AG3 battery.DC drive power LED dimming and technology

　　It is easy to adjust the brightness of the LED by adjusting the forward current. The first thing that comes to mind is to change its driving current, because the brightness of the LED is almost directly proportional to its driving current.

　　How to adjust forward current

　　The simplest way to adjust the LED current is to change the current detection resistor in series with the LED load. Almost all DC-DC constant current chips have an interface for detecting current, which is controlled by comparing the detected voltage with the reference voltage inside the chip. constant current. However, the value of this detection resistor is usually very small, only a few tenths of ohms. If you want to install a potentiometer of a few tenths of ohms on the wall to adjust the current, it is impossible, because the lead resistance will also be a few tenths of ohms. Therefore, some chips provide a control voltage interface, and changing the input control voltage can change its output constant current value.

　　Adjusting the forward current will shift the chromatogram

　　However, adjusting the brightness by adjusting the forward current will cause a problem, that is, while adjusting the brightness, its spectrum and color temperature will also be changed. Because currently white LEDs are produced by using blue LEDs to excite yellow phosphors, when the forward current decreases, the brightness of the blue LEDs increases while the thickness of the yellow phosphors does not decrease proportionally, resulting in an increase in the dominant wavelength of the spectrum. For example: when the forward current is 350mA, the color temperature is 5734K, and when the forward current increases to 350mA, the color temperature shifts to 5636K. When the current further decreases, the color temperature will change to warmer colors. Of course, these problems may not be a big problem in general actual lighting. However, in the LED system using RGB, it will cause color deviation, and the human eye is very sensitive to color deviation, so it is not allowed.

　　Adjusting the current will cause serious problems that make the constant current source unable to work.

　　However, in actual implementation, dimming by adjusting forward current may cause a more serious problem. We know that LEDs are usually driven by DC-DC constant current driving power sources, and this type of constant current driving source is usually divided into two types: boost type or buck type (of course there is also a buck-boost type, but due to low efficiency , expensive and not commonly used). Whether to use a boost type or a buck type is determined by the relationship between the power supply voltage and the LED load voltage. If the power supply voltage is lower than the load voltage, the boost type is used; if the power supply voltage is higher than the load voltage, the buck type is used. The forward voltage of an LED is determined by its forward current. It can be seen from the volt-ampere characteristics of LED that changes in forward current will cause corresponding changes in forward voltage. To be precise, a decrease in forward current will also cause a decrease in forward voltage. Therefore, when the current is lowered, the forward voltage of the LED also decreases. This changes the relationship between supply voltage and load voltage.

　　For example, in an LED lamp with an input of 24V, eight 1W high-power LEDs are connected in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V. Then 8 LEDs connected in series are 26.4V, which is higher than the input voltage. Therefore, a boost-type constant current source should be used. However, in order to dim the light, the current is reduced to 100mA. At this time, the forward voltage is only 2.8V. If 8 units are connected in series, it is 22.4V, and the load voltage becomes lower than the power supply voltage. In this way, the step-up constant current source cannot work at all, and the step-down type should be used. For a boost-type constant current source, it must work in a step-down mode. In the end, the LED will flicker. In fact, as long as a boost-type constant current source is used, flickering will almost certainly occur as long as the brightness is adjusted to a very low level when adjusting the forward current. Because the LED load voltage at that time must be lower than the power supply voltage. Because many people don't understand the problems, they always look for problems in the dimming circuit, which is futile.

　　Using a step-down constant current source will cause fewer problems, because if the original power supply voltage is higher than the load voltage, when the brightness is lowered, the load voltage will decrease, so a step-down constant current source is still needed. However, if the forward current is adjusted to a very low forward current, the load voltage of the LED will also become very low. At that time, the voltage step-down ratio will be very large, which may also exceed the normal operating range of this step-down constant current source, which will also cause it to Doesn't work and causes flickering.

　　Working at low brightness for a long time may reduce the efficiency of the buck constant current source and increase the temperature rise, making it unable to work.

　　Most people may think that downward dimming reduces the output power of the constant current source, so it is unlikely to increase the power consumption and temperature rise of the buck constant current source. As everyone knows, the reduction in forward voltage caused by reducing the forward current will reduce the step-down ratio. The efficiency of the step-down constant current source is related to the step-down ratio. The greater the step-down ratio, the lower the efficiency and the greater the power consumption on the chip.

　　Accurate dimming cannot be obtained by adjusting forward current

　　Because the forward current and light output are not completely proportional, and different LEDs will have different forward current and light output relationship curves. Therefore, it is difficult to achieve precise light output control by adjusting the forward current. An LED is a diode, which can switch quickly. Its switching speed can be as high as microseconds or more. It is unmatched by any light-emitting device. Therefore, as long as the power supply is changed to a pulse constant current source and the pulse width is changed, the brightness can be changed. This method is called pulse width modulation (PWM) dimming. If the pulse period is tpwm and the pulse width is ton, then its working ratio D (or hole ratio) is ton/tpwm. Changing the working ratio of the constant current source pulse can change the brightness of the LED.

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