Various types of transformers are developed for adapting different functional demands, so that when designing transformer, it should consider many properties, such as, transformer ratio, current and loss, and dimension, for example, as the output power and the properties of the transformer are qualified, naturally, the smaller the size, the more different kinds of electric equipment the transformer can be adapted thereto. However, as the transformer works, a large amount of current will be generated and the voltage will have an apparent variation, so that the transformer should conform to severe safety regulation before using. And, for conforming to the safety regulation, on one hand, the transformer is always enlarged for providing sufficient insulation distance for the coils, and thus, obviously, under the same electric characteristic, the miniaturization of the transformer is restricted by the safety regulation. On the other hand, for conforming to safety regulation, most power supply select isolation transformer, such as, TWP No. M314404, entitled “Driving device with isolation transformer used in LCD back light source” which is composed of a DC power supply, a square wave switcher, an isolation transformer and a driver transformer. In this patent, the primary side of the isolation transformer is fluctuated by input power, and for controlling the output power, the power at the primary side of the isolation transformer is constantly changed. And, the secondary side which is not connected with the primary side transmits power only through electromagnetic induction for maintaining the stability and quality of the power which is received by the load. Further, the isolation transformer is used to separate the different voltage potentials of the power input terminal and the load terminal so as to ensure the safety thereof. Another example is TWP No. M334349, entitled “Light tube driving device”, in which the light tube driving device includes an inverter, a first transformer, a second transformer, at least one light tube, a control unit and a third transformer. In this patent, FIG. 2 disclosed that two transformers (32, 34) are utilized to transmit energy between the high voltage inverter (30) and the light tube (36) and also to isolate the power fluctuation at the primary and the secondary sides. However, the use of two transformers increases the cost and also the volume.
Consequently, the size and the adaptability of the transformer are restricted by the insulation strength and the power waveform at the primary and the secondary sides.