MSc. Stefan Preimel
Application Engineer, High Voltage MOSFET, Infineon Technologies
Smaller, smarter, and more powerful are market trends for low-power switched-mode power supply (SMPS) applications. Smaller requires the SMPS to have a more compact form factor – and, thus higher power density. Smarter requires intelligent system-to-system/system-to-human communication to ensure the systems are environment- and human-friendly. More powerful requires the SMPS to output more power to supply energy to many different devices in order to save cost for consumers.
These trends expose engineers to new SMPS design challenges. Because of higher output powers and smaller form factors leading to higher power densities, engineers need to spend more effort improving efficiency and managing overall system heat. At the same time, longer product lifetimes require engineers to optimize SMPS designs at a system level to improve reliability.
Benefits due to these market trends are obvious. A smaller size leads to space saving and makes systems more portable. Energy saving due to higher efficiency and smarter design helps consumers save money on electricity bills, as well as supporting a ‘greener’ world – as less power supplies are needed at home thus there is less pollution in production and less waste when it comes to end-of-life disposal.
Adapters and chargers, LED lighting, audio SMPS, auxiliary power and industrial power are typical low-power SMPS markets where flyback topology is the most common topology. High-voltage (HV) MOSFETs play a fundamental role in this topology, which means that finding the right MOSFET is critical to meeting the challenges posed by market requirements.
Factors to consider when choosing HV MOSFETs for flyback based application
Determining the ‘right’ MOSFET for a flyback topology requires a solid, system-level understanding as well as the way MOSFET is used in the application. State-of-the-art flyback designs switch HV MOSFETs at 40 kHz – 100 kHz to reduce the need for EMC optimization, while other designs switch at higher frequencies in order to achieve size reductions associated with smaller magnetic components. Better light-load efficiency requires lower switching losses and high full-load efficiency requires lower low conduction losses.
For switching losses, Qg and Eoss are important MOSFET device parameters. Qg is closely linked with the effort to drive MOSFETs and smaller values leads to lower driving losses. Eoss is the energy consumed by the MOSFET when it switches on. RDS(on) is an important device parameter and determines conduction losses, which plays a critical role at full-load condition.
MOSFET ESD ruggedness is one area that is gaining more and more attention when it comes to MOSFET selection, as it helps to improve production yield and reduce field returns. Robustness, ease of driving and design-in also have to be considered by engineers who are under pressure to deliver fast time-to-market. VGS(th) and its deviation needs to be considered here: lower VGS(th) makes MOSFETs easier to drive while a smaller VGS(th) deviation gives designers more freedom in SMPS design.