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in turn results in a transient increase in the output voltage of the supply, and then the process
repeats. This type of instability can be recognised by the characteristic short pulses of current
separated by longer periods of zero current. Sometimes this instability can be avoided by setting
the load threshold well below the open circuit output voltage of the source. Adding resistance in
the connection between source and load may also be helpful.
Another possible form of oscillation can arise if the source has a large output capacitor; this is
characterised by a sawtooth voltage waveform, and is often called a relaxation oscillation.
The operation of the Slow Start circuit is modified in Constant Voltage mode to implement one
common technique to avoid triggering instability. When the load is initially enabled with Slow Start
active, its effective voltage setting starts at 80V (which should be above the open circuit voltage
of the attached source) so no current flows. The slow start circuit progressively reduces the
voltage setting (as determined by the configured Slew Rate). No current flows until the setting
falls past the output voltage of the source. Depending on the output resistance of the source the
current will then gradually increase until the source has been pulled down to the final voltage
setting and the slow start circuit no longer has any effect, leaving stable operation at the desired
operating point.
If Constant Voltage mode cannot be made stable, it is possible to use the offsetting capability of
Constant Resistance mode as described above. The Dropout voltage level is set to just below the
required voltage and the resistance level setting is used to define the slope resistance. Even
quite small values of resistance setting will reduce the gain far below that of voltage mode and
allow stable operation to be obtained.
Multiple Unit Operation
It is possible to operate two loads in parallel in Constant Current mode, which will double both the
current handling and power dissipation capability of a single unit. The connections to the source
should be matched as well as possible.
Note that additional stability issues may arise, because of phase response differences between
the units; the use of more than two units in parallel is not recommended. Multiple unit operation
should not be attempted in any operating mode other than Constant Current.
Zero Volt Operation
Although this unit is designed with very low internal resistance (less than 25mΩ) to enable
operation down to low voltages at high currents, there are occasions when a load is needed
capable of conducting the full current down to zero voltage. This can be achieved by connecting
an additional voltage supply in series with the source under test to boost the voltage at the input
to the load. It is strongly recommended that a diode should also be included (in series) in the
circuit, to ensure that reverse current cannot flow.
The additional supply must be capable of providing the full load current and must have a dynamic
performance that does not impair the stability of the combination. If possible, the use of batteries
is recommended, but note that the direction of the current discharges the battery, so care must
be taken to avoid full discharge.
External voltage sensing must be used, with the sense leads connected to the actual source
under test. The additional supply must provide a voltage of at least the minimum operating
voltage of the load (at the current concerned), plus any voltage drop in the inter-connections, but
the maximum voltage permitted by the external sense circuit is 6 Volts.
Note that if the circuit does not include the series diode recommended, then it is possible for the
combination of the additional supply and the load (in a low impedance condition) to apply a
reverse voltage to the source under test. It is strongly recommended that a switch, capable of
disconnecting the full load current, should be included in the circuit.
The load has a diode across the input terminals that will conduct current if reverse
polarity is applied to the load, even if the load input is disabled.
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