With the development of large-scale integrated circuits, the volume and weight of electronic equipment are getting smaller and smaller, while the traditional continuous regulated power supply cannot meet the optimal design of the whole machine due to its large size and low efficiency. Switching power supply is a new type of power supply with small size, light weight and high reliability. It has the outstanding advantages of low power consumption and high efficiency, and its efficiency can reach more than 75%. Switching power supply to replace linear power supply is an inevitable trend of industrial development.
The switching power supply is composed of main components such as high-frequency magnetic core, high-frequency capacitor, high back-voltage high-power transistor, power rectifier diode and control circuit. Among them, the rectifier diode is the key component, because its power consumption is the largest, accounting for about 30% of the power consumption, so the rectifier diode is required to have the characteristics of small forward voltage drop and short switching time under high-speed and high-current operation.
In fact, there are two main types of rectifier diodes that are widely used: one is a fast recovery rectifier diode, and the other is a Schottky barrier diode (SBD). Generally speaking, the performance of Schottky barrier diodes is better than that of fast recovery diodes. Due to its excellent characteristics such as low voltage and high current, low power consumption, and high-speed switching, it is used as rectifier and freewheeling in high-frequency rectification, switching circuits and protection circuits. components, which can greatly reduce power consumption, improve circuit efficiency and frequency of use, and reduce circuit noise. Therefore, in the low voltage range, SBD has eliminated the fast recovery rectifier diode. However, due to the metal barrier, the SBD cannot withstand higher voltages. Therefore, in a large voltage range, a fast recovery rectifier diode is indispensable.
Next, the two are compared in terms of reverse recovery time trr, reverse voltage and forward voltage, as follows.
1. Comparison of reverse recovery time trr
The forward characteristic or reverse characteristic of the PN junction depends on the height of the potential barrier inside the junction, or, in other words, depends on the carrier distribution inside the junction. Because a certain barrier height corresponds to a certain carrier distribution. When forward biased, the potential barrier height is very low, and there are many carriers inside the junction, which greatly exceeds the equilibrium value; when reverse biased, there are few carriers inside the junction, which is lower than the equilibrium value, so the PN junction is forward biased. When turning to reverse bias, there must be a process similar to capacitor discharge called reverse recovery process. The time required to go through this process is the reverse recovery time trr. Obviously it is related to the lifespan of excess minority. The N region of the fast rectifier is usually gold-doped to reduce the minority carrier lifetime and obtain a shorter reverse recovery time.
The forward current of the Schottky barrier diode SBD is mainly the injection of "hot" electrons from the semiconductor into the metal, and the minority hole injection current is a negligible part. After the bias is reversed, the electrons injected into the metal also in principle return to the semiconductor, but this still requires the electrons to maintain sufficient energy to exceed the barrier. Under forward bias, the energy of the electrons injected into the metal under the action of electron bias is higher than the Fermi level of the metal by a potential barrier height, and this part of the energy disappears in a very short time due to the collision in the metal .
Therefore, after the reverse bias is applied, these hot electrons can only return to the semiconductor within this order of magnitude, so the minority carrier storage effect of the PN junction does not actually exist in the SBD, and the reverse recovery time is mainly determined by the external It is determined by the circuit and not by the intrinsic electronic processes related to the conduction mechanism. Therefore, the reverse recovery time of SBD is much shorter than that of PN junction. The SBD has a differential capacitance effect similar to the PN junction, which has a certain influence on the reverse recovery time. In order to reduce the differential capacitance, the effective area of the SBD is required to be reduced as much as possible, and the doping concentration of the corresponding epitaxial layer is low.
2. Comparison of reverse voltage
In general, metal-semiconductor contacts are generally used to form Schottky barriers. However, due to the existence of SiO2 layer at the contact interface when metal contacts semiconductors, the contact resistance and surface state density increase significantly, resulting in greatly improved device performance. reduce.
To solve this problem, a new process technology, the metal silicide-silicon contact barrier process, is used to form very reliable and repeatable Schottky barriers. At the same time, new process technologies such as guard ring structure are adopted, which greatly improves the reverse characteristics of Schottky diodes and shows ideal volt-ampere characteristics. However, due to the metal barrier, the barrier layer is relatively thin. Therefore, it cannot withstand higher voltages. The fast recovery rectifier diode has the characteristics of high substrate resistivity, thick epitaxial layer, and deep junction diffusion, and its pressure capacity can reach several thousand volts, which is why the fast recovery rectifier diode cannot be completely replaced.
3. Comparison of forward voltage drop
The special structure of the Schottky diode determines that its body resistance is small, and its forward voltage drop is much lower than that of the fast recovery rectifier diode, so its power consumption is low.
In production practice, there is a contradiction between the improvement of VF and trr of the fast recovery rectifier diode. That is to say, when VF is as low as possible, its trr must rise. Similarly, if trr is improved, VF will rise. The reduction of power consumption is unfavorable, and usually only a compromise can be made between the two, which is also a disadvantage of fast recovery rectifier diodes. The Schottky barrier diode SBD, because it is in contact with gold half, determines that its trr itself is very small. Under normal circumstances, improving VF does not make trr worse. Therefore, Schottky barrier diodes are beneficial to reduce power consumption.
As a rectifier diode, Schottky diodes have the following advantages over PN junction rectifier diodes:
1. The Schottky diode is a diode made of the potential barrier formed by the contact between the metal and the semiconductor. It is different from the diffused PN junction diode. Its forward voltage drop is only half of the PN junction, so the power consumption can be reduced by half.
2. The Schottky diode uses a majority carrier device, and there is no transient recovery characteristics of PN junction diode barrier minority carrier injection and storage, which makes the recovery characteristics of the two obvious under the same application conditions. The difference is that the recovery time is about 1:50. Since the recovery time of SBD is much shorter, its switching speed is twice as fast as that of a PN junction diode. The shorter the recovery time of the diode, the smaller its average power loss. .
To sum up, in switching power supplies, Schottky diodes are more ideal than fast recovery diodes, and their power consumption can be reduced by more than half, which further improves the efficiency of the power supply. Therefore, Schottky diodes are indispensable for switching power supplies. It is widely used in voltage regulators, rectifiers, inverters, UPS, etc., and can also be used as fast clamping diodes.
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