A flicker-free, electrolytic capacitor-free AC-DC LED driver that reduces LED current

　　1. Introduction

　　As countries around the world gradually ban the import and sale of ordinary incandescent lamps, new, green, efficient and long-life LED lighting technology has achieved unprecedented development [1]. Long life is one of the greatest advantages of LED lighting, and its average service life reaches 80,000-100,000 hours [2]. For single-stage LED driver power supply, if it is powered by AC power, in order to achieve a high power factor (Power Factor, PF) and meet the harmonic requirements of IEC61000-3-2 [3], LED lighting requires a power factor correction converter (Power Factor Correction, PFC). When the power factor is 1, the input current is a sine wave with the same phase as the input voltage, so its input power presents a pulsating form of twice the input frequency. For LEDs with constant output power, in order to match the imbalance of instantaneous input and output power, a storage capacitor is required. The energy storage capacitor is very large, and most of them are electrolytic capacitors. However, the service life of electrolytic capacitors is only about 10,000 hours [4], which is the main component affecting the overall life of the LED driver power supply. In order to improve the service life of AC-DC LED driver power supply, it is necessary to remove the electrolytic capacitor. Appropriately reduce the power factor to reduce the input power pulsation, such as injecting third and fifth harmonics into the input current [5, 6], so that the energy storage capacitor can be reduced. Use pulsating current to drive the LED, so that the instantaneous input and output power are the same or close, which can reduce or eliminate the energy storage capacitor [7-10]. Pulsating current driving LED is generally used for landscape or street lighting, and is not suitable for some occasions with high requirements on light source quality. Using inductors as energy storage elements can replace or reduce energy storage capacitors, but the energy storage density of inductors is small, their volume is large, and there will be losses [11]. Increasing the voltage wave on the energy storage capacitor can reduce the capacitance of the capacitor [12-14]. Reference [13] proposed a frequency-free

　　The AC-DC LED driver power supply without electrolytic capacitor is shown in Figure 1. It consists of a PFC converter, a bidirectional converter and a CL filter. The inductor Lo and the capacitor Co form a low-pass filter to prevent the current harmonics of the switching frequency and its multiples from flowing into the LED. Therefore, the capacitor Co here does not play the role of energy storage. A film capacitor or a ceramic capacitor with a smaller capacity can be used. At this time, the PFC output current i'o contains a pulsating current with twice the input frequency. In order to make the LED drive current io a constant DC current, a bidirectional converter is connected in parallel at the output end of the PFC converter, and the input current ib of the bidirectional converter is equal to the AC component of the PFC output current with twice the input frequency, thus solving the flicker problem of LED lighting. The DC side capacitor Cdc of the bidirectional converter adopts the method of reducing the capacitance value when the voltage of the energy storage capacitor is large ripple. Under the condition of the same pulsating voltage, in order to further reduce the capacitance value, it can be

　　To appropriately increase the DC average voltage of Cdc.

　　Based on the flicker-free and electrolytic capacitor-free AC-DC LED driver power supply, this paper analyzes the nonlinear problem caused by the DC-side capacitor ripple of its bidirectional converter to the duty cycle of the bidirectional converter switch tube. Since the traditional linear control method cannot provide this part of the content, it is ultimately reflected in the increase of the steady-state error of the bidirectional converter current tracking, causing the LED drive current to be distorted. In response to the problems existing in the flicker-free and electrolytic capacitor-free AC-DC LED driver power supply, this paper proposes an improved control strategy in the bidirectional converter to reduce the impact of the nonlinear problem caused by the DC-side capacitor ripple, improve the ability of the bidirectional converter to track the sinusoidal AC reference, and eliminate the distortion of the LED current.

　　2. Basic Concepts of Flicker-Free and Electrolytic Capacitor-Free AC-DC LED Driver

　　Reference [13] analyzes in detail the working principle of the flicker-free and electrolytic capacitor-free AC-DC LED driver power supply. This article only briefly introduces it. The PFC converter here uses a discontinuous current mode (DCM) flyback converter, as shown in Figure 2.

　　The flyback converter uses average current control to achieve the purpose of constant average output. From the above analysis, it can be seen that due to the absence of electrolytic capacitors, the pulsating current contains an AC component with twice the input frequency, which will cause the LED to flicker. For this reason, a bidirectional converter is connected in parallel to the filter capacitor Co of the flyback converter. This article uses a Buck/Boost bidirectional converter, as shown in Figure 3. After adding the bidirectional converter, Lo mainly flows through DC current, and its high-frequency current ripple is small. Therefore, it can be considered that the voltage across the capacitor Co, that is, the voltage on the input side of the bidirectional converter, is equal to the voltage across the LED Vo.

