Free-wheeling diode

Figure 1

Refer figure 1, the D1 have a lot of name for it. It called flyback diode, snubber diode, free-wheeling diode, suppressor diode, or catch diode. It have a lot of name, but its purpose only one, use to de-charge the solenoid (L1) energy.

Refer figure 2, looking at the inductor equation, V = L di/dt. At initial (2a), the current start flow into inductor, the inductor will generate the voltage (EMF / electromotive force) to block current flow. When current is constant / continuous (DC), the voltage across the inductor is 0V (short circuit). When circuit open instantly (2b), will cause the current flow across inductor change instantly. The energized inductor will generate voltage (back EMF) same as source direction. The inductance, current change and time will determine voltage that generated by the energized inductor. This spike voltage can up to few hundred volt. 

Figure 2

Faraday's Law of induction, any change in the magnetic environment of a coil of wire will cause a voltage (EMF) to be induced in the coil. When current start flow across solenoid, the solenoid generate the magnetic field and EMF at the same time. Until the current flow across solenoid is constant, the EMF disappear (0V), but magnetic field still exist. When the inductor disconnect from current source, the solenoid magnetic field have decay (change), the back EMF is generate at this time.

Back to figure 1, if without the free-wheeling diode, when the Q1 turn off. The solenoid L1 may generate few hundred volt and top up with supply voltage (12V). This voltage will exceed the Q1 collector breakdown voltage, and Q1 will damage.

When free-wheeling diode connect to circuit, the back EMF will reduce, due to the continued circulation of current in the inductor and diode.

Choosing the Free-wheeling diode

The smallest spike voltage to transistor Q1 is the best, but what diode parameter can help to reduce spike voltage?

1. Diode turn on voltage (diode forward voltage)

    => diode easy to turn on will have lower spike voltage.

2. Diode turn on speed (diode turn on speed)

    => spike voltage is voltage change in short period. dV/dt

         faster diode turn on, the lower spike amplitude.

         the diode a will conduct at low voltage, but diode b will conduct at high voltage. that mean, it need higher peak forward current.

3. Peak forward current

    => The larger EMF generated, the larger current flow across the diode.

         Just a concept, if diode in turn condition is equal to 100 ohm resistor, the spike voltage is around 100V.

         I forward (pk) | min = (100/100) A = 1A

4. Reverse voltage

    => the reverse voltage must higher than supply voltage (e.g. 12V in figure 1).

Low-cost applications unconcerned with efficiency often use 1N4007 series power diodes as free-wheeling diodes.