Simple MOSFET question

[mc]

Is there any reason why a n-type enhancement MOSFET can't be used to swith the postive side to a load?
After much googling, all the examples I can find use n-type to switch the return/ground side, and p-type to switch the supply/power side, and my head now hurts.
I've thrown together a quick simulation in LTspice, and it works, but I'm wondering if there's something obvious I've missed.

My reason for asking, is I need to disconnect a 2 cell LiPo balancer circuit when the charger is turned of to reduce battery drain (charger/balancer/batteries are all built into the device), and the simplest way I can think of doing it is by using 3 n-types to disconnect the battery cells from the balancer circuit. When outside power is applied and the charger kicks in, power is supplied to the mosfets and the balancer is connected. Or is there some other option I've missed?

[Psi]

Do you mean putting the load from the source pin to ground  (source follower) ?

Doing so means you have current gain but no voltage gain and you must overcome the gate threshold voltage.
So, for example, if the fet has a gate threshold voltage of 3V then 7V on the gate will result in a max of 4V across your load (source-ground).

[Zero999]

Is there any reason why a n-type enhancement MOSFET can't be used to swith the postive side to a load?

It can but the gate voltage needs to be higher than the source, by the threshold voltage, in order to turn it fully on, otherwise the source voltage will float at the gate voltage, minus the gate threshold, at the current being drawn.

For example, suppose the MOSFET has a threshold voltage of 4V, at the required load current (which we assume is constant regardless of the voltage) and the power supply voltage is 14V. If the gate is connected to the positive rail, the load voltage will be 10V so there will be 4V lost across the MOSFET. In order to turn the MOSFET fully on, the gate voltage needs to be at least 18V. If you attempted to drive the MOSFET from a 5V, the load voltage would only be 1V.

The problem is, if may be inconvenient to add another higher voltage power supply, just to turn on a MOSFET. For AC applications bootstrapping can be used but it requires additional components. Attached is a simple solution using a transistor but the turn off time is very slow.

[electroguy]

Do you mean putting the load from the source pin to ground  (source follower) ?

Doing so means you have current gain but no voltage gain and you must overcome the gate threshold voltage.
So, for example, if the fet has a gate threshold voltage of 3V then 7V on the gate will result it a max of 4V across your load (source-ground).

+1
Also, your voltage at the gate pin will have to be higher than your input voltage to the drain pin, because when the fet turns on, it's low on resistance means the drain and source pin voltages are very similar (yes i know it depends on the "on resistance" and your load current, etc....).
It's ok if you have 2 different voltages available for you to use (ie: dual power supply) but if you only have 1 input voltage, it won't work the best, as you will struggle to get the gate pin voltage to exceed the V-G-S required...

[electroguy]

...

Hero999 described it better than i did. I didn't see his post till after i finished mine. sorry.

[Zero999]

Whoops I forgot, the example of bootstrapping I posted is really just a voltage doubler used for DC operation. The MOSFET can be slowly turned on/off by applying a 20kHz signal to the base of the BJT.

Attached is the bootstrap I was talking about. I'll post the LTSpice file, if you like. The problem is the on time is slow and the load has to pass enough current to keep the capacitor charged.

[djsb]

How does the highside N Bootstrap circuit work? My initial guess is that the diodes and capacitor form a pump circuit of some kind. And I assume this circuit will only work with an AC signal driving it?

David.

P.S How does your bootstrap demo circuit work? Yes, Please post the LT spice file. Thanks.

[Zero999]

How does the highside N Bootstrap circuit work? My initial guess is that the diodes and capacitor form a pump circuit of some kind.

Yes, although a bootstrap wasn't really the correct term, it's just a charge pump voltage doubler.

And I assume this circuit will only work with an AC signal driving it?

Yes, apply a 20kHz waveform to the base of the transistor and the MOSFET will turn on, once the gate capacitance is charged enough.

P.S How does your bootstrap demo circuit work? Yes, Please post the LT spice file. Thanks.

There you go. Try simulating it and looking at the voltages across different components and varying the frequency and duty cycle. You'll see that there's a limit on the maximum on time which depends on the load current.

Code: [Select]

Version 4
SHEET 1 880 680
WIRE 512 -176 160 -176
WIRE 288 -128 -240 -128
WIRE 416 -128 288 -128
WIRE 160 -112 160 -176
WIRE -240 -64 -240 -128
WIRE 288 -48 288 -128
WIRE 416 -48 416 -128
WIRE 160 32 160 -32
WIRE 240 32 160 32
WIRE -240 64 -240 16
WIRE 160 64 160 32
WIRE 416 64 416 16
WIRE 512 64 512 -176
WIRE 512 64 416 64
WIRE 416 96 416 64
WIRE -176 112 -240 112
WIRE -64 112 -96 112
WIRE 64 112 0 112
WIRE 96 112 64 112
WIRE 64 144 64 112
WIRE -240 160 -240 112
WIRE 160 192 160 160
WIRE 288 192 288 48
WIRE 288 192 160 192
WIRE 416 192 416 160
WIRE 416 192 288 192
WIRE 64 224 64 208
WIRE 160 224 160 192
WIRE 160 224 64 224
WIRE 160 272 160 224
WIRE -240 288 -240 240
WIRE 160 384 160 352
FLAG -240 64 0
FLAG 160 384 0
FLAG -240 288 0
SYMBOL nmos 240 -48 R0
SYMATTR InstName M1
SYMATTR Value IRL3915
SYMBOL Misc\\EuropeanResistor 144 -128 R0
SYMATTR InstName R1
SYMATTR Value 100k
SYMBOL voltage -240 -80 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12V
SYMBOL diode 400 -48 R0
SYMATTR InstName D1
SYMBOL Misc\\EuropeanResistor -80 96 R90
WINDOW 0 5 56 VBottom 0
WINDOW 3 27 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 1Meg
SYMBOL Misc\\EuropeanPolcap 400 96 R0
SYMATTR InstName C1
SYMATTR Value 100µF
SYMBOL Misc\\EuropeanResistor 144 256 R0
SYMATTR InstName RL
SYMATTR Value 100R
SYMBOL voltage -240 144 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(0 12V 0 0 0 100m 200m)
SYMBOL npn 96 64 R0
SYMATTR InstName Q1
SYMBOL cap 0 96 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 100n
SYMBOL diode 80 208 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
TEXT -226 376 Left 0 !.tran 10


[mc]

Thanks for the replies guys.
I knew there was something I was missing, so time to go and think of other solutions