Design of intelligent driving system for LED lamps

0 Introduction

LED lighting is the mainstream of energy saving in the world, and high-power LED lighting is the mainstream trend of lighting systems in the future. High-power LED has the characteristics of high brightness, energy saving, environmental protection, safety and high stability, and saves 60% to 70% of electricity compared with traditional light sources.

Traditional sound and light controlled delay controllers can well control lights. When it is dark or at night, they can effectively achieve "lights turn on when people come and turn off when people leave". However, since the switch uses a mechanical controller such as a relay, it is easy to be damaged due to frequent switching in places with a lot of traffic.

Based on the existing high-power LED spotlight products on the market, an intelligent lighting system was designed that can be used in corridors, stairs, classrooms and other places.

The system is powered by a 24 V / 3 A switching power supply, and multiple LED intelligent lighting modules can be distributed at the lower level (see Figure 1). The controller uses AT89C2051, and the constant current driver uses PT4115 chip. The on and off of the lights and the brightness are realized and changed by dimming.

The DIM terminal of the PT4115 chip can use PWM dimming to adjust the output current from 0 to 100%. Based on this feature, an intelligent dimming module integrating voice control, light control and time control is designed. Its functions are:

(1) When there is light (natural light), the light will not turn on regardless of whether there is sound or not.

(2) When there is no light (natural light), if someone passes by and makes a sound, the controller first determines the time period. If it is completely dark (defined as the first time period), the LED will emit 100% brightness. In the rest of the time period, when the sky is still bright, the controller can output a 50% duty cycle square wave, so that the LED can emit 50% brightness. This can further save energy and improve power efficiency.

Figure 1 Block diagram of LED intelligent drive system

1 Constant current drive based on PT4115

1.1 PT4115 chip introduction

PT4115 is a step-down constant current source in continuous inductive current conduction mode, used to drive one or more series LEDs. PT4115 input voltage range is 6 V ~ 30 V, output current is adjustable, up to 1. 2A. Depending on the input voltage and external devices, PT4115 can drive up to tens of watts of LED. PT4115 has a built-in power switch, uses high-end current sampling to set the average current of LED, and can accept analog dimming and a wide range of PWM dimming through the DIM pin. When the voltage of DIM is lower than 0. 3 V, the power switch is turned off, and PT4115 enters a standby state with extremely low operating current.

1. 2 LED spotlight driving circuit

When V IN is powered on, the initial current of the inductor (L) and the current sampling resistor (RS) is zero, and the LED output current is also zero (see Figure 2). At this time, the internal power switch is turned on and the potential of SW is low. The current flows from V IN to ground through the inductor (L ), current sampling resistor ( RS ), LED and internal power switch. The slope of the current rise is determined by V IN, inductor (L ) and LED voltage drop, generating a voltage difference VCSN on RS. When (V IN-VCSN ) > 115 mV, the internal power switch is turned off, and the current flows through the inductor (L ), current sampling resistor (RS ), LED and Schottky diode (D ) at another slope; when (V IN-VCSN ) < 85mV, the power switch is turned on again, so that the average current on the LED is IOUT = (0. 085 + 0. 015) /2 ?RS = 0. 1 /RS. If the dimming function is not used, the DIM pin can be left floating, and the set maximum current can be output at this time.

Figure 2 LED spotlight driving circuit

2 72W switching power adapter design based on Top249Y

2.1 Transformer design

The following points should be noted in the design of high-frequency transformers: 1) In the design of high-frequency transformers, at the maximum output power, the magnetic induction intensity in the magnetic core should not reach saturation to avoid distortion when the signal is large. 2) During transients, high-frequency chain leakage inductance and distributed capacitance will cause surge current, peak voltage and pulse top oscillation, which will increase losses and, in severe cases, damage the switch tube. At the same time, when the output winding has many turns and many layers, the influence of distributed capacitance should be considered. Reducing distributed capacitance is beneficial to suppressing the interference of high-frequency signals on the load. It is difficult to reduce distributed capacitance and leakage inductance for the same high-frequency transformer at the same time. Appropriate capacitance and inductance should be ensured according to different working requirements.

2.2 Switching power supply circuit

In Figure 3, C6 is an X-type capacitor that filters out the series-mode interference between power grids. L2 is a common-mode suppressor that can filter out common-mode interference, and C1 is an input filter capacitor. R11 uses a 2MΩ resistor value to achieve undervoltage and overvoltage detection, while providing voltage feedforward to reduce the output voltage frequency ripple.

The DC voltage range of TOP249Y in this circuit is 100~450V. Once this voltage range is exceeded, TOP249Y will automatically shut down. Resistor R10 uses a 20.5K resistor value to set the current limit value externally to just slightly higher than the full-load peak current when operating at low voltage, allowing the use of a smaller transformer core while avoiding core saturation during startup and output load transients. VR1 is a transient voltage suppressor tube, model P6KE200, and capacitor C11 is connected in parallel with it to reduce the loss of the Zener clamp. D1 is a blocking diode model UF4006. The target clamping voltage average is about 180 VR4, R5, R6 are sampling resistors for the output voltage. After sampling, they are compared with the internal reference voltage of TL431 to generate an error voltage, which is then fed back to the control pin C through the PC817A optocoupler, thereby changing the output duty cycle of TOP249Y and stabilizing the output voltage.

Figure 3 72W switching power adapter

3 Intelligent controller based on AT89C2051

The intelligent controller circuit based on AT89C2051 is shown in Figure 4, which is mainly composed of a sensor unit, an AD conversion unit, and a controller unit. The AT89C2051 chip is used to process the signal data after shaping from the signals detected by the sound control and light control sensors, and then control the LED driver. In this circuit, the p3.0 ​​and p3.1 ports of AT89C2051 are used for input signal detection, and the remaining 13 ports can select output control. The software flow chart is shown in Figure 5.

Figure 4 Intelligent controller circuit diagram

Figure 5 Software Flowchart

4 Summary

The LED spotlight intelligent drive system designed by the author can effectively monitor and detect changes in the surrounding environment, and turn off and on the light source and dim the light in time. Compared with the traditional sound and light controlled delay switch lighting system, this system can not only save a lot of electricity, but also greatly improve the electricity efficiency of its unique dimming module. This system has a good application prospect in engineering.