Design and analysis of inductive LED driver and high power LED lamp design and heat dissipation scheme

LED street lights meet the general direction of “energy saving and emission reduction” and have become the focus of attention of local governments. The Ministry of Science and Technology carried out the "Ten Cities and Ten Thousand Miles" LED lighting demonstration project, and Guangdong Province proposed the "Thousands of Miles" plan and so on. The Electronic Components Technology Network understands that while local governments are vigorously supporting LED streetlights, the quality of high-power LED Driver power supplies has become a shortcoming that hinders the development of high-power LED lighting.

An LED is an LED with diode characteristics that can only be energized in one direction. Generally, the LED brightness output is proportional to the LED current, but the white LED will appear saturated under high current, and the luminous efficiency is greatly reduced or even failed. Therefore, the LED current cannot exceed its specification. In addition, the LED brightness output is inversely proportional to the temperature, so the heat dissipation and well-designed heat dissipation system should be minimized during use.

White LEDs can be connected in series or in parallel, and each of these solutions has advantages and disadvantages. The disadvantage of the parallel mode is that the LED current and brightness cannot be automatically matched. The series maintains the inherent matching characteristics but requires a higher supply voltage. Since the forward voltage drop of white LEDs is 3 to 4V (typical), the battery voltage of most portable electronic devices is not enough to drive the LEDs, whether in parallel or in series, so an independent power supply is required.

In a series configuration, the number of LEDs is limited by the highest voltage of the driver. If the maximum voltage is 40V, according to the forward voltage of the white LED in the series configuration, this maximum voltage can drive up to 13 white LEDs, and the driving current ranges from 10 to 350 mA in continuous state. The advantage of this configuration is that a series of white LEDs can carry current with a single line; the disadvantage is that when the PCB space is limited (especially at high power), the current density on the copper wire is a problem, and if in the series mode one The white LED fails and all white LEDs are turned off. However, from a design point of view, if there is a white LED, there is a need to increase the battery voltage to n&TImes; Vp, so a boost structure must be used. Inductive components can be used to accurately monitor the current slope, limiting the EMI generated by uncontrolled transient currents. A typical boost topology is shown.

To solve the problem of LED heat dissipation, we should mainly start from two aspects - before and after packaging, it can be understood as LED chip heat dissipation and LED lamp heat dissipation. The heat dissipation of the LED chip is mainly related to the selection and process of the substrate circuit. This article does not elaborate on this, but mainly introduces the heat dissipation of the LED lamp. Any LED will be made into a luminaire, so the heat generated by the LED chip will always be dissipated into the air through the outer casing of the luminaire. If the heat dissipation is not good, because the heat capacity of the LED chip is small, a little bit of heat accumulation will quickly increase the junction temperature of the chip. If it is operated at a high temperature for a long period of time, its life will be shortened very quickly. However, in order for this heat to actually lead the chip to the outside air, there are many ways to go. Specifically, the heat generated by the LED chip comes out of its metal Heat Sink block, first passes through the solder to the PCB of the aluminum substrate, and then passes through the thermal conductive glue to the aluminum heat sink. Therefore, the heat dissipation of LED lamps actually includes two parts of heat conduction and heat dissipation. However, there is also a different choice for the size and location of the LED lamp housing. Now mainly by the following cooling methods:

1. Aluminum heat sink fins: This is the most common way to dissipate heat by using aluminum fins as part of the enclosure to increase heat dissipation.

2. Thermally conductive plastic shell: Fill the heat-conducting material during injection molding of the plastic shell to increase the heat conduction and heat dissipation capability of the plastic shell.

3. Air hydrodynamics: The use of the shape of the lamp housing to create convective air, which is the lowest cost to enhance the heat dissipation.

4. Fan: The interior of the lamp housing is cooled by a long-life and high-efficiency fan, which has low cost and good effect. However, replacing the fan is cumbersome, and this method is not suitable for outdoor use, so this design is relatively rare.

5. Heat pipe: Using the heat pipe technology, the heat and LED chips are led to the heat sink fins of the casing. This design is often used in large fixtures (such as street lights, etc.)

6. Surface radiation heat treatment: Radiation heat treatment on the surface of the lamp housing, the simplest is to apply Zhisheng Weihua radiation heat dissipation coating, which can radiate heat away from the surface of the lamp housing.