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. Resonant transformer and ultra-small CCFL-Inverter 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.
ワイヤレス給電問題解決のカギはCCFLインバータ回路の歴史の中に隠されている
●初期のnon-leakage type transformerの時代は二次側の共振を利用せず、Baxandall oscillatorの時代。効率は良くない。
●CCFLインバータ回路は電圧共振回路(Baxandall oscillator)の一次側駆動と二次側の直列共振の組み合わせから始まる。(1993年頃〜螢謄ノリウム)
■ワイヤレス給電においてはオークランド大学とダイフク(Baxandall oscillator)から始まる。(1993年頃)
※ともにP-P回路方式に相当して二次側共振は直列共振周波数を使う
●一次側の電圧共振回路(Baxandall oscillator)は二次側の直列共振周波数と相性が悪いことに気づく(平成8年通産省補助事業実験)
■オークランド大学ジョンボーイズも電圧共振回路(Baxandall oscillator)と二次側共振との相性が悪いことをチュートリアルの中で述べている。
●CCFLは一次側を固定周波数駆動にして二次側共振周波数と合わせる。(他励型)N-P方式に該当。(2000年頃〜O2Micro,テクノリウム)
■WiTricityは駆動周波数を85KHzに固定するようにライセンス契約書で強要する。(2023年現在)←WiTricityはイマココ (CCFLインバータの他励型に相当)
●CCFLは電流共振型回路にして一次側電流位相検出で二次側共振周波数をトラッキングする。S-S方式に該当 (2000年頃〜Monolithic Power Systems)
●PLLで同期を取る方法をO2Microとテクノリウム-富士電機でそれぞれ独自に検討。応答速度が遅いのでリリースにならず。←ワイヤレス給電の研究アプローチはイマココ
●応答速度ではPLL方式よりも電流位相で直接スイッチングする方法に軍配。(2001年頃〜Monolithic Power Systems)
US6114814の図
一次側検出電流共振型 ZCS-ONのSwitching方式
ワイヤレス給電の次の通過点は必ずここに来る
●CCFLは一次側の共振が二次側の共振周波数のトラッキングの障害になることを次第に認識し始め、一次側共振を止める。S-S方式からN-S方式に移行という意味。(2001年頃〜Monolithic Power Systems)
●Linfinityは一次側の共振が有効と主張して意見対立。そのまま取り残される。
●CCFLは二次側共振から位相フィードバックする電流共振型回路に行き着く。N-S方式+電流共振回路に相当。(2007年〜テクノリウム)
■ワイヤレス給電は途中のS-S方式(CCFLインバータでいえば他励型:2000年頃に相当)で停滞している、という状態である。これはCCFLインバータ回路における"Over the following decade, it became apparent that resonance on the primary side was unnecessary."の最中に該当する。

●ワイヤレス給電は今後、送電側と受電側との両側共振から受電側共振のみの二次側共振へ
ワイヤレス給電は、電力を送る側と受ける側の間に物理的な接続を必要としない技術である。
 ワイヤレス給電の方式には、送受電側の両方に共振器回路を配置する両側共振と、受電側だけに共振回路を配置する二次側共振がある。両側共振は送受電間の距離が長くても効率が高いと言われる技術であるが、駆動周波数やコイルパラメータの調整が難しい。二次側共振は送受電間の距離が近い場合、効率で両側共振よりも有利になる。
 このような方式の選択は、冷陰極管インバータ回路の技術の発展史と似ている。冷陰極管インバータ回路は、液晶ディスプレイやバックライトなどに使われていた電源回路である。
初期の回路では、二次側回路の共振を利用していなかったが、その後一次側、二次側とも共振する回路に移り、最後には二次側回路の共振だけを利用する方が効率や安定性が良いことがわかるという経緯を辿った。
 この経緯を見れば、ワイヤレス給電も同じように二次側共振だけを利用する方が良いことがわかる。その理由は、N-S方式やN-P方式と呼ばれる二次側共振の方式が最も優れているからである。N-S方式とは、送電側は共振させず、受電側の共振コンデンサを受電側のコイルを直列に接続する方式である。N-P方式とは、送電側は共振させず、受電側の共振コンデンサを受電側のコイルと並列に接続する方式である。これらの方式は電流共振回路と組み合わせることによって、送受電間の距離や位置に関係なく高い効率を実現できる。
ワイヤレス給電は今後さまざまな分野で応用される可能性がある。その普及のためには先行する事業(CCFLインバータ回路)の先例に準じて最適な方式を選択し、実用化に向けた開発を急ぐ必要がある。N-S方式かN-P方式がその答えであり、S-S方式やS-P方式は回り道の途中経過に過ぎない。