Abstract: This paper introduces the current development status and prospects of high-power LED (Light-Emitting Diode) lighting systems, and designs an LED driver power supply with power factor correction. This lighting power supply uses the TEA1713 half-bridge resonant converter as the core controller, integrating various functional sub-circuits. The input AC voltage range is 90~265V, and it can drive high-power LED lighting lamps with a load voltage of 33V and a load current of 1.8~2.4A. The EMC (Electromagnetic Compatibility) test results of this power supply are studied through the three elements of electromagnetic interference: interference source, transmission path, and sensitive equipment. A brief analysis of the considerations and key points that should be paid attention to during the design is also provided.
1. Introduction
LED (Light-Emitting Diode) technology is becoming increasingly mature, and its development in the lighting industry has become a major trend, bringing broad application prospects for the advancement of energy-saving lamps. LED lighting has advantages such as energy saving, high efficiency, environmental protection, and long lifespan. In particular, lighting products represented by high-power LED lamps have very promising development prospects in the lighting market. However, its inherent weaknesses cannot be ignored: high temperature environment will shorten the lifespan of its electrolytic capacitors, thereby affecting the output load capacity and easily reducing the utilization rate of LED lamps. To overcome these shortcomings, this design uses the TEA1713 (half-bridge resonant converter) power driver chip as the core controller. Moreover, for high-power applications, it is generally necessary to reduce power harmonics, so corresponding peripheral circuits are added to form a high-power LED driver power supply without electrolytic capacitors [1].
2. Overall Structural Design
Based on the design requirements, the overall structure of the LED lighting fixture's driver power supply is shown in Figure 1:
Figure 1 Overall block diagram of LED driver power supply
2.1 Introduction to Main Circuit Structure
The TEA1713 (half-bridge resonant converter) integrates a PFC (Power Factor Correction Controller) and a resonant controller, eliminating the need for electrolytic capacitors in circuit design. It features power factor correction and continuous current control. It can be used to design low-cost, long-life LED driver power supplies with a power factor down to 0.9, high efficiency, and a wide dimming range. It also enables fast and smooth power-on, achieving flicker-free operation.
The working principle is as follows: the switching transistor is controlled by a high-frequency pulse width modulation signal. When the driving power supply is working, the power switching transistor is in the on or off state, and there is a very small power generated by the switching loss of the semiconductor device. During the operation, the input DC voltage is chopped to form a pulse voltage with equal amplitude. The duty cycle of the pulse voltage is adjusted by the controller, and the amplitude of the AC square wave is adjusted by the transformer. In addition, the number of output voltage groups is increased by increasing the number of secondary windings of the transformer. Finally, after rectification and filtering, the AC power is converted into DC power [2].
The principle of the drive controller is equivalent to an error amplifier. Its function is to maintain stable output. A voltage pulse conversion unit is added before the power switching transistor. The secondary side of the switching transformer senses high-frequency voltage, which is then rectified and filtered before being sent to the load. The control circuit receives the output of the feedback loop to control the duty cycle of the PWM, and finally achieves stable output.
2.2 Introduction to TEA1713
The TEA1713 half-bridge resonant converter integrates multiple functions such as power factor correction (PFC), capacitive mode protection, and adaptive dead-time control into one unit. The integration of the two controllers makes them easier to work with, minimizing the number of external components (no controller interface connection is required), and providing users with an effective solution to reduce power supply harmonics.
TEA1713 has a self-starting function and multiple protection functions: such as input overvoltage protection to ensure the power supply's ability to withstand input voltage faults and surges; output overvoltage protection to prevent damage to the power supply when the load is suddenly disconnected; and significant hysteresis thermal shutdown function to ensure that the average temperature of the PCB board is within a certain safe range under all conditions [3].
The topology used in this design features primary-side detection and output current regulation, ensuring a high power factor and low harmonic current for the drive power supply. Because the current sensing circuit is integrated within the chip, no external sensing circuit is needed on the primary side, thus reducing component and power losses. Its mode is isolated flyback and continuous conduction, which avoids the need for secondary optocoupler feedback components and secondary current sensors, significantly simplifying the circuit design.
