Nonlinear Resistors

download Nonlinear Resistors

of 42

  • date post

    20-Feb-2015
  • Category

    Documents

  • view

    277
  • download

    6

Embed Size (px)

Transcript of Nonlinear Resistors

Nonlinear Resistors Introduction (1)type varistor physical effect grain boundary phenomena in semiconducting ceramicsy grain boundary phenomena in semiconducting ferroelectric ceramics self heating effects hopping condution in metal oxides self heating effects material n-doped ZnO n-doped BaTiO3 102...103 S/m spinells e.g. (Ni,Mn)3O4 10-2 S/m

PTC

NTC

glossary Thermistor: NTC: PTC: Varistor: thermally sensitive resistor negative temperature coefficient positive temperature coefficient variable resistor[Schaumburg 1990]

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 1, 11.02.2006

IWE

Nonlinear Resistors Introduction (2)

NTC

PTC

Varistor

PTC-heater with metalized surface

arrangement of PTCs and radiators

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 2, 11.02.2006

IWE

Varistor Variable Resistor (Voltage dependent Resistor)

symbol varistor

U[www.staratech.com]

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 3, 11.02.2006

IWE

Varistor Nonlinear I/V-Characteristic

I IK UK UK U

varistor: variable resistor voltage dependent resistor with symmetrical I/V-chracteristic

IK

K

I = K Ugeometry parameter K

nonlinearity coefficient

materials: SiC, ZnO typical -values: SiC: 5...7 ZnO: 30...70

[Heywang 1984]

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 4, 11.02.2006

IWE

Varistor Equivalent Circuitmicrostructure path I path II path I equivalent circuit path II

equivalent circuity grain

RD CD

RP CP 2 RK

secondary phase

grain boundary

CD 240 nF/cm K 1 cm P 80 kcm CP 90 nF/cm[Ein 1982]

RD CD varistor

RP CP leakage currentleakage current RP, CP in parallel

grain- (RK) and grain boundary (varistor) (RD) resistance RD and CD in parallel RD and CD are voltage dependent

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 5, 11.02.2006

IWE

Varistor Defects and Band-Structure in ZnO-Varistorenpoint defects in ZnO (2-dimensional view) VZn EA2 = 2,8 eV EA1 = 0,7 eV electroneutrality n + [VZn] + 2[VZn] = p + [VO] + 2[VO] VO, VO: VZn, VZn: e, (n): h, (p): O-vacancies Zn-vacancies electron (-concentration) hole (-concentration) (ED1, ED2) (EA1, EA2) y D: donor A: acceptor band structure with energy levels of schottky-defects (EA, ED) EC ED1 = 0,05 eV ED2 = 2,0 eV EV

e VO

Eg = 3,2 eV

Materials and Devices in Electrical Engineering[Heywang 1984 ]

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 6, 11.02.2006

IWE

Varistor Grain Boundary Propertiesprocessing (sintering temperature 1300 C) high temperature equilibrium: oxygen vacancy excess charge carrier: n = [VO] + 2[VO] operation (operating temperature < 100 C) low temperature equilibrium: [ VZn ] + 2[ VZn ] [ VO ] + 2[ VO ] n 3 eV acceptors

EF EV

EF EV tunneling effect

100 nm[Fasching 1994]

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 9, 11.02.2006

IWE

Varistor Summary

varistor effect in doped ZnO-ceramic is related to grain boundary properties, ZnO-grains exhibit a high conductivity varistor effect only at grains exhibiting a direct contact varistor effect starts at a voltage of ~ 3,0 0,5 V / grain boundary grain boundary forms a potential barrier of 0,6 eV. time constants of several nanoseconds technical varistor systems: ZnO + Bi2O3 + MnO2 + Co3O4 + Sb2O3 + Cr2O3 + additional additives (up to 10 components to optimize secondary properties)

[Ein 1982 ]

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 10, 11.02.2006

IWE

Varistor Application: Overvoltage ProtectionUS = 9,8 KV UB = 200 V Ri = 10 voltage U / V I 104 100 % with SiC-varistor 103 25 % with ZnO-varistor 3% operating voltage 102 consumer the voltage peak US(t) increases the voltage U for a short time U(t) = US(t) + UB - Ri I(t)Materials and Devices in Electrical Engineering[Heywang 1984 ]

without varistor

varistor U

10-1 101 103 current / A

logarithmic graph of U(I) at operating voltage at overvoltage US = 9,8 KVInstitut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 11, 11.02.2006

