Design and Control of a Single-Stage LED Driver with Low Output Current Ripple

Abstract

Light emitting diode (LED) has the merits of low energy consumption, environmental harmless, long life-time utilization, and so on. It is a new generation of lighting technology after the inventions of the incandescent bulb, fluorescent tube, and high intensity discharge (HID) lamp [1]-[6]. A driver with a high power factor, long-lifetime utilization, low-cost, and low-package size is a key technology that should be developed for LED lamps [7]-[12]. Nowadays in the market, the driver for LED lamps usually cannot match the requirement mentioned above. Therefore it is meaningful to study and focus on this topic. One of the more complex aspects topics of LED drivers is the single-stage power converter with low output current ripple. Typically, more output capacitors could be installed in order to obtain the low output current ripple. However, the package size of LED lamp does not permit it. A high current ripple usually leads to a high light flicker, due to the characteristics of LED. For incandescent bulbs, high light flicker does not occur because of after-glow [13]-[14]. LED lamps have disadvantages when compared with the traditional lighting-source. Even a high light flicker with low frequency is harmful to peoples’ eyes [15]-[16]. Therefore, a driver circuit with a low output current ripple and small output capacitor value should be discovered [17]-[24]. Usually, the output capacitors are electrolytic ones, whose life-time is around several thousand hours. If we use other kinds of capacitors, such as ceramic ones, whose life-time is longer than that of electrolytic one, then the whole package of the LED lamp could be named as a long life-time LED lamp. However, the capacitance of ceramic capacitors is generally below 10 uF. An output capacitor with such a small value cannot filter the low frequency current ripple in the conventional single-stage power converter topology, such as Flyback. To solve this problem, a new topology or control scheme of power converters should be studied. Another advanced topic for LED drivers is the digital controlled single-stage power converter [25]-[32]. In conventional converters, the control chip of the driver circuit is usually analog. The analog chip manufacturers design the control chip and offer the application solution. The power converter circuit controlled by digital chips has the advantages of programmability, low sensitivity to variations and noise, low-costs of design and devices. It is the future trend of power converter circuit design. In the first part of this dissertation, the problem of large output current ripple or large light flicker is mentioned. A standard of Energy Star® for light flicker is introduced. Then a full scope of the nowadays single-stage power converter circuit for LED lamps is reviewed. A survey study for previously proposed single-stage power converter circuits with low output current ripple is also conducted. The disadvantages of pre-existing method for solving this problem are fully explained. The purpose and originality of our study, namely achieving a single-stage single-switch PFC circuit compatible with the primary side control, for low LED current ripple, is offered in the end of this part. Then, a new circuit with small output current ripple is proposed and theoretical analysis of the circuit based on a small-signal model is conducted. Through the theoretical analysis it can be found what are the factors determined the output current ripple. An experimental board was built for verifying the theoretical results. The experiment results demonstrate that the proposed circuit could achieve a huge output current ripple reduction, which is now below 15% compared with the conventional one with the current ripple around 80%. Also the power efficiency of proposed circuit could be above 85% and the power factor could be above 0.9, which means our target is met. A design guideline for this circuit is also presented. Finally, the digitally controlled LED driver was proposed. The digitally controlled method was applied to the single-stage power factor correction (PFC) circuit proposed. Basic information about the digital control chip is introduced. A design procedure of digital controller for dual loop control is provided. A microchip microcontroller is programmed to realize the control algorithm. The experimental prototype is also built and tested. The circuit achieves a power factor above 0.9 and power efficiency above 80%. The proposed circuit has a value to be a product in the healthy lighting market, for its lower output current ripple, which means lower light flicker. Meanwhile it maintains the basic requirement for single-stage PFC LED driver, such as at least 85% power efficiency and at least 0.9 power factor. It is also compatible with the primary side control, which is popular among the current commercial application. In a word, it is a good solution for LED lighting company. |LED 는 낮은 에너지 소비 및 환경적 무해성, 긴 수명 동의 장점들을 가지고 있다. 이 기술은 백열 전구와 형광등, HID 맴프의 말띵에 이은 차세대 조명 기술이다. 고역률 및 긴 사응시간, 저바응, 작은 패키지 사이즈를 가진 드라이버는 LED 램프를 위해 반드시 개발되어야 하는 핵심 기술이다. 요즘 시장에 나온 LED 램프용 드라이버는 대부분 위에 제시한 요구조건을 총족하지 못한다. 따라서 이 주제에 대해 초점을 맞췄다.

낮은 출력 전류 리플을 가진 단상 컨버터는 LED 드라이버에 관련된 상당히 복잡한 주제돌 중 하나이다. 일반적으로 낮은 출력 전류 리플을 확보하기 위해서 더 많은 출력 커패시터롤 설지한다. 그러나 LED 램프는 패키지 크기 때문에 그럴 수가 없다. 일반적으로 높은 전류 리폴은 LED 의 특성으로 인해 플리커 현상을 초래한다, 백열 전구는 잔광으로 인해 플리커 현상이 발생하지 않는다. LED 램프는 기존의 조명과 비교할 때, 여러 단점들을 가지고 있다 심지어는 빈도가 낮더라도 플리커 현상이 발생하면 사람의 눈에 해로울 수 있다.

