Circuit Description - TTI LD300 Service Manual

Circuit Description

In the following descriptions, items in bold are a cross reference to a section with that name.
System Overview
Although the load can operate in various modes – constant current, voltage, power, resistance or
conductance, it is fundamentally a constant current load. The other modes (all except constant
voltage) are implemented by measuring the value of the voltage across the load terminals and
using an analogue function multiplier (in some modes, as a divider) to calculate the magnitude
of the current required to satisfy the defined condition. For example, in constant conductance
mode I = V × G where V is the instantaneous terminal voltage and G is a voltage (from the control
circuits) defining the value of conductance required.
The power is dissipated in two parallel FET power stages mounted on the heatsinks. These have
separate gate driver circuits (on the main PCB), and each has a current sense resistor in its
source which provides both local feedback to the gate driver and an input to the current sensing
circuit which controls the overall load accuracy.
There is a common control voltage input to the two gate drivers from the comparison amplifier,
which determines the overall magnitude of the load current. The local feedback from each sense
resistor to its gate driver provides a wideband control path, and matches the power dissipation in
the two FETs.
All control circuits in the unit are voltage controlled. The full-scale voltage FS (derived from the
internal reference) corresponds to a theoretical setting of (for example) 81·92 Amps; it is
nominally 2378mV. In practice, because of end-resistance in the control potentiometers, this
value cannot be reached, and the rated full-scale is 80 Amps, represented by nominally 2323mV.
In other modes the control voltage relates to the corresponding unit: to Ohms in constant
resistance mode, Watts in constant power mode, Siemens (Amps per Volt) in constant
conductance mode and Volts in constant voltage mode. In all modes except power, two ranges
are provided.
A feature of this load is its ability to test the transient performance of the user's supply by creating
load transients. These transients are changes between two preset levels, level A and level B, in
the level select circuit. A slew rate generator produces straight line transitions between these
two levels with a slope defined by the slew rate control settings. The interval between the
transitions can be controlled by an internal transient oscillator with adjustable frequency and
duty cycle, or by an external logic signal applied to the remote control input.
Under remote analogue control the user can generate any transients required by applying a
suitable waveform to the remote input. This waveform still passes through the slew rate generator
to impose a defined slew rate on the output waveform.
Another facility is Low-voltage Dropout, which makes the unit stop conducting current when the
terminal voltage falls below a level set by the user; this can be used to avoid over-discharge of
batteries. A related facility is slow start which makes the load current ramp up slowly to its final
value when enabled and ramp down slowly to zero when the unit is disabled.
There are a number of protection circuits within the unit, detecting excess power, current, voltage
or temperature. When any of these conditions arise a Fault Latch is set to prevent the unit
conducting current.
Note that the unit is designed so that the load input terminals are not grounded. The system 0V
line is tied to the negative terminal of the load input by connections to the FET PCBs. Be aware,
when taking measurements, of the possibility of voltage drops caused by heavy currents in the
load circuit wiring and avoid creating ground loops.
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