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millisecond or (at very slow rates) Watts per second. The shape of the current transition is
therefore not necessarily linear in any mode other than Constant Current (CC).
In addition to the usual numeric prompt this menu also shows the calculated theoretical transition
time considering the present Slew Rate setting and the difference between Level A and Level B.
This theoretical value takes no account of any dependency of the actual transition time on the
source and load characteristics, impedances and interconnection inductance which may occur,
particularly at fast slew rates. The user must ensure that the transition time which results from the
values entered is not shorter than the minimum attainable transition time in the present mode,
which is documented in the Specification. It is emphasised that severe overshoots can occur if
the slew rate is set faster than the combination of source and load is capable of supporting
(which may be slower than the value in the Specification, which applies to optimal conditions),
see the section on ‘Fast Slew Rate Limitations’ below.
Pressing the Range soft-key produces a display of the limits of available slew rate for the present
operating mode and range. If a slew rate value is entered that falls outside the parameter limit
range, an error message is displayed, followed also by a display of these range limits.
The bandwidth of the power stages of the load is reduced (by changing the compensation
networks) when the slew rate is set to less than 0·1% of the maximum slew rate for the given
load mode and range. For example, on the 80A range in constant current mode, the maximum
slew rate setting is 2.5A/us, so the bandwidth is reduced when the slew rate is set < 2.5A/ms.
This change is made even if the transient facilities are not being used, and alters the dynamic
behaviour of the unit. This may improve stability with some difficult combinations of source and
load characteristics.
Upon completion of the slew rate setting update, press the Back soft-key to return to the
transient menu or the
Home key to return to the home screen.
Slow Slew Rate Limitations
There is a lower limit to the slew rate value that can be used, which is determined by the
combination of slew rate, frequency, duty cycle, and the difference between the two levels. If the
requested transition time (which is the difference between the level settings divided by the slew
rate) is longer than the available time (which is the oscillator period multiplied by the smaller duty
factor), then the transition will not complete before the oscillator initiates a return to the other
level. As a result, the intended level setting will never be reached. In this case, Slew is displayed
in the status line as a warning.
Fast Slew Rate Limitations
In practice there are a number of limitations on the fastest slew rate actually attainable. One is
the minimum transition time of the power stages of the load (which depends on the operating
mode – see the Specification). If a combination of a fast slew rate and a small change in level
imply a transition time shorter than this, then the settling time of the power stage will dominate.
The dynamic behaviour of a source and load combination at high slew rates depends on many
factors, particularly interconnection inductance and the damping factor of feedback loops. In
addition, the response of the power stages of the load is slower when operated at very low or
high currents, or at low voltages. In many circumstances, a lower slew rate setting will be needed
to avoid aberrations.
If an attempt is made to set a slew rate faster than is possible in the circumstances, then
significant overshoots and extended settling times can arise. When configuring fast transitions
approaching the limits of the capabilities of the unit, it is strongly recommended that the current
monitor output be viewed on an oscilloscope to verify the results actually obtained. Particular
care should be taken to avoid a fast transition down to a low current, as this may result in the
power stage entering the cut-off state, which incurs a long recovery time and possibly multiple
current pulses.
In CR (constant resistance) mode, where the current is inversely related to the resistance
transition, it is particularly difficult to predict the maximum useful slew rate setting.
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