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Electronics => Electronics => Topic started by: izua on April 25, 2008, 10:39:22 AM

Title: commanding MOS/FETs
Post by: izua on April 25, 2008, 10:39:22 AM

I'm planning to build an efficient stepper controller.
I want to understand how to command MOSFETs efficiently so they don't heat at all - I've seen it done for DC motors and steppers sucking 3-4 amps with the transistors cool to touch. (I think they were IRF or IRFZ family). I've been told that one factor is having a very low drain-source resistance while another factor is pumping a bigger voltage into the gate.

I haven't been able to find any more info on this, but I have a few BUZ11 power mosfets around to play with (rated at 30A, 50V, 75W and 150/300ns switching time).
What I want to do is a stepper control circuit for <4Amps, with the transistors cool.
Ideas, suggestions, any experiments anyone done before? thanks.

Title: Re: commanding MOS/FETs
Post by: Rebelgium on April 25, 2008, 12:00:50 PM

Quote from: izua on April 25, 2008, 10:39:22 AM

I've been told that one factor is having a very low drain-source resistance while another factor is pumping a bigger voltage into the gate.

Those are the only ones I know to.
Having the max allowed gate voltage ensures your FETS will conduct for 100%, resulting in less heat.

Title: Re: commanding MOS/FETs
Post by: izua on April 25, 2008, 12:10:23 PM

I'll need that voltage from somewhere. and an extra pair of transistors to drive that voltage from my uc.
what is the characteristic that determines at what voltage the transistor goes into saturation?

Title: Re: commanding MOS/FETs
Post by: GHF on April 25, 2008, 12:35:20 PM

Quote from: izua on April 25, 2008, 12:10:23 PM

I'll need that voltage from somewhere. and an extra pair of transistors to drive that voltage from my uc.
what is the characteristic that determines at what voltage the transistor goes into saturation?

You don't have to do all that. There are MOSFET drivers that take care of the higher voltage and and be controlled straight from a micro's pins.

-Xo W.

Title: Re: commanding MOS/FETs
Post by: izua on April 25, 2008, 12:39:18 PM

I haven't found any for sale, that's the problem.
Then again, I might be looking for the wrong stuff. Any names?

Title: Re: commanding MOS/FETs
Post by: Admin on April 27, 2008, 01:43:51 PM

Quote

what is the characteristic that determines at what voltage the transistor goes into saturation?

see mosfet datasheet for a graph at the bottom ;)

Quote

I haven't found any for sale, that's the problem.
Then again, I might be looking for the wrong stuff. Any names?

go to digikey or mouser.com and type 'mosfet driver', you will find a few thousand :P


As for choosing a mosfet, if its a one-off design, just get over-rated mosfets. No harm in that . . .