Wayne Kerr AP10030, User & Service Manual

Page: 33 / 70

Wayne Kerr AP 1kW Power Supplies • Issue 2 33
(control board diagram). This allows for a more precise control of the width of the
narrow pulses which are needed when a very low power output is demanded.
When this ramp reaches a value determined by the voltage on pin 9 of the comparator,
IC2-C (control board diagram), the drive pulse is switched off. The voltage on pin 9 of
the comparator is dependent on the output voltage of the unit (or output current if in
current mode). To provide a maximum power limit, a second comparator (IC2-D) is
‘OR’ed with the volts/current comparator. When the ramp (i.e. primary current) reaches a
pre-set value the pulse to the FETs is switched off, no matter what is demanded from the
load. At switch on, the power limit point rises exponentially to its normal value by means
of C16, R55, etc. (control board diagram) and the switching off of TR13. This limits the
power into the load immediately after switch on.
Dynamic Bleed
IC16, TR21, TR22 and TR20 etc. (control board diagram) comprise the dynamic bleed
circuit. Under low load conditions, when the output is required to go to a lower value or
off, the output of IC5-C goes low. This switches on the bleed circuit. This discharges the
output capacitors at a rate of 100V per second. If the drive pulses are inhibited (e.g.
overvoltage, off or overtemperature) the dynamic bleed circuit is activated to ensure that
the output capacitors discharge. To allow the discharge to continue after the mains has
been switched off, C57 provides power for a limited period.
Servo Control
a) Constant Voltage (CV)
In constant voltage mode operation, the output voltage is divided by R184, P10, R183
and R182 and fed to either the overvoltage trip comparator, IC22 (control board
diagram), or the control amplifier, IC13 pin 2. The positive input of this amplifier (pin 3)
has a voltage applied to it via IC24-D pin 12. This applied voltage can be varied
depending on the output required. The output of IC13 is applied, via R57, to pin 9 of the
ramp comparator, IC2-C and determines the peak ramp voltage and its duration.
Feedback is used to keep the output voltage constant. This is achieved by keeping the
inputs to IC13 equal (the output of IC13 controls the pulse width of the FET ‘on’ time).
The voltage on pin 3 of IC13 is variable by the front panel voltage control potentiometer
or, in external mode, by external resistors or a voltage. In local operation and in
resistance programming, the control voltage is produced by passing a constant current
through either the front panel potentiometer or the external programme resistors. The
constant current source is formed from the circuits of IC19-C, TR23, TR24, etc. (control
board diagram).
b) Constant Current (CI) & Automatic Changeover
In constant current mode operation, the output current is converted to a voltage by the
current sense resistors R406/7/8 (power conversion diagram). This voltage is applied to
the inputs of IC12 (control board diagram). IC12 and associated circuitry serves two
functions, i.e. amplification and also rejection of any common mode voltage introduced
by the finite resistance of the leads from the sense resistor to the sense terminal. The
output from IC12 is taken to pin 2 of the current comparator IC15 and compared with the
voltage on its pin 3. This voltage is set by the current limit required. When the voltage on