Flyback diode

A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, or catch diode [http://www.hp9825.com/html/internal_printer.html] ) 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.

Induction at the Opening of a Contact

According to Lenz's law, if the current through an inductance changes, this inductance induces a voltage so the current will go on flowing as long there is energy in the magnetic field. If the current can only flow through the air, the voltage is therefore so high that the air conducts. That is why 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. Energy is dissipated in this arc primarily as intense heat which causes undesireable premature erosion of the contacts. Some energy is also lost from the system as a whole and from the arc as a broad spectrum of electromagnetic radiation, in the form of radiowaves and light. These radiowaves can cause undesirable clicks and pops on nearby radio receivers.

Working Principle

The latter diagram shows just what was described before; when the switch is turned off at 1ms, the same current that flowed through the R1 before now has to flow through R2 in the first moment, too; but due to the values of these resistors, the voltage at R2 has to be 1000 times as high. You may notice that the voltage spike appears negative where the diode would be placed. Thus, placing a diode reversely to the voltmeter would cause a current through the diode. The current would flow in the loop of L1, R1 and the diode.

That way, a flyback diode provides a discharge current path for the energy stored in the load, which allows that energy to dissipate slowly (see the diagram), rather than appearing as a voltage spike as it dissipates instantaneously. Due to the characteristic of the diode that its forward voltage is never greater than roughly 0.7V, the voltage at the switch is also limited.

Deduction

The voltage at an inductor is, by the law of electromagnetic induction and the definition of inductance::$V\_L\; =\; -\; \{dPhi\_B\; over\; dt\}\; =\; -\; L\; \{dI\; over\; dt\}$

If there is no flyback diode but only something with a great resistance (such as the air between two metal contacts), say, R2, we will approximate it as::$V\_\{R\_2\}\; =\; R\_2\; cdot\; I$If we open the switch and ignore $V\_\{CC\}$ and $R\_1$, we get::$V\_L\; =\; V\_\{R\_2\}$or:$-\; L\; \{dI\; over\; dt\}\; =\; R\_2\; cdot\; I$which is a differential equation with the solution::$I(t)\; =\; I\_0\; cdot\; e^\{-\; \{R\_2\; over\; L\}\; t\}$

We observe that the current will decrease faster if the resistance is high, as that of air is.

Now if we open the switch with the diode in place, we only need to consider L1, R1 and D1.For $I\; >\; 0$, we can assume::$V\_D\; =\; const$so::$V\_L\; =\; V\_\{R\_1\}\; +\; V\_D$which is::$-\; L\; \{dI\; over\; dt\}\; =\; R\_1\; cdot\; I\; +\; V\_D$whose solution is::$I(t)\; =\; I\_0\; cdot\; e^\{-\; \{R\_1\; over\; L\}\; t\}\; -\; \{1\; over\; R\_1\}\; V\_D$We can calculate the time it needs to switch off by determining for which $t$ it is $I(t)\; =\; 0$.

Applications

Flyback diodes are used whenever inductive loads are switched off by silicon components: in relay drivers, H-bridge motor drivers, and so on. A switched-mode power supply also exploits this effect, but the energy is not dissipated to heat but used for pumping additional charge through the load.

Design

Schottky diodes are preferred in flyback diode applications, because they have the lowest forward drop (0.2V rather than 0.7V for low currents) 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 [http://www.ixys.com/images/technical_support/Application%20Notes%20By%20Topic/FREDs,%20Schottky%20and%20GaAS%20Diodes/IXAN0044.pdf] .

Low-cost applications unconcerned with efficiency often use 1N4004 or 1N4005 or 1N4007 power diodes as flyback diodes.

External links

* [http://groups.google.com/group/sci.electronics.design/browse_thread/thread/f67136cd905c4fce/6ab2ddf92059d817 sci.electronics.design : "diode for relay coil spikes and motor shutoff spikes?"]