Parameters not taken into account to this point are internal electronics dissipation, peak dissipation during dimming and power factor. Though it is possible to create a higher level of dimmer compatibility using a design with extreme low power factor and high mains harmonics at dimmed position, the current trend is not to exploit this loophole because it does not comply with future and current quality standards, and is misleading in terms of energy consumption. It is recommended that the reactive power of a dimmable LED lamp remains below two times the undimmed real power over the complete dimming range.
For internal electronics dissipation, most designers specify only the driver efficiency without use of a dimmer. There are, however, other parameters to consider:
• Total peak dissipation of the lamp. This determines the thermal design of the lamp, and thus the size of the heat sink and lamp size. Total peak dissipation occurs mostly with leading edge dimmers at large opening angles. Additional losses required to maintain a stable mains line signal add to the total lamp dissipation, and depending on the dimming curve, the lamp power can exceed nominal power.
• Peak dissipation of the internal electronics. Electronics construction and components must be designed such that peak dissipation can be handled. Though provisions can be made to distribute additional dissipation during dimming over a larger number of components, all dimmable led lamps have additional circuitry to provide additional load to keep dimmers operational and stable.
• Lamp power at lowest dimming angle. In order to achieve energy savings, the user expects the lamp to draw substantial less power when dimmed. However, this power cannot be zero, because the lamp still has to provide sufficient load to keep the dimmer operational and stable. A lamp power less than 1 watt is recommended.
Figure 2: Electronics dissipation versus dimming angle of a number of dimmable 230V LED lamps. Click on image to enlarge
Figure 2 shows peak electronics dissipation powers between 4.5 and 1.5 W. The lamp output power can be utilized as complementary load to the electronics in the dimmer. As such, a tradeoff exists between dimming curve, dimming range, and internal electronics dissipation. Some manufacturers utilize this in an active way: The IC senses if unstable operation is reached at a certain dimming angle, and provided the electronics dissipation bandwidth is fully used, the lamp output power, and thus light amount, is increased again until stability is reached.
Future quality standards already anticipate this method, defining minimum dimming level below 50 percent as compliant. From an end user perspective, the lamp operates stably, but the dimming range is dissatisfying. Also, the dimming curve can be shifted towards small opening angles. This way, electronics dissipation at large opening angles can be reduced, but a dead band will occur when controlling the dimmer at large opening angles. It is evident that these aspects and tradeoffs become more critical at lower lamp powers. At higher wattage lamps, one could utilize the headroom to improve dimming curve and controllability again.
About the author
Victor Zwanenberg is a senior application engineer at NXP Semiconductors. Victor has over 25 years experience in various fields of electronics design and production. Since 2003, Victor has specialized in the AC/DC LED driver electronics field and retrofit LED lamp design.
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