MOSFET QUESTIONS

[large_ghostman]

ok ive been reading up and watching video's but i have got in a bit of a tangle, we are talking gen purpose N channel MOSFETS here like the irf540?(i think thats the part number) some of the video's gave me the impression that it wasnt just a case of say putting 10V-12V on the gate to fully turn it on, but there had to be a voltage difference of 10V (or so) from GATE to SOURCE.
is this correct?

I had it in my mind that i could use say a P channel transistor with its base connected to pic 5V and drop 12V through it into the GATE of the mosfet and drive a 12V load through the mosfet (obviously i have omitted talk of resistors on the transistor for simplicity), but if i have understood what i have learned so far today this wouldnt work, can someone tell me if the video is infact correct as it wasnt a lecture type vid more a vid by someone on the kitchen table.

 

[audioguru]

The IRF540 is an ordinary Mosfet that has its maximum on-resistance spec'd on its datasheet only when the gate-source voltage is 10V. If you apply a gate-source voltage of only 5V from a PIC then some will barely turn on and others will turn on fairly.

An ordinary P-channel Mosfet also has its maximum on-resistance spec'd when its gate-source voltage is 10V but the polarity is the opposite to an N-channel Mosfet. You can make the source connected to +12V and the load connected from its drain to 0V. Then when its gate is +10V to +12V it is turned off and when its gate is 0v to +2V then it is turned on. If its gate is at +5V from a Pic then most of them will always be turned on fairly and they will never turn off.

"Logic level Mosfets" are available like the IRL540 that has its maximum on-resistance spec'd when its gate-source voltage is 4V and 5V.

 

[large_ghostman]

i cant get to the pc to do a schematic at the mo, but i was thinking of driving a transistor from the pic and using the output of the transistor to drive the mosfet gate with 12V, i am trying to sim it in LT spice, i really need to get to grips with that then i could answer questions like this in a sim

 

[audioguru]

The +5V and 0V from a PIC can turn on and off an ordinary NPN bipolar transistor through a series current-limiting resistor to its base. Its collector connects to the gate and can turn on an ordinary P-channel Mosfet that has its source connected to +12V. Its load is from its drain to 0V. A resistor from gate to source on the Mosfet turns it off when the NPN transistor turns off.
Do it like this:

 

[large_ghostman]

thank you Mr AG thats great it gives me something to work with. how about if i want to drive a N channels mosfet?

 

[audioguru]

An ordinary N-channel Mosfet has its source connected to 0V and a load connected between its drain and the load's supply voltage (+12V).
0V at the gate turns it off and +12V turns it on.
A PIC can turn an NPN transistor on and off and it drives a PNP transistor with its emitter at +12V. The collector of the PNP transistor drives the gate of the Mosfet from 0V to +12V.

 

[large_ghostman]

thank you AG, that has cleared up the confusion on the video's! now i have something to sim, i am still reading up on mosfets then i might need to ask about datasheet stuff .
i think i now have the drain and source thing sorted! i just have to remember its talking about electrons and wich way they move across the depletion area! slowly getting there but it is a bit hard having to rethink stuff. the anology used with a tap is confusing but now i am thinking in electron flow terms it all fits better

 

[large_ghostman]

ok first question after reading up. I picked a random part that i have for no particular reason and i am looking at the datasheet as all my questions are datasheet related, the part is IRF540N the datasheet is this one


now according to the sheet it has a VDSS of 100V, so i take that to mean Voltage from DRAIN to SOURCE? or the max voltage you are meant to put through it.

then we have this table

and it shows a VGS +-20V, now my question for this
is the 20V-+ the max gate voltage allowed or the max difference of gate voltage depending on what the voltage is between Drain and SOURCE? for example does it mean that if i have a VDSS of 100V i have to have a gate voltage no lower than 80V?
i am not sure exactly what it means, my feeling is its the max GATE voltage

 

[audioguru]

I picked a random part that i have for no particular reason and i am looking at the datasheet as all my questions are datasheet related, the part is IRF540N the datasheet is this one
Now according to the sheet it has a VDSS of 100V, so i take that to mean Voltage from DRAIN to SOURCE? or the max voltage you are meant to put through it.

That is the drain to source voltage that will cause some of them to suddenly conduct current when they shouldn't and get very hot. So do not allow more than 100V in your circuit (it will be safer if you do not allow more than 80V).

then we have this table

and it shows a VGS +-20V, now my question for this
is the 20V-+ the max gate voltage allowed

It is the maximum allowed gate to source voltage. Its on-resistance is spec'd at a gate-source voltage of 10V so limit the voltage to 10V to 15V.

or the max difference of gate voltage depending on what the voltage is between Drain and SOURCE? for example does it mean that if i have a VDSS of 100V i have to have a gate voltage no lower than 80V?
i am not sure exactly what it means, my feeling is its the max GATE voltage

Maximum VDSS is completely separate from maximum VGS. For this N-channel Mosfet they are usually both a positive voltage.

