Owner`s manual

4.7.1.2 Inverter
The boost transformer has two secondary windings, 22 turns, center tapped at 11 turns. The two secondary
windings, which are rectified, make two separate outputs of +212.5V and –212.5V each. These DC voltages are
applied to a filter “Capacitor” PCB, which connects to the inverter IGBTs. The primary has eight turns, center tapped
at 4 turns. The secondary inverter DC bus voltage is regulated by a TL431 shunt regulator and an optical coupler,
which feeds back the control signal to the primary (battery) side electronics. The control of this Current Feed, Center
tapped, Push Pull booster is by what is called “Average Current Mode Control”. A write up of this basic technique
can be found in the old “Unitrode” application manual.
4.7.1.2.1 Dual Inverters
There are two identical inverters. The two inverters track one another because the inverters IGBTs are switched
ON and OFF at the same time and their DC bus voltages are nearly identical. Their outputs are applied to a two
winding (tightly coupled) output choke, which forces the two inverters outputs to track one another. The inverter’s
IGBTs switch between +212V and -212V at a 20KHz rate. To generate the 120VAC sine wave output voltage, the
“Pulse Width Modulated” (PWM) voltage (+/-212V) is filtered by an “L/C” filter, inductor being in the “power module”
chassis, the capacitor in the “receivers” AC raceway printed circuit board. If the duty cycle is 0.5, that is, the time
at the +212V rail is equal to the time at the –212V rail, the output will average out to zero (25uS at +212V, 25uS at
–212V). If the IGBT switch is at +212V for 37.5uS, at –212V for 12.5uS (duty cycle of 75%), then the output voltage
must average out to be +106V ((+212-V)*37.5uS =(V+212)*12.5uS), or V=106). To obtain 120Vrms (+/-170V peak),
the duty cycle must range between 0.099 to 0.909. The duty cycle can be calculated from the following flux balance
formula. V1*T1=V2*T2, and T1+T2=T, Duty=T1/T, where V1 is the voltage across the output choke when the
positive switch (IGBT, Q3) is ON and V2 is the voltage across the inductor when the negative switch (IGBT, Q4) is
ON. T1 and T2 are time at each supply rail, +212.5V or –212.5V. Control of the inverter is through what is called
“Average Current Mode Control”.
4.7.1.2.2 Inverter Output
The inverter’s output filter capacitors (50uF/250V on each 120VAC output) are located in the receivers “AC
raceway”. “Hot swap” of the power modules, while the system is operating, can be accomplished with no arcing of
the power module’s connector pins. If the capacitors were in each power module, connector arcing would occur
when the power module is installed or removed. Output fuses and disconnect relays are located in the output of
each power module. The relay driver is an “avalanche rated” high voltage FET, which is allowed to “avalanche”
during turn OFF of the relays. This will minimize the turn OFF time of the relays. The relays will not be energized
until the inverters DC bus voltage is greater than +/-190V. As indicated previously, fuses are placed in series with
each of these inverter output filter capacitors. In the event of a capacitor failure (generally shorting) the fuse will
blow, thus preventing system down. In a system with three “power modules”, a failure of one capacitor will not affect
system performance. The fuses are located in the “cold” side of the capacitors, that is, in the “NEUTRAL” and
“ACLO” side. A “blown fuse” indicator is provided on the front panel of the LCD display panel.
4.7.1.2.3 Inverter Control
As indicated earlier, the inverter is a voltage controlled current source. Control input to the power module is from
the Voltage Error Amplifier, located on the “processor” printed circuit board. Control voltage scale factor is approx-
imately 10A/V in a 120VAC system. Thus the power modules can be easily paralleled to increase the output power
capability of a system. The control input to the power module is filtered, buffered by an operational amplifier, and
then applied to a current error amplifier, the other input to this current error amplifier is the output current of the
power module as sensed by a “LEM” LA125-P current sensor. This current error amplifier operates in what is called
“average current mode control”. The current error amplifier’s output is summed with a triangular wave sweep signal,
which sweeps between +5.2V and –5.2V at a 20KHz rate. This summed signal is now applied to a high-speed
comparator, the output of which control optically coupled IGBT gate drivers.
3.5 to 21 kVA N+1 Inverter
Theory of Operationpage 4 — 8