LED lighting design LED electrical characteristics and simple drive circuit

LED lighting has received widespread attention as a new generation of lighting. Relying solely on LED packaging cannot produce good lighting fixtures. This article mainly explains how to use LED characteristics to design from the aspects of electronic circuits, thermal analysis, and optics .

LED

LED is a type of electronic diode, and its main structure is PN junction . As shown in Figure 1, when voltage is applied to the two ends of the LED, the electrons will absorb energy and transfer to the valence electron band, and then release the absorbed energy. This released energy is light. The wavelength and color of the emitted light are determined by the potential difference of the semiconductor .

Figure 1 LED operation principle

LED applications

LED has the advantages of high luminous efficiency , long life, light weight, and no harmful substances. High luminous efficiency can increase the service life of the battery, which is very suitable for portable products. The service life of LED is 2 to 3 times that of ordinary incandescent lamps (some data show that it is 40 times). A typical example of using LED because of its long life is traffic lights . In addition, since LED lights have a very fast response speed, they are also suitable for car brake lights. The design freedom of LED lamps is very high, and it can not only adjust the brightness but also the color. LED electronic characteristics LED has similar characteristics to general silicon diodes. Voltage is applied between the positive and negative electrodes. When the applied voltage reaches a critical value, current is generated in the LED and it begins to emit light. When the voltage exceeds this critical value, the current increases sharply. The critical voltage value of white LED is about 3.5V. The critical voltage values ​​of red, green, and blue LEDs are shown in Table 1.

Table 1

We use circuit simulation to illustrate the electronic characteristics of LEDs and the drive circuit. Circuit simulation is a simple method to model the electronic design of LEDs and resistors and simulate the operation of the circuit on a computer . The circuit simulation PSp ic e A/D from Cadence is used here . The LUXEON series LX3-PW71 from Philips Lighting is also used .

First, we will verify the electronic characteristics of the LED using the circuit in Figure 2. To obtain the voltage-current characteristics, a DC analysis is required.

Figure 2 Circuit for verifying voltage-current characteristics

The analysis results are shown in Figure 3.

Figure 3 Analysis results obtained using PSpice A/D: Voltage-current characteristics

The results of verifying the voltage-current characteristics of LXM3-PM71 show that the current is about 354mA at 3.0V.

Drive circuit

Incandescent lamps use a voltage-driven circuit design. LEDs use a current-driven circuit design, which adjusts brightness and color by controlling the current in the LED. The electronic drive in lighting fixtures such as incandescent lamps and fluorescent lamps cannot be directly applied to LED lighting fixtures. In order to maximize the advantages of LEDs, it is necessary to design a drive circuit suitable for LEDs. Moreover, even if a highly efficient LED is used, if the efficiency of the drive circuit is not high, it will also affect the luminous efficiency of the entire lighting fixture. The design of LED lighting fixtures is roughly divided into the following three stages. 1. Optical design, 2. Thermal design, 3. LED drive circuit Design. Here we first introduce the LED drive circuit design.

- Resistor drive circuit

The following introduces the design method of LED resistor drive. The LED, resistor, and DC voltage source are connected as shown in Figure 4. Based on the voltage value and voltage drop (forward voltage) of the DC voltage source, the resistance value corresponding to the current in the LED light emission is calculated. The calculation formula is as follows:

For example, the LXM3-PW71 is driven by a 5V voltage source. The forward voltage of the LED is 3.0V, and when the current is 350mA, the resistance should be 5.71. Figures 4 and 5 show the circuit in the circuit simulation operation and its analytical results. From Figure 5, we can see that the voltage in the LED is about 3.0V and the current is about 350mA.

Figure 4 Resistive drive circuit Figure 5 Transient analysis results

- LED forward voltage deviation

This circuit does not consider the deviation of the forward voltage, which is an important design factor of the LED circuit . There will always be deviations in the electronic design characteristics of LEDs. Even if the design is the same model, the same characteristics cannot be obtained if they are measured separately . The deviation varies depending on the LED, and there are cases where the deviation in the forward voltage exceeds 15%. The forward voltage of the LXM3-PW71 fluctuates between 4 and 2.5V. In this way, if the previous resistor-type drive circuit is used, the current will also vary greatly depending on the forward voltage of the LED.

High- power lighting fixtures usually require multiple LEDs. If you simply connect these LEDs in series and drive them with resistance, the circuit will not work. For example, if you connect three LXM3-PW71 in series and drive them with 12V, if the three LEDs have the maximum forward voltage at the same time, they will not work due to insufficient voltage because the total voltage is 12V. And even if the current is the same, it does not mean that all LEDs will emit the same light. The same is true for parallel LEDs.

So is there any way to solve the problem of LED forward voltage deviation?

There is only one answer, which is to classify LEDs. During the design and manufacturing process, different characteristics will cause deviations. Under the same conditions (same production line, same day), the errors between products manufactured in the same batch are usually much smaller than the errors with products manufactured in other batches. LED classification is to arrange LEDs of the same model in the order of forward voltage deviation. The Bin table corresponding to the forward voltage is recorded in the database of LXM3-PW71 (Figure 6). For example, when using Bin Code C LEDs, the forward voltage is between 2.79 and 3.03V.

Figure 6 Bin table corresponding to forward voltage

- Constant current source drive circuit

To adjust the brightness of the LED, the current of the LED needs to be increased or decreased. Since the sensitivity of the human eye to light is a logarithmic relationship, when adjusting the current, the increase or decrease rate of the current changes exponentially with the brightness of the LED. Therefore, the resistor type drive circuit is not the most suitable for lighting fixtures. Even if the voltage is biased, a constant current source drive circuit is required to maintain a constant current.

Figure 7 Constant current source drive circuit

Figure 8 is an example of a constant current circuit using semiconductors and diodes. To keep the base potential of the semiconductor constant (about 1.2V) when the supply voltage value changes, two diodes (D1, D2) are needed. The current of the LED is connected to the resistor on the emitter. Calculation shows that the collector and emitter currents of the semiconductor are basically the same. In this example, in order to maintain the LED current at 350mA, a 1.48 ohm resistor is needed.

Figure 8 Simple constant current source drive circuit

Even if the input voltage changes, the voltage of diodes D1 and D2 is stable, and the base potential of the semiconductor is stable, ensuring that the current of the LED is 350mA. The actual input voltage is changed between 5V and 24V through circuit testing. The test results are shown in Figure 9.

Figure 9 Voltage change verification results of constant current source drive circuit

When the input voltage changes between 5V and 24V, the voltage of diodes D1 and D2 will also change slightly. From the central waveform of Figure 9, it can be seen that this slight change occurs at the base of the diode. The current and change in the LED can be read from the top waveform.

In order to maximize the advantages of LED, the PWM type drive circuit design method should be used in the drive circuit design. In the next chapter " LED drive circuit design (4) pulse modulation PWM circuit detailed explanation ", we will mainly focus on the PWM type drive circuit.