Mosfet switching

[colin mac]

I'm multiplexing 7-segments from the o/p of a micro. I want to use Mosfets instead of BJTs to enable the 7-segs. My question is are Mosfets fast enough?

Thanks.

 

[Hank Fletcher]

It's a pretty generic question, because the frequency characteristics of MOSFETs vary from part to part. What's your MOSFETs' part number?

I'd err on the side of saying they'll be just fine. The human eye is much slower than most transistors can switch - anything above 50Hz looks as smooth as to appear permanently on. If you want to make a video of your display (so taking into account the flicker of the viewing monitor/TV, or the frame rate of the camera), just a little bit faster than that might be desirable, so that the image of the display still appears smooth (flicker free).

 

[Papabravo]

MOSFETS come in two flavors, N-channel and P-channel. The primary difficulty among first time users is not having a CLEAR picture of what it means to turn one on. The second difficulty is in designing a drive circuit that can move charge on to and off of the gate terminal.

Beginning circuit designers think in terms of voltage. Advanced designers think in terms of current.

 

[colin mac]

I looked in the datasheet and the turn on/off time is lower than 140ns, so I reckon that's quick enough since the segments will be on for milliseconds a piece.

I tested the mosfet and it works great at lighting up the segments when I apply
0V to the gate, and turns off again when the gate is high. I only needed one resistor on the drain so it's much simpler also.

 

[Nigel Goodwin]

I looked in the datasheet and the turn on/off time is lower than 140ns, so I reckon that's quick enough since the segments will be on for milliseconds a piece.

Multiplexing a display is really a very slow procedure, you could do it mechanically never mind using a MOSFET - speed of the device isn't going to be a problem.

 

[colin mac]

Sounds good. Take it easy guys.

 

[Papabravo]

Yes well the problem is not speed with respect to your application. I'm guessing you have a P-channel MOSFET since you said it is turned on when the gate is at 0V. Since you did not provide us with a schematic it is difficult to say if there will be a problem associated with turning the device off. The switching speed of the device quoted in the datasheet makes some assumptions about the ability of the driving circuit to move charge off the gate. The problem with the turnoff may be an excessive amount of time in the linear region leading to excessive heat dissipation. I don't know if you will have this problem or not. It is the thing to watch out for.

 

[colin mac]

Yes, they are IRf9530 P-Channel.

I applied a square wave to the gate, from a signal generator, to flash an led.
I noticed flicker until I reached 44Hz. I also tested my micro, with one 7-seg. I turned the mosfet on and off at a low frequency, and sent out a single digit, and it worked as expected.