LEDs offer higher efficiency and longer life than traditional lighting sources, so the technology is becoming the latest solution to reduce indoor or outdoor lighting energy consumption. The same is true in street lighting systems that require higher efficiency and longer life at lower cost. Switching power supplies that supply LED lights should also be highly efficient and durable to ensure the same operating life as LED luminaires. Resonant converters are one of the most popular power supply topologies for this type of application because resonant converters improve efficiency and reduce EMI compared to previous power supply topologies.
Soft switching is an important feature of resonant converters. However, the use of body diodes in resonant converters can sometimes lead to system failure. The charge stored in the body diode should be completely released to avoid large current and voltage spikes, including the high dv/dt and di/dt in these topologies. Therefore, the key parameters of the power MOSFET (such as Qrr) and reverse recovery dv/dt will directly affect the dynamic performance of the resonant converter. This article will discuss the overall solution for switching power supplies for LED street lighting. The new resonant controller combined with the new power switch provides an efficient solution for LED lighting power without compromising converter robustness and cost effectiveness.
High efficiency with resonant converters
A variety of DC/DC power conversion topologies can be used to reduce switching losses, device stress on power MOSFETs, and radio frequency interference (RFI) while achieving high power density. In these topologies, a resonant converter that uses a body diode of a MOSFET to achieve zero voltage switching can achieve higher efficiency. In particular, the LLC resonant converter achieves high efficiency at high input voltages and low voltage stress on the secondary rectifier because the secondary has no inductance. In addition, the LLC resonant converter guarantees zero voltage switching (ZVS) without load. Zero-voltage switching technology significantly reduces switching losses while dramatically increasing efficiency. In addition, the zero-voltage switch effectively reduces switching noise, allowing the use of small-sized EMI filters.
With these unique properties, LLC resonant converters are becoming a popular topology for many applications including LED street lighting. The FAN7621S provides everything necessary to build a reliable, rugged LLC resonant converter. The device includes a high-side gate drive circuit, a precision current-controlled oscillator, a frequency-limiting circuit, soft-start, and built-in protection to simplify design and increase productivity.
The FAN7621S features a variety of protection features such as overvoltage and overcurrent protection (OVP/OCP), abnormal overcurrent protection (AOCP), and internal thermal shutdown (TSD). Due to the special application requirements of LED street lighting systems, all protections are self-starting. The high-voltage side gate drive circuit has a common mode noise cancellation function. This excellent noise immunity ensures stable operation of the system. When the output is shorted, the operating point of the latest resonant converter can also be moved to the zero current switch (ZCS) region. Figure 1 shows how the work point moves. In this case, the zero voltage switch is no longer active and the MOSFET will conduct a particularly large current. The biggest disadvantage of zero current switching operation is the hard switching of the conduction point, which causes the MOSFET body diode to generate reverse recovery stress.
The body diode turns off at very large dv/dt, producing a high reverse recovery current spike. These spikes are 10 times higher than the steady state current. Such a large current causes an increase in loss and heats up the MOSFET. The rise in junction temperature will cause the dv/dt performance of the MOSFET to decrease. In extreme cases, the MOSFET can be damaged and the system malfunctions.
Figure 1: LLC resonant converter operating point based on load conditions
The latest MOSFET technology
The body diode of a MOSFET typically has a long reverse recovery time and a large reverse recovery charge. Despite its poor performance, such body diodes are often used as freewheeling diodes because of their simple circuit and do not increase system cost in applications such as resonant converters. As more and more applications use intrinsic body diodes as key components in the system, Fairchild has designed a highly optimized power MOSFET for the resonant converter under an in-depth analysis of the MOSFET failure mechanism. This MOSFET improves the body diode's durability and stores less energy in the output capacitor. As shown in Table 1, the reverse recovery charge (Qrr) of the new UniFET II MOSFET family is significantly reduced by 50% and 80% compared to the alternative.
Table 1: Comparison of key indicators of the device under test