3. Circuit Design
3.1 Flyback Converter Circuit
The flyback converter circuit offers a power range that adequately meets design requirements, boasting advantages such as low cost, simple circuitry, and high reliability. Based on the TEA1713, its operating principle is as follows: when the power switch is turned on, electrical energy is first stored in the primary winding of the high-frequency transformer; only when the switch is turned off is electrical energy supplied to the secondary winding. It is worth noting how to address its low efficiency; typically, soft-switching technology or a sandwich-winding method for the transformer is employed. Its topology is shown in Figure 2.
Figure 2 Flyback converter circuit
The technical specifications require that the input voltage of this power supply be suitable for the input voltage range of AC85V to 265V, with a large voltage range that is adjustable. It also requires high current regulation accuracy. Therefore, the influence of current harmonics and noise generation cannot be ignored, so a filter circuit has been added.
3.2 Input Filtering
The input filter circuit includes a fuse, a varistor, common-mode inductors LF1 and LF2, and a filter capacitor. The varistor clamps the maximum voltage that may be generated during the differential-mode surge test and can effectively prevent lightning surge impact. The filter capacitor includes an X capacitor and a decoupling capacitor. The X capacitor is mainly used to filter out series-mode interference, and its capacitance is generally 0.01uF-0.47uF. The decoupling capacitor can provide a low-impedance path for the primary switching current. The diode rectifier bridge rectifies the AC line voltage. This circuit can effectively suppress common-mode and differential-mode ripple interference [3]. In addition, in order to ensure that the power factor is above 0.9, the capacitance should not be too large, otherwise it will increase the leakage current and affect the withstand voltage test.
3.3 Feedback Circuit
Feedback control is affected by errors and temperature drift, and the bias winding voltage is used to indirectly reflect the output voltage level. For example, the current transfer ratio Iout/Iin (Ioutput/Iin) of an optocoupler drifts with temperature and gradually deteriorates with increasing operating time. In this circuit design, an error adjustment resistor is used to convert the bias voltage into current injected into the feedback (FB) pin of U1.
3.4 Output Rectification
High-frequency square wave voltages caused by leakage inductance and distributed capacitance can generate voltage spikes or surge currents during transients, leading to pulse top oscillations. This can have several adverse consequences, such as excessively high peak voltages, increased losses, and damage to the switching transistor. Therefore, it is essential to reduce leakage inductance and distributed capacitance. Furthermore, reducing distributed capacitance can suppress high-frequency signal interference to the load. To address loss issues, Schottky barrier diodes can be selected to improve efficiency. Output filtering using capacitors can ensure that the total value of the capacitors equals 40% of the average LED ripple current.
3.5 Active Attenuation and Passive Discharge Circuit
For issues such as limited dimming range and/or flickering caused by inconsistent thyristor triggering, and severe oscillations caused by surge current when the thyristor is turned on, active attenuation circuits and passive discharge circuits are used to address these problems [4]. The active attenuation circuit limits the surge current flowing into and charging the capacitor when the thyristor is turned on. The passive discharge circuit ensures that the input current is always greater than the holding current of the thyristor, preventing switching oscillations at the beginning of each conduction angle. For non-dimming applications, these two circuits can be omitted.
4. EMC testing of the power supply
Electromagnetic interference is the degradation of equipment or system performance caused by electromagnetic disturbance. The interference source, transmission path and sensitive equipment are the three essential elements for electromagnetic interference. In the hardware of electronic equipment, such as cables, inductors, capacitors, resistors, printed lines on PCB boards, mutual inductance components, grounding plates, internal wires, etc., all have antenna functions. These components can transmit energy in various ways, such as electric fields, magnetic fields or electromagnetic fields. During operation, this energy is coupled into the circuit, interfering with the normal operation of the equipment [5].
In the EMC test of this power supply, both the device and the EUT were reliably grounded. During commissioning, the wiring positions of the common-mode inductor and safety capacitor were changed. The test frequency range was 9kHz to 30MHz, and the test data is shown in Figure 3. In the test graph, the red line is the standard line (red line 1 represents the peak limit, red line 2 represents the average limit), blue line 3 represents the peak measurement value, and green line 4 represents the average measurement value. The judgment criterion is: when the green waveform is below red line 1 and the blue waveform is below red line 2, with a margin of more than 3dB, the test is considered合格 (qualified).
