Mosfet High or Low side

[100mhz]

As far as a driver , would this be considered high or low side?
And, am I wrong in thinking that an enhanced P-Channel Mosfet is driven differently than a standard P-Channel Fet? Source Voltage turns it off, ground turns it on?

 

[KlausST]

Hi,

The FET (switch) is in the high side. --> high side switching

With p-ch FETs:
* more negative Vgs makes it more conductive
* more positive Vgs makes it less conductive

Enhancement mode p-ch FET are non conductive with zero Vgs. They need negative Vgs to get conductive
Depletion mode FETs are conductive with zero Vgs. They need positive Vgs to become non condictive. With negative Vgs they become even more conductive.

MOSFETs usually are enhancement mode FETs.

Klaus

 

[100mhz]

Thanks Klaus! A very thorough answer. I've been searching for the last 4 hours for a suitable high side driver. Used the parametric search on digi-key and mouser but as soon as I choose high side P-channel it reduces the results to a very minimum. None of which quite fits my requirements.
Since a P-channel only has one way to turn it on and off I can't quite comprehend why there is a difference in high side and low side drivers. I noticed a few drivers that claim to work high and low side.
I also noticed in the datasheets that drivers are rated in amps. My FET is rated at 6 amps so would the driver need to be rated at 6 amps also?
Should anyone be familiar with drivers and could suggest one, my specs are ;
Rated for up to 30 volts.
Logic level input. 3v to 5.5v?
Rise / Fall Time (Typ) 25ns, 25ns (or better)

The circuit I'm working on is currently using a icl7660 and 555 to switch the FET. It's clocked at 10khz and without a heatsink doesn't get warm. What I'm trying to do is remove the 7660/555 so I can control the frequency with an MCU.

Thanks

 

[FvM]

Don's see how a ICL7660 could serve as PMOS gate driver.

I'm not aware of a dedicated PMOS gate driver IC. High side NMOS gate drivers are intended to drive the gate voltage above positive supply, which is inappropriate for PMOS. A low side driver with respective supply and level shifter might work, or a discrete driver circuit.

If the switch duty cycle is less than 100 percent with known maximal pulse width, a NMOS switch with bootstrap high side driver could be used instead of the PMOS switch. For 100 % duty cycle a NMOS high side driver with charge pump.

 

[KlausST]

Hi,

The usual way is to use an N-ch Mosfet and a driver IC. For high speed switching I recommend at least 0.5A driver capability.
But in detail you need to calculate with the Mosfet parameters.
Those driver ICs usually contain a bootstrap circuit. It generates the drive supply voltage for the high side Mosfet.
The only drawback of the bootstrap circuit is, that it can't work with 100% duty cycle.
If you are just driving a high side Mosfet you maybe need to add some circuitry to ensure that the bootstrap capacitor becomes charged properly.

Look at IR2110. It may be too slow for yor requirements...but a good point to start.
Driver IC manufacturers have selection guides, distributors, too.

Klaus

 

[treez]

you coudl just make the mcu output switch a low side signal fet or bjt to switch the p channel on and off....ie make it pull the gate down to turn it on, vice versa to turn it off.
The fet is running 6 amps max......as long as you can get the gate voltage low enough to allow the transconductnace of the fet to conduct that much current then you are fine.......

- - - Updated - - -

youll have to discharge the fet junction capacitances quick enough to give you your 25ns rise or fall....so let that be the decider of the resistors that interface from your low side signal transistor to your fet gate.

 

[CataM]

youll have to discharge the fet junction capacitances quick enough to give you your 25ns rise or fall....so let that be the decider of the resistors that interface from your low side signal transistor to your fet gate.

Do you have any reference on how to design that ?

 

[treez]

https://reipooom.files.wordpress.com/2011/08/mosfet-must-read-2-slup169.pdf

pages 1 to 9 of the above show the fet capacitances.......charge current get by ohms law and also use i = cdv/dt

 

[d123]

Hi,

Do you have any reference on how to design that ?

These are two well-known documents with a variety of design ideas:

Design And Application Guide For High Speed MOSFET Gate Drive Circuits

Chapter 5 Gate and Base Circuits and Protection