How safe is your power supply?

How safe is your power supply?
How safe is your
power supply?
Introduction
Overvoltage (OVP) is an
important security feature
in power supplies. By
using this feature, the
device under test (DUT)
can be protected from a
higher voltage than the
DUT was designed to
handle. It also lets users
address a variety of
application in electronics
manufacturing and
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 since it
allows the user to select the
maximum safe current or
voltage for the particular
load device.
Overvoltage Crowbar
The new Agilent U8000
series power supply is
short-circuit proof and can
operate under any current
overload condition
indefinitely without risks of
internal damage. It uses an
overvoltage crowbar in
which the crowbar circuit is
connected across the output
terminals, and provides
protection against any
output overvoltage
condition which might
occur because of operator
error or failure within the
power supply or load.
This application note is
going to show you the
architecture of OVP and its
applications and what you
need to pay attention when
using this feature.
An operator error or a
component failure within
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 insures 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
present-day 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
short enough to avoid
damage to the load.
The sense circuit can be a
simple bridge or voltage
divider network that
compares the output
voltage to some internal
crowbar reference voltage.
The best trigger circuit is
the one that turns the SCR
on the fastest. Fastest SCR
turn-on is accomplished by
a fast rise-time pulse circuit
such as a blocking
oscillator or Schmitt trigger.
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 activate.
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.
• An auxiliary winding is
included on the blocking
oscillator transformer for
connection to an
There are several beneficial
features for crowbar
circuits:
• An overvoltage indicator
appear on front panel
screen when the SCR
fires; the indicator
conducts a holding
current to prevent the
SCR from oscillating on
and off.
additional crowbar.
Tandem crowbar operation
is then available for
coincident firing of all
crowbars in a system.
Attention when using
OVP
-Connecting power supply
in series
One OVP application is
connecting power supplies
in series for higher voltage
• The crowbar circuit
output. Set the OVP
creates an extra current
shutdown voltage in each
path during normal
unit so that its shuts down
operation of the supply,
thus changing the current at a voltage higher than its
output voltage during autothat flows through the
tracking operation. When a
current monitoring
master unit shuts down, it
resistor. A diode keeps
programs any slave units to
this extra current at a
zero output. When a slave
fixed level of which
compensation can then be unit shuts down, it shuts
down only itself, and the
made in the constant
current comparator circuit. master continues to supply
output voltage.
• 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.
-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 change 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 insure that
current only flows out of
the power supply.
crowbar the output,
protecting output loads
from the hazardous ESD
current.
- False shut downs
Conclusion
The voltage at the output
terminal can be increased
inadvertently a variety of
ways: 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. Set the
OVP shutdown voltage 4%
higher or at least +2.0V
above the desired output
voltage to avoid false
shutdowns from loadinduced transients.
Overvoltage is an important
feature for power supplies.
It does not only protect you
and your device under test,
it also protects your
investment by preventing
down time of your test
system due to testing fault.
A common cause of false
OVP shutdowns is an OVP
limit set too close to the
output voltage when you
are using remote sense.
Remote sense will increase
the voltage at the output
terminals to compensate for
a voltage drop in the wire
between the power supply
and the unit under test.
Long wire runs and small
gauge wires can create a
significant voltage drop.
Another possible cause of
cause OVP false shutdowns
is strong electrostatic
discharge to the power
supply, which can trip the
OVP limit and eventually
Agilent U8000 series power
supply come with a
thoughtful design that
provide you with value
pack features at affordable
price.
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