TDK-EPC has joined STMicroelectronics to develop a reference design for LED lamp ballasts for use in street lighting. TDK-EPC offers all its key electronic components such as capacitors, varistors and inductors. The power supply has an output of 130 W at an output voltage of 48 V.

LEDs are known for their high efficiency and long operating life. This makes them increasingly popular for lighting technology, where they are considered to be innovation drivers. The reason: they make a decisive contribution to reducing the energy consumption of lamps for indoor and outdoor use. This applies especially to street lighting, where efficiency, operating life, maintenance and energy consumption play a key role in reducing overall costs. In order to ensure maintenance-free operation, the corresponding ballast for an LED lamp must be just as efficient and have a similarly long operating life as the LEDs themselves.

Two-stage reference design

The new reference design developed by TDK-EPC and STMicroelectronics (Figure 1) consists of two stages: a front-end power factor correction (PFC) stage with the L6562AT controller from STMicroelectronics and an LLC resonant converter based on the L6599AT power IC.

Figure 1: Main board with rectifier, PFC stage and resonant converter

The use of film capacitors assures very high reliability and a long operating life.

The key properties of the design include a very high efficiency of over 90 percent, a broad range of input voltage, i.e. an AC line voltage of between 85 and 305 V, as well as high reliability and a long operating life. As the reliability or mean time between failures (MTBF) of power supplies depends on the failure rate of the electrolytic capacitors used, this reference design takes an innovative approach: it makes exclusive use of film capacitors instead of electrolytic ones for smoothing and storage in the DC link circuit. The developers have also considered the derating of the components, thus reducing their load in line with Recommendation MIL-HDBK- 217D. With the use of the new L6562AT and L6599AT ICs from STMicroelectronics, the number of active components was also minimized, thus increasing the MTBF and simultaneously optimizing the total cost of the components. Thanks to the high efficiency of the circuit, the PFC stage needs only a small heat sink. The reference design is additionally protected against overload and short circuit, open-circuit operation at every stage and overvoltages at the input. The system automatically restarts after a fault.

Key properties

• Extended European AC input voltage range of 85 to 305 V at a frequency of 45 to 55 Hz
• Output voltage of 48 V at 2.7 A
• Long operating life thanks to EPCOS film capacitors
• Power harmonics to EN61000-3-2 Class C
• Efficiency at nominal load of over 90 percent
• Electromagnetic compatibility to EN55022 Class B, EN55015
• Safety: double insulation to EN60950, SELV

Stage 1: PFC circuit

The PFC stage, which operates in transition mode, acts as a precontroller to supply energy to the resonance stage at 450 V. The PFC power stage is a conventional boost converter connected to the output of the rectifier bridge. It also includes the boost inductor, rectifier diode and output capacitors. The PFC output capacitors are EPCOS film capacitors of 5 µF / 800 V. A MOSFET acts as the boost switch. The board is equipped with an input EMC filter that extracts the interference caused by the boost stage. The power factor correction is performed by the L6562AT controller, a small and low-cost component that operates over the wide temperature range required for external applications.

Stage 2: Resonant converter

The L6599AT controller performs the active function in the buck converter. It comprises all the functions required for the correct regulation of the resonant converter. It has a constant duty cycle of 50 percent and uses the frequency as the control parameter. An integrated magnetic approach with a series inductor is used in the transformer, thus obviating an external inductor for generating the resonance. The choice for the secondary winding was a transformer configuration with center tapping and Schottky power rectifiers of STPS10150CG type. EPCOS film capacitors with a rating of 4.7 µF / 63 V are also used here for smoothing. A small LC filter that suppresses the RF ripple completes the output part. The output voltage is regulated and stabilized via a feedback network. Figure 2 shows the complete circuit diagram of the power supply, and the materials are listed in Table 1.

Figure 2: Complete circuit diagram of the LED power supply

Table 1: Material list of TDK-EPC products

Efficiency measurement

Table 2 shows the overall efficiency determined at an AC line voltage of 230 V / 50 Hz and 115 V / 60 Hz with various loads. At a voltage of 115 V and full load, the overall efficiency is 90.96 percent, rising to 93.39 percent at 230 V. If the efficiency is measured at 25, 50, 75 and 100 percent load to the ES-2 standard and the mean efficiency is calculated, the result is 91.04 percent at 230 V and 89.52 percent at 115 V. This means that the converter can operate at a high efficiency not only at full load but also at the lower loads typical of strongly dimmed LEDs.

Table 2: Efficiencies at various loads