According to different requirements, Power level and the overall system drive the best LED solutions are also different.
The disadvantages are as follows:
1. Brightness is a function of voltage. V changes substantially alter the current, then change the LED brightness.
2. This solution can not use LED's efficiency advantage. As compared with the LED's resistor, R is very large, most of the power is consumed in the R.
For High Power LED
For high-power LED, in order to maximize out LED efficiency advantage, a current controlled solution is required. Figure 2 shows an example of such a solution.
V • I Chip PRM and VTM rectifier voltage converter is used to provide a stable voltage. In order to use the PRM and VTM to the LED power supply, modifying the PRM's work is needed to provide a stable current. This is done by using a current amplifier and compensator to achieve.
Compared with the conventional method, using the PRM and VTM to provide a constant current has several advantages. VTM can be used in the system load point of the current doubler. According to the following formula, VTM output current proportional to its input current, the ratio is fixed turns ratio K, IOUT = IIN / K.
Therefore, in the current controlled applications detecting and controlling the VTM input current to control its output current can be reached. Current needs smaller and lower detection sensors which will consume less power and improve overall efficiency. Similarly, high efficiency and high power density makes the LED system small, low temperature, and make the energy of output per watt to the highest number of lumens.
An important additional advantage is that flowing through the LED's current (IOUT) is not a function of input voltage. Therefore, in our example the car headlights, as long as the current flows through the LED remains constant, regardless of how much battery voltage, LED brightness will remain constant. This is because the PRM is a change to the variable V with the negative resistance, thus providing a way is required to keep current constant.
More importantly, PRM is the resistance of effective resistance, not the true resistance, which means that power loss is very small, and it is not a function of effective resistance. Therefore, most power is consumed in the LED, making it the solution to use the same LED as effective and efficient.
Some of the shortcomings of this solution, including its complexity significantly higher than the solution shown in Figure 1 is higher, so it's correct implementation require more attention and control. Increased complexity also brings higher costs, so this solution is more suitable for high-power LED to save power (and power LED running costs) which could compensate for the increased costs and lost complexity more easily.
Finally, the power LED with different requirements, different from simple to complex solutions available. Figure 1 shows a simple resistive current limiter, low cost, but in terms of its low efficiency, high power LED is not suitable.
Figure 2 shows the current source can be adjusted to achieve the best efficiency and size, but they require higher costs and increased complexity. Adjustable current source provides additional advantages, such as from input voltage fluctuations, the overall system goals may be important in terms of these additional advantages may also be important. LED power supply designers should be noted that LED can be used to power the whole system of different programs and goals.