MGE UPS Systems S3 - Manual (Page 45)

As indicated, two identical inverters are used to produce
either 120 or 240VAC depending on parallel or series
connection of their outputs. Each inverter is a conventional
half bridge, that is, only two FET switches are used in each
inverter.
See Figure 3-3. The output of each inverter is
applied to a coupled choke. Since the choke acts as a
transformer, the two windings of the choke must have the
same voltage at any given time. Therefore if all four of the
200 VDC capacitors are at the same voltage, the output of
the two inverters must be identical (within 2% of one
another).
Note that the FET switches are driven in
synchronization. Control of the inverter is unique in that the
inverter uses what is called "Average Current Mode Control".
This is actually a control of the choke current and results in
excellent dynamic response to step loads, overloads, and
short circuits.
F i g u r e : D u a l I nv e r t e r a n d Wa v e fo r m D i a g r a m
3- 3
3.1.2 Dual Inverters
86-153061-00 3 — 5 Theory of Operation
Owner’s Manual S3 Static Inverters 1kVA