Specifications

Current Overshoots Above the Current Limit Setting
Question: Why does the power supply allow transient current overshoots above the current limit setting?
Answer: Agilent power supplies can operate in either constant voltage (CV) or constant current (CC) over the rated
output voltage and current. However, most are designed as constant voltage sources. This means that these units turn
on in constant voltage mode. There is no command for constant current operation. Whether you set the current (Iset)
to 0 or any other value, the dc source operates in the CV mode first, and will generate maximum voltage set by
voltage (Vset) until the operating mode of the unit is determined by the voltage setting, the current setting, and the
load resistance. Thereafter, the power supply will automatically transition back and forth between CV and CC mode
as necessary based on changes in the load resistance, the voltage setting, or the current setting.
Furthermore, the transition from CV to CC is not instantaneous. The power supply’s internal control loop has a finite
slew rate characteristic, which introduces a time delay in mode crossover. Switching power supplies respond more
slowly (typically 100’s of milli-seconds) than linear supplies (typically 100’s of micro-seconds). During this
transition period, the current can exceed the current limit by a significant amount.
This should not be surprising. The output capacitor, present on every power supply, is outside the control loop, and
is capable of supplying very large currents into the load as a result of resistance changes. This type of situation is
possible with most power supplies to a greater or lesser extent. To prevent this characteristic from becoming a
problem, there are a few precautions you should observe.
First choose the smallest power supply that will reasonably do the job. If possible, use a linear rather than a switcher.
Choosing a larger supply than necessary may provide flexibility, but it also increase the potential for problems.
Next, don’t “hot switch” the device under test (DUT) onto a power supply output that is already at working voltage,
especially if the DUT represents a capacitive load. Instead, first program the power supply to 0.0amps, then connect
the DUT, then re-program the supply to the desired current limit.
Lastly, don’t turn the power supply output on with a sensitive load connected. The power supply will initially turn on
in the CV mode. The only way to turn the unit on in CC mode is by placing a short across the output and then
enabling or turning the output on, and finally removing the short.