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Looking at your schematic... Running a simulation...
One end of the secondary is connected to the node between IGBT's. It only conducts when either left-hand IGBT is switched on.
This technique allows the capacitors to send juice to the transformer, in the reverse direction. Is this the method that lets you charge the battery? Looks clever.
The IGBT's need to be switched at precise moments, to coincide with the mains waveforms. Do you locate the control circuit? Can you confirm whether it works all right? Or is that the part that is broken?
When the inverter is on C1 and C2 are charged to +/- 325V. U4 and U6 generate the sine output. U3,U5 are off.
When the mains is on, U4,U6 are off, the integral diode inside these transistors charges C1 and C2 to +/- 325V. U3 and U5 switched alternatively at high frequency and generate AC for the transformer. U1 and U2 are switched in synch to U3/U4 rectify the voltage that charges the battery.
Wow.. This is really Interesting. But the charging Mode is not clear to me. I don't really understand the conduction path yet. I mean what happens at positive and negative cycle of AC mains and how the capacitors are charged. I just checked how the capacitors are charged in Voltage doubler, and applied same to mains mode charging. But this is confusing me. The polarity on the transformer is also puzzling. How do i start analyzing it?
Assume that pin 2 is N and pin 1 is L
When L is positive the current flows through the antiparallel diode of U4 and charges C1 to +325V. When L is negative C2 is charged via the diode of U6 to -325V.
The voltage on leg 4 of the transformer never exceed the voltages on the caps so D1-D4 are off.
Falstad's animated interactive simulator is ideal for examining behavior in this sort of situation where the concept of operation is not apparent.
* The transformer is not utilized symmetrically here. Only one half of the secondary (right-hand in schematic above) carries substantial current. I wonder if the original design had a symmetrical arrangement?
* Re the switching devices on the right hand of the H-bridge, they carry current upwards. This may or may not be too much of a burden for the mosfets' body diode.
* The clocks are on a 25% duty cycle, turning on near the peaks of incoming mains AC. This is to improve efficiency. When duty cycles were 50%, current flowed in the wrong direction near the zero crossings.
As you said, one half of the transformer is used for the inverter and the other half for the battery charger. I guess the idea is that since the inverter and the charger never work together it saves having 2 transformers.
It is not an H bridge. The 2 left transistors (IGBT) are for the charger only, they are completely off in inverter mode. The 2 transistors on the right are on only in inverter mode, in charger mode they are off (no gate drive), only the intrinsic diodes are used to charge the caps.
The diode part of mosfet is rated for the same current and voltage as the mosfet.
In inverter mode the duty cycle of the 2 transistors vary from 0 to 100% at 50Hz rate to form a sine wave.
In charger mode the transistors on the left generate high frequency from the +/- 325VDC on the caps. The duty cycle is controlled in order to regulate the charge current.
Sync with mains isn't needed because the supply to the transistors is DC from the caps.