| Light emitting diode(LED) is recognized as the fourth generation lighting source with the advantages of environmentally friendly, high efficiency, long lifetime and rich in color, etc. A high-quality and high-efficiency LED driver is of crucial importance in order to maximize the advantages of the LED and guarantee the quality and performance of LED lighting. This thesis is dedicated to the optimal control of the bidirectional converter in the electrolytic capacitor-less AC-DC LED driver.The electrolytic capacitor-less AC-DC LED driver is composed of a power factor corretction(PFC) converter and a bidirectional converter. The bidirectional converter is connected in parallel with the output to absorb the second harmonic component in the PFC output current to avoid LED flicker. In this thesis, a buck/boost bidirectional converter is adopted. The voltage ripple of the storage capacitor in the bidirectional converter is intentionally designed to be high to reduce the capacitance, so that film capacitor is used instead of electrolytic capacitor to prolong the lifespan. To ensure proper operation of the buck/boost bidirectional converter, the instantaneous voltage of the storage capacitor must be always higher than the output voltage of the LED driver at any load. Since the voltage of the storage capacitor is related to the output power, an adaptive voltage control scheme for the bidirectional converter is proposed to decrease the storage capacitor voltage as the load becomes lighter, thus obtaining higher efficiency throughout the load range. Two implementation methods for the adaptive voltage control scheme are presented, including the adaptive average voltage control and the minimum voltage control. Hence, the mimimum value of the storage capacitor voltage is reduced to a value which is a little higher than the output voltage of the LED driver. Thus, the swithing loss and core loss can be minimized. In addition, the mathematical model of the bidirectional converter is analyzed and a small-signal model is obtained. The closed-loop design of the bidirectional converter is performed based on the model.A 33.6 W prototype has been built and tested in the lab. The experiment results are presented to verify the effectiveness of the adaptive control scheme for the bidirectional converter and the light-load efficiency is improved. Besides, the small signal model of the bidirectional converter is validated. |
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