Figure 3 Preliminary measurement results of power terminal interference voltage
As shown in Figure 3, the initial test results of the power supply terminal interference voltage are seriously out of standard, especially in the frequency band of 0.1MHz to 1MHz, where the maximum exceedance exceeds more than ten dB. Moreover, the driving power supply of the LED lamp is a constant current switching power supply. Possible reasons include: the differential mode filtering of the switching power supply may be insufficient, causing the test curve to exceed the limit in the frequency band of 0.1MHz to 1MHz. Therefore, we should consider how to reduce the noise of differential mode interference. In addition, the presence of high-frequency interference sources inside the driving power supply, excessively long internal cables, or poor grounding of the circuit board are also reasons for exceeding the limit. Therefore, we need to check the internal structure of the prototype and the circuit board wiring in order to provide corresponding solutions [6].
The internal wiring of the prototype was sorted out according to the following principles [7]: input lines and output lines should be separated as much as possible; high-frequency signal lines and low-frequency signal lines should be separated as much as possible; attention should also be paid to the application of soft switching technology in switching power supplies, electromagnetic compatibility design of printed circuit board wiring, etc. After taking the above measures, the power supply terminal interference voltage was retested, and the test results are shown in Figure 4 (the curves of each color are represented in Figure 3).
The frequency points of the power terminal disturbance voltage retest results in Figure 4 have been greatly improved.
The efficiency of the power supply was tested across the entire voltage range at room temperature. The test results show that the output efficiency is above 80% across the entire voltage range, and the output current is very stable. The test results are shown in Table 1.
frequency Hz | Vin(VAC) (Input voltage) | Pin (W) (Input power) | Vout (V) (Output voltage) | Iout (A) (Output current) | Pout (W) (Output power) | efficiency (%) | PF value (Power factor) |
50 | 90 | 91.03 | 33.82 | 2.395 | 80.9 | 80 | 0.92 |
50 | 100 | 90.77 | 33.81 | 2.394 | 81 | 81 | 0.92 |
50 | 125 | 90.64 | 33.76 | 2.395 | 80.8 | 82 | 0.92 |
50 | 140 | 90.56 | 33.75 | 2.39 | 80.7 | 82 | 0.92 |
50 | 165 | 90.81 | 33.73 | 2.388 | 80.6 | 81 | 0.92 |
60 | 180 | 90.78 | 33.77 | 2.387 | 80.61 | 82 | 0.98 |
60 | 215 | 90.71 | 33.69 | 2.386 | 80.49 | 82 | 0.98 |
60 | 230 | 90.72 | 33.71 | 2.387 | 80.6 | 83 | 0.98 |
60 | 245 | 90.41 | 33.70 | 2.39 | 80.45 | 85 | 0.98 |
60 | 265 | 90.41 | 33.70 | 2.39 | 80.4 | 85 | 0.98 |
Table 1 Power Efficiency Test Table
5. Conclusion
This paper designs a high-power LED driver power supply that avoids the problem of electrolytic capacitor failure after prolonged operation. It also features power factor self-correction. Its high efficiency and high power factor fully meet the market demand for widespread LED lighting, demonstrating broad application prospects and significant practical value. Furthermore, this paper addresses some issues encountered during EMC testing of this power supply, providing corresponding rectification ideas and conducting experimental verification with good results. This can serve as a reference for subsequent EMC testing of LED driver power supplies. However, the widespread adoption of LED lighting is a complex issue, and the LED driver power supply is only one component. In addition, knowledge from the traditional lighting industry, LED device knowledge, and other factors should be comprehensively considered to provide a broad development space for the application of energy-saving lamps.
About the author: Kong Qiangqiang (1985—) Male, from Zoucheng, Shandong Province, holds a master's degree and is an engineer. He is mainly engaged in research on the performance and safety regulations of lighting fixtures, as well as rectification.
