It is easy to adjust the brightness of LED by adjusting the forward current in OFweek semiconductor lighting network. The first thing that comes to mind is to change its driving current, as the brightness of an LED is almost directly proportional to its driving current.

1. Method of adjusting forward current

The simplest way to adjust the current of an LED is to change the current detection resistance in series with the LED load. Almost all DC-DC constant current chips have an interface for detecting current, which compares the detected voltage with the reference voltage inside the chip to control the constant current. But the resistance value for this detection 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, as the lead resistance will also have a few tenths of an ohm. So some chips provide a control voltage interface, and changing the input control voltage can change its output constant current value.

2. Adjusting the forward current will cause chromatographic shift

However, using the method of adjusting forward current to adjust brightness can cause a problem, which is that while adjusting brightness, it will also change its spectrum and color temperature. Because currently, white LED is generated by exciting yellow fluorescent powder with blue LED. When the forward current decreases, the brightness of blue LED increases while the thickness of yellow fluorescent powder does not decrease proportionally, resulting in an increase in the main wavelength of its spectrum. For example, when the forward current is 350mA, the color temperature is 5734K, but when the forward current increases to 350mA, the color temperature shifts to 5636K. When the current further decreases, the color temperature will change towards warm colors.

Of course, these issues may not be considered a major issue in general practical lighting. However, in LED systems using RGB, color deviation can occur, and the human eye is very sensitive to color deviation, so it is also not allowed.

3. Adjusting the current will cause serious problems that prevent the constant current source from working

However, in practical implementation, using the method of adjusting the forward current to dimming may create a more serious problem.

We know that LEDs are usually driven by DC-DC constant current driving power supplies, and these constant current driving sources are usually divided into two types: boost or buck (of course, there are also boost and boost types, but they are not commonly used due to low efficiency and high price). Whether to use a step-up or step-down 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, use a boost type; If the power supply voltage is higher than the load voltage, a step-down type is adopted. The forward voltage of LED is determined by its forward current. From the volt ampere characteristics of LEDs, it can be inferred that changes in forward current will cause corresponding changes in forward voltage. Specifically, a decrease in forward current will also cause a decrease in forward voltage. So when the current is lowered, the forward voltage of the LED also decreases accordingly. This will change the relationship between the power supply voltage and the load voltage.

For example, in an LED light fixture with an input of 24V, 8 high-power 1W LEDs are connected in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V. Therefore, if 8 LEDs are connected in series, it is 26.4V, which is higher than the input voltage. So a boost type constant current source should be used. However, in order to adjust the brightness, the current is reduced to 100mA, and the forward voltage at this time is only 2.8V. Eight connected in series 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 a step-down type should be used instead. For a boost type constant current source, it is not possible to operate it under voltage reduction, and eventually the LED will flicker. In fact, as long as a boost type constant current source is used, when adjusting the forward current for dimming, as long as the brightness is adjusted to a very low level, flickering phenomenon will almost always occur. Because at that time, the LED load voltage was definitely lower than the power supply voltage. Many people, because they do not understand the problems, still have to look for problems in the dimming circuit, which is futile.

The problem of using a step-down constant current source will be less, because if the original power supply voltage is higher than the load voltage, and the brightness is lowered, the load voltage will decrease, so a step-down constant current source is still needed. But if the forward current is adjusted to a very low level, the load voltage of the LED also becomes very low. At that time, the voltage reduction ratio is very large, which may exceed the normal working range of this voltage reduction type constant current source, and it may also make it unable to work and cause flickering.

4. Working at low brightness for a long time may cause a decrease in the efficiency of a step-down constant current source and an increase in temperature rise, rendering it unable to function

Most people may think that dimming downwards is to reduce the output power of a constant current source, so it is impossible to cause an increase in power consumption and temperature rise of a step-down constant current source. Little did they know that the decrease in forward voltage caused by the decrease in forward current would result in a decrease in the voltage reduction ratio. The efficiency of a step-down constant current source is related to the step-down ratio. The higher the step-down ratio, the lower the efficiency, and the greater the power loss on the chip.

5. Adjusting the forward current cannot achieve 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. So it is difficult to achieve precise optical output control by adjusting the forward current.

LED is a diode that can achieve fast switching. Its switching speed can reach over microseconds. It is incomparable to 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, its brightness can be changed. 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 (also known as porosity ratio) is ton/tpwm. Changing the working ratio of the constant current source pulse can change the brightness of the LED.