따라서 낮은 출력 전류 리풀과 작은 출력 커패시터 값을 가진 드라이버가 개발되어야 한다, 일반적으로 출력 커패시터는 전해 커패시디이고, 수명은 수천 시간에 달한다, 전해 커패시터보다 수명이 긴 다른 종류의 커패시터를 사용한다면 긴 수명의 LED 램프가 될 수 있다. 그러나 일반적으로 세라믹 커괘시터의 정전용량은 10 uF 이하이디. 이런 낮은 값의 출력 커패시터로는 Flyback 컨버터와 같이 기존의 단상 토플로지에서 발생하는 낮은 주파수의 전류 리플을 줄 일수 없다. 이 문제를 해결하기 위해서는 컨버터의 새로운 토폴로지 또는 제어 기범이 연구되었다. LED 드라이버에 관한 또 하나의 주제로는 디지털 제이 단상 컨머터가 있다. 기존 컨버터의 컨트롤러는 일반적으로 아날로그 방식이다. 아날로그 컨트롤러는 제조엄체들이 컨트룰러를 설계하고 어플리케이션 솔루션을 제공한다. 디지털 침이 제어하는 컨버터 희로느 프로그래밍 가능성, 그리고 변화 및 노이즈에 대한 낮은 민감도, 낮은 설계 비용 등 여러 장점을 가지고 있다. 따라서, 전력 변환 회로 설계에 있어 새로운 방식이 되었다

본 논문은 최근 LED 램프용 단상 컨버터 희로에 대한 모든 촉면둘을 검토한다. 앞에서 제안한 낮은 출력 전류 리플을 가진 단상 컨버터에 대한 조사 연구도 진헹하였디.

그리고 낮은 출력 전류 리플을 가진 새로운 희로를 제안하였다. 또한 소신호 모텔에 기반한 이론적 분석을 제시하떴디. 본 논문에서 제안한 이론을 검중하기 위해 실험용 보드를 제작하었디. 이 희로에 대한 설계 가이드라인 또한 제시하였디.

마지막으로 디지털 방식으로 제어되는 LED 드라이버롤 제안하었디. 이 디지딜 제어 방식욘 제안 된 단상 역률 개선 희로에 적용하었다. 마이크로 컨트롤러는 해당 체어 알고리즘을 구현하기 위해 프로그래밍 하였다. 실험용 시작픔도 제작 후 테스토하었디.; Light emitting diode (LED) has the merits of low energy consumption, environmental harmless, long life-time utilization, and so on. It is a new generation of lighting technology after the inventions of the incandescent bulb, fluorescent tube, and high intensity discharge (HID) lamp [1]-[6]. A driver with a high power factor, long-lifetime utilization, low-cost, and low-package size is a key technology that should be developed for LED lamps [7]-[12]. Nowadays in the market, the driver for LED lamps usually cannot match the requirement mentioned above. Therefore it is meaningful to study and focus on this topic. One of the more complex aspects topics of LED drivers is the single-stage power converter with low output current ripple. Typically, more output capacitors could be installed in order to obtain the low output current ripple. However, the package size of LED lamp does not permit it. A high current ripple usually leads to a high light flicker, due to the characteristics of LED. For incandescent bulbs, high light flicker does not occur because of after-glow [13]-[14]. LED lamps have disadvantages when compared with the traditional lighting-source. Even a high light flicker with low frequency is harmful to peoples’ eyes [15]-[16]. Therefore, a driver circuit with a low output current ripple and small output capacitor value should be discovered [17]-[24]. Usually, the output capacitors are electrolytic ones, whose life-time is around several thousand hours. If we use other kinds of capacitors, such as ceramic ones, whose life-time is longer than that of electrolytic one, then the whole package of the LED lamp could be named as a long life-time LED lamp. However, the capacitance of ceramic capacitors is generally below 10 uF. An output capacitor with such a small value cannot filter the low frequency current ripple in the conventional single-stage power converter topology, such as Flyback. To solve this problem, a new topology or control scheme of power converters should be studied. Another advanced topic for LED drivers is the digital controlled single-stage power converter [25]-[32]. In conventional converters, the control chip of the driver circuit is usually analog. The analog chip manufacturers design the control chip and offer the application solution. The power converter circuit controlled by digital chips has the advantages of programmability, low sensitivity to variations and noise, low-costs of design and devices. It is the future trend of power converter circuit design. In the first part of this dissertation, the problem of large output current ripple or large light flicker is mentioned. A standard of Energy Star® for light flicker is introduced. Then a full scope of the nowadays single-stage power converter circuit for LED lamps is reviewed. A survey study for previously proposed single-stage power converter circuits with low output current ripple is also conducted. The disadvantages of pre-existing method for solving this problem are fully explained. The purpose and originality of our study, namely achieving a single-stage single-switch PFC circuit compatible with the primary side control, for low LED current ripple, is offered in the end of this part. Then, a new circuit with small output current ripple is proposed and theoretical analysis of the circuit based on a small-signal model is conducted. Through the theoretical analysis it can be found what are the factors determined the output current ripple. An experimental board was built for verifying the theoretical results. The experiment results demonstrate that the proposed circuit could achieve a huge output current ripple reduction, which is now below 15% compared with the conventional one with the current ripple around 80%. Also the power efficiency of proposed circuit could be above 85% and the power factor could be above 0.9, which means our target is met. A design guideline for this circuit is also presented. Finally, the digitally controlled LED driver was proposed. The digitally controlled method was applied to the single-stage power factor correction (PFC) circuit proposed. Basic information about the digital control chip is introduced. A design procedure of digital controller for dual loop control is provided. A microchip microcontroller is programmed to realize the control algorithm. The experimental prototype is also built and tested. The circuit achieves a power factor above 0.9 and power efficiency above 80%. The proposed circuit has a value to be a product in the healthy lighting market, for its lower output current ripple, which means lower light flicker. Meanwhile it maintains the basic requirement for single-stage PFC LED driver, such as at least 85% power efficiency and at least 0.9 power factor. It is also compatible with the primary side control, which is popular among the current commercial application. In a word, it is a good solution for LED lighting company.