Application Notes - Aim TTi LD300 Fonctionnement

This chapter is intended to give helpful information concerning practical applications of the unit.
All electronic loads are subject to the impact of source characteristics, interconnection inductance
and feedback loop characteristics, and the following sections will assist in understanding the
factors involved.
Sources
Batteries are a low impedance source; apart from the possibility of inductance in the
interconnecting leads, they are generally easy to use in conjunction with an electronic load. The
dropout facility should be used with batteries that can be damaged by total discharge.
Electronic supplies have active feedback networks whose dynamic characteristics often interact
with the load. When that load is itself an active network whose dynamic characteristics depend
on the nature of the source, it will be apparent that the behaviour of the resulting system can be
impossible to predict.
Some common problems:
Constant Power mode has an inherent negative resistance characteristic that can result in
oscillation with some source impedances.
Many "constant current" sources based on feedback circuits are only high output impedance at
low frequencies. As the frequency increases, the output impedance falls. In fact, such units often
have a significant capacitor across the output terminals.
Many supplies have L-C output filters to reduce noise; these introduce extra phase shift into the
overall source – load combination and can give rise to instability. If there is no damping across
the inductor, a resonant circuit can be formed which allows oscillations to build up to significant
amplitude.
When using transient frequencies of a few kHz, it is often possible to excite the natural frequency
of the feedback circuit in the supply. This can have unexpected results, in extreme cases leading
to destruction.
Sources with asymmetrical characteristics can cause instability; many electronic power supplies
can source current to increase their terminal voltage rapidly, but cannot sink current and so can
only reduce their terminal voltage slowly. Equally, the load can only sink current and reduce the
voltage across its terminals; it relies on the source to pull the voltage up.
Simple supplies have just a transformer, rectifier and a large reservoir capacitor across the
output terminals. When fed from standard 50 or 60 Hz mains supplies this capacitor will not be
recharged until the peak of the next mains cycle. Such a supply has no pull-up capability between
mains cycles.
Wound machines have substantial inductance and slow response times. Transient response
testing of such sources should only be attempted at low slew rates.
Inductive sources
If a source has significant inductance, then, whenever the load current falls, a voltage transient
will be generated which might exceed the voltage rating of the load. The unit is fitted with
varistors designed to absorb non-repetitive transients up to 80 Joules, but repetitive energy up to
only 2 watts. If the energy is likely to exceed either limit then some form of external protection
must be added, perhaps a catching diode across the inductor itself.
When operating in Constant Power, Conductance or Resistance modes, the conductivity of the
load changes as the applied voltage changes; this magnifies the effect of any voltage transients
caused by inductance.
Inductive sources also increase the possibility of instability, as discussed below.
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Application Notes