Single Stage Offline LED Driver

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1 Single Stage Offline LED Driver Jianwen Shao STMicroelectronics 375 E.Woodfield Rd., Suite 400 Schaumburg, IL 6073 Phone: Abstract: A non-isolated soft-switched high power factor offline LED driver is introduced in the paper. The Buck- Boost converter is chosen for this application due to its simplicity and low cost. The converter operates with constant peak current for constant power control, and in transition mode (boundary mode between CCM and DCM) to achieve soft switching. High power factor is achieved by reshaping the peak current nearby the zero crossing of input AC line. Keywords single stage, LED driver, buck-boost converter, softswitching I. INTRODUCTION With the rapid development of high brightness LED, SSL (Solid State Lighting) starts to penetrate residential markets other than just a few niche markets. For example, there is a big potential market for residential application of SSL. CFL (Compact Fluorescent Lamp) retrofit is one of them. The standardization of SSL products helps to lead the growth of the market. In September 2007, the US Department of Energy (DOE) issued its Energy Star criteria for SSL products. To meet the Energy Star specifications for SSL products, the power factor of the power supply needs to be higher than 0.7 for residential application. For CFL retrofit application, the cost, size, and reliability are very important. To achieve a high power factor, one could use either passive PFC (Power Factor Correction) or active PFC. Typically, the passive PFC requires large passive components; it is difficult to fit the small size needed for the retrofit application. The tradition active PFC circuit will require a two-stage topology, e.g. boost stage for PFC, then buck or flyback for the current regulation of LEDs. Obviously, the cost of two stages is high. Reference [2] presented a single-stage converter for LCD back lighting using LEDs. The concept can be used for CFL retrofit application. However, the method used to improve power factor in that paper would cause significant power variation when the input voltage varies. It relies on a delay caused by the RC filter in the current sensing to shape the current, which will be impacted by the amplitude of input voltage. A new method of shaping the current waveform is present in this paper. Due to its simplicity and low cost, a non-isolated buck-boost converter is chosen in this LED lighting application. For this particular design, the LED string is 8 one watt white LEDs in series. Isolation is not required. The goal of the design is high power factor, high efficiency, simplicity, and low cost. II. CIRCUIT DESIGN Figure shows the proposed single stage LED driver. If the inductor current is constant, the power of the converter is constant, which will be illustrated in the following. Since the LED string is a constant voltage load, we can get constant current in the LED string. It makes it possible to get rid of the LED current sensing, which simplifies the circuit design. If the inductor current is constant, then the power factor of the circuit will be very poor. The input current waveform will be greatly distorted near line zero crossing. If there is a way to reduce the current amplitude near line zero crossing, the power factor can be improved. A PFC controller L6562A is used to achieve this purpose. The idea is to reduce the current amplitude near line zero crossing; in this way, the power factor is improved. The L6562A is a current mode PFC controller operating in transition mode (boundary mode between CCM and DCM). Its linear multiplier enables the converter to shape the AC input current waveform following the input voltage. The Block diagram of L6562A is shown in figure /09/$ IEEE 582

2 AC R4 + - L + - LEDs R2 R3 Vcc MULT ZCD Q R INV CS L6562A Rs Fig. Schematic of the single stage LED driver Fig2. Block diagram for L6562A The proposed circuit is running at a constant peak current. The current sensing voltage is set for V, which is the clamping voltage of the current sensing comparator of L6562A. The voltage of the LED string is sensed on INV pin of L6562A through a coupled winding. The turns-ration of the coupledinductor is designed in such way that the feedback voltage is lower than 2.5V in normal operating conditions, so the error amplifier is saturated at maximum level, which will set the max current sensing voltage to V. Therefore, the peak current of the inductor is fixed at a designated level determined by the sensing resistor value. Since the peak current of the inductor is constant, the input power is constant. And as the LED voltage is considered constant, the current into the LED can be considered constant as well. If the load (LEDs) is open, and the reflected output voltage on INV pin is above a certain level, the controller will shut down to provide open load protection. If the load is shorted, the controller will be shut down since Vcc power is lost. If the inductor current is controlled at a constant level all the time, then the power factor will be very poor. The multiplier of L6562A is used to reshape the current /09/$ IEEE 583

