Editor's note: The development of incandescent light bulbs is nearing the limit, and its 95% heat loss has caused many users to worry. In contrast, the performance of fluorescent lamps is slightly better, but the power consumption is still as high as 80%. Neither of these technologies can be compared to white LEDs, which theoretically have an energy efficiency level of more than 80%. Not only that, the life of LEDs is much longer than traditional light sources.
　　 Therefore, as general lighting applications gradually shift to LED technology, energy consumption will be greatly reduced. According to a recent study by the US Department of Energy, white LEDs will be widely adopted in 2025, and global power consumption will be reduced by 10%, saving up to $10 billion in electricity bills.
Despite the many advantages of white LEDs, the design of LED driver circuits faces significant challenges. Space requirements and heat dissipation requirements are limited by design. Finally, designers must also carefully consider the impact of EMI requirements on their design.
In low-power (≤3W) lighting applications, designers use off-the-shelf, non-isolated, inductor-based buck and lift switch mode power supplies. This article will compare the two topologies and discuss their advantages and disadvantages.
Two topologies
Figure 1 shows the LinkSwitch-TN device configured as a basic buck converter (Figure 1a) and a basic buck-boost converter (Figure 1b). Simplify design complexity in the converter stage by integrating a power MOSFET, oscillator, simple on/off control, a high voltage switch current source, frequency jitter, cycle-by-cycle current limit, and thermal shutdown circuitry on a single IC And reduce the number of components. LinkSwitch-TN devices are self-powered via the drain pin, eliminating the need for a bias supply and associated circuitry. It is a cost-effective alternative to linear and capacitive buck non-isolated power supplies with output currents less than or equal to 360mA, providing excellent input voltage regulation and load regulation. It is more efficient than the passive component power scheme, and the power factor is higher than the capacitive buck scheme.
Figure 1 – Basic LinkSwitch-TN structure
The buck converter shown in Figure 1 has a number of advantages. First, it maximizes the available output power and inductance of the selected LinkSwitch-TN device. It also reduces the voltage stress on the power switch and freewheeling diode. In addition, the average current flowing through the output inductor is slightly lower than the average current in a similar buck-boost converter.
Compared to buck converters, buck-boost converters have the advantage that their output diodes are connected in series with the load. In a buck converter, if a MOSFET has a short-circuit fault, the input is tied directly to the output. In the case of such a buck-boost converter, a reverse biased output diode blocks the path between the input and output.
In both converters, the AC input is rectified and filtered by D1, D2, C1, C2, RF1 and RF2. Two diodes enhance input surge withstand and conducted EMI performance. Designers should use a fusible flame retardant resistor as RF1, but a flame retardant resistor can be used as the RF2. The on/off control in the Linkswitch-TN device is used to regulate the output current. Once the current into the feedback (FB) pin exceeds 49μA, the MOSFET switch will be disabled to enter the next switching cycle.
Reduce heat
The main challenge in designing LED driver circuits is heat dissipation. Even with more efficient technology than incandescent technology, 3W circuits will reach temperature levels that compromise device integrity. Moreover, the integration of drive electronics into a standard GU10 lamp holder with severe limitations can also present severe thermal challenges. The only way for the designer to solve this problem is to conduct heat to the screw-in socket of the bulb. In the above topology, a thermal shutdown circuit is added to the LinkSwitch-TN device to disable the power MOSFET when the junction temperature exceeds 142 °C, thereby preventing potential damage to the LED. Once the junction temperature drops by 75°C, the MOSFET will automatically turn back on.
The buck-boost topology is slightly less efficient than the buck topology because power is not transmitted to the output every time the MOSFET switch is turned on. Therefore, it generates more heat than the buck topology. But the difference is not obvious.
To ensure that the circuit topology meets thermal regulation requirements, the designer installs the power supply assembly into the lamp holder and then measures the temperature of the LNK306DN source pin. Ideally, the temperature of the source pin should not exceed 100 °C. Measurements at an ambient temperature of 25 ° C indicate that the source pin temperature will exceed 100 ° C when the Vin value rises to 265 VAC. In light of these results, the designer concluded that there may be thermal limits on some additional heat sinks, such as placing the LED heat sink on top of the U1 SO-8C package.
Control EMI
LED driver electronics circuits must meet stringent EN55022B/CISPR22B conducted EMI requirements. In view of the high switching frequency of the switching IC and the limited size of the GU10 socket, these requirements present a major challenge for the bulb designer. In a buck-boost circuit topology, an EMI noise current loop flows from the MOSFET to the output diode, the output capacitor, and then back to the input capacitor; in a buck circuit configuration, the current loop flows from the MOSFET to the freewheeling diode and then back to the input. The capacitor is therefore shorter than the loop in the former. Therefore, the above situation makes it more difficult to slightly reduce the noise in the buck-boost design.
To comply with industry EMI specifications, the engineer decided to split the drive electronics into two boards: the converter board at the top and the input rectification/EMI filter board at the bottom (see Figure 2). Then they placed a Faraday shield between the two boards. The shield electrically connected to the converter board contains a single-sided copper-platinum area PCB that is the same size as the bottom input rectification/EMI filter board. Using this design to drive three LEDs, the test results show that conducted EMI is about 7dBμV at the worst case input voltage of 230VAC, which is lower than the industry EMI requirements.
Figure 3? Conductive EMI at 115VAC for similar products
These two circuit topologies have similar advantages from a cost perspective. Importantly, a typical design requires the use of approximately 25 devices and allows the use of off-the-shelf, low-cost inductors instead of custom transformers.
There is an important difference in designing a current sense feedback loop. Current feedback limits the LED current during normal operation. Designers can use the FB pin to directly detect the voltage drop across the sense resistors to meet current sensing requirements. However, due to the 1.65V voltage at the FB pin, unacceptable dissipation occurs in the GU10 housing. Therefore, if designers use a buck circuit topology, they must purchase some additional low-power signal devices for the feedback loop. This additional investment typically includes two ceramic capacitors, two NPN surface mount transistors, and four high precision thick film resistors. But it must be noted that all of these devices are only a small incremental cost.
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