One feature we include in our Agilent system DC power
supplies for providing additional safeguard for overload-sensitive DUTs is over
current protect, or OCP. While some may think this is something separate and
independent of current limiting, OCP actually works in concert with current
limiting.
Current limiting protects overload-sensitive DUTs by
limiting the maximum current that can be drawn by the DUT to a safe level.
There are actually a variety of current limit schemes, depending on the level
of protection required to safeguard the DUT during overload. Often the current
limit is relatively constant, but sometimes it is not, depending on what is
best suited for the particular DUT. Additional insights on current limits are
provided in an earlier posting, entitled “Types of current limits for
over-current protection on DC power supplies“.
By limiting the current to a set level may DUTs are
adequately protect from too much current and potential damage. When in current
limit, if the overload goes away the power supply automatically goes back to
constant voltage (CV) operation. However, current limit may not be quite enough
for some DUTs that are very sensitive to overloads. This is where OCP works together
with the current limit to provide an additional level of protection. With OCP
turned on, when the DC power supply enters into current limit OCP takes over
after a specified time delay and shuts down the output of the DC power supply. The
delay time is programmable. This prevents OCP from shutting down the DC power
supply from short current spikes and other acceptably short overloads that are
not considered harmful. Like over voltage protect or OVP, after tripping the
output needs to be disabled and an Output Protect Clear needs to be exercised
in order to reset the power supply so that its output can be re-enabled. Unlike OVP, OCP can be turned on and off and
its default is usually off. In comparison, OVP is usually always enabled and
cannot be turned off. A typical OCP event is illustrated in Figure 1.
Figure 1: OCP operation
Hi Ed, I found this in an Agilent Power Supply user Manual:
ReplyDelete"Note that the time it takes the output to go into CC mode varies - depending on the magnitude of the over-current condition compared to the current limit setting. For example, if the over-current is only slightly greater than the current limit setting, it may take several tens of milliseconds, depending on the power module type, for the output to set the CC status bit. If the over-current is significantly greater than the current limit setting, it may only take a few milliseconds or less, depending on power module type, for the output to set the CC status bit. To determine when the output will shut down, you must add the time it takes for the CC status bit to the over-current protection delay time. If the over-current persists beyond the sum of these two time intervals, the output will shut down."
My questions are:
- Why does this happen?
- Is there anyway to reduce this time? What do you recommend doing for stringent in-rush requirements for monitoring a capacitor bank charging?