Straight Facts about Linear MOSFETs and their Applications BY José Padilla, Director Product Marketing; Aalok Bhatt, Product Marketing Engineer; Vladimir Tsukanov, Senior Principal R&D, Littelfuse-BODO POWER SYSTEM

Straight Facts about Linear MOSFETs and their Applications Linear-mode applications such as class-A audio amplifiers, active DC-link discharge, battery charge-discharge, inrush current limiter, low-voltage DC motor control or electronic loads demand the power MOSFETs to be operated within the current saturation region. Standard MOSFETs are prone to ETI when used in linear-mode applications leading to possible device destruction. Linear MOSFETs are the most appropriate choice for linear-mode applications to ensure reliable operation. José Padilla, Director Product Marketing; Aalok Bhatt, Product Marketing Engineer; Vladimir Tsukanov, Senior Principal R&D, Littelfuse

Introduction – What is linear-mode operation and why is it required Power MOSFET’s output characteristic can be divided in to three distinct regions namely ohmic region, non-linear region, and saturation or active region as displayed in Figure 1. In the ohmic region, the drain current ID is directly proportional to the drain-source voltage VDS for a given gate-source voltage VGS. The MOSFET acts as a resistor in this operation mode with a value equal to its on-state resistance RDS(ON). In the non-linear region, the MOSFET’s resistance behaves non-linearly and the rate of increase of ID with VDS slows down. In the active region, the MOSFET’s channel is saturated with majority charge carriers. In this region, ID is independent of VDS. ID is governed only by VGS and it remains constant for any given VDS. In other words, the MOSFET exhibits the behavior of a constant current sink. This operating mode is commonly known as linear operation mode of power MOSFETs. In this operating mode, the MOSFETs typically dissipate higher power levels than they would in the more common switched-mode applications due to simultaneous occurrence of high voltage and current[1].

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