4. Theory of Operation
4-36
EMI Filter Board:
The EMI filter board provides the necessary filtering for conducted
emissions both into and out of the FMS2000. Both common mode and
single ended filtering are employed. The board also contains transient
protection. AC power enters the system through the EMI board where it is
filtered. The filtered voltage then leaves the board to go to the circuit
breaker, and to AC/DC power supply board. The voltage then comes back
from the circuit breaker and is further filtered and transient protection is
provided at this point. This filtered voltage is then applied to the driver
boards.
Driver ‘A’ and ‘B’ Boards:
The driver boards provide the power to the heat exchanger and isolate the
main electronics from the AC line. Each driver board contains two
Insulated Gate Bipolar Transistors, IGBT’s, which are used as power
switches. Driver circuits that monitor for unsafe operating conditions such
as over current, IGBT desaturation, and under voltage control these
devices. One of the driver boards contains a zero-cross detection circuit
that provides a square wave output in phase with the AC power input on the
EMI board. The IGBTs are driven with signals derived from the heater
control board.
Heater Control Board:
The heater control board contains the following functions: heater driver
signal generation, non-volatile memory, power measurement, fan speed
control, over temperature for the heatsink, watchdog and system interface.
The heater control board receives control data from the computer board
through a serial interface, as well as line phase information and driver
circuit fault monitoring signals from driver boards. The computer data
determines the frequency at which the heater is to be driven, and the duty
cycle at which the power is to be supplied to the heater. The line phase
information, supplied by the zero cross circuitry, coordinates the IGBT
commutation sequence with the AC line polarity. A Field Programmable
Gate Array (FPGA) derives pulse width modulated signals and
complementary synchronous rectifier control signals, which drive the
IGBT’s. The operating frequency of the heater driver, which is determined
during factory calibration, is the series resonant frequency of the ferrite core
heater winding and resonant capacitors.
The heater control board measures the power that is applied to the heat
exchanger. This is accomplished by measuring the magnetic field behind
the heat exchanger rings with a "pickup winding". The voltage across the
pickup winding squared is proportional to the power induced into the heat
exchanger. The pickup winding is loaded to achieve an impedance match,
and to convert the induced current to a RMS voltage.
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