# A New Circuit Design of AC/DC Converter for T8 LED Tube
## 1 Introduction  
LEDs have a lot of characteristics such as high luminous efficacy, energy saving properties and long lifetimes. These advantages have allowed LED lighting to replace other types of lighting quickly in the marketplace. Furthermore, as regulations for the use of traditional lighting such as incandescent lamps and florescent lamps become increasingly stronger, the market share of LED lighting is expected to increase rapidly [1; 2; 3]. An LED lighting product generally consists of various mechanical parts, an LED module and an LED converter. An LED converter should provide stable and accurate current to the LED module to achieve good optical performance, since the luminance variation of LEDs depends on the variation of the current supplied to LEDs. In addition, the LED converter requires high power efficiency, high power factor (PF), low total harmonic distortion (THD), low total cost, and low light flicker [4; 5]. The light quality of LED lighting is mainly determined by the driving method and the key components of the LED converter. Due to the low circuit complexity and low cost, a single-stage power factor correction (PFC) driving method is usually used in many types of low-power LED lightings, including LED tubes. However, the output current inevitably has a double line frequency ripple and light flicker is generated by variation of the luminance. Various studies show that low-frequency light flicker caused by large output current ripple can adversely affect human health and cause headaches, visual fatigue and epileptic attacks [6; 7; 8; 9]. Figure 1 shows a block diagram of a typical single-stage buck-boost LED converter, which is composed of many function blocks. An AC voltage with 50/60 Hz is supplied to abridge rectifier through a line filter, and a full-wave rectified sinusoidal voltage is supplied to the DC link capacitor \(\text{C}_{\text{LINK}}\). Switching is conducted for power factor correction to obtain high power factor, and smoothing is performed to reduce the output ripple and control the output current [5]. The smoothing to remove the ripple is not perfect, so the single-stage PFC driving method always makes a low-frequency output ripple, and output current varies between a maximum peak and minimum peak [10]. If the light has cyclic variation in the amplitude, percent flicker is very useful to represent the level of light flicker, which is defined as [11]:  
\[\text{Percent Flicker}\left(\%\right)=\frac{A-B}{A+B}\times 100 \tag{1}\]  
where A and B are the maximum and minimum luminance, respectively. The Equation (1) shows that the low percent flicker represents good performance. Several methods can be considered to improve the light flicker caused by the double line frequency output ripple [12; 13; 14; 15]. One of the obvious ways is to increase the link capacitor \(\text{C}_{\text{DC}}\); however, regulatory requirements for PF and THD performance may not be met. Therefore, this method is usually suitable for non-regulated low power LED lighting. Another method is to increase the output capacitance for the smoothing operation. However, a large electrolytic capacitor is needed to remove the output ripple completely, which increases the system size and cost. Therefore, this is not suitable for small lighting applications with small circuit space. Various converter topologies have been proposed to remove current ripple [16; 17; 18]. In [16], a flicker-free LED converter composed of PFC flyback converter and a bidirectional buck/boost converter to reduce the current ripple was proposed to reduce the current ripple. The output current waveform is almost flat, while high power efficiency is maintained [16; 17]. However, due to the high circuit complexity, it cannot be applied to applications such as T8 LED tubes. A multiplexing ripple cancellation was proposed by adding a ripple cancellation unit to achieve flicker-free operation in [18]. Although this solution is very competitive, it is still too large to mount inside a G13 base with a diameter of 28 mm for low-power LED tubes [19].