To the application, lighting systems consisting of solar cells, batteries, controllers and LEDs are gaining more and more attention. However, since the price of electricity generated by solar cells is relatively expensive at present, power saving control should be adopted for LED lighting systems. This paper mainly discusses several LED control methods, analyzes the working principle of these kinds of circuits, finds the control mode of comparative power saving, and emphasizes the control mode suitable for solar LED lighting system. The fourth generation light source white LED belongs to semiconductor Light emitting device. Its luminous mechanism, structure and production process are completely different from incandescent lamps, fluorescent lamps and HID lamps. A semiconductor light-emitting diode is an injection-type electroluminescent device. As a novel semiconductor light source, its main features are 111: low voltage drive, small size, good directionality, firm structure, high light efficiency (currently up to 10 lifetimes). Long (more than 20,000h), fast response and other characteristics, is one of the most competitive new energy-saving light sources. LED energy consumption is small, is an energy-saving light source, the current white LED's light efficiency has reached 251mW, more than The level of ordinary incandescent lamps, and now LED technology is developing rapidly. LED lights have created lighting for home lighting, highway lighting, mobile lighting, military lighting, equipment lighting, and construction, stage, surgery, etc. 121. Because of LED The characteristics of low-ultraviolet radiation (without filter glass) make the white LED light source gradually being used in lighting. It is composed of solar photovoltaic power storage battery, controller and LED. The solar battery generates electric energy under the control of the controller. The battery is charged, and under the control of the controller, the stored energy of the battery is changed to an appropriate level to drive the LED to emit light. In the system, because solar cells are expensive and batteries are expensive, in addition to their large size and heavy weight, it is especially important to improve efficiency (in fact, where LED lighting is used, mainly to save energy, non-solar-powered LED lighting systems also emphasize efficiency. From the perspective of improving efficiency, there are three main problems in solar lighting systems that are worthy of focus. One is how to make solar cells charge the battery with maximum power, and the other is how to make the required brightness of LEDs with minimum energy consumption. The third is how to reduce the loss of light efficiency of LEDs combined. The first question will be introduced in a future article, this article focuses on several ways to improve the efficiency of LED driving. Because the luminous flux emitted by a single LED is not big enough Therefore, whether for lighting or for signals, the lighting device or the LED signal is equipped with a large number of LEDs connected in series and in parallel (the low heat release of the LEDs allows them to be arranged in a compact display). Structure) This of course increases the difficulty of control, but their control principles are similar.
1 direct control mode LED direct control mode circuit as shown, R is the current limiting function, VT control LED work, for example, the rated working current of the LED is 20mA, long-term operation beyond this current, easy to make the LED semiconductor chip burn Bad. In order to ensure that the operating current of the LED is 20nA, the size of the current limiting resistor can be determined according to the magnitude of the power supply voltage and the tube voltage drop of the VT. It can be found that the control method is easy to implement according to the control requirements, but since the current limiting resistor R consumes power, the higher the power supply voltage, the greater the power consumed by R. For example, a large number of TTL circuits use a 5 volt power supply, which consumes 50% or more of the electrical energy. On the other hand, if it is intended to reduce the resistance loss by lowering the driving voltage or connecting the LEDs in series, this is effective within a certain range, but if the resistance is too small, it will be difficult to stabilize the flow, and a slight fluctuation of the voltage will cause a significant change in the luminance of the light.
Because this control method is relatively simple, it is a kind of LED control method that is currently used more. However, this type of control is unscientific for solar-powered or other energy-saving LED lighting systems.
