Hello,
I salvaged some circuit boards from discarded equipment (a Dukane Ultrasonic Generator). It has 4 transistors on a heat sink in an H-bridge configuration, and the driver board has a transformer at the heart of it (labeled U1 in the attachment). The transformer has 10 leads: 3 and 8 are grounded, 5 and 6 connect to control wires from another board, 2 and 9 connect to the load (unknown on another board), and the remaining leads connect to the base resistors for the H-bridge.
I want to understand how this works and maybe use it in a project, but I don't know what the transformer is doing. All my search results showed a transformer as the load of the bridge, but not on the control side.
Any ideas or links to similar circuits would be greatly appreciated.

 

Google the phrase, "transformer coupled gate drive"
Or, click here: http://www.ti.com/lit/ml/slup169/slup169.pdf
and start on page 30.

Your transistors are MOSFETs or IGBT's. The transformer provides a means to drive both the low side and high side MOSFETs without more than the basic controller IC.