4. Theory of Operation
4-37
This voltage is applied to a voltage-to-frequency converter, which produces
a frequency that is proportional to the output power. A counter then
generates a number that is used by the system computer to control the
output power under software control, thus closing the control loop. The
counter is continuously monitored in hardware for a number that exceeds
the maximum power level. If this power level is reached, the system stops
pumping and heating, and sounds alarm with ‘Over Power Fault’ message
on screen. The heater control also monitors the AC power supplied to the
system. If dangerous levels of noise are present or the line frequency
changes the control circuit will stop heating and wait for stable conditions.
If stable conditions are not obtained then a fault is generated. When the
system is operating in battery mode, the heater does not function, and the
system is allowed to pump only at a flow rate of 50 ml/min maximum.
The fan control circuitry supplies three levels of energy to the fan. The
speed of the fan is governed by the computer and an over temperature
switch mounted on the heatsink. The computer controls the fan speed
based on the output power that it requests. If the over temperature switch
is activated, the fan will go as fast as possible regardless of the computer
command, and the computer is notified of the over temperature condition. If
the over temperature condition exists for a fixed period of time, pumping
and heating stop. When the power switch is turned off, and the system is
plugged into an AC line, the fan will continue running at a low speed to vent
the machine during battery charging.
Heatsink Assembly:
The heatsink assembly contains an aluminum bonded fin heatsink and fan
to cool the IGBT's. It also holds the driver boards and heater control board.
This entire assembly mounts to the EMI board. The fan bracket and holes
on the EMI board are used to mount the module into the system.
OPERATION:
The power module receives information from the computer board via a
serial peripheral interface (SPI). A watchdog constantly monitors the flow of
information back and forth between the power module and the computer. If
the computer detects improper communication, heating and pumping will
stop. If the driver boards send any fault information to the heater control
board, the heater control board will try to correct the problem. If the
problem persists, the board will stop responding to the system commands,
stop pumping and heating, and output error information to the display.
To Next Page
Loading...
