Original question:
Hi,
With reference to TI's SLUA560C, schematic on page 8, I am seeking design guideline to determine selection of diodes D6 and D11.
These are two power diodes in the full bridge section.
Please help me with -
1. What role do they play in the power conversion cycle ?
2. What are the design guidelines in selecting them ?
Thanks and Regards,
Sanjay
Hello Sanjay
The Shim inductor LS appear in series with the transformer leakage inductance LLK. The series combination of these inductances has two functions
It reduces the di/dt in the secondary circuit when the system changes state. This reduces rectifier reverse recovery turn-off losses. These losses occur even when SRs are used because the body diodes of the SRs are conducting at the instant when the current reaches zero.
It stores energy which is used to drive the ZVS transitions and reduces the minimum load current at which ZVS is achieved. This is especially important for the ZVS transition in the PA leg.
Sometimes the leakage inductance of the transformer alone is sufficient to store enough energy to allow ZVS operation over the required load current range but usually a physically separate component, LS is needed.
The purpose of the diodes DA and DB is to clamp the transformer primary winding at either VIN or GND and thus prevent a resonant ringing between the inductance LS + LLK and stray capacitance in the transformer windings which would otherwise cause high amplitude voltage spikes to appear across the rectifiers. When the diodes are present, the resonance, between the leakage inductance of the transformer LLK and its stray capacitance is unclamped, but the energy is much less and the resulting voltage spikes are lower.
Regards
Colin
