CCFL inverter
From Wikipedia, the free encyclopedia
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.
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[edit] Characteristics
- Small form and structure
- Switchover efficiency over 80%
- Adjustable light
[edit] 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.
[edit] 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 the 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 stages of the inverter circuit for a cold cathode fluorescent lamp, it 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 as a step-up transformer. In addition, the leakage inductance of the step-up transformer in the inverter circuit was such that it reduced an output voltage on the secondary side of the transformer and was not desirable, and thus was made as small as possible.
As a result, the resonant frequency of the secondary side circuit of the transformer in the past was decided to have no relation with the operating frequency of the inverter circuit. Thus, the resonant 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 generation of technology“ is shown. It isn't used any more. The more recent inverter circuit was invented by Hitachi electronics in Japan, and has 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 the primary side circuit. A step-up transformer with a greater leakage inductance value is suitable for use in this case.
The shape of the transformer which is actually used in the so-called three-time resonance circuit is a 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 larger leakage inductance value.
In any case, this idea (refer to figure of the “past generation of 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 (Lsc 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 harmonic 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 the conversion efficiency of the inverter circuit is considerably improved, and also the step-up transformer is further miniaturized.
In the next stage more dramatic miniaturization and high efficiency promotion of the step-up transformer have been realized by using the resonant transformer. The present design 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 notebook type personal computer.
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.
