The original power supply has various forms, but no matter which kind of power supply, it is generally not possible to directly supply power to the LED . Therefore, to use LED as the illumination source, we must first solve the power conversion problem. The LED is actually a current-driven low-voltage unidirectional conductive device. The LED driver should have DC control, high efficiency, PWM dimming, overvoltage protection, load disconnection, small size, and ease of use. The following must be observed when designing a power converter that supplies power to the LED.
1 Since the LED is a unidirectional conductive device, the LED should be powered by a direct current or a unidirectional pulse current.
2 Since the LED is a semiconductor device with a PN junction structure and has a barrier electromotive force, this forms a turn-on threshold voltage, so the voltage applied to the LED must exceed this threshold voltage, and the LED is fully turned on. The threshold voltage of high-power LEDs is generally above 2.5V, and the voltage drop of LEDs during normal operation is 3~4V.
The current and voltage characteristics of the 3LED are non-linear. Because the current flowing through the LED is numerically equal to the electromotive force of the power supply minus the potential of the LED, and then divided by the total resistance of the circuit (the sum of the internal resistance of the power supply, the lead resistance, and the resistance of the LED body), the current flowing through the LED It is not proportional to the voltage applied across the LED.
4 Since the PN junction of the LED has a negative temperature coefficient, the barrier electromotive force of the LED will decrease when the temperature rises. Therefore, the LED cannot be directly powered by the voltage source, and current limiting measures must be taken. Otherwise, as the temperature of the LED increases, the current will become larger and larger, causing damage to the LED.
5 The ratio of the current flowing through the LED to the luminous flux of the LED is also non-linear. The luminous flux of an LED increases as the current flowing through the LED increases, but it is not proportional, and the later the luminous flux increases. Therefore, the LED should be operated at a current value with a relatively high luminous efficiency.
In addition, LEDs, like other light sources, can withstand limited electrical power. If the electric power applied to the LED exceeds a certain value, the LED may be damaged. Due to differences in production process and material properties, the barrier electromotive force of the same type of LED and the internal resistance of the LED are not exactly the same, which leads to inconsistent voltage drop during LED operation, and the LED barrier electromotive force has a negative temperature coefficient. Therefore, LEDs cannot be used in parallel directly.
There are four cases for powering the LED with the original power supply: low voltage drive, transient voltage drive, high voltage drive, and mains drive. Different situations have different solutions for the implementation of power converter technology. The following briefly describes the methods of driving the LEDs by the above several types of power sources.
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