Aim TTi LD300 Fonctionnement page 25

Source inductance
Source and interconnection inductance has a major impact on the behaviour of the load: the
fundamental characteristic of an inductance is that it resists any change in current. As the current
rises, the inductance generates an emf that reduces the voltage across the load, often to the
point where the load saturates. Whenever the voltage falls below about 3V the transconductance
of the power stage changes considerably, changing the damping factor of the feedback loop, and
the dynamic behaviour changes markedly. As the current falls, the inductance generates an emf
that increases the voltage across the load terminals; this in turn affects the conductivity of the
load in operating modes that are voltage dependent.
Shunt capacitance
The load can only sink current, and 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.
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 before it starts to
conduct, followed by a ringing as the source responds.
Source Characteristics
The purpose of transient testing is to examine the behaviour of any feedback loops within the
source. If the response of the source is under-damped, then in general the use of an active load
will accentuate the effect. This is particularly true in the modes where the load responds to
changes in voltage. At particular transient frequencies the load may excite resonances in L-C
filters or match the natural frequency of a feedback loop. This can result in considerable reaction
from the source.
Implementation
The following sections give a brief description of the way each mode operates, and give some
guidance of the effect that has on the application of the load.
Constant Current Mode
The load has two power stages (each a large FET) in parallel; each stage has local current
feedback to ensure equal power sharing. Overall current feedback to an earlier stage is used to
enhance accuracy. The sensed voltage signal is only used for the meters. Ideally, the operation
of the power stages would be independent of the applied voltage, but in practice, both the gain
and the inter-electrode capacitance of the FETs vary with operating point, particularly at low
voltages (below about 3V) and at low and high currents. This results in slower response and
different stability conditions and dynamic behaviour in these regions.
Constant current mode is normally used in conjunction with low impedance power supplies, and
is normally quite stable unless there is significant inductance in either the interconnections or the
source. The load is designed to support higher current slew rates in Constant Current mode than
in all the other modes; this makes it particularly critical to have low inductance connections.
Constant Voltage Mode
Because the power stages of the load are fundamentally a current sink, Constant Voltage mode
operates in an entirely different manner to all other modes. The difference between the sensed
voltage and the required voltage is applied to an integrator with a short time constant. The output
of this integrator (which is, in effect, a guess at the current required) drives the power stages.
The operation of this mode depends entirely on feedback action. The transconductance of the
load (the change in load current caused by a small change in sensed voltage) is very high,
resulting in very high system gain.
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