Design and implementation of LED street lamp drive and intelligent dimming system

1 Introduction

LED is considered to be the fourth generation of green light source. It is a solid cold light source with many advantages such as high efficiency, long life, safety and environmental protection, small size, and fast response speed. It has been widely used in urban landscape decoration, traffic signals and commercial advertisements. In recent years, with the continuous development of manufacturing technology, the performance of high-power and high-brightness LEDs has been continuously improved, and the price has been continuously reduced. At present, to achieve the same brightness effect, the power consumption of LED is about 1/10 of that of incandescent lamps and 1/2 of that of fluorescent lamps [2]. All these have led to its application in general lighting, and it has great development prospects. It has a great trend to replace traditional light sources such as incandescent lamps and fluorescent lamps. The application of LED lamps at the World Expo can be said to represent this direction. LED dimming can save energy. The driving and dimming of high-brightness white light LEDs have been a hot topic in recent years. This paper has conducted some research in this regard and designed an LED street lamp driver and intelligent dimming system with power factor correction.

2 LED characteristics, driving requirements and dimming methods

The theoretical luminous efficacy of LED is 300lm/W. The current laboratory level is 260lm/W, and the market level is above 120lm/W. The general conduction voltage of high-brightness LED is about 3.0~4.3V, but its core is still PN junction, and its volt-ampere characteristics are the same as those of ordinary diodes. When the voltage applied to the LED is less than its conduction voltage, almost no current flows through the LED. But when the LED is turned on, its forward current changes exponentially with the forward voltage, and a small voltage fluctuation will cause a large current change. In the conduction zone, the voltage rises from 80% to 100% of the rated value, and the current rises from 0% to 100% of its rated value.

Figure 1 Relationship between LED relative luminous flux and forward current

Figure 1 is a graph showing the relationship between the relative luminous flux of an LED and its forward current IF. It can be seen from the figure that the luminous flux of an LED is proportional to its forward current, so it is possible to control the brightness of the LED by controlling the forward current of the LED. If the LED is driven by a constant voltage source, a small voltage change will cause a large current change, so constant voltage driving is only suitable for low-power applications with low requirements. In applications with high requirements and high power, LEDs must be driven by constant current. Studies have shown that the brightness of an LED decreases with working time, and after the brightness decreases, the luminous efficacy decreases with the increase of current. The brightness of an LED is in a saturation relationship with the driving current. When the current of an LED reaches 70% to 80% of its rated current, a large proportion of the current is converted into heat energy. Therefore, the driving current of an LED should be 70% to 80% of the rated working current. In constant voltage driving or PWM dimming, the maximum current should not exceed 3 times the minimum current, otherwise the impact current will greatly reduce the service life of the LED [8]. At present, the power of a single LED on the market is not large, mostly below 10W. The actual use for lighting is to form an LED array by connecting multiple LEDs in a certain direction or in series or parallel.

It can also be seen from Figure 1 that the brightness of the LED can be changed by changing the current of the LED. There are two ways to change the current, and there are two corresponding ways to dim the LED. One is to continuously adjust the size of the current in the LED to change the brightness of the LED. This method is called analog dimming, and the current in the LED is continuous; the other is to change the brightness of the LED by changing the ratio of the time when the current flows through the LED to the time when it is turned off. When the current flows through the LED, the current is constant, and when it is turned off, the current flowing through the LED is zero. This method is called PWM dimming, which is a fast switching of the LED at a frequency that is imperceptible to the human eye. The switching frequency should not be less than 100Hz. When the average current flowing through the LED is the same, the two dimming methods have the same effect. Since the LED will emit the purest white light when the current is a certain size, there will be color deviation as the current deviates from this value. In addition, the response time of the LED is only a few nanoseconds to tens of nanoseconds, which is very suitable for occasions with frequent switching, so the LED dimming method is better to use PWM dimming. In addition, this method is also conducive to the heat dissipation of the LED.

