Flyback diode

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A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, or catch diode[1]) is a diode used to eliminate flyback, the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed. A flyback diode provides a discharge current path for the energy stored in the load, which allows that energy to dissipate slowly, rather than appearing as a voltage spike as it dissipates instantaneously. In mechanically-switched circuits, the near-instantaneous dissipation which occurs without a flyback diode is often observed as an arc across the opening mechanical contacts, resulting in the energy's dissipation as electro-magnetic energy including everything from visible light to a wide spectrum burst of radio-frequency noise, while causing premature erosion of the contacts.

Applications: Flyback diodes are used whenever inductive loads must be switched off: in relay drivers, H-bridge motor drivers, and inside the switched-mode power supply.

Design: Schottky diodes are preferred in flyback diode applications, because they have the lowest forward drop and the fastest (but still "soft") reverse voltage recovery. In applications which require higher "reverse blocking voltage" than Schottky diodes offer, the "gold doped diode" is preferred as the next most efficient diode choice[2].

Low-cost applications unconcerned with efficiency often use 1N4004 or 1N4005 or 1N4007 power diodes as flyback diodes (sci.electronics.design : "diode for relay coil spikes and motor shutoff spikes?").