Freewheeling or Flyback Diode – Working and Purpose

What is Freewheling Diode?

A freewheeling diode is basically a diode connected across the inductive load terminals to prevent the development of high voltage across the switch. When the inductive circuit is switched off, this diode gives a short circuit path for the flow of inductor decay current and hence dissipation of stored energy in the inductor. This diode is also called Flywheel or Flyback diode.

Purpose of Freewheeling Diode

The main purpose of freewheeling or flyback diode is to free wheel the stored energy in inductor by providing a short circuit path. This is necessary else a sudden decay in circuit current will give rise to high voltage across the switch contacts and diode.

For better understanding, let us consider the figure below.

When switch S is closed, the steady state current I through the circuit is (V/R) and hence the stored energy in inductor is (LI²)/2. When this switch S is opened, the current will suddenly decay to zero from steady value I = (V/R). Due to this sudden decay of current, a high reverse voltage (as per lenz’s law) equal to L(di/dt) will appear across the inductor terminals and hence across the diode and switch. This will lead to sparking across the switch contacts. If this reverse voltage exceeds the Peak Inverse Voltage of diode, then it may get damage.  To avoid such occurrences, a diode, called freewheeling or flyback diode is connected across the inductive load RL as shown in figure below.

For better understanding of how freewheeling diode prevents such occurrences of over voltages and when it come into play, let us focus on the working principle and circuit diagram of flyback diode.

Working Principle of Freewheeling Diode:

In figure-1, inductive load RL is connected to the DC source through switch SW and diode D. When this switch SW is closed at t = 0 sec, current starts to flow through the load. This current builds up in the inductor and reaches its steady value after some time (roughly 2-3 time constant).

When the switch SW is opened at t = 0 (say), current in the circuit tends to decay through the load. This decay of current through inductor results in development of a reverse voltage equal to L(di/dt) across the inductor terminals. This reverse voltage across inductor terminal makes freewheeling diode forward biased. Thus freewheeling diode behaves like a closed switch as shown in figure below.

Thus, the main circuit current is transferred to the circuit consisting of freewheeling diode FD, R and L as shown in above figure. In this new circuit, the current will exponentially decay to zero. This decaying current in the circuit is given as below.

Thus we see that, freewheeling diode dissipates the stored energy in inductor by providing a short circuit path. It also provides a shorted path for exponentially decay of circuit current. Thus high voltage is not induced. Therefore, the switches and diode is protected from the high voltage.