Electronic – Safe dead time limit on 19Khz PWM sine wave inverter


I'm buiding a 60Hz sine wave inverter based on 19Khz PWM generated by a PIC16F883 (lookup table) using a half bridge with 4 x IRF2807 – switching a pair of low-side/high-side. I'm using my own designed MOSFET driver with discrete transistors. I'm feeding the bridge with 12V and using a 700VA transformer to step up the voltage from 12V to 127VAC.
I still have some small wave distortion specially with inductive loads such as a fan motor (in the picture, an example with a 60W/127VAC fan).

Question: Looking at the oscilloscope screen capture, do you guys think I should go further in reducing the deadtime to enhace the wave quality and still be safe to avoid shoot thru current? Or should I keep it and go for the final PCB, since the circuit is still on protoboard, so parasitc inductances may be present.
Each vertical division means 10us in the PWM wave. I fixed the duty cyle just to capture the image.

Thank you!

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Best Answer

Try putting a (non-inductive) sense resistor in series with the inductive load and measuring the voltage across it. The voltage across this resistor will then be in phase with the current, which will be phase-shifted from your voltage with the inductive load. You may see that the distortion occurs at the point where the (lagging) current is zero; if so you could be looking at the wrong place for your solution. If this is the case, your voltage's PWM artifacts can't be fixed with dead time.