There are numerous benefits to replacing traditional screw-in incandescent bulbs with LED-based alternatives. Typically, a small number of LEDs (ranging from 5 to 9) are connected in series and powered by a single power supply that converts the line voltage into a lower DC voltage, usually around tens of volts, with a current range of approximately 350 to 700 mA. When designing such systems, it's crucial to consider how best to isolate users from potentially dangerous line voltages. This isolation can be achieved either within the power supply itself or during the LED installation process.
In some low-power applications, physical isolation of the LEDs is a common approach, as it allows the use of more cost-effective, non-isolated power supplies. Figure 1 illustrates a typical method for replacing an incandescent bulb with an LED lamp. In this design, the power supply is non-isolated, meaning that the insulation required for user safety is integrated into the lamp housing rather than the power supply unit. This compact design presents significant challenges, especially regarding space constraints and heat dissipation. Since the power supply is embedded inside the housing, it becomes difficult to manage heat effectively, which can reduce overall efficiency.
Figure 2 shows a non-isolated circuit that powers LEDs using a 120 V AC input. The circuit includes a rectifier bridge that feeds into a buck power stage. The buck regulator operates in an "inverted" configuration, with the power switch Q2 in the main loop and the loop diode D3 connected to the power supply. Current regulation is achieved through a source resistor while the power switch is active. Although this design is relatively efficient—achieving 80% to 90% efficiency—it has several drawbacks that limit its performance.
When the power switch is turned on, it must carry the full output current. When it is off, the current flows through the loop diode, resulting in additional losses. Additionally, the current-sensing resistors R8 and R10 drop about 1 volt each. These voltage drops are significant compared to the 15 to 30 V across the LEDs, further reducing efficiency. More importantly, these losses contribute to increased temperatures within the bulb.
The ability of LEDs to illuminate gradually is closely tied to their operating temperature. For instance, at 70°C, an LED’s light output may decrease by 30% after more than 50,000 hours, and at 80°C, this reduction occurs much faster, within just 30,000 hours. Since many LED bulbs are housed in cylindrical enclosures that restrict airflow, heat dissipation becomes even more challenging, making the thermal management issue more complex. Effective cooling is essential to maintain both the longevity and performance of the LED lighting system.
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