3 LED drive circuit

3.1 LED drive circuit classification

From the perspective of LED driving power supply, its driving can be divided into AC/DC type and DC/DC type. LED requires DC power supply. When AC power supply is used, AC must be converted into DC before driving LED, so we only need to study DC/DC type. The driving methods of DC/DC type LED can be divided into resistor current limiting type, linear voltage regulated power supply type, capacitor charge pump circuit and inductor switch conversion circuit. Resistor current limiting connects the resistor and LED in series, and drives the LED lamp through the voltage division of the resistor to limit the current. This method cannot guarantee the control accuracy, and a large amount of electric power is wasted on the resistor. It is only used in low-voltage occasions with low requirements. The linear voltage regulated power supply has higher accuracy than the resistor current limiting type, but it also has the problem of low efficiency, and is not used much in practice. In practice, charge pump circuits and inductor switch conversion circuits are more commonly used.

The charge pump circuit uses the cumulative effect of capacitors on charges to store electrical energy, and uses capacitors as energy coupling elements. By controlling the high-frequency switches of power electronic devices, the capacitors are switched to store energy in part of the clock cycle and release energy in the remaining time of the clock cycle. This method is to obtain different output voltages through different connection methods when charging and discharging the capacitors. The inductive switching conversion circuit is also called a switching power supply. It changes the output voltage by controlling the time relationship between the on and off of the power switch tube. Inductors and capacitors are generally used as filtering elements to stabilize the output. In comparison, the charge pump type uses fewer components, has low cost and small size, but it uses more switching elements and has relatively low efficiency. The output voltage varies in the range of 1/3 to 3 times the input voltage, and the output power is small, so it is mostly used in low-power occasions; while the switching power supply has relatively fewer switching elements, high efficiency, can achieve a wide range of voltage output, and the output voltage is continuously adjustable and the output power is large, so it has a wider range of applications, especially in medium and high power occasions. It is the first choice. There are many topologies of switching power supplies. The most commonly used ones in LED drive circuits are Boost circuits, Buck-Boost circuits and Buck circuits.

3.2 Design of LED street light drive circuit based on HV9931

There are already some LED driver chips. LED street lights are relatively powerful and are powered by AC power. The state stipulates that when the power reaches a certain value, a power factor correction device must be installed. In addition, the designed LED street lights must have a dimming function to save energy. Based on the above considerations, Supertex's HV9931 driver chip is selected here. HV9931 is an 8-pin PWM integrated controller with the following functions and features: (1) The output current is constant, suitable for LED constant current driving; (2) It allows a wide range of DC input voltage from 8 to 450V, and has a large step-down ratio, so no transformer is required when using AC power to drive LED lamps; (3) It has a power factor correction function, which can obtain unity power factor and low input current harmonics, meet national regulations, and is environmentally friendly; (4) It has PWM dimming and analog dimming functions, which can easily achieve dimming control when driving LEDs, meeting energy-saving requirements; (5) The oscillator has two working modes: fixed frequency and fixed off time.

Figure 2 LED street light driver circuit based on HV9931

The driving circuit is shown in Figure 2, which is a switching power supply driving method.

The main circuit is a single-stage single-switch non-isolated constant current output Buck-Boost-Buck circuit. The Buck-Boost circuit composed of L1, C1, D1, D5 and Q1 is the input stage, working in discontinuous conduction mode; the Buck circuit composed of C1, Q1, D2, D4 and L2 is the output stage, working in continuous conduction mode. The two stages share a power switch tube Q1. The capacitor C1 is equivalent to a load for the input stage and a DC power supply for the output stage. The system step-down ratio is the product of the two-stage step-down ratios, so that a lower voltage output can be achieved without a transformer when powered by the mains. When the switch Q1 is turned on, the input stage Buck-Boos current path is: rectified voltage → D1 → L1 → Q1 → Rs1, the current in L1 increases linearly, and the output stage Buck circuit current path is:

C1→Q1→Rs2→LED→L2, C1 provides energy; when Q1 is turned off, the input stage current path is: L1→C1→D5→D1, the energy in L1 is transferred to C1, due to the existence of D1, the current of L1 cannot be reversed, and the current of L1 drops to 0 and then becomes intermittent; the output stage current path is: L2→D4→LED, due to different parameter settings, the current in L2 not only does not become 0, but also has relatively small fluctuations.

