Aim TTi LD300 Fonctionnement page 26

Constant Voltage mode is intended for use with true high impedance current sources. The
presence of shunt capacitance can form a relaxation oscillator, where the load takes a pulse of
current and then cuts off until the source recovers. Asymmetrical sources (those which can only
pull up, not down) make this problem worse. Electronic supplies operating in constant current
mode often have high impedance only within the bandwidth of a feedback loop. At higher
frequencies, a shunt capacitor reduces the output impedance considerably; a combination of this
load and such a source is often unstable.
If Constant Voltage mode cannot be made stable, it is possible to use the offsetting capability of
Constant Resistance mode. The DROPOUT control is set to the required voltage and the
resistance level setting is used to define the slope resistance. Increasing this setting will reduce
the gain and perhaps allow stable operation to be obtained.
Constant Power Mode
Constant Power mode is implemented by using an analogue divider to divide the required power
by the sensed voltage to calculate the necessary current. The effect of this is to attempt to satisfy
a demand for more power by increasing the conductivity of the load and raising the current.
Because of source (and wiring) resistance, the terminal voltage will fall as the current rises;
provided the power (the product of terminal voltage and current) rises as the current increases
then the load will function as expected.
The maximum load power that can be drawn from a given source occurs when its terminal
voltage has fallen to half the open circuit voltage. As the current increases beyond this point, the
fall in voltage outweighs the increase in current and the power falls. The load then latches into a
condition of hard conduction, with maximum current and almost zero voltage: it is attempting to
increase the power level by increasing the current, but as the supply is already delivering its
maximum current this attempt fails. The only way to recover from this situation is to disable the
load input or the source output.
Constant Power mode has the characteristics of a negative resistance (the current increases as
the voltage falls) and 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.
In transient operation, if the source is constant voltage (with low source impedance), then the
current will follow the changes in power demand and the response will be very similar to constant
current mode. If the source voltage falls as the power demand increases, then the current has to
increase more than proportionally and the current slew rate rises; this will limit the maximum
useful power slew rate to a setting below the point at which the ERR indicator lights.
Constant Conductance and Resistance Modes
In both these modes, an analogue multiplier-divider is used to derive the current required from
the sensed voltage. In Conductance mode the current required is calculated by multiplying the
sensed voltage by the required conductance; in Resistance mode the current required is
calculated by dividing the difference between the sensed voltage and the dropout voltage setting
by the required resistance.
In both cases, the current rises as the applied voltage rises. At equivalent resistance and
conductance settings, the path from the voltage sense input through to the power stage is the
same, so the two modes will exhibit similar stability characteristics.
In transient operation, the two modes are very different. In Conductance mode, the current
required linearly follows the changing conductance value and the behaviour is fundamentally
similar to constant current mode. In Resistance mode, the required current is inversely
proportional to the linearly changing resistance value, so the resulting current waveform is very
non-linear, rising rapidly at the low resistance part of the cycle. This rapid rise accentuates the
effect of inductance in the interconnecting leads and can easily lead to bottoming and
overshoots. Resistance mode is best used at higher voltages and modest currents.
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