TTI LD300 Service Manual page 16

Main PCB
Two sets of four-pin test points give access to the most important signals on the board: TP2 has
FS, IFB, VS and 0V while TP3 has PROD, REMV, DMD and 0V. TP1 has the internal oscillator
signal (when it is in use).
Voltage Sensing
The internal (IC19) and external (IC32) voltage sense amplifiers are very similar. Each is a fully
differential circuit to reject common mode influences and has a gain of 20/680 to provide an
output scaling of 2353mV at 80 Volts. The accuracy of the offset null adjustments on these
amplifiers is critical for the accuracy of the unit at low voltages in constant power, resistance and
conductance modes. SW2 (on the back panel) selects the required signal for the Voltage
Feedback signal VFB and the Voltage Sense signal VS (which are virtually identical).
C106 & C107 provide high frequency coupling from the internal sense connection onto the
external sense signals to reduce the effect of long external sense connections on the stability and
high frequency behaviour of the unit.
The external sense circuit is designed to allow the use of backing-off batteries to extend the
performance capabilities of the load down to zero volts. Comparator IC46-A monitors the
difference between the two sense voltages and trips the fault latch if it exceeds about 8 volts.
The internal voltage sense signal VINT (which reflects the actual terminal voltage) is always used
to check the power dissipation and voltage fault conditions.
Comparator IC46-C has a threshold equivalent to 87·5 volts and triggers the transient over-
voltage clamp circuit Q9 (which is part of the comparison amplifier). This circuit is designed to
clamp the terminal voltage and absorb energy from small inductances in the load circuit. If the
energy involved is excessive it will cause the power limit multiplier to trip the fault latch.
Comparator IC46-D has a threshold equivalent to 96 volts and trips the fault latch to protect the
unit against excessive voltages. This does not prevent the terminal voltage being applied to the
FETs, but it ensures that they are not conducting. There are also Varistors across the FETs, which
will start to conduct (at typically 120V) to absorb transient energy; these are not intended to
conduct continuously, and will be destroyed if excessive DC voltage is applied.
Current Sensing
The current is monitored by two 5mΩ 4-terminal resistors in the FET sources. To obtain the
required accuracy IC22 is used in a differential configuration to sum the voltages across the
sense terminals of the two resistors and disregard any common mode voltages. It provides a gain
of 3000/510 from each input, so its output voltage is nominally 2353mV at 80 Amps (40 Amps in
each FET); this is the signal IFB. The accuracy of the offset null adjustment of this amplifier
(VR12) greatly affects the accuracy of the unit at low currents. The voltage across each sense
resistor is also separately used by the differential feedback input of each gate driver IC21 and
IC30.
Current Monitor Output
To avoid difficulties with ground loops, the current monitor output is driven by a circuit designed to
provide an accurate voltage between the terminals regardless of common mode voltages applied
at the outputs (within the working range of the circuit – about ±3 V). In essence IC45-B is a
current source and IC45-A is a current sink, combining to pass a current through R128 (and its
parallel trimmer) to develop the required output voltage. The nominal output impedance is 600Ω ;
VR26 adjusts this to calibrate exactly the required gain to obtain the output scaling of 4V at 80
Amps.
Because of the cross-coupled nature of this circuit, almost any fault will make it saturate, so fault
finding is best done by inspection.
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