Analysis on the L discharge principle and application of EEF

1. Introduction
EEFL is the abbreviation of External Electrode Fluorescent Lamp. Its structural feature is that there is no electrode inside the lamp tube. In addition to the working gas and the inner surface coated with a phosphor layer, the glass tube can be said to be an "empty tube". The outer surface of the two ends of the EEFL glass tube is exposed with a conductive layer or metal sleeve, forming an external electrode that has an important influence on the discharge of the lamp tube, and the name EEFL also comes from this.

Figure 1. Schematic diagram of the basic structure of EEFL
1 glass tube 2 three-primary color phosphor layer 3 external electrode 4 working gas (Ne, Ar, Hg mixed gas)

In order to have a more intuitive understanding of the structure and performance characteristics of EEFL, let's do a few experiments first. Take a glass tube with a diameter of 3mm, and after the normal exhaust process and fill it with 80Torr neon gas and seal it off, it looks like a clean and bright "empty" glass tube. At both ends of this transparent glass tube, wrap a layer of aluminum foil close to the outer surface of the glass tube, and then connect the aluminum foil at both ends of the glass tube to the output end of the cold cathode fluorescent lamp CCFL inverter power supply. When the power switch is turned on, the neon gas in the glass tube is immediately ignited and discharges to emit bright red light, just like a thin tube neon lamp. In fact, it can be said that this is an external electrode neon lamp.
If the gas in the above glass tube is replaced with Ne, Ar, Hg mixed gas, and a layer of phosphor layer is coated on the inner wall of the glass tube, the above external electrode neon lamp becomes an external electrode fluorescent lamp EEFL. Connect the output end of the corresponding CCFL inverter power supply to its external electrode, and it can be ignited and emit light like a CCFL.
In fact, any fluorescent lamp, after the conductive layer is coated on the outer surface of the two ends of the glass tube to form the external electrode, constitutes a certain type of EEFL, which can be lit by a high-frequency power supply with an output of 1-3 kilovolts. It can be seen that the distance between EEFL and us is so close!
Through the above, we can really feel the first outstanding feature of EEFL, that is, simple structure, simple process and low cost.
The second outstanding feature of EEFL is in terms of electrical performance, that is, EEFL lamp tubes can be used directly in parallel, that is, directly in parallel ignition. When it is necessary to light up multiple EEFLs at the same time (such as the direct backlight source of LCD color TV), it is often only necessary to use a high-frequency driving power supply to directly light up several to dozens of EEFLs, so it is very easy to use and the cost is low. It should be noted that almost all gas discharge lamps cannot be used directly in parallel. It is rare that EEFL gives us such convenience!
In addition to the above, many data report that EEFL also has higher brightness and longer life than CCFL, so it is believed that EEFL is the product of CCFL technology progress and is a new light source that emerged in the early 21st century and will replace CCFL.
Is EEFL really cheaper and better than CCFL and about to replace CCFL? Judging from the actual development in recent years, this is not the case yet. Although people are very interested in and looking forward to EEFL, and there are indeed many companies that trial-produce and try out EEFL, it is still in the trial stage and its production and sales volume is far from comparable to CCFL. Why? It is because the performance consistency and stability of my country's EEFL products are not good enough, especially EEFL has high requirements for its supporting high-frequency drive power supply. Many existing products cannot meet the use requirements and cannot be used normally. Therefore, the current development of EEFL faces the problem that the technology of both lamp tubes and supporting circuits needs to be further improved.

