Driving Logic Level Mosfets directly from micro controller port, is it possible ?

I

ItsMike

New Member

• Jun 17, 2011

• #1

Hey everyone,

I wonder if it is possible to drive a logic level mosfet directly from a micro controller.
The IRLZ44N has a Vgs(TH) of 2v, while the µC supplies almost 5v at 25mA or so.

Will that be enough to drive the mosfet into saturation fast enough for a 20KHz+ pwm ?
What gate charge do I need to calculate the current needed to drive the gate ?

Thanks in advance.

 

Ian Rogers

User Extraordinaire

Forum Supporter

Most Helpful Member

• Jun 17, 2011

• #2

I have used the IRL520N TTL MOSFET which connects directly to a micro. I used the PIC18F452 no problem When I moved to the 4520 it didn't work, Microchip explained that the new chips don't latch properly as there is a build up of capacitance and the gate stays high..

 

I

ItsMike

New Member

• Jun 17, 2011

• #3

The question is will it be able to drive the mosfet into saturation fast enough for 20KHz+ pwm.

 

Ian Rogers

User Extraordinaire

Forum Supporter

Most Helpful Member

• Jun 17, 2011

• #4

Must it be a mosfet? I dropped them and I now use Darlington with the same current capability..The trouble here is the larger the device the slower it goes.. are you pulling much amps from the mosfet?

 

I

ItsMike

New Member

• Jun 17, 2011

• #5

About 4-5A.
The thing about Darligtons is that they have high VCE of up to 2v, and that's a lot of power wasted on heat.

 

ronv

Well-Known Member

Most Helpful Member

• Jun 17, 2011

• #6

You can use Qg (total gate charge) to estimate turn on time. It is the turn on time in nanoseconds that 1 amp of gate current will provide. In the case of your transistor it is 48. So 48/.025 = 1920 ns. add the rise time etc and you have a little over 2 us, not to good for a 20 Khz PWM.

PS. They never seem to be as fast in real life as the spec.

 

Last edited: Jun 17, 2011

crutschow

Well-Known Member

Most Helpful Member

• Jun 17, 2011

• #7

ronv said:

You can use Qg (total gate charge) to estimate turn on time. It is the turn on time in nanoseconds that 1 amp of gate current will provide. In the case of your transistor it is 48. So 48/.025 = 1920 ns. add the rise time etc and you have a little over 2 us, not to good for a 20 Khz PWM.

Click to expand...

It somewhat depends upon the current and voltage you are switching with the PWM. A 2µs rise and fall time is about 8% of the 20kHz period. For a rough calculation of the average power dissipated by the transistor from the switching time, multiply 8% by the MOSFET supply voltage times ½ the peak current (0.08 x V x I/2).