h bridge

[aamir1]

dear friends !! i am designing a h bridge circuit for a 4A motor i want to use mosfets for that is it necessary to use ir2110 I.C for that??? and one more thing in which region does the mosfet should be driven ??? linear or saturation ?? help pzl

 

[dougy83]

You would use the IR2110 if the voltage you are driving the motor with is large; otherwise it is not necessary, but can still be used nonetheless. You would normally operate the MOSFETs in the linear region unless you want them to get hot.

 

[aamir1]

the motor is of 12 volts and the driving voltage is 5v which will come from controller is it necessary to use ir2110 ?

 

[koolguy]

Hi, in my site you can get the circuit using mosfet...
i am using both P and N-MOS but it can be done with 4 n mos also.

 

[audioguru]

Most Mosfets need a 10V or 12V gate-source voltage to completely turn on. With a gate-source voltage of only 5V then you need "logic-level" Mosfets.
You should amplify the 5V driving voltage with a transistor so it becomes a 12V driving voltage. Then you can use ordinary P-channel and N-channel Mosfets.

 

[shortbus=]

You would normally operate the MOSFETs in the linear region unless you want them to get hot.

Did you mean to say this? I was always understood that the "linear region" of a mosfet was the area of operation that made heat, high resistance. And that the "saturated region" was the area of operation that was fully turned on, lowest resistance.

Can some one comment on this?

 

[ericgibbs]

Did you mean to say this? I was always understood that the "linear region" of a mosfet was the area of operation that made heat, high resistance. And that the "saturated region" was the area of operation that was fully turned on, lowest resistance.

Can some one comment on this?

hi,
The linear region will generate a higher dissipation in the MOSFET

The OFF or saturated region should generate the least dissipation.

E

 

[audioguru]

Mosfet operation is the opposite to bipolar transistor operation.
A Mosfet is in the "saturation" region when it is not turned on hard. It is in the "ohmic" or "linear" region when it is turned on hard.

 

[shortbus=]

Thanks for answering, but one of you says one thing and one the other. But AG's graph shows the opposite of what he verbally says, but reinforces what Eric says. Now which is it?

 

[audioguru]

Thanks for answering, but one of you says one thing and one the other. But AG's graph shows the opposite of what he verbally says, but reinforces what Eric says. Now which is it?

No.
My graphs show that the Mosfet is a linear amplifier when it is in the "saturation" region and it is a switch when it is in the "ohmic or linear" region.
A Bipolar transistor is the opposite.

It heats when it is not switched on hard because then it has voltage and current. When it is switched on hard it has current but its voltage is very low so it does not heat much.

 

[jpanhalt]

It may be a bit confusing, but AG has some pretty good support:

Source: Application Note 558 (Fairchild Semiconductor)
Ralph Locher

ON-RESISTANCE RDS(on)
The on-resistance of a Power MOSFET is a very important parameter because it determines how
much current the device can carry for low to medium frequency (less than 200kHz) applications.
After being turned on, the on-state is defined simply as its on-state voltage divided by on-state
current. When conducting current as a switch, the conduction losses P are:
(9)
To minimize RDS(on), the applied gate signal should be large enough to maintain operation in the
linear or ohmic region as shown in Figure 8. (emphasis added)

Sorry, I do not have the actual link, but have attached the pdf. Perhaps the confusion is between gate "saturation"(i.e., the voltage to turn it on completely) and mosfet saturation.

John

View attachment mosfet heating_AN-558.pdf

 

[dougy83]

Did you mean to say this? I was always understood that the "linear region" of a mosfet was the area of operation that made heat, high resistance. And that the "saturated region" was the area of operation that was fully turned on, lowest resistance.

Yes, I meant to say that. AG has already posted the graph of the MOSFET operating regions, which clearly show that in the linear/ohmic region, there is less power dissipated by the MOSFET.

