Research On Passive Precise Current Sharing Method Of Multi-output LED Driver

As a new and efficient solid light source, Light-emitting diode (LED) has significant advantages of long life, high efficacy, environmental friendliness, etc. LED is expected to be the fourth generation of electric light souce after the thermal radiation light source, the low-pressure gas discharge lamp and the high intensity discharge (HID) lamp. With the increased attention to energy saving and environmental protection, the LED lighting market is rapidly expanded and looks to be an irreversible tendency in the near future. The rated power of individual LED module is limited due to the packaging technology and the thermal management limitation. Thus a large number of LEDs are required in a single system to achieve expected luminance value. It is important to keep the current in each LED identical for the same luminance and thermal performance. The series structure is the easiest way to balance the current in LEDs. But this will result in low reliability because the whole system will be invalid once one LED occurs open circuit failure. Meanwhile it leads to high voltage stress and safety issue. But the current sharing ability is poor in LEDs because the LED’s exponential voltage-current characteristic and the negative temperature coefficient of its forward voltage drop, thus current balancing technique is necessary in multi-string LEDs. The feeding bus of the LED driver is divided into DC bus and AC bus in this paper. Improved passive precise current sharing method is proposed for high power and medium to low power LED lighting application respectively by comparing the current sharing ability, cost, loss and convenience of expansion, etc.A new rectifier for multi-output LED driver is proposed based on directly charge exchanging and balancing principle in the secondary capacitors for high power LED dirver. The proposed rectifier structure has the minimum quantity of secondary charge balancing capacitors. The charge balancing principle guarantees the current sharing between two neighboring LED connected to the capacitor. The charge exchanging between adjacent capacitor ensures the precise current sharing among each LED module. It has the advantanges such as low cost, high precision, high reliability, etc. It is appropriate for the topologies in which the primary side could equal to an AC current source.Based on the proposed current sharing method, a four elements LLCC resonant topology is proposed by combining the LLC topology and smaller DC blocking capacitance. The DC blocking capacitor is connected with the LLC resonator in series and used as a resonant element. The capacitance is smaller and the cost is reduced in the proposed topology. There is no DC bias in magnetizing current because of the capacitor on the secondary side. Thus an asymmetrical PWM control LLCC resonant converter is proposed. Because the frequency is much higher in light load condition for PFM (Pluse-Frequency Modulation) controlled LLCC resonant topology. The PWM control strategy could achieve higher efficiency over wider output voltage and current range.In medium to low power LED lighting application, the series-cascaded transformer structure is proposed for flyback converter. The problems of complex electrical connections in secondary side and inconvenient quasi-resonant control are solved when the series-cascaded transformer structure is adopted in flyback topology. The first stage transformer transfer the energy while the second stage transformers realize current sharing by series primary windings. A more accurate current sharing method is proposed for the series-cascaded transformer structure in flyback converter. Parallelling capacitor across the rectifier diode could decrease the current deviation by utilizing the resonant between the capacitor and the magnetizing inductor of the second stage transformer. Precise current sharing is achieved by the proposed structure and method. Meanwhile the control of primary side and the electrical connection is simplified, thus the cost is decreased.