What is Fast Recovery Diode

Fast recovery diodes are indispensable and important components in power circuits, but improper selection can cause overheating of the device and electromagnetic compatibility (EMC) problems of the circuit. This article starts with the basic characteristics and main parameters of fast recovery diodes, analyzes their working status, and proposes the basis and principles for selection.

What is fast recovery diode?

Fast Recovery Diode (FRD) is a diode with fast reverse recovery time. It is widely used in power supply circuits and high-speed switching circuits. It can improve rectification efficiency, reduce power supply noise, reduce switching losses, and improve circuit reliability and stability. This article will introduce the role of fast recovery diodes in detail from the following aspects.

fast recovery diode

Basic structure of fast recovery diode

The PN junction of a diode (formed by doped semiconductor) is to do differently on both sides of an intrinsic semiconductor, so that one side becomes a P-type semiconductor and the other side forms an N-type semiconductor. At the junction of the two, a very thin special conductive layer is formed. This special conductive layer is called a PN junction. The main characteristic of the PN junction is unidirectional conductivity.

The PN junction chip is packaged with plastic packaging material, glass, metal and other shells, and electrode leads are added to form a semiconductor diode. The electrode drawn from the P area is the anode (A), and the electrode drawn from the N area is the cathode (K). When the diode is applied with a forward voltage, the PN junction becomes narrower, the forward current is large, the forward resistance is small, and it is in a conduction state; when the diode is applied with a reverse voltage, the PN junction becomes wider, the reverse current is small, and the reverse resistance is large. is closed. The structure is shown in the figure below.

Fast recovery diode structure diagram - PN junction

The working principle of fast recovery diode

In the power circuit, when the AC voltage passes through the rectifier, the output voltage will change multiple times in one cycle, and the fast reverse recovery time of the fast recovery diode can ensure that the current has reversed before the start of the next cycle, thereby reducing the output voltage fluctuations. In addition, fast recovery diodes have the advantages of large current, low reverse leakage current, and low capacitance, which can improve rectification efficiency and reduce power supply noise.

In high-speed switching circuits, fast recovery diodes are used to protect other devices from excessive reverse voltage and sharp reverse current impacts. When a switch turns off momentarily, the inductor generates a reverse electromotive force, and if it is not protected by a diode with a fast reverse recovery time, the reverse current can flow through other devices, causing them to be damaged. The fast recovery diode can quickly cut off the reverse current to protect other devices.

Comparison between fast recovery diodes and ordinary diodes

Compared with ordinary diodes, fast recovery diodes have the following advantages:

(1) Shorter reverse recovery time: The reverse recovery time of fast recovery diodes can reach as few as tens of nanoseconds, while ordinary diodes require several microseconds to reverse recovery. This makes fast recovery diodes suitable for use in high frequency, high speed and precision circuits.

(2) Suitable for high-speed switching circuits: The fast reverse recovery time and low capacitance of fast recovery diodes allow it to withstand high-speed and high-frequency switching operations, while ordinary diodes cannot meet these requirements.

(3) Reduce the fluctuation peak voltage: The fluctuation peak voltage of a fast recovery diode is relatively stable during reverse recovery and is not prone to high-frequency oscillation, while the fluctuation peak voltage of ordinary diodes decreases significantly during reverse recovery and is prone to high-frequency oscillation.

(4) Improve rectification efficiency: The short reverse recovery time of fast recovery diodes can reduce switching losses and sharp current impacts in the power circuit, thereby improving rectification efficiency.

How to choose fast recovery diode

There are currently three series of conventional fast recovery diodes on the market: fast recovery diodes (FR series, fast recovery rectifier), high efficiency diodes (HER series, high efficiency rectifier), and ultrafast diodes (SF series, super fast rectifier). Detailed parameters see table:

Fast recovery diode classification

Typical values of main parameters	Switching time TRR (ns)	Forward voltage drop Vf (V)	Reverse voltage withstand VRRM (V)
Fast recovery diode	150-500	1.05	600-1000
High efficiency diode	50-75	1.38	600-1000
Ultrafast diode	20-35	1.4	200-600

The selection of fast recovery diodes in the line mainly considers reducing the power consumption of the diode in the line, preventing the output efficiency from decreasing and reducing electromagnetic oscillation. It is necessary to select a suitable fast recovery diode in terms of reverse recovery time, forward voltage drop and reverse withstand voltage.

