Aim-TTI LD400, Инструкция по эксплуатации

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Shunt capacitance
The load can only sink current, so it can only pull the voltage at its terminals down. The source
must pull the voltage up, including providing charging current to any capacitance across the
terminals. If the total current available is more than sufficient to charge this capacitance at the
slew rate required, then the load will continue to conduct the excess current during the transition
and the behaviour will be as expected. However, if the source cannot charge the capacitor at the
required slew rate, then the load will cut-off until the final voltage is reached. There will then be
an overshoot as it starts to conduct, followed by a ringing as the source responds.
Stability of Source and Load Combinations
This instrument is optimised for accuracy under constant load conditions by using a high gain
feedback loop. 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
reduces the bandwidth of the loop and adds additional phase shift. 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.
Constant voltage mode operates in a completely different way, by integrating the voltage error to
create a value for the current demand. This results in a system with extremely high gain and
significant potential for instability.
Many supplies have L-C output filters to reduce noise; these introduce extra phase shift into the
overall source and load combination and can increase the possibility of instability. If there is no
damping across the inductor, a resonant circuit can be formed which allows oscillations to build
up to significant amplitude.
Remedial Actions
The compensation networks of the power stages in the load are changed when the Slew Rate is
set to less than 0.001 times the maximum slew rate for the given load mode and range. For
example if constant current mode is selected and set to its high settings range (up to 80A), the
maximum slew rate setting is 2.5A/us, hence the compensation networks are changed at slew
rates settings below 2.5A/ms. Even if the transient facilities are not being used, this change in
compensation reduces the bandwidth and may make the source and load combination 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 network consisting of a capacitor and a resistor in series (sometimes
called a Zobel network), 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 lowest
possible input capacitance. It can be added externally: values around 2·2µF and 5
Ω
are
common, but note that this must be a non-inductive power resistor capable of dissipating a few
watts. A flat film type is best – wire wound resistors are not suitable.
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.