In order to ensure that the illumination-level white LED can not only get good application, but also can obtain high efficiency, firstly, it needs to meet certain application conditions, and secondly, it needs to adopt adaptive drive circuit to meet the parameter matching requirements of LDE work. .
First, application requirements
1. The drive circuit is a special power supply specially designed for LEDs. It has a simple circuit structure, a small footprint, and a high conversion efficiency.
2. The output electrical parameters (current, voltage) of the drive circuit should match the technical parameters of the driven LED to meet the requirements of the LED, and have a high precision constant current control and a suitable voltage limiting function. When multiple outputs are used, the output of each channel must be individually controllable.
3. It has a dimming function with good linearity to meet the requirements of LED illumination brightness adjustment for different applications.
4. In the abnormal state (LED open circuit, short circuit, drive circuit failure), the circuit can have corresponding protection for the circuit itself, LED and user.
5. When the drive circuit works, it has less interference to the normal operation of other circuits and meets the relevant electromagnetic compatibility requirements.
Second, linear drive applications
The linear drive application is one of the simplest and most direct drive applications. In the illumination-level white LED application, although there are problems such as low efficiency and poor adjustability, the circuit is simple and compact, and can meet general requirements, so it is widely used in some specific occasions.
1. Regulated power supply Vdd+ ballast resistor R mode Figure 1 shows the driving mode of the regulated power supply + ballast resistor. The advantage of the circuit is that the structure is simple and the cost is low. The additional loss on the resistor in series with the LED is large, and the power consumption of the linear regulated power supply Vdd itself is also large, so the overall efficiency of the two is superimposed ("50%"), and the LED current The control precision is low and the brightness is not adjustable, so it is generally only used in 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 driving mode is the improved way of Figure 3 driving mode, its advantages not only improve the control accuracy of the LED current, but also the brightness of the LED can also be changed The on/off ratio of the electronic switch S is adjusted. However, since the series resistors and the linear steady current power supply have higher additional losses, the overall efficiency obtained is still low, and the specific application range is greatly limited.
Third, switch drive application
Switched drive can achieve good current control accuracy and high overall efficiency, switching drive.
Application methods are mainly divided into two categories: buck and boost.
1. Buck-type switch drive mode: The buck switch drive is applied to the case where the power supply voltage is higher than the terminal voltage of the LED or when multiple LEDs are driven in parallel.
The main principle of the circuit is to use the chopping current obtained by the electronic switch S that is required to be turned on and off to obtain the current lf value required to meet the LED operation, and to pass through the LED through the negative feedback action of the current (current sampling by R0). The current If is stable within a certain range, and can have a certain dimming function. The function of the inductor L in the figure is to perform the energy storage after the S is turned on and the freewheeling action after the S is turned off to reduce the fluctuation of the current flowing through the LED If. 2. Step-up switch drive The step-up switch drive is applied to the case where the power supply voltage is lower than the terminal voltage of the LED or when multiple LEDs are driven in series.
The working principle of the circuit is to use the on/off function of the electronic switch S that is turned on and off as required. When S is turned on, the power supply Vdd stores energy to the inductor L. After the S is turned off, the polarity of the voltage on the L is reversed and the power supply voltage Vf is superimposed to obtain the current value If and the voltage value Vr required to meet the LED operation, and the current negative feedback is passed. The function (current sampling by R0) makes the current If flowing through the LED stable within a certain range, and can have a certain range of dimming functions.
Fourth, dimming application
Illumination-level white LEDs are not suitable for adjusting the brightness of their illumination by linear hand-changing segments. The amplitude of the current If should be the same (the operating current of the LED), and only the mode of the current pulse j in the unit time is changed. Dimming, because it does not change the spectrum of its light and causes the color cast of white light. Commonly used dimming applications mainly use the following methods: 1. Pulse width modulation mode: Pulse width modulation is a common way to adjust the brightness of LEDs. By changing the width of the rectangular pulse current applied to the LED, the average current obtained on the LED is changed over a wide range, and a wide range of dimming effects can be obtained.
2. Frequency modulation mode: Frequency modulation is another way to adjust the brightness of LEDs. Keep the width of the rectangular pulse current (the amplitude is constant) added to the LED unchanged, by changing the rectangular pulse added to the LED per unit time; the number of currents in the LED makes the average current obtained on the LED larger. Changes occur within the range, resulting in a wide range of adjustments in LED brightness.
3. Position angle modulation mode: The bit angle modulation uses a series of pulses containing binary sequences, and each bit width of the sequence pulse is extended according to the proportion of its bit value. By changing the rectangular pulse added to the LED per unit time; the medium current occupies the width extended by the bit value, so that the average current obtained on the LED changes over a 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 to replace ordinary illumination. Although it is subject to the current price, it restricts the speed of the application to a certain extent, but because of its excellent performance, The continuous improvement of R&D technology can predict that the illumination-grade white LED must have a good application prospect.
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