Aim-TTI LD400, Инструкция по эксплуатации

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both the gain and the inter-electrode capacitance of the FETs vary with operating point,
particularly at low voltages (below about 3V) and at either low or high currents. This results in
slower response and different stability conditions and dynamic behaviour in these regions,
whatever the operating mode.
The other operating modes first derive the current required according to the instantaneous
source voltage applied and then use the power stages to conduct that current.
Constant Current Mode
As described above, this is the fundamental operating mode of the power stages of this
instrument, so it has the simplest feedback loop and the widest bandwidth. The sensed voltage
signal is only used for the meters and protection. Constant current mode is normally used in
conjunction with low impedance power supplies, and will be quite stable unless there is significant
inductance in either the interconnections or the source. Because of the wider bandwidth it is
particularly critical to have low inductance connections in this mode.
Note that the load cannot be used in constant current mode to test a constant current power
supply, as this combination has only two stable conditions: if the load setting is below the supply
limit then the supply will not be in constant current operation and will deliver its maximum output
voltage, whereas if the load setting is above the limit of the supply then the load will saturate at
its minimum operating resistance with the supply delivering its designed current. The best way to
test a constant current supply is to use the load in constant resistance mode, with a suitable
setting of the Dropout Voltage offset, as described below.
Constant Power Mode
Constant Power mode is implemented by using an analogue divider to divide the specified power
setting by the actual sensed voltage to calculate the necessary current. The power stages then
adjust their conductivity in order to obtain this current. If the source voltage falls then the load will
seek to keep the same power level by reducing its resistance to raise the current. The fact that
the current rises as the voltage falls means that the load is acting as a negative resistance. This
behaviour is also exhibited by most switch-mode power supply circuits.
This characteristic raises the possibility of a latch-up condition if the source has a significant
output impedance. To explain this, consider the possibility that the source voltage falls slightly
(perhaps because of noise) – the load responds by increasing the current to maintain the power
level. This causes a further reduction in the terminal voltage of the source (because of its internal
impedance), so the increase in power is less than expected. The load responds to this by
reducing its resistance even more, in an attempt to increase the current and obtain the required
power. A cross-over point is reached when the fall in voltage outweighs the increase in current
and the load cannot draw the required power. This leads to the latch-up condition, with the load at
its minimum resistance (nearly a short-circuit), the voltage across it almost zero, and the source
is delivering its maximum current into the almost short-circuit load. The status line of the display
will be showing the
Low Voltage warning.
If the source impedance is purely resistive then this condition will be triggered when the source
terminal voltage falls to half its open circuit voltage (this is the maximum power transfer condition
of classical electrical theory). More commonly, it will also be triggered immediately if the source
reaches a current limit, or enters constant current operation.
The only way to recover from this situation is to disable either the load input or the source output.
The vast majority of electronic sources will start in a current and power limited state at power-up,
so, to avoid immediately entering the latch-up condition, it is necessary to use the slow start
facilities of the load to constrain the power demand while the source builds up its output voltage.
As Constant Power mode has the characteristics of a negative resistance, the possibility always
exists of forming a negative resistance oscillator in combination with the output impedance of the
source. In practice, constant power mode normally operates well in conjunction with sources
designed to supply such a load.