Valleylab Force FX-8C, Руководство по обслуживанию

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Power Supply/RF Board
4-20 Force FX-8C Service Manual
The SYS_ECON signal from the microcontroller controls the output
voltage level. This 0 to 5 Vdc signal sets the reference for the PWM control
loop. An external op-amp (U7B) is used for gain and integration, since
common mode voltage limitations in U5’s internal op-amp preclude its
use over the full range of 0 to 5 V. The internal op-amp is connected as a
follower. SYS_ECON is compared to the feedback voltage from the output
divider (R34, R35, and R49), and an error signal (ECON) is sent to the
PWM microcontroller. In addition to the error signal, U7B and the
associated R-C networks provide lead-lag loop compensation to increase
the bandwidth of the regulator beyond that of the output L-C filter.
Note : U7A is used for random gain switching in the Spray mode and is
configured for unity gain in all other modes.
The output of U5 is a pair of 180° out-of-phase signals that are pulse width
modulated by comparing ECON with the internal oscillator ramp
waveform. At the start of an oscillator cycle, an output is turned on. It
turns off when the ramp voltage crosses the ECON level. The two output
signals from U5 (pins 11 and 14) feed the MOSFET drivers (U3A and
U3B).
R36 and C42 set the U5 oscillator frequency to approximately 170 kHz.
C45 controls the ramp-up of the pulse width at power on for slow start
control. Transformer T2 limits the power transformer primary current,
protecting against faults in the DC/DC switcher power stage and faults in
circuitry downstream of the switcher. The output of T2 is rectified
(CR3–CR6), filtered (R30 and C30), and fed to the current limit pin
(pin 9 of U5). During an overcurrent condition the U5 current limit
function resets the slow start circuit, resulting in the output cycling from
on to off until the current falls. Pin 9 of U5 is also used for remote shut
down of the DC/DC switcher through U6A and CR8. The shut down
signal comes from the main microcontroller on the Control board.
The resistor divider on the high voltage DC output formed by R52 and
R53 is used for dosage error sensing.