I have an EMC2101 driving an N channel mosfet that sends a pwm signal to the pwm pin of a fan. See the schematic below:
Also see the waveforms below. The low side I would exepct to be 0V but it is going about -0.5V when the duty cycle is high and as the duty cycle drops, the negative voltage gets closer to 0.

This is measured on the drain of the mosfet which is connected to the pwm pin of a fan that has an internal pull up to 5v.

The offset on the scope/probe is 0V.

 

Because the FET is acting as an inverter in this configuration.

Gate goes high, current flows, drain goes low.

 

Your probe might need to have its frequency compensation adjusted.

 

You have the probe coupling on AC, not DC, so the display tends to center.

 

You have the probe coupling on AC, not DC, so the display tends to center.

It’s on DC. In the picture it shows the setting so I don’t think it’s that.

 

Your probe might need to have its frequency compensation adjusted.

Thanks that’s a new one for me. I’ll look into it and maybe try another probe if I can’t figure it out.

 

It’s on DC. In the picture it shows the setting so I don’t think it’s that.

Yes, you are correct. My mistake. I saw the "AC" and not the highlighted DC.
As they say, "Got to be good-lookin', 'cause he's so hard to see".

 

Just a thought, is your CRO probe GND right at the FET and not over a length of wire that will be carrying current?

 

There are many possibilities making it an artefact on the oscilloscope.
Do you have the oscilloscope probe set to x1 or x10 attenuation?
As Dick says, the frequency compensation on the probe is not properly adjusted.

 

There are many possibilities making it an artefact on the oscilloscope.
Do you have the oscilloscope probe set to x1 or x10 attenuation?
As Dick says, the frequency compensation on the probe is not properly adjusted.

It is set to 10x. I’ll be looking into adjusting the frequency compensation today. Thanks!

 

It is set to 10x. I’ll be looking into adjusting the frequency compensation today. Thanks!

It is not that difficult. There is a probe compensation test signal at the right bottom corner of the oscilloscope.
Connect your probe to this signal. Trim the probe compensation on the probe for the flattest top and bottom of the signal trace on the screen.
(Do this for each probe. It is best to use one probe for each channel, i.e. don't move the probes around. This way the compensation is done with the probe on that channel. Use the coloured rings on the probes to match the probe with the channel.)



You want the scope trace to look like the last one on the right.