Introduction of LED dimming technology for DC drive power

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

Method of adjusting forward current

The easiest way to adjust the current of the LED is to change the current detection resistance in series with the LED load. Almost all DC-DC constant current chips have an interface to detect the current. The detected voltage is compared with the internal reference voltage of the chip to control Constant current. However, the value of this detection resistor is usually very small, only a few ohms, if you want to install a potentiometer on the wall to adjust the current is unlikely, because the lead resistance will also have a few ohms. Therefore, some chips provide a control voltage interface. Changing the input control voltage can change the output constant current value.

Adjusting the forward current will shift the chromatogram

However, using the forward current method to adjust the brightness will cause a problem, that is, it will change its spectrum and color temperature while adjusting the brightness. At present, white LEDs are produced by exciting blue phosphors with blue LEDs. When the forward current decreases, the brightness of blue LEDs increases and the thickness of yellow phosphors does not decrease proportionally, thereby increasing the dominant wavelength of its spectrum. For example, when the forward current is 350mA, the color temperature is 5734K, and when the forward current is increased to 350mA, the color temperature shifts to 5636K. When the current is further reduced, the color temperature will change to a warmer color. Of course, these problems may not be a big problem in general actual lighting. However, in the LED system using RGB, it will cause the color shift, and the human eye is very sensitive to the color deviation, so it is also not allowed.

Regulating current will cause serious problems that make the constant current source inoperable

However, in the specific implementation, dimming with the method of adjusting the forward current may cause a more serious problem. We know that LEDs are usually driven by DC-DC constant current drive power, and such constant current drive sources are usually divided into two types of boost type or buck type (of course there are buck-boost type, but due to low efficiency , Expensive and not commonly used). Whether to use the boost type or the 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 LED forward voltage is determined by its forward current. It can be seen from the volt-ampere characteristics of LEDs that the change in forward current will cause a corresponding change in forward voltage. Specifically, a decrease in forward current will also cause a decrease in forward voltage. Therefore, when the current is turned down, the forward voltage of the LED decreases accordingly. 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, eight 1W high-power LEDs are used in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V. Then 8 series connected is 26.4V, which is higher than the input voltage. Therefore, a boost type constant current source should be used. However, in order to adjust the light, the current is reduced to 100mA. At this time, the forward voltage is only 2.8V, and the eight connected in series are 22.4V. The load voltage becomes lower than the power supply voltage. In this way, the boost type constant current source cannot work at all, and the buck type should be used. For a boost-type constant current source, it must not work in step-down, and finally the LED will flicker. In fact, as long as a boost type constant current source is used, when dimming with forward current, dimming will almost certainly occur as long as the dimming is very low. Because the LED load voltage at that time must be lower than the power supply voltage. Because many people do not understand the problems, they still have to find problems in the dimming circuit, which is futile.

The problem of using a buck constant current source will be less, because if the original 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 required. But if it is adjusted to a very low forward current, the load voltage of the LED also becomes very low. At that time, the step-down ratio is very large, and it may exceed the normal operating range of this step-down constant current source. Cannot work and flashes.

Working at low brightness for a long time may reduce the efficiency of the step-down constant current source and increase the temperature rise to prevent it from working

Most people may think that dimming down is to reduce the output power of the constant current source, so it is impossible to cause the power consumption of the buck constant current source to increase and the temperature rise to increase. It is not known that the reduction of the forward voltage caused by the reduction of the forward current will lower the step-down ratio. The efficiency of the buck constant current source is related to the buck ratio. The larger the buck ratio, the lower the efficiency, and the greater the power consumption of the chip.

Adjusting the forward current does not allow precise dimming

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 accurate light output control by adjusting the forward current. LED is a diode, it can achieve fast switching. Its switching speed can be as high as more than microseconds. 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 method. If the pulse period is tpwm and the pulse width is ton, then its working ratio D (or called aperture 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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