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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 (as described
above) the current has to increase more than proportionally and the current slew rate rises; this
will limit the maximum useful power slew rate.
Constant Conductance and Resistance Modes
In both these modes, the 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 specified conductance; in Resistance mode the current required is
calculated by dividing the difference between the sensed voltage and the dropout voltage setting
by the specified 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, changing rapidly at the low resistance part of the cycle. This rapid change
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.
Dropout Voltage and Resistance Mode
The use of the Dropout voltage setting as an offset in Constant Resistance mode allows flexibility
in constructing load characteristics for particular circumstances. For example, setting a low value
of resistance and a significant value of dropout voltage yields a characteristic similar to a string of
LEDs or a Zener diode and provides an alternative to Constant Voltage mode (see below) but
without the extreme stability problems of that mode.
Constant Voltage Mode
Constant Voltage mode is more likely to exhibit instability than any other mode, especially when
used in conjunction with electronically regulated sources. It is primarily useful with true wideband
current sources which maintain their high output impedance at all frequencies. It will also operate
satisfactorily with resistive sources of moderate impedance, such as photovoltaic cells.
The behaviour required in Constant Voltage mode is the opposite of the fundamental operation of
the power stages of the load, which are intrinsically a voltage independent current sink, so it is
implemented 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 therefore depends entirely on feedback action.
The presence of the integrator means that the low frequency transconductance of the load (the
change in load current caused by a small change in sensed voltage) is very high: many
thousands of Amps per Volt. This combines with the output resistance of the source to produce a
system with extremely high loop gain. High frequency instability can result in the normal way if
the phase shift around the loop reaches the threshold for oscillation before the gain has rolled off
below unity. Generally such oscillations will be roughly sinusoidal, at a frequency of many kHz.
The addition of a series CR (Zobel) network across the load, as discussed above, may eliminate
such instability. Alternatively, series resistance between source and load might be helpful.
A more common instability arises from the transient behaviour of the source. The simplest
visualisation of this is to start with the load suddenly increasing its current (perhaps because the
source voltage has just risen above the set point). This increase in current causes a transient
reduction of the output voltage of the source (depending on its transient response) which causes
the voltage to fall below the setting of the load, which in response ceases to conduct current. This
