The flyback diode and its applications

Question

So in an application like this:

The coil of the motor will, by Lenz's Law, induce a very large negative voltage across its terminals (without the diode in place). Would this voltage be very negative at the left terminal of the inductor and very positive at the right? In this way the diode will conduct, but the negative voltage in this case would be directly connected to the power supply.

Also, how are the flyback diodes in this way utilized[L293 - H Bridge IC]:

It seems that the two left diodes will always be reversed biased, but what about the right-side two? Why are the left-side diodes even present if they are always reversed bias? It seems that the large negative voltage would forward bias one of the diodes to the right (whichever terminal is induced to a large negative potential), but what benefit does that have? Also, current flow back into the H-Bridge circuit does not appear very practical.

Perhaps I am looking at this all wrong.

Answer 1

In your first example, before the switch opens, there is a current flowing through L. When the switch opens the current wants to continue flowing in the same direction so this has to create a positive voltage on the right-hand side of L relative to the left hand side (which is fixed at +Vs).

This positive voltage causes current to continue to flow and this current finds the path of least resistance through diode D. It continues to flow until all the magnetic energy in the inductor is gone then current = 0 and the voltage on the right hand side of L is the same as the left hand side i.e. +Vs.

In the 2nd example with the H bridge, all the diodes are needed because it has to cater for scenarios when the motor is deactivated i.e. pin 3 and pin 6 are open circuit. This is the simplest way to explain it I think. The motor current may have previously been upwards or downwards so all four diodes are needed to catch the back emf when the motor is switched off.

Also, current flow back into the H-Bridge circuit does not appear very practical.

That's how they work.

Answer 2

Flyback diodes provide a path for current from the motor's inductivity to continue rather than cause voltages that can kill circuits and solenoids (the latter by arcing). For mere circuit/coil protection, a straightforward flyback diode is fine and in an idealised circuit, the result would be "idling": a mechanically unloaded motor would not have anywhere for the energy to go and keep turning.

For a stepper motor, you want the motor to stop. The "open" flyback circuit in your motor diagram feeds flyback diodes into the supply voltage. If the supply voltage is an ideal voltage source/sink, the motor thus has a power drain tantamount to the supply voltage times the induced current (which should start out equal to the motor's operating current).

However, this requires the voltage supply (minus other active loads) to be able to actually sink that current. Not all voltage supplies are able to gracefully deal with reverse current. Buffering capacitors on the circuit board might defuse the situation somewhat and limit the possible voltage increase even when the power supply just blocks reverse current rather than sinking it.