| Summary: | LED is widely used in many applications, such as indoor lighting, backlighting and street lighting, etc. For these application, multiple LED strings structure is adopted for reasons of cost-effectiveness, reliability and safety concerns. Several methods and topologies have been proposed to drive multiple LED strings. However, the output current balance and efficiency are always the two major concerns for LED driver.
A simple two-stage multi-channel LED driver is proposed. It is composed of a buck converter as the first stage and a multi-channel constant current (MC3) CLL resonant converter as the second stage. For the CLL resonant converter, the magnetizing inductance of the transformer can be as large as possible. Therefore, the magnetizing current of the transformer has little influence on the output currents. In addition, the currents of two LED strings driven by the same transformer is balanced by a DC blocking capacitor. As a result, the current balance among LED strings is very good, even if the load is severely unbalanced. Meanwhile, the current flowing through the external inductance Lr1, instead of the magnetizing current is used to help the primary-side switches to achieve ZVS. Therefore, large magnetizing inductance is good for current balance and properly designed Lr1 is helpful for ZVS achievement. These properties of MC3 CLL are preferred to drive multi-channel LED strings.
In the design procedure of MC3 CLL resonant converter, the parasitic junction capacitor of the secondary-side rectifier is taken into account. It influences the operation during dead time significantly when the voltage step-up transformer is applied. The junction capacitors of the secondary-side rectifiers, and the output capacitors of the primary-side switches will resonate with the inductor Le2 during the dead time. Finally, this resonance impact the ZVS achievement of the primary-side switches. Therefore, the inductors Lr1 and Le2 should be designed according the charge needed to achieve ZVS with considering the resonance.
Additionally, the control strategy for this two-stage structure is simple. Only the current of one specific LED string is sensed for feedback control to regulate the bus voltage, and the currents of other LED strings are cross-regulated. Furthermore, the MC3 CLL is unregulated and always working around the resonant frequency point to achieve best efficiency. The compensator is designed based on the derived small signal model of this two-stage LED driver. Due to the special electrical characteristics of LED, the soft start-up process with a delayed dimming signal is adopted and investigated. With the soft start-up, there is no overshoot for the output current.
Finally, a prototype of the two-stage LED driver is built. The current balance capability of the LED driver is verified with the experiment. Good current balance is achieved under balanced and severely unbalanced load condition. In addition, the efficiency of the LED driver is also presented. High efficiency is guaranteed within a wide load range. Therefore, this two-stage structure is a very promising candidate for multi-channel LED driving applications. === Master of Science
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