0 Preface For DC power supplies, in order to improve their performance, or to achieve the purpose of small and light power DC power, high current high frequency rectification problems are often encountered. Especially for low voltage (臆24 V) DC power supplies, this problem is even more prominent. Typical examples are: high-frequency DC plating power supplies. Due to the controllability of the output waveform, the high-frequency DC electroplating power supply not only greatly speeds up the electroplating speed, but also greatly improves the quality of the electroplating layer, and at the same time greatly reduces the volume of the power supply device, and the power saving effect is remarkable. 1 Proposal of the topic The fast recovery rectifier diodes used in previous high-frequency electroplating power supplies were Schottky diode structures. This fast-recovery rectifier diode makes full use of the Schottky diode's majority carrier conduction, so the positive and negative recovery time is short, achieving high-frequency high-efficiency rectification. The positive recovery time of a power diode is understood to be the time required for a power diode that has not been turned on to change to the fully open state when the forward current suddenly forces through it (called forced turn-on). Before the power diode is fully restored to the on state, the forward voltage drop during forward recovery is much higher than the voltage drop during full turn-on, which can cause circuit voltage spikes. The reverse recovery time of a power diode is understood to be the time it takes for a forward conducting power diode to return to a blocking state when its voltage is suddenly reversed (called forced shutdown). Power diodes will generate large reverse currents and large power losses during reverse recovery, which is undesirable for the development and application of power diodes. A power diode with a long reverse recovery time is similar to a power diode with a large parasitic capacitance, and a power diode with a long positive recovery time is similar to a power diode with a large parasitic inductance. This topic uses a fast recovery rectifier diode made of the usual PiN structure to achieve high frequency rectification and plating applications. Under the premise of ensuring that both the forward and reverse recovery times meet the basic requirements, the fast recovery rectifier diode does not generate large reverse current and large power loss during the reverse recovery time, and does not generate during the forward recovery time. Excessive circuit voltage spikes (in other words, minimize parasitic capacitance and inductance). In addition, the high current characteristics, especially the high surge current, are realized, and high-current high-frequency rectification and application are realized. 2 Advantages and disadvantages of Schottky diode structure The contact between the metal and the lightly doped semiconductor is a rectifying contact, also known as a Schottky barrier contact. A device made using such a rectifying contact is called a Schottky diode. The transport of charge in a Schottky diode is accomplished by majority carriers. Therefore, the phenomenon associated with minority injection, extraction of excess carriers, and recombination does not occur during the turn-on and turn-off processes. Therefore, the use of Schottky diodes at high frequencies has advantages. 2.1 Advantages of Schottky Diodes 1) The reverse recovery time and the positive recovery time are both short; 2) At a low current density (JF < 10 A/cm2), there is a lower on-state voltage than a rectifier diode of the P+-n-N+ structure. 2.2 Shortcomings of Schottky diodes 1) The breakdown voltage is usually less than 100 V at a limited area contact; 3 basic technical solutions The technical solution adopted in this project is based on the scientific research results of high current density rectifier diodes for electric welding machines [4], such as the selection of single crystals, diffusion methods and technical requirements, ohmic contact of multi-layer metallization, and blasting of countertops. Polyimide passivation protection, shell design, etc. are mostly borrowed directly, and are developed through improved programs, so the entire development work has taken shortcuts. The life p has an approximate ideal distribution; the 12 Mev electron irradiation is used to reduce the minority lifetime of the base region to the vapor deposition of titanium-nickel-gold on both sides of the silicon wafer, and the surface is sandblasted, and then the sand is cleaned and cleaned. The imide passivation protection, the intermediate test, the ceramic ring filled with nitrogen cold pressure welding package molding, and then fully tested the electric heating parameters, dynamic parameters qualified, and finally made a high-frequency electroplating DC power supply dedicated power rectification fast recovery diode. 