Explore the dimming technology of DC power LED

Adjusting the brightness by adjusting the forward current method To change the brightness of the LED, it is easy to implement. The first thing that comes to mind is changing its drive current because the LED's brightness is almost directly proportional to its drive current.

1. How to adjust the forward current The simplest way to adjust the LED's current is to change the current sense resistor in series with the LED load. Almost all DC-DC constant current chips have an interface for detecting current, which is the detected voltage and chip. The internal reference voltage is compared to control the current constant. However, the value of this sense resistor is usually very small, only a few tenths of a ohm. It is impossible to adjust the current by installing a zero-potentiometer potentiometer in the wall, because the lead resistance will also be a few tenths of a ohm. Therefore, some chips provide a control voltage interface and change the input control voltage to change the output constant current value.

2, adjust the forward current will make the chromatogram shift but adjust the brightness with the method of adjusting the forward current will have a problem, that is, while adjusting the brightness will change its spectrum and color temperature. Because current white LEDs are generated using blue LEDs to excite yellow phosphors, when the forward current decreases, the blue LED luminance increases and the thickness of the yellow phosphor does not decrease proportionately, 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 increases to 350mA, the color temperature shifts to 5636K. When the current further decreases, the color temperature changes to warm colors.

Of course, these problems may not be a big problem in general practical lighting. However, in an LED system using RGB, color shift is caused, and the human eye is very sensitive to the color deviation, and therefore it is also not allowable.

3. Regulating the current will cause serious problems that make the constant current source unable to work. However, in a specific implementation, adjusting the light by adjusting the forward current may cause a more serious problem.

We know that LEDs are usually driven by DC-DC constant-current driving power sources. Such constant-current driving sources are usually classified into either step-up or step-down types (of course, buck-boost type, but due to low efficiency) , expensive but not commonly used). Whether the boost type or the step-down type is determined by the relationship between the power supply voltage and the LED load voltage. If the supply voltage is lower than the load voltage, the boost type is used; if the supply voltage is higher than the load voltage, the step-down 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 the forward current changes will cause a corresponding change in the forward voltage. Specifically, the decrease in the forward current also causes a decrease in the forward voltage. Therefore, when the current is turned down, the forward voltage of the LED also decreases. This will change the relationship between the supply voltage and the load voltage.

For example, in an LED lamp with an input of 24V, 8 high-power LEDs of 1W are used in series. At a forward current of 350mA, the forward voltage of each LED is 3.3V. Then 8 series is 26.4V, which is higher than the input voltage. Therefore, a boost type constant current source should be used. However, in order to dim, the current is reduced to 100mA. At this time, the forward voltage is only 2.8V, 8 series is 22.4V, and the load voltage becomes lower than the power supply voltage. This type of step-up constant current source will not work at all, but a step-down type should be used. For a boost-type constant current source, it must work at a reduced voltage, and finally the LED will flicker. Actually, as long as a constant-current source of a boost type is used, when dimming with a normal-direction current, as long as the luminance is adjusted to a very low level, flicker will almost certainly occur. Because at that time the LED load voltage must be lower than the supply voltage. Many people still have to go through the circuit of dimming to find problems because they do not understand the problems. It is futile.

The problem of using a buck constant current source is less, because if the original power supply voltage is higher than the load voltage, when the brightness is low-key and the load voltage is reduced, a buck constant current source is still needed. But if you adjust to a very low forward current, the load voltage of the LED becomes very low. At that time, the step-down ratio is very large. It may also exceed the normal operating range of this buck constant current source, and it also makes it Can't work but flashes.

4, long-term work in low brightness may make the buck constant current source efficiency decreases temperature rise and can not work. Most people may think that the downward dimming is to reduce the output power of the constant current source, so it is impossible to cause buck-type constant The power consumption of the flow source increases and the temperature rises. As everyone knows, when the forward voltage is reduced when the forward current is reduced, the step-down ratio is reduced. 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 of the loss on the chip.

5, adjust the forward current can not be accurately dimmed because the forward current and light output is not completely proportional to the relationship, and different LED will have different forward current and light output curve. Therefore, it is difficult to achieve accurate light output control by adjusting the forward current.

The LED is a diode that enables fast switching. Its switching speed can be up to microseconds. It is unmatched by any light emitting device. Therefore, as long as the power supply is changed to a pulsed constant current source, the 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 duty ratio D (or the hole ratio) is ton/tpwm. Changing the duty ratio of the constant current source pulse can change the brightness of the LED.