In voltage mode the system works differently – IC43 acts as an error integrator. Its output
changes to adjust the conductivity of the FETs, in turn changing the current drawn through the
source impedance of the user's supply, until the terminal voltage matches the required value.
Because the FETs invert, the connections to IC43 appear reversed – the required voltage is
applied to its negative input and the voltage feedback signal is applied to positive input. The gain
of the feedback loop is extremely high, and is critically dependent on the nature and magnitude of
the user's source impedance, which provides the drain load for the FETs, and determines the
stability of the overall system.
Output Enable
Over-riding control of FET stage conduction is provided by the DRIVE signal from the input
enable and fault logic. When DRIVE is low, Q8 turns on and makes all the transmission gates in
IC42 and 44 open circuit by setting their E input high. IC43 has its positive input pulled to ground
by Q14, while R274 sources current through D22 onto its negative input and causes its output to
go negative until Q7 turns on (at about -0·7V) to conduct this current. The outputs of the gate
drivers IC21 & 30 also go negative (until D2 and D3 conduct) and the FETs are completely turned
off. The unit cannot conduct any load current from the user's source while DRIVE is low.
The power limit and low voltage dropout circuits operate by pulling down the positive input of
the comparison amplifier through Q4 and Q5 respectively.
Power Limit Multiplier
This is another log-antilog analogue multiplier, using the transistors in IC27 and the op-amps IC28
and IC29. The principle of its operation is the same as the function multiplier described above.
IC28-A adjusts the base-emitter voltage of transistor IC27-A until its collector current equals the
current from VINT through R100. IC29-A does the same with IC27-D to match the current
proportional to IFB through R107. IC27-B carries a scaling current from R95. Therefore the
collector current of IC27-C is proportional to the product of VINT and IFB, divided by a calibration
value set by VR18. IC29-B converts this current to a voltage across R109 that is proportional to
the power being dissipated in the load.
R214 provides an offset below ground equivalent to the protection threshold, approximately 340
Watts. When the power exceeds this level, D16 conducts the current into R109 so the output of
IC29-B is about 0·7V above ground. This starts to turn on Q4, which acts as a variable resistor
and reduces the demand level at the input of the comparison amplifier by causing a voltage
drop across R230; this additional feedback mechanism attempts to control the power in the load
at this limiting threshold. The positive voltage at PLIM turns on Q11, which pulls down WARN,
and makes the panel lamp show orange by driving both RED and GRN.
If the limiting action fails, and the power continues to rise, eventually (at about 360 Watts) the
voltage passes the threshold of comparator IC58-A and the fault latch is tripped.
Low Voltage Dropout
The control signal OFFV from the front panel passes through IC48-C, which disconnects it from
the internal signal DROPV in voltage mode (the facility is redundant in that mode, as the load
does not conduct below the set voltage anyway). DROPV is used by the slow start comparator
IC46-B. When the control is fully counter-clockwise (perceived by the user as zero) XOLO
provides a small negative voltage at DROPV to ensure that the output of this comparator is high
even when VS is 0V.
IC23-A inverts DROPV to produce a corresponding negative voltage at VXO; R263 provides
slightly over unity gain, to match the scaling of the voltage sense signal VS. Normally D7
conducts the output signal, but when DROPV goes negative (because the control is at zero) then
D8 conducts (holding the amplifier output at +0·7V) and D7 turns off to allow R264 to hold VXO at
exactly 0V. VXO is also used by the function multiplier in resistance mode.
19
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