LED cooling solutions

The radiator of the LED is a key issue that the designer must consider. The light decay of the LED is related to the working time and current of the LED and the temperature of the LED when it is working. Under the correct working condition, the working life of LED can reach at least 50,000 hours. Generally, it is easy to reach 100,000 or even hundreds of thousands. However, once the ambient temperature is too high and the working current is too large, the life of the LED will drop sharply. , And even cause permanent damage. In general low-power LED applications, you don’t need to consider heat dissipation. But at present, medium-power LEDs with a working current of 60-150mA have appeared. At this time, designers are required to consider the heat dissipation of the LED during normal operation. Fortunately, the outer diameter is 8mm. , A white-emitting LED with a working current of 150mA has thicker pins, which can basically use its sturdy pins for heat dissipation, but for larger power such as 1W and above 3W, 5W, or even more than 10W LEDs must Adding a heat sink is considered a normal working environment.

    Most electronics enthusiasts know that after the PN junction semiconductor device is conducting, the junction voltage VF decreases with the increase in ambient temperature, that is: -2mv/℃, which is called the negative temperature effect of the PN junction. This feature can be used to make a temperature sensor , But this feature is a fatal flaw in luminous applications, directly affecting its luminous efficiency. Luminous brightness, luminous chromaticity. As a popular example, when the normal temperature is 25℃, the best LED working current is 20mA. When the ambient temperature rises to 85℃, the junction voltage VF decreases, the working current increases sharply to 35-37mA, and when the temperature drops to -40℃, the junction voltage VF increases. , The best working current will be reduced from 20mA to 8-10mA. The luminous brightness will also decrease with the decrease of the current, which can not reach the illumination required by the place.

    In order to avoid the above-mentioned undesirable phenomena, it is generally necessary to adopt some measures in LED-related products. If the LED is mounted on the heat sink, or the fan is cooled by air cooling, or the LED is powered by a constant current source, the current will not increase due to the rising temperature of the LED to prevent the vicious temperature of the PN junction, or use both methods. Practice has proved that the heat sink is indeed an effective measure for high-power LED spotlights and LED street lights. However, when LED lights enter the homes of ordinary people, the following problems are encountered: whether the heat dissipation plate and air cooling can be concentrated in the space of an ordinary lamp holder; using integrated circuits or constant current source circuits composed of many components, its life and reliability will be reduced Regardless of the LED, the reliability of the entire system according to the "barrel theory" will depend on some "shortcomings" such as temperature rise.

    "Radio" magazine introduced Nanjing Huaju Electronics Co., Ltd. specially developed a thermistor with a positive temperature coefficient for LED applications in series with a negative temperature characteristic LED to form a resistance type load with a very small temperature coefficient, which is a cheap and good effect. s Choice. Once the working voltage is determined, the current in the series circuit will not change with temperature. In layman's terms, when the LED current increases with the increase in temperature, the thermistor also increases with the temperature and the resistance becomes larger, which prevents The loop current increases. When the LED decreases with the temperature and the current decreases, the thermistor also decreases with the temperature decrease, preventing the decrease of the loop current. When matched properly, when the ambient temperature changes in the range of -40°C to 85°C, the optimal operating current of the LED will not change significantly. Compared with the constant current source composed of integrated circuits and other components, the thermistor temperature compensation method has similarities, but the biggest difference is that the constant current problem of LED is solved with one component, and its price, volume, life, etc. The advantages are self-evident.