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Issues Building a Full-Bridge Inverter

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Uzi

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Hi,
I am building a Full-Bridge Inverter using four IGBT and four drivers for each IGBT. Driver is HCPL-3120, all four drivers are supplied with 15VDC isolated supply. I am switching using M bed micro controller LPC1768 generating two PWM signal working alternately, first signal turns IGBT 1 and 2 on and second turns IGBT 3 and 4. Inverter is supplied with roughly 36V while each driver with 15V. I have uploaded picture of schematic where four IGBT can be seen and one driver although in actual four drivers are used. After testing circuit i am unable to get a sine wave however I have upload snaps of my output. I haven't put any load yet and I am not getting any signal on DC coupled scope, Ac coupled scope is just giving a bit of noise while output voltage is approx. just 2Vp-p. So what I am doing wrong here and what type of output i should be expecting from this circuit .
Any help here is much appreciated.
 

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Without an output filter you should see a square wave being generated. The frequency of this will depend on what code you're running to determine the switching.

Have you confirmed that the gate drives are working? I would just start off by confirming that you are able to turn on the IGBTs one by one. Just connect an Ohmmeter across each IGBT to see it switching.
 
I have followed voltage for pwm signal to IGBT, there is voltage going in from driver to IGBT gate however at emitter voltage is very low i dont know why
 
You need to feed your +15V to the +pin of each of your two high side drivers through two diodes. When each side of the h-bridge is at ground, the capacitor will charge to +15, and then carry the 15 volts with respect to the emitter pin voltage as the emitter climbs up to +36V, at which time the gate will be 51V. Depending of the power needs of the driver and the IGBT gate capacitance you are driving, you may need a larger cap, as it needs to maintain most of the 15V charge for the duration of the high side ON pulse.
 
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