2. Discharge Principle of EEFL
The EEFL structure is so simple, it is so easy to light up, and it is so convenient to use it in parallel directly, which makes people interested in it and want to know the truth. In addition, the current task of improving the performance of EEFL is imminent. Therefore, it is necessary to study the discharge principle of EEFL. Only by correctly understanding the basic process of gas discharge in EEFL and improving the structure and process of EEFL based on basic concepts can good results be achieved.
However, in the papers and materials related to EEFL that I have seen, the discharge principle of EEFL is always mentioned briefly, with only a few general words, and no more accurate analysis and discussion. For example, some only mention that it belongs to electromagnetic induction electrodeless discharge, which means that it is the same as the electrodeless fluorescent lamp that people are familiar with, which is obviously wrong. Because it is impossible for EEFL to have a strong high-frequency magnetic field through the lamp tube to induce a high-frequency electric field sufficient to ignite the discharge; some people mentioned that the high-frequency electric field is coupled into the lamp tube through the external electrode of EEFL and the capacitance of the tube wall, causing the gas discharge in the tube, which belongs to high-frequency non-polar discharge; some people believe that the high-frequency voltage applied to the external electrode is passed through the glass medium to generate gas discharge, which belongs to dielectric barrier non-polar discharge, etc. According to the definition that "any discharge method in which the electrode is not exposed to the ionized gas is called non-polar discharge", there are no electrodes installed in the EEFL tube, only external electrodes installed on the outer surface of the two ends of the glass tube, which seems to be non-polar discharge.
However, in-depth analysis of the gas discharge process of EEFL shows that the electrode process is still indispensable to maintain the high-frequency discharge of EEFL. Specifically, EEFL still has a cathode that emits electrons and a cathode potential drop that accelerates electrons and positive ions. To illustrate this point, let us review the analysis of several high-frequency discharge situations in gas discharge theory.
The discharge generated by gas under the stimulation of high-frequency energy is called high-frequency discharge, and its current density j is:
j=[nee2υ/me(υ2+ω2)]E (1)
where E is the electric field strength, ω is the angular frequency of the external high-frequency electric field, υ is the effective electron collision frequency, and υ is calculated by the following formula:
υ＝3.19×109×P/√Te0.5 (2)
where P is the gas pressure (mm Hg), and Te is the electron temperature (K).
According to the relative size of the angular frequency ω of the external high-frequency electric field and the effective electron collision frequency υ, high-frequency discharge can be divided into three cases: ω <<υ, ω≌υ, and ω>>υ. When ω<<υ, a situation similar to low-frequency discharge occurs. At this time, the movement of electrons in the high-frequency electric field drifts toward the instantaneous anode of the high-frequency electric field like a group of bees floating in the wind; when the electric field is reversed, it drifts in the opposite direction. There are still plasma zones and cathode drop zones in the discharge space, and cathode emission of electrons is still required to maintain high-frequency discharge in this case. The difference between it and low-frequency discharge is that the period of high-frequency electric field change is less than the time required for deionization of the discharge space. Therefore, the plasma zone does not have time to disappear, and the polarity of the high-frequency electric field changes, which only affects the cathode drop zone current on both sides of the plasma zone.
When the frequency of the external high-frequency electric field is greatly increased to ω≌υ and ω>>υ, the movement of electrons in the discharge will undergo fundamental changes. The electrons in the plasma zone are affected by the high-frequency electric field and move back and forth continuously, increasing the probability of collision ionization of electrons. At this time, the electrode process is no longer required to maintain a stable discharge, that is, there is no need for the cathode to emit electrons, no cathode is needed, and there is no cathode drop zone, thus forming a non-polar discharge under the action of the high-frequency electric field. This example of high-frequency non-polar discharge occurs in radar antenna switches, whose operating frequency is about 1000 MHz, and the gas pressure in the discharge tube is no more than 20 Torr.
In contrast to the above theoretical analysis of high-frequency discharge, the well-known electronic ballast fluorescent lamp ECFL and cold cathode fluorescent lamp CCFL (both with operating frequencies of 20KHZ-100KHZ) belong to the case of ω<<υ. The discharge has a stable plasma zone and a cathode drop zone necessary for maintaining the discharge. The electron emission of the cathode plays a vital role in the discharge.
Let us return to the gas discharge in EEFL. The operating frequency of EEFL is the same as that of CCFL (20KHZ-100KHZ), and the type of gas filling and the gas pressure are similar to those of CCFL. Therefore, their discharge types are also similar (belonging to the category of ω<<υ). Specifically, the discharge in EEFL still requires the existence of a cathode and a cathode drop zone to maintain a stable discharge. However, where is the cathode of EEFL? The outer electrode is separated from the discharge gas by the glass tube wall, and it is impossible for it to serve as the cathode of gas discharge to emit electrons. I believe that the inner wall surface of the glass tube opposite to the outer electrode is the inner electrode of EEFL. When a certain outer electrode is in the positive potential half cycle, the corresponding inner wall glass surface attracts and receives electrons, and accumulates wall charge; when it is in the negative half cycle, it attracts and accelerates positive ions, and is bombarded by positive ions to produce secondary electron emission, forming a cathode potential drop zone. Some people may ask, glass is an insulator, can it play the role of an electrode? I think it is completely possible. The glass surface can receive electrons and build up a wall potential, which has long been clearly stated in the analysis of the discharge process of AC plasma displays; on the other hand, it is also recognized that glass can produce secondary electron emission when bombarded by positive ions. Since a high-frequency voltage is applied, the potential change of the inner wall can be transmitted to the outer electrode and then to the external circuit through the capacitance formed by the glass wall, and there is nothing unreasonable.
It should be emphasized that the inner electrode of the above EEFL formed by the inner wall of the glass opposite the outer electrode has a prominent feature, that is, it is in the shape of countless "micro-islands". After all, the inner surface of the glass is not a metal electrode, and its surface is not conductive. No matter which point on the inner surface attracts electrons to form wall charges (or attracts positive ions and generates secondary electron emission to accumulate positive charges), there can be no electrical communication between points. We can call it a "micro-island" electrode. Therefore, it can be considered that the inner electrode of the EEFL is composed of countless "micro-island" electrodes on the inner wall of the glass corresponding to the outer electrode.
In summary, we can draw a schematic diagram of the equivalent electrode structure of the EEFL, as shown in Figure 2:

