Flyback Diode - Working Principle

Working Principle

In its most simplified form with a voltage source connected to an inductor with a switch, we have 2 states available. In the first steady-state, the switch has been closed for a long time such that the inductor has become fully energized and is behaving as though it were a short (Figure 1). Current is flowing "down" from the positive terminal of the voltage source to its negative terminal, through the inductor. When the switch is opened (Figure 2), the inductor will attempt to resist the sudden drop of current (dI/dt is large therefore V is large) by using its stored magnetic field energy to create its own voltage. An extremely large negative potential is created where there once was positive potential, and a positive potential is created where there was once negative potential. The switch, however, remains at the voltage of the power supply, but it is still in contact with the inductor pulling down a negative voltage. Since no connection is physically made to allow current to continue to flow (due to the switch being open), the large potential difference can cause electrons to "arc" across the air-gap of the open switch (or junction of a transistor). This is undesirable for the reasons mentioned above and must be prevented.

A flyback diode solves this starvation-arc problem by allowing the inductor to draw current from itself (thus, "flyback") in a continuous loop until the energy is dissipated through losses in the wire and across the diode (Figure 3). When the switch is closed the diode is reverse-biased against the power supply and doesn't exist in the circuit for practical purposes. However, when the switch is opened, the diode becomes forward-biased relative to the inductor (instead of the power supply as before), allowing it to conduct current in a circular loop from the positive potential at the bottom of the inductor to the negative potential at the top (assuming the power supply was supplying positive voltage at the top of the inductor prior to the switch being opened). The voltage across the inductor will merely be a function of the forward voltage drop of the flyback diode. Total time for dissipation can vary, but it will usually last for a few milliseconds.

In these images we observe classic signs of back EMF and its elimination through the use of a flyback diode (1N4007). The inductor in this case is a solenoid connected to a 24V DC power supply using 20 awg wire. Each waveform was taken using a digital oscilloscope set to trigger when the voltage across the inductor dipped below zero. In Figure 1 we see the voltage as measured across the switch bounce/spike to around -300 V. In figure 2, a Flyback diode was added in antiparallel with the solenoid. Instead of spiking to -300 V, the flyback diode only allows approximately -1.4 V of potential to be built up. -1.4V is a combination of the forward bias of the 1N4007 diode (1.1 V) and the foot of wiring separating the diode and the solenoid. The waveform in Figure 2 is much less bouncy than the waveform in Figure 1. In both cases, the total time for the solenoid to discharge is a few milliseconds.