4 P+-i-N+ power diode frequency characteristics improvement It is self-evident that the on-state characteristics of P+-i-N+ power diodes at high current densities are much better than those of Schottky diodes. The problem is how to increase its frequency characteristics to the level of the Schottky diode. Increasing the speed of the turn-on and turn-off process, that is, trying to shorten the time from off to on, especially from on to off, ie shortening the forward recovery time tfr and the reverse recovery time trr. 4.1 Shorten the positive recovery time t (fr improve the turn-on feature) According to the international standard for rectifier diodes, the forward recovery time tfr is defined as: when a predetermined step forward current is applied immediately following a zero voltage or other specified reverse voltage condition, the forward voltage rises to the first specified value instant. And the time interval from the fall of its peak VFRM to the second specified value of the final stable value of the forward voltage. as shown in picture 2. 2) The highest peak voltage at turn-on is mainly caused by the additional voltage L·di/dt of the device spur (also called parasitic) inductance at the time of the current rise rate and the junction voltage (including the voltage of the high and low junctions). Obviously controlling the generation of excessive stray inductance is key. The use of a well-designed flat structure, the shell is designed as a thin shell without umbrella (also called skirt), which is necessary to reduce the stray inductance of the assembly and ensure that the VFRM value is not high. In general, the effect of turn-on on high frequency applications is much less than the reverse recovery time and the reverse recovery of charge at turn-off. 4.2 Reduce the reverse recovery time t (rr improve the shutdown characteristics) According to the international standard of rectifier diodes, the reverse recovery time trr is defined as: when switching from forward to reverse, from the moment of current zero-crossing to the moment when the reverse current is reduced from the peak IFM to the specified low value (see Figure 3). The time interval shown). The measures taken during the development process are: 2) first low-temperature expansion of platinum and then use the high-concentration phosphorus-silicate glass absorption on the cathode surface to control the minority carrier lifetime. The essence of soft recovery: under the premise of the reverse recovery time, the soft factor FRRS increases, that is, the reverse recovery current fall time trrf increases, and the essence is that the reverse recovery charge Qr is reduced (that is, the maximum reverse recovery current) Reduce), thus achieving the purpose of not generating excessive reverse current and excessive energy loss when turned off. 3) Using a silicon single crystal with uniform cross-sectional resistivity, the space charge region width is uniform, and the junction capacitance is small, which is one of the measures that does not generate excessive reverse current and excessive energy loss when turned off. 4) Deliberately making the surface concentration of the anode region lower than that of the cathode region is also one of measures to increase the softness factor and reduce the reverse recovery charge. 5 Test of device parameters The device developed by the test was exemplified by a device with a diameter of 48 mm / 3 000 A / 200 V. The measured results are listed in Table 1. The test results and customer field application show that the products developed are in line with the requirements of fast recovery power diodes for high frequency electroplated DC power supplies. 6 Conclusion The successful development of the P+-i-N+ structure high current density high frequency rectifier diode has undoubtedly provided a good choice for the design and manufacture of the power supply device, and can be applied to a large current high frequency rectifier device with an output DC voltage of 12 V. It is characterized by improved high-frequency rectification performance, reduced power supply unit size, and a large increase in output current. If the epitaxial wafer and the anode ultra-thin emitter structure are used, the application frequency will be higher, which is more conducive to improving the performance of the electroplating power supply, and further reducing the device volume and reducing the energy consumption. With the increasing emphasis on power devices in the country, vigorous research and development of high-performance diodes will be an important task for power semiconductor workers in China.
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To this end, to improve the high frequency application capability of the rectifier diode, it is important to focus on the improvement of the shutdown characteristics.
1) By taking the absorption of phosphorus silica glass, borosilicate glass and slow cooling, first increase the life of the minority to The purpose is to increase the lifetime of electrons to the minority
To ensure that the voltage drop during high frequency application is not too large, and the forward turn-on time tfr is short.
And there is an ideal distribution in the base area, followed by electronic irradiation to achieve the final shutdown requirements to reduce the minority life control technology, which not only ensures the long-term application reliability of the device, but also ensures the reverse recovery time requirements, and It is also possible to make the device soft-off, that is, the soft factor FRRS increases, and the reverse recovery charge Qr decreases.