Introducing DC drive power LED dimming technology

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

　　Methods for regulating forward current

　　The simplest way to adjust the current of an LED is to change the current detection resistor in series with the LED load. Almost all DC-DC constant current chips have a current detection interface, which compares the detected voltage with the reference voltage inside the chip to control the constant current. However, the value of this detection resistor is usually very small, only a few tenths of an ohm. It is unlikely to install a few tenths of an ohm potentiometer on the wall to adjust the current, because the lead resistance will also be a few tenths of an ohm. 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 color spectrum

　　However, there is a problem with adjusting the brightness by adjusting the forward current, that is, while adjusting the brightness, its spectrum and color temperature will also change. Because the current white light LED is produced by using blue light LED to excite yellow phosphor, when the forward current decreases, the brightness of the blue light LED increases, but the thickness of the yellow phosphor does not decrease proportionally, so that the main wavelength of its spectrum increases. 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 is further reduced, the color temperature will change to warm colors. Of course, these problems may not be a big problem in general practical lighting. However, in the RGB LED system, 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 the forward current may cause a more serious problem. We know that LEDs are usually driven by DC-DC constant current drive power supplies, and this type of constant current drive source is usually divided into two types: boost type or buck type (of course there is also buck-boost type, but it is not commonly used due to low efficiency and high price). Whether to use boost type or 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 the LED is determined by its forward current. From the volt-ampere characteristics of the LED, it can be seen that changes in the forward current will cause corresponding changes in the forward voltage. To be precise, a decrease in the forward current will also cause a decrease in the forward voltage. Therefore, when the current is lowered, the forward voltage of the LED will also decrease. This will change the relationship between the power supply voltage and the load voltage.

　　For example, in an LED lamp with an input of 24V, 8 1W high-power LEDs are used in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V. Then the 8 series voltage is 26.4V, which is higher than the input voltage. Therefore, a boost 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, and the 8 series voltage is 22.4V, and the load voltage becomes lower than the power supply voltage. In this way, the boost constant current source cannot work at all, and the buck type should be used. It is not possible for a boost constant current source to work at a buck, and the LED will flicker in the end. In fact, as long as a boost constant current source is used, when dimming by adjusting the forward current, flickering will almost certainly occur as long as the brightness is adjusted to a very low level. Because at that time, the LED load voltage must be lower than the power supply voltage. Many people always look for problems in the dimming circuit because they do not understand the problem, which is futile.

　　There will be fewer problems with a buck constant current source, because if the power supply voltage is higher than the load voltage, when the brightness is lowered, the load voltage is reduced, so a buck constant current source is still needed. However, if the forward current is adjusted to a very low level, the load voltage of the LED will also become very low. At that time, the step-down ratio is very large, which may exceed the normal working range of this buck constant current source, and it will also cause it to fail to work and produce flicker.

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

　　Most people may think that dimming down means reducing the output power of the constant current source, so it is impossible to increase the power consumption of the buck constant current source and increase the temperature rise. However, when the forward current is reduced, the forward voltage reduction caused by reducing the forward current will reduce the step-down ratio. The efficiency of the buck constant current source is related to the step-down ratio. The larger the step-down ratio, the lower the efficiency and the greater the power consumption on the chip.

　　Adjusting the forward current does not allow for precise dimming

　　Because the forward current and light output are not completely proportional, and different LEDs have different forward current and light output curves. Therefore, it is difficult to achieve precise light output control by adjusting the forward current. LED is a diode, which can achieve fast switching. 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, the brightness can be changed by changing the pulse width. 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 aperture ratio) is ton/tpwm. Changing the working ratio of the constant current source pulse can change the brightness of the LED.