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Posted on 17 July 2019

Novel Low Leakage Current and Low Forward Voltage Drop Triangular Diode

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Protecting solar cells from reverse avalanche

In solar panel applications the solar cells during periods of shadow can be permanently damaged due to reverse biasing of the cell into avalanche breakdown. IXYS introduces a new triangular rectifier diode connected anti-parallel to the solar cell as a bypass diode in a solar charging circuit to protect the solar cells from experiencing high leakage currents in reverse blocking which can permanently damage the solar cells during periods when the cells are experiencing darkness or shadow. The bypass diode would be forward biased during shadow periods safely protecting the solar cells from reverse avalanche. The device shows low forward voltage drop and stable blocking characteristics with low reverse leakage current after high temperature reverse bias and humidity test. Furthermore the diode withstands cosmic radiation levels found in space.

By J.V. Subhas, Chandra Bose and Peter Ingram IXYS Semiconductor, Lampertheim, Germany

 

The solar cell panel is an environmental friendly device that can charge a battery without noise or pollution. Solar cells do not require expensive fuel to run and are powered by the limitless free energy of the sun. Therefore they are attractive devices for generating energy in a wide range of applications from household appliances to space shuttles. However solar cell panels require maintenance such as monitoring of the charging current and keeping the panel clean and free of dirt to receive the sun’s full illumination. If the charging current has reduced compared to when the panel was installed one has to check the electrical connections and solar cell array or panel. When sunlight falls on solar cells in an array each cell will be forward biased. If one or more of the cells are shadowed by tree, clothes or in space a satellite antenna, the shadowed cell may become reversed biased because of the voltage generated by the unshadowed cells. Reverse biasing a cell can cause degradation in cell performance or complete cell damage. To overcome such problems one can use protective bypass diodes. Bypass diode should give zero loss or low energy loss that is low forward voltage drop and low leakage current during reverse mode (1-2). Depending upon application each solar cell can have a bypass diode or a single bypass diode connected across several cells. The solar cell and bypass diode are connected in an anti parallel configuration, with the cathode and the anode of the solar cell connected to the anode and the cathode of the bypass diode. The bypass diode will be reverse biased when the cells are illuminated. If cell is shadowed it becomes reverse biased. The bypass diode connected across the shadowed cell becomes forward biased. The current will flow through the bypass diode instead of through the shadowed cell. The current will continue flowing through the solar cell array. The bypass diode limits the reverse bias voltage generated across the shadowed cell thereby protecting the shadowed solar cell, increasing the life span of the solar cell panel.

Two different diode technologies could be used for the bypass diode, Schottky and p-n junction diodes. Both technologies have advantages and disadvantages.

The schottkys forward voltage drop is lower than the p-n junction diode, however it has the disadvantage of higher leakage current when the diode is reverse biased (3-4). When the solar cell is operating in forward bias the bypass diode is reverse biased. Any leakage current going through the bypass diode reduces the total current. Therefore leakage current of the Schottky diode needs to be kept to a minimum in the nano amps range (less than 1uA). However getting leakage current less than 1uA using a schottky diode is difficult because the junction occurs at the metal semiconductor interface and is dependent on the surface conditions of the semiconductor prior to the deposition of the metal. Factors affecting the interface include the thickness of the oxide on the semiconductor surface and possible organic contamination can also affect the barrier height at the metal semiconductor junction. Process tolerances and semiconductor surface variations can lead to an increase in forward bias voltage and reverse bias leakage current.

The p-n junction diode technology has several advantages over the schottky diode. Doping levels of the p and n layers can be tightly controlled easily. The fact that the junction is also made in a controlled diffusion environment allows for variation in forward voltage drop to be minimised. Reverse bias leakage currents of less than 1uA are easily obtained with p-n junction diode technology and achieving leakage current in the nano amps range is not an issue. The p-n junction diode also has the added advantage of better fabrication process repeatability compared to the schottky diode technology. However forward voltage drop of a p-n junction diode is slightly higher than Schottky diode. Therefore we believe that to increase life span of a solar cell, the p-n junction diode is the best solution. Therefore IXYS Semiconductor introduces the new low loss triangular diode specially designed to easily fit in a solar cell panel as shown in Figure1a and Figure 1b.

