New high-efficiency offline LED drivers with transformer coupled recycle snubber

Abstract

LEDs are rapidly becoming a popular lighting solution for offline application due to their high lighting ef-ficacy and environmental friendliness. This thesis focuses on high-efficiency offline LED drivers.

In Chapter II, a high-efficiency offline isolated LED driver based on a flyback converter is presented. Among the various isolated topologies, the single-stage flyback converter is widely used for residential lighting applications due to its simplicity and low cost. A well-known problem associated with flyback con-verters is that the leakage inductance energy of the transformer leads to high voltage stress at turn-off. Ac-cordingly, a snubber network is usually adopted in a flyback converter to protect the power switch. The pas-sive RCD snubber is widely used owing to its simplicity. However, the leakage inductance energy is dissipative and the turn-off switching of the power switch involves hard switching, which limit the efficiency. On the oth-er hand, the leakage inductance energy is recycled and the turn-off ZVS operation is fully performed with LC snubber with an intermediate voltage source. In spite of its good performance, additional components hinder the adoption of LED drivers. Accordingly, a novel passive turn-off snubber with a simple structure which supports the soft-switching of the power switch as well as the recovery of the leakage inductance energy is proposed. It is termed a transformer coupled recycle snubber. With the current state of converter topology, control schemes have emerged as another important issue. In order to reduce the complexity and cost, primary-side regulation schemes have been reported. The average LED current is calculated and predicted at the primary side, and the regulation is performed without a direct connection to the secondary side. Conventional PSR flyback-type LED drivers regulate the LED current so as to stabilize the luminance against variations of the forward voltage drop of the LEDs. Regulating the LED power, on the other hand, has the additional advantage of offsetting the LED from the effects of tempera-ture modulation and LED aging. Accordingly, a primary-side power regulating control scheme is proposed for isolated LED drivers as well. Test results of the proposed LED driver show an efficiency level that exceeds 83% (up to 90%) in an out-put range of 5 W to 10 W with an input range of $85 V_{AC}$ to $265 V_{AC}$.

In Chapter III, a high-efficiency offline non-isolated LED driver based on a buck type converter is pre-sented. Non-isolated LED drivers have been studied in recent years, and they are mainly categorized by two types: current source type and switched-mode power supply type. In the current source type LED driver, it has a simple structure and simple operation principle. However, this type of LED driver is hard to use for a wide range of input source voltages. On the other hand, switched-mode power supply type needs some dis-crete components, including an inductor, it maintains high efficiency for a wide input voltage range. Accord-ingly, to achieve high efficiency for a wide input range, a buck type converter is used for the proposed LED driver. In a buck type converter, if no auxiliary circuits are adopted to maintain the simplicity, then the power switch operates on hard switching, which lowers the efficiency. On the other hand, for the soft-switching op-eration, an additional snubber inductor (in passive type) or an additional power switch with relative control blocks (in active type) is necessary, which increases the complexity of the LED driver. Accordingly, in order to enhance the reliability as well as the power efficiency further, the transformer coupled recycle snubber is properly adopted for the soft-switching operation of the power switch. Test results of the proposed LED driver show the efficiency of over 85% (up to 93%) in the output range from 5 W to 10 W with the input range from $85 V_{AC}$ to $265 V_{AC}$.