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 electronic devices powered by direct current sources such as batteries. CCFL inverters are small, have a power conversion efficiency over 80%, and offer adjustable light output. They are widely used for backlights for LCDs or for rear lighting in advertising signs.
History
Voltage resonant self-excited type circuits have been widely used as inverter circuits for cold cathode fluorescent lamps. This is sometimes referred to as the "Royer circuit".[1] 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.[2]
In the early designs of inverter circuits for cold cathode fluorescent lamps, the resonance method of a secondary circuit was not utilized at all. Instead, a so-called non-leakage type transformer with small leakage inductance was used as a step-up transformer. The size of transformers in inverter circuits during this era was large compared to the power they handled, and their power conversion efficiency was not so good.
Leakage inductance (more precisely, short-circuit inductance) was disliked because it caused a reduction in the output voltage on the secondary side of the transformer. This was considered undesirable, and it was thought to be necessary to reduce it as much as possible. At the same time, the parasitic capacitance of the LCD backlight was also disliked as it caused a reduction in the lamp current.[3] In the next era, methods were invented to dramatically improve efficiency by resonating these disliked components with each other.[4][5]
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. 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. [6] This piezoelectric inverter circuit is very small and, at the same time, has very high efficiency compared to the CCFL inverter circuit that uses a non-leakage transformer. As a result, it was expected to be widely used as the LCD backlight inverter for notebook PCs. However, this inverter method is extremely expensive, and there are many failures due to damage of the piezoelectric element, so it was gradually replaced by the winding-type ultra-small inverter circuit which is low-cost, high-efficiency, and highly reliable.
Ultra-small inverter
In 1992, a Japanese inventor discovered the power factor improvement effect, which led to a significant reduction in the size of the CCFL inverter circuit. This effect is achieved by resonating the secondary side of the step-up transformer, which reduces the magnetization current of the primary winding and allows for a drastic reduction in the number of turns in the primary winding. Moreover, the ferrite core can be uniquely shaped so that the magnetic path is elongated and the cross-sectional area is smaller compared to a standard non-leakage transformer.[7]
Over the following decade, it became apparent that resonance on the primary side was unnecessary.[8] Several IC manufacturers then competed to develop new methods, resulting in the creation of two types of drive ICs that used resonance only on the secondary side. 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.[9][10][11]
For the room illumination
For room illumination purposes, CCFL technology has been increasingly adopted due to its high efficiency, comparable costs to compact fluorescent lighting, long lifespan, and eye-friendly light emission. Inverter circuits based on external excitation or current resonant circuits inherited from LCD backlight technology are utilized to power the CCFL. The current resonant circuit utilizes current phase feedback to track changes in the resonant frequency of the secondary side caused by human interaction and adjust the driving frequency accordingly to maintain stable lighting performance.
See also
External links
- Example of the conventional type (using the big cube shape transformer):Jim Williams, "A fourth generation of LCD backlight technology: Component and measurement improvements refine performance", Linear Technology Application Note 65, November 1995.
- Example of the advanced type (using the small elongated shape transformer):OZ960 Notebook Inverter Schematic in the datasheet, 2002
References
- ^ Royer Oscillator Circuit United States Patent 2783384, 1954
- ^ P.J. Baxandall, "Transistor Sine-Wave LC Oscillators", International Convention on Transistors and Associated Semiconductor Devices, 25 May 1959, fig 5, p. 751
- ^ Jim Williams, "A fourth generation of LCD backlight technology: Component and measurement improvements refine performance", Linear Technology Application Note 65,P7, November 1995.
- ^ Inverter circuit for use with discharge tube United States Patent 5495405, 1993,"Also, since a capacitive component and an inductive component in the secondary side circuit of the step-up transformer cancel out with each other, the power factor is improved so that a reactive current flowing through the primary winding (collector winding) of the step-up transformer is reduced and a loss due to a copper loss is decreased and the efficiency of the inverter circuit becomes higher."
- ^ JIEC seminar, LCD backlight inverter and measures against parasitic capacitance, held on May 20 1997 (In Japanese)
- ^ 50 Years Of Piezoelectric Transformers. Trends In The Technology[1][2]
- ^ Miniaturization of the Inverter for Cold-cathode type Fluorescent-lamp using the Inductance Isolation Effect in the Transformer.[3], Jan. 1997
- ^ Begun the recognition of the secondary resonance principle (although it was incomplete): LX1686 Direct Drive CCFL Inverter Design, Fig 2,2000
- ^ Background of lawsuit of MPS v. O2Micro[4], November 2006.
- ^ Technology Trends for Small Backlight Inverters (Special Feature Backlight Technology) in Japanese [5], April 2003
- ^ The latest trends in LCD backlight inverters and Phase-synchronous coupling transformer type inverter in Japanese