The original power supply has various forms, but no matter what kind of power supply, generally can not directly power the LED. Therefore, to use LED as a lighting source, we must first solve the problem of power conversion. LED is actually a current driven low voltage unidirectional conductive device, LED driver should have DC control, high efficiency, PWM dimming, overvoltage protection, load off, small size, and easy to use and other characteristics. The following matters must be noted when designing the power converter to power the LED.
① Because the LED is a unidirectional conductive device, it is necessary to use direct current or unidirectional pulse current to power the LED.
② Because the LED is a semiconductor device with a PN junction structure, with a barrier electromotive force, which forms the threshold voltage, so the voltage value added to the LED must exceed this threshold voltage, the LED will be fully switched on. The threshold voltage of high-power LED is generally above 2.5V, and the voltage drop of LED in normal operation is 3 ~ 4V.
③ The current and voltage characteristics of LED are non-linear. Because the current flowing through the LED is numerically equal to the electromotive force of the power supply minus the barrier electromotive force 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 LED body resistance), the current flowing through the LED is not proportional to the voltage added at both ends of the LED.
④ Because 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 can not be directly powered by a voltage source, and current limiting measures must be taken, otherwise with the rise of the temperature when the LED is working, the current will become larger and larger, resulting in damage to the LED.
⑤ The ratio of the current flowing through the LED and the luminous flux of the LED is also non-linear. The luminous flux of the LED increases with the increase of the current flowing through the LED, but it is not proportional, and the more the luminous flux increases, the less. Therefore, the LED should be made to work under a current value with relatively high luminous efficiency.
In addition, LED is also like other light sources, the electrical power it can withstand is limited. If the electrical power applied to the LED exceeds a certain value, the LED may be damaged. Due to the differences in production process and material characteristics, 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 pressure drop when the LED is working, plus the LED barrier electromotive force has a negative temperature coefficient, so the LED can not be directly used in parallel.
With the original power supply to the LED power supply there are 4 cases: low voltage drive, transition voltage drive, high voltage drive, mains drive. Different situations have different schemes in the realization of power converter technology. The following is a brief introduction to the above several power drive LED methods.
1. Low voltage drive LED
Low voltage drive is to drive the LED with a voltage lower than the LED positive voltage drop, such as using an ordinary dry battery or nickel-chromium\/nickel-metal hydride battery to drive the LED, its normal power supply voltage is 0. 8 to 1. Between 65V. When driving LED with low voltage, it is necessary to increase the voltage enough to make the voltage value of LED conduction for low-power lighting devices such as LED, low voltage drive method is a common use, such as LED flashlights, LED emergency lights, energy-saving table lamps and so on. Due to the limitation of the capacity of a single battery, the low-voltage drive power supply generally does not require a lot of power, but requires the lowest cost and relatively high conversion efficiency, considering that it may sometimes be necessary to work with a No. 5 battery, so it also has the smallest volume. The best technical scheme is to choose capacitive boost converter.
2. Transition voltage drives LED
The transition voltage drive means that the voltage value of the power supply to the LED changes around the LED voltage drop, which may sometimes be slightly higher than the LED voltage drop, and sometimes it may be slightly lower than the LED voltage drop. If the power supply is composed of a lithium battery or two lead-acid batteries in series, the voltage is above 4V when the battery is fully charged, and the voltage is below 3V when the battery discharge is near the end, the typical application is LED mining lamps.
Transition voltage drive LED power conversion circuit to solve the problem of boost, but also to solve the problem of buck, in order to work with a lithium battery, but also need to have as small a volume and as low as possible cost. Under normal circumstances, its power is not large, the highest cost-effective circuit structure is inductive up-buck converter.
3. High voltage drive LED
High voltage drive refers to the voltage value of the power supply to the LED power supply is always higher than the voltage drop of the LED, the common power supply is 6V, 12V, 24V battery. Typical applications of this method include solar lawn lights, solar garden lights, motor vehicle lighting systems, etc. High voltage drive LED to solve the voltage reduction problem, because the high voltage drive is generally powered by ordinary batteries, will use a relatively large power, such as motor vehicle lighting and signal lighting, so there should be as low a cost as possible. The best circuit structure of the converter is inductive buck converter.
4. LED driven by mains power