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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 1 General Electrical MOV Training 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 2 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. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 3 MOV Definition and General Electrical Circuit Protection 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. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 4 MOV Definition and General Electrical Circuit Protection 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 5 MOV Definition and General Electrical Circuit Protection 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 The MOV depends on the source impedance to effectively clamp. Transient on Line 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. Energy Dissipated Clamp Voltage VLINE Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 6 General Electrical MOV Training 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 7 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 8 MOV Characteristics and Device Physics Electrical Characteristics 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. Temperature Dependence in the Leakage Region The above table outlines a suggested program of testing to verify MOV transient and pulse ratings with a minimum of testing. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 9 MOV Characteristics and Device Physics Device Constructions Optical Photomicrograph Cross section of a polished and etched varistor element Schematic depiction of the microstructure of a MOV Grains of conducting ZnO averaged size “d” are separated by intergranular boundaries. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 10 MOV Characteristics and Device Physics Device Operation Theory Thermal Barrier vs Applied Voltage Chart This varistor theory draws its inspiration from semiconductor transport theory. Energy band diagram of a ZnO Grainboundary ZnO junction Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 11 General Electrical MOV Training 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 12 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. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 13 MOV General Electrical Power Applications Examples 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. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 14 MOV General Electrical Power Applications Examples 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. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 15 MOV General Electrical Power Applications Examples Noise Suppression In this circuitry, an MOV would be used to reduce the noise generated from the mechanical contacts switching on and off. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 16 MOV General Electrical Power Applications Examples Motor Protection In this application, an MOV would be used to protect the transient for the motor from its insulation breakdown. Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 17 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 18 General Electrical MOV Training 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 19 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 20 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 21 MOV General Elec Applications Product Selection MOV DC Selection Table Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 22 General Elec MOV Training Section 5 Littelfuse MOV Product Road Map EMI/ESD Array 0805 Array SMD 0402 Integrated Fusing IndTMOV Pb-Free 1449 3rd Pb-Free Industrial SM 20 Pb-Free 25mm Leaded TMOV iTMOV 2005 TMOV 25mm Pb-Free 2006 2007 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 23 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 Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved. 24