Aim TTi LD300 Fonctionnement page 24

Grounding
A scope will often be used to view the voltage and current waveforms, particularly when using the
transient capabilities of the load to investigate the behaviour of a source. Take care to select a
suitable point to connect the scope ground, as voltage drops on the interconnecting cables
(particularly transients caused by inductance) can give misleading results. The Current Monitor
Output from the load can be used to avoid multiple grounds, as it provides common mode
rejection (as long as the voltage is kept within a few volts of the load negative terminal). If
possible, the best ground point is usually the negative terminal of the source.
Note that if the load is used with a source having the positive terminal grounded, then any
instrument attached to the Current Monitor output must be fully floating.
Stability
This load is optimised for accuracy under constant load conditions and has high internal
feedback. Because of this, the possibility exists for combinations of source, interconnection and
load characteristics to give rise to instability. There are three major potential causes: inductance
in the wiring between source and load (or an inductive output impedance of the source),
capacitance in parallel with the connection between source and load (including an output
capacitor within the source) and the characteristics of active feedback circuits within the source.
In Constant Power, Conductance and Resistance modes, the system includes an analogue
multiplier used by the load to derive the current requirement from the instantaneous voltage. This
adds additional phase shift into the loop. In general, Constant Current mode is the most likely to
be stable, but in some cases instability can be avoided by using a different mode. The conditions
that affect the dynamic behaviour of the load in transient operation also lead to instability, and
some of the suggestions in the sections below may be found helpful.
Remedial Actions
The compensation networks of the power stages in the load are changed when the SLEW RATE
RANGE switch is put in the Lo position. Even if the transient facilities are not being used, this
change in compensation may make the load stable.
If instability arises, observe the voltage waveform across the load with a scope: if at any point the
voltage rises above the open circuit emf of the source, then there must be an inductive element
present to form a resonant circuit. Some means must be found to insert damping into this circuit.
One technique is to use a snubber network (consisting of a capacitor and a resistor in series),
across the input terminals of the load. Many electronic loads have such a network built-in; it is
omitted from this load to maximise its versatility by offering the minimum possible input
capacitance. It can be added externally: values around 2·2µF and 5Ω are common; note that this
must be a power resistor (capable of handling a few watts) constructed with non-inductive
technology – a flat film type is best.
Dynamic Behaviour in Transient Operation
When the transient capabilities of the load are used, the dynamic behaviour of the source and
load combination during the transitions depends on similar considerations to those affecting
stability: series inductance, shunt capacitance and feedback loop characteristics. Proper
operation depends on the load neither saturating nor cutting off at any point in the cycle. The
faster the slew rate sought, the more likely it is that aberrations will occur on the transitions.
Because of changes in the transconductance of the FETs, the dynamic behaviour of the power
stages changes at both low and high currents, and also at low voltages when the inter-electrode
capacitance increases considerably. In general, behaviour is optimum in the middle of the current
range (5 to 60 Amps) and at voltages between about 3 volts and 3 volts below the open circuit
voltage of the source.
Attempting to achieve a slew rate beyond the capabilities of a source and load combination will
result in substantial overshoot and ringing. Reducing the slew rate, sometimes by just a small
amount, will improve the response considerably.
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