Brochure

GMOV™ Hybrid Overvoltage Protection Component Introduction
Metal Oxide Varistors (MOVs) are popular bipolar ceramic
components typically used for overvoltage protection
on AC and DC power lines. MOVs operate as non-linear
resistors exhibiting a high resistance in their off-state.
An MOV operates when the voltage applied to the MOV
exceeds its maximum operating voltage (MCOV). As the
voltage increases across the MOV, its resistance decreases
quickly. In its fully on-state, the MOV clamps the voltage
transient and then returns to its highly resistive off-state
after the voltage triggering event has subsided. The term
"varistor" is a generic term for a voltage-variable resistor
and is often used to describe an MOV.
MOVs are manufactured using a variety of proprietary zinc
oxide formulations due to their non-linear characteristics
as well as their ability to support a wide array of voltages
and currents. MOVs have been a widely accepted and
popular overvoltage protector for many years. However,
with the growth of electronic devices used in harsh and
uncontrolled environments, MOVs are being exposed more
often to transient events that can cause degradation and
premature catastrophic failure of the component.
Degradation and catastrophic failure can occur if an MOV
is subjected to transient surges or temporary overvoltage
(TOV) conditions exceeding its rated maximum values. The
life of an MOV is defined as the time required to reach a
thermal runaway condition, which is a catastrophic failure
mode resulting in either a short circuit, open circuit or
some residual linear resistance. In some cases, the MOV
can become a fire hazard requiring the need for additional
fusing or thermal cutoff components.
Events such as lightning surges or switching transients
exceeding an MOV’s current ratings can begin to decrease
its varistor voltage, resulting in increased standby/off-state
leakage and watt loss heating. Additionally, increased
system voltage swells (TOVs) applied for durations longer
than microseconds can lead to physical and chemical
changes within the multiple grain boundaries of the MOV.
With decreased varistor voltage, increased leakage and
compromised ability to dissipate heat caused by these
events, the MOV is more likely to enter into a thermal
runaway condition, thus ending the MOV's operational life.
Bourns designed its innovative GMOV™ hybrid overvoltage
surge protection components as a way to provide enhanced
performance and reliability compared to standard MOV
components. Engineers combined Bourns® innovative
and space-saving Gas Discharge Tube (GDT) with FLAT®
technology with an MOV to create a compact and robust
hybrid component that is size and footprint compatible with
standard 14 mm and 20 mm MOVs.
In this series configuration, the MOV and GDT components
are capacitively coupled. Under low frequency conditions,
the voltage limiting of the GMOV™ component would be
equal to the sum of the voltage limiting of the individual
MOV and GDT components.
Under a high-rate voltage ramp condition, the situation is
different. When exposed to a high-rate voltage ramp similar
to what is experienced during a lightning or switching
transient event, most of the voltage ramp appears across
the GDT first, due to its relatively low capacitance compared
to the MOV. When the GDT sparks over, it then charges the
MOV capacitor and it begins to clamp and conduct. When
in full conduction, the combined voltage limiting of the pair
will be approximately the MOV clamping voltage. During
full conduction, the MOV resistance is still sufficient enough
to limit the follow-on current to the GDT allowing for turn-
off and reset of the GMOV™ component.
INTRODUCTION
DEGRADATION AND FAILURE of MOVs
HOW IT WORKS
BOURNS® GMOV™ COMPONENT DESIGN
A GMOV™ component functions when the GDT isolates the MOV
from the circuit until such time as the voltage exceeds the turn-on
voltage of the GDT. This isolation prevents the MOV from being
damaged and prematurely failing from temporary overvoltage
conditions below the turn-on voltage of the GDT.
CIRCUIT DIAGRAMCONSTRUCTION
U
Zinc Oxide Disc
GDT Disc
Lead Material:
Tin-plated Copper
Coating: Epoxy Resin