Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

The emerging field of semiconductor lighting, which made the engineers who are specializing in power electronics, optics and thermal management (mechanical engineer), become the hot talent.

The total power electronic engineers are used to ensure that the output voltage or current with high precision, but in terms of LED driver design is not a good habit. FPGA and the DSP and the like, such as the number of loads need to lower the core voltage, which in turn requires more stringent controls to prevent a high error rate. Therefore, the digital power supply rails will usually control the tolerance within ± 1% or smaller than their nominal value, the absolute value that can also be used, such as 0.99V to 1.01V. In the design of the traditional habits of the introduction of the power LED driver design usually brings the problem is: In order to achieve the strict control of output current tolerance will waste more electricity and the use of more expensive devices.

Cost pressures
The ideal power is the cost is not high, efficiency can reach 100% and does not occupy space. Power electronics engineer used to listen to the views from customers, they will do their utmost to meet those requirements, in a bid to the minimum space and system design within the budget. To control the LED current accuracy and digital load to the accuracy of the same supply voltage, it will be a waste of electricity, and also waste costs. 100mA to 1A is currently the most current range of products, in particular the current 350mA (or, more precisely, the semiconductor junction photovoltaic current density 350mA/mm2) is the thermal management and lighting efficiency compromise between the often adopted. Control LED driver IC is silicon, so in the range of 1.25 V there is a typical band gap. To 1.25V at up to 1% tolerance, it requires the voltage range of ± 12.5mV. This is not difficult to achieve, to achieve this tolerance, or better tolerance of low voltage reference circuit or power supply control IC variety, low prices. When the control output voltage, can be used at very low power precision, resistors get feedback about output voltage (as shown in Figure 1a). To control the output current, we need feedback on ways to make some adjustments, as shown in Figure 1b. This is currently the only and most simple means to control the output current.

After thorough research, you will find a major drawback of this approach, they are: the load and feedback circuit between the two are identical. Reference voltage is added to a resistor in series with the LED, this means that the reference voltage or the LED current is higher, and the greater is the resistance of the power consumption. Therefore, the first generation of dedicated LED driver ICs to be much lower than the reference voltage current product, which is similar to the battery charger. Lower voltage means lower power consumption, and also means smaller, cheaper, more low-loss current sense resistor. As shown in Figure 1b simple low-end feedback environment, 200mV conventional voltage selection. However, to be achieved in the 200mV reference voltage tolerance of ± 1%, you need a very high price of integrated circuits, when compared to the nominal reference voltage tolerance is ± 2mV. Although this is not impossible to achieve, higher accuracy requires a higher cost. ± 2mV the need for high-precision voltage reference tolerance required for the production, testing and classifying technology, this time, additional costs should be spent on more intelligent LED drive. The value of the cost of the new addition of a feedback loop, with the help of the loop, you can use light output (instead of current output) to control LED driver.

Measuring light output
LED manufacturers have made it clear that, with a luminous flux is proportional to the current. All using the same current drive LED, each LED will then produce the same luminous flux. Therefore, the power electronics engineer reached a conclusion: the current high precision is required. In this way, they forget the light output of the lumen and the flux value (instead of amperes) is the focus. It is easy to measure current, while the relative measure of light and the need for expensive large-scale equipment, as shown in Figure 2, the integrating sphere, and most electronic engineers do not quite understand the integral ball.
 
In addition, even if the tolerance is ± 0.1% of the current source (the price will be quite high) has a huge market value, it produces the actual light output in a strict, no effect on the capacity difference. LED luminous flux observed the sub-file, which determines it. Table 1 shows the optical power the world's top three semiconductor manufacturers in the high end of the cold white LED 350mA and 25 C flux under the conditions of sub-file results. Note the last column is the tolerance of the sub-file, on average, but not all sub-files within the flux tolerance.
 
Table 1, the results of the world's top three power optoelectronic semiconductor manufacturer of high-end 350 mA in cool white LED and 25℃ flux are under the sub-file.