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I have designed a PCB with a FAN73933 half bridge driver. It is designed to drive 2 IGBTs (FGH40N60SMD) with +- 170V rails. I have attached a schematic for better understanding. The problem is I can not seem to get the high side to turn on. The system uses a boot strapping method to boost the gate voltage for the high side.

When I scope the boot capacitor I see a steady ~12V, which is my Vcc for testing. My rail voltage is +12 and 0V so all my GND,COM,-V are tied together. I'm driving the system with a simple Arduino for testing, its outputting a 1khz 50% duty wave with ~10% dead time, this was verified on a scope. The low side gate driving wave form is as expected and I believe the low side is turning full on.

The high side seems to never have any voltage on the gate. That being said, as i crank up the frequency above ~20khz I begin to pull a lot of power. I'm using a 50 Ohm test load so the max current draw should be ~ .5A I'm seeing ~6A based on a shunt resistor.

What I don't understand is that there appears to never be any gate voltage so i cant be having a shoot through. Also the controller has shoot through prevention, it will not turn both gates on at once. I'm really confused, all the signals look OK except for the bootstrap and the high side gate.

The boot diode is a 600V Schottky also. One item to note is when I start pulling a lot of power the high side IGBT will get warm.

Half Bridge Driver. Note +170 = 12V and -170 = GND. Also the bootstrap diode is 600V

Any input would be appreciated.

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  • \$\begingroup\$ If the schematic you show is correct, you need to connect Vs (pin 11) to the load. See the application circuit in fairchildsemi.com/datasheets/FA/FAN73933.pdf \$\endgroup\$
    – WhatRoughBeast
    Dec 28, 2014 at 18:46
  • \$\begingroup\$ WOW! I looked at that application note many times when troubleshooting and missed that. I'd bet that's the problem. Looking at my schematic I assumed it internally connected and disconnected. I'll try this see how it fairs. Thanks. \$\endgroup\$
    – MadHatter
    Dec 28, 2014 at 18:51
  • \$\begingroup\$ yup, second this. The IGBT right now doesn't have an emitter return. \$\endgroup\$
    – user16222
    Dec 28, 2014 at 19:02
  • \$\begingroup\$ Valid point on the no emitter return Jon, as soon as you said that it now stands out plane as day. \$\endgroup\$
    – MadHatter
    Dec 28, 2014 at 19:09

1 Answer 1

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As WhatRoughBeast and JonBR said, it is the fact that Pin 11 is not connected to the output. I made the connection and it worked as expected for 5 seconds till my wire slipped and I blew out the driver.

Amazing how fast another set of eyes can find a mistake.

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