High-power Three-phase FCML converters with power factor
corrector (PFC) at hundred-kilowatt levels are widely used in electric
vehicles (EVs) charging systems such as multi-port EV fast chargers [1].
The control strategy of FCML converters is proposed in [2]. However, the
FCML converter still has a flying capacitor voltage start-up problem [3],
If the FCML converter is without the start-up procedure, we can only start
the circuit by slowly increasing the input voltage to avoid excessive switch
voltage and inrush current. This thesis proposes a start-up procedure
simulation for high-power three-phase FCML converters. In the start-up
procedure, first, we make a table to list all the switching combinations and
how they charge the capacitors. Second, we use the table data to charge
each of the capacitors to its theoretical value, when the output capacitor is
charged to the maximum input voltage. Third, use the level-changing
method to pre-charge all the flying capacitors. As a result, the flying
capacitor voltage of each phase can be pre-charged in the start-up
procedure and greatly reduce the inrush current. Compares with FCML
without a start-up procedure, the maximum inrush current during start-up
is greatly decreased. A start-up procedure that pre-charges the flying
capacitors is proposed. In summary, this pre-charge procedure requires no
additional hardware to avoid causing excessive voltage stress on the
switches, making an FCML converter that can be used in practical
applications.

Reference
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