Tweet

Posted on 05 November 2019

SiC MOSFETs Improve PV and UPS Inverters and Industrial Power Supplies

Free Bodo's Power Magazines!

 

 

 

Since the release of the first generation of SiC MOSFETs in 2011, these wide bandgap power devices have taken a major step forward in the commercial market place, including the introduction of a second generation MOSFET product family.

This new generation device’s improved performance, as well as the fact that there are now multiple sources of SiC MOSFETs available in the market, has changed the economics for several applications, notably photovoltaic (PV) and uninterruptible power supply (UPS) inverter design, as well as industrial power supplies.

By Paul Kierstead, Cree

The second generation of SiC MOSFETs available in 2013 from Cree and others now includes devices rated at 1200V/ 320†, 160, 85, and 25mOhm, along with devices rated at 1700V / 1Ohm and 40mOhm. These higher-rated next generation MOSFETs not only enable their use in applications ranging from 60W to 1MW (primarily in industrial power supply, solar inverter, UPS inverter, and motor drive markets), but they are also the industry’s most efficient 1200-1700 V power transistor technology available in terms of measured device losses. SiC MOSFETs exhibit a fraction of the switching losses associated with commonly used silicon IGBTs. Conduction losses are also reduced in most applications. The latest generation of SiC MOSFETs has improved on the already substantial (factor of 7-10 lower losses) technical advantages over silicon devices, while device costs have been reduced by nearly half in 2013. The result has been several recent public announcements of mass production using these new SiC MOSFETs by manufacturers of solar inverters and industrial power supplies. This article compares different economic and performance trade-offs for applications using the SiC MOSFET relative to other conventional solutions.

In PV inverters, SiC MOSFETs are rapidly being designed in, but whereas a few years ago, the reason to specify SiC devices was part of a push to increase efficiency, the latest generation of SiC MOSFETs is being used primarily to cut inverter costs. SiC MOSFET energy efficiency and high switching frequency capability are allowing increased power density, reduced cost of thermal management and smaller, lighter, less expensive magnetic components. For example, Figure 1 illustrates a commercial 11kW PV inverter released in 2013 using SiC MOSFETs1. These PV inverters typically operate with a boost stage, a power or inverter stage, and an auxiliary power supply, and SiC MOSFET technology impacts each of these segments. (UPS inverters face similar needs, although with some differences in terms of requirements to deliver feedback to the grid, and handle over-current pulses.). Using SiC MOSFETs with very low switching losses allows designers to keep efficiencies high with relatively simple topologies such as two-level instead of three-level, fewer components, and standard gate drive products.

11kW PV inverter released in 2013 using SiC MOSFETs

Inverter Boost Stage

In a typical inverter boost stage, a 200 to 400VDC solar panel input (for PV) is boosted to an output voltage of 450 to 1,000VDC. The power electronics design engineer’s goal is to keep the system efficiency as high as possible, but at the same time achieve as low cost as possible through size, weight and component reduction. Work published at the 2013 PCIM2 and in Bodo’s Power Systems3 detailed the use of SiC MOSFETs and diodes in the boost stage to cut costs by 10-20% compared to conventional technologies, and at the same time gain extremely high efficiency (+ 99%) for a 10kW interleaved topology operating between 60 and 100 kHz. Compare that to efficiencies in the mid 98% range at no more than 20 kHz for typical silicon boost designs. SiC MOSFETs provide better efficiency at much higher frequencies, which enables the key cost reduction in magnetics. The initial boost work published at 10kW has since been scaled to 50kW as shown in Figure 2, and is targeted for future publication. The 50 kW boost shown in Figure 2 is only 7 kg, and a volume of only 450mm x 250mm x 155mm. The SiC 50 kW boost shown in Figure 2 is up to half the weight and half the volume of a typical silicon implementation. Throughout the 5-50kW range, the performance and economic benefits to using SiC MOSFET and diode technology are similar.

Initial boost work published at 10kW has since been scaled to 50kW

Si IGBT technology is typically limited to the 20 kHz range due to their switching losses, even when using high speed IGBT devices (IGW40N120H3). Moving to SiC MOSFET technology allows the frequency to scale up to between 60 and 100 kHz, with improved or constant efficiency depending on the optimization chosen for the particular design. This higher frequency operation, in turn, enables the inductor costs to be cut substantially, resulting in as much as a 20% overall bill of materials (BOM) cost reduction for the boost stage.

Additionally, the operating temperature, weight and efficiency of the system are dramatically improved by specifying the SiC MOSFET over conventional Si technology, with power density typically possible to be over one kW/kg for lower power (up to 100 kW) systems which is three times higher than many commercial Si-based systems today in the same power range. Improved kW/kg translates directly to BOM cost reduction, and reduced shipping, installation and maintenance costs. As illustrated in Figure 3, the normalized results using SiC MOSFETs in a 10 kW boost reference design, relative to the Si IGBT technology results in significant reductions in cost (~20% less), weight (~50% less), and operating temperature (20-30% less). Exact results will vary depend the design engineer’s implementation of SiC MOSFETs, but clearly the system cost can be cut significantly.

