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SEMI D35-1103 E © SEMI 2003 8 ~ Inverter High v oltag e side Ground Electrode Minimal discharge Stray capacit ance ~ Inverter High v oltag e side Ground Electrode Minimal discharge Stray capacit ance ~ Inverter High v ol…

SEMI D35-1103

RELATED INFORMATION 1

STARTING VOLTAGE OF COLD CATHODE FLUORESCENT LAMP

(CCFL)

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

R1-1 Phenomenon of CCFL Discharge

Starting

R1-1.1 Although the phenomenon of CCFL discharge

starting shows the similarity to that of low gas pressure

discharge starting, a high frequency lighting circuit

(inverter) with approximately 30–100 kHz is mostly

used to discharge and so it is affected by stray

capacitance.

R1-1.2 When the secondary voltage of inverter is

stepping up from a sufficiently low voltage gradually,

an emission area appears near the electrode connected

to higher voltage side. (See Figure R1-1 1

Phase).

This means that the discharge starts between the

electrode and ground level of discharge circuit through

the stray capacitance, and a spark after dark current

takes place in the discharge space in the tube. In the

discharge, the other electrode is the tube wall (glass or

fluorescent substance); the formation of dielectric

barrier discharge and the existence of the secondary

electron supply system to the discharge space can be

considered and thus the discharge seems sustainable.

Therefore, the emission around the electrode is judged

to be the emission mainly by negative glow of glow

discharge.

R1-2 From Discharge Starting to Stable

Lighting Mode

R1-2.1 After the discharge takes place near the

electrode and the secondary voltage of the inverter

increases gradually, the emission area near the

electrode grows longer accordingly. (See Figure R1-

2.) This means that a positive column is generated by

applied voltage to have such the length as corresponds

to the value of resistance based on the applied voltage.

When the voltage is applied furthermore, the emission

area (i.e. positive column) between two electrodes is

generated finally. (See Figure R1-3 2

Phase.)

However, if the tube is thin and long and the stray

capacitance is relatively greater to the discharge circuit,

the ratio of current that flows through the stray

capacitance is still large at this phase. With the

increase of the inverter secondary voltage, the ratio of

current that flows in the tube also increases and most of

the current flows in the tube because the impedance of

discharge path decreases on the contrary due to

negative characteristics of discharge. (See Figure R1-4

Phase.)

R1-2.2 Each voltage applied at these three phases may

be called starting voltage at the first phase, or stable

discharge voltage or tube current at the third phase, for

example. Sometimes it could be hard to distinguish the

second phase from the third phase. In addition, since

the second phase is hardly observed by the tube designs

(for the tube length, outer and inner diameters of tube,

pressure and composition of filler gas, and peripheral

members of tube), it looks as if the first phase shifted to

the third phase suddenly. This is because the stray

capacitance is so big that the current cannot flow into

the tube, the discharge surges immediately after the

current flows into the tube, the impedance of the

discharge path decreases due to the negative

characteristics and a large amount of current flows in

the tube all of a sudden.

R1-2.3 From the viewpoint of discharge phenomenon,

the voltages applied to the tube at the first and third

phases are important. Especially the voltage applied at

the third phase should be noted practically. (The

voltage applied at the second phase may be noted also.)

R1-3 Cautions at Discharge Starting

R1-3.1 At the discharge starting,  coefficient of

discharge gas and  coefficient of electrode are

involved. These coefficients contain the pressure of

discharge gas (particle density) as a parameter.

Especially when the mercury is filled in, the control of

ambient temperature becomes very important because

the mercury vapor (particle density) pressure changes

sensitively to the ambient temperature.

R1-3.2 In addition, the lighting circuit is usually built

for AC, the reduction of unnecessary stray capacitance

base on frequency shall be required. As the impact of

stray capacitance becomes greater especially when the

tube is thin and long, sufficient consideration shall be

given.

SEMI D35-1103

Inverter

High voltage side

Ground

Electrode

Minimal discharge

Stray capacitance

Inverter

High voltage side

Ground

Electrode

Minimal discharge

Stray capacitance

Inverter

High voltage side

Ground

Electrode

Minimal discharge

Stray capacitance

Inverter

High voltage side

Ground

Electrode

Minimal discharge

Stray capacitance

Figure R1-1

Phase of CCFL Discharge Starting

Positive column

Figure R1-2

From 1

Phase to 2

Phase of CCFL Discharge Starting

~~~~

Figure R1-3

Phase of CCFL Discharge Starting

~~~

Figure R1-4

Phase of CCFL Discharge Starting

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