Selection of reverse recovery time TRR

According to the frequency of the diode in the line, select the diode with the smallest TRR. The general experience is: the reciprocal of the frequency/100 (ns). For example, in a 100K HZ circuit, it is advisable to choose a fast recovery diode below 100ns. At the same time, try to choose a diode with "soft" recovery characteristics. The "softness coefficient" is generally not marked in the manufacturer's specifications. If the application circuit has requirements for EMC and frequency noise, you can select a diode with a "softer" recovery by testing the reverse recovery curve.

Selection of forward voltage drop VF

Try to choose a diode with a small VF to reduce the forward turn-on power consumption. This is the most important heat generation and power consumption factor in circuits with low switching frequency. But there is an inverse relationship between VF and TRR. Therefore, in a circuit with a small frequency, if the TRR of an ordinary fast recovery diode can meet the line requirements, it is not necessary to choose an ultrafast diode to avoid increasing forward power consumption.

Select the appropriate reverse voltage

In the selection of fast recovery diodes, circuit designers generally choose diodes with small TRR (small switching power consumption), small VF (small forward power consumption), and large VRRM (large reverse voltage). As the reverse withstand voltage VRRM of the fast recovery diode is larger, VF will also increase. Generally, it is sufficient to select a reverse voltage with a margin of more than 50%. If the working current of the diode in the line is small and the forward power consumption is not large, in order to increase the ability to withstand surge voltage, you can choose a diode with a higher withstand voltage regardless of the forward voltage drop VF.

Application Fields of Fast Recovery Diodes

Fast recovery diodes are mainly used in power supply circuits and high-speed switching circuits, such as switching power supplies, high-speed recovery rectifier circuits, frequency converters, motor drive control and other fields. The specific application occasions are as follows:

(1) Frequency conversion control: With the development of modern intelligent manufacturing, it is widely used in AC motor drive control. Because AC motors have high transmission efficiency, stable operation, and are adaptable to various load requirements, they are used more and more widely. As an important component in the frequency converter, fast recovery diodes can make the entire control system more accurate and sensitive.

(2) Switching power supply: Switching power supply is an essential power supply type in modern electronic equipment and is commonly used in digital electronic products such as computers and mobile phones. Since the switching power supply needs to work in high-speed switching mode, fast recovery diodes need to be used to reduce switching distortion and reverse stress.

(3) High-speed recovery rectification circuit: In the development of semiconductor technology, fast recovery diodes have replaced Schottky diodes and become the ideal choice for high-speed recovery rectification circuits. It can reduce switching noise, reduce switching losses, and increase energy utilization.

The future development trend of fast recovery diodes

With the continuous development of electronic technology, people's requirements for fast recovery diodes are becoming higher and higher. In the future, fast recovery diodes will have the following development trends:

(1) Increase current density: Increase the current density of fast recovery diodes to adapt to higher power power supply and switching applications.

(2) Reduce the reverse leakage current: reduce the reverse leakage current of the fast recovery diode and improve its reliability and stability.

(3) Continue to increase switching speed: Improve the reverse recovery time and switching speed of fast recovery diodes to adapt to higher frequency switching applications.

(4) Improve the radiation resistance performance: improve the radiation resistance performance of fast recovery diodes to meet more stringent environmental requirements.

In summary, fast recovery diodes have the advantages of fast reverse recovery time, low reverse leakage current, low capacitance, etc., and are widely used in power supply circuits and high-speed switching circuits. In the future, with the continuous advancement of electronic technology, fast recovery diodes will continue to play an important role.

4 Conclusion

The selection of fast recovery diodes in the circuit needs to take into account the reduction of heat consumption and the prevention of line oscillation. One is to select an appropriate TRR according to the line frequency, and the softer the recovery curve is, the better; the other is to choose a diode with a smaller VF, and at the same time, the larger the reverse withstand voltage VRRM, the better. It should be noted that TRR increases with temperature, so the rate of change of TRR at high temperature during power-on also needs to be considered.