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Posted on 01 October 2019

Advances in SiC Rectifiers

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SiC can eliminate the need for many other components

One of the biggest advances in silicon carbide rectifier technology is the creation of better material in larger sizes. By moving from 3-inch wafers to 4-inch, suppliers have achieved greater manufacturing efficiency and are driving down the costs of SiC devices.

By John Palmour, Executive VP of Advanced Devices, Cree Inc.

 

In the past, cost issues were driven by the fact that SiC is a very difficult material to create, infinitely more difficult to make than Si because of the high processing temperatures and pressures that need to be used. Unlike silicon, silicon carbide has no molten state, so manufacturers must work with gases and controlled temperature gradients that make the process difficult and costly.

The new 4-inch wafers make the creation of SiC power devices more cost-effective. If you’re processing devices that are about 4mm x 8mm on small-diameter wafers, the device yield of each wafer is limited. As the diameter of that wafer increases, manufacturers can produce more devices at one time. By using the 4-inch wafer, 40% less manufacturing capability is needed to manufacture the same number of die, making SiC devices more cost-effective than ever before.

The material quality also continues to greatly improve, allowing device manufacturers to design and create very large devices. Cree has introduced a very large 1200-volt, 50-amp device — the largest commercially available device in SiC. This development will enable many higher power conversion systems to use SiC now, especially in large inverters that would be used to drive power trains on hybrid electric vehicles, large solar inverter systems, and wind generation systems. Those applications were closed to SiC devices before because manufacturers could not yield a large enough device in appropriate quantities.

The improvements in material quality and increase in size are especially important in what has become the main consideration for most in the power industry — energy efficiency.

SiC may initially cost more when compared to alternative materials, but it can eliminate the need for many other components in power supplies, which can save in overall costs. For example, power supply makers can use a smaller EMI filter and reduce the number and size of the MOSFETs used with a SiC Schottky diode, so the net cost today is usually lower for a SiC-based solution than a silicon-based solution.

Cree uses SiC to manufacture high-voltage Schottky diodes. There is no minority carrier recombination, which leads to zero reverse recovery currents. There is, however, a very small junction capacitance charge. The magnitude of this charge is negligible in comparison to the equivalent reverse recovery charge in a Si PiN device, and it is also independent of temperature, forward current and switching di/dt. These Schottky diodes also have zero forward recovery voltage and turn on immediately. These switching characteristics also have the often-overlooked benefit of greatly reducing EMI. These devices eliminate diode-switching losses in a power conversion system, which in turn greatly reduces the turn-on losses in the associated switch that has to commutate the reverse recovery currents associated with Si PiN devices. Because of this increased efficiency and performance, SiC Schottky diodes are the ideal solution for applications where energy efficiency is the major concern.

According to Report #: DOE/EIA-0484 (2007), energy demand in the United States is expected to increase by 19% within the next ten years, and in developing countries demand is expected grow even faster. Because of this, energy users are demanding improved efficiency from the products they purchase. Even just a 1% improvement in efficiency of a power supply can save energy and money. For example, a 1000W power supply that is operational 18 hours per day, 365 days per year, in an area with an electricity cost of $0.10/kWH will generate energy costs of $657. A 1% efficiency improvement will save $6.57 annually. Consider the savings that $6.57 for every single power supply provides to an application such as a server farm. Considering that electricity costs are now the single largest cost of running a large data center, the savings from incorporating a slightly more expensive SiC Schottky diode into the power supply can recoup the incremental costs several times over in one year.

SiC devices are expected to become increasingly important in the global effort to become more efficient. Today, Schottky diodes up to 1200V packaged to 20A are available, and large area Schottky diode die up to 1200V/75A are producible. SiC MOSFETs, BJTs and PiN diodes for high voltage are all in development. For high-voltage, hightemperature power devices, SiC is the more efficient material of choice due to a higher breakdown field, lower specific on-resistance, faster switching, better thermal conductivity and higher temperature operation.

In a power supply, SiC diodes offer lower switching losses to improve system efficiency, and because of this, they reduce the system size by requiring fewer and smaller additional components, which can lower overall system costs.

How do these features make power supplies more efficient? Lower switching losses can be used to raise the operating frequency and reduce the overall size of the converter. It will also lead to lower operating temperatures for the semiconductor devices, resulting in higher MTBF. A lower switching loss will lead to component reductions that can translate to a higher power density for the power supply.

Increased efficiency will be the driver in future advancements in SiC rectifiers. The technology is a perfect fit for optimizing the perform- ance of energy-saving products such as hybrid electric vehicles and solar energy systems.

Currently, in hybrid electric vehicles, a separate liquid cooling system, comprised of a electric water pump, pump control electronics, pump motor drive electronics, hoses, wiring and a separate radiator, is needed for the power electronics. But with SiC devices’ lower loss, coupled with higher temperature capability, there could be a reduction or elimination of the need for liquid cooling, allowing for weight reduction (and energy savings). SiC devices can operate at high voltage with better performance than lower-voltage silicon devices, which would optimize the electric power train for better acceleration. Further, switching to SiC Schottky diodes would result in these vehicles’ reduction in electric motor drive losses. Overall, coupled with as much as a 50% reduction in inverter losses, there is a projected fuel efficiency improvement of 5 to 15%.

Another example of what is driving advancements in SiC technology to the next level is SiC Schottky diodes’ use in solar energy systems. In such a system, solar panels collect the sun’s energy and convert it to a positive DC voltage, which is boosted to a fixed DC voltage by means of a boost converter switching at high frequency. Cree’s SiC Schottky diodes eliminate the boost diode switching losses and greatly reduce the MOSFET or IGBT turn-on loss. This significantly improves the boost section efficiency. An inverter then converts the fixed DC voltage to a usable AC voltage of fixed frequency. The SiC Schottky diodes eliminate diode-switching losses in the free-wheeling diodes of this section along with reducing IGBT turn-on losses. Inverter efficiency is significantly improved. Silicon-based inverters typically operate at close to 96% average efficiency. With a more efficient system, more of the energy delivered by the solar panels gets converted to usable electricity. With SiC devices, the inverter’s average efficiency can be boosted up to 97.5%. This represents up to a 25% reduction in inverter losses.

The consensus estimate from available sources shows that electricity accounts for more than 39% of the world’s energy use. In the next few years, advances in SiC technology, especially rectifiers, are expected to have a dramatic impact in optimizing usage in all segments, including motion applications, IT equipment, lighting, heating, and cooling.

 

 

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