CCFL inverter

A CCFL inverter is a device (an inverter) for providing drive power to a Cold Cathode Fluorescent Lamp (CCFL). CCFLs are often used as inexpensive light units in electrical devices.

Characteristics

#Tiny form and structure
#Switchover efficiency over 80%
#Adjustable light

Applications

#widely used in backlights for LCDs such as in notebook computers
#used in ultrathin lamp cases, such as the rear lamp for advertising signs
#used in auxiliary illumination and lighting devices

Electric parameters should be strictly matched during design in order to ensure the rated life of the tube.

History of the Technology

As for the inverter circuit for a cold cathode fluorescent lamp, a collector resonance type circuit has been widely used. This collector resonance type circuit is referred as another name to as the "Royer circuit" in some cases. However, the proper definition of the Royer circuit is such that the inversion of a switching operation is performed in a state in which a transformer is saturated. Thus, the inverter circuit which performs the inversion operation by utilizing the resonance on the collector side is desirably referred to as the "collector resonance type circuit" or the "collector resonance type Royer circuit" in distinction from the Royer circuit. In the early stage of the inverter circuit for a cold cathode fluorescent lamp did not utilize the resonance method of a secondary side circuit at all, and the so-called closed magnetic circuit type transformer having a small leakage inductance was used in a step-up transformer. In the background of the times, the so-called closed magnetic circuit type transformer method a transformer having a small leakage inductance in terms of recognition of a person skilled in the art. In addition, the leakage inductance of the step-up transformer in the inverter circuit was recognized such that it reduced an output voltage on a secondary side of a transformer and was not preferable, and thus was desirably as small as possible.

As a result, a resonance frequency of the secondary side circuit of the transformer in the background of the times was decided to have no relation with an operating frequency of the inverter circuit. Thus, the resonance frequency of the secondary side circuit used to be set to a much higher frequency than the operating frequency of the inverter circuit so as to exert no influence on the operating frequency of the inverter circuit. In addition, a ballast capacitor Cb is essential for stabilization of a lamp current.

In the middle stage, with respect to the inverter circuit for a cold cathode fluorescent lamp, an inverter circuit shown in the figure of the “CCFL Inverter circuit of the past technology“ is known. But, it wasn't used any more. However, this inverter circuit was advanced and disclosed by the Hitachi media electronics in Japan, and had come into world wide use as the so-called three-time resonance circuit in which as shown in the figure as the “advanced technology“, the resonance frequency of the secondary side circuit is three times as high as an oscillation frequency of a primary side circuit. A step-up transformer in which a leakage inductance value is increased to some degree is suitable for one used in this case.

The shape of the transformer which is actually used in the so-called three-time resonance changed flat shape. Thus, though the magnetic path structure is closed, the leakage of the magnetic flux is considerably more than that of the conventional one. That is, that transformer has a large leakage inductance value.

In any case, this technical idea (refer to figure of the “past technology“) is such that the leakage inductance value of the step-up transformer is increased to some degree, whereby a resonance circuit is structured by using a leakage inductance (Ls in the figures) and a capacitance component obtained on the secondary side of the step-up transformer, and a resonance frequency of the resonance circuit is set to a frequency three times as high as the operating frequency of the inverter circuit in order to generate a third-order harmonics in the secondary side circuit, thereby obtaining a lamp current waveform having a trapezoid shape. A ballast capacitor Crb in this case, though being a ballast capacitor, functions as a part of a resonance capacitor.

As a result, as disclosed by the invention of the Hitachi media electronics in Japan, the conversion efficiency of the inverter circuit is considerably improved, and also the step-up transformer is further miniaturized. In addition, this technical idea about the three-time resonance has become the basis of the collector resonance type inverter circuit for a cold cathode fluorescent lamp from recent years up to the present time. Thus, it is not too much to say that the technique concerned is utilized in the great majority of a considerable number of collector resonance type inverter circuits which currently come into world wide use.

In the next stage, one of the invention was disclosed in Japan, whereby more dramatic miniaturization and high efficiency promotion of the step-up transformer have been realized by using the resonant transformer. The present invention began to be widely implemented in about 1996, and thus has greatly contributed to the miniaturization and high efficiency promotion of the inverter circuit used in a note type personal computer. And the present invention highly improved the reliability of the CCFL inverter circuit.The invention concerned is the invention such that the operating frequency of the inverter circuit and the resonance frequency of the secondary side circuit are made nearly agree with each other. The leakage inductance value of the step-up transformer is further increased than in the case of the three-time resonance. And at the same time the capacitance component of the secondary side circuit is increased, thereby realizing the invention concerned.

External links

# [http://www.casemodgod.com/ccfl_inverter_rant.htm One of the trouble] which used the conventional CCFL inverter


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