Flyback diode (freewheeling diode)

오늘은 Flyback diode (freewheeling diode) 에 대해 알아보도록 하자 

 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. 

source - wikipedia.com

프리윌링 다이오드는, flyback 을 제거하는데 쓰이는 다이오드이다. flyback 이 무엇이냐구요 ? 서플라이 전압이 갑자기 줄어들거나 제거되었을 때에 생기는 인덕터쪽의 부하에 걸리는 갑작스런 전압 스파이크를 말한다.

In its most simplified form with a voltage source connected to an inductor with a switch, we have 2 states available. In the first steady-state, the switch has been closed for a long time such that the inductor has become fully energized and is behaving as though it were a short (Figure 1). Current is flowing "down" from the positive terminal of the voltage source to its negative terminal, through the inductor. When the switch is opened (Figure 2), the inductor will attempt to resist the sudden drop of current (dI/dt is large therefore V is large) by using its stored magnetic field energy to create its own voltage. An extremely large negative potential is created where there once was positive potential, and a positive potential is created where there was once negative potential. The switch, however, remains at the voltage of the power supply, but it is still in contact with the inductor pulling down a negative voltage. Since no connection is physically made to allow current to continue to flow (due to the switch being open), the large potential difference can cause electrons to "arc" across the air-gap of the open switch (or junction of a transistor). This is undesirable for the reasons mentioned above and must be prevented.

위 회로를 보게 되면 이렇다. 두 가지 상황을 생각해 볼 수 있는데, 첫번째는 steady-state 이다. 장시간 동안 스위치가 연결되어
있어, 인덕터가 쇼트된 것처럼 작동을 한다. 그런데 스위치가 열리게 되면 인덕터는 저장되어진 magnetic field 에너지를 이용하여 자기만의 전압을 생성하여 갑작스런 전류 drop 을 방해하려 한다. 그 다음 (이부분은 잘 이해가 안되는데) 어쨌든, 큰 voltage potential 차이가 생기며 회로가 open 되어있어 전류가 흐르지 못하므로 arc 가 공기중으로  오픈된 스위치에서 발생할 수가 있다. 따라서 이 현상을 막기 위해 플라이백 다이오드를 달아준다. 그렇게 되면 전류들이 다이오드를 통해 인덕터와 저항으로 소모될때까지 마구마구 흐르게 된다. 
(뺑뺑이...-,- ) 회로가 closed 되면 다이오드는 없는 존재처럼 사라져 버린다. (reverse biased against the power supply) 
 

A flyback diode solves this starvation-arc problem by allowing the inductor to draw current from itself (thus, "flyback") in a continuous loop until the energy is dissipated through losses in the wire and across the diode (Figure 3). When the switch is closed the diode is reverse biased against the power supply and doesn't exist in the circuit for practical purposes. However, when the switch is opened, the diode becomes forward biased relative to the inductor (instead of the power supply as before), allowing it to conduct current in a circular loop from the positive potential at the bottom of the inductor to the negative potential at the top (assuming the power supply was supplying positive voltage at the top of the inductor prior to the switch being opened). The voltage across the inductor will merely be a function of the forward voltage drop of the flyback diode, and the distance between the diode and the inductor (according to National Instruments, the flyback diode should be no greater than 18 inches from the inductor). Total time for dissipation can vary, but it will usually last for a few milliseconds.