Compared with traditional lighting technologies such as tungsten bulbs, fluorescent tubes and LEDs, OLEDs have the advantages of having slim physical sizes, wide color variety, and physical bendability. These characteristics of OLEDs avail the technology to many new applications, such as decorative and effect lighting, which were not practically achievable in the past. For example, an OLED lighting module, without the accompanying power convertor and mechanical supporting structure, may have a thickness of less than 1 mm. On the other hand, its relatively high price at the present limits its application in general lighting purposes.
Similar to LED lighting and unlike tungsten bulbs and fluorescent tubes, OLED lighting requires to be driven by electric current as power source and brightness control. Therefore, a power converter is required to provide a controlled electric current. In the applications of decorative OLED lighting, the power sources are often 12-48V DC voltage sources. A power converter is then used to convert the constant voltage to a controlled constant current. A conventional power converter uses a single-phase buck current converter as shown in FIG. 1. It converts a direct current (DC) voltage to the controlled constant current.
To maintain the slimness and bendability of an OLED lighting panel (i.e. within 1.5 mm thickness including mechanical supporting structure), the power converter needs to be thin as well. Otherwise, the power module cannot be integrated into the OLED panel and must be installed separately and connected to the OLED panel with long wires. Long wires carrying constant current are not desirable as there can be substantial energy loss in the transmission. More importantly, the wires of individual panels have to be separated due to its constant current nature, and cannot be combined to save space and cost.
To build a 1.5 mm thick power module, all components needed to be 1.0 mm or less in height as 0.5 mm is normally reserved for the flexible printed circuit board (PCB) and mechanical mount. Among the electronic components within a power module, inductors with low profiles are the least available components. For instance, a normal surface mount 15 uH 0.6 A inductor already has a thickness of 1 mm. As another example, a low-profile 60 uH 1.2 A inductor has a thickness of 5 mm, while an inductor with a normal profile is even thicker.
On the other hand, an OLED lighting panel usually has a large surface area (i.e. 100 mm×100 mm is commonly found and 300 mm×100 mm is forthcoming) that can easily conceal a power module there behind. This means that there are more space allowance in the length and width dimensions, but very limited in the height dimension for the power module. As such, the inductor's thickness is the critical factor, and there is no practical solution in the art currently to enable slim power modules for OLED lighting due critically to the thickness of normal inductors.
A possible but undesirable solution is to replace one thick inductor by multiple smaller thin inductors, so that the thickness of the power converter can be reduced in the expense of having a larger occupying area in the length and width dimensions. For example, as shown in FIG. 2, the 60 uH 1.2 A inductor is replaced by a network of sixteen 15 uH 0.6 A inductors. Since the height of each 15 uH 0.6 A inductor is only about 1.0 mm, the power converter with this network of inductors may meet the thickness requirement. However, the total magnetic material used is doubled as compared with the single 60 uH 1.2 A inductor. Also, the much higher component count increases material cost and assembly cost significantly. The amount of magnetic material used is a major factor affecting the size of the inductors.