Freewheel Diodes
Signal diodes can be used in the variety of clamping, protection and wave shaping circuits with the most ordinary form of clamping diode circuit being one which uses the diode linked in parallel with a coil or inductive load to prevent the damage to delicate switching circuit by suppressing the voltage spikes and/or transients which are generated when load is suddenly turned "OFF". This type of diode is commonly known as a "Free-wheeling Diode" or Freewheel diode as it is commonly called.
The Freewheel diode is taken in use to protect solid state switches like power transistors and MOSFET's from the damage by reverse battery protection as well as protection from the highly inductive loads like relay coils or motors, and an instance of its connection is shown below.
Use of the Freewheel Diode
Modern fast switching, power semiconductor devices needs fast switching diodes like freewheeling diodes to protect them from the inductive loads like motor coils or relay windings. Each time the switching device shown above is turned "ON", the freewheel diode changes from the conducting state to the blocking state as it becomes reversed biased. However, when the device quickly turns "OFF", the diode becomes forward biased and collapse of the energy stored in the coil makes current to flow through freewheel diode. Without protection of the freewheel diode high di/dt currents would take place causing a high voltage spike or transient to flow around the circuit perhaps damaging the switching device.
Formerly, the operating speed of semiconductor switching device, either MOSFET, transistor, IGBT or digital has been impaired by addition of the freewheel diode across inductive load with Schottky and Zener diodes being used instead in some of the applications. But in the past few years however, freewheel diodes had regained importance mainly because of their improved reverse-recovery characteristics and use of super fast semiconductor materials capable of operating at high switching frequencies.
Other types of advanced diodes which are not included here are PIN Diodes, Photo-Diodes, Tunnel Diodes and Schottky Barrier Diodes. By the addition of more PN junctions to the basic two layer diode structure other types of the semiconductor devices can be made. For instance a three layer semiconductor device becomes Transistor, a four layer semiconductor device becomes Thyristor or Silicon Controlled Rectifier and five layer devices called as Triacs are also available.
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