MOSFET question

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clcheung
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Joined: Mon Apr 14, 2008 5:39 am

MOSFET question

Post by clcheung » Mon Apr 14, 2008 9:26 am

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
Posts: 27
Joined: Mon Apr 14, 2008 5:39 am

Post by clcheung » Tue Apr 15, 2008 3:22 am

problem fixed. Sorry to bother you guys.

TCPIP_v6
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Joined: Sun Aug 31, 2008 12:39 pm

Post by TCPIP_v6 » Fri Sep 05, 2008 7:22 pm

Hi clcheung,

What did you do to fix the problem?

clcheung
Posts: 27
Joined: Mon Apr 14, 2008 5:39 am

Post by clcheung » Sat Sep 06, 2008 1:20 am

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
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Post by guru » Mon Sep 15, 2008 8:50 pm

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.

clcheung wrote: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.

jharvey
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Post by jharvey » Thu Oct 01, 2009 10:24 am

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.

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