I have some noise being introduced in my circuit when I PWM a large motor. The noise was greatly increased when installed the flyback diode on the motor. If I remove the diode the noise is reduced but the MOSFET get very hot quickly. With the flyback diode installed the MOSFET runs cool but the noise causes all sorts of strange issues with my circuit. I'm driving the motor on the high side. Any suggestions on how to clean up the noise?
I will post a picture of my circuit shortly

Red line is my PWM output to the motor from the MOSFET
Yellow line is my 12V+ Power Rail

 

That's probably the back EMF of the motor. The diode does not clamp instantly and the path through the diode may increase that. You might want to use a faster diode. Another thing you might want to try is adding a snubber (capacitor in series with a resistor) in parallel with the windings.

 

The glitch only generated when the positive edge raising, what is the schematic?

 

Hi,
Try to add a resistor gate to source a 5k ohms.Can you give the mosfet part number.

 

Mosfet is: IRLB8721PBF-ND

 

The output of LM339 are open collectors, so you have to add the pull up resistors as 10K to the +12V, if you didn't add the pull up resistor then when the output of comparators goes high then they will get into the high impedance status, so the inputs of max620 will be like floating, and the input floating state caused the noise and made the mosfet generated the heat, you should in parallel the diode with motor to protects the mosfet.


LM339 datasheet page 10.

 

Back to the glitch where did you place the diodes? And while at it, what kind of diode are you using?

 

Back to the glitch where did you place the diodes? And while at it, what kind of diode are you using?

Currently the diode is placed right at the motor power connection. I originally had it on the circuit board on the source pin of the mosfet but it was generating more noise that way. I would prefer to have the diodes on the circuit board for ease of installation. Diode is SB5100

 

The output of LM339 are open collectors, so you have to add the pull up resistors as 10K to the +12V, if you didn't add the pull up resistor then when the output of comparators goes high then they will get into the high impedance status, so the inputs of max620 will be like floating, and the input floating state caused the noise and made the mosfet generated the heat, you should in parallel the diode with motor to protects the mosfet.

View attachment 120655
LM339 datasheet page 10.

Thanks for the information, I'm grateful. I actually changed from the LM339 to the TLC3704CN that has a push-pull output. I need to update my schematic.

 

Hi,
Try to add a resistor gate to source a 5k ohms.Can you give the mosfet part number.

Thanks for the suggestion.
I added the 5K resistor from gate to source and I also tried gate to drain with no change to the noise.

 

The fact that you are getting the spikes on the leading edge (turn-on) of the high side driver is very strange.
Normally the spike is during turn-off.

Post the schematic showing how the motor is connected to the circuit with the diode.

 

How much current do these motors draw? (I infer from your schematic there are three of them, plus a pump.) I ask because I'm concerned about possibly inadequate filtering on your 12 power, your ground topology, and how you might be routing the motor wiring.

If these motors draw a lot of current, you could easily have inductively coupled interference, which seems possible looking at your scope traces.

 

How much current do these motors draw? (I infer from your schematic there are three of them, plus a pump.) I ask because I'm concerned about possibly inadequate filtering on your 12 power, your ground topology, and how you might be routing the motor wiring.

If these motors draw a lot of current, you could easily have inductively coupled interference, which seems possible looking at your scope traces.

Grateful for your time here.
Keep in mind that the schematic is outdated and that is why I was concerned about posting it(I'm working on updating it). I was switching the low side of the motors but I have changed to switching the high side. I changed to a high side mosfet driver.
The motors can draw 1-5 amps each (depending on motor speed chosen). Max amp draw for all motors and pump could be 20amps but typically they run at 7-10 amps total for all motors and pump.

Motor wiring is: high(+) side goes to each mosfet source pin. Ground(-) side goes to vehicle mass ground.

Open to suggestions

 

I think if I were trying to PWM that much current into motors, I'd be tempted to split the overall circuit into two blocks: a "control" block containing the comparators, the CD4060 and the LM556, and a "power" block containing the MOSFETs, the MOSFET drivers and motor connections, with opto-isolators between the comparator outputs and the MOSFET driver inputs. I'd power the two circuit blocks with separate feeds from the main power input and give the control block its own voltage regulator, with the control block power heavily filtered.

Maybe one of our members with more power system experience than I can make some other (or better) suggestions, but that's my take on it.

 

I changed to a high side mosfet driver.

What's the reason for that?
High side drive usually requires a more complex drive circuit if you are using N-MOSFETs.

 

What's the reason for that?
High side drive usually requires a more complex drive circuit if you are using N-MOSFETs.

Good question. This is for an automotive environment and all the motors grounds are connected to the vehicle body(mass ground). When I was switching the low side I had to install relays on each motor to break the ground and 12v+ connection so I could control it...lots of relays.

 

I think if I were trying to PWM that much current into motors, I'd be tempted to split the overall circuit into two blocks: a "control" block containing the comparators, the CD4060 and the LM556, and a "power" block containing the MOSFETs, the MOSFET drivers and motor connections, with opto-isolators between the comparator outputs and the MOSFET driver inputs. I'd power the two circuit blocks with separate feeds from the main power input and give the control block its own voltage regulator, with the control block power heavily filtered.

Maybe one of our members with more power system experience than I can make some other (or better) suggestions, but that's my take on it.

Great suggestion. The mosfets are actually located on a separate board with separate power rails. Hmmm I wonder if the high side driver (max620) outputs can be opto-isolated to the mosfets gate. The mosfet high side driver is located on the main control board currently and not the board with the mosfets

 

Here is an updated schematic that shows the motors and other changes

I don't see the charge pump capacitors that the MAX620 driver requires.
Did you just leave them off the schematic?

And I'm still puzzled as to how a spike is generated when the MOSFETs turn ON.
Anyone have any thoughts on that?

 

Hmmm I wonder if the high side driver (max620) outputs can be opto-isolated to the mosfets gate.

Definitely NOT. Optocouplers are logic-level devices and are not suitable for driving MOSFET gates.

And I'm still puzzled as to how a spike is generated when the MOSFETs turn ON. Anyone have any thoughts on that?

All I can think of is that the chopping of the motor current by the PWM is being coupled into the rest of the circuit inductively. I can't think of any other reason.

 

Good catch. Yes, I left them off the schematic for now, just ran out of time. The charge pump circuit is working well. I just for the charge-pump I copied the circuit from the data sheet. The driver is boosting gate voltage +11v higher then Vcc and is fully saturating the mosfets.