3 amplitude near line zero crossing to improve the power factor. The rectified sine waveform is sampled at MULT pin. The output of the multiplier, which is the current setting, will be lower than the setting level near line zero crossing. In this way, the power factor of the converter is improved significantly. Design procedure: The input voltage: Vin(θ)= 2 * Vin *sin( θ ), Vin=20v. The output voltage: 8 LED in series, Vout=54v The output current: Iout=350mA. The design variable is the peak current of inductor, Ipk, and the inductance L, When the Q is turned on, the inductor will be charged to Ipk. The on time: L * Ipk Ton ( θ ) = () Vin( t) The off time will be: L * Ipk Toff ( θ ) = (2) Vout The period of the switching cycle: L * Ipk L * Ipk T ( θ ) = Ton( θ ) + Toff ( θ ) = + Vin( θ ) Vout (3) The duty cycle D: Ton( θ ) Vout D( θ ) = = (4) T( θ ) Vout + Vin( θ ) The frequency is: fsw( θ ) = T (5) Vout * Vin( θ ) = ( ) L * Ipk Vin( θ ) + Vout The switching frequency will vary during line cycle, which is good for reducing EMI. The max frequency happens at peak input voltage: Vout * Vpk fswmax = ( ) L * Ipk Vpk + Vout (6) The input power: π 2 P = * L * Ipk * fsw( θ ) dt (7) 0 2 π P = * Ipk * D( θ ) * Vin( θ ) dθ (8) 2 0 The integration term of (8) is a constant value, so the power is determined by Ipk only. There is no simple solution form for the integration term. We can use the average value of input voltage and duty cycle to do the estimation. After Ipk is calculated, the inductance can be determined according to the desired switching frequency range. The average input voltage over half cycle at 20v AC line: Vave = π Vpk * sin( θ) dθ = 08V (9) 0 The average duty cycle: Vout Dave = = Vave + Vout Therefore we have: Pin Ipk = =.2A * Vave* Dave 2 After the current is determined, we need to choose the right inductance. The inductance will affect the running frequency. We choose the max switching frequency max fsw around 50KHz. From equation (6), we can have: Vout * Vpk L = ( ) fswmax* Ipk Vpk + Vout (0) L=200uH. The operation of the converter is as following. The period of t0 to t3 is near line zero crossing. The period of t4 to t7 is around peak line voltage. [t0,t], Q is turned on at time t0. The inductor current reaches the peak at t. Near line zero crossing, the peak amplitude will be lower than the constant value Ipk /09/$ IEEE 584

4 [t,t2] Q is turned off at t. The inductor current will decrease to zero at t2. [t2,t3] the drain voltage of Q starts to fall at t2, and reaches zero at t3. The ZCD pin of the controller will detect the ZCD signal low and turn on Q again at t3. Q is turned on at zero voltage. [t4,t5], Q is turned on at time t4. The inductor current reaches the peak at t4. The peak amplitude is the constant value Ipk. [t5,t6] Q is turned off at t5. The inductor current will decrease to zero at t6. [t6,t7] the drain voltage of Q starts to fall at t6, and reaches minimum value at t7, but it won t reach zero. The ZCD pin of the controller will detect the ZCD signal low and turn on Q again at t7. Q is turned on at reduced voltage. Inductor current 0.5A/div Vmult 2v/div Fig4. The inductor current and multiplier input. Figure 5 shows the input voltage and current. Input voltage 00v/div Envelop of peak inductor current Inductor current Q Vds Input current 200mA/div ZCD signal Fig5. The input voltage and input current. T0 t t2 t3 t4 t5 t6 t7 Fig.3 Illustration of key waveforms of the converter The MOSFET Q operates zero voltage turn-on when instant input voltage is lower than output voltage. It is turned on at reduced voltage when input voltage is higher than output voltage. Therefore, it is a partial soft-switched converter. III. RESULTS Figure 6 shows output LED s current and voltage. LED current 200mA/div LED voltage 20v/div The efficiency of the circuit is 88%; the power factor is Key waveforms are shown below. Figure 4 shows the envelope of inductor current and input signal at MULT pin. Fig6. LED voltage and LED current /09/$ IEEE 585

5 Figure 7 shows the MOSFET switching waveforms. When input line voltage is lower than output voltage, zero voltage turn-on is achieved. When the input voltage is higher than output voltage, MOSFET is turned on at reduced voltage Inductor Current 0.5A/div Inductor Current 0.5A/div Vds 50V/div Vds 50V/div Vgs 0V/div Vgs 0V/div When Vout >Vin When Vout<Vin Fig.7. switching waveform of MOSFET Q IV. CONCLUSIONS Running at transition mode provides the benefit of lower switching loss and spread of EMI spectrum. The buck-boost converter achieves the constant power by operating at a constant peak current, and high power factor is achieved by reshaping the current waveform near the zero crossing of the line voltage. This single stage buckboost converter provides a cost effective solution for the offline non-isolated LED application. REFERENCE: [] Energy Star Requirement for SSL, [2] In-Hwan Oh, A Single-Stage Power Converter for a Large Screen LCD Back-Lighting, APEC [3] Datasheet of L6562A, STMicroelectronics /09/$ IEEE 586

AN3111 Application note

AN3111 Application note Application note 18 W single-stage offline LED driver Introduction With the rapid development of high brightness LEDs, SSL (solid state lighting) has begun to move from being a niche market to penetrating