Normalized results using SiC MOSFETs in a 10 kW boost reference design

Inverter Power Stage or Inverter Stage

In the inverter stage, unlike the boost stage, the operating frequency is generally lower to minimize EMI. However, a typical high-frequency PV inverter power stage using SiC MOSFETs can be targeted to 48 kHz, whereas Si-based PV inverter power stages are typically limited to operation at 16kHz or less. Thus, although inductor costs are reduced, it is a less significant reduction than is possible in the boost stage.

For the power stages, either two-level, T-type or similar topologies are available for SiC-based inverters. Unlike Si-based inverters, switching losses are typically reduced by a factor of seven or more, thus scaling to 48+kHz allows the design engineer to maintain acceptable (~ 98% +) power inverter stage efficiencies regardless of topology. Hence, the choice of topology then becomes driven by cost, rather than performance concerns. For example, SiC enables equal or better performance in a simple two-level hard switched topology than more complicated multi-level topologies while providing equivalance advantage of higher switching frequency. For central inverters, the DC link voltage is also being pushed up to 1,500 V with 1,000 V output, and this move also favors SiC for simpler topologies due to the very low switching losses of 1,200 and 1,700 V SiC MOSFETs. In general, using SiC MOSFET technology enables a reduction in inductor costs, and the topology can be chosen based on other cost factors, such as number of components, drivers, and so on, so as to achieve the maximum high-frequency performance for the minimum cost. In addition, lower part counts and simpler designs can contribute to improved system reliability and lifetimes.

In the power stage, there is one additional way SiC MOSFET technology can lower costs. In addition to the savings realized by reducing the inductor size by raising the frequency to 48+ kHz in a simple PV inverter stage topology like two-level or T-type, it is further possible to eliminate the external anti-parallel SiC Schottky diode. By using the SiC MOSFET’s third quadrant operation4, the body diode is only on during dead time, thus the channel of the MOSFET, allowing for symmetric conduction, lowers conduction losses by 55-60% relative to the body diode operation alone. In this manner, both inductor costs and SiC diode costs are reduced for a typical 48 kHz power stage.

Auxiliary Power Supply

To illustrate the savings possible in the auxiliary power supply stage, a 60 W reference board has been made available [5] demonstrating the advantages of using a 1700 V, 1 Ohm SiC MOSFET in a simple single-switch flyback topology as compared to a conventional siliconbased two-switch flyback topology. The simpler single-switch flyback topology made possible with SiC MOSFET technology results in the architecture having fewer components, lower costs (only one driver instead of two drivers, for example), better efficiency (enabling cooler, more reliable operation while completely eliminating the heat sink cost), and, of course, lower magnetic costs due to the higher frequency operation. Additionally, voltage clamps or snubbers can be eliminated, since the SiC MOSFET has 1700 V of breakdown voltage (with a comfortable margin) compared to Si-based FETs, which are typically limited to 1500 V.

60 W auxiliary power supply evaluation board

Figure 4 shows a 60 W auxiliary power supply evaluation board, which illustrates these advantages. Note the SiC MOSFET is shown on a heat sink, but in many actual designs, the heat sink may not be needed. It is important to point out these auxiliary power supply advantages are not only limited to UPS and PV inverters, but can be applied to motor drives as well.

Summary

Applying SiC MOSFET technology in the boost, inverter power, and auxiliary power supply stages, can result in anywhere from a 5% - 25% reduction in overall inverter costs for PV and UPS inverters, depending on the architecture chosen and volume considerations. Additionally, the inverter weight can be reduced substantially (~50%), as SiC MOSFET technology can achieve a 1kW/kg power density for lower power systems up to 100 kW. This weight reduction allows for even more savings in shipping and installation costs, in addition to the BOM reduction.

References:
[1] “Cree SiC MOSFETs Enable Next-Generation Solar Inverters from Delta Energy Systems,” Press Release, http://www.cree.com/news-and-events/cree-news/press-releases/2013/april/delta-pv-inverters, April 17, 2019.
[2] J. Liu, “Increase Efficiency and Lower System Cost with 100 kHz, 10 kW Silicon Carbide (SiC) Interleaved Boost Circuit Design, PCIM 2013, May 14, 2019.
[3] J. Liu and K. L. Wong, “Silicon Carbide (SiC) 10 kW Interleaved Boost Converter Achieves 99.1% Peak Efficiency,” Bodo’s Power Systems, p. 38, Dec. 2012.
[4] R. Callanan, J. Rice, and J. Palmour, “Third Quadrant Behavior of SiC MOSFETs,” APEC 2013, Technical Session 26, March 21, 2019.
[5] P. Kierstead, J. Liu, “Silicon Carbide MOSFETs Provide Simple, Rugged, Low Cost Solutions to High Voltage Auxiliary Power Supplies,” Darnell Power Forum, Ft. Worth, Texas (USA), September 2013.

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 791 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.