　　The bidirectional converter adopts double closed-loop control. In order for the Buck/Boost bidirectional converter to work properly, it is necessary to ensure that the minimum voltage of the DC side capacitor Cdc is higher than the input voltage Vo. The voltage outer loop controls the average value of the DC side capacitor voltage, and its output is added to the given current reference iref (the AC component with twice the input frequency obtained by sampling the secondary current of the flyback converter) according to the proportional coefficient k as the reference of the current inner loop. The current inner loop adopts average current control to make the average value of the input current of the bidirectional converter track the current reference. Then the current at the output end of the flyback converter is shunted, and the AC component with twice the input frequency flows into the bidirectional converter. From formula (4), we get:

　　It is found that due to the constant average value of the DC side capacitor voltage, the DC component in the duty cycle does not change with the load; as the load increases, the capacitor ripple increases, and the low-frequency component of the required duty cycle increases rapidly, which makes the linear current regulator insufficient to provide this part of the low-frequency component, and can only compensate these low-frequency components by increasing the steady-state error of current tracking, which eventually leads to the distortion of the LED output current. If the load Po=Pmax, then the change of the amplitude of each harmonic of the duty cycle under different capacitor values ​​under full load is shown in Figure 7. When the load and the DC side capacitor voltage are constant, as the capacitance decreases, the low-frequency component of the duty cycle increases rapidly. Therefore, it is possible to consider designing a nonlinear controller based on the expression of the duty cycle, so as to achieve the difference-free tracking of the sinusoidal current reference of the bidirectional converter without affecting the stability of the system, and reduce the influence of the duty cycle nonlinearity on the LED output current.

　　3.2 Implementation of improved control strategy

　　The idea of ​​variable duty cycle control has been applied to high power factor DCM PFC converters [15]. This method is applied to the control circuit of bidirectional converters. Observe equation (15). If the duty cycle of the bidirectional converter switch Q1 is changed according to the theoretical value during the power frequency cycle, the DC side capacitor voltage of the bidirectional converter will change according to the theoretical form, and the input current will also change with an AC reference of twice the input frequency. Since the bidirectional converter switches Q1 and Q2 are complementary, in order to simplify the implementation, the control switch Q2 is selected here. From equation (15), its duty cycle d' can be obtained as:

　　4. Simulation Verification

　　In order to verify that the improved control strategy can reduce the steady-state error of the bidirectional converter current tracking and reduce the distortion of the LED drive current, a flicker-free and electrolytic capacitor-free AC-DC LED driver power supply with the improved control strategy was built using Saber software. Its main parameters are as follows: the AC input voltage is 220 VAC/50Hz, the full-load output average current Io=0.7A, the output voltage Vo=48V, the bidirectional converter inductance is 1.4mH, the DC side capacitor is 4.7μF, the average voltage is 150V, and the sawtooth wave amplitude Vm=3V. Figures 9 and 10 show the simulation waveforms of the secondary current with high-frequency components filtered out, the inductor current of the bidirectional converter, the LED output current, and the energy storage capacitor voltage under full load and half load conditions, respectively. It can be found that the peak-to-peak value of the full-load output current is 110mA, which is 15.7% of the average value of 700mA; the peak-to-peak value of the output current at half load is 22mA, which is 6.3% of the average value of 350mA, and the LED output current is highly distorted.

　　Figures 11 and 12 show the simulation waveforms under full load and half load respectively when the improved control strategy is adopted. At this time, the peak-to-peak value of the output current under full load is 13mA, which is 1.9% of the average value; the peak-to-peak value of the output current under half load is 7mA, which is 2.0% of the average value. Figures 13 and 14 show the spectrum of the full load and half load output current before and after the improvement respectively. It can be found that the improved control strategy can greatly reduce the low-frequency component in the LED drive current and suppress the distortion of the LED output current. The simulation results verify the correctness and effectiveness of this method.

　　5. Conclusion

　　This paper conducts a steady-state analysis of the Buck/Boost bidirectional converter in the flicker-free and electrolytic capacitor-free AC-DC LED driver power supply, and analyzes the nonlinear problem of the bidirectional converter caused by the DC side capacitor voltage ripple. In order to reduce the tracking error of the bidirectional converter input current to the twice-power frequency AC current reference, an improved variable duty cycle nonlinear control strategy is proposed to improve the original LED drive current distortion problem.