TTI CPX200, Service Manual, Page: 11 / 30

Primary Current Control and Primary Current Limit
The primary current is sensed by transformer T2 on the main board; the primary is in series with
T1 and the secondary is fed to D2/R2 on the control board via PJ5 pins 1 and 2. D2 rectifies the
signal which appears as a voltage across R2 in parallel with VR5/R22; R1 and D1 allow the
current transformer to be reset on each cycle. The ramp voltage developed across R22,
corresponding to the ramp current in the primary of T1/T2, is fed to IC5 pin 4, the +current sense
input of the PWM controller, via VR5/R22. This arrangement provides pulse-by-pulse primary
current control, giving better overall voltage loop control by improving line regulation and load
response.
In normal operation the pulse is terminated at a point determined by the voltage on IC5 pin 5, the
error amplifier input; the greater the demand from the voltage control loop via the error amplifier,
the higher the primary current will ramp before the pulse is terminated. There is, additionally, an
overriding limit mechanism in IC5 which terminates the pulse when the ramp at pin 4 reaches a
certain level, regardless of a demand for more power by the signal on pin 5. Since VR5 ‘scales’
the voltage applied to IC5 pin 4, VR5 effectively sets the primary current limit which in turn sets
the secondary power limit, see Calibration section.
Front Edge Blanking
Front edge blanking is provided by Q1, C2, R4, D24 and C3. Front edge blanking prevents the
switching spikes caused by main board FETs Q1 and Q2 being fed into pin 4 of IC5 via the
primary current limit circuit (T2, D2, R2, VR5, R22 and R3).
Measurement and Display
The measurement system and display are controlled by a microcontroller IC11. The measurement
of output and preset values of voltage, current and OVP is performed by the 12 bit analog to
digital converter IC12. The measurement rate is controlled by the 4·0MHz ceramic resonator XL1
connected between pins 22 and 23 and the buffered version of this 4MHz signal at pin 25 is used
as the clock to the microcontroller IC1. The ADC, IC12, is a dual slope converter and provides
just over 8 readings per second when clocked at 4MHz. The ADC is run in continuous mode and
the status signal on pin 2 is read by the microcontroller every 6ms. When a reading is ready the
microcontroller reads the 12 bit binary value and then converts it to 7 segment BCD and stores it
ready to be sent to the display. After each reading the microcontroller switches the input
multiplexers IC13 and IC14 to the next required input. In this way it is possible to read and display
any of the following:
Preset Volts
Preset Current
OVP
Output Volts
Output Current
The multiplexers are controlled by the latch, IC6, on the front panel board which also drives the
additional indicator LEDs; this latch is driven by the microcontroller, IC11.
The decision on what to measure and display at any time is taken by the microcontroller and in
order to do this correctly a number of status signals and switches are monitored on a regular
basis. These are:
VLIMDA from IC4-B
ILIMDA from IC4-A
OVPTRIPA the OVP trip signal
OVPKEYA the OVP key signal
OUTPUTA the output on/off switch signal
All these signals may be read by the microcontroller as required.
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