Chap 5. Dielectric and Insulators

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Chap 5. Dielectric and Insulators
2
5. Dielectrics and Insulators
ltPART I CAPACITIVE APPLICATIONSgt 5.3 THERMAL
SHOCK RESISTANCE 5.4 CAPACITORS
5.4.1 Capacitor characteristics 5.4.2
Non-ceramic capacitors 5.4.3 Ceramic
capacitors
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5.3 THERMAL SHOCK RESISTANCE
TSR? ??ceramic? ??? ????? ???? ??? ??? ???.
one being that a failure criterion which depends
on the application has to be adopted. real
difficulties a useful guide to TSR is the
quantity (thermal conductivity, strength,
Youngs modulus, coefficient of linear
expansion) ?? ceramic?? ?? ??(
) ???? TSR? ???? ?? , but this is
only a rough guide and the matter of TSR,like
electric strength, is extremely complex.
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5.4 CAPACITORS
Fulfill various function -Blocking
-coupling -decoupljng -AC-DC separation
-filtering -energy storage -blocking
direct current but pass alternating current
-charge storage capability photoflash unit of a
camera
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5.4.1 Capacitor characteristics

• Volume efficiency
• measure of the capacitance that can be
  accommodated in a
• given size of capacitor.

??? capacitor? ?? A ??? ?? h
Volume efficiency
? volume efficiency? ????? ??, ??? ??? ??? ???.
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5.4.1 Capacitor characteristics

1. Volume efficiency

But ? (breakdown
value)? ??? ???. ??? ??? ?? ? ??.
Working voltage lt normal breakdown voltage
maximum permissible energy density
Used at high fields
2
2
e
e
E
CU

