AN-4161 — Practical Considerations of Trench MOSFET Stability when Operating in Linear Mode

Semiconductor technology has achieved remarkable results through its evolution over the years. Today’s devices have significantly improved performance, especially in reduced drain-source on-state resistance, lower gate charge, and faster switching speeds. The overall system low-power consumption and higher performance is the name of the game in today’s competitive world. Most power MOSFET devices are used as switches in high-frequency applications where switching speed is an essential application requirement. They exhibit better efficiency as a result of lower on-state and dynamic switching losses, which are generated during the very short switching intervals. They also display better electro-thermal stability characteristics because of the positive temperature coefficient of the onstate resistance (RDS(ON)) and the breakdown voltage (BVDSS). These attributes are desirable to limit the possibility of a thermal runaway situation. However, these desirable characteristics are not ideal in applications where the device is operated in Linear Mode. Transconductance (gFS) is high, which makes devices prone to electro-thermal instability, especially when operated in Linear Mode. The thermal instability situation is more pronounced at low drain current (ID), which is influenced by the progressive scaling down of power MOSFET die size. Secondly, the threshold voltage (VTH) has negative temperature coefficient, which makes it impossible to maintain a constant drain current (ID) without a negative feedback. [1] The positive temperature coefficient of RDS(ON) does not represent all the factors for stable operation. These tradeoffs can lead to a phenomenon known as “hot-spotting” in power MOSFETs, which can be destructive to the device. Even if the device has a well-designed heat sink, the hot spots are difficult to control because heat sinks work well only to reduce the total mean junction temperature and hot spots are more concentrated on a specific area of the power MOSFET cell structure.