CCFL inverter
A CCFL inverter is an electrical inverter that supplies alternating current power to a cold cathode fluorescent lamp (CCFL). CCFLs are often used as inexpensive light units in electrical devices that are powered by direct current sources such as batteries. CCFL inverters are small, have power conversion efficiency over 80%, and offer adjustable output of light. They are widely used for backlights for LCDs, or for rear lighting in advertising signs. History. As for the inverter circuit of a cold cathode fluorescent lamp, voltage resonant self excited type circuit has been widely used. This is sometimes referred to  as the "Royer circuit". However the proper definition of the Royer circuit requires that the inversion of a switching operation be performed in a state in which the transformer is saturated. An inverter circuit which performs the inversion operation by utilizing resonance in the collector circuit of a transistor is preferably referred to as the "Baxandall converter" in distinction from a true Royer circuit. The early designs of an inverter circuit for a cold cathode fluorescent lamp did not utilize the resonance method of a secondary circuit at all. Instead the so-called non-leakage type transformer having a small  leakage inductance was used as a step-up transformer. Transformers size in inverter circuits of this era were large compared to the power they handle, and their power conversion efficiency was not so good. The leakage inductance (more precisely, short-circuit inductance) was hated as it reduced the output voltage on the secondary side of the transformer. This was not desirable and was thought and believed to need to be as small as possible. At the same time, the parasitic capacitance of the LCD backlight was also hated as a cause of reduced the lamp current. In the next era , methods were invented to dramatically improve efficiency by resonating these hated components with each other. Development of LCD backlight inverters and piezoelectric inverter circuits. With the development of liquid crystal display technology, the CCFL inverter circuit was also required to be miniaturized. However, in an inverter circuit using a conventional non-leakage transformer, when trying to miniaturize the transformer, the magnetization current of the primary winding increases and heat generation increases. So It was thought that there is a limit to the miniaturization of the transformer. Therefore, major electric manufacturers competed to develop inverter circuits for CCFL using a piezoelectric  transformer which invented by Rosen, and the miniaturization of the inverter circuit was about to be realized. This piezoelectric inverter circuit is very small and at the same time, it is very high efficiency compared to the CCFL inverter circuit that uses a non-leakage transformer. As a result, it was seen that it will be widely used as the LCD backlight inverter for notebook PCs. But, this inverter method is extremely expensive, and there are many failures due to damage of the piezoelectric element, so it was gradually replaced by winding-type ultra-small inverter circuit which is low -cost, high-efficiency and high reliability. Ultra-small inverter. When the power factor improvement effect was discovered by a Japanese individual inventor in 1992, the CCFL inverter circuit was dramatically downsized. This is the effect that when the secondary side of the step-up transformer is resonated, the magnetization current of the primary winding is reduced, which eliminates to makes it possible to drastically reduce the number of turns of the primary winding. In addition, the ferrite core can be made into a unique shape, that the magnetic path is very long and the cross-sectional area is small compared  to the standard non-leakage transformer. In the next 10 years from such a invention, it became clear that resonance on the primary side was unnecessary, several IC manufacturers competed for development, and finally two types of drive ICs using only the resonance on the secondary side were developed. With O2 Micro International and MPS (Monolithic Power Systems), two drive methods called external excitation type and current resonant type combined with this elongated shape transformer had become widespread. And they became occupied almost 100% of the world's notebook LCD backlight inverters. For the Room lighting. Due to its good  efficiency, CCFL technology has expanded into room lighting. Costs are similar to current compact fluorescent lighting, it has a long life, and the light emitted is easier on the eyes. And the external excitation circuits or current resonant circuits inherited from the technology of LCD backlights are used as inverter circuits to light them. In the current resonant circuit that is utilizing current phase feedback, the resonant frequency of the secondary side is automatically tracked when the parasitic capacitance which caused around the CCFL changes due to the approach of a human hand, and the stable lighting state of the  CCFL is kept.