In order to ensure that the lighting level white LED can not only be well applied, but also can obtain high efficiency, the first is to make it meet certain application conditions, and the second is to use the appropriate drive circuit to meet the requirements of the LDE working parameters. .
First, the application requirements 1, the drive circuit is a special power supply for LED power supply, to have a simple circuit structure, a small footprint, and a high conversion efficiency.
2. The output electrical parameters (current, voltage) of the drive circuit must be matched with the technical parameters of the LED to be driven to meet the requirements of the LED, and have a high precision constant current control and appropriate voltage limiting function. For multiple outputs, each output must be individually controlled.
3, with a good linearity of the dimming function to meet the different applications of the LED light brightness adjustment requirements.
4, in the abnormal state (LED open circuit, short circuit, drive circuit failure), the circuit can have a corresponding protective effect on the circuit itself, the LED and the user.
5. When the drive circuit is working, it will not interfere with the normal operation of other circuits and meet the relevant electromagnetic compatibility requirements.
Second, linear drive applications Linear drive applications is the most simple and most direct drive application. In lighting-level white LED applications, there are problems such as low efficiency and poor adjustability. However, due to its simple circuit and small size, it can meet the general requirements, so it is used in some specific applications.
1. Regulated power supply Vdd + ballast resistor R mode Figure 1 shows the driving mode of the regulated power supply + ballast resistor. The circuit has the advantages of simple structure and low cost. The additional loss on the resistor in series with the LED is larger, and the power consumption of the linear regulator power supply Vdd itself is also large, so the overall efficiency obtained by the superposition of the two is low (<50%), and the LED current is The low control precision and the uncontrollable brightness are generally used only in the occasions with low power and short time lighting, such as LED flashlights and emergency lighting.
2, steady current power supply Vdd + ballast resistor R + electronic switch S mode: Figure 2 shows the drive mode is an improved mode of Figure 3 drive mode, its advantages not only improve the LED current control accuracy, but also the brightness of the LED can be changed The on/off ratio of the electronic switch S is adjusted. However, due to the high losses of series resistance and linear steady-state power supply, the overall efficiency obtained is still very low, and the specific application range is greatly limited.
Third, the use of switch-driven drive switch-type drive can get a good current control accuracy and high overall efficiency, switch-driven.
The application is mainly divided into two categories: buck and boost.
1. Step-Down Switch Drive Mode: The step-down switch drive is used for applications where the power supply voltage is higher than the LED's terminal voltage or when multiple LEDs are used in parallel drive conditions.
The main principle of the circuit is to use the chopped current obtained by the electronic switch S that is switched on and off in accordance with the requirements to obtain the current lf that meets the requirements of the LED during operation. The negative feedback effect of the current (current sampling by R0) causes the LED to flow through the LED. The current If is stable within a certain range and at the same time can have a certain dimming function. The role of the inductor L in the figure is to play the freewheeling action after the S-opening and the S-off, in order to reduce the fluctuation of the LED current If flowing through. 2. Step-up switching driver Step-up switching driver is used for applications where the supply voltage is lower than the LED's terminal voltage or when multiple LEDs are used in series.
The working principle of the circuit is to use the on-off function of the electronic switch S which is on and off as required. When S turns on, the power supply Vdd stores energy to the inductor L. After the S turns off, the polarity of the voltage on L is reversed and the supply voltage Vf is superimposed to obtain the current value If and the voltage value Vr required when the LED is working, and the negative current feedback is achieved. The function (sampling of current by R0) makes the current If flowing through the LED stable within a certain range and can simultaneously have a range of dimming functions.
Fourth, dimming applications Lighting level white LED is not suitable to use the linear hand to change the segment to adjust the brightness of the light, should use the current If the amplitude of the same (LED operating current), only change If, current pulse per unit time The j width is used for dimming because it does not change the light spectrum of the light and cause the white color cast. Commonly used dimming applications mainly use the following methods: 1. Pulse width modulation: Pulse width modulation is a common way to adjust the brightness of an LED. By changing the width of the rectangular pulse current added to the LED, the average current obtained by the LED can be changed within a relatively large range, and a wider range of dimming effects can be obtained.
2. Frequency modulation: Frequency modulation is another way to adjust the brightness of the LED. Keeping the width of the rectangular pulse current (invariable amplitude) added to the LED unchanged, by changing the rectangular pulse added to the LED per unit time; the number of medium currents makes the average current drawn on the LED larger Changes in the range, so that a large range of LED brightness adjustment.
3. Bit Angular Modulation: Bit Angular Modulation uses a string of pulses that contain a binary sequence, and each bit width of the sequence pulse is scaled according to the ratio of its bit value. By changing the rectangular pulse added to the LED per unit time, the width of the current occupying the value of the bit, so that the average current obtained by the LED changes within a relatively large range to adjust the brightness of the LED.
In short, the illumination-grade white LED is a high-power solid-state light-emitting device that can be used as an alternative to general lighting. Although subject to the current price, it restricts the application speed to some extent, but due to its excellent performance, With the continuous advancement of R&D technology, it can be predicted that lighting-grade white LEDs must have a good application prospect.
