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What is the difference between Schottky diodes and fast recovery diodes?

The basic principle of the Schottky diode is to block the reverse voltage with the formed Schottky on the contact surfaces of metals (such as lead) and semiconductors (N-type silicon wafers). Schottky and PN junction rectification principle of a fundamental difference. Its pressure level is only about 40V. Its specialty is: switch speed is very fast: reverse recovery time is particularly short. Therefore, a switching diode and a low-voltage high-current rectifier diode can be fabricated.

Schottky Diode It is a diode with a Schottky characteristic "metal semiconductor junction". Its forward starting voltage is low. Its metal layer in addition to tungsten material, you can also use gold, molybdenum, nickel, titanium and other materials. The semiconductor material used silicon or gallium arsenide, mostly semiconductors. This device is electrically conducted by a majority of carriers, so its reverse saturation current is much greater than that of minority carriers. Since the memory effect of the minority carriers in the Schottky diode is small, the frequency response is limited to the RC time constant, so it is ideal for high frequency and fast switching. Its operating frequency up to 100GHz. Also, MIS (metal-insulator-semiconductor) Schottky diodes can be used to fabricate solar cells or light-emitting diodes.

Schottky diodes use metal and semiconductor contacts formed by the potential barrier to control the current. Its main feature is a low forward voltage drop (0.3V to 0.6V); in addition it is more involved in conduction, which is less than the sub-devices have faster response speed. Schottky diodes are commonly used in the gate circuit as a triode collector diode to prevent the transistor from entering the saturation state and reduce the switching speed.

Schottky barrier diode reverse recovery time is very short (can be as little as a few nanoseconds), forward conduction voltage drop of only about 0.4V, while the rectifier current can reach several thousand amperes. These excellent features are fast enough to restore diodes. The structure principle of the Schottky rectifier is very different from that of the PN junction rectifier. The PN junction rectifier is usually called the junction rectifier, and the metal-semiconducting rectifier is called the Schottky rectifier. In recent years, Al-Si Schottky diodes made with silicon planar processes have also been available, which not only saves precious metals, significantly reduces costs, but also improves the consistency of parameters. Schottky rectifiers use only one carrier (electron) There is no accumulation of minority carriers on the outside of the barrier, so that there is no charge storage problem (Qrr → 0), so that the switching characteristics are improved. Its reverse recovery time has been reduced to less than 10ns. But its reverse pressure value is low, generally not more than 100V. So suitable for low voltage, high current conditions work. The use of its low pressure drop this feature, can improve the low voltage, high current rectifier (or freewheeling) circuit efficiency.

Fast recovery diode refers to the reverse recovery time is very short diode (5us below), the process of more use of gold-doped measures, the structure of the PN junction structure, and some use of improved PIN structure. Its forward pressure drop higher than the ordinary diode (1-2V), reverse voltage more than 1200V in the following.

From the performance can be divided into fast recovery and ultra fast recovery two levels. The former reverse recovery time of hundreds of nanoseconds or longer, the latter in 100 nanoseconds, fast recovery diodes in the manufacturing process using gold, simple diffusion process, can get a higher switching speed, but also Can be a high voltage. The current fast recovery diode is mainly used in the inverter power supply rectifier components. Fast recovery diodes with good switching characteristics, reverse recovery time is short, forward current, small size, easy installation and so on. Ultrafast recovery diode SRD is developed on the basis of fast recovery diodes, and its reverse recovery time trr value is close to the Schottky diode index. They can be widely used in switching power supply, pulse width modulator (PWM), uninterruptible power supply (UPS), AC motor frequency control (VVVF), high frequency heating and other devices, for high frequency, high current freewheeling diode or Rectifier, is extremely promising power, electronic semiconductor devices.

Reverse recovery time, what is the reverse recovery time? When the voltage applied to the diode immediately from the forward to reverse, the current flowing through the device can not be instantaneous from the forward current into the reverse current.At this time, The positive injections of minority carriers (holes) are extracted by the strong electric field of the space charge region. Since the density of these holes is higher than the density of the hole in the base region, the reverse bias will produce a much larger The reverse current of the leakage current, that is, the reverse recovery current IRM. At the same time, the intensification of the process is also accelerating the decrease of these additional carrier densities until the accumulation of additional carriers in the base region completely disappears The current is reduced and stabilized to the reverse leakage current, and the time of the whole process is the reverse recovery time. The reverse recovery time trr is defined as the time at which the current transitions from zero to the specified low value. It is a measure of high-frequency continuous flow and rectifier performance of the important technical indicators. Reverse recovery current IF is forward current, IRM is the maximum reverse recovery current. Irr is the reverse recovery current, usually Irr = 0.1IRM. When t ≤ t0, the forward current I = IF. When t> t0, since the forward voltage on the rectifier device suddenly becomes a reverse voltage, the forward current decreases rapidly, and at t = t1, I = 0. Then the rectifier current flows through the reverse current IR, and IR gradually increases; at t = t2 time to achieve the maximum reverse recovery current IRM value. Then by the role of forward voltage, the reverse current gradually decreases, and t = t3 time to reach the specified value Irr. The reverse recovery from t2 to t3 is similar to the discharge process of the capacitor.

Fast recovery, ultra-fast recovery diode structural characteristics Fast recovery diode internal structure and ordinary diodes, it is in the P-type, N-type silicon material to increase the base area I, constitute P-I-N silicon. Because the base area is very thin, the reverse recovery charge is small, not only greatly reduces the trr value, but also reduces the transient forward voltage drop, so that the tube can withstand very high reverse operating voltage. Fast recovery diode reverse recovery time is generally hundreds of nanoseconds, forward voltage drop is about 0.6V, forward current is several amperes to several thousand amps, reverse peak voltage up to several hundred to several thousand volts. The reverse recovery charge of the ultrafast recovery diode is further reduced so that its trr can be as low as several tens of nanoseconds. The fast recovery and ultrafast recovery diodes below 20A are mostly in the TO-220 package. Measuring the reverse recovery time, the DC current source for the provisions of the IF, the pulse generator through the DC capacitor C plus pulse signal, the use of electronic oscilloscope observed trr value, that is, from the time I = 0 to IR = Irr time Experience of the time. Suppose that the reverse return charge is Qrr. For the trr ≈ 2Qrr / IRM, when the IRM is constant, the reverse recovery charge is smaller and the reverse recovery time is shorter.

Conventional detection method In amateur conditions, the use of multimeter to detect fast recovery, ultra-fast recovery diode one-way conductivity, and whether the internal open circuit, short circuit fault, and can measure the positive conduction voltage drop. If equipped with megger, can also measure the reverse breakdown voltage. Example: Measure an ultrafast recovery diode with the main parameters: trr = 35ns, IF = 5A, IFSM = 50A, VRM = 700V. The multimeter dial to R × 1 file, read the forward resistance of 6.4 ?, n '= 19.5 grid; reverse resistance is infinite. Further, VF = 0.03 V / div × 19.5 = 0.585 V was obtained. Prove that the pipe is good.


1) some single tube, a total of three pins, the middle of the empty feet, usually cut off at the factory, but there are not cut.

2) If there is a tube in the tube damage, can be used as a single tube.

3) When measuring positive pressure drop, use R × 1 file. If the R × 1k file, because the test current is too small, much lower than the normal working current of the tube, so the measured VF value will be significantly lower. In the above example, if you choose R × 1k file measurement, the forward resistance is equal to 2.2k ?, then n '= 9 grid. The calculated VF value is only 0.27V, well below the normal value (0.6V). Fast recovery diode recovery time is 200-500ns ultra fast diode recovery time is 30-100.