When electronic load is used for the applications of high power/low voltage/fast load change, the following consideration is necessary to avoid the oscillation of Power Supply-Electronic Load system.
(1) The Reasons of oscillation:
As there is line inductance from the output terminals of power supply to the input terminals of electronic load, this inductance will get the energy 1/2Leqi2 when the current flows from power supply to electronic load. When the current changes from one value i1 to another value i2, the 1/2Leq(i22-i12) energy will need to be increased (i2 is larger than i1) or reduced (i2 is smaller than i1) in the line inductance. If the increased energy from power supply is larger than the needed energy of the line inductance, then the surplus energy needs to become loss by the loop resistances including Rs, and Req if the current go to steady state and become direct current (DC). When the surplus energy does not become loss efficiently, it will become the circular energy in the circuit loop of power supply-electronic load and produce the current oscillation of the power supply-electronic load system. When the amount of line inductance is large, generally long connection line, or large current change in the short time, which causes more surplus energy in the line inductance, this oscillation becomes more serious. In applications involving high power and fast change load, the system will be needed to deal with larger surplus energy which will cause the system to oscillate more easily if the loop resistances are not large enough. And in the application of low voltage, the compensated capacitor (Cc shown in the figure1) can support or store smaller compensative energy comparing to high voltage application if the value of the compensated capacitor is the same in both cases. When the compensative energy is smaller than the needed energy for the line inductance, the oscillation will be produced.
(2) The methods to protect the system from the oscillation:
Basically the best method to protect the system from the oscillation is reducing the amount of the line inductance such as using short connection line between power supply and electronic load. And also larger line resistance will help to reduce the possibilities of the oscillation because more loss caused by the line resistance will damp the oscillation. In another way, to keep the system stable, a compensated capacitor paralleled with the Mosfets of electronic load is used in our design of electronic load, by which the needed energy (increased or reduced) for the line inductance can be supported from or temporarily stored in the compensated capacitor when the current command change from one value to another value. By the compensated capacitor the system can more easily go to the steady state. However, if the surplus energy in the line inductance and temporary stored in the compensated capacitor is too large to become loss efficiently by the loop resistances, the oscillation also will be produces.
