MOSFET Source follower

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simayonis

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Hi guys,

I attached an NMOS source follower circuit which works..... however i need to understand what is the requirement for the NMOS to switch on in this configuration? Perhaps something to do with Vgs?
 
It is dependent on part number but generally for low ON resistance you have to have gate at least 10 volts higher then source. If you are using source follower to switch power it means you must have a gate supply 10-15 vdc higher then supply voltage you are switching.

Look up the Rs versus Vgs for your particular MOSFET.
 
in my configuration (NMOS) my source is not connected to ground and thats why its a source follower. so your saying that my gate has to have around 10~15V higher than the drian to switch off the mosfet? yes?
 

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If M1 is properly turned on, the source would be within mV of its drain (15.9xxV). The way to get it properly turned on is to get its gate 10V more positive than its source, i.e. ~26V. The only way to do this is to have another (more positive) power supply, or use a charge-pump derived off the +16V.
 
I have actually built this on a breadboard, it actually works with this current configuration. however i could not give any logical explanation....it works in the simulation as well though...
 
Try drawing an Amp or two of output current; then tell us if it works...

Hint: figure out the power dissipation in M1.
 
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Sorry mike i didnt quite get you, but i have attached the results from simulation.
Red is the output from the source... Its switching meaning its working?
Green is the inductor current, since it has riples meaning its discharging/charging?

My confusion now is between a normal configuration of NMOS whereby the source is grounded... if that were the case i can easily find out what is needed for Vgs.

But for source follower, Im not sure the requirement for it. Like the theoretical explanation. But from my analysis of the simulation, a NMOS source follower configuration works like a normal PMOS configuration (Source to Supply)...

Hope you can help me with this
 

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I think it's just like an emmiter follower except where you have a Vbe drop you now have the FET threshold voltage (few volts). Could be wrong though.
 
The 12v peak on your picture should be 16v peak if switch is low resistance to 16v supply. You will have a lot of power dissipation on MOSFET and you probably have a lot of ripple on output.

Your switching power conversion efficiency is going to be very poor.

The -1.1 vdc is the drop on the diode. (Which is fairly high. You should change to a schottky diode with sufficient current handling capability).
 
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i seee.. so this configuration its not that good... but what i want to know is why does it work? can you tell me from this configuration. how do i calculated what is needed to turn on/off the nmos in source follower mode?
?
because my circuit is a buck converter.. the output voltage will be lower.
 
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As has been stated, to properly turn on a MOSFET the gate-source voltage needs to typically be 10V, or 10V greater than your source voltage. That's why N-MOSFETs are not normally used for positive output buck converters, since that gate voltage needs to be 10V above the input voltage. If you use less voltage on the gate in a follower mode, there will be a significant voltage drop across the transistor when it is on, increasing it's dissipation and reducing the power supply efficiency.

A P-MOSFET is much easier to use, since with a positive voltage on the source, you just need to ground the gate to fully turn on the MOSFET (for an input voltage of 10V or greater).
 
crutchow,

i know what you mean but i dont know why that circuit configuration didnt work with a PMOS.... ive built it on a breadboard it didnt work however, in simulation it worked quite well. the problem back then was that when the gate was driven to mebe +15V +0V the drain voltage of the PMOS only outputs 0.5Vpeak to peak. so it couldnt switch on/off. I really dont know the reason why... i then cahnge it to a NMOS with a source follower circuit then my buck converter works.... lol....
 
A PMOS should work.

But I can't help you with what might be wrong unless you can post the circuit along with the transistor type.
 
but this circuit is working ya? its just that it has bad efficiency? but curtschow... for this configuration... u said Vgs > 10 for nmos right... but my Vs i wont know its voltage isnt it? Vd i know its 16V, Vg ?? Vs??? I mean imagine I havent designed that circuit and am still calculating the values needed to turn on/off this switch. what are the values of Vg needed? with Vd = 16V? Vs ????? Im talking about the same NMOS source ofllower circuit i posted..thanks..
 
With VD=16V, Vg should be at least 26V for proper turn on of the NMOS and good efficiency of the regulator.
 
Vs will be the threshold voltage (turn on voltage of the FET) below the gate.
 
It's probably your experience but why do you say Vgs needs to be at least 10V.
I just today tested SSH7N90A and it gives max current at Vgs = 5V. Specs say threshold max 3.5V .
(5A until it heats up, then 3A)

I tried up to 12 V when cold but I can't get specified 7A.
I only have up to 36 V Vds test conditions.
 
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crutschchow:

I analyzed the "source" voltage of the configuration in practical... it was turning on properly... if it weren't the oscilloscope would show either a distorted pulse or straight line but it was a nice pulse...

ronv:

but for my configuration i dont knwo what is Vs.... my Vs source is not connected to ground? i only know Vd. and Vg.

Vlad777:
Datasheet says that for Rds(on) Vgs = 10...
 
I said Vgs should be 10V for good saturation because that's how many MOSFETS are rated. The threshold voltage, the point at which it is just starting to turn on, is much less than that, of course.

If you look in the data sheet for the particular MOSFET you are using, it will give you the Vgs voltage required to deliver the rated current at minimum "ON" voltage.
 
ok chow:

So meaning it doesnt matter what type of configuration that my NMOS is in (Source grounded or Source follower config). All i need to see is Vgs ya? For example the output of buck converter needed is 6V, so meaning that wil be my Vs? and from there if Vg is somewhere around 14V~18V it can drive my nmos to on properly? because Vgs more than or equal to around 10V?

to off it on the other hand my Vgs needs to be 0 right? meaning, Vg can be 6V or less right?
 
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