The circuit works in the peak current mode. The oscillator makes GATE output high level, making Q1 conduction. CS1 and CS2 terminals are the reverse input terminals of the two voltage comparators inside HV9931. The same-direction input terminals of the two comparators are grounded inside the chip. The circuit detects the input current and output current simultaneously through CS1 and CS2 terminals. CS1 is the input current signal detection terminal, and CS2 is the output current signal detection terminal. As long as the voltage on one of these two terminals is lower than the ground, the GATE terminal outputs a low level and Q1 is turned off. VDD is the reference voltage output pin of the chip. Rs1, Rcs1 and Rref1 can program the maximum peak current in L1, and Rs2, Rcs2 and Rref2 can program the output current. In the cycle of AC power, the duty cycle and switching frequency can be considered unchanged, so the input current peak envelope is a sine wave, and the average current is a sine wave, which can realize power factor correction.

C2 is the input capacitor, used as a high-frequency bypass. If a large capacitor is used, the circuit loses the power factor correction function. RT corresponds to the internal oscillator. There are two ways to connect it, setting a constant operating frequency and a constant off time. The figure uses the constant off time connection. The PWMD pin is the digital dimming signal input pin. When the pin is high, the circuit works normally. When the pin is low, the GATE pin always outputs a low level, the switch tube Q1 is off, and the drive circuit does not work.

4 Intelligent dimming device

If the designed system can change its brightness according to the intensity of the ambient light, it will save energy accordingly and meet the requirements of current low-carbon life. When the ambient light is the worst, the PWM duty cycle of the dimming signal of the design system is close to 100% (a certain margin is left to prevent the temperature from rising too high) to make the LED brightest and meet the illumination requirements; when the ambient light changes, the PWM duty cycle of the dimming signal will automatically change according to the intensity of the external light, making the LED darker accordingly, but the illumination meets the requirements; in addition, the brightness can be appropriately reduced when there are fewer people in the middle of the night.

To achieve this requirement, a good light intensity sensor is required. Photoresistors have poor linearity and low frequency response, while phototransistors have high sensitivity, but poor temperature characteristics and linearity. The system design uses TLS2561 as a photoelectric intensity sensor. TLS2561 is close to the human eye's response to brightness, and can directly convert light intensity signals into digital signal outputs. It has a programmable interrupt function and a standard I2C interface, and can be easily connected to a single-chip microcomputer. The single-chip microcomputer uses Microchip's PIC16C62. This single-chip microcomputer has stable performance, PWM output, and can easily realize I2C bus communication. The light intensity sensor TLS2561 converts the light signal into a digital signal and transmits it to the single-chip microcomputer. The single-chip microcomputer processes it to generate a dimming PWM signal, which is sent to the PWMD terminal of HV9931 to realize PWM dimming. The dimming part is shown in Figure 3.

Figure 3 Dimming system block diagram

The system design also takes into account the influence of temperature. Under the same current, the luminous flux of LED will decrease as the PN junction temperature increases, which will also affect the service life of LED. Therefore, a temperature sensor is added to the system, and the temperature signal is also sent to PIC16C62 for processing, which will also affect the duty cycle of the PWM dimming signal it outputs. This is not the main problem of this design and will not be described in detail.

5 Experimental Results

The system design power is 72W, using 72 1W high-brightness LED lamps, 24 in series and then in parallel. When the maximum light intensity is output, the PWM dimming signal duty cycle is designed to be 90%, and the average total current in the LED is measured to be 932mA; when the LED drive circuit is working, the switch tube operating frequency is 100kHz, the drive circuit efficiency is 75%, the input current THD is less than 20%, the power factor is greater than 0.9, and the photoelectric conversion efficiency is about 95lm/W; the PWM signal frequency for dimming is 120Hz, which saves about 9% energy compared to not using the intelligent dimming circuit. Figure 4 shows the current waveform in the LED when the dimming PWM duty cycle is 50%.

Figure 4 LED current waveform when the dimming PWM duty cycle is 50%

6 Conclusion

The LED street lamp drive circuit designed in this paper is powered by AC power and does not require a power transformer, which greatly reduces the size of the drive circuit. The drive circuit realizes constant current drive and has a PFC function, which meets the current requirements of green environmental protection. In addition, the drive circuit has high conversion efficiency and is relatively novel. The intelligent dimming circuit uses PWM dimming, and the LED emits purer white light without color deviation. The energy-saving effect of the intelligent dimming circuit is quite significant. The actual circuit designed has a good prospect and market, but the disadvantage is that the cost is slightly high. However, with the continuous improvement of LED manufacturing technology and the continuous deepening of the research on the drive dimming circuit, I believe this problem will be well solved.