LED lights meet EMC and power quality standards based on highly integrated ICs

When comparing lighting technologies, the most meaningful metric is luminous efficacy, expressed in lumens per watt (Lm/W). Incandescent and halogen lamps are particularly lacking in this regard, with rated luminous efficacy of 15Lm/W to 20Lm/W, while compact fluorescent lamps are typically rated at 50Lm/W. But in the past few years, HB (high-brightness) LEDs have even surpassed this figure, and are expected to reach as much as 150Lm/W by 2012. In addition to being more energy-efficient, LED lamps also offer advantages such as longer operating life and lower operating and maintenance costs. As a result, HB LED lamps are expected to become important household and commercial products in the next few years. Due to the difficulty of fitting the LED power driver circuit into a standard lamp housing, some early LED bulbs did not have internal filters, which made them non-compliant with EMC standards. In addition, many of these LED bulbs used inefficient capacitor-step-down power supplies rather than switching ballasts. This approach can cause an imbalance in the AC mains current, which can lead to power quality problems in some equipment.

Design goals

This design is used to power a string of three HB LEDs rated at 300mA (equivalent to a standard 10W incandescent lamp). Under normal operating conditions, the output voltage is clamped at about 9.5Vdc by the forward voltage drop of the series LEDs , but the circuit must reach 12Vdc to allow for performance variations of the diodes. Its topology is a switching constant current offline buck regulator capable of operating over the entire 85

The 300Vac MOSFET operates within the universal input range of 265Vac and the line voltage frequency of 47Hz to 64Hz. Other design goals include high efficiency, low cost and compliance with EN55022A EMI requirements. The design can be integrated into standard lamp housings (both Edison screw-in bulbs and bayonet halogen bulbs), making it very easy to replace existing bulbs. EMI compliance considerations

Due to space and cost constraints, many LED lamps on the market are not designed to meet conducted EMI specifications. However, the design in this article utilizes the frequency modulation feature integrated into PI's LinkSwitch-TN power conversion IC, so a smaller EMI filter can be used.

Design Details

The LinkSwitch-TN LNK306DN integrated power conversion IC from Power Integrations contains a fully integrated 700V power MOSFET, eliminating the need for external power devices. The offline non-isolated buck topology can operate in continuous conduction mode at a maximum frequency of 66KHz. The frequency is modulated using 4kHz peak-to-peak frequency jitter, which simplifies the design requirements for EMI filters. Although this design uses a buck topology, this IC can also be configured as a buck-boost converter. A very critical point is that the LinkSwitch-TN LNK306DN uses a small SO-8 package, which is a great advantage for this application structure design.

The design principle of the converter and EMI filter is shown in Figure 1. Based on the voltage drop between the current sensing resistors R8 and R10, the current control loop is set to the desired constant current value. Although the standard design supports a current of 300mA, it can easily adapt to an output current of up to 360mA. Q1 and Q2 can amplify the detected voltage drop so that a current sensing resistor with a lower resistance value can be used to minimize power dissipation. The EMI filter adopts a p-type topology and contains a fusible flame-retardant resistor RF1 for overload protection.

Only 25 components are needed to design the converter and EMI filter, and no PCB board and connector components are required. The biggest challenge in the electrical design of this application is the structural design, especially the design of integrating both the converter and EMI filter into a standard lamp housing. However, the design fits into both the Edison screw-type lamp holder (E27) and the halogen bayonet lamp holder (GU 10). The dimensions of the halogen bayonet lamp holder are shown in Figure 2.

If the circular PCB was large enough to hold all the converter and EMI filter components, it would not fit into the lamp holder. Therefore, it was decided to divide the design into two circular PCBs, one for the converter circuitry and the other for the EMI filter. The final diameter of the converter board was 19.66 mm, while the final diameter of the EMI filter was 16.91 mm. The boards were then stacked and interconnected with discrete wiring to complete the assembly. Although the design functioned as required, it still had conducted emissions issues. Due to the proximity of the two PCBs, switching currents would couple from the converter board to the EMI filter board, degrading the performance of the EMI filter. This problem was solved by placing a "shielded" PCB between the two boards. The third board was just a layer of copper with no circuitry. It was electrically connected to the junction of the negative output of the EMI filter and the negative input of the converter. The final assembly then consisted of three circular boards stacked together. Adding a third board was simple and cost-effective, and not only solved the coupling problem, but also met the EMI performance requirements.

performance

This reference design will have a circuit efficiency of over 62% with a nominal input voltage of 120Vac. At 220/240Vac, the efficiency will be over 56%. Conducted EMI characteristics are shown at 115Vac and 230Vac input voltages using quasi-peak and average readings according to EN55022A limits . Under worst-case conditions, this circuit passes the standard with a 7dB margin when configured for 230Vac input voltage. The margin is higher when the input voltage is 115 VaC. Additional EMI plots are provided in the reference test report along with waveforms during normal operation and startup.

in conclusion

Despite certain physical limitations,  it is entirely possible to integrate the electronic ballast of a high-performance HB LED lamp into a standard lamp housing in a cost-effective manner and still comply with EMI regulations and power quality standards. In addition, application guides, design tools, and the newly launched LED lighting application site can provide extensive support for the design. The emergence of the RDK-131 reference design will surely accelerate the pace of new HB  LED lighting products to market and greatly alleviate the current concerns about the lack of products that meet EMI standards.