drops below 7.5V.

Anyway, so there's those, and there are also voltage boosters available for RC stuff which are also cheap and will boost say 4.5V to 5V and supply up to 1A.  And both of these are tiny, about 1x1", so I'll be able to squeeze them into my handheld devices.  And they're much more efficient and won't get hot. 

So if I use those, I can choose whichever type I need for the application.  And they're already designed for me.  And they're cheap.

There's just one problem with them.  They don't have an Enable pin.  There's no way to shut them off with a low power switch.

For my 12V application, this isn't an issue.  For that, the people using my boards will have an external audio amp hooked up and that will need a large switch on it just the same as these UBECs will.  For my smaller applications though, that becomes an issue.  I will need to develop or buy some voltage regulator boards for those devices.  Ones which will have that third pin on the power connector going to the Enable pin.   

But for my 12V application, my users also want to be able to turn their lights and sound on and off with small switches located away from where the main power switch would be.  So I still need a way for the board to be shut down by an external switch.  Now if I had an external regulator of my own design, I could wire that Enable pin up ad just make the switch switch that.  But I'm not going to have time to design one.  And even if I did it would cost too much to have 250 of them made.  It would probably cost something like $10 per board, even more costly than that simple switcher.

So, putting the MOSFET on the board to act as a switch is kind of a kludge.  It allows people to put their device in a kind of standby where the UBEC is powered and their external amp is powered, but my boards which control the lights and sounds and servos are not powered.

I am not sure this is the best solution, but I'm extremely short on time now.  I really need to have these boards ready by the first week of October.  Which means they need to go to manufacturing asap so I can get one back to test and then have them assemble the rest once that checks out.

Kinda screwed up big time.  Just trying to find an acceptable solution at this point, rather than a perfect one.


Good luck. I hate it when I find a mistake just as the boards are about to go out (or just went out) :smiley-eek-blue:

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