TB8100 Service Manual Power Management Unit Circuit Description 207
© Tait Electronics Limited September 2006
PFC Control
The control circuitry for the boost power supply is located on the PFC
control card which plugs into the AC converter board. This circuitry is
designed to achieve a power factor of near unity. The power supply for the
PFC control circuitry is derived from the secondary winding on the main
boost choke.
High Voltage to DC
(HVDC)
The regulated 400VDC from the PFC circuitry is converted to a regulated
28VDC output using a forward converter.
The forward converter transformer provides galvanic isolation between the
input and output. The primary is switched between 400V and 0V via two
power MOSFETs.
The output from the transformer is rectified and filtered before being fed to
a current sense resistor.
HVDC Control This secondary control circuitry consists of a voltage and current amplifier,
which provides a combined error signal. This error signal is fed via an opto-
coupler to the PWM (pulse width modulation) controller IC, which is part
of the primary control circuitry.
The power supply for the HVDC control circuitry is derived from the
secondary winding on the main transformer. The HVDC control circuitry
is located on a separate card which plugs into the AC converter board.
Current Sense and
Output Filter
The current sense resistor provides the current output level to the
microprocessor for monitoring purposes and to supply data to the Service
Kit. The current output level is also used by the analogue current control
loop to limit the maximum output current.
After the current shunt the regulated 28VDC output is split and filtered to
provide two DC outputs: a high current output for the PA, and a low
current output for the reciter. The high current output is electronically
protected, and the low current output is protected by a 2.5A self-resetting
fuse.
The low current output for the reciter is normally supplied by the standby
power supply. If this card is not fitted, the high current PA output is also
supplied to the low current reciter output via a diode.
The current sense and output filter circuitry is also used by the DC converter
to provide a common output stage for the PMU.
Microprocessor
Control
The microprocessor is located on the HVDC control card and is referenced
to the output (secondary side) of the converter. The microprocessor is used
by both the AC and DC modules and is fitted to all PMU models.
The microprocessor controls the output voltage to the fan via a MOSFET
and dropping resistor to provide the 24VDC output.
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