I have struggling for about a week with this problem and I am not sure how to fix it , so stack exchange community please help :).

Ok so basically I am making a h-bridge using 4-N channel mosfets, and in order to drive the high side I am using IRS2001 High and Low side drivers.

Here is my schematic : enter image description here

Larger pic : https://i.sstatic.net/ayE0L.png

Here is an updated schematic of how my signals are inputted :

enter image description here

Larger pic : https://i.sstatic.net/EkNCB.png

I am using a 5kHz PWM signal to switch on for about 0.5secs and then turn off the mosfets to drive the motor in one direction and vice versa for the other direction.

The motor seems fine but when I probe the input and out of the gate driver I see problems.

The low output seems to couple with the high side output even though the high input signal is clean as shown in the oscilloscope output below.

Another question i have is what are the best capacitors to be using for this mosfet driver, at the moment i am using an electrolytic type capacitor around 10uF, is this applicable?

enter image description here

  • The green signal is the HO ( high output signal)
  • The yellow signal is the HIN ( high input signal)

enter image description here

  • The blue signal is the LIN( low in signal)

Here is where I am probing

enter image description here

So my question really is , why is this happening and how can I solve this ? Thanks

  • 1
    \$\begingroup\$ How are you probing? You have large currents, and high edge rates, which means your probe ground can easily couple in to your probed "signal" very easily. You're not using the ground clip wire, are you? \$\endgroup\$ Commented Sep 17, 2013 at 10:31
  • 2
    \$\begingroup\$ Why does your motor appear to have four leads? Also, do you understand that the high output is relative to Vx? If Vx is varying, the high output will vary, too, regardless of what's happening on HIN. \$\endgroup\$
    – Dave Tweed
    Commented Sep 17, 2013 at 11:14
  • \$\begingroup\$ Umm yeah i am using the ground clip wire , should I not ? I'll post a picture of how i'm doing it in a minute let me take a photo on my iPhone . \$\endgroup\$
    – subz
    Commented Sep 17, 2013 at 11:21
  • \$\begingroup\$ @DaveTweed was in a hurry messed up on paint , should be just TWO leads , What is this Vx you are referring to ? \$\endgroup\$
    – subz
    Commented Sep 17, 2013 at 11:23
  • \$\begingroup\$ Vx is the pin between LO and HO on the driver chip. \$\endgroup\$
    – Dave Tweed
    Commented Sep 17, 2013 at 11:30

1 Answer 1


I think you are measuring the correct. Because high side MOSFET is off and low side MOSFET is switching middle point(VX in your case) voltage will swing between battery voltage and ground.

If you see low pin voltage and vx pin voltage they should be complimentary to each other.

Please post the low side gate voltage and VX voltage (zoomed)

  • \$\begingroup\$ 'measuring the correct' what? Or do you mean 'your measurement is correct'? \$\endgroup\$
    – RJR
    Commented Apr 11, 2016 at 10:40

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