8 - Manual setup
Load Modes
47
SMU4000 Series Instruction Manual
Load
Power
(L
P
)
CNFG >
[Source Measure Action] Manual Setup > [Overall] Mode > LP Mode
When Load Power mode is selected, the SMU simulates a load where the power consumption is
independent of the applied voltage. This is true of many types of equipment that incorporate
switch-mode regulators. This mode may be particularly suitable for testing power sources of
portable devices such as Lithium-ion batteries.
Load Power mode is implemented by the very fast digital feedback loop, used to divide the
specified power setting by the actual sensed voltage to calculate the necessary current. If the
external source voltage falls, then the load will seek to keep the same power level by reducing
its resistance to raise the current. The fact that the current rises as the voltage falls means that
the load is acting as a negative resistance. This behaviour is also exhibited by most switch-mode
power supply circuits.
This characteristic raises the possibility of a latch-up condition if the external source has a
significant output impedance. To explain this, consider the possibility that the external source
voltage falls slightly (perhaps because of noise) – the load responds by increasing the current to
maintain the power level. This causes a further reduction in the terminal voltage of the external
source (because of its internal impedance), so the increase in power is less than expected. The
load responds to this by reducing its resistance even more, in an attempt to increase the current
and obtain the required power. A cross-over point is reached when the fall in voltage outweighs
the increase in current and the load cannot draw the required power. This leads to the latch-up
condition, with the load at its minimum resistance (nearly a short-circuit), the voltage across it
almost zero, and the source is delivering its maximum current into the almost short-circuit load.
If the source impedance is purely resistive then this condition will be triggered when the source
terminal voltage falls to half its open circuit voltage (this is the maximum power transfer
condition of classical electrical theory). More commonly, it will also be triggered immediately if
the external source reaches a current limit or enters constant current operation.
The only way to recover from this situation is to disable either the load input or the external
source output.
To avoid immediately entering the latch-up condition, it may be necessary to reduce the slew
rate setting to constrain the power demand while the source builds up its output voltage.
As Load Power mode has the characteristics of a negative resistance, the possibility always exists
of forming a negative resistance oscillator in combination with the output impedance of the
external source. In practice, constant power mode normally operates well in conjunction with
sources designed to supply such a load.