Fig. 2: MOSFET used as constant current source. The current flow through the fan is controlled exclusively by the
VGS voltage at the MOSFET gate.The red point in the output characteristics diagram on the right hand side indicates
a potential operation point of the MOSFET in the linear region.
Partial linear mode operation – e-fuse, load-switch
In e-fuse applications or load-switches the MOSFET can be used to maintain slow turn on avoiding high inrush
currents. For e-fuse applications an e-fuse controller senses the current and controls the current flow by varying the
MOSFET VGS voltage. In doing so the MOSFET briefly operates in linear mode. The VGS voltage is being increased
only slowly until the full current flows and finally the MOSFET is being operated in the ohmic region. The diagram on
the right part of Fig. 3 illustrates the three operation states of the MOSFET in the output characterstics diagram.
At first the MOSFET is off and the entire voltage drops across the MOSFET. Then the VGS voltage is being
increased continiously and drain current starts to flow. At this point the MOSFET operates in linear mode which is
shown as 2nd operational state in the diagram. Here the SOA becomes relevant. Finally the MOSFET is fully
enhanced (turned on) and it is being operated in the ohmic region. The same three operational states occur when
the MOSFET is used as a load switch. Critical for the MOSFET is the time in linear mode which depends on the
load-switch controller (or the e-fuse controller) timing. Typical timings are in the range of μ-seconds and may even