How to design a led filament ?
The LED filament consists of multiple series-connected LEDs on a transparent substrate, referred to as chip-on-glass (COG). These transparent substrates are made of glass or sapphire materials. This transparency allows the emitted light to disperse evenly and uniformly without any interference. An even coating of yellow phosphor in a silicone resin binder material converts the blue light generated by the LEDs into light approximating white light of the desired colour temperature—typically 2700 K to match the warm white of an incandescent bulb. Degradation of silicone binder and leakage of blue light are design issues in LED filament lights. A benefit of the filament design is potentially higher efficiency due to the use of more LED emitters with lower driving currents. A major benefit of the design is the ease with which near full "global" (360°) illumination can be obtained from arrays of filaments, but two zones emitting less light appear diagonal to the substrate. Flickering of dimmable filament LED lamp as interference with a digital camera frame rate The lifespan of LED emitters is reduced by high operating temperatures. LED filament bulbs have many smaller, lower-power LED chips than other types, avoiding the need for a heatsink, but they must still pay attention to thermal management; multiple heat-dissipation paths are needed for reliable operation. The lamp may contain a high-thermal-conductivity gas (helium) blend to better conduct heat from the LED filament to the glass bulb.The LED filaments can be arranged to optimize heat dissipation. The life expectancy of the LED chips correlates to the junction temperature (Tj); light output falls faster with time at higher junction temperatures. Achieving a 30,000 hour life expectancy while maintaining 90% luminous flux requires the junction temperature to be maintained below 85 °C.Also worth noting is that LED filaments can burn out quickly if the controlled gas fill is ever lost for any reason. The power supply in a clear bulb must be very small to fit into the base of the lamp.[8] The large number of LEDs (typically 28 per filament) simplifies the power supply compared to other LED lamps, as the voltage per blue LED is between 2.48 and 3.7 volts. Some types may additionally use red LEDs (1.63 V to 2.03 V). Two filaments with a mix of red and blue is thus close to 110 V, and four are close to 220–240 V, compared to the mains voltage reduction to between 3 V and 12 V needed for other LED lamps. Four filaments are usually used, and the appearance is similar to an overrun carbon filament lamp. Typically, a mix of phosphors is used to give a higher color rendering index (as distinct from color temperature) than the early blue LEDs with yellow phosphor. The simple capacitive or resistive dropper power supply used by some cheaper bulbs will cause some flickering at twice the frequency of the mains alternating current, which can be difficult to detect,[8] but possibly contributes to eyestrain and headaches. Also, due to the falling reactance of a capacitor with increasing frequency, the current delivered by capacitive power supplies increases with mains frequency, e.g., from 50 to 60 Hz.