1 Introduction
"Low-carbon" life is a concept advocated at present. For lighting, the application of LED is a concrete manifestation. LED has the advantages of environmental protection, energy saving, high efficiency, long life, safety and reliability. To this end, it is necessary to understand the operating conditions, working principle, driving method and typical application of LED.
2, LED working principle
To design a driver circuit, you must first master its working principle. The brightness of the LED is mainly related to VF and IF. The volt-ampere characteristics of the LED are shown in Figure 1, where VF is the forward voltage drop of the LED and IF is the forward current. When the forward voltage exceeds the threshold (ie, the turn-on voltage, as shown in Figure 1.7V), it can be approximated that IF is proportional to VF. As can be seen from the figure, the highest IF of the LED can reach 1A, and the VF is usually 2 V to 4V.
Figure 1: Relationship between VF and IF of LED
The forward voltage drop of the LED varies widely (up to 1V or more), and the VF-IF curve in the above figure shows that a small change in VF causes a large change in IF, which causes a large change in brightness. Therefore, usually the luminescence properties of LEDs are described by a function of current, not as a function of voltage. However, the output voltage of the general rectifier circuit also changes with the fluctuation of the grid voltage. It can be seen that the constant voltage source driving cannot ensure the consistency of the LED brightness and affect the characteristics of the LED. Therefore, the LED driver is usually driven by a constant current source.
3, LED drive technology
It is known from the working principle of the LED that a constant current source is required to drive the LED to maintain an optimum brightness state. The task of driving is to maintain constant current characteristics while maintaining low power consumption. In order to meet the above requirements, the commonly used method of controlling current is: controlling the current by adjusting the size of the current limiting resistor; adjusting the current by adjusting the reference voltage on the current limiting resistor; PWM modulation realizes current control. The driving technology of LED is very similar to the technology applied in switching power supply. LED driving circuit is a power conversion circuit, but the output is constant current instead of constant voltage. In any case, a constant and average current is output, and the ripple current is controlled within a certain range.
(1) Current limiting method
As shown in Figure 2, this is a conventional circuit. After the grid voltage is stepped down, rectified, and filtered, the LED is stabilized by resistor current limiting. The fatal disadvantage of this circuit is that the power dissipation on the resistor R directly affects the efficiency of the system, plus the transformer loss, the system efficiency is about 50%. When the supply voltage fluctuates within ±10%, the current flowing through the LED will vary by ≥25% and the power on the LED will vary by more than 30%. The advantage of resistor current limiting is that the design is simple, the cost is low, and there is no electromagnetic interference; however, the current changes brightness with the change of VF, the efficiency is very low, and the heat dissipation is difficult.
Figure 2: Current limiting method
The diode is changed to a hollow straight line, the capacitance is a parallel straight line, the '+' sign is deleted, the symbol is changed to italic, and the footnote is changed to lowercase normal
(2) Voltage regulation method
Figure 3 is based on Figure 2 plus an integrated voltage regulator MC7809, so that the output voltage is basically stable at 9V, the current limiting resistor R can be used very small, will not cause the voltage instability of the LED. However, this circuit is still inefficient. Because the pressure drop on the MC7809 and R1 still accounts for a large proportion, the efficiency is about 40%. This is not an energy-saving lighting product. In order to achieve stable operation of the LED while maintaining high efficiency, low-power current limiting components and circuits should be used to improve system efficiency. The advantages of the linear voltage regulation method are simple structure, low external components, medium efficiency, and low cost.
Figure 3: Voltage regulation
The diode is changed to a hollow straight line, the capacitance is a parallel straight line, the '+' sign is deleted, the symbol is changed to italic, and the footnote is changed to lowercase normal