Diodes are semiconductor-based devices that allow current in only one direction. They are one of the most used electronic components used in modern circuits. They are used for switching, rectification, protection, etc. There are different types of diodes used in modern circuits for their specific function. A fast recovery diode is one of the many types of diode used for its fast reverse recovery time.
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A fast recovery diode is a semiconductor-based PN junction diode that has a very fast reverse recovery time. It is also known as a fast diode or fast switching diode. Since it has a very fast reverse recovery time as compared to a conventional diode, it is best suited for high-frequency applications.
The diode conducts current in forward bias and blocks current in reverse bias. But due to the stored charge in its junction, it is unable to immediately block current when the applied voltage changes from forward to reverse bias. Due to stored charge carriers, it conducts in the reverse direction for a short duration of time called reverse recovery time.
A conventional diode has a large reverse recovery time and it is unable to block a high-frequency signal as the reverse half cycle conducts during the reverse recovery time. Therefore, the fast recovery diode is used for rectification of high-frequency signals.
The reverse recovery time Trr of the fast recovery diode ranges from tens to hundreds of nanoseconds. Whereas the reverse recovery time of a conventional PN junction diode resides in tens of microseconds. Therefore fast diode is used for rectification of signals up to 100 kHz.
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The symbol of a fast diode (for example FR-106, FR-107 etc. diode) is the same as a conventional diode because both of them have similar operations except for their switching speed.
It is a PN junction diode having a similar structure as a conventional diode. P-type semiconductors and N-type semiconductors are joined together to form a PN junction.
The only difference between them is that the fast diode has recombination centers in its semiconductor material. The recombination centers help in recombining the stored charge carriers and reducing their lifetime. Thus the junction is quickly drained of the stored charge and the recovery reverse time is reduced.
Gold is added as recombination centers in the semiconductor material such as gallium arsenide (GaAs). It greatly reduces the reverse recovery time as compared to a conventional diode. However, there is a limitation because the reverse current increases with an increase in the number of recombination centers. Therefore the amount of gold being added is taken into consideration.
A fast recovery diode has the same operation as a conventional diode as they both are used for rectification. When an AC signal passes through a diode, the positive half cycle passes through it while the negative half cycle is blocked. However, it doesn’t immediately block the negative half cycle. It requires an amount of time to recover from the conduction state to the blocking state.
As we know that the time period of a signal is inversely proportional to its frequency (T = 1/f). Time period means the time required to complete one cycle. Low-frequency signal has a large time period i.e. they are slow. Any conventional diode can rectify them as they do not require a small recovery time.
However, high-frequency signals have a very short time period and the duration of a negative half cycle is very small. A conventional diode having a large recovery time will conduct the fast negative half cycle. Therefore they require a diode that has a very small recovery time to block the negative half.
The fast recovery diode achieves this small recovery time by adding gold as recombination centers in its semiconductor. During the conduction state, the electrons and holes flow to the opposite side of the PN junction. When the applied voltage reverses, the electrons and holes start to move to their sides.
Electrons are fast and they recover pretty quickly but the holes take time as they are heavier than electrons. They recombine with each other and form the depletion region that blocks the current flow. The recombination centers placed at a short distance in the semiconductor help in the recombination of the holes thus providing a fast recovery time.
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The reverse recovery time greatly depends on the forward current. Forward current is the current flowing through the diode in forward bias.
In forward bias, electrons and holes move to the opposite side of the PN junction to flow out of the opposite terminals. The number of these electrons and holes depends on the forward current. A large forward current requires a large number of electrons and holes.
The greater the number of electrons and holes on their opposite sides, the larger the time taken by them to move to their sides. Thus the reverse recovery time is increased.
The n-layer consists of the majority of charge carriers electrons while the p-layer consists of holes. In forward bias, the electrons move to the p-layer while the holes move to the N-layer. When the applied voltage is reversed, the majority charge carriers move to their original side. But the holes are heavier.
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Therefore, during the construction, the n-layer of the semiconductor is designed larger as compared to the p-layer and filled with traps (recombination centers). Usually, Gold is used to catch the holes moving back and reduce the recovery time.
The reverse recovery of a diode can be classified into two types
During hard recovery, the reverse current decrease suddenly whose magnitude oscillates creating noise as well as causing power loss. Hard recovery is illustrated by the given graphs.
The reverse recovery time is fairly small but there is a large noise generated in the system.
During soft recovery, the reverse current softly and smoothly reduces to zero. There is no abrupt change in the reverse current, as a result, there is no noise generated in the system. The reverse recovery time is comparatively large with low power loss but the system has no noise.
Here are some electrical characteristics of a fast recovery diode.
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Such a type of FRD is specifically designed for its high speed even at the cost of high forward voltage. It has an extremely small reverse recovery time around 25 ns. But the forward voltage drop is fairly large in the range of 3 to 3.6 volts. It does have the disadvantage of high forward voltage but still, it is used for its high switching speed. Another advantage of ultra high-speed FRD is its soft recovery characteristics.
The given graph shows the comparison between the fast recovery diode and the conventional diode. It shows the change in the diode current with respect to time during the switching from forward bias to reverse bias.
As seen in the graph, the reverse current in the fast recovery diode quickly reduces to zero due to the recombination centers. Whereas in a conventional diode, the reverse current takes a long time to reduce to zero.
Advantages
Here are some advantages of a fast recovery diode
Disadvantages
Here are some advantages of a fast recovery diode
Generally, a fast recovery diode is used in the high-speed application.
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A fast recovery diode (FRD) is a type of power diode that has a short reverse recovery time for high-frequency rectification. The recovery time is the finite amount of time taken by a diode to discharge (during conducting state, it stored the charges) when switching from the conducting to the blocking state. They are designed to handle high voltage and high current levels than the conventional diode.
Fast recovery diode consists of two terminals - anode & cathode, and functions the same as that of a conventional diode, but differs in construction. The major construction difference is the presence of the recombination centers in the fast recovery diode. A conventional PN junction diode is formed by joining n-type and p-type semiconductor materials. Whereas in the fast recovery diode, Gold (Au) is added as a dopant to the Gallium Arsenide (GaAs) semiconductor. This leads to an increment of the number of recombination centers in the forbidden band, which results in a reduction of reverse recovery time at the instant of switching. This construction difference ensures the fast recovery diode exhibit a very small reverse recovery time than a normal diode.
Fast recovery diodes are ideal for use in applications such as switching power supplies, DC-DC converters, AC-DC adaptors/chargers, inverters, motor control & drives, power converter & inverter for wind turbines, commercial, construction & agricultural vehicles (CAV), UPS, and PFC.