IWE

Varistor

ZnO-varistorsMaterials and Devices in Electrical Engineering

varistors for high voltage applicationsInstitut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 12, 11.02.2006

IWE

PTC Positive Temperature Coefficient Resistor

symbol PTC

[www.atpsensor.com]

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 13, 11.02.2006

IWE

PTC Temperature Dependency of a ResistorRmin, Rmax minimum and maximum resistance, change of sign of the temperature coefficient Rmax/Rmin effective resistance increase max. 107 TB nominal temperature of the device at which RB = 2 Rmin TB values: -30...+250 C

Rmax 106 resistance R /

104 PTC-behavior resistance increases several orders of magnitude 100 TB 200 temperature / C 300

102 RB 100 0 Rmin

TKR: temperature coefficient of resistance

TKR =

1 dR , TKR values of 5...70% K-1 R dTInstitut fr Werkstoffe der Elektrotechnik

Materials and Devices in Electrical Engineering[Heywang 1984]

Nonlinear Resistors.ppt, slide: 14, 11.02.2006

IWE

PTC Defect Chemistry of Barium-Titanate (BaTiO3)

system BaTiO3 La-doping on the Ba-site oxygen ion O2 LaBa

barium vacanciesBaBa + 2e / + 1 O2 ( g ) 2// VBa + BaO

oxygen vacanciesx OO VO + 2e / +

barium ion Ba2+ // VBa VO

1 O2 ( g ) 2

-

electroneutrality// [e] + 2 VBa 2 VO + LaBa

acceptors

donors

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 15, 11.02.2006

IWE

PTC Electrical Properties of Grain Boundaries in Barium-TitanateBaTiO3 insulating grain boundary area acceptor concentration barium vacancy concentration and profile is determined by the temperature profile during cooling down fast cooling down thinner insulating grain boundary area

donor concentration

grain

slow cooling down fast cooling down

[LaBa] 2[VBa]

n charge carrier concentration

x

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 16, 11.02.2006

IWE

PTC Charge Carrier Transport at Grain Boundariesgrain boundary (KG) p-type doping AKG grain n-type doping grain n-type doping hight of the schottky-barrier WB at the grain boundary EB = e0 [ Agb ]2 8n 0 r [Agb]: surface charge at the grain boundary n conductivity (tunneling current)

reduced charge carrier (electrons) concentration EB EC EV

+ + - + + + + - + + + E -

~ exp x

EB k T

grain boundary resistance E Rgb ~ exp B k T

xMaterials and Devices in Electrical Engineering Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 17, 11.02.2006

IWE

PTC Phase Transition in the PTC Temperature Rangetemperature dependency of the grain boundary resistance does not explain the observed temperature dependence. E Rgb ~ exp B k T

phase transition in the PTC temperature range

resistance R /

NTC 106 104 102 100 0

PTC

NTC

100 200 temperature / C

300[Siemens]

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 18, 11.02.2006

IWE

PTC Compensation of the GB Potential Barrier due to spontaneous PolarizationT < TCgrain boundaryPs + + + + + + + + Ps -

T > TC

grain boundary-

-

EB

EB

negative grain boundary charge compensated by polarisation charge EB >

50Materials and Devices in Electrical Engineering

250Institut fr Werkstoffe der Elektrotechnik

T / C

Nonlinear Resistors.ppt, slide: 19, 11.02.2006

IWE

PTC Equivalent Circuit

equivalent circuit

summary

grain boundaryPs + + + + + + + + Ps -

PTCs consist of polycrystalline ferroelectric compounds, mostly metal-oxides like BaTiO3. The PTC-effect is based on the insulating grain boundary (schottkybarrier) and their interaction with the temperature dependent ferroelectric porperties. The negative grain boundary charge is compensated either by a space charge (paraelectric region high resistivity) or by a polarisation charge (ferroelectric region low resistivity).

-

Rgrain

Rgb

Rgrain

Cgrain

Cgb

Cgrain

Materials and Devices in Electrical Engineering

Institut fr Werkstoffe der Elektrotechnik

Nonlinear Resistors.ppt, slide: 20, 11.02.2006