LED lighting design (3) LED electrical characteristics and simple drive circuit

(From MONOist) (CCS Compilation)
CYBERNET Application Systems Division Prabhakar Abbigeri

LED lighting design (1) LED lighting basic LED lighting design (2) indispensable "heating solution"
LED lighting design (3) LED electrical characteristics and simple drive circuit LED lighting design (4) pulse modulation PWM circuit detailed

LED lighting has received wide attention as a new generation of lighting. Simply relying on LED packaging does not make good lighting fixtures. This article mainly explains how to use LED characteristics to design from electronic circuits, thermal analysis and optics.

led

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

Figure 1 LED operating principle

LED application

LED has the advantages of high luminous efficiency, long life, light weight and no harmful substances. High luminous efficiency increases the life of the battery and is suitable for products that are carried around. The service life of LEDs is 2 to 3 times that of ordinary incandescent lamps (some data shows 40 times). A typical example of LEDs used for long life is traffic lights. In addition, since the reaction speed of the LED lamp is very fast, it is also suitable for the brake lamp of the automobile. The design freedom of LED luminaires is very high, not only to adjust the brightness, but also to adjust the chromaticity.

LED electronic characteristics

LEDs have similar characteristics to typical silicon diodes, applying a voltage between the positive and negative electrodes. When the applied voltage reaches a critical value, a current is generated in the LED to start emitting light. When the voltage exceeds this threshold, the current increases sharply. The threshold voltage of the white LED is about 3.5V. The red, green, and blue LED threshold voltage values ​​are shown in Table 1.

Table 1

We use circuit simulation to illustrate the electronic characteristics of the LED and the driver circuit. Circuit simulation is an easy way to model the electronic design of LEDs and resistors and simulate the operation of a computing circuit on a computer. Here is the circuit simulation PSpice A/D from Cadence. At the same time, Philips Lighting's LUXEON series LX3-PW71 was used.

First use the circuit of Figure 2 to verify the electronic characteristics of the LED. To obtain voltage-current characteristics, DC analysis is required.

Figure 2 Verify voltage-current characteristic circuit

The analysis results are shown in Figure 3.

Figure 3 Analytical results based on PSpice A/D: voltage-current characteristics

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

Drive circuit

Incandescent lamps use a voltage-driven circuit design. The LED is designed with a current-driven circuit that adjusts the brightness and chromaticity by controlling the current in the LED. Electronic drives in lighting fixtures such as incandescent and fluorescent lamps cannot be directly used in LED lighting fixtures. In order to maximize the performance of the LED, it is necessary to design a drive circuit suitable for the LED. Moreover, even if an LED with high luminous efficiency is used, if the efficiency of the driving circuit is not high, the luminous efficiency of the entire lighting fixture is affected. 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 introduce the LED driver circuit design.

- Resistive drive circuit

The following describes the method of LED resistance type drive design. The LED, resistor, and DC voltage source are connected as shown in Figure 4. Based on the voltage value of the DC voltage source and the voltage drop (forward voltage), the resistance value of the corresponding current in the LED illumination is calculated. Calculated as follows:

For example, the above LXM3-PW71 is driven in a voltage source of 5V. The LED has a forward voltage of 3.0V and a current of 350mA. The resistance should be 5.71. 4 and 5 show the circuit in the circuit simulation operation and the analysis result thereof. From Figure 5, it can be seen that the voltage in the LED is approximately 3.0V and the current is approximately 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 as an important design element of the LED circuit. There must be deviations in the electronic design characteristics of the LEDs. Even if the design of the same model is measured separately, the same characteristics cannot be obtained. The deviation differs depending on the LED, and there is a case where the deviation in the forward voltage exceeds 15%. The forward voltage of LXM3-PW71 fluctuates between 4 and 2.5V. Thus, if the previous resistive drive circuit is used, the forward voltage of the LED will be different and the current will vary greatly.

High-power lighting fixtures typically require multiple LEDs. Simply connecting these LEDs in series, if the resistor is driven, there will be a circuit failure. For example, three LXM3-PW71s are connected in series and driven by 12V. If the three LEDs have the maximum forward voltage at the same time, since the total voltage is 12V, it will not operate due to insufficient voltage. And even if the current is the same, it does not mean that all the LEDs are illuminated. The same is true for parallel LEDs.

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

There is only one answer, and the LEDs are graded. In the design and manufacturing process, different characteristics can cause deviations. Mass production under the same conditions (same production line, same day), the error between products manufactured in the same batch is usually much smaller than that of other mass-produced products. LED grading is to arrange the same type of LEDs in the order of deviation of the forward voltage. A Bin table corresponding to the forward voltage is recorded in the database of LXM3-PW71 (Fig. 6). For example, when using the Bin Code C LED, the forward voltage is between 2.79 and 3.03V.

Figure 6 Binding table corresponding to the forward voltage

- Constant current source type drive circuit

Adjusting the luminosity of the LED requires increasing or decreasing the current of the LED. Since the sensitivity of the human eye to light is logarithmic, when the current is adjusted, the rate of increase and decrease of the current changes exponentially with the brightness of the LED. Therefore, the resistive drive circuit is not optimal for lighting fixtures. Even if the voltage is deviated, it is necessary to use a constant current source type drive circuit for a constant current.

Figure 7 constant current source type driving circuit

Figure 8 is an example of a semiconductor and diode constant current circuit. The supply voltage value is changed, and in order to keep the potential of the base of the semiconductor constant (about 1.2 V), it is necessary to use two diodes (D1, D2). The current of the LED is connected to the resistor on the emitter. The calculation shows that the collector and emitter currents of the semiconductor are substantially the same. In this example, in order to maintain the LED current at 350 mA, a 1.48 ohm resistor is required.

Figure 8 Simple constant current source type driving circuit

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

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

When the input voltage varies between 5V and 24V, the voltages of the diodes D1 and D2 also change slightly. As can be seen from the central waveform of Figure 9, this slight change occurs at the base of the diode. Read the current and changes in the LED from the top waveform.

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

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