A transformer has become an essential electronic component for voltage regulation into required voltages for various kinds of electric appliances.
In the power supply system of the new-generation electric products such as LCD televisions, leakage inductance transformers (e.g. LLC transformers) become more and more prevailing. The use of the leakage inductance transformer may reduce damage possibility of the switch, minimize noise and enhance performance.
FIG. 1 is a schematic exploded view of a conventional leakage inductance transformer. As shown in FIG. 1, the transformer 1 comprises a bobbin 11, a covering member 12, and a magnetic core assembly 13. A primary winding coil 111 and a secondary winding coil 112 are wound around the bobbin 11. The output terminals 113, 114 of the primary and the secondary winding coils 111, 112 are directly wound and soldered on pins 115, which are perpendicularly extended from the bottom of the bobbin 11. The covering member 12 is used for partially sheltering the upper portion of the bobbin 11 in order to increase the creepage distances between the primary winding coil 111, the secondary winding coil 112 and the magnetic core assembly 13. The magnetic core assembly 13 includes middle portions 131 and leg portions 132. The middle portions 131 are accommodated within a channel 116 of the bobbin 11. The bobbin 11 is partially enclosed by the leg portions 132. Meanwhile, the transformer 1 is assembled.
Please refer to FIG. 1 again. After the covering member 12 is placed over the bobbin 11 to shelter the bobbin 11, the creepage distances between the primary winding coil 111, the secondary winding coil 112 and the magnetic core assembly 13 are increased. The use of the covering member 12, however, increases the overall height of the transformer 1. In addition, the required inductance is determined according to the turn numbers of the primary winding coil 111 and the secondary winding coil 112. If the diameter of the primary winding coil 111 or the secondary winding coil 112 is too large, the overall volume of the transformer 1 is increased as the turn numbers are increased. That is, it is difficult to minimize the conventional transformer 1.
In addition, after the transformer 1 is assembled, an air gap (not shown) is defined between the corresponding leg portions 132. The air gap is formed between the primary winding coil 111 and a secondary winding coil 112. If the secondary winding coil 112 is in a short-circuit condition, the magnetic path possibly causes individual loop. Under this circumstance, the leakage inductance of the transformer 1 fails to be stably controlled.
Due to that the bobbin 11 of the transformer 1 has only one single-trough first winding section for winding one primary winding coil 111 and one single-trough second winding section for winding one secondary winding coil 112, and the bobbin 11 has limited space for winding coils, and the turn numbers of the primary winding coil 111 and the secondary winding coil 112 are also limited. Therefore, the transformer 1 has a limited maximum power output of 100 Watts. When a power converter having a relatively high power output of 400 Watts is designed, four transformers 1 connected in series or in parallel must be employed in the power converter. It is obvious that the manufacturing cost of the power converter will be increased due to the usage of four set of bobbins 11 and four set of magnetic core assemblies 13 of the four transformers 1.