Development of fast recovery rectifier diodes for high frequency electroplating

0 Introduction

"For DC power supplies, in order to improve their performance or to make them small and lightweight, they often encounter the problem of high-current and high-frequency rectification. This problem is particularly prominent for low-voltage (~24 V) DC power supplies." "Typical examples include high-frequency DC electroplating power supplies, etc."

The high-frequency DC electroplating power supply not only greatly accelerates the electroplating speed and greatly improves the quality of the electroplating layer due to the controllability of its output waveform, but also greatly reduces the size of the power supply equipment, thus achieving a significant power-saving effect.

1. Proposal of the topic

In the past, the fast recovery rectifier diodes used in high-frequency electroplating power supplies were all Schottky diode structures. This fast recovery rectifier diode fully utilizes the advantage of the majority carrier conduction of Schottky diodes, so the forward and reverse recovery time are short, to achieve high-frequency and efficient rectification.

The forward recovery time of a power diode is understood as the time it takes for a power diode that has not yet turned on to change to a fully turned-on state when a forward current is suddenly forced through it (called forced turn-on). Before the power diode fully recovers to the turned-on state, the forward voltage drop during the forward recovery period is much higher than the voltage drop in the fully turned-on state, which may cause circuit voltage spikes.

The reverse recovery time of a power diode is understood as the time required for a forward-conducting power diode to return to the blocking state when the voltage across it suddenly reverses (called forced shutdown).

The power diode will generate a large reverse current and a large power loss during the reverse recovery period, which is undesirable in 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 forward recovery time is similar to a power diode with a large parasitic inductance.

This project uses a fast recovery rectifier diode made of a common PiN structure to achieve high-frequency rectification and electroplating 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 a large reverse current and large power loss during the reverse recovery time, and does not generate excessive circuit voltage spikes during the forward recovery time (in other words, the parasitic capacitance and inductance are minimized). Then, the large current characteristics, especially the advantages of high surge current, are brought into play to achieve high-current high-frequency rectification and application.

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. The device made using such a rectifying contact is called a Schottky diode.

The transport of charge in Schottky diodes is accomplished by majority carriers. Therefore, phenomena associated with minority carrier injection, extraction and recombination of excess carriers do not occur during the turn-on and turn-off processes. Therefore, using Schottky diodes at high frequencies has advantages.

2.1 Advantages of Schottky diodes

1) Both reverse recovery time and forward recovery time are short;

2) At low current density (JF<10 A/cm2), it has a lower on-state voltage than the P+-n-N+ structured rectifier diode.

2.2 Disadvantages of Schottky Diodes

1) At a limited contact area, the breakdown voltage is usually less than 100 V;

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 welding machines [4]. For example, the selection of single crystals, diffusion methods and technical requirements, multi-layer metallized ohmic contacts, table sandblasting modeling and polyimide passivation protection, tube shell design, etc. are mostly directly borrowed and developed through improved solutions, so the entire development work took a shortcut.

The life carrier p has an approximately ideal distribution; then 12 Mev electron irradiation is used to reduce the minority carrier lifetime in the base region to titanium-nickel-gold vapor-deposited on both sides of the silicon wafer, and the surface is sandblasted and shaped. After that, the sand is removed, cleaned, corroded, polyimide passivation protection is performed, intermediate testing is performed, and the ceramic ring is filled with nitrogen and cold-pressed and welded for packaging. After comprehensive testing of the electric and thermal parameters and the dynamic parameters are qualified, it is finally made into a power rectifier fast recovery diode dedicated to high-frequency electroplating DC power supply.

4 Improvement of frequency characteristics of P+-i-N+ power diode

It is self-evident that the on-state characteristics of the P+-i-N+ power diode at high current density are much better than those of the Schottky diode. The question is how to improve its frequency characteristics to make it close to the level of the Schottky diode. Improving the speed of the turn-on and turn-off process means doing everything possible to shorten the time from off to on, especially from on to off, that is, shortening the forward recovery time tfr and the reverse recovery time trr.

