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SEMI D35-1103 E © SEMI 2003 10 second, approximately initial electron s with the order of 10 4 particles per second will be required. R2-3.4 Measuring  under various cond itions helps us to understand the dark character…

SEMI D35-1103

RELATED INFORMATION 2

DARK CHARACTERISTICS OF DISCHARGE LAMP

NOTICE: This related information is not an official part of SEMI D35 and was derived from Japan Backlight TF.

R2-1 Start of Discharge Lamp

R2-1.1 When a discharge lamp is activated, there must

be electrons (initial electrons) in the discharge space

generally. In order to start up, it is necessary to apply

an electric field, accelerate the initial electrons, excite

and ionize gaseous particles, and establish discharges

(keep up discharges) in the discharge space. At that

moment, the initial electrons are said to have relation to

the continuity of discharges at random, and the

combinations of the status of a lamp when

manufactured, history of on/off of lighting, various

factors of a discharge lamp, and lighting circuits are

involved.

R2-1.2 For these reasons, in terms of the start-up of a

discharge lamp, initial electrons are required and there

are probability factors to be considered, and so the

discharge start-up time delay should be observed

statistically.

R2-2 Dark Characteristics of Discharge Lamp

and Remedy

R2-2.1 A discharge lamp usually starts up in a bright

place even when the initial electron source is not

provided especially. It is considered that this is because

the external light gives energy to materials (gas,

electrode, etc.) inside the discharge space to emit

electrons (photoelectrons) as an initial electron.

However, in order to activate the discharge lamp in a

dark place, the photoelectron cannot be used. There are

electron sources such as natural radiation as the other

initial electrons (cosmic rays and radian from the earth)

but these are not enough to get initial electrons at

present.

R2-2.1.1 Since the initial electrons in the discharge

space in the darkness decrease greatly, the discharge

start-up time delay, which is from the voltage-applied

time to the start-up time, tends to be salient. This is

defined as dark characteristics.

R2-2.2 In order to start up a discharge lamp within a

limited time, the method to supply positively initial

electrons in a discharge space is needed. For example,

regarding a fluorescent lamp for ordinary lighting

purpose, its negative electrode is preheated before the

start-up. As a result, thermoelectrons, which are emitted

from the emitter applied to the negative electrode, are

used as an initial electron. For a glow starter,

bremesstrahlung electrons by radioisotope (RI) built in

the electrodes are employed. In addition, there are

other methods such as applying high-voltage to supply

electrons from the electrodes (field emission) and

supplying photoelectrons by turning on another light

source (light bulb) located nearby a discharge lamp

before its start-up.

R2-2.3 In terms of a discharge lamp used as an LCD

backlight, the conventional remedies have not been

utilized because it must have compact cathode

electrodes, the RI is hardly used from the

environmental view point, the output voltage for an

inverter of high-frequency lighting circuit is limited,

and the space is too small to built another light source

in it. Thus, unique methods have been studied.

Nowadays, the followings, such as exposing a metal

oxide that emits exoelectrons

in the discharge space,

forming spattering layers

on the wall near cathode

electrodes by lighting a discharge lamp for a certain

time, and making a compound of materials with low

work function

into electrodes or placing it near

electrodes, are performed.

R2-3 Theory on Discharge Start-up Time Delay

R2-3.1 The equation on the discharge start-up time

delay is as follows.

 = 1/(P·Q) ----------------------------- (1)

where

: average discharge start-up time delay (sec)

P: percentage of one electron that discharges ( /particle)

Q: number of initial electrons (particle/sec)

R2-3.2 In Q factors mentioned before, such as the

photoelectrons

, thermoelectrons by external light,

electrons emitted by field emission, electrons by natural

radiation, electrons by RI, and exoelectrons, and in P

factors, such as the voltage applied to a discharge lamp,

filler gas pressure, kind of gases, electrode materials,

impact of residual impure gases and concessionary

shape of electrode, are involved.

R2-3.3 Here, if a maximum value “1” is set to P by

adjusting a discharge lamp and lighting circuit and if at

least one particle of electron per second exists in a

discharge space,  will be 1 second. Usually P is

considered to be the order of 10

-3.

If  becomes

approximately 1 second, the initial electrons with order

of 10

particles per second will be required, and for 0.1

SEMI D35-1103

second, approximately initial electrons with the order of

particles per second will be required.

R2-3.4 Measuring  under various conditions helps us

to understand the dark characteristics and improve the

remedies.

