A, LED drivers of major classes
(A) DC drive
According to their different roles and purposes, they can be divided into three kinds.
(1) DC step-down drive. The basic principle is to use switching devices with electrical resistance device limiting the external power supply step-down LED drivers later work. Buck drives in series of simple structure, high conversion efficiency. The main disadvantage of this drive is damaged when the main switching devices, high current directly through the LED to LED burned. Obviously, such a program when the supply voltage is lower than the voltage drop across the load can not be used.
(2) DC step-up drive. Its basic principle is to use switching devices with electrical energy storage devices boost resistance to limiting the way to work. Step-up drive the transformation efficiency is relatively high. The more significant advantage is its failure will not damage the LED. Step-up drive can only be used in the load tube pressure drop is always higher than the supply voltages. In the load control power supply voltage drop below the case, the drive will be out of control, direct-current through the LED to LED burned. DC low voltage input used when a small number of LED drivers can use step-down step-up drive can also be used, as no parallel or less parallel series LED. If the number of multi-LED boost drivers should be used is not as much as possible in parallel or less parallel series LED.
(3) converter based drive. It is used with high frequency transformer switching devices to achieve energy transfer from primary to secondary, and a voltage / voltage conversion-driven light-emitting diode. This drives the output voltage from the input voltage constraints, in accordance with the required number of any series of LED design, application flexibility, suitable for use in power supply voltage fluctuations in the vicinity of the voltage drop across the load situation is also suitable for supply voltage and load voltage drop across the very different situation. The disadvantage is that the circuit complexity, conversion efficiency than the above two types of drive slightly lower. If the input DC voltage drive low power tube can be used in series step-down drive, drive high-power tube should be selected from a safety point of view the structure of the drive converter.
(B) AC drive
AC drives are also divided according to different application buck, boost, converter type three circuit types. AC drives and DC drives in addition to the difference between the AC input rectifier filter to do outside, from a security perspective, there are still isolated and not isolate a problem.
Second, LED lighting design thermal considerations
Twenty-five watts of LED lighting system design is generally used for lamps, spotlights the living room, family dining lamp, night light, etc., even so, most less than twenty-five watt low-power LED lighting applications will also require a certain degree of miniaturization. This often leads to a higher power density, although its power is not great. In this case, the necessary measures for adequate thermal management must be improved to provide the mechanical structure.
In addition, high electrical efficiency also helps reduce power consumption. And another to work long hours to prevent overheating of the LED idea is to use dimming solution. In fact, in this power range, LED lighting will replace the halogen and compact fluorescent lamps. In addition, in order to get rid of heat problem, we must get rid of temperature-sensitive passive components.
However, the majority of LED driver solutions are derived from the power supply topology, and on this basis, it should take into account the temperature range restriction, because generic products are usually based on commercial standards, but the lights had to be able to adapt to the harsh environment to ensure .
Third, LED lighting design solutions in the thermal control
In the rapid development of LED lighting design, most people focus on the bright (HB) LED dimming control strategy. However, HB LED lighting applications, the essential requirement that we will be more attention to thermal control. Although the LED manufacturers by a substantial increase in the number of lumens per watt of HB LED lighting design is to reduce technical barriers, but light output, there is still more energy to be distributed out into the heat. Therefore, a thermal management of the overall strategy to ensure that the LED can control the heat as a function of temperature. With incandescent, tungsten bulbs, high power LED does not radiate heat. In contrast, LED to the heat transfer to the PN junction of the LED package on the heat the metal pieces. As the heat generated by the LED distribution using conduction, the heat needs a longer, more expensive path to fully distributed into the air. Currently, HB LED general lighting business is the biggest obstacle of a heat problem. Therefore, the ability to effectively and completely solve the problem can be said that the key to winning customers.
Electronic control circuit must be able to deal with trigger point and gain setting. In fact three LED need to be able to cope with potential heat sources: self-heating, and LED electronic temperature controls. Using variable resistance as a method of dimming of HB LED devices is also impractical, because too much resistance on the power consumption, while the need for a dedicated winding resistance. Resistor and LED in the position very close to the case, the resistance of the additional heat generated heat will only make the problem worse. Pass element is a transistor can also be seen in its power transistors, rather than a variable resistor.
With this method, generate a few responses, and for thermal control and luminance-defined negative or positive temperature coefficient thermistor, provides more flexibility. The use of LED thermal control thermistor easiest implementation uses a PTC element. PTC element increases with temperature increase of the nominal low resistance, until the trigger point.