4.1 Shorten the forward recovery time t (fr improves the turn-on characteristics)

According to the international standard of rectifier diodes, the forward recovery time tfr is defined as: when a specified step forward current is applied immediately after zero voltage or other specified reverse voltage conditions, the time interval between the moment when the forward voltage rises to the first specified value and the moment when it drops from its peak value VFRM to the second specified value close to the final stable value of the forward voltage. As shown in Figure 2.

2) The highest peak voltage at turn-on is mainly composed of the additional voltage L·di/dt of the device stray (also called parasitic) inductance when the current rise rate occurs and the junction voltage (including the voltage of the high and low junctions). Obviously, controlling the generation of excessive stray inductance is the key. Here, a sophisticated flat-plate structure is adopted, and the tube shell is designed as a thin shell without an umbrella (the umbrella is also called a skirt). These are necessary measures to reduce the assembly stray inductance and ensure that the VFRM value is not high.

Generally speaking, the impact of turn-on on high-frequency applications is far less significant than the impact of reverse recovery time and reverse recovery charge during turn-off.

Therefore, in order to improve the high-frequency application capability of rectifier diodes, the focus should be placed on improving the turn-off characteristics.

4.2 Reduce the reverse recovery time t (rr) to improve the turn-off characteristics

According to the international standard for rectifier diodes, the reverse recovery time trr is defined as the time interval from the moment the current crosses zero to the moment the reverse current decreases from the peak value IFM to the specified low value (as shown in Figure 3) when switching from forward to reverse.

The measures taken during the development process are:

1) By adopting phosphosilicate glass and borosilicate glass absorption and slow cooling, the minority carrier lifetime is first increased to the purpose of increasing the minority carrier lifetime of electrons to ensure that the voltage drop is not too large during high-frequency applications and the forward turn-on time tfr is short.

2) First, platinum is diffused at low temperature and then absorbed by high-concentration phosphorus-silicate glass on the cathode surface to control the minority carrier lifetime and have an ideal distribution in the base region. Finally, electron irradiation is used to achieve the final shutdown requirement. This control technology of reducing the minority carrier lifetime not only ensures the reliability of the device in long-term application, but also meets the requirements of reverse recovery time, and can make the device softly shut down, that is, the soft factor FRRS increases and the reverse recovery charge Qr decreases.

The essence of soft recovery: Under the premise that the reverse recovery time remains unchanged, the soft factor FRRS increases, that is, the reverse recovery current fall time trRF increases. In essence, the reverse recovery charge Qr decreases (that is, the maximum reverse recovery current decreases). This achieves the purpose of not generating excessive reverse current and excessive energy loss during shutdown.

3) Use silicon single crystals with uniform cross-sectional resistivity to make the space charge region width uniform. Small junction capacitance is also one of the measures to avoid excessive reverse current and excessive energy loss when turning off.

4) Intentionally making the surface concentration of the anode area lower than that of the cathode area is also one of the measures to increase the soft factor and reduce the reverse recovery charge.

5. Device parameter testing

The developed and produced devices were tested, taking a device with a diameter of 48 mm/3 000 A/200 V as an example, and the measured results are recorded as listed in Table 1.

Test results and customer on-site applications show that the products developed and produced meet the requirements of fast recovery power diodes for high-frequency electroplating DC power supplies.

6 Conclusion

The successful development of the P+-i-N+ structure high current density high frequency rectifier diode undoubtedly provides a good choice for the design and manufacture of power supply devices. It can be used in high current high frequency rectifier devices with an output DC voltage of 12 V. Its characteristics are improved high frequency rectification performance, reduced power supply device size and greatly increased output current.

If epitaxial wafers and ultra-thin anode emitter structure are used, the application frequency will be higher, which will be more conducive to improving the performance of electroplating power supply, and further reducing the size of the device and reducing energy consumption.

As the country pays more and more attention to power devices, vigorously carrying out the research and development of high-performance diodes will be an important task for my country's power semiconductor workers.