Half Bridge Strange Transformer Waveform When Unloaded

It is the inductance kicking back. The primary inductance, I think you said there is no secondaries.

In this data sheet, with a typ. G-S voltage of 2.9V the Drain current is 250uA. The MOSFET is not on! It is just thinking about being on.

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Here is a graph of Vsg verses Id. It you have a GS voltage of 3.6V the part tears open at 0.05A. With Vgs=4.1V you can get maybe 1.5A at the very most. Vgs=5V you can get maybe 6A.
If you want the part to run cool drive the gate hard. 5V minimum and 10V is better.

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This is how we normally do half bridge. Use two N-MOSFETs. The bottom Gate-Source voltage is 0 to VCC. The top G-S Gate voltage is slightly lower. There are many versions of Half Bridge Gate Drivers.
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