MOSFET: N- or P-, enhance or deplete?

Started by Bob Engelhardt June 16, 2017
Here's really basic question for s.e.BASICS:
I want to use a MOSFET to control a load, via a switch.  The tricky part 
is that I need load current when the switch is open and no current when 
the switch is closed.  What I am confused about is what type of MOSFET 
to use, and its orientation in the circuit (S & D connected which way?)

                 +
                 |
                 L
                 |
                 |
        /      |-
   + --/ o--+--|
            |  |-
            R    |
            |    |
            G    G

I could put a BJT inverter before the MOSFET, but is there a way that I 
don't have to?

Thanks,
Bob
Bob Engelhardt wrote on 6/16/2017 12:37 PM:
> Here's really basic question for s.e.BASICS: > I want to use a MOSFET to control a load, via a switch. The tricky part is > that I need load current when the switch is open and no current when the > switch is closed. What I am confused about is what type of MOSFET to use, > and its orientation in the circuit (S & D connected which way?) > > + > | > L > | > | > / |- > + --/ o--+--| > | |- > R | > | | > G G > > I could put a BJT inverter before the MOSFET, but is there a way that I > don't have to?
Can the switch be wired to ground instead of V+? Tie the resistor to V+ and an open switch lets current flow in the load and a closed switch stops current from flowing in the load. -- Rick C
On 06/16/2017 12:37 PM, Bob Engelhardt wrote:
> Here's really basic question for s.e.BASICS: > I want to use a MOSFET to control a load, via a switch. The tricky part > is that I need load current when the switch is open and no current when > the switch is closed. What I am confused about is what type of MOSFET > to use, and its orientation in the circuit (S & D connected which way?) > > + > | > L > | > | > / |- > + --/ o--+--| > | |- > R | > | | > G G > > I could put a BJT inverter before the MOSFET, but is there a way that I > don't have to?
The simple approach is to put the resistor from the gate to the positive supply, and the switch to ground. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net

"Bob Engelhardt" <BobEngelhardt@comcast.net> wrote in message 
news:oi11mm0etq@news6.newsguy.com...
> Here's really basic question for s.e.BASICS: > I want to use a MOSFET to control a load, via a switch. The tricky part > is that I need load current when the switch is open and no current when > the switch is closed. What I am confused about is what type of MOSFET to > use, and its orientation in the circuit (S & D connected which way?) > > + > | > L > | > | > / |- > + --/ o--+--| > | |- > R | > | | > G G > > I could put a BJT inverter before the MOSFET, but is there a way that I > don't have to?
The source follower configuration is non inverting. Assume the load is from source to GND and you pull the source up to pass current through the load - the source will be up at Vdd and the drive will have to be at least VGSthr higher than Vdd. You can get away with a high side driver, but many bootstrap the gate supply, those only work if the output is a train of pulses. If the PSU is an old iron core transformer, you can use a charge pump voltage doubler to get the gate supply - you might get away with it on a SMPSU secondary, but it can be a bit more tricky. If the load must hang from Vdd, use the opposite channel polarity MOSFET - all the same basic principles aply.
Ian Field wrote on 6/16/2017 4:39 PM:
> > > "Bob Engelhardt" <BobEngelhardt@comcast.net> wrote in message > news:oi11mm0etq@news6.newsguy.com... >> Here's really basic question for s.e.BASICS: >> I want to use a MOSFET to control a load, via a switch. The tricky part >> is that I need load current when the switch is open and no current when >> the switch is closed. What I am confused about is what type of MOSFET to >> use, and its orientation in the circuit (S & D connected which way?) >> >> + >> | >> L >> | >> | >> / |- >> + --/ o--+--| >> | |- >> R | >> | | >> G G >> >> I could put a BJT inverter before the MOSFET, but is there a way that I >> don't have to? > > The source follower configuration is non inverting. > > Assume the load is from source to GND and you pull the source up to pass > current through the load - the source will be up at Vdd and the drive will > have to be at least VGSthr higher than Vdd. You can get away with a high > side driver, but many bootstrap the gate supply, those only work if the > output is a train of pulses. If the PSU is an old iron core transformer, you > can use a charge pump voltage doubler to get the gate supply - you might get > away with it on a SMPSU secondary, but it can be a bit more tricky. > > If the load must hang from Vdd, use the opposite channel polarity MOSFET - > all the same basic principles aply.
You aren't looking at it the right way. The difference between putting the load on the source or the drain is one of voltage polarity. In both cases the current flows when the gate is more positive and turns off when the gate is negative. One thing he could do is to use a P-channel device. That would turn on when the gate is more negative and off when the gate is more positive. I didn't think of that before... lol -- Rick C
On 6/16/2017 2:28 PM, rickman wrote:

> Can the switch be wired to ground instead of V+? ...
No - it does other stuff too and a double pole isn't available in the style.
On 6/16/2017 7:25 PM, rickman wrote:
...
> One thing he could do is to use a P-channel device. That would turn on > when the gate is more negative and off when the gate is more positive. > I didn't think of that before... lol >
That's the kind of answer that I was looking for - thanks. So it would be like this (P-channel, enhancement mode)? : + | / |- S + --/ o--+--| | |- D R | | | G L | G Switch open: Vgs = -V+, MOSFET on Switch closed: Vgs = 0, MOSFET off
On 6/17/17 12:44 PM, Bob Engelhardt wrote:
> On 6/16/2017 7:25 PM, rickman wrote: > ... >> One thing he could do is to use a P-channel device. That would turn on >> when the gate is more negative and off when the gate is more positive. >> I didn't think of that before... lol >> > > That's the kind of answer that I was looking for - thanks. So it would > be like this (P-channel, enhancement mode)? : > > + > | > / |- S > + --/ o--+--| > | |- D > R | > | | > G L > | > G > > Switch open: Vgs = -V+, MOSFET on > Switch closed: Vgs = 0, MOSFET off
YUP!!
>"So it would
be like this (P-channel, enhancement mode)? : " It has to be enhancement mode or you will need a voltage source higher than the FET source to turn it off.
On 6/17/2017 3:25 AM, jurb6006@gmail.com wrote:
>> "So it would > be like this (P-channel, enhancement mode)? : " > > It has to be enhancement mode ... >
Thanks. Furturlec has a bunch of P-channel MOSFETs & I picked IRFL9110 more or less randomly. Looking at the IR spec sheet for it, it doesn't say "enhancement": http://www.futurlec.com/Transistors/IRFL9110pr.shtml The Mouser listing for it does say it's enhancement, but why isn't it on the spec sheet? Is it because enhancement is the default mode & it "goes without saying"? Bob