Adaptive LED street light power control system

LED lighting has outstanding advantages such as energy saving, environmental protection and long life. It has become a LED street lamp that replaces traditional lighting technology with the development of future lighting application technology. It needs to solve many technical problems: effective packaging and heat dissipation of light source, twice Light distribution problems, stable and reliable power control system problems, etc. The effective packaging and heat dissipation of the light source mainly solves the problem of the light output efficiency of the LED and the service life of the light source; the secondary light distribution solves the subjective and objective factors, and involves knowledge in many fields such as behavior, psychology, etc., where light comfort is physical quantity and psychology. A comprehensive indicator combining quantity and quantity.

Simply put, secondary light distribution is to solve the problem of suitable illumination distribution of light, suitable environmental illumination, suitable wall illumination, uniformity of brightness, prevention of glare, color temperature and color rendering of light source. The power control system addresses LED lighting drive, efficiency and stability issues. 1 As far as we know, more than 80% of the current LED street lights are caused by the power control system, which affects the service life of LED street lights. Therefore, it is an effective method to reduce the failure rate of LED street lamps by developing an efficient, stable and reliable power supply control system.

According to the volt-ampere characteristic curve of the LED, a small voltage change causes a large change in the current, resulting in a large change in the luminance of the LED. Due to the negative temperature characteristic of the semiconductor device, the input current increases with the input voltage constant, resulting in an increase in the amount of heat generated, causing the temperature rise of the LED, and forming a positive feedback, which may cause system failure and burn the circuit. Therefore, the high-power LED driving power supply mainly uses constant current driving. This paper designs an adaptive LED street light power control system. The brightness sensor is used to sense the brightness of the environment. The clock perpetual calendar uses the environment to turn on and off the street light period. It is flexible according to the region and season. The control system has temperature detection and The fault detection function automatically reduces the output current when the temperature is too high, reduces the power consumption and reduces the temperature, and prolongs the service life of the street lamp. The adaptive power control system has received good results in the LED streetlight road lighting application of the Expo Park.

2 adaptive LED street light power control system design 2.1 high-power LED street light structure Expo street lighting LED street light using 1 watt LED chip consists of 7 series 6 and requires a street lamp normal working voltage range of 10%, constant current is 2A, system The efficiency is more than 85%, and the power factor is over 90%. 2.2 Adaptive LED street lamp power control system As far as we know, the widely used LED power supply control mainly active PFC boost converter + DC / DC converter two-stage solution, two The stage PFC converter uses two switches and two controllers (one power factor controller and one PWM controller). This kind of scheme has complicated circuit, low efficiency and high cost, and the highest efficiency of the power supply cannot reach about 82%, which undoubtedly affects the effect of LED energy saving. In addition, LH) street lamps inevitably will appear in the process of burning a certain LED so that the current in the string of LEDs drops to zero, resulting in the current flowing through other LEDs exceeds its safe use range, shortening the service life of LED street lamps .

Considering the above factors, this paper designs an adaptive LED street lamp power control system, which uses a single-stage PFC function to isolate the flyback LED street lamp constant current control to improve the efficiency of the power supply. The online fault detection circuit is used to realize any LED light. Burning out ensures that the output current of other circuits falls within the safe range and extends the life of the LED. The composition of the entire system is as shown.

Power Control System Diagram A PFC single-tube flyback switching power supply circuit replaces the secondary circuit of the PFC boost converter and DC/DC converter, which simplifies the circuit structure and improves the circuit efficiency accordingly. The power driver is designed based on the active PFC control chip L6562, and the BOOST boost circuit is improved to the BOOST-BUCK type isolated flyback switch circuit. The control circuit is based on STC's STC12C5406AD chip, plus peripheral optical sensors, temperature sensors, perpetual calendar chips and fault detection circuits to intelligently adjust the output current and control the brightness. This chip uses MCS-51 as the core, comes with eight AD input ports, two PWM output ports, and the peripheral interface circuit is very simple.

The L6562 chip is a current-mode PFC controller with a starting current of 70uA and an operating current of 4mA. The internal high-linear multiplier is embedded with an AC input current total harmonic distortion (THD) optimization circuit that effectively controls the crossover distortion of the AC input current. And the error amplifier output ripple distortion provides very low THD and higher harmonic components over a wide range of AC line input voltages and a large load range.

3 circuit principle and implementation 3.1 circuit principle traditional operation in the transition mode of the current-type switching power supply chip output voltage is Uo = UinTon / (TpN) (Uin is the input line voltage, Ton is the switch tube conduction time, Tp is the switching cycle , N is the primary number of turns Np / secondary turns Ns) P. Primary peak current Ton / Lp (Lp is the primary inductance of the transformer XPFC circuit input voltage is a sine wave, so that the input current is also in phase with the sine wave Ton should not change with Vin. To achieve the goal of constant current and constant voltage output, Ton should be determined by the feedback constant current or constant voltage signal. Therefore, the conduction time of the PFC controller is determined by the feedback signal, and the threshold current is turned off. The input voltage Vin is determined. Vin determines that the waveform of the input current is a sine wave, and the feedback signal determines the amplitude of the sine wave. Thus, the constant current or constant voltage output can be achieved and the input current is sinusoidal.

