Driving and Application of White Light LED

In order to ensure that lighting-grade white light LEDs can not only be well applied but also achieve high utilization efficiency, first of all, it is necessary to meet certain application conditions, and secondly, it is necessary to adopt appropriate driving circuits to meet the parameter matching requirements of LDE work.

1. Application requirements

1. The driving circuit is a special power supply designed for LEDs. It should have a simple circuit structure, a small footprint, and high conversion efficiency.

2. The output electrical parameters (current, voltage) of the driving circuit must match the technical parameters of the driven LED, meet the requirements of the LED, and have high-precision constant current control and appropriate voltage limiting functions. When there are multiple outputs, each output must be able to be controlled separately.

3. It has a dimming function with good linearity to meet the requirements of LED brightness adjustment in different applications.

4. In abnormal conditions (LED open circuit, short circuit, drive circuit failure), the circuit can provide corresponding protection for the circuit itself, LED and user.

5. When the driving circuit is working, it has little interference with the normal operation of other circuits and meets the relevant electromagnetic compatibility requirements.

2. Linear drive application

Linear drive application is the simplest and most direct drive application mode. In lighting-level white light LED applications, although there are problems such as low efficiency and poor regulation, it is widely used in some specific occasions because of its simple circuit, small size and ability to meet general requirements.

1. Regulated power supply Vdd + ballast resistor R mode Figure 1 shows the driving mode of regulated power supply + ballast resistor. The advantages of the circuit are simple structure and low cost. 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. Therefore, the overall efficiency obtained by superimposing the two is very low (<50%), and the control accuracy of the LED current is low, and the brightness cannot be adjusted. Therefore, it is generally only used in occasions with low power and short-time lighting, such as LED flashlights and emergency lighting.

2. Regulated current power supply Vdd + ballast resistor R + electronic switch S mode: The driving mode shown in Figure 2 is an improved mode of the driving mode in Figure 3. Its advantages not only improve the control accuracy of the LED current, but also the brightness of the LED can be adjusted by changing the on-off ratio of the electronic switch S. However, due to the high additional losses of the series resistor and the linear regulated current power supply, the overall efficiency is still very low, and the specific application range is greatly limited.

3. Switching drive application

Good current control accuracy and high overall efficiency can be obtained by using switch type drive.

The application methods are mainly divided into two categories: buck and boost.

[page]1. Buck switch drive mode: Buck switch drive is used for applications where the power supply voltage is higher than the terminal voltage of the LED or 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 turned on and off as required to obtain the current lf value that meets the requirements of the LED when it is working. Through the negative feedback of the current (current sampling by R0), the current If flowing through the LED is stabilized within a certain range, and it can also have a certain dimming function. The role of the inductor L in the figure is to store energy when S is turned on and to continue the current after S is turned off, so as to reduce the fluctuation of the current If flowing through the LED.

2. Boost switch drive Boost switch drive is used for applications where the power supply voltage is lower than the terminal voltage of the LED or 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 in the inductor L. After S is turned off, the voltage polarity on L is reversed and superimposed with the power supply voltage Vf to obtain the current value If and voltage value Vr that meet the requirements of the LED operation. Through the current negative feedback function (current sampling by R0), the current If flowing through the LED is stabilized within a certain range, and a certain range of dimming function can also be achieved.

4. Dimming application method

Lighting-grade white light LEDs are not suitable for adjusting their brightness by changing the pulse width in a linear manner. Instead, the amplitude of the current If (the working current of the LED) should be kept constant, and only the width of the current pulse j per unit time should be changed. This is because it will not change the spectrum of the light and cause the color cast of the white light. Common dimming applications mainly use the following methods: 1. Pulse width modulation: 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 can be changed within a larger range, and a larger range of dimming effects can be obtained.

2. Frequency modulation: Frequency modulation is another way to adjust the brightness of LEDs. Keep the width of the rectangular pulse current (amplitude unchanged) applied to the LED unchanged, and change the number of rectangular pulses applied to the LED per unit time; the average current obtained by the LED changes in a larger range, so that the LED brightness can be adjusted in a larger range.

3. Bit angle modulation: Bit angle modulation uses a series of binary pulses, and the width of each bit of the pulse sequence is extended according to the proportion of its bit value. By changing the width of the rectangular pulse applied to the LED per unit time; the current occupies the extended width of the bit value, the average current obtained on the LED changes in a larger range to adjust the brightness of the LED.

In short, lighting-grade white light LED is a high-power solid-state light-emitting device that can be used to replace ordinary lighting. Although it is restricted by the current price, which to a certain extent restricts the speed of application, due to its excellent performance, with the continuous advancement of research and development technology, it can be predicted that lighting-grade white light LED will definitely have good application prospects.