Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

First, through adjusting Forward current to adjust the brightness

It is easy to change the brightness of LED power supply. First thinking about is to change its drive current, because the LED's brightness is almost directly proportional to drive current. Picture 1 shows the Cree's XLampXP-G of the output relative brightness and forward current.

We can know that if 350mA as 100% light output, then 200mA about 60%, 100mA about 25%. Therefore, adjust current can adjust the brightness easily.

1.1 The way in forward current regulation

The easiest way to adjust LED’s current is to change the current testing resistor which is LED load in series (Picture 2a).  Almost all DC-DC constant current chip interfaces have a test current and the on-chip voltage reference compared to control current constant. But the sense resistor value is usually very small, only a few tenths of Europe, if you want to install a few tenths of the wall of the pot in Europe, it is unlikely to regulate the current, because the lead resistance there will be a few tenths of Europe. Therefore, some chip provides a control voltage interface to change the input control voltage can change the value of output current. Such as Linear Technology's LT3478 (Picture 2b), as long as the change in the ratio of R1 and R2, it can also change its output with constant current value. 

1.2 Chromatography will shift forward current
However, the method of transferring forward current used to adjust the brightness will have a problem, that is the brightness adjustment will also change its spectrum and color temperature. Because the current white light LED is excited by blue LED light which generated $ phosphor, when the forward current decreases, the blue LED brightness increase in the thickness of $ phosphor thinning is not in proportion to its growth in the main wavelength spectrum, the specific examples shown in Picture 3. 

When the forward current is 350mA, the main wavelength is 545.8nm; when the forward current is reduced to 200mA, the main wavelength is 548.6nm; when the forward current is reduced to 100mA, the main wavelength is 550.2nm.
Changes in forward current can also cause changes in color temperature (Picture 4). 
  
When the forward current is 350mA, the color temperature is 5734K, while the forward current increased to 350mA, the color temperature shifts to the 5636K. Current is further reduced, color temperature turned to warm color.

Of course these issues in general may be not a big problem in actual lighting. However, the use of RGB LED system can cause color shift, while the human eyes are very sensitive to the color deviation. Therefore it can not be allowed.

1.3 Adjust the current constant will cause current source can not work which is a serious problem

However, in the specific implementation, the method of adjusting the forward current dimming may have a more serious problem.

We know that is usually used LED constant current driver DC-DC power supply to drive and such constant current drive source is usually divided into
step-up or step-down (of course both type, but because of low efficiency and high prices, they are not common.) Which one should be chose is determined by the relations between the voltage of power supply and load voltage of LED. If the supply voltage is lower than the load voltage, choose to boost; if the power supply voltage higher than the load voltage, choose to buck. The LED forward voltage is determined by its forward current. From the LED's voltage characteristic (Picture. 5), it shows that changes in forward current will cause a corresponding changes in forward voltage. The decrease of forward current can also cause the decrease of forward voltage. So, when the current reduction, LED forward voltage will follow decrease. This will change the relationship of supply voltage and load voltage. 

For example, in a 24V input LED lamp, using 8 high-power 1W LED in series. When forward current is 350mA, each LED's forward voltage is 3.3V. Then the 8 series is 2 * V, higher than the input voltage. So it should boost current source. However, in order to be dimming, the current is 100mA, at this time, the forward voltage is only 2.8V, 8 together the voltage is 22.4V, and the load voltage becomes lower than the supply voltage. This step-up current source simply can not work, but it should be a buck. For a step-constant current source must be that it works in step-down is not enough, the last LED will flicker. In fact, as long as the step-up current source used in the forward current by adjusting dimming, just transferring to low light will certainly give rise to flicker. Because then the LED load voltage must be below the supply voltage. Many people do not understand the issues, and always going from the dimmer circuit to find the problem, it is futile.

A buck will cause less problem of constant source because if the original power supply voltage higher than the load voltage, when the brightness is to low-key, the load voltage is lower, they still need to step-down constant current source. But if you adjust to a very low forward current, LED load voltage becomes low, then step-down ratio is very large, this may be beyond the normal step-down current source scope of work, and even make it flicker so that can not work.

1.4 Exposure to low light may cause reduced efficiency step-down current source and increases the temperature rise even it can not work

People may think that down-toward adjust to reduce the current source output power, it can not cause the power step-down current source and increase the temperature increased. Few of them know when the forward current caused by reducing the forward voltage drop will reduce the step-down ratio. The efficiency of step-down current source and the step-down ratio is related, step-down ratio the greater, the efficiency the lower, the power consumption on the chip bigger. Picture 6 is curve of SLM2842J efficiency and step-down ratio. 
 
Picture 6 the relationship between efficiency of step-down current source and the step-down ratio

The input voltage in the picture is 35V, output current is 2A, when the output voltage is 30V, and the efficiency can be as high as 97.8%. But when the output voltage is down to 20V, the efficiency is reduced to 96%; when the output voltage drops to 10V, the efficiency is reduced to 92%. In all three cases, although the output power of the order of 60W, 40W and 20W, the loss of power was the order of 1.2W, 1.6W, 1.6W. In later two cases, the power increases 33%. If the constant current module's cooling system designed to be very critical, an increase of 33% of the dissipation power is likely to cause the junction temperature rise, so as to cause over-temperature protection are unable to work, may also be serious attributed to the chip burned.

1.5 Adjust the forward current can not be accurately dimming

Because the forward current and light output is not entirely directly proportional relationship, and different LED is with different current and light output of the forward curve. Therefore, the method of adjusting the forward current is difficult to control of light output precisely.