Thanks for answering, but one of you says one thing and one the other. But AG's graph shows the opposite of what he verbally says, but reinforces what Eric says. Now which is it?

Nothing reinforces what Eric said. Eric misspoke. AG was consistent with the graph.

To work out the power dissipated by the MOSFET from the graph, for a given trace, simply multiply the voltage by the current for each point on the trace. From this you can see where a suitable operating point (or region) will be.

 

[shortbus=]

This is blowing my mind. I've always understood that you need to use the correct Vgs to get the mosfet (when used as a switch) out of the ohmic/linear region. That this was done to lessen the amount of heat produced in the mosfet.

Now you guys are saying that the linear region gives the least heat in the mosfet. Going to have to read the PDF that jpanhalt posted.

 

[audioguru]

The terms for a Mosfet are The Opposite of an ordinary transistor.
An ordinary transistor is saturated when it is turned on hard. Then it has a very low Vce so it does not heat much.
When an ordinary transistor is linear then it has a fairly high Vce and a fairly high current so it gets hot.

A Mosfet does not saturate. Instead it fully turns on and it is in the "ohmic or linear" region. Then it has a very low voltage across it so it does not heat much.
When a Mosfet is partially turned on then it is in the "saturation" region and has voltage across it and current through it so it gets hot.

They should not say a Mosfet is in the "saturation" region which is confusing. Instead they should say it is "partially turned on".

 

[BeerBelly]

They should not say a Mosfet is in the "saturation" region which is confusing. Instead they should say it is "partially turned on".

A transistor, mosfet,switch,relay, all dissipate less heat when either fully on or fully off. It's the in between fully on and off where the heat is wasted.

 

[ericgibbs]

A transistor, mosfet,switch,relay, all dissipate less heat when either fully on or fully off. It's the in between fully on and off where the heat is wasted.

hi BB,
I agree with your comments.

I also agree with 'agu's remarks [post14] regarding the ambiguous way saturation is defined.

But the bottom line in simple terms,, operating a MOSFET in the linear region will generate a higher dissipation in the MOSFET compared to being used in switching mode.

E

 

[shortbus=]

OK, I know I'm not near as informed on this stuff as the rest of you. But the graphs that AG posted and the link that jpanhalt (jp) gave still don't make sense to me.

All of the graphs in mosfet data sheets are basically the same. The ohmic/linear region is at the left side and a very small part of the scale. In the link from jp it gives an example of a mosfet that has a 30V rating but the ohmic/linear region stops at ~1.5V, and then goes into the saturated region. It , the example given in the link, is a logic level mosfet and this 1.5V is where the curve starts on the graph for Vgs 4.5V the rated value.

What am I missing? How is the graph supposed to be read? Surely the ohmic/linear/useful voltage of a mosfet isn't that small?

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?

Please help an old dumb guy make sense of this. @Moderators, I'm sorry this ended up hijacking the OPs thread. Maybe you should make it into a new thread?

 

[ronv]

It does mess with your mind.

"One should also note that in comparing
MOSFET operation to Bipolar transistor, the linear and saturated regions are just the opposite."

It's all in the name.

Try this write up. It shows to make a linear amplifier the mosfet needs to be in the saturation region. Cute...

https://www.electro-tech-online.com/custompdfs/2013/07/ch06.pdf

When used as a switch, hard and fast is the rule as power is dissipated in the area between full off and full on.

 

[alec_t]

Does this help? It shows FET dissipation as the gate voltage is ramped up from 0 to 10V. I don't recall seeing the max power region referred to as the 'saturation' region.

 

[shortbus=]

OK, after much reading and Googling I may have found the answer. It depends on that you want the mosfet to do.

AG's and jp's answer is correct for amplification and small signals. But not for power switching. For power you want to get in to the fully turned on, fully saturated or as this application note calls it "active region". Pages #8, #9.

So for the OPs case he wants it in the active (switch) not ohmic/linear region.