Broadcast Electronics AM-2.5E, Руководство пользователя

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4-14
4-72. A dc operating supply for optical coupler U17 and buffer U19 is generated by bridge
rectifier D25. D25 full-wave rectifies an ac potential from ac transformer T1 into an
unreg20V supply. The supply is applied to +18V regulator U16. U16 outputs a
+18V supply which is further regulated to a 5V operating potential by a resistive divider
and a zener diode. The 5V supply is applied to optical couplers U17 and buffer U19.
4-73. Control of the regulator circuit is provided by the switching regulator power control PWM
circuit, a current sampling circuit, and a voltage and current feedback correction circuit.
Together, the circuits function in a closed-loop to control regulator operation. The
switching regulator power control PWM circuit is designed to produce two out-of-phase
square wave signals with varying duty cycles. The duty cycle is varied in response to the
signal from the correction circuit. The output of the control circuit is applied to optical
coupler U17. U17 provides isolation between two different ground circuits. The output of
the coupler is applied to gate drive inverting buffer U19. The output of U19 is applied to
the gates of IGBT switching regulator transistors Q21 and Q22. Q21 and Q22 are
operated to on for a specific time duration to regulate the output voltage for varying load
conditions.
4-74. The regulator circuit output voltage is directed by a 1 kHz power control PWM signal
from the controller. The signal is applied to the correction circuit on the power supply
circuit board. In addition to the 1 kHz PWM signal, a voltage and current sample from
the regulator output is routed to the voltage and current feedback correction circuit. The
correction circuit responds by evaluating the output samples and the power control signal
and generating a correction voltage. The voltage is applied to the switching regulator
power control PWM circuit to adjust the output of the regulator.
4-75. The output of the transistor switching regulator circuit is applied to filter inductor L2.
Protection of the transistors from switching transients during turn on/off operation is
provided by clamp diode D32. Clamp diode D32 protects the transistors by limiting
positive peak voltages. The output of the regulator circuit is applied to circuitry on RF
power module modulator circuit boards.
4-76.
MODULATOR CIRCUIT BOARD.
4-77. The B- leg from the power supply circuit board is routed to circuitry on the RF power
module modulator circuit board. The B- leg is applied to relay K1 on the modulator
circuit board. K1 is controlled by a fault detector circuit. The relay is designed to
immediately remove the supply from the forward converter circuit during a fault
condition. The output of the relay is applied to the converter circuit.
4-78. Transistors Q1 and Q2 are the switching devices in the forward converter circuit. The
circuit is controlled by the audio PWM signal from the driver circuit board. The circuit
operates by switching the applied B- leg at a 122 kHz to 135 kHz rate. The duty cycle of
the PWM signal is 40% with no modulation. The output of the forward converter circuit
is applied to an LC PWM low-pass filter network. Protection of the transistors from
switching transients is provided by clamp diodes D2 and D3. D2 and D3 limit the positive
peak transients appearing on the output.
4-79. A sample of the modulator circuit output is routed to a fault detector circuit. The circuit
will respond to a fault by: 1) routing a control signal to relay K1 to disconnect the
amplifier circuit from the B- supply and 2) route a control signal to the power supply
circuit board to momentarily mute the power supply. After a short delay, a control signal
is routed to the power supply to enable the supply to provide power to RF module 2.
4-80. The B+ leg of the supply is also routed to the modulator circuit board. The B+ leg is
applied to fuse F1 and is the common for the dc voltages contained in the power supply
circuit board switching regulator circuit and the RF power modules. F1 is provided to
protect clamp diodes D2 and D3 from overcurrent conditions.