Figure 2 Schematic diagram of EEFL equivalent electrode structure
1 External electrode 2 Discharge space 3 Glass capacitor 4 "Micro-island" electrode There is a view accepted by most people that the high-frequency electric field is introduced into the EEFL discharge space through the glass wall capacitor by the external electrode, and directly acts on the plasma zone of the discharge to produce and maintain stable high-frequency non-polar discharge. We will not discuss the above-mentioned theory's negation of this view, but analyze it from the perspective of counter-proof. If the high-frequency electric field can be introduced to maintain stable discharge without the action of the electrode, then why must the CCFL, which has the same discharge conditions as the EEFL, rely on the electrode process to maintain stable discharge when the high-frequency electric field is directly introduced into the discharge space by the electrode? If the electrode process is really insignificant, then why does the electrode process of the CCFL concentrate so much power and have an extremely important impact on the discharge performance?
In order to examine whether the inner wall electrode of the EEFL exists, we can judge by observing whether there is a cathode drop zone on the inner wall surface corresponding to the outer electrode of the EEFL. The actual photograph taken is shown in Figure 3. From Figure 3, it can be clearly observed that the luminescence of the inner wall of the glass in the plasma zone is very different from the luminescence of the inner wall of the glass corresponding to the external electrode. The inner wall surface corresponding to the plasma zone does not emit red light, while the inner wall surface corresponding to the outer electrode emits red light similar to the CCFL electrode surface. The reason is very clear. The electron energy in the plasma zone of the fluorescent lamp is low and cannot cause neon atoms to be excited. Only when the electrons in the cathode drop zone are accelerated into high-energy electrons can the neon atoms emit red light when they are excited and return to transition.

Figure 3 Photograph of the luminescence of the inner wall of the EEFL glass

In addition, we dissected the EEFL lamp after thousands of hours of life test. The surface condition of the inner glass wall corresponding to the plasma zone did not change much, while the inner glass wall corresponding to the outer electrode was seriously blackened. This is obviously the result of positive ion bombardment of the cathode.
Finally, we comparatively measured the light efficiency curves between the output luminous flux and output power of CCFL and EEFL with the same structure (same glass, same diameter, same production process), and found that their shapes and data are quite close, which also shows that the same type of discharge is generated in EEFL and CCFL, and there is no significant difference in the discharge principle. I will not elaborate on this. The
above analysis of the EEFL discharge principle may have important practical significance, that is, to improve the performance of EEFL, we should pay great attention to the surface condition of the inner wall at both ends of the lamp tube as the inner electrode, and what kind of process, material or even adding a certain coating may significantly improve the performance and life of EEFL.

3. Application prospects of EEFL
EEFL is very popular as an ultra-thin light box for advertisements, photos, and pictures, as well as the backlight source of LCD color TVs in flat-screen TVs, especially the direct backlight source of large-screen LCD color TVs. As long as its performance is reliable, it is likely to be listed as the preferred product by many users. The reason is the unique advantage mentioned above that it can be used directly in parallel, and the lower price brought about by its lower production cost.