Tuesday, March 12, 2013

What is a power supply’s over current protect (OCP) and how does it work?

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

When powering DUTs, either on the bench or in a production test system, it is always imperative that adequate safeguards are taken to protect both the DUT as well as the test equipment from inadvertent damage. Over current protect or OCP is yet another of many features incorporated in system DC power supplies you can take advantage of to protect overload-sensitive DUTs from damage during test!

1 comment:

  1. Hi Ed, I found this in an Agilent Power Supply user Manual:

    "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?