Instructions

9.2 Stability of Source and Load Combinations

This instrument is optimised for accuracy under constant load conditions by using a high gain

feedback loop. Because of this, the possibility exists for combinations of source, interconnection

and load characteristics to give rise to instability. There are three major potential causes:

inductance in the wiring between source and load (or an inductive output impedance of the

source), capacitance in parallel with the connection between source and load (including an output

capacitor within the source) and the characteristics of active feedback circuits within the source.

In Constant Power, Conductance and Resistance modes, the system includes an analogue

multiplier used by the load to derive the current requirement from the instantaneous voltage. This

reduces the bandwidth of the loop and adds additional phase shift. In general, Constant Current

mode is the most likely to be stable, but in some cases instability can be avoided by using a

different mode. The conditions that affect the dynamic behaviour of the load in transient operation

also lead to instability, and some of the suggestions in the sections below may be found helpful.

Many supplies have L-C output filters to reduce noise; these introduce extra phase shift into the

overall source and load combination and can increase the possibility of instability. If there is no

damping across the inductor, a resonant circuit can be formed which allows oscillations to build

up to significant amplitude.

9.2.1 Remedial Actions

The compensation networks of the power stages in the load are changed when the Slew Rate is

set to less than 0.001 times the maximum slew rate for the given load mode and range. For

example if constant current mode is selected, the maximum slew rate setting is 500A/ms, hence

the compensation networks are changed at slew rates settings below 500A/s. Even if the

transient facilities are not being used, this change in compensation reduces the bandwidth and

may make the source and load combination stable.

If instability arises, observe the voltage waveform across the load with a scope: if at any point the

voltage rises above the open circuit emf of the source, then there must be an inductive element

present to form a resonant circuit. Some means must be found to insert damping into this circuit.

One technique is to use a network consisting of a capacitor and a resistor in series (sometimes

called a Zobel network), across the input terminals of the load. Many electronic loads have such

a network built-in; it is omitted from this load to maximise its versatility by offering the lowest

possible input capacitance. It can be added externally: values around 2·2µF and 5Ω are

common, but note that this must be a non-inductive power resistor capable of dissipating a few

watts. A flat film type is best – wire wound resistors are not suitable.

9.3 Dynamic Behaviour in Transient Operation

When the transient capabilities of the load are used, the dynamic behaviour of the source and

load combination during the transitions depends on similar considerations to those affecting

stability: series inductance, shunt capacitance and feedback loop characteristics. Proper

operation depends on the load neither saturating nor cutting off at any point in the cycle. The

faster the slew rate sought, the more likely it is that aberrations will occur on the transitions.

Because of changes in the transconductance of the FETs, the dynamic behaviour of the power

stages changes at both low and high currents, and also at low voltages when the inter-electrode

capacitance increases considerably. In general, behaviour is optimum at the lower end of the

current range (100mA to 4A) and at voltages > 25V.

Attempting to achieve a slew rate beyond the capabilities of a source and load combination can

result in substantial overshoot and ringing. Reducing the slew rate, sometimes by just a small

amount, will often improve the response considerably.