Nautel FM10, Technical Instructions

Page: 105 / 200

10 000 WATT FM BROADCAST TRANSMITTER
FM10
Page 6-4
15 November 1999
purposes. Output level and voltage regulation is
controlled by switching FETs on the power supplies.
FET gate drive is supplied by the power supply
control PWB (A5A1) based on feed-back (RF power
module supplies) or feed-forward (IPA supply)
control techniques. A
PA Volts Sample (J1-1),
representative of the voltage applied to the RF power
modules, is applied to the power supply control PWB
for use in feed-back regulation of switching power
supplies A6 through A11. An
IPA Volts Sample
(A12J1-1), representative of the voltage applied to
the IPA module, is applied to the power supply
control0 PWB for use in feed-forward control
regulation of the IPA switching power supply. Each
switching power supply contains a thermistor whose
output (J1-9) is applied to the power supply control
PWB to sense excessive module temperature.
6.3.7 POWER SUPPLY CONTROL PWB
(A5A1): See figure SD-3. The power supply control
PWB provides controls and monitors the operation of
switching power supplies A to F (A6 to A11) as well
as the IPA switching power supply (A12). Refer to
the power supply control PWB service instruction
manual for detailed information.
6.3.7.1 PA Volts Control: The PA Volts Control
signal (J1-2/3), provided by the control/monitor PWB
(A14), is a low level DC control voltage which
determines the output level of the switching power
supplies A thru F (A6 thru A11). This low level
control voltage is used to develop a bi-phase pulse
duration modulated (PDM) FET drive signal which is
applied to switching FETs in switching power
supplies A thru F [
FET Drive (Source) and FET
Drive (Gate) ]. The pulse duration (duty cycle) of
this PDM drive signal determines the on/off ratio of
the switching FETs and hence the output voltage
level of the switching power supplies. The output of
each switching power supply (A thru F) is applied to
a corresponding RF power module (A thru F) and its
magnitude will determine the RF power module's
output power level. A sample of the output voltage
from each switching power supply (
PA Volts Sample )
is fed back and monitored by the power supply
control PWB. This feed-back voltage sample is
compared with the
PA Volts Control voltage from the
control/monitor PWB to allow output voltage
adjustment and to also maintain a stable output
voltage at the desired level.
6.3.7.2 IPA Volts Control: The IPA Volts
Control signal (J3-1/2), provided by the control/
monitor PWB (A14), is a low level DC voltage which
determines output level of the IPA switching power
supply. This voltage is used to develop a pulse
duration modulated (PDM) FET drive signal applied
to the switching FETs in the IPA switching power
supply [
IPA FET Drive (Source) and IPA FET Drive
(Gate) ] at J10-2/1. The pulse duration (duty cycle)
of this PDM drive signal determines the on/off ratio
of the switching FETs and hence the output voltage
level of the IPA switching power supply. The output
of the IPA switching power supply is applied to the
IPA module and its magnitude determines the IPA
module's output power level. A sample of the output
voltage from the IPA switching power supply (
IPA
Volts Sample at J2-3/4) is fed back and monitored by
the power supply control PWB. The IPA B+ Volts
Sample , representative of the B+ voltage applied to
the IPA switching power supply, is monitored by the
power supply control PWB at J2-1/2 and is used in
the feed-forward output control of the IPA switching
power supply. This voltage, and the
IPA Volts
Control voltage from the control/monitor PWB,
allows for IPA supply output voltage adjustment and
output voltage stabilization with B+ reservoir voltage
variation using a feed-forward technique.
6.3.7.3 Charging Relay Control: Each switching
power supply (PA and IPA) contains a reservoir
capacitor, which must be charged when the
B+ Volts
supply is first applied to the supply, and a relay
(initially de-energized), which controls the charge/
discharge status of the reservoir capacitor. Each
capacitor charges thru a resistor in the supply when
the corresponding
PWR MDL
circuit breaker ( A thru F
or IPA ) is turned on. The voltage across the
capacitor [between Pwr Sply Capacitor (+) and Pwr
Sply Capacitor (-) ] is monitored by the power supply
control PWB. The voltage at each Pwr Sply
Capacitor (+) input will be regulated to 56 VAC and
the voltage at each Pwr Sply Capacitor (-) input will
initial be the B+ volts level and decay to 0.0 VDC
exponentially as the reservoir capacitor charges.
Initially, the voltage difference between
(+) and (-)
inputs will be negative and the associated PDM
Inhibit A signal will be high (14 VDC), inhibiting the
FET gate drive applied to the associated PA
switching power supply. An open circuit will be
applied to the
Pwr Sply Relay Control output. The
relay in the PA switching power supply will remain