Nautel XR12, Troubleshooting Manual, Page: 87 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-28 Page 2-19
RF drive power supply PWB (62 V) (NAPS10
C
)
See electrical schematic Figure SD-31.
The RF drive power supply PWB is a boost type switching power supply which provides a regulated
dc supply voltage for the transmitter's RF drive amplifiers. A n48V supply is applied to the
drain of switching FET Q1 through inductor L2. The gate of FET Q1 is driven with pulses from
pulse width modulation (PWM) switching power supply control U1. This device is a fixed-frequency
PWM control circuit, incorporating the functions required for the control of a switching power
supply. Internal to the device is a sawtooth oscillator which is set to a nominal frequency of 100 kHz
by external components R6 and C6. The output, PWM control pulses to the gate of FET Q1, is
generated by the comparison of this sawtooth waveform with a feedback voltage sample from the
junction of U2-Cathode and the bias voltage created by
VOLTAGE CONTROL
potentiometer R3. The
resultant variable pulse width output (U1-8) is applied to the gate of FET Q1. Q1 turns on and off,
ringing inductor L2 with the appropriate pulses to maintain the desired set dc output voltage. Diode
pair U2 rectifies the signal and inductor L3 and capacitor C9 provide filtering for the
RF Drive B+
output voltage. A sample of the dc output voltage level is applied through zener diode CR1 to the
base of transistor Q2 which provides an alarm signal should the RF drive power supply fail. The
RF
Drive B+ Fail alarm output is normally at ground potential and switches to an open circuit condition
should a failure occur in the RF drive power supply.