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Introduction of LED drive power principle
2018-11-20 18:24:04
The graph below shows the relationship between forward voltage drop (VF) and forward current (IF). From the curve, it can be seen that when the forward voltage exceeds a certain threshold (about 2V), i.e. the on-voltage, it can be approximated that IF is proportional to VF. The table shows the electrical characteristics of the current super bright LED. According to the table, the highest IF of super bright LED can reach 1A at present, and VF is usually 2 to 4V.
Because the light characteristics of LED are usually described as a function of current rather than voltage, and the relation curve between luminous flux (V) and IF, constant current source driver can better control brightness. In addition, the range of forward voltage drop of LED is relatively large (up to 1V), and the VF-IF curve in the figure above shows that small changes of VF will cause large changes in IF, which will lead to greater changes in brightness. Therefore, the use of constant voltage source driver can not guarantee the consistency of LED brightness, and affect the reliability, life and optical decay of LED. Therefore, super bright LED is usually driven by constant current source.
Below is the relationship between temperature and luminous flux (V) of LED. From the figure below, it can be seen that luminous flux is inversely proportional to temperature. The luminous flux at 85 C is half that at 25 C, and the output at 140 C is 1.8 times that at 25 C. Temperature changes also have a certain impact on the wavelength of the LED. Therefore, good heat dissipation is the guarantee of keeping constant brightness of the LED.
The following is a picture of the relationship between temperature and luminous flux of LED.
Introduction of general LED drive circuit
Due to the limitation of the power level of the LED, it is usually necessary to drive multiple LEDs simultaneously to meet the brightness requirements. Therefore, a special driving circuit is needed to light the LED. The following is a brief introduction of the LED concept driving circuit.
The resistance current limiting circuit is shown in the following figure. The resistance current limiting driving circuit is the simplest driving circuit. The current limiting resistance is calculated by the formula below.
Vin is the input voltage of the circuit: VF is the forward current of IED; VF is the voltage drop of LED when the forward current is IF; VD is the voltage drop of anti-reverse diode (optional); y is the number of LED in each series; x is the serial number of parallel LED.
The linearized mathematical model of LED can be obtained from the above figure.
Formula: Vo is the opening voltage drop of a single LED; Rs is a linearized equivalent series resistance of a single LED. The calculation of upper limit current resistance can be written as
When the resistor is selected, the relationship between the IF of the resistance current limiting circuit and the VF is
It can be seen from the above formula that the resistance current limiting circuit is simple, but when the input voltage fluctuates, the current through the LED will also change, so the regulation performance is poor. In addition, because the power loss of resistance R is xRIF, the efficiency is low.
Introduction of linear regulator
The core of linear regulator is to use power triode or MOSFFET working in linear region as a dynamic adjustable resistance to control load. There are two types of linear regulators: parallel and series.
The parallel linear regulator shown in figure a below is also called shunt regulator (only one LED is shown in the figure, actually the load can be multiple LEDs in series, the same below). It is parallel to the LED. When the input voltage increases or the LED decreases, the current through the shunt regulator will increase, which will increase the voltage drop on the current limiting resistance. The current through LED remains constant.
Because shunt regulators need to be connected in series with a resistor, the efficiency is not high, and it is difficult to achieve constant regulation in the case of a wide range of input voltage changes.
The following figure B shows a series regulator. When the input voltage increases, the dynamic resistance of the regulator increases to keep the voltage (current) constant on the LED.
Because the power transistor or MOSFET has a saturated on-voltage, the minimum input voltage must be greater than the sum of the saturated voltage and the load voltage, so that the circuit can work correctly.
Introduction of switch regulator
The driving technology is not only limited by the input voltage range, but also has low efficiency. When used in low power ordinary LED driver, the current is only a few mA, so the loss is not obvious. When used for driving high-brightness LED with current of several hundred mA or even higher, the loss of power circuit becomes a serious problem. Switching power supply is the most efficient energy conversion device at present, and it can achie