It is easy to change the brightness of led by adjusting the forward current. The first thought is to change its driving current, because the brightness of LED is almost directly proportional to its driving current.
1. Method of adjusting forward current
The simplest way to adjust the LED current is to change the current detection resistance in series with the LED load. Almost all DC-DC constant current chips have a current detection interface to compare the detected voltage with the reference voltage inside the chip to control the constant current. However, the value of this detection resistance is usually very small, only a few tenths of a ohm. It is impossible to install a potentiometer with a few tenths of a ohm on the wall to adjust the current, because the lead resistance will also have a few tenths of a ohm. Therefore, some chips provide a control voltage interface. Changing the input control voltage can change its output constant current value.
2. Adjusting the forward current will offset the chromatography
However, using the method of adjusting the forward current to adjust the brightness will produce a problem, that is, while adjusting the brightness, it will also change its spectrum and color temperature. At present, white LEDs are generated by exciting yellow phosphor with blue LED. When the forward current decreases, the brightness of blue LED increases, while the thickness of 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 decreases further, the color temperature will change to warm color.
Of course, these problems may not be a big problem in general practical lighting. However, in the LED system using RGB, the color deviation will be caused, and the human eye is very sensitive to the color deviation, so it is not allowed.
3. Current regulation will cause serious problems that make the constant current source unable to work
However, in the specific implementation, dimming by adjusting the forward current may cause a more serious problem.
We know that led is usually driven by DC-DC constant current driving power supply, and this kind of constant current driving power supply is usually divided into boost type or step-down type (of course, there is also voltage rise and fall type, but it is not commonly used due to low efficiency and high price). Whether the step-up type or step-down type is adopted 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 shall be adopted; If the power supply voltage is higher than the load voltage, the step-down type is adopted. The forward voltage of LED is determined by its forward current. From the volt ampere characteristics of LED, the change of forward current will cause the corresponding change of forward voltage. Specifically, the decrease of forward current will also cause the decrease of forward voltage. Therefore, when the current is turned down, the forward voltage of the LED decreases. This changes the relationship between the supply voltage and the load voltage.
For example, in an LED lamp with an input of 24V, 8 1W high-power LEDs are connected in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V. Then the eight LEDs in series are 26.4V, which is higher than the input voltage. Therefore, boost constant current source should be used. However, in order to adjust the light, reduce the current to 100mA. At this time, the forward voltage is only 2.8V, and the eight are connected in series to 22.4v. The load voltage becomes lower than the power supply voltage. In this way, the step-up constant current source can not work at all, and the step-down type should be used. For a step-up constant current source, it is not possible to work in step-down, and finally the LED will flicker. In fact, as long as the step-up constant current source is adopted, flicker will almost certainly occur as long as the brightness is adjusted to a very low level when the forward current is used for dimming. Because the LED load voltage must be lower than the power supply voltage at that time. Because many people do not understand the problems, they always have to look for problems from the dimming circuit, which is futile.
The problem of using step-down constant current source will be less, because if the original power supply voltage is higher than the load voltage, when the brightness is low, the load voltage is reduced, so step-down 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 will be very large, which may also exceed the normal working range of this step-down constant current source and make it unable to work and flicker.
4. Working at low brightness for a long time may reduce the efficiency of step-down constant current source and increase the temperature rise
It is generally believed that downward dimming is to reduce the output power of the constant current source, so it is impossible to increase the power consumption and temperature rise of the step-down constant current source. However, when the forward current is reduced, the forward voltage will reduce the step-down ratio. The efficiency of 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.
5. Precise dimming cannot be achieved by adjusting the forward current
Because the forward current and optical output are not completely proportional, and different LEDs will have different forward current and optical output curves. Therefore, it is difficult to achieve accurate optical output control by adjusting the forward current.
LED is a diode, which can realize fast switching. Its switching speed can be as high as microseconds. It is unmatched by any light-emitting device. Therefore, as long as the power supply is changed into a pulse constant current source, its brightness can be changed by changing the pulse width. This method is called pulse width modulation (PWM) dimming. If the period of the pulse is TPWM and the pulse width is ton, the working ratio D (or porosity ratio) is ton / TPWM. Changing the working ratio of the constant current source pulse can change the brightness of the LED.
Responsible editor; zl