What is a Fast Recovery Diode?

Fast recovery diode

A fast recovery diode is a diode which has a quick recovery time.

What a quick recovery time is will be explained below.

One of the most common applicadtions for a diode is to rectify 60Hz sine waves. This is the frequency of normal AC power lines in the United States. The diode rectifies this current so that it can be changed from AC current to DC current, since most electronic devices work off of DC power. So the diode rectifies this 60Hz AC sine waves so that it can converted into DC. 60Hz is a very low frequency. Most standard conventional diodes are designed so that they give their best performance at relatively low frequencies.

When a diode is dealing with AC signals, such as rectifying them, a certain finite amount of time is required for a diode to recover from one signal to the next.

AC signals come nonstop at the diode:

AC signals with fast recovery diode

As you can see in the illustration, the diode is battered with AC signals coming at it. Most diodes can handle low frequency signals because since the time periods of each cycle is not very long, they aren't very fast. This is why in low-frequency applications, the recovery time of a diode is not particularly significati. Time period is inversely proportional to the frequency, according to the formula, T= 1/f.

However, the greater the frequency of the signal, the shorter the time period of each cycle. The shorter the time period, the less time the diode has to recover from each cycle. At very high frequencies, this can become a problem, since most diodes cannot recover quickly enough from each cycle. This happens in high-frequency applications, such as in television flyback circuits. In cases like these, the recovery time can become very crucial.

This is because the diode must respond to very short-duration spikes with a very brief rest period between adjacent spikes. An ordinary diode could cause erratic or incorrect operation of the circuit. For better and more reliable performance in high-frequency circuits, a special-purpose diode called a fast recovery diode is used.

Below shows the different in recovery time between a fast recovery diode and a standard diode:

Fast recovery diode chart

An ordinary diode takes a few microseconds (µs) to recover. Again, this is fine for low-frequency applications. However, a fast recovery diode takes just nanoseconds (ns) to recover. This is crucial in very high frequency applications. You can see in the above chart how much quicker fast recovery diodes recover.

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