Wide bandgap technology to maximize efficiency and power density in high-voltage LED lighting
PFC stage is followed by a non- isolated inverse buck stage with CC/CV control. In the cases where isolation is needed, a resonant power converter (LLC, LCC) or a flyback converter can be used depending on the output power requirements of the application. The PFC boost converter generates a regulated DC bus voltage on its output (higher than the peak of the input AC voltage) and passes this higher DC bus voltage to the inverted buck converter stage. The stepdown operation is quite simple. When the switch in the buck is on, the inductor voltage is the difference between the input and output voltages (V IN – V OUT ). When the switch is off, the catch diode rectifies the current and the inductor voltage is the same as the output voltage. MasterGaN system in package (SiP) for LED drivers Along with power density and efficiency, a key challenge for high-voltage lighting applications is the complexity of the design. With the use of wide bandgap semiconductors like GaN, the power density and efficiency of the circuit can be increased. ST’s MasterGaN family addresses that challenge by combining the high- voltage smart-power BCD-process gate drivers with high-voltage GaN transistors in a single package. MasterGaN allows for an easy
Figure 1 shows a high-level architecture of an LED lighting application that will serve as a baseline example for applying GaN wide bandgap technology. Although wide bandgap materials can be implemented across the application, the high-voltage current generator buck, highlighted in green, will be the focus to leverage wide bandgap technology for maximizing efficiency and power density. Most lighting applications require high power factor and low harmonic distortion across a wide AC input voltage range. In this case, it is preferred to implement a PFC boost to provide a clean 400 VDC input for the LED driver and meet power quality requirements. There are multiple options for a front end PFC boost converter; transition mode (TM), continuous conduction mode (CCM) as well as others. Transition mode is characterized by variable frequency operation and zero current switching at turn on of the power MOSFET. Other advantages are simple design, small inductor size, and no reverse recovery of the boost diode. The
main challenges are high peak and RMS input current, which also results in a larger EMI filter as the power increases. CCM, instead, provides fixed frequency operation. The boost inductor current always has an average component, besides near zero crossing points. The inductor is designed for 20-30% ripple, resulting in a smaller EMI filter compared to TM operation. This also means a larger boost inductor and a smaller EMI filter for the same output power when compared to TM operation. The main challenges are more complex control and the need for an ultrafast soft recovery diode or SiC diode. Consequently, the CCM PFC is generally more expensive than a TM PFC. Ideally, a zero reverse recovery switch can be used in place of the rectifying diode in CCM PFCs. This makes GaN transistors very good candidates for this application. Isolation is optional and can be introduced between the input stage and the second stage of power conversion. In this example, isolation is not used, and the input
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