I am working with a "tried and true" H-Bridge MOSFET driver circuit. I have used this design 2 times before with no problems. Now I am having an issue that I cannot seem to solve. The only difference in the current iteration over the others is that my gate driver is 1.5" away from the MOSFET as opposed to 0.25" away. To compensate for this distance I am only running my PWM period at 16kHz instead of my usual 60-100kHz.

High Side MOSFET Gate Driver Signal

I am seeing a strange waveform on my high-side gate signal. After changing the capacitance of the bootstrap caps a higher value the period of the on-off problem goes up. A smaller value speeds up the period. My actual PWM duty and period are correct during this "on" time. It appears that the bootstrap caps are slowly loosing charge until the high side turns off to recharge and the process starts over. Aprox (800mV) However if I use a super small capacitance the on-off pattern continues only to disrupt the actual PWM. I have check all of the microcontroller signals and they are perfectly clean and consistent.

Any suggestions of where to go from here?


  • 1
    \$\begingroup\$ A schematic would REALLLLLY help us mate. Also, why are there bootstrap capacitors used? As far as I know, they will only slow down, and interfere, with your driving signals. \$\endgroup\$
    – KyranF
    Apr 25, 2014 at 9:42
  • \$\begingroup\$ Not related to the question but have you requested a deletion of your account on Stack Overflow? It's just your accounts are linked so if you have I'm pretty sure unless you change your profile (you can do it just here) it will be deleted here as well. \$\endgroup\$
    – PeterJ
    Apr 25, 2014 at 9:45
  • \$\begingroup\$ Bootstrap caps are needed to drive the highside MSOFET fairchildsemi.com/ds/FA/FAN7842.pdf Edit: Forgot this is part of stackoverflow... Disregard my question please. My account will be deleted. \$\endgroup\$ Apr 25, 2014 at 9:51
  • \$\begingroup\$ @PleaseDeleteME - only if you want it to be, if you edit your profile here and press the "save" button your account here still stay and only your SO account will be deleted. \$\endgroup\$
    – PeterJ
    Apr 25, 2014 at 10:05
  • \$\begingroup\$ @PleaseDeleteME lol.. well it is possible you dont even need a MOSFET driver IC like that - the boot strap could be killing your performance - also do you need to switch so fast? Switching loss is higher the higher you turn MOSFETs on and off.. \$\endgroup\$
    – KyranF
    Apr 25, 2014 at 10:06

1 Answer 1


Answer: I was being negligent in trying all variables. The gate driver needs a load to charge the bootstrap caps. Soooo running at 20mA gives me the previous waveform. At >800mA it looks as it should.

The FAN7842 requires a load (current flow) to operate properly.


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