A blue LED light source driving circuit design

Semiconductor (LED) lighting technology, known as "green lighting", has developed rapidly. LED has the advantages of low power consumption, long service life, small size, and green environmental protection. Through the continuous research and development of high-intensity blue light LEDs, several generations of devices with increasingly higher brightness have been produced. The efficiency of LEDs based on silicon carbide bare die materials launched around 1990 was about 0.04lm/W, and the light intensity emitted rarely exceeded 15 mcd. The first practical LED based on GaN appeared in the mid-1990s. There are still many companies producing GaN LEDs on different substrates (such as sapphire and silicon carbide), which can emit light in colors such as green, blue or violet. The invention of high-brightness blue LEDs has made it possible to realize true-color advertising displays that can display true-color, full-motion video images. With the continuous advancement of technology, high-power and high-brightness blue light LEDs with a power of up to 100 W and a luminous flux of up to 2,000 lm can be produced. Therefore, as a technology with broad application prospects, it is necessary to conduct in-depth research on it. The underwater imaging system is based on the low-loss window of seawater for blue-green light (480-540 nm). It can be used not only in ordinary water, but also in turbid water, and even in the dark seabed. This paper is mainly based on the research of high-power blue LED underwater imaging system. The high-power blue LED (≥50 W) light source drive circuit is an important part of the research and analysis of underwater imaging system based on high-power blue LED.

1 Basic Principles and Circuit Design

For underwater imaging systems, the choice of light source is crucial. The stability, uniformity, response speed and wavelength range of the light source are the key to system imaging. The light emitted by the blue light LED passes through the water medium-target reflection-water medium-receiving optical system, and finally forms an image on the imaging sensor (CCD, ICCD). This determines that the LED must work in a fast pulse driving mode. The driving circuits on the market generally work in a slow pulse driving mode (μs level), which is far from meeting our requirements. Therefore, we must solve the problem of fast pulse driving circuits by ourselves, especially to achieve high-power fast pulses in the order of ns to drive high-power LEDs. Based on the actual needs of this experiment, we have developed this fast pulse driving circuit. The basic function of this circuit is to generate a low-frequency, fast, high-power driving current pulse signal. The frequency and period of the output pulse have high accuracy and good stability. The pulse signal width is adjustable in 5 levels within 20 to 100 ns, and the output works at a reference frequency of 50 Hz. The pulse signal passes through the driving circuit, outputs a fast pulse signal to drive the output current switch, and generates a large current pulse signal to drive a high-power LED. In order to achieve the design requirements and objectives, we use a crystal oscillator circuit to generate a high-precision frequency/period reference pulse as a built-in signal source. By dividing the reference pulse multiple times, an output pulse signal of the required frequency is generated, so that a pulse signal with a stable frequency can be obtained, and frequency selection is achieved at the same time. The divided pulse signal is then passed through a pulse width shaping circuit to achieve pulse width adjustment, and then passes through a current switching circuit to generate a large current pulse to make the LED light up. The drive circuit is mainly composed of the following parts: signal source, frequency division circuit, frequency selection, pulse width circuit, TTL output, drive circuit and current switching circuit. Its block diagram is shown in Figure 1.

2 Main unit circuits and principles

As mentioned above, the circuit is mainly composed of signal source circuit, frequency division circuit, frequency selection, pulse width shaping circuit, drive circuit, high-power current switch and other unit circuits. The signal generated by the built-in signal source is sent to the drive circuit to generate a drive signal after frequency division and pulse width shaping, which drives the high-power current switch to control the high-power blue LED light source circuit. The synchronized TTL output signal can be used as a detection signal or a trigger and drive signal for other circuits.

2.1 Signal source circuit

A quartz crystal multivibrator can be formed by connecting a quartz crystal and the coupling capacitor of an asymmetric multivibrator in series. The oscillation frequency of this circuit depends on the natural frequency f0 of the quartz crystal and has nothing to do with the parameters of the external resistor. The coupling capacitor plays the role of fine-tuning and compensating the oscillation frequency. We use a quartz crystal with a natural frequency f0 of 10 MHz to form a built-in signal source. The frequency/period accuracy of its output pulse is high and the stability is good.

2.2 Frequency division circuit and pulse shaping circuit

We get a 10MHz pulse signal from the crystal oscillator circuit, and we get the 50Hz reference pulse we need by dividing it by 10 five times and dividing it by 2 once. All the divisions are done by the decimal counter 74HC390. The shaping circuit uses the DS1040-100 programmable pulse signal shaping device. The integrated circuit has an input terminal and two output terminals with opposite polarities. The frequency selection circuit sends the 50 Hz output signal to the input terminal of the DS1040. The width of the output pulse signal is controlled by the three control terminals (P0, P1, P2) on the DS1040-100. By setting the dip switch, we can select 5 different widths of pulse signals, such as 20, 40, 60, 80, and 100ns. The shaping signal has good stability and meets the needs of the underwater imaging system.