What are the differences between fast recovery diode and switching diode
Fast recovery diode structure: PN junction structure and improved PIN structure are adopted. Its forward voltage drop is higher than that of ordinary diode (0.5-2V), and its reverse voltage withstand is mostly below 1200V. The performance can be divided into two levels: fast recovery and ultra fast recovery. The reverse recovery time of the former is hundreds of nanoseconds or longer, while that of the latter is less than 100ns (nanoseconds).
Schottky diode is a diode based on the potential barrier formed by the contact between metal and semiconductor, which is called Schottky Barrier Diode for short. It has a forward voltage drop (0.4-1.0V), a short reverse recovery time (2-10ns nanoseconds), and a large reverse leakage current, low withstand voltage (generally lower than 150V). It is mostly used in low-voltage occasions. The difference between Schottky diode and fast recovery diode is that the recovery time of the former is about 100 times smaller than that of the latter, and the reverse recovery time of the former is about a few nanoseconds!
The former has the advantages of low power consumption, high current and super high speed! Of course, the electrical characteristics are all diodes! High switching speed and high withstand voltage can be obtained by using gold doping and simple diffusion in the manufacturing process of fast recovery diodes. At present, fast recovery diodes are mainly used as rectifier components in inverter power supplies
Schottky diode: the reverse withstand voltage is low (generally less than 150V), the on state voltage drop is 0.3-0.6V, and the reverse recovery time is less than 10nS. It is a "metal semiconductor junction" diode with Schottky characteristics. Its forward starting voltage is low. In addition to materials, the metal layer can also be made of gold, molybdenum, nickel, titanium and other materials. Its semiconductor material is silicon or gallium arsenide, mostly N-type semiconductor. This device is conductive by most carriers, so its reverse saturation current is much larger than that of PN junction conductive by a few carriers. Because the storage effect of minority carriers in Schottky diode is very small, its frequency response is only limited by RC time constant, so it is an ideal device for high frequency and fast switching. Its operating frequency can reach 100GHz. In addition, MIS (metal insulator semiconductor) Schottky diodes can be used to make solar cells or light emitting diodes.
Fast recovery diode: It has a forward conduction voltage drop of 0.8-1.1V, a reverse recovery time of 35-85nS, and a fast conversion between conduction and cut-off, which improves the use frequency of the device and improves the waveform. The fast recovery diode is manufactured by gold doping and simple diffusion, which can achieve high switching speed and high voltage withstand. At present, fast recovery diode is mainly used as rectifier in inverter power supply.
Fast Recovery Diode (FRD) is a new type of semiconductor device introduced in recent years. It has the advantages of good switching characteristics, short reverse recovery time, large forward current, small size, simple installation, etc. Superfast Recovery Diode (SRD) is developed on the basis of fast recovery diode. Its reverse recovery time trr value is close to that of Schottky diode. They can be widely used in switching power supply, pulse width modulator (PWM), uninterruptible power supply (UPS), AC motor variable frequency speed regulation (VVVF), high-frequency heating and other devices, as high-frequency, large current freewheeling diode or rectifier tube, and are very promising power and electronic semiconductors.
1. Performance characteristics
Reverse recovery time
The reverse recovery time tr is defined as the time interval between the current passing through the zero point and the forward conversion to the specified low value. It is an important technical index to measure the performance of high-frequency freewheeling and rectifier devices. IF is the forward current and IRM is the maximum reverse recovery current. Irr is the reverse recovery current, which usually specifies Irr=0.1 IRM. When t ≤ t0, the forward current I=IF. When t ＞ t0, the forward voltage on the rectifier suddenly changes to the reverse voltage, so the forward current decreases rapidly. At t=t1, I=0. Then the reverse current IR flows through the rectifier, and the IR gradually increases; The maximum reverse recovery current IRM value is reached at t=t2. After that, under the action of forward voltage, the reverse current gradually decreases and reaches the specified value Irr at t=t3. The reverse recovery process from t2 to t3 is similar to the capacitor discharge process.
2) Structural characteristics of fast recovery and ultrafast recovery diodes The internal structure of fast recovery diodes is different from that of ordinary diodes. It adds base I between P-type and N-type silicon materials to form P-I-N silicon chips. Because the base region is very thin and the reverse recovery charge is very small, not only the trr value is greatly reduced, but also the transient forward voltage drop is reduced, so that the tube can withstand a high reverse operating voltage. The reverse recovery time of fast recovery diode is usually several hundred nanoseconds, the forward voltage drop is about 0.6V, the forward current is several amperes to several thousand amperes, and the reverse peak voltage can reach several hundred to several thousand volts. The reverse recovery charge of the ultra fast recovery diode is further reduced, making its trr as low as tens of nanoseconds.
Most of the fast recovery and ultra fast recovery diodes below 20A are packaged in TO-220. In terms of internal structure, it can be divided into single tube and double tube (also called double tube). There are two fast recovery diodes inside the tube. According to the different connection methods of the two diodes, there are also two types of tubes with common cathode and common anode. Figure 2 (a) shows the outline and internal structure of the C20-04 fast recovery diode (single tube).
The fast recovery diode with tens of amperes is generally packaged with TO-3P metal shell. The pipes with larger capacity (hundreds of amperes to thousands of amperes) are packaged in bolt type or flat plate type.
2. Detection method
1) Measure reverse recovery time
The specified IF is supplied by the DC current source. The pulse generator adds the pulse signal through the isolated capacitor C. The trr value observed by the electronic oscilloscope is the time from I=0 to IR=Irr.
Assume that the reverse recovery charge inside the device is Qrr, and the relationship is: trr ≈ 2Qrr/IRM. When IRM is constant, the smaller the reverse recovery charge, the shorter the reverse recovery time.
2) Routine testing methods
Under amateur conditions, the multimeter can be used to detect the unidirectional conductivity of fast recovery and ultra fast recovery diodes, as well as whether there are open circuit and short circuit faults inside, and to measure the forward conduction voltage drop. If equipped with a megger, the reverse breakdown voltage can also be measured.
Example: measure an ultra fast recovery diode. Its main parameters are: trr=35ns, IF=5A, IFSM=50A, VRM=700V. Set the multimeter to R × First gear, read the positive resistance as 6.4 Ω, n '=19.5 grids; The reverse resistance is infinite. Further calculate VF=0.03V/lattice × 19.5=0.585V。 Prove that the pipe is good.
matters needing attention:
1、 Some single tubes have three pins in total, and the middle one is empty, which is usually cut off when leaving the factory, but some are not cut.
2、 If one of the pipes is damaged, it can be used as a single pipe.
3、 When measuring the conduction pressure drop, R must be used × First gear. If R is used × For gear 1k, the test current is too small, which is far lower than the normal working current of the tube, so the measured VF value will be significantly lower. In the above example, if R is selected × When measured at 1k gear, the positive resistance is equal to 2.2k Ω. At this time, n '=9 grids. The calculated VF value is only 0.27V, which is far lower than the normal value (0.6V).