18
System Circuit Boards
- The system circuit boards are shown in Figure 10. The circuitry represented
here is primarily located on the left side of the UPS and consists of all PC-boards in the unit (except the
IGBT drivers that are located on the heat sink) and power circuitry located in the same vicinity. The
external battery breaker BR2 (included only in units requiring external battery cabinets) and the main
battery breaker BR1 are shown in the top of this diagram. The switch next to these two breakers is a
DC contactor. It is commanded by the microcontroller and is used for connecting the batteries to the
inverter. Its physical location is in the top left of the cabinet, behind the battery breakers. Following is a
list of the control PC-boards that appear in this figure:
PC-P:
The PC-P board is the main power supply board. It takes the DC battery voltage to produce a
+24VDC source via the use of a step-down chopper. This voltage source is used to energize various
power components such as the DC contactor. It also supplies power to an onboard high frequency
inverter, which produces a 20V/25kHz square wave. This square wave is used to power all of the PC-
boards in the unit. Boards requiring power contain small, high frequency transformers that provide
needed galvanic isolation between the circuitry on the board and the rest of the UPS system. The
output of each transformer is rectified and regulated to the desired voltage levels needed by that board.
PC-L:
The PC-L board is the logic power supply. This board generates the highly regulated and filtered
±
12V and +5V used to support the main control logic from the 20V/25kHz square wave power source.
PC-C:
The PC-C board houses the UPS 16-bit microcontroller that controls the entire UPS operation.
This board contains circuitry to condition all system signals that are monitored by the microcontroller as
well as driver circuitry to allow the microcontroller to control all system components. The
microcontroller’s firmware is stored in two EPROMs (Erasable Programmable Read Only Memory),
marked “H” and “L”. These EPROMs contain the program executed by the microcontroller to control the
UPS operation.
FP1:
FP1 is the front panel mounted on the door of the UPS. It consists of the 8 line by 40 character
liquid crystal display (LCD) and a small interface PC-board PC-F. PC-F contains the three status lights
(LEDs), the keypad interface and the audible alarm associated with the front panel.
PC-I:
The PC-I board contains interface circuitry that convert high system voltages (DC and AC) to
control level voltages that can be further processed by circuitry on the PC-C control board. It also
contains precharging resistors, which are used to charge the large amount of capacitance of the
inverter in a controlled manner when the UPS is powered up. Once this capacitance is charged to a
voltage that is close to that of the batteries, the DC power contactor is closed to complete the
connection between the batteries and the power inverter.
PC-R
: The PC-R board provides the rear panel interfacing for the serial communications port, the
custom I/O and the relay contacts. It is located on the rear panel of the unit.
PC-S:
The final board shown is PC-S, which is the shunt board. It measures the current coming from
the internal batteries using resistive shunt SH1. This board is only included in systems with internal
batteries.
PC-X, PC-Y, PC-Z:
These 3 identical PC boards are located on the heat sink and are not shown in
Figure 10. They connect to PC-C through connectors P1, P2, and P3, respectively, of PC-C. These
boards convert the IGBT control signals produced by PC-C to isolated gate signals needed to turn on
and off the IGBT power devices. Each board controls three IGBTs: the two IGBTs in one of the three
poles of the power inverter and one of the three AC line IGBT switches needed to connect/disconnect
the UPS from the AC utility.
