Nexperia details merged silicon carbide PiN Schottky structure for 650V diode
Nexperia has developed a Silicon Carbide (SiC) Schottky diode technology for power applications with higher performance and a thinner die.
The Merged-Pin Schottky (MPS) design combines a Schottky diode and a P-N diode connected in parallel to more easily meet the latest energy efficiency standards.
The first part to use the MPS technology is the 10 A, 650 V PSC1065K SiC Schottky diode. This is an industrial-grade part for switched-mode power supplies, AC-DC and DC-DC converters, battery-charging infrastructure, uninterruptible power supplies and photovoltaic inverters.
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The P-doped areas are implanted in the drift zone of a conventional Schottky structure, forming a P-ohmic contact with the metal at the Schottky anode and a P-N junction with the lightly-doped SiC drift or epi-layer.
Under reverse bias, the P-wells ‘push’ the general area of maximum field strength downwards into the almost defect-free drift layer, away from the metal barrier with its imperfections, reducing the overall leakage current.
The P-wells’ physical placement and area and doping concentration affect the end characteristics, with forward voltage drop traded against leakage and surge currents. As a result, an MPS device can operate at a higher breakdown voltage with the same leakage current and drift layer thickness.
The surge current performance of SiC devices comes from the unipolar nature of the devise and relatively high drift layer resistance, and the MPS structure can also improve this parameter. The reason for this is that the differential resistance of a bipolar device is lower than that of a unipolar type.
Under nominal operation, the Schottky part of the MPS diode conducts almost the entire current to effectively behave like a Schottky diode, delivering the same benefits during switching. During high transient surge current events, the voltage across the MPS diode increases beyond the built-in P-N diode cut-in voltage, which begins to conduct with lower differential resistance. This diverts current, limiting dissipated power and relieving the MPS diode from thermal stress.
Without the P-N diode, the Schottky diode alone would have to be significantly over-dimensioned to allow for transient over-current events in the target application and while parts could be paralleled (or extra circuitry added) to limit over current this would increase costs.
The dimensions and doping of the P-wells create a trade-off between forward voltage drop in regular operation and surge withstand capability. The best choice depends on the application, and Nexperia is planning a range of diodes to suit a wide range of hard- and soft-switching applications.
The MPS structure also benefits dynamic switching. One significant advantage over silicon-based P-N diodes relates to reverse recovery behaviour. The reverse recovery charge is one of the main contributors to power loss in silicon fast recovery diodes, adversely affecting converter efficiency. Several parameters affect this, including diode turn-off currents and junction temperatures. In contrast, only majority carriers contribute to the overall current flow in SiC diodes, meaning they exhibit almost constant behaviour, with little of the non-linear performance of silicon fast recovery diodes.
This makes it easier for power designers to predict their behaviour because they do not need to consider various ambient temperatures and load conditions.
The MPS diode provides additional advantages derived from reduced die thickness during fabrication. The unprocessed SiC substrate is N-doped, and SiC epitaxial layers are grown to form the drift region. The substrate starts with a thickness of up to 500 µm, but after epitaxy, this adds unnecessary electrical and thermal resistance in the current and heat-flow path to the back-side metal. This increases forward voltage drop and junction temperature for a given amount of current.
The MPS approach is to ‘thin’ the underside of the substrate by grinding it. Material quality and grinding precision are crucial during this process step to avoid non-uniformities and consequent performance degradation of the diode (which could result in devices failing in the field). Furthermore, advanced manufacturing expertise is required because of the hardness of SiC (9.2 to 9.3 on the Mohs scale, compared to 6.5 for silicon).
Overall the merged design gives temperature-independent capacitive switching and zero recovery behaviour which gives a higher figure-of-merit (QC x VF). It also provides robustness against surge currents that eliminates the need for additional protection circuitry. These features significantly reduce system complexity and enable hardware designers to achieve higher efficiency with smaller form factors in rugged high-power applications.
The SiC Schottky diode is encapsulated in a Real-2-Pin (R2P) TO-220-2 through-hole power plastic package. Additional package options include the surface mount (DPAK R2P and D2PAK R2P) and through-hole (TO-247-2) with a real 2-pin configuration that enhances reliability in high-voltage applications at temperatures up to 175 °C.
“We are proud to offer a high-performance SiC Schottky diode that ranks among the top tier of currently available solutions. In an increasingly energy-conscious world, we are bringing greater choice and availability to the market as demand for high-volume, high-efficiency applications increases significantly,” said Katrin Feurle, Senior Director of the SiC Product Group at Nexperia.
Nexperia plans to grow its portfolio of SiC diodes by including automotive-grade parts that operate at 650 V and 1200 V voltages with currents in the 6-20 A range. Samples and production quantities of the SiC diodes are available now.
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