0
b
r
w
2
h
2
2
V
table 5.1(p.190) capacitor? ?? volumetric
efficiency ???.
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5.4.1 Capacitor characteristics
(b) DC resistance
Charged capacitor will discharge through its own
resistance.
remaining charge, original charge
Time constant of the capacitor Depend on the
dielectric material
depend on dielectric except high working
voltage(gt1kV)
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5.4.1 Capacitor characteristics
(C) Equivalent parallel and series resistance
dissipation factor DC leakage resistance ,
dielectric losses
(lead, electrode resistances)
???, but depond on ,
temp.
the matter is more complex than appears at first
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5.4.1 Capacitor characteristics
(d) Resonance frequency
examine how the equivalent circuit for a
typical capacitor respond to frequency Mica
capacitor, assumed to be independent of frequency.
ltlt 1
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5.4.1 Capacitor characteristics
(d) Resonance frequency
increase decrease and approximates
to
Resonance effect places an upper Limit on
the freq. above resonance the reactance of a
capacitor is inductive( )
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5.4.1 Capacitor characteristics
(e) Breakdown and degradation
breakdown in a capacitor results in the
replacement of a reactive insulating
component by either a low-resistance short
circuit or an open circuit.
breakdown field lt intrinsic dielectric
strength breakdown field governed by the
concentration of flaws(metallic inclusion, ??,
???? ?? ?? etc.) and the design of the electrode
structure.
??? ?? ???? ?? ???? ??? polymer? ??? ???
metal cans ?? ???? ????. degradation? ageing?
?? ageing is due to the presence of
ferroelectric domains and to a change in the
mobility of the walls between them as time
passes. changes that occur in the properties
breakdown ??? ???? ??? ?? ?? ?? ???? ?? ????
???? ??? ????.
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5.4.2 non-ceramic capacitors
(a) polymer-film capacitors
dielectric ???? ????? ??films
(polymer,paper,both)? ???? ??. ?? ???? ? or
???? ?? ? ?? ???? ?? ???? cans?? epoxy ???
??. ( ?? 0.025 )
25 50? ?? ???. polystyrene capacitor low
(lt ) ????. ?? ???
frequency-selective circuit? ?? ?? power-factor
correction, filter circuit ??.
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5.4.2 non-ceramic capacitors
(b) Aluminium electrolytic capacitors
???? ??? ??? ??(?? 99.99, 50 ) ???
porous paper ???? ?????? paper ?? ???? ?? ??
? ?? ?????? ?? ?? ??. ???? ??? ??????(
)? ???, ???? ??,????? ???, ???? ???? ? ???
????? ??? ???? ????? ?? ??? ????? ?? ?? ???
???? ??? ??. ????? ??? ?? ?? ?????? ??? ???,
????? ??? ???? ??? ????? ????? ??? ?? ?? ????? ??
???? ?? ?? ??? ??
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5.4.2 non-ceramic capacitors
(c) Tantalum electrolytic capacitors
??? tantalum ??? ???? ?? ?????. tantalum
powder? ??( )?? ??? ?? ??? ??? ????
??. ??? ? ?? ??. ?? ?? ??.(??? ??? ?? ???
???? ?? ??? ??? ???? ???? ??? ??)
???? ?? ???? ?? ???? ??? ???? ??? ?? ?????
?? ??? ??? ??, ???? ?? ?? ?? ?? ???? ???????
???? spike ??? ??? ??? ???? ?? ??? ????? ????
????? ?? ???? ?? ??
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5.4.2 non-ceramic capacitors
(d) Mica capacitors
??(mica)? ???? ??? ???. ??? ????? ?? ????
????, ??? ??? ???? ??? ?????? ????? ???? ??
??. ????? ????? ?????? ?? ?? ?? ??? ??? ??
??, ???.
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5.4.3 ceramic capacitors
(a) Classes of dielectric
Class I
low and medium permittivity ceramics ????(
) 15 500 dielectric factor lt0.003
temperature coefficient -2000 100
Class II
high permittivity ceramics ????( ) 2000
20000 dielectric factor lt0.03 class I?
?? ??, ??, ???? ??
Class III
?? ? ? ???? ??. class II? ??? ????? working
voltage(225V)?? ??? ???? ?? ??. ?? ???
??.
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5.4.3 ceramic capacitors
(b) Form and fabrication
(i) Discs and tubes
discs can be formed by dry pressing calcined
and milled powers containing some 5-10vol. of
an organic binder. They can be cut from
extruded ribbon or band-cast tape.
tube are formed by extrusion. Advantage of
being less fragile than flat pieces and are
more suitable in some type of circuit assembly.
after sintering , tubes are coated with silver.
lead and electrode inductance can be less in
discs than in tubes so that discs have some
advantage at higher frequencies. the two shapes
are similar in volumetric efficiency since their
bulk largely consists of the encapsulating
resin. disc and tubular shape are used for all
classes of dielectric since they are lowest in
cost.
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5.4.3 ceramic capacitors
(i) Discs and tubes
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5.4.3 ceramic capacitors
(ii) Multilayer capacitor
??? ???? tape casting??? ? ???? ??? ???? ?
?? ??? ?? ? ??? ?? ? ???? ??? ceramic
capacitor? ????? ???? ?? ? ???? ??.
??? ?? ??? capacitance? ??? ???? ??? ???? ???
??? ??? ??? ??. ??? ?? ?? ?? ? ? ? capacitance?
??.
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5.4.3 ceramic capacitors
(ii) Multilayer capacitor
??? ?? 20 ? ??
?? Dielectric class I or class II
?? Electrode ???? ???? Termination
???? ??? ?? ?? ??.
????? three process the wet, the dry, the
fugitive electrode. fig.5.12 ????.
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5.4.3 ceramic capacitors
(ii) Multilayer capacitor
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5.4.3 ceramic capacitors
(iii) High-voltage and high-power capacitors

• (a)? ?? ???? ??? ????? field? ??? ??? ??.
• Breakdown??
• (b)? ?? ??? ?? ???? field? ??? ????.
• ?breakdown strength ??
• ?high-power capacitor ?? ??.

Fig.5.15(p.204)?? high-power capacitor? ???? ??.
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5.4.3 ceramic capacitors
(iii) High-voltage and high-power capacitors
the ceramic in a high-power capacitor is disc
,cylindrical, pot shaped.
???? cavities? ?? ????? ?? ???? ????? ?? ???
??? ??? ???? ??? ??? ??? ??? density? ???
??. ??? ?? 3 ??. class I dielectric ??
capacitance 2pF12nF peak voltage 3kV
rated power 0.25500kVA
0.00050.001