The latest breakthrough in LED technology has enabled people to produce LEDs that can output a wide spectrum of light, rather than just traditional red and green derivatives. Therefore, the absolute number of LEDs used in products (lighting and other aspects) is increasing exponentially.
From flashlights to car headlights to backlighting of LCD TV displays, LED applications are expanding into new areas. In short, the most important driving force behind the rapid growth of LED applications is the efficiency gain brought by LED technology (that is, the energy consumption per watt can provide higher lumen lighting output).
Power requirement
LEDs have different power requirements than previous incandescent lamps. In addition, depending on the power level and overall system requirements, the best solution for driving LEDs is different.
In traditional lighting solutions (such as incandescent lighting), the load is resistive. The power consumption and the corresponding brightness of the lamp are a function of the voltage applied to the lamp and the resistance of the lamp (according to Ohm's law).
The light intensity increases or decreases as the input voltage changes. Taking the car headlight as an example, when the engine is started, the battery voltage is low, so the headlights will be darkened; once the engine starts running, the battery voltage will recover and the headlights will return to the standard brightness.
LEDs are fundamentally different from traditional incandescent lamps. The brightness of the LED is controlled by adjusting the LED current, and the resistance of the LED load varies with the applied load. LEDs require a constant current instead of constant brightness and resistance to maintain a constant brightness. The method of achieving the above effects differs depending on the power level of the LED.
For very low power LEDs, a very simple circuit as shown in Figure 1 is basically sufficient. Compared to R, the effective resistance of the LED is very small, so the current flowing through the LED is determined by V/R. Its disadvantages are as follows:
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Figure 1 Simple Power Solution for Resistor-Limited LEDs
â— Brightness is a function of voltage. The change in V essentially changes the current, which in turn changes the brightness of the LED. Going back to the original example of a car headlight - the light will still be weak at the beginning of the engine.
â— This solution cannot take advantage of the efficiency of LEDs. Because R is very large compared to the resistance of the LED, most of the power is consumed on R.
High power LED
For high-power LEDs, in order to maximize the efficiency advantages of LEDs, a current-controlled solution is needed. Figure 2 shows an example of such a solution.
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Figure 2 Current controlled LED power supply solution
The V·I chip PRM rectifier and VTM voltage converter are used to provide a stable voltage. In order to power the LEDs using PRM and VTM, it is necessary to modify the way the PRM works to provide a stable current. This is achieved by using a current amplifier and a compensator.
There are several advantages to using PRM and VTM to provide a constant current compared to conventional methods. The use of VTM in the system enables current multiplication at the point of load. According to the following formula, the output current of VTM is proportional to its input current, and the ratio is a fixed turns ratio K, IOUT = IIN/K.
Therefore, in current controlled applications, the output current of the VTM can be controlled by detecting and controlling its input current. Detecting lower currents requires smaller sensors that consume less power and improve overall efficiency. Similarly, high efficiency and high power density make the entire LED system small, low temperature, and maximize the lumens output per watt of energy consumed.
An important additional advantage is that the current flowing through the LED (IOUT) is not a function of the input voltage. Therefore, in our example of a car headlight, as long as the current flowing through the LED remains constant, the brightness of the LED will remain constant regardless of the battery voltage. This is because the PRM is a variable negative resistance that can change with V, providing a way to keep the current constant.
More importantly, the resistance of the PRM is the effective resistance, not the real resistance, which means that the power loss is very small and not a function of the effective resistance value. Therefore, most of the power is consumed on the LEDs, making this solution as efficient and efficient as the LEDs it uses.
Some of the shortcomings of this solution include its complexity significantly higher than the solution shown in Figure 1, so its proper implementation requires more attention and control. The increased complexity also brings cost increases, so this solution is more suitable for high-power LEDs, and the power saved in high-power LEDs (and the cost of electricity in LED operation) can more easily compensate for the increased cost and the complexity.
Finally, there are different solutions ranging from simple to complex for different power requirements of LEDs. The resistive current limiter shown in Figure 1 is simple and low cost, but it is inefficient for high power LEDs and is not suitable.
The adjustable current source shown in Figure 2 optimizes efficiency and size, but they require higher cost and increase complexity. Adjustable current sources offer additional advantages, such as being unaffected by input voltage fluctuations, which may or may not be important for overall system goals. Designers of LED power supplies should be aware of the different options available to power LEDs and the goals of the entire system.
Editor: China Lighting Network Feng Yaoyuan
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