After the diode is turned on, the voltage is slightly increased, the current flowing through the LED rises rapidly, and the luminous intensity of the LED increases linearly with the increase of the current, and the LED whose linear intensity rises with the increase of the current is suitable for the pulse. Work under the circumstances, not easy to heat. In the case where the average current and the DC current are equal, the pulse working luminous intensity is larger, combined with the characteristics of the light-emitting diode response speed, so the control circuit shown is used to analyze the working principle of the circuit, and it is known that the control voltage K is low in the pulse signal. At the level, VT is cut off, and the voltage across the LED is the sum of the supply voltage and the inductance back EMF, so the circuit is an LED boost control circuit. When the control voltage V is high for the pulse signal, the control tube VT is turned on, and the current flowing through the inductor is R is the equivalent resistance of the loop, which means that the smaller L is, the larger the R is, the faster the current rises. The maximum current is equal to /R. When VT is turned off, the current passes through the LED without abrupt changes, causing the LED to start working. The resistance is large, the current rises fast, but the maximum current is small, the energy storage of the inductor is small, so the supply resistor and the LED consume time are shorter; while the inductance is large, the current rises slowly, the current has not reached the maximum value, and the triode has been cut off. The energy storage of the inductor is also small, so that the time required to supply the resistor and the LED is short. This requires a reasonable match between the parameters of the circuit and the pulse frequency, so that the maximum pulse operating current of the LED should be such that the temperature of the chip junction junction is equal to the maximum junction temperature can be expressed as /= AeUT, U is the voltage across the PN junction, I Is the current flowing through the diode, A is the reverse saturation current, Ur is called the temperature voltage equivalent, the volt-ampere characteristic of the LED is as shown, as seen from the disk - when the applied voltage is greater than the threshold voltage and the switch The saturation voltage drop and the cutoff frequency fT of the switching tube. It is the LED voltage waveform measured when the inductance values ​​are different and the switching period is the same. From the point of view, the voltage waveform is a 3-step buck control mode buck control mode circuit. For example, the circuit is analyzed. When VT is turned on, the current flows from the power source E through the inductor L and the switch VT to the ground, which is the inductor current. In the process of establishment, the law of current change can be expressed by formula (1). When VT is cut off, the energy in the inductor is discharged through the LED, and the variation law of the discharge current is discontinuous, indicating that it is in the conduction phase. The electric energy stored in the inductor is not enough to be supplied to the LED, and oscillation occurs in the voltage waveform. Since the optical flux density increases proportionally with the increase of the current density, the luminance of the LED is weakened, which indicates the pulse frequency and the frequency. The selection of the air ratio should consider the size of the inductor. If the inductance is too large, the inductor current will be established for too long when the VT is turned on, so that the energy stored in the inductor is not large enough during the entire conduction period, resulting in LED. The brightness is reduced; the inductance is too small, although the current in the inductor is easy to reach the maximum when the VT is turned on, but the energy storage in the inductor is still insufficient to provide sufficient power to the LED. It is the LED voltage waveform when the inductance parameters and other parameters in the control circuit are more reasonable.
I is the current at the VT cutoff, R is the sum of the wire resistance of the inductor and the equivalent resistance of the LED. For the same initial current, the larger the R, the greater the power of the 4 conclusion resistor, so that the energy storage is faster by the resistor. Consumed, because the internal resistance of the LED is basically constant, it can save energy by reducing the internal resistance of the inductor; the larger the L, the larger the energy storage capacity of the inductor, and the longer the discharge time, the longer the LED illumination time, with With the extension of the discharge time, the discharge current is gradually reduced, the operating current flowing through the LED is also reduced, and the luminous intensity is also weakened. In order to make the LED have a certain luminous intensity, the pulse frequency and duty ratio should be controlled. Consider the power supply voltage, the size of the inductor, and the opening of the LED.
The lighting system is generally designed to meet the requirements of solar panels and accumulators in the case of rainy days for 3 to 5 consecutive days. Thus, for the three LED controllers discussed above, the direct control method is uneconomical.
In all three control modes, the pulse signal can be used to control the conduction of the switch tube. When the duty cycle of the pulse signal is constant, the average current of the pulse is equal to the DC current pulse, which can make the LED emit more light, and when the maximum peak current is allowed. The maximum pulse operating current of the LED should be considered so that the temperature of the PN junction region of the chip is equal to the maximum allowable junction temperature. In order to ensure the best luminous efficiency of the LED in the pulse mode, the parameters in the control circuit should be optimized.
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