 

[large_ghostman]

THANKS AG seems i was on the right path then, next job is to look at the graphs and work those out, very slowly getting there!

 

[audioguru]

THANKS AG seems i was on the right path then, next job is to look at the graphs and work those out, very slowly getting there!

Beware of the graphs on the datasheet for a Mosfet because they are for a "typical" one that you cannot buy. Some Mosfets will be much better or much worse than a "typical" one. You need to look at the printed minimum and maximum guaranteed spec's on a datasheet so that ALL your circuits work properly, not just your circuits that have "typical" parts.

Go to an electronics distributor and order a "typical" Mosfet. They will laugh at you because you get whatever they have. Maybe they have not had a "typical" one for months.

 

[large_ghostman]

that has made me feel better! the graphs always put me off! i will concentrate on the other bit's, thanks for the info

 

[MrAl]

Hi,

There are a couple tricks you can use to drive a MOSFET gate with a PIC, but you're better off using the "logic level" MOSFETs which can turn on with a gate voltage of 5v or even less. That's the best way i think if you want to just use one transistor (the MOSFET). Of course you can drive an NPN transistor and have that drive the MOSFET, but then that's another transistor to worry about.

There is also at least one PIC chip that has at least one output pin that is an open collector. You might be able to use that to drive an ordinary MOSFET, but it's been so long since i looked at this i cant remember what the part number is. I know it is one of the lower end PIC chips though, probably in the 16F series.
I am pretty sure it is only one pin too, but it could be more. It also has no protection diode to Vcc.

But remember that when driving MOSFET gates from low current sources like PIC chips or even smaller transistors, the switching speed is not going to be very fast as compared to using a standard MOSFET driver chip made specifically for driving MOSFETs. Low current drive could have significant degrading effects for applications like power supplies, although other apps such as turning an LED on will usually not be affected unless it has to turn on and off very fast to transmit a signal for example.

 

[large_ghostman]

thanks for that, its something i will keep in mind, the application at the moment is switching a fan on/off but not at speed just simple on for a few seconds then off for maybe 30-60 seconds

EDIT
the other application maybe to switch my heater on/off when its on back up power, as the normal mode will be AC but on backup power it will probably be DC, again not very fast switching maybe 1/2 second on and off for 10-15 seconds (depending)

 

[MrAl]

Hi,

Yes that sounds ok for a fan, unless you plan to do PWM which may require faster switching to reduce power losses in the MOSFET device.

 

[rumpfy]

Your post 8 and the reply in 9 was about 'Absolute Maximum Ratings'. These AMR values are NOT to be exceeded without the danger of destroying the device.
Your post 1 was about using a mosfet as a switch, and the AMR are only half the story to help you to select a suitable device. There are many threads here, where the way to cause a transistor to switch and carry a certain current, are discussed. A bipolar transistor needs a certain level of base current to support a level of collector current. These are 'current' devices. The forward gain characteristic is defined as 'beta' which is Ic/Ib, and is dimensionless.
With a fet, the level of drain current is supported by a certain level of gate to source voltage, and the forward characteristic is defined by Id/Vg. This is a 'conductance' and is has the dimensions of mA/volt or milli Siemens(mS) or for big FET's, in Siemens(S).
So, when you use a fet as a switch, it needs to have an adequate Vgs, and not necessarily the maximum the device can withstand. So you need to ask the question; 'what is my drain current and what is the corresponding drive voltage'. When using these things in high speed circuits, you need to consider the rate of change of charge removal from the gate. From your classes in physics, you will remember that current is the rate of change of charge, so this also is a consideration in device selection, because I= C x de/dt. C will be the gate charge.
Data sheets are a hugely important thing to get to understand, and they are full of traps and pitfalls. So you need to keep asking specific questions.

 

[large_ghostman]

thanks that really helpful info! now i know roughly what i need to look out for! for most stuff we are looking low I, normally under 2A mostly under 1.5A, one or two things i need them for are around 5-8A but thats a one off

 

[misterT]

Add this to your eBook collection: **broken link removed**

 

[large_ghostman]

thanks mr T that one is good enough to print out and go in the sacred folder lucky for me we have a duplex printer so i can print on both sides of the paper and save space

 

[large_ghostman]

i am going to have to get better with google so i can find this kind of stuff! maybe i just dont use the right search terms??? i dont know but i never seem to get this kind of thing pop up in a search!