Instructions

9. Application Notes

This chapter is intended to give helpful information concerning practical applications of the unit.

All electronic loads are subject to the impact of source characteristics, interconnection inductance

and feedback loop characteristics, which can give rise to unexpected instability or poor dynamic

behaviour. The information given here will assist in understanding the factors involved. The initial

sections below cover general considerations, while later sections provide greater detail on the

particular characteristics of each operating mode.

9.1 Sources

Batteries are a low impedance source and, apart from the possibility of inductance in the

interconnecting leads, they are generally easy to use in conjunction with an electronic load. The

dropout facility should be used to protect batteries from being damaged by excessive discharge.

Electronic supplies have active feedback networks whose dynamic characteristics often interact

with the load. When that load (like this instrument) also includes an active feedback controlled

network, whose dynamic characteristics in turn depend on the nature of the source, it will be

apparent that the behaviour of the resulting system can be impossible to predict.

9.1.1 Source resistance

If a source has significant resistance (including the resistance of the connecting leads), so that

the voltage falls as the current rises, it is important to ensure that the voltage across the load

terminals remains at all times above the minimum permissible operating voltage.

The particular considerations concerning source resistance which apply in Constant Power mode

are discussed in the section covering that mode below.

9.1.2 Source inductance

Source and interconnection inductance has a major impact on the behaviour of the load: the

fundamental characteristic of an inductance is that it generates an emf to oppose any change in

current. As the current rises, the emf generated by the inductance reduces the voltage across the

load terminals, perhaps to the point where the load saturates. Whenever the voltage falls below

about 25V the transconductance of the power stage changes considerably; this changes the

damping factor of the feedback loop and alters its dynamic behaviour, possibly giving rise to

overshoots or even oscillation.

Whenever the load current falls, the inductor will generate a voltage transient which might exceed

the voltage rating of the load. The unit is fitted with gas discharge tubes designed to absorb non-

repetitive transients. Even if the overvoltage detector disables the load input, these GDTs remain

connected, so if large energy levels are likely then some form of external protection must be

added, such as a catching diode across the inductor in the source.

9.1.3 Shunt capacitance

The load can only sink current, so it can only pull the voltage at its terminals down. The source

must pull the voltage up, including providing charging current to any capacitance across the

terminals. If the total current available is more than sufficient to charge this capacitance at the

slew rate required, then the load will continue to conduct the excess current during the transition

and the behaviour will be as expected. However, if the source cannot charge the capacitor at the

required slew rate, then the load will cut-off until the final voltage is reached. There will then be

an overshoot as it starts to conduct, followed by a ringing as the source responds.