Typical view of a triangular DFP32-005 bypass diode

Typical view of a triangular diode in wafer level before metallisation

The triangular diode has a silicon rich oxide, which helps to achieve a stable blocking voltage characteristics. The chip was designed with a total area of 32 mm2 and an active area of 29.6 mm2. Devices have seen both top (anode) and bottom (cathode) side silver metallisation. Fig.2 and Fig.3 shows forward voltage drop and reverse leakage current characteristics of a diode at 50V and 100V at 25°C to 150°C respectively. Forward voltage drop and leakage current measurements were taken using WM515C SCHUSTER ELEKTRONIK and SCHUSTER BVM625 measuring equipments.

Typical forward voltage drop characteristics of a triangular bypass diode

Typical leakage current of a triangular bypass diode during blocking voltage

Figure 4 shows increase in maximum reverse current (Irm) with rate of change of current (di/dt) during switching conditions of 25 °C, 50V, and 2A.

Switching characteristics of a triangular diode at room temperature

Diode Qualification or Reliability

Reliability

Reliability is defined as the ability of a device to conform to its electrical and visual/mechanical specifications over a specified period of time under specified conditions with a specified level of confidence.

Prior to the official release of a new device for mass manufacturing, it must pass qualification through a series of lifetime reliability tests. The actual reliability of a device cannot be accurately determined with standard visual and electrical measurement techniques. New device qualification most often requires several sets of samples for different reliability tests. The most important reliability tests for the electrical stability of the chip are High Temperature Reverse Bias (HTRB) and Humidity test. There is a further test known as a welding test, which is carried out to ensure stable device characteristics during assembly in the solar panel application. For space application the bypass diode has to pass a particle radiation exposure test.

HTRB: This test check the ability of the samples to withstand a reverse bias while being subjected to the maximum ambient temperature that the parts are rated to withstand.

Humidity: This test checks the ability of the package and chip to resist moisture penetration. The sample is loaded into an environmental chamber. The relative humidity is then increased from 85 to 100 percent at elevated temperature.

HTRB and Humidity test samples are randomly selected from 25 processed wafers. The HTRB test is carried out at 80% of rated voltage at 125 or 150°C. The device characteristics including breakdown voltage and leakage current are measured pre and post test to monitor for signs of device degradation. The test was conducted for up to 1000 hours and readings were taken once every 4h. Test results showed that there was no increase in leakage current between pre and post measurement results.

The humidity test was conducted at 85°C and at 85% relative humidity for 168 hours. The device characteristics are remeasured after cooling down for 2 to 3 hours. Pre and post measurement results show that there is no increase in leakage current.

Welding and exposure to particle radiation test: The welding test is particular to schottky diodes. The schottky junction occurs at the metal semiconductor interface and is therefore sensitive to high temperature welding of the contact metal, which has the possibility of changing the interface characteristics which can lead to an increase in reverse bias leakage current. In a p-n diode the junction formed between the p-n junction is inside the silicon and is therefore not affected by the welding process and consequently shows no increase in the reverse bias leakage current due to the welding process. The p-n junction bypass diodes are not, or much less, sensitive to the welding process.

The particle irradiation test involves exposure to both electrons and protons. The pre and post test results showed that all devices had forward voltage drop and reverse leakage currents within the specification limit. The devices have been qualified through successful completion of the above tests. The bypass diode qualification has also been successfully completed by the customer as per their application test conditions.

Conclusion

We have demonstrated from the practical results that it is possible to get low reverse leakage current and low forward voltage drop diode from triangular bypass diode design. A p-n junction diode is better suited to this application due to better fabrication process repeatability and much lower reverse leakage currents. Diode qualification test results show that they are robust and reliable for both normal and space applications. This diode can also be used to prevent the energy stored in the batteries from leaking back out to the solar panel in darkness or during shadow periods. IXYS can design different chip sizes to meet the customer requirements upon request. With the introduction of the DFP32-005 triangular diode IXYS expands its product range to include solar panel bypass protection diodes.

Reference:

1.Forward and Reverse Stable HiPer Fast Recovery Diodes. J.V. Subhas Chandra Bose and Peter Ingram. POWER ELECTRONICS EUROPE issue4 – May/June 2008.
2. Multi junction cells with monolithic bypass diodes. Sharps, P.R. Stan, M.A. Aiken, D.J. Clevenger, B. Hills, J.S. Fatemi, N.S. Photovoltaic Energy Conversion 2003. May 2003 Volume 1.
3. M.S. Tyagi, Introduction to Semiconductor Materials and Devices. John Wiley C Sons, New york 1991.
4. B.J. Baliga, Power Semiconductor Devices, PWS publishing, 1996.

 

 

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