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1 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved.

General Electrical MOV Training

Training Agenda

1. MOV Definition and Circuit Protection

2. MOV Characteristics and Device Physics

3. MOV General Electrical Power Application Example

4. MOV Product Selection

5. Littelfuse MOV Product Road Map

6. MOV Technology Challenges



Section 1 MOV Definition and Circuit Protection MOV Definition

– A MOV is a voltage suppression device that filters and clamps the transient in the electrical circuit

Circuit Protection Concepts

– Threats

• Repetitive Transients

• Random Transients

• General Electrical Line Transients

– Effects of Voltage Transients

• Effects on Semiconductors

• Effects on Electro-mechanical Contacts

• Effects on Insulation

– Transient Testing and Standards

• Regulatory Requirements

• UL Regulation

– Transient Suppression Method

• Clamping

• Crowbar

• Filters

• Transient Suppression Compared


MOV Definition and General Electrical Circuit Protection

MOV Definition

– A Metal Oxide Varistor (MOV) is a voltage suppression device that filters and clamps the transient in an

electrical circuit. A Varistor is a Variable Resistor, sometimes referred to as Voltage Dependant Resistors

(VDRs) by some manufacturers.

– An MOV is a voltage dependent, nonlinear device which has an electrical behavior similar to back to back

zener diodes.

– When exposed to high voltage transients, the MOV’s impedance changes many orders of magnitude from

a near open circuit to a highly conductive level, thus clamping the transient voltage to a safe level.

– The potentially destructive energy of the incoming transient pulse is absorbed by the varistor, thereby

protecting vulnerable circuit components.

– An MOV is composed primarily of zinc oxide with small additions of bismuth, cobalt, manganese and

other metal oxides. The structure of the body consists of a matrix of conductive zinc oxide grains

separated by grain boundaries providing P-N junction semiconductor characteristics. These boundaries

are responsible for blocking conduction at rated voltage and are the source of the nonlinear electrical

conduction at higher voltages.

– Electrical properties of the MOV are controlled mainly by the physical dimensions of the varistor body.

The energy rating is determined by volume, voltage rating by thickness, and current capability by area

measured normal to the direction of current flow.



Circuit Protection Needs in General Electrical Systems

– A sudden change in the electrical conditions of any circuit will cause a transient voltage to be

generated from the energy stored in circuit inductance and capacitance. The rate of change in

current in an inductor will generate a switching-induced transient voltage.

– Energizing the transformer primary

• When a transformer is energized at the peak of the supply voltage, the coupling of this voltage

step function to the stray capacitance and inductance of the secondary winding can generate

an oscillatory transient voltage with a peak amplitude up to twice the normal peak secondary

voltage.

– De-Energizing the transformer primary

• The opening of the primary circuit of a transformer generates extreme voltage transients.

Transients in excess of ten times normal voltage have been observed across power

semiconductors when this type of switching occurs.

– Fault with inductive power source

• If a short develops on any power system, devices parallel to the load may be destroyed as the

fuse clears.

– Switch arcing

• When current in an inductive circuit is interrupted by a contactor, the inductance tries to

maintain its current by charging the stray capacitance.



Transient Sources

– Major power system switching disturbances, such as capacitor bank switching

– Minor switching near the point of interest, such as an appliance turnoff in a household

– Resonating circuits associated with switching devices, such as thyristors

– Various system faults, such as short circuits and arcing faults



MOV as Transient Suppression Device

There are two major categories of transient suppressors:

a) Diverting transients away from sensitive loads and thus limit the residual voltage

– Voltage Clamping type

– Crowbar type

b) Attenuate transients and preventing them from propagating into the circuit to be protected

– Filters inserted in series within a circuit

– Attenuates high frequency and allows the signal or power flow to continue undisturbed

MOV as a voltage clamping device

– Circuit is essentially unaffected by the presence of

the device before and after the transient for any

steady-state voltage below the clamping level of the

MOV used.

– The voltage clamping action results from the

increased current drawn through the device as the

voltage across it rises. The clamping of the voltage

results from the increased voltage drop across the

source impedance.

The MOV depends on the source impedance

to effectively clamp.

VLINE

Transient on Line

Energy Dissipated

Clamp Voltage



Section 2 MOV Characteristics and Device Physics

Measurement of Varistor Characteristics

– Electrical Characteristics

• Nominal varistor voltage VN

• Maximum Clamping Voltage VC

• DC Standby Current ID

– Maximum ratings

• Continuous / Rated RMS and DC Voltage

• Continuous Power Dissipation

– Thermal Characteristics

• De-rating

– Signal Integrity Characteristics

• Capacitance at 1MHz

Varistor Construction and how it affects the related Varistor characteristics

– Varistor Construction

– Varistor Energy Band

– Varistor Thermal Barrier


MOV Characteristics and Device Physics

Electrical Characteristics

Nominal Varistor Voltage VN

This is measured at a DC test current, In of 1mA

for product models.

V-I Characteristics

– Leakage Region

– Normal Operation

– Upturn Region



Electrical Characteristics

Temperature Dependence in the Leakage

Region

Continuous Power Dissipation

Since MOVs are used primarily for transient

suppression purpose, their power dissipation rating

has been defined and tested under transient

impulse conditions as outlined above.

