OpenServo.com

clcheung · Joined: 14 Apr 2008 Posts: 27

Hi,

I have tried to use openservo on MG995. At this stage I just use IRF9450 and IRF540 to test it out instead of the IRF7307 or IRF7309. My knowledge on MOSFET is quite limited.

So far I am able to get it up and running well. However, I find that the temperature of the MOSFET goes up quickly to 100 degree C, even those the motor loading is not that high. I am wondering if it is a wrong pick of MOSFET or it is a normal behavior ? If using the small footprint IRF730x, will the temperature be lowered ?

It seems the temperature rising is caused by the PWM switching, i.e., even my motor do not move much, but whenever the PWM output is not zero, the MOSFET start to burn.

FYI, I am using the V2 schematic.

Many thanks.

clcheung · Joined: 14 Apr 2008 Posts: 27

problem fixed. Sorry to bother you guys.

TCPIP_v6 · Joined: 31 Aug 2008 Posts: 3

Hi clcheung,

What did you do to fix the problem?

clcheung · Joined: 14 Apr 2008 Posts: 27

Hi

I made a silly mistake in the schematic so everything works fine after fixing that.

I believe any MOSFET will work fine on the openservo schematic except that you may need to tune up on the delay time during direction switching.

guru · Joined: 03 Jan 2006 Posts: 128 Location: St Pete Beach, FL

This problem can occur from floating mosfet inputs, thus the hbridge can be slightly active in both directions. (I experienced this designing hbridges.) Also, of course, a mistake in the software can make it happen too, but usually this makes them smoke, not just get very hot.

angelrita123 · Joined: 23 Sep 2009 Posts: 1

Ok... I've been playing with some hardware the past few months and I'm shocked at how much I forgot just by doing firmware over the years.

I need to run a 3.3V logic line to a P-Channel MOSFET. Its running to the gate and I have 5V at the Source. I will be drawing .08mA when active... I'm not switching this on at any fast rate.

I am running this model is switchercad and noticing different MOSFET's giving different results... mainly when trying to turn the fet off with 3.3V some can some wont and I cant seem to find what is the determining factor.

I would then like to get some MOSFET's from digikey.

Example: I run my simulation with...
Fairchild: FQB11P06,FDC5614P works! FDS9933A,FDS4953 doesnt

I am not sure if it has to deal with the Rds(on), Qgate ????
Well... thanks for your input in advance. Confused

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jharvey · Posted: Thu Oct 01, 2009 10:24 am Post subject:

I haven't done much modeling via spice or others. Some things you should keep in mind. The driving pin will drive hard at first to compensate for the capacitive components. So I might recommend driving it via resistor to help prevent spikes. Also a drain resistors is often good, something between gate and drain for low side driving. I see you mention P channel, that implies high side driving. Where's the load you are trying to drive?

Here's a good refresher about MOSFET's

http://en.wikipedia.org/wiki/Mosfet

For MOSFET's I typically recommend the PIP3104 as a general purpose. I like it because of it's over voltage protection scheme.