h bridge

[jpanhalt]

I think you are getting closer, but you still seem to be confusing "fully turned on" with operating the mosfet in its saturated region. When the mosfet/gate is not fully turned on, it is not acting as a switch. Increasing or decreasing the gate voltage will affect the resistance between the drain and source (Rds). With a fixed voltage on the drain, you will see the current (Id) change as a function of the gate voltage relative to source (Vgs).

As a switch, when the mosfet is fully turned on, Vds should be quite small (Id*Rds).* If you see a large and increasing Vds with minimal increase in Id (i.e., the mosfet is in its saturated region), the mosfet effectively cannot handle more current and heat is produced.

John

*This fact explains why Vg must exceed Vd if an N-channel mosfet is used in the high configuration, i.e, load is attached to source.

 

[audioguru]

In many electronic websites the thing they always say is turn on the mosfet fast and hard, to get it out of the linear region as fast as you can. Are they all wrong?

Yes they are all wrong.
When a Mosfet is turned on hard as a switch it is in the "linear or ohmic" region. It gets hot when it is an amplifier in the "saturation" region.
The graphs show this.
The terms "linear and saturation" are the opposite for a bipolar ordinary transistor.

EDIT: when a Mosfet is turned on hard with an on-resistance of only 0.05 ohms and it has 30Amps flowing then it heats with (30 squared x 0.05 ohms =) 45watts which is very hot. The voltage across it is (30A x 0.05 ohms =) 1.5V.

 

[BeerBelly]

when a Mosfet is turned on hard with an on-resistance of only 0.05 ohms and it has 30Amps flowing then it heats with (30 squared x 0.05 ohms =) 45watts which is very hot. The voltage across it is (30A x 0.05 ohms =) 1.5V.

I now try to visualize a MOSFET turned on soft with say 5 ohms and 30 amps across it
(30 squared x 5 ohms =) 4500watts
(30A x 5 ohms =) 150V

 

[alec_t]

The terminology is indeed unfortunate. Perhaps it helps to think of the MOSFET "saturation region" as the characteristic curve region in which the channel is becoming increasingly saturated with charge carriers, the "linear or ohmic region" being where the channel is fully saturated.

 

[audioguru]

I now try to visualize a MOSFET turned on soft with say 5 ohms and 30 amps across it
(30 squared x 5 ohms =) 4500watts
(30A x 5 ohms =) 150V

Why would you want to blow up a Mosfet by using it to heat your home?

 

[alec_t]

.....maybe not heat your home (though 4.5kW would help, albeit very briefly!), but a FET could be handy as a cheap, convenient, compact, controllable heater in some circumstances, e.g. for laboratory specimens, micro-environments .

 

[shortbus=]

Yes they are all wrong.
When a Mosfet is turned on hard as a switch it is in the "linear or ohmic" region. It gets hot when it is an amplifier in the "saturation" region.
The graphs show this.
The terms "linear and saturation" are the opposite for a bipolar ordinary transistor.

EDIT: when a Mosfet is turned on hard with an on-resistance of only 0.05 ohms and it has 30Amps flowing then it heats with (30 squared x 0.05 ohms =) 45watts which is very hot. The voltage across it is (30A x 0.05 ohms =) 1.5V.

AG, go back and look at the graphs you posted and read my post #17. How can the ohmic/linear region be the only useable part of a mosfet? All of the graphs show the Vds as a very low value in the linear region. the higher voltages are into the saturated region. This is the part I don't understand.

Or is the whole area above the Vgs value included into the linear region? Would you ge able to show on one of the graphs the complete linear region for a mosfet?

 

[dougy83]

Re post 17, you're asking about the max voltage rating of the FET, Vds. This the the maximum recommended voltage between drain and source; it doesn't mean you can have the max current flowing through it as well. Vds has nothing to do with the linear region; the linear region (in terms of Vds, as you put it) will be determined by the applied Vgs.

As for the region being "small", for switching, you only want to use the linear region (with as low a Vds as practicable) for the on state, and the off state.

Or is the whole area above the Vgs value included into the linear region? Would you ge able to show on one of the graphs the complete linear region for a mosfet?

The linear region is to the left of the green/brown/red arc shown in the plots provided by AG.

 

[audioguru]

AG, go back and look at the graphs you posted and read my post #17. How can the ohmic/linear region be the only useable part of a mosfet?

A Mosfet can be a turned on switch (in the ohmic/linear region or it can be an amplifier in the saturated region.

All of the graphs show the Vds as a very low value in the linear region

Yes because it is turned on hard. The datasheet lists its typical and maximum on-resistance when it is turned on hard.

the higher voltages are into the saturated region.

Yes because it is only partly turned on and is an amplifier. Then of course it has a higher voltage across it. It cannot amplify when it is turned on hard because as an amplifier its output must be able to swing up and down.

Or is the whole area above the Vgs value included into the linear region? Would you ge able to show on one of the graphs the complete linear region for a mosfet?

The term "linear" is very confusing when talking about Mosfet operation. Instead you should ask about when it is a switch that is turned on hard. Or ask about when it is an amplifier that is partly turned on.

 

[shortbus=]

Well after more reading and rereading, I had a "head slap" moment. I've been looking at the Vds and Id values as the circuit or supply values going TO the mosfet. (head slap) The Vds is the voltage drop, due to the Rds(on) in the mosfet. Like the voltage drop of any normal resistor in the circuit.

Do I have it correct this time? This stuff is hard to learn, when you don't have the actual background of theory. And need to do it in a non personal, one on one, face to face way.

 

[alec_t]

The Vds is the voltage drop, due to the Rds(on) in the mosfet. Like the voltage drop of any normal resistor in the circuit.

Yes. And that's why it's called the 'ohmic region'. Vds is linearly proportional there to the current. Hence the alternative name 'linear region'.