[1] A flyback diode (or freewheeling diode) is any diode in paralle to an inductor to eliminate flyback, the sudden voltage spike seen across an inductive load if its supply current is suddenly reduced or interrupted. It is used in circuits where inductive loads are controlled by switches, and in switching power supplies and inverters.
A flyback diode parallel to an inductor must meet 2 criteria to be a flyback diode:
Inductive kick protection: A flyback diode's main purpose is to provide a safe discharge path for current when the inductive circuit switch opens, dissipating or recirculating the energy to prevent high voltage spikes.
Reverse bias during regular operation: The diode must be placed in reverse bias relative to the supply, by placing its cathode to the supply's positive end and the anode to the negative terminal or ground.
Normal operation operation with closed switch
An inductor's polarity usually follows the voltage supply's polarity.
By placing a flyback diode's cathode to a power supply's positive terminal and the anode grounded or to the supply's negative terminal, with the switch closed, the positive voltage hits the cathode, which pushes against the diode's internal PN junction, forcing diode into reverse-bias.
[Q1]
This is due to the higher positive voltage hitting the cathode (n-type), attracting the free electrons from the PN junction while the lower potential anode (p-type) attracts the positive holes from the junction, widening the depletion region and blocking current flow.
The diode stays off to block current, acting as an open circuit. So all supply current flows across the inductor.
Inductive kickback with switch opened The inductor's magnetic field collapses, causing the voltage polarity across the inductor to flip, which now forward-biases the diode, letting current to safely recirculate through the diode loop til the energy dissipated.
[2]
Inductive flyback in depth An inductor fundamentally, opposes sudden current change (ΔI/Δt) and its behavior is governed by induction law:
V = L*dI/Dt
L = inductance
dI/Dt = current's derivative
Opening the switch breaks the circuit path via a sudden change, forcing the current to drop from a steady state down to 0 roughly instantly (Δt = 0). As Δt is near 0, as the denominator, the derivative dI/Dt becomes a high negative value.
With the inductor coil's field collapsed the energy stored is released. This cuts across the inductor's wire turns, converting the stored magnetic energy back to electric energy.
After this, by nature, the inductor forces current to flow in the original direction by briefly works as a power source by flipping its voltage polarity. This governed by Lenz’s Law, stating an induced electromotive force (voltage) always oppose the magnetic flux change that produced it.
Inductive kickback without flyback diode Without a flyback diode, opening the switch causes inductive kick back while high voltage spike occur, with negative voltage spikes, causing dangerous arcing across the switch, especially mechanical ones or relays, and damaging it.sing dangerous arcing across the switch, especially mechanical ones or relays, and damaging it.
[2]
[Q1] Fig. 1
Motor circuit
With the PB released, the base current is cut, bringing the BJT to cutoff mode. Since BJT is off, the current from the supply to the ground the emitter connects to is blocked, no current is present, turning off the motor.
By pressing the PB, current can flow to the base and turn on the BJT to saturation, with a drop of ~0.7 V at the collector-emitter junction, to allow current to flow to ground, turning on the motor.
By releasing the PB once again, an inductive kickback is induced, cutting the current across the circuit again and cutting off the motor coils.
When a motor is deenergized, its magnetic field collapses rapidly. Since a motor uses inductors, this abrupt collapse induces the flyback voltage across the motor terminals, damaging it in these ways (if no flyback diode is used):
Voltage spike/Inductive kickback is the induced voltage spike flowing in the opposite direction of the original current and can reach dozens to hundreds of volts—far beyond the circuit's operating voltage.
Transistor/Switch sreakdown: Without a path to safely dissipate, the high voltage arc across the switching device exceeds its collector-emitter breakdown voltage, permanently damaging/shorting the transistor.
Insulation breakdown: Severe repeated voltage spikes can degrade the insulation winding in the motor coils overtime, leading to internal short circuits.
By connecting the flyback diode in reverse-bias to the supply, it becomes forward-biased during the inductive spike, in the case of Fig. 1's circuit, when the button is pressed. The diode provides a safe low-resistance loop for the stored magnetic energy to flow and dissipate safely, avoiding high voltages form destroying the BJT's collector-emitter junction.
[Q1] ELE8941 Test 1 exam set 1: Torque Wheel Travel Optical Interrupts (Canva)
[Q2]