TTI CPX200, Руководство по обслуживанию

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Control Section
The control circuitry is contained entirely on the control (upper) board with the exception of the
current transformer (T2) which is fitted on the main board and the user controls themselves which
are on the front panel board.
Voltage Control Loop
The feedback voltage for the voltage control loop is fed from the main board via PJ4 pin 5
(+SENSE A) and PJ4 pin 8 (-SENSE A); R12/C21 and R11/C19 (all on the main board) ensure
stability when long leads are used with remote sense. Usual operation is with the sense
connections made directly at the output terminals but up to 1V drop in the connecting leads can
be corrected for by sensing remotely. PTC1 and PTC2 (on the main board) provide an alternative
permanent sense connection if no other sense connection is made. If, during remote sense
operation, the output terminals become disconnected from the load, PTC1 and PTC2 also provide
current limiting protection when the load attempts to draw current from the sense terminals.
IC2 buffers and inverts the front panel voltage controls VR1, VR2 to give approximately –2·62V at
maximum output (35V). IC1 is a differential amplifier with a voltage gain of 13·3 set by R18, R20
and R19, R21, VR4. VR4 trims the differential gain which ensures voltage regulation is
maintained during remote sense operation when there is a voltage difference between –SENSE
and –OUTPUT, see Calibration section. The output of the op. amp. IC1 drives the input diode of
the opto coupler OPT1. This is a low noise device and helps to prevent switching noise and pick-
up from getting into the control loop. The output of the opto-coupler drives current through R13;
the voltage developed across R13 is fed to IC5 pin 5 the input of the error amplifier of the PWM
controller, IC5. This amplifier is configured as a voltage follower with a gain of one. IC5 then
converts this input signal into a pulse width modulated squarewave which is used to drive the two
power FETs Q1 and Q2, via IC1 (all on the main board).
Current Control Loop
The current control loop uses the voltage across the current sense resistor R14 (main board)
which is fed to the control board via PJ4 pin 7 (A) and PJ4 pin 2 (ISENSE-A). IC6
provides a gain of 10 and its output is fed to the high gain amplifier IC17, the other input of which
(ISETA) is derived from the front panel current limit control, VR4, via PJ8 pin 2. When the current
sensed through R14 is below the set level, the output of IC17 is high, driving the maximum
current into opto OPT3 via R6 which in turn develops the maximum voltage across R11 at pin 1 of
IC5. When the output current approaches the set level the output of IC17 falls and the voltage
across R11 drops. This voltage is fed to IC4 pin 1, the current limit adjust input of the PWM
controller, and acts to reduce the peak current in the primary as the voltage across R11 falls,
irrespective of the voltage control loop.
Overvoltage Protection (OVP)
The overvoltage trip point is set by preset VR3 on the front panel board (it is a screwdriver
adjustment through the front panel). This voltage (OVPSETA) is fed via PJ8 pins 4 and 8 to the
inverting input of comparator IC4-C; the output voltage (OVA) is sensed at the
common-mode choke L7 (main board) and fed to the other input of IC4-C via PJ4 pin 6. When the
output voltage exceeds the set OVP the output of IC4-C goes high, turning on Q4 and Q3 which
latches the comparator in the high state. When the comparator output goes high, current is driven
into the opto OPT2 via D9; the output of the opto goes to 5V, pulling pin 16 of IC5 high which
shuts it down. The power supply needs to be switched off to allow the OVP circuit to reset.
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