full-bridge signal distortion

[MsStateDawg]

I am building a full-bridge, single phase inverter that uses PWM to switch IRF740 transistors [Vdss = 400V, Id = 10A] at 18kHz. The goal is to produce a single-phase, 120V, 60Hz sine wave with less than 5% total harmonic distortion. By examining the unfiltered output of the full-bridge inverter with an o-scope, the signal can be seen. In fact, the sine wave becomes more apparent when an LC low-pass filter is placed on the load. The problem I am having is that the negative half wave of the output signal (from the inverter) begins to disappear as the DC link voltage is increased from 33.22 VDC to 35.06 VDC.

The waveforms are shown at the following link:

**broken link removed**

Does anyone know what could possibly cause the negative half wave of the output sine wave to become distorted as shown in the pictures? Thanks in advance for any help.

homepage: **broken link removed**

 

[Styx]

what dead times are you using between switches in a leg?

Is yr supply going into current limit?
What are you using for gate-drive?

I'm going along the lines of shoot-though here. Probably not but start with the usual suspects.