The emergence of the emerging field of semiconductor lighting has made engineers who specialize in three areas of power electronics, optics and thermal management (mechanical engineering) a sought-after talent. At present, there are not many engineers with experience in three fields, and this usually means that the background of the system engineer or the overall product engineer is related to these three areas, and they also need to cooperate with engineers in other fields as much as possible. System engineers often bring their habits or accumulated experience in the original field into the design work, which is similar to what an electronic engineer who mainly studies digital systems turns to solve power management problems: they may rely on Simple simulation, do not directly test the power supply on the test bench, because they do not realize that the switching regulator needs to carefully check the layout of the board; in addition, if it has not passed the test bench test, the actual work situation It is difficult to match the simulation.
In the process of designing LED luminaires, when the system architecture engineer is an electronic power expert, or the power supply design is contracted to an engineering company, some common habits in standard power supply design will appear in the LED driver design. Some habits are useful because LED drivers are very similar in many ways to traditional constant voltage sources. Both types of circuits operate over a wide range of input voltages and large output power. In addition, these two types of circuits are connected to different connections such as AC power, DC regulated power rails or batteries. The challenge.
Power electronics engineers are accustomed to always want to ensure high accuracy of output voltage or current, but this is not a good habit for LED driver design. Digital loads such as FPGAs and DSPs require lower core voltages, which in turn require tighter control to prevent higher bit error rates. Therefore, the tolerances of digital power rails are usually controlled within ±1% or less than their nominal values, and can also be expressed in absolute values, such as 0.99V to 1.01V. When introducing the design habits of traditional power supplies into the field of LED driver design, the usual problem is that in order to achieve tight control of the output current tolerance, more power will be wasted and more expensive devices will be used, or both. It.
Cost pressure
The ideal power supply is not expensive, the efficiency can reach 100%, and it does not take up space. Power electronics engineers are accustomed to listening to customers, and they will do their utmost to meet those requirements in an effort to design the system within the smallest space and budget. In the LED driver design is no exception, in fact it faces greater budget pressure, because the traditional lighting technology has been fully commercialized, and its price has been very low. Therefore, it is very important to spend every penny under the budget. This is also the place where some power electronics designer engineers are “misguided†by old habits.
Controlling the accuracy of the LED current to the same accuracy as the supply voltage of the digital load wastes both power and cost. 100mA to 1A is the current range for most current products, especially the current 350mA (or, more precisely, the current density of the optoelectronic semiconductor junction is 350mA/mm2), which is a compromise between thermal management and lighting efficiency. The integrated circuit that controls the LED driver is silicon-based, so there is a typical bandgap in the 1.25 V range. To achieve a 1% tolerance at 1.25V, a voltage range of ±12.5mV is required. This is not difficult to achieve, and a variety of low-cost voltage reference circuits or power control ICs that can achieve such tolerances or better tolerances are inexpensive. When controlling the output voltage, a high-precision resistor can be used to feed back the output voltage at very low power (as shown in Figure 1a). In order to control the output current, some adjustments need to be made to the feedback mode, as shown in Figure 1b. This is the only and simplest means of controlling the output current.
Figure 1a: Voltage feedback; Figure 1b: Current feedback (click on the image to view the high-resolution image)
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