This technical article introduces five dimmer control technology of DC power LED.

1. Forward current adjustment method

The simplest method of adjusting LED current is to change the current resistance which is connected with LED load. Almost all the DC-DC constant current chips have a test current of interface, comparing the detected voltage and chip internal reference voltage to keep current constant. But the sense resistor values are usually very small, only several ohm. It is impossible for you want to put this kind of potentiometer to adjust current, because there will be a fraction of the lead resistance. Therefore some chips provide a control voltage interface, and when you change the input control, the output voltage constant current value is changed too.

2. Adjust forward current will distract chromatography.

However, adjust the brightness with forward current methods will have a problem that is at the same time of adjustable brightness its spectrum and color temperature can also be changed. Because the white LED is done with lincom LED excitation yellow phosphor, forward current, small blue LED brightness increased, but the yellow phosphor is not thinning according to proportion, increasing the dominant wavelength of the spectrum.

3. Adjust the current may make the constant current unable to operate.

We know that the LED is usually use DC - DC constant current power to drive, and this kind of constant current drive source is usually divided into step-up or step-down type (of course, there is also lifting pressure type, but is not commonly used due to the low efficiency and high cost). What type to choose is decided by the relationship between power supply voltage and LED load voltage. If the power supply voltage is lower than the load voltage, you can use the step-up one; or else use the step-down one. And the LED forward voltage 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, to be exact, the decrease of the forward current will cause the decrease of the forward voltage. So when the current lower, the forward voltage of LED with reduced. This will change the relationship between the power supply voltage and load voltage.

4. Long time working at low light level may reduce the efficiency of step-down constant current and rise its temperature so that make it fail to work.

People may think that reduce the light is to lower the output power of the constant current source, so it is impossible to cause step-down type constant current source temperature increase by the increasing power consumption. But the reduction of forward voltage caused by reducing forward current will depress the ratio. And the efficiency of the step-down type one is associated with the ratio: the larger the ratio, the greater the efficiency and the greater the power consumption of loss on a chip.

5. Adjust forward cannot get accurate dimming

Because the forward current and optical output is not fully proportional relationship, and different leds have different forward current and optical output curve, it is difficult to use the method of adjusting the forward current to achieve precise control of light output. LED is a diode, which can realize fast switch. The switching speed of it can be as high as above microseconds, which is unmatched by any light emitting device. Therefore, change the power supply into a pulse constant-current source and use the method of pulse width, you can change the brightness. This method is called pulse width modulation (PWM) dimming method. If pulse cycle is TPWM, pulse width is ton, and then its work ratio D is ton/TPWM. Change the work of constant flow source and you can change the brightness of the LED.