What is a Logic Level Power MOSFET ?

snr090428Power MOSFET transistors are voltage controlled switches.  They begin to turn on at their rated “gate threshold voltage” or Vgs(th).  Most of the datasheet ratings are given for a fully saturated condition, typically 10 volts.  There is also a maximum allowable gate voltage, Vgs, that is typically 20 volts.  Any voltage greater than that will pierce the thin metal oxide layer that insulates the gate from the rest of the silicon die, destroying the device.  This is why uninstalled MOSFETS can be so sensitive to electrostatic discharge (ESD) damage.

A typical half-bridge or h-bridge driver chip will apply a nominal 12 volts to the MOSFET gates.  This ensures that the transistor is fully on, while allowing a comfortable margin below the maximum voltage.  This margin is needed because the effective gate voltage can fluctuate due to transients caused by fast switching, high currents and inductance in the package leads and PCB layout.

What if you want to drive a MOSFET gate directly from a microcontroller or other logic circuit?  A standard power MOSFET will not turn on fully at 5 volts.  Instead, you would use a “logic level” MOSFET, which is designed to fully saturate with only 5 volts applied to the gate.  However, there is a price to pay.  The lower threshold voltage comes with a lower maximum voltage, often only 10 volts.  Obviously, if a logic level MOSFET is placed in a circuit designed for a standard MOSFET without any other modifications, it will be destroyed.

Image courtesy of International Rectifier Corp.