The two main features in system power supplies that have
traditionally protected DUTs from too much current are the current limit and the
over current protect (OCP). When a device, for any of a number of reasons,
attempts to draw too much current, the current limit takes control of the power
supply’s output, limiting the level of current to a safe level. An example of
current limit taking control of a power supply output is shown in Figure 1.
Figure 1: Current limit protecting a DUT against excess
current.
For those devices that cannot tolerate a sustained
current at the current limit level, the over current protect can be set and
activated to work with the current limit and shut down the power supply output
after a specified delay time. This will protect a DUT against sustained current
at the limit. An example of an OCP
shutting down a power supply output for greater protection against excess
current is shown in Figure 2.
Figure 2: OCP protecting a DUT against excess current
We have talked about the current limit and OCP in
previous posts. For more details on how the OCP works, it is worth reviewing “What
is a power supply’s over current protect (OCP) and how does it work?” (Click here to review)
Sometimes it is desirable to have something that is in between
the two extremes of current limit and OCP. One middle-ground is a fold-back current
limit, which cuts back on the current as the overload increases. More details
about a fold-back current limit are described in a previous posting “Types of current limits for
over-current protection on DC power supplies” (Click here to review). One thing
about a fold-back current limit is the DUT and power supply will not be able to
recover back into constant voltage (CV) operation unless the DUT is able to cut
way back on its current demand.
Another
type of current limit behavior that operates between regular current limit and
OCP is one that shuts down the output, like OCP, but only temporarily. After a
set period of time it will power up the output of the power supply again. If
the DUT is still in overload, the power supply will shut down again. However,
if the DUT’s overload condition has gone away, it will be able to restart under
full power. In this way the DUT is protected against continuous current and at
the same time it the power supply is not shut down and requiring intervention
from an operator.
While
this type of current limit is not normally a feature of a system DC power
supply, it is possible to implement this functionality in the N6900A/N7900A
Advanced Power System (APS) using its expression signal routing feature. This
is a programmable logic system that is used to configure custom controls and
triggers that run within the APS. Here the expression signal routing was used
to create an auto-restarting current shutdown protect in the example shown in
Figure 3.
Figure 3: Custom auto-restarting current shutdown protect
configured for N6900A/N7900A APS
A custom control was created in the expression signal
routing that triggers the output transient system to run if the current limit
is exceeded for longer than 0.3 seconds. A list transient was programmed into
the APS unit to have its output go to zero volts for 10 seconds and then return
to the original voltage setting each time it is triggered. In this way the
output would pulse back on for 0.3 seconds and then shut back down for another
10 seconds if the overload was not cleared. The custom trigger signal was
graphically created and downloaded into the APS unit using the N7906A software
utility, as shown in Figure 4.
Figure 4: Creating custom trigger for auto-restarting
current shutdown protect on APS