According to this principle, the circuit schematic of this circuit design such as efficiency: 85% r6. Power factor: greater than 90% Ip-3.3 transformer design transformer design power control system key part. The input current waveform for the power system. To make the input current waveform and the input voltage in phase with the sine wave, the circuit principle shows that the MOS tube conduction time is constant during one half-wave period, and the transformer primary inductance Lp is determined during the off-time. The higher the switching frequency, the greater the loss on the transformer core. Limiting the switching frequency at the peak at the highest input voltage to 100 kHz will effectively control the loss of the transformer core. According to this, the inductance Lp of the primary side is set as follows: (Vor is the voltage of the secondary reflection back to the primary) The determination of the transformer primary turns Np. According to Faraday's law, the induced voltage Uin=da>/dt, at both ends of the inductor is the total magnetic flux = NpBSm (B is the magnetic flux density). dB=Uindt/(NpSm), dt is the on-time Ton, then Np=UinTon/(dBSm)UinTon is the total magnetic flux generated by the switch once, equal to the peak value of IpLp.Ip occurring at the lowest input voltage At this point, you should not enter the magnetic saturation state. For PC40 cores, the dB should be less than 0.3 Tesla. For reliability, this circuit sets the maximum flux density to 0.2 Tesla. The determination of the maximum peak turns ratio N and the number of secondary turns Ns. The turns ratio is determined by the voltage Vor that is reflected back to the primary by the secondary. In this circuit, the reflected voltage is taken as 200 V. The turns ratio = Vor / Vo = 200 / 25 = 8. Secondary resistance.

Calculation of primary maximum current rms Irms and selection of wire diameter. The primary current rms value is maximum when the output power maximum input voltage is lowest.丨rms=Po/(Vacri)=48/(1870.85) The wire diameter is selected according to the empirical formula d=0.5 =0.27 mm. This is the minimum value of the primary wire diameter.

Calculation of secondary current rms Isrms and selection of wire diameter. The average output of the secondary output is 2A. When the current waveform is a triangular wave, its effective value will be greater than the average value, which is about 1.6 times.

>/i=0.5=0.9mm. This is the minimum of the secondary wire diameter.

Primary auxiliary winding turns Naux selection. The output voltage of the auxiliary winding is supplied to Vcc, and the operating voltage range of Vcc is 10~22V. Therefore, when the output voltage is 24V, the output voltage of the auxiliary winding is preferably in the middle and lower position of the working voltage range, while ensuring normal operation while trying to Reduce the power consumption, take 13V, then the primary auxiliary winding resistance Naux = Ns13 / 24 = 4 匣.

4 control program design control program flow as shown. When the control chip is powered on and reset, the initialization program is executed first, and then the function modules are sequentially executed. When the program is executed to the end, the loopback returns to the initialization. The watchdog timer is reset every cycle. In this way, the program can be automatically restored in case of interference with the program, so as to avoid a crash.

The main loop is executed once per lm, and each time the time is read back from the perpetual calendar chip DS1302. The DS1302 is powered by the grid while charging during normal operation, and is powered by the battery during a power outage. Due to its operating current of only 400nA, the battery life is very long. The control chip communicates with it via a synchronous serial interface to read the current time.

Temperature, brightness, and fault detection are done by reading the AD sample values. To reduce interference, the software averages 16 samples with an interval of 1 ms as the final sample value. Thus, even if there is interference, its weight is only 1/丨6, which is not enough to affect the result.

Brightness control is accomplished by controlling a PWM signal output from the chip. The control chip outputs a PWM signal that can be divided into 256 levels according to the situation on the spot, and becomes a threshold value of the DC signal control output current through a second-order low-pass filter network. The output current value is obtained by sampling a 0.1£1 resistor. The sampled voltage value is 0.2V at 2 currents, at which point the power dissipation is only 0.4 watts.

The RS232 communication interface is directly connected to the computer.> The remote Zigbee network can also be used for remote transmission and control of data, in preparation for the next step of network wireless remote control.

4 power control system test In verifying the performance of LED street light power control system, this paper uses the power factor, efficiency and output voltage, current and other parameters of the test system under rated load conditions. After half an hour of operation of the power system, the front-end MOSFET temperature is about 35.8C, the back-end MOSFET is about 41.6C, and the transformer is about 34C. The test results are shown in Table 1. Vin(min) and Vin(max) are the minimum and maximum input voltages at which the drive power can output 24V/2A normally.

Table 1 Power Control System Performance Test Table Input Voltage (V) Power Factor Output Voltage (V) Output Current (A) Efficiency is the measured waveform of the input voltage and current of the power control system. The orange line is the input voltage waveform, and the blue line is Input current waveform. The input current is a non-sharp, sinusoidal waveform similar to voltage, and the measured power factor is above 97%.

For the voltage output waveform of the secondary and auxiliary windings of the transformer, a flat-topped sine wave with a frequency of 100 Hz, the flat top display output voltage is constant, and the sine wave bottom shows the input voltage as a sine wave.

It can be seen that the ripple of the output voltage is small and meets the design requirements. According to these measurement data, this circuit has a good constant current characteristic within a wide range of input voltage variation, and can fully meet the requirements of LED constant current driving. Achieving the requirements of efficient constant current.

The average top-f spring average bacteria 漤 front screen g-contained voltage output waveform 5 Conclusion The adaptive LED street light power control system designed in this paper has high efficiency, low cost and strong stability compared with the traditional two-stage power drive system. The advantages are that the output current can be automatically adjusted according to the fault phenomenon and temperature conditions to ensure stable operation of the LED. The power control system received good results in the road lighting application of the Expo Park LH).

In addition, the LED street lamp power control system developed in this paper also has the Zigbee wireless remote control capability, which realizes communication with Zigbee through the system control chip to complete the distributed control.

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