86 Application Programs
Application 5. Providing Time-Varying Current Limiting
Overview Of Application
To provide current limit protection which varies as a function of time, multiple thresholds on current limit are required.
Having multiple thresholds can provide a high limit to protect the DUT during its power-up in-rush with automatic
switchover to a lower limit to protect the DUT during its steady state operation.
For this example, the DUT is a printed circuit assembly. This assembly is being tested prior to installation in the end
product. The module provides power to the assembly, which will undergo a functional test. The assembly has capacitors
on-board, and when power is applied, the in-rush current approaches 4 A. After the capacitors charge, which takes about
500 milliseconds, the steady state current settles to 600 mA. See Figure B5-1.
The MPS can address this application using dwell-paced Lists. In this case, the List will consist of a set of current limits
and dwell times, because the voltage will remain constant throughout the test.
Once power has been applied, the first current limit, which provides protection to a shorted DUT while still allowing high
current in-rush to occur, will remain in effect for the dwell time. Then the current limit will switch to its next setting in the
List. The result is a current limit which changes with time and provides protection as the DUT current requirements drop
off to their steady state value. When the dwell time expires for the last current limit in the List, the current limit stays at this
value until reprogrammed. Thus, the actual value of the last dwell time is not important. The last current List point would
be the current limit for the steady state operation during the test of the DUT. See Figure B5-2 for how the MPS implements
this protection.
Throughout List execution, overcurrent protection will be enabled. If at any time the module goes into CC, the output will
be disabled, the test stopped, and the DUT protected.
MPS Features Used
• 20-point current List.
• Dwell time.
• Dwell-paced Lists.
• Disable the output on a change in internal status.
�� Stop the List on a change in internal status.
• Change the voltage on trigger.
• Trigger on a GPIB trigger command.
• Overcurrent protection.
• Active downprogramming.
Advantages/Benefits Of The MPS Solution
By using Lists, the module changes its current limit automatically, so that the computer is not devoted to reprogramming the
current limit.
The output can change faster when dwell paced than when the computer must explicitly reprogram each change.
Overcurrent protection can disable the output before the DUT is damaged.
By letting the modules monitor status, the CC condition will be responded to faster than if the computer was responsible for
stopping the test.
The sequence is simpler to program (no timing loops).
By using dwell times, the timing of each point is accurate and repeatable because the computer does not provide timing for
the sequence.
When the output is disabled, the active downprogrammers in the module output can quickly discharge the module’s output
capacitors and any capacitance in the DUT.