In order to take the advantage of LED general lighting in energy saves and brightness regulating requirement from customer, it is necessary to use dimming technology in LED general lighting application. In offline LED dimming application, there are two kinds of most popular dimmers in the market. One is forward-phase dimmer (Triac dimmer); the other is reverse-phase dimmer (Transistor dimmer). The dimmer is used to control the input AC power of the LED lamp. The output of the dimmer includes both the supply power to LED lamp from the AC line and the dimming phase information.
For AC line dimmers, there are two kinds of dimming control schemes: analog dimming and PWM dimming methods.
For analog dimming, it picks up the pulse width information from the output of the dimmer and pass through a low-pass filter to get the related DC voltage signal. The DC voltage signal is proportion with the pulse width information from the output of the dimmer. The DC voltage signal is used to control the output current from a LED driver and to implement analog dimming. The advantage of analog dimming is simple and no output noise; the disadvantage is that there is color shift as the LED driving current decreases.
For forward-phase dimmer (Triac dimmer), there is minimum holding up current and it is related with the power level of the dimmer. The higher power is, the higher holding up current is in general. In analog dimming, as it is in deep dimming level, that is, low output LED driving current. It is possible to make the input current of the LED driver less than the required holding up current and it makes the conducting angle of Triac dimmer unstable and the output waveform unstable. It is the output of the dimmer unstable that makes the output current of the LED driver with jitter and the output light from the driven LED with flicker. Another issue with analog dimming is there is confliction between response capability and modulation related with line frequency.
For PWM dimming, it picks up the output pulse width information from the dimmer and processes the information and generates the related PWM signal to control the output duty-cycle of the LED driving current from the LED driver in order to change the output power. The advantage of the control scheme is that the output is a PWM signal and it only controls LED turn-on time and doesn't impact LED driving current amplitude. With the control scheme, due to LED driving current constant, it has less color shift issue. So far there are some PWM dimming scheme, e.g. National semiconductor's LM3445 proposal control scheme. It can output 400 Hz PWM control signal (it has no relation with the line frequency) to implement PWM dimming control scheme. In order to implement the control scheme, it requires that the input of LED driver has high enough input power or energy to output the related LED driving current. In general, to meet the requirement, it needs to add one or couple big electrolytic capacitors to store the energy to make sure that as PWM control signal comes, the input of the LED driver can offer the required power and the LED driver can output the related LED driving current.
In integrated LED lamp application, due to space limitation, the LED driver needs to operate under higher environment temperature. In order to keep high reliability of the LED driver and long lifetime of LED lamp, it hopes that there is no electrolytic capacitor in the LED driver due to the lifetime of electrolytic capacitor sensitive with the operating temperature.
In order to keep the advantage of PWM dimming control scheme and make the LED driver high reliability and the LED lamp long lifetime, it is required that the frequency of PWM dimming control scheme is related with or the same as the output frequency of dimmer; and the output PWM dimming pulse is covered by the output pulse from the dimmer further. In this way, it can guarantee that as PWM dimming pulse comes, the output from the dimmer can offer enough power to the LED driver and the LED driver can output the required LED driving current even there is jitter from the dimmer.