Integrated power stage LED and constant current source circuit integrated design

1 Introduction At present, power-level LED products can be implemented in two ways: First, a single large-area power-level LED chip package is used. In the United States and Japan, 5W chips have been introduced to the market, and low-voltage and high-current constant-current driving power supplies are required. The price is also relatively high; the other is the use of low-power chip integration to achieve power-level LED, Japan Matsushita Electric has developed 20W integrated LED products. However, since the power stage LED operates under the condition of low voltage and high current, there are many technical problems that are difficult to solve for the long-distance constant current driving power supply, such as large line power consumption and low system reliability. In the national science and technology research project undertaken by the company, we integrated the newly designed DIS1xxx series of floating-voltage constant current integrated diodes and LED chips through thick-film integrated circuit technology to solve the constant current power supply of integrated power-level LEDs in use. Power supply problems, technical indicators such as current stability, temperature drift and reliability, are in line with project requirements. 2 The main parameters are designed to use a single crystal silicon wafer as the substrate, and a silicon dioxide insulating layer and an aluminum conductive reflective layer are formed on the silicon wafer by a bipolar integrated circuit process, and a plurality of LED chips, SMD RC components and the DIS1xxx series are floated. The voltage constant current integrated chips are integrated. Through the photolithography and diffusion process, a reverse voltage stabilizing diode is formed on the single crystal silicon layer for discharging static electricity to improve the antistatic capability of the LED. We designed a 5W power stage integrated LED, using 80 0.3mm × 0.3mm LED blue chip, white light by coating YAG phosphor, the main technical parameters are: Input voltage range Vin: DC 150 ± 5 V; Constant operating current Io : 20 mA × 2 mA; current stability error △ Io: < ± 5 %; constant current temperature drift ΔIT : < 5 μA / ° C; antistatic voltage: VEDS ≥ 1500 V; electric power: Pm ≥ 5 W (with heat sink Light efficiency ≥17 Lm/W; Thermal resistance: RΘ≤16°C/W (including silicon substrate and copper heat sink); 3 Circuit structure design 3.1 Circuit principle design Circuit principle design (Fig. 1) uses two DIS1020A floats The constant current integrated diode is divided into two constant current driving 40 LEDs in series, and the working current of each channel is 20mA. On the silicon substrate, 16 56V/10mA Zener diodes were fabricated by diffusion process to absorb and discharge static electricity, and the LEDs were protected from electrostatic breakdown. In the circuit, the designed capacitors and diodes are mainly used to absorb harmonics, pulses and other interference signals from the external power supply, reduce the influence of these interference signals on the product, and improve product reliability and work environment adaptability.

3.2 Hybrid integrated design uses silicon substrate and copper heat sink structure design (Figure 2), 80 LEDs are designed as 10×8 matrix structure, each 10 LEDs and a set of 2 Zener diodes form a unit, the bottom surface of the silicon substrate For the p-region of the Zener diode, the n-region is connected to the positive and negative electrodes of each group of LEDs through an aluminum conductive retroreflective layer, and ohmic contact is achieved by an alloy process. SMD capacitors C1, C2, C3 and diode D1 are designed in the peripheral area to reduce the effects of light absorption and shading.

3.3 Temperature gradient design of heat sink In order to improve the reliability of the product, 1 mil gold wire is used for bonding ball bonding. Due to the large number of LEDs and the large area of ​​the silicon substrate, the heat in the central part of the silicon substrate cannot be transferred to the heat in time. Sinking causes the temperature to rise, resulting in a decrease in the luminance of the LED at the center. To this end, the new alloy technology is used to design the copper heat sink structure, which reduces the thermal resistance RΘ of the heat sink and the temperature gradient dT(x,y)/dL, so that the heat in the central part of the silicon substrate can be transmitted to the heat sink in time. The heat is dissipated through the outer casing to improve the reliability of the product. The silicon substrate has a rectangular structure with a thickness of 0.3 mm, and its thermal resistance can be described by the following formula [1] RΘ={ln[(a/b)( a+2x)/(b+2x)]}/ 2k(ab Where a and b are the length and width of the silicon substrate, respectively; x is the thickness of the silicon substrate; k is the thermal conductivity of the silicon substrate. 4 Main parameters test results 4.1 Thermal resistance and temperature gradient test and mass production products have been proved to meet the design requirements. Table 1 shows the results of the thermal resistance RΘ, the position x, y, and the temperature gradient dT(x, y)/dLx, dT(x, y)/d Ly measured by the photothermal resistance scanning method [2]. At the origin, X, -X, Y, and -Y were tested at a distance of 1 mm.

4.2 Temperature Drift Figure 3 is a plot of the temperature drift parameter ΔIT of the product versus the constant operating current Io. The test environment temperature of product temperature drift is -50 to 100 °C, with 25 °C/40 mA as the reference. The maximum temperature drift at 100 °C is 1.64 μA/°C, and the maximum temperature drift at -50 °C is 1.49 μA/°C. Under normal environmental conditions (-25 ~ 50 °C), the average temperature drift is 0.94μA / °C, fully meet the design specifications and actual use requirements.

4.3 Luminous efficiency and antistatic Because the power level LED chip area is large, the high temperature generated during operation is difficult to conduct in time, and the temperature of the LED chip and pn junction is too high, resulting in a sharp drop in luminous efficiency with increasing power and temperature. Therefore, in the integrated power stage LED design, the silicon substrate structure design with high thermal conductivity and the heat sink design of copper alloy technology are adopted, so that the thermal resistance is greatly reduced, and the luminous efficiency of the product is improved, averaging 18 to 20 Lm. /W. The anti-static design has obvious effects. 50 products are placed in an environment with static electricity of 1600-1800V, and work continuously for 48 hours without static breakdown. 5 Conclusion The integrated design of the product is successful, so that the integrated power-level LED reduces the requirement of constant-current driving power supply during the application process. It can be directly driven by the regulated power supply, and the requirements for the regulated power supply are also loose. When changing within ±10%, the LED can ensure normal operation under constant current conditions, thus improving the reliability of the LED. At the same time, the product can greatly simplify the design of its power frequency driving power supply circuit in the actual use process. Because of the small driving current circuit, the loss of the long distance power supply line is very low, especially suitable for use in long distance power supply. .