LOW SIDE MOSFET Driving

[ScratchBuild]

low side mosfet

Hi everyone,

I just need to understand an elementary question here please. I understand how to drive a HIGH side N-Channel mosfet in an H-Bridge and why the Gate voltage must be 10 Volts or higher.

Can any anyone please explain the gate source voltage on a low side N-Channel mosfet in an H-Bridge configuration please. What is the voltage at the gate that must be applied to handle the maximum current on the low side?

Regards

Nic

 

[FvM]

low side high side mosfet difference

Hello,

there's no difference in Ugs requirement for high- or low-side driver. The differences may be in driver circuit, cause high-side driver must be floating. If you have e.g. optoisolated drivers with internal power supply (they are used mainly for high power MOSFET and IGBT bridges), high- and low-side drivers are using identical circuits.

To my opinion, low-side driver circuit is typical more simple for non-isolated driver, cause you don't need a level shift nor a floating supply.

E.g. IRF supplies integrated drivers for MOSFET bridges, the have instructive application literature, I remember.

Regards,
Frank

 

[ScratchBuild]

Hi there,

I think I figured it out, and the problem is with the floating power supply. With low side MOSFETS you need 0V + 10-15V to fully drive the MOSFET. But with the High Side the gate supply must be Vcc + 10-15V.

I could not see the difference because I have a supply of 7.2V and needed to get the Vcc+10V from a charge pump. In this case a need the charge pump for the low side too because I have to get to a minimum of 10V Vgs.

But I think I do understand it now.

Regards

Nic

 

[ScratchBuild]

Hi everyone,

OK, I want to drive these MOSFETS from 7.2 - 14V batteries. So I need a charge pump that can output at least 25mA or higher at 18V, it needs to also put out Vcc + 11V more or less. The MAX622 does this but only puts out +- 2mA at 18V, so it does not have enough power to drive the MOSFET fast enough. Can anyone direct me to a charge pump that are able to do this and do come in a SO or SOT23 package.

My second problem is that most of the MOSFET driver IC's can only handle up to 15V operational, like the TC4427, Does anybody know of any chip that can handle my specs at the top.

Oh yes, the Mosfets I want to drive is the IRF1404S.

Regards

Nic

 

[ScratchBuild]

Hi there,

If I calculated the current for an mosfet to be 10A @40•C Ambient standing in free air from the datasheet. I for instance find that I have to have 5Vgs on the gate to allow for this current to flow. Say I know the current will never go above 10A. Do I have to only provide for the 5V gate voltage or do I still have to saturate it not to get hot?

As far as I gather the Rds is decreasing with Vgs and for that reason it would still be advantageogus to fully turn the MOSFET on.

P=I²R

and therefore P would be minimised and therefore temperature as well.

Regards

Nic