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Keysight Technologies
Overvoltage Protection in Power Supplies



                             Application Note
Introduction                                 Overvoltage Crowbar
Overvoltage (OVP) is an important
security feature in power supplies.          An operator error or a component failure in the regulating feedback loop can drive
By using this feature, we can protect        a power supply's output voltage to many times its preset value. The function of the
the device under test (DUT) from a           crowbar circuit is to protect the load against this possibility. The circuit ensures
higher voltage than the DUT was              that the power supply voltage across the load will never exceed a preset limit.
designed to handle. OVP is useful in         This protection is valuable because of the extreme voltage sensitivity of today's
a variety of applications in electronics     semiconductor devices.
manufacturing and the educational
market.                                      The basic elements used in most crowbars are some method of sensing the output
                                             voltage, a silicon controlled rectifier (SCR) that will short the output, and a circuit
Overvoltage protection is also available     that will reliably trigger the SCR within a time period that is brief enough to avoid
if required during constant current          damage to the load.
operation. The constant voltage/
constant current (CV/CC) and                 The sensing circuit can be a simple bridge or voltage divider network that compares
constant voltage/current limit (CV/          the output voltage to the internal crowbar reference voltage. The best trigger circuit
CL) automatic crossover circuitry is         is the one that turns on the SCR most quickly. The fastest SCR turn-on is
ideal for these purposes because it          accomplished by a fast rise-time pulse circuit such as a blocking oscillator or
allows you to select the maximum safe        Schmitt trigger.
current or voltage for your particular
load device.

Keysight Technologies, Inc. E36001 and
U8000 series power supplies are short-
circuit proof and can operate under any
current overload condition indefinitely,
without the risk of internal damage.
It uses an overvoltage crowbar in
which the crowbar circuit is connected
across the output terminals. This
provides protection against any output
overvoltage condition that might occur
because of operator error or failure
within the power supply or load.

This application note explains the
architecture of OVP and describes its
applications. It discusses what you
need to pay attention to when you use
this feature.
                                             Figure 1. General crowbar overvoltage protection circuit


                                             Figure 1 shows the general crowbar overvoltage protection circuit. The circuit
                                             compares the output voltage with a reference voltage +V. The overvoltage
                                             potentiometer adjusts the reference voltage on the comparison amplifier and sets
                                             the voltage level at which the crowbar will be activated.

                                             Normally, the overvoltage control is located on the front panel and can be adjusted
                                             from approximately 20% to 120% of the maximum rated output voltage of the power
                                             supply. When the output voltage exceeds the reference, the comparison amplifier
                                             triggers the blocking oscillator, which then sends firing pulses to the SCR. When
                                             the SCR fires, it places a very low impedance across the output, reducing the
                                             voltage to near-zero.
1. Refer to literature 5989-4104EN for the
   list of power supplies that have OVP


                                                                    2
Crowbar circuits have several             OVP application
beneficial features:

 

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