enter image description hereenter image description hereCircuit Setup with connection of power supply to circuit and drivers, line 48 is where output voltage is being measured.All four IGBT have their own drivers with setting shown for one.

I'm building a Full-bridge inverter with input of up to 36V, Four IGBT are used for switching and embed micro controller for PWM. To drive IGBT I'm using four HCPL-3120 chips. After testing the circuit, I'm having an issue of very low output voltage somewhere around 1V irrespective of input voltage however I got something close to a sine wave. Also I want to know if it is necessary for opto-couplers to have isolated supplies.

  • \$\begingroup\$ what frequency sinewave are you seeing? because you shouldn't actually see a sinewave at the moment \$\endgroup\$
    – user16222
    Apr 7, 2015 at 16:19
  • \$\begingroup\$ @JonRB, its not sine wave I have uploaded shots of signals i got and not sure about it. \$\endgroup\$
    – uzair
    Apr 7, 2015 at 16:53
  • \$\begingroup\$ Thats scope pickup (are you in a 50Hz country by any chance). With a resistive load and correct PWM pattern you will ONLY see a squarewave across the resistor. AndyAKA has already posted enough about correcting the gatedrive so I don't want to digress away from that \$\endgroup\$
    – user16222
    Apr 7, 2015 at 20:56
  • \$\begingroup\$ Yea im in Uk its 50Hz,@JonRB and @AndyAKA help from both of you is much appreciated. \$\endgroup\$
    – uzair
    Apr 7, 2015 at 22:03

1 Answer 1


Here's the data sheet for the driver and here's a typical circuit used to drive IGBTs: -

enter image description here

You should be able to see where you have gone wrong - you haven't coupled pin 5 to the emitter of IGBT1. Step and repeat for the other devices.

Also i wanted to know is it necessary for optocouplers to have isolated supply.


  • \$\begingroup\$ Actually I have made that connection but i forgot to put it in drawing but thanks for notice, After following voltage through circuit, in collector for IGBT 1 voltage is comming in but at emitter im loosing voltage. By the way im using IGBT FGH40T655SHDF \$\endgroup\$
    – uzair
    Apr 7, 2015 at 14:09
  • \$\begingroup\$ do you have the other 4 IGBT's being driven? You are only showing one gate-drive you need 4 and two complementary PWM signals \$\endgroup\$
    – user16222
    Apr 7, 2015 at 14:23
  • \$\begingroup\$ @uzair you are losing credibility with your poor diagram and mistakes. If you want this solved you need to be more precise with what you show and tell us. Is the Vcc to the top IGBT on an isolated supply AND are both "top" drivers isolated from each other? \$\endgroup\$
    – Andy aka
    Apr 7, 2015 at 14:33
  • \$\begingroup\$ @JonRB I have included in my question i am using 4 drivers for each IGBT however I have shown one in my drawing, All four have its own power supply of 15V. I am doing PWM using an mbed which generates two signals one going to IGBT 1 and 4 while one going to 3 and 4. Both works alternately. I have also included dead time in my code aswell. \$\endgroup\$
    – uzair
    Apr 7, 2015 at 15:04
  • \$\begingroup\$ @Andyaka I have provided all driver with isolated supply(Vcc) by the way is it necessary for all four or just top drivers are required to have? I hope these two comments clear some things up. Thanks for your support. \$\endgroup\$
    – uzair
    Apr 7, 2015 at 15:06

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