Improving bulb efficiency with high-voltage LEDs

There are many benefits to using LEDs as a light source instead of screw-in incandescent bulbs. Generally, small (5-9) LEDs are connected in series and a power supply is used to convert the line voltage to a low voltage (usually tens of volts) with a current of about 350 to 700mA. Determining how to best isolate the user from the line voltage requires careful consideration and trade-offs. Isolation can be implemented in the power supply or during the LED installation process. Physical isolation of the LEDs is a common approach in some low-power designs because it allows the use of lower-cost non-isolated power supplies.

Figure 1 shows a typical LED lamp replacement approach. The power supply in this example is a non-isolated power supply, which means that the isolation to protect the user from high voltages is embedded in the package rather than in the power supply. Obviously, the space for the power supply is extremely small, which poses a packaging challenge. In addition, the power supply is buried inside the package, which hinders heat dissipation and affects efficiency.

Figure 1 Light bulb replacement makes the power supply space extremely small

Figure 2 shows a non-isolated circuit for powering LEDs from a 120 volt AC source. It consists of a rectifier bridge that powers a step-down power stage. The step-down regulator is an “inverted version” with the power switch Q2 in the loop and the catch diode D3 connected to the source. During the on-time of the power switch, the current is regulated through a source resistor. Although this is fairly efficient (80%-90%), this circuit has several drawbacks that limit efficiency. When on, the power switch must carry the full output current, and when the power switch is off, the output current flows through the catch diode. In addition, the voltage across the current sense resistors R8 and R10 is approximately 1 volt. All three of these voltage drops are large compared to the 15 to 30 volt LED voltage and will limit the power supply efficiency. More importantly, these losses contribute to the temperature rise of the bulb. The ability of the LED to emit light decreases over time, and this ability is closely related to the operating temperature of the LED. For example, at 70°C, the time it takes for an LED's light output to decrease by 30% is more than 50,000 hours, while at 80°C, it is only 30,000 hours. The heating problem is further complicated by the fact that the bulbs are mounted in "tubes" that tend to block heat dissipation and prevent convection cooling.

Figure 2 Buck regulator implements a simple offline LED driver

LED manufacturers have created higher voltage emitters by connecting several LEDs in series on a common substrate. These high voltage emitters offer either lower cost or higher power efficiency. Using these high voltage products, we only need to use a set of rectifiers and a ballast resistor, which allows a lower cost power supply method. Although this power supply can produce a fairly good power factor, the efficiency is very low because a large part of the input voltage is used in the ballast resistor, resulting in 30%-50% of the LED power loss. However, it can be used in some small and low power applications. However, in some high power applications, the low efficiency makes it useless. Figure 3 shows an alternative method: it uses a boost power supply. Most of the circuit is the same as the above method. However, the switch, diode and current sensing losses are much smaller, resulting in efficiencies as high as 90% to 95%. In addition, this circuit has a good power factor of 97%.

Figure 3 Using a boost power supply to improve LED driver efficiency

Figure 4 is a photograph of the power supply depicted in the schematics of Figures 1-2. Even though the power supplies produce roughly the same output power, there are some significant differences that affect the size of the power supplies. The inductor size of the boost supply is significantly smaller because of the lower energy storage requirements. The buck supply has a larger resistor than the boost supply. This resistor is a simulated load resistor (R20 in Figure 2) that determines when the dimmer turns on the silicon controlled rectifier (SCR). This is required because the dimmer has an electromagnetic interference ( EMI ) suppression capacitor next to the triac switch component , which has a higher voltage than the power supply when it is not loaded. This disturbs the power supply and causes unstable dimming. This is not necessary with the boost supply because the LEDs are connected to the input through the boost inductor, providing enough load for them to be a non-issue. The back of the board is not shown in the figure, but as the schematic shows, the buck supply has more low-level circuitry. As a result, the boost supply has lower power consumption, which is extremely important in space-constrained applications such as LED bulb replacements.

Figure 4: Boost power supply is smaller and more efficient

In summary, high voltage LEDs can help increase the life of screw-in LED bulbs due to their low power consumption and low temperature rise. This is achieved by using a boost power supply instead of a buck power supply, thereby improving power efficiency. The losses of a boost power supply are about half of those of a buck regulator. In addition, a boost power supply has fewer components, better power factor, is smaller, and is easier to dim using a triac component.