*1: When a material emits electrons, the energy

greater than work function must be given to

electrons in the material but here the electrons

are such energies emitted as low heat and light.

The alpha alumina is said to emit exoelectrons

and it is actually used as an initial electron

source for a cold cathode lamp. However, the

details are still unknown.

*2: The mercury seems to interact with the

spattering layer to create the exoelectrons

source.

*3: The cesium (Cs) and the like with low work

function applied to an electrode is used as

compound.

*4: The question still remains if photoelectrons can

be supplied when energy more than work

function is given to a discharge lamp material

by external light (visible light). The electron by

external light may be also considered as an

exoelectron.

SEMI D35-1103

RELATED INFORMATION 3

PRECAUTIONS AGAINST MEASURING METHOD OF COLD

FLUORESCENT LAMP

NOTICE: This related information is not an official part of SEMI D35 and was derived from Japan Backlight TF.

R3-1 Introduction

R3-1.1 The cold fluorescent lamp (CCFL) is turned on

usually by using a compact DC/AC convertible power

supply called inverter. The CCFL has been developed

with top priorities such as a narrow diameter, space-

saving shape and high brightness for its application and

so the inverter has been requested to have a low profile

space-saving shape with high efficiency as well.

R3-1.2 Understanding these requirements are satisfied

and the characteristics such as high voltage, high

frequency and micro current are compassable as trade-

off in the CCFL, we should handle it with care.

R3-2 Stray Capacitance and Leakage Current

R3-2.1 Stray Capacitance

R3-2.1.1 The stray capacitance is defined as a

characteristic that behaves as if it had the electrostatic

capacity that system does not provide and its volume is

too unstable to be measured quantitatively

(unintentional electrostatic capacity element). If a

formula for electrostatic capacity between two parallel

copper wires is put into an equation, it will be nearly

expressed as follows;

C = Q/V = (0)/(log ((da)/a) ------ (1)

where

C: electrostatic capacity (stray capacitance)

d: distance between wires

a: diameter of wire

R3-2.1.2 According to the above-mentioned equation

(1), it is understood that the stray capacitance does not

affect the frequency but is inversely proportional to

logarithm of the distance between wires. (This means

that the closer the distance between high potentials

becomes, the more the stray capacitance increases, and

the longer the wire becomes, the stray capacitance

increases as well.)

R3-2.2 Leakage Current

R3-2.2.1 The leakage current is the inverter secondary

current that flows to the above-mentioned stray

capacitance. (It equal to the inverter secondary current

except for that of CCFL.) Suppose “C” is the stray

capacitance around CCFL and “I” is the current that

flows to “C” and sinusoidal AC voltage is applied, the

bellow-equation is obtained.

I = ·C·V = 2f·C·V ------ (2)

where

I: leakage current (true AC current)

f: AC frequency

C: stray capacitance (electrostatic capacity)

V: tube voltage (true AC voltage)

R3-2.2.2 According to the above-mentioned equation

(2), the leakage current increases proportionally to the

increase of the frequency, stray capacitance, or

potential difference.

R3-2.2.3 Since the stray capacitance and leakage

current increase or decrease by each parameter such as

the length of high voltage wire, distance between the

wire and the conductor of ground, lighting frequency

and potential difference, it is recommended that

measuring conditions should be specified and

standardized as much as possible.

R3-3 Temperature Characteristics

R3-3.1 The CCFL is quite sensitive to mercury

temperature characteristics because its fluorescent

substance emits light receiving ultraviolet rays from

mercury particles as activation energy.

R3-3.2 If the temperature of mercury particles is

regarded as the same as the CCFL wall temperature

when lighting, and suppose the tube temperature is

25°C at 0°C of ambient temperature against wall

temperature at 50°C, approximately 1/10 of saturated

steam pressure difference will take place and actual

luminance of the tube will be approximately 1/2. On

the contrary for the high temperature side, suppose the

wall temperature is 100°C against the ambient

temperature 70°C the steam pressure difference which

is greater by 20 times will take place and the actual tube

luminance is approximately 2/3.

R3-3.3 In this manner, the temperature and tube

luminance have peak characteristics within a certain

temperature range because the partial pressures of Ar,

Ne and Hg have the optimum density against the

lighting tube current. When the temperature is low, the

luminance decreases due to the lack of absolute amount

of ultraviolet rays caused by insufficient mercury

molecules.