The above table outlines a suggested program of

testing to verify MOV transient and pulse ratings

with a minimum of testing.



Schematic depiction of the microstructure of a MOV

Grains of conducting ZnO averaged size “d” are

separated by intergranular boundaries.

Device Constructions

Optical Photomicrograph

Cross section of a polished and etched varistor

element



Energy band diagram of a ZnO Grain-

boundary ZnO junction

Device Operation Theory

Thermal Barrier vs Applied Voltage Chart

This varistor theory draws its inspiration from

semiconductor transport theory.



Section 3 MOV General Electrical Power Applications Examples

– Power Supply Protection

– SCR Motor Control

– Contact Arcing (due to inductive load)

– Noise Suppression

– Protection of Transistors (switching inductive loads)

– Motor Protection


MOV General Electrical Power Applications Examples

Power Supply Protection

The MOV should always be placed as close as possible to the source of the

transient and in front of all components to be protected.

In this example, an MOV is used to protect the power supply again transients.



SCR Motor Control

In this circuitry, an MOV would be used to protect the rectifier and SCR against

transients when the power is switched off.



Contact arcing due to inductive load

In this circuitry, an MOV would be used to protect the transient for relay and therefore

extend the service life of the relays.



Noise Suppression

In this circuitry, an MOV would be used to reduce the noise generated from the

mechanical contacts switching on and off.



Motor Protection

In this application, an MOV would be used to protect the transient for the

motor from its insulation breakdown.


MOV Applications Protection Examples

Global Lab Capabilities • Qualification of all LF products

• UL-Approved Customer Testing in ISO 17025 Lab (Des Plaines)

– High power (AC/DC up to 1KV/50KA) UL approvals available in DP

– Telcordia approvals in DP planned (2008)

• Verification of Telcordia, ITU, IEC, FCC, and other industry, regulatory, and safety standards

– Verification to various OC and OV standards

• Insure application meets standards before submitting for approval

• Customer Application testing

– Assistance with design-in and performance verification

• Help with selection of appropriate technology and rating

– Application troubleshooting

• Assistance insuring proper OV/OC and primary/secondary protection coordination

– Competitive evaluations

• Competitive or technology performance comparisons

– Reliability & Tin Whisker data/testing



Section 4 MOV General Electrical Application Product Selection Varistor Product Selection

– Varistor Type/Series Selection

• Identify Varistor Regulatory Requirements

• Identify Varistor Dimensional Requirements

• Identify Varistor Surge Rating Requirement

• Identify Varistor Capacitance

– Varistor Rating Selection

• Identify Varistor Voltage Rating Requirement


MOV General Elec Application Product Selection

Varistor Product Selection Table

APPLICATION EXAMPLE TYPICAL SERIES SELECTED

TV/VCR/White Goods Office Equipment ZA, LA, UltraMOV, "C" III, CH, MA and ML Series

Motor Control ZA, LA, UltraMOV, "C" III, HA, HB, NA,

BA, BB, DA and DB Series

Transformer (Primary Protection) ZA, LA, UltraMOV, "C" III, BA, BB, DA,

DB, HA, HB, and NA Series

Instrumentation MA, ZA, ML, MLN and CH Series

Automotive (Primary / Secondary

Protection )

ZA, CH and AUML Series

Noise Suppression MA, ML, MLN, MLE, CH, ZA, LA, UltraMOV and "C" III

Series

Power Supply LA, UltraMOV, "C" III, ZA, HA, HB, NA, BA, BB, DA,

and DB Series

Transient Voltage Suppressor AC Power

Strip

TMOV, LA, UltraMOV, "C" III, Series

AC Distribution Panels LA, UltraMOV, "C" III, HA, HB, and NA Series

ESD Protection MLE, ML, MLN Series


MOV General Elec Applications Product Selection

MOV AC Selection Table

VOLTAGE

(V)

ENERGY

(J)

PACKAGING AND

OTHER

CONSIDERATIONS

PREFERRED

SERIES

130 - 1000 11 - 360

Through-Hole Mounting

Low/Medium AC Power

Lines

LA

"C" III

UltraMOV

130 - 275

11 - 23 Surface Mount

Leadless Chip

CH

130 - 750 270 - 1050 High - Energy Applications

Shock / Vibration

Environment

DA

HA, HB

NA

DB

130 - 880 450 - 3200 Rigid Terminals

Primary Power Line

Heavy Industrial

BA

1100 - 2800 3800 -

10000

Rigid Terminals

Heavy Industrial

BB


MOV General Elec Applications Product Selection

MOV DC Selection Table


General Elec MOV Training

Section 5 Littelfuse MOV Product Road Map

Leaded TMOV iTMOV

Pb-Free

SM 20

Industrial

IndTMOV

Integrated

Fusing

SMD

0805 Array

EMI/ESD Array

Pb-Free

Pb-Free

Pb-Free

2007 2006 2005

1449 3rd

0402

25mm TMOV

25mm


General Elec MOV Training

Section 6 General Elec MOV Technology Challenges

– Higher Surge Ratings in Smaller Packages

– Multiple Devices in One Package

– Varistor Technology Combined with Other Technologies in the Same Package

– Higher Operating Temperatures

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