Keysight Technologies
Overvoltage Protection in Power Supplies
Application Note
Introduction
Overvoltage (OVP) is an important
security feature in power supplies.
By using this feature, we can protect
the device under test (DUT) from a
higher voltage than the DUT was
designed to handle. OVP is useful in
a variety of applications in electronics
manufacturing and the educational
market.
Overvoltage protection is also available
if required during constant current
operation. The constant voltage/
constant current (CV/CC) and
constant voltage/current limit (CV/
CL) automatic crossover circuitry is
ideal for these purposes because it
allows you to select the maximum safe
current or voltage for your particular
load device.
Overvoltage Crowbar
An operator error or a component failure in the regulating feedback loop can drive
a power supply’s output voltage to many times its preset value. The function of the
crowbar circuit is to protect the load against this possibility. The circuit ensures
that the power supply voltage across the load will never exceed a preset limit.
This protection is valuable because of the extreme voltage sensitivity of today’s
semiconductor devices.
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
that will reliably trigger the SCR within a time period that is brief enough to avoid
damage to the load.
The sensing circuit can be a simple bridge or voltage divider network that compares
the output voltage to the internal crowbar reference voltage. The best trigger circuit
is the one that turns on the SCR most quickly. The fastest SCR turn-on is
accomplished by a fast rise-time pulse circuit such as a blocking oscillator or
Schmitt trigger.
Keysight Technologies, Inc. E36001 and
U8000 series power supplies are shortcircuit 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
beneficial features:
–– An overvoltage indicator appears
when the SCR fires; the indicator conducts a holding current to
prevent the SCR from oscillating
on and off.
–– The crowbar circuit creates an
extra current path during normal
operation of the supply, thus
changing the current that flows
through the current monitoring
resistor. The diode keeps this extra current at a fixed level where
compensation can then be made
in the constant-current comparator circuit.
–– In pre-regulate supplies, the crowbar turns off the pre-regulator
circuit when the SCR fires, reducing the voltage drop across the
series regulator and the current
flow through the SCR.
–– An auxiliary winding is included
on the blocking oscillator transformer for connection to an additional crowbar. Tandem crowbar
operation is then available for
coincident firing of all crowbars in
a system.
OVP application
Connect power supply in series
One of the OVP application is to connect power supplies in series for higher
voltage output. Set the OVP shutdown voltage in each unit so that it shuts down
at a voltage higher than its output voltage during auto-tracking operation. When a
master unit shuts down, it programs any slave units to zero output. When a slave
unit shuts down, it shuts down only itself, and the master continues to supply
output voltage.
Battery charging
Some power supplies contain an overvoltage protection circuit, which will short
the power supply’s output if an overvoltage condition occurs. The OVP circuit
is typically a crowbar SCR that operates independently of the power supply’s
regulation circuit. When you use a power supply to charge a battery, it is possible
to charge the battery to a voltage greater than the OVP limit. Once the OVP limit
is exceeded, the SCR will short the output and continuously sink a large current
from the source, possibly damaging the power supply. To avoid damaging the power
supply, connect a diode in series with the power supply to ensure that current only
flows out of the power supply.
False shutdowns
The voltage at the output terminal can be increased inadvertently in a variety of
ways such as from the front panel or by a change in the load. False OVP shutdowns
can occur if the OVP shutdown voltage has been set too close to the power
supply’s operating voltage. You should set the OVP shutdown voltage 4% higher
or at least +2.0 V above the desired output voltage to avoid false shutdowns from
load-induced transients.
False OVP shutdowns occur commonly when the OVP limit is set too close to
the output voltage when using remote sensing. Remote sensing will increase the
voltage at the output terminals to compensate for the voltage drop in the wire
between the power supply and the DUT. A longer cable can create significant
voltage drops due to low resistance. Another possible cause of OVP false
shutdowns is strong electrostatic discharge (ESD) applied to the power supply,
which can trip the OVP limit and eventually crowbar the output, protecting output
loads from the hazardous ESD current.
Conclusion
Overvoltage protection is an important feature for power supplies. It can protect
your DUT from a voltage higher than it was designed to handle.
By knowing the architecture of a power supply’s overvoltage crowbar circuit and its
applications, you can easily apply and fully utilize this feature for many applications.
3
4 | Keysight | Overvoltage Protection in Power Supplies - Application Note
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© Keysight Technologies, 2013 - 2014
Published in USA, August 3, 2014
5989-9395EN
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