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Forum Index : Electronics : Basic Dumb Mosfet Question.

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Davo99
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Joined: 03/06/2019
Location: Australia
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Posted: 04:44am 06 Aug 2019
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I have spent ( too much) time looking on the net but can't find an answer to a basic Question.

Are Mosfets ( one type or the other) like conventional relays where you can use a low input current to switch a Much Higher one?

I thought that was their basic premise but mucking around with some I pulled out of an old GTI, all the legs seem to draw roughly the same power which doesn't make sense to me. Maybe I'm using the wrong Fets or they are stuffed or it's just the way they work.

Knowing if My expectations are correct or not will be the first thing in determining if I am going about things the wrong way to start.

Thanks for any education of the dummies.
 
mackoffgrid

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Posted: 05:13am 06 Aug 2019
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  Davo99 said  Are Mosfets ( one type or the other) like conventional relays where you can use a low input current to switch a Much Higher one?


That statement better describes a conventional Transistor, NPN or PNP.

The physics of Mosfets is even more weird but, simply - you apply a voltage to the gate and that voltage will determine the resistance of the Drain to Source path. Unusually, 10v is enough to lower the resistance to its minimum amount which is referred to as Rds. One of the favourite Mosfets among the Ozinverters is the HY4008 and it has a Rds of 2.9 milli ohms.

BTW, If you simply apply a static DC voltage to the Gate, it will consume negligible power.

Cheers
Andrew
 
Poppy

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Joined: 25/07/2019
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Posted: 06:56am 06 Aug 2019
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A little quotation from
Charles Platt
Encyclopedia of Electronic Components Volume 1
O´Reilly, 2013

  Quote  
A field-effect transistor creates an electric field to
control current flowing through a channel in a
semiconductor. MOSFETs of microscopic size
form the basis of complementary metal oxide
semiconductor (CMOS) integrated circuit chips,
while large discrete MOSFETs are capable of
switching substantial currents, in lamp dimmers,
audio amplifiers, and motor controllers. FETs
have become indispensable in computer electronics.

A bipolar transistor is generally thought of as
a current amplifier because the current passing
through it is controlled by a smaller amount of
current passing through the base. By contrast, all
FETs are considered to be voltage amplifiers, as
the control voltage establishes field intensity,
which requires little or no current. The negligible
leakage through the gate of an FET makes it ideal
for use in low-power applications such as
portable hand-held devices.


Actually you can use both as simple switches and if they are "power" versions then comparable to a simple relay, but generally without galvanic isolation.


Edited 2019-08-06 16:59 by Poppy
Andre ... such a GURU?
 
Davo99
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Posted: 07:40am 06 Aug 2019
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Thank you Andrew.
I am putting power on the gate and it's sucking a lot of power.
Least I know that's not the way it should be and can try and find what I am doing wrong.

Would it be in the ball park to say a mosfet can be like a variable resistor depending on the voltage applied to the gate as far as the amount of power that will pass between the source and the drain?

Thanks again.
 
Davo99
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Posted: 07:43am 06 Aug 2019
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Thanks Poppy.
Enlightens me a bit too.
 
Poppy

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Posted: 08:05am 06 Aug 2019
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What component is it exactly?

Andre ... such a GURU?
 
poida

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Posted: 10:48am 06 Aug 2019
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  Davo99 said  Thank you Andrew.
I am putting power on the gate and it's sucking a lot of power.
Least I know that's not the way it should be and can try and find what I am doing wrong.

Would it be in the ball park to say a mosfet can be like a variable resistor depending on the voltage applied to the gate as far as the amount of power that will pass between the source and the drain?

Thanks again.


Oh yes, for sure.
I built a constant current DC load using some special mosfets.
I can control the current the device consumes with a potentiomer
from zero to about 8 Amps, and with voltages from about 4V to 50V.



Some MOSFETs are very good switches and others are more useful when used in the intermediate gate drive voltages, becoming the variable resistor.
The constant current device of mine is the latter.

In posts here on the forum I explored how easy it was to destroy the
switching optimised type MOSFETS by running them so that they are partly
switched on.
The HY4008 was very easy to kill in these tests.
The constant current thing I made uses NEC K3304 MOSFETS which are quite happy to
run at any gate voltage, producing large amounts of heat.
I drive the gates via an op-amp output such that the op-amp ensures the voltage drop
across the large resistors equals the voltage out of the potentiometer.
This way I have a current control. (this device has been invaluable in helping me
understand switching power supplies and inverter theory)

In the application of inverters and power conversion & control in general,
MOSFETS are always driven in such a way as to ensure they are either full ON or full OFF and to take as little time as possible when changing states.
When OFF the MOSFET consumes zero energy and when fully ON it also consumes
timy amounts of energy. It's the time spent going from one to the other state
where most of the heat is generated.
wronger than a phone book full of wrong phone numbers
 
mackoffgrid

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Posted: 12:38am 07 Aug 2019
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Peter,

Would you mind posting a schematic (sketch) of your circuit.

Cheers
Andrew
 
Warpspeed
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Posted: 01:05am 07 Aug 2019
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  poida said  
  Davo99 said  

Some MOSFETs are very good switches and others are more useful when used in the intermediate gate drive voltages, becoming the variable resistor.
The constant current device of mine is the latter.

In posts here on the forum I explored how easy it was to destroy the
switching optimised type MOSFETS by running them so that they are partly
switched on.
The HY4008 was very easy to kill in these tests.


Indeed very true. If you are running a mosfet that is only partially turned on, there is absolutely no disadvantage in using a mosfet that has a much higher Rdson rating.
In fact, it is an advantage, as the current sharing over the silicon die will be better.
So the higher voltage rated mosfets offer some advantages for linear operation and will be more robust. The thing to look for is the power dissipation figure. That is going to be the limiting factor for linear operation rather than the max current rating.
Cheers,  Tony.
 
poida

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Posted: 05:38am 07 Aug 2019
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Mack:
two DC terminal pairs, one for 12V supply, one for the DC load.
I use the LM358 as a comparator to control the cooling fan.
And place the NTC on the heatsink, I put it on the top of one of the FETS

This is from memory. Probably only need 50R gate resistors, maybe a 100nF cap.
The power FETS are tricky. You can not use the usual power MOSFET such as HY4008.
They will be weak, and blow ever so quietly under low loads.
(about 1/5 of what you expect from the spec's safe operating area.

The NEC K3304 I salvaged from a dead plasma TV or something are very good for this.
No idea what else to use.
IGBTs were suggested.

I have had 150W continuous on this thing, peaks of 200W
10A at 20V is reasonable, 5A at 30V all day.

wronger than a phone book full of wrong phone numbers
 
mackoffgrid

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Posted: 05:55am 07 Aug 2019
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Thanks
I was going to use standard Power Mosfet but I'll be more careful now IGBT makes good sense.



Cheers
Andrew
 
poida

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Posted: 09:01am 07 Aug 2019
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and the 100nF cap is probably not needed.
But have a go and see what happens.
The feeling of Absolute Power when you twiddle the pot's knob
to go from 0.5A to 5A, just like that, is intoxicating.

I'd love to figure out some sort of variable load for 240V AC, from 0 to 5kW
again controlled by a pot. I want to test the inverters.
I wonder if a large salt water bath, with 2 large plates, connected to a
3kW Areosharp toroid via a low voltage secondary might work.
Have a motor dip in more or less under some control mechanism..
wronger than a phone book full of wrong phone numbers
 
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