Tutorial: The emerging technology of wide bandgap SiC & GaN power transistors
01 April 2022
Ignys_Tutorial_wide bandgap SiC & GaN power transistors
New technology for high voltage power transistors is starting to reach the market, with two new materials – the next generation of wide bandgap MOSFET (metal oxide semiconductor field-effect transistor) power transistors, silicon carbide (SiC) & gallium nitride (GaN) – set to precipitate huge changes within power conversion systems.
This tutorial was originally featured in the April 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
Here, Dr Nicholas Shattock, Senior Embedded Electronics Engineer at electronics design & embedded software development experts, Ignys will outline how this emerging technology of wide bandgap SiC and GaN transistors will pave the way for the next generation of mains voltage power converters for electronic devices, resulting in significant improvements in size and power efficiency…
These new materials will allow for smaller and more efficient power converters – but what current technology is used? Since the invention of the transistor in 1954 by Bell Laboratories, solid state power conversion has used silicon as the base semiconductor material. Solid state power converters have sparked a revolution in power density, efficiency and performance. And this technology has been refined and improved over the last 60 years – but it is now reaching a stage where the fundamental physical limits of silicon as a power semiconductor material are limiting any significant further improvement.
Transistors – and how they are changing
In order to change one form of electrical energy to another, at high efficiency, a solid state power converter will use a transistor to chop up the voltage or current from the supply, and feed this into a filter to remove the switching harmonics. The size of this filter, either inductive or capacitive depending on the topology, is dictated by the switching speed of the transistor.
What are the key advantages of using SiC & GaN
The key advantage of these new technologies is that SiC and GaN devices can switch on and off much faster, when compared to traditional silicon MOSFETs of a similar voltage and current rating. This allows for a reduction of switching losses and an increase in switching frequency. In addition, SiC and GaN can operate at much higher temperatures than silicon, increasing reliability and reducing volume yet further.
The effect on frequency & volume
Since the invention of the closed core transformer in 1885 by Ganz Works, AC electrical power conversion has traditionally relied on the use of low frequency transformers, operating at 50Hz or 60Hz. However the transformers used are large, expensive and heavy – and are therefore not well suited for use in modern day electronic devices. In addition to providing a regulated DC voltage, a rectifier and linear regulator need to be used, limiting the efficiency.
To overcome these issues, solid state power converters were developed, allowing the use of transformers or inductors operated at 10,000 Hz or greater. As the core cross sectional area is limited by magnetic saturation from the Voltage Time Area (VTA) of the AC wave, a higher frequency will allow for a smaller core. Even allowing for the use of high frequency magnetic materials, with a lower maximum flux density, such as ferrite, there will be a significant reduction in core size when compared with a 50Hz transformer. The impact of this can be observed by comparing the size and power rating of a mobile phone charger from the 2000s to one made today. Such an example might see an improvement of around a 7x reduction in weight and a 3x reduction in volume, while producing 2x the output power.
Ignys_Tutorial_wide bandgap GaN power transistors
The next generation of power semiconductor materials have the potential to allow for greater than an order of magnitude improvement in power density over the current technology, as switching speeds can be increased well in excess of 100,000Hz.
The effect on power losses
With power transistors, you can lose energy one of two ways: conduction losses when the current is flowing through the device; and switching losses as you turn it on or off.
As the voltage rating of the transistor increases, the size of the depletion region needs to be increased to stop the uncontrolled flow of current from avalanche breakdown. SiC and GaN have an intrinsically higher voltage rating for a given depletion region size. This is due to material properties, with a bandgap of more than triple that of silicon, which has a bandgap of 1.1eV: SiC’s bandgap is 3.2eV; and GaN’s bandgap is 3.4eV.
When designing a power converter, the average switching losses and conduction losses should be approximately equal. In a power converter, the duty cycle is rarely more than 50%, and therefore the instantaneous conduction losses can be between 2 and 10 times the average. The conduction losses can be reduced by increasing the number of transistors in parallel.
The instantaneous switching losses are incredibly large (thousands of Watts), due to the high voltage and currents involved. However, these switching losses only occur when the transistor is operating in the linear region, when it is neither on or off. If the transistor takes too long to transition between the on and off state, these losses can easily overheat the device and destroy it.
The new SiC and GaN devices not only reduce switching time by an order of magnitude, reducing the switching losses, but due to the reduced physical size of the devices, the conduction losses are also halved.
The new materials mean you can operate devices at a switching frequency ten times greater and operate with lower losses. Magnetic materials can be ten times smaller.
Ignys_Tutorial_wide bandgap GaN power transistors_application shot
In summary, the magnetic components are half the size, and the volume taken up by the transistors is smaller, while the performance and efficiency are significantly increased.
Where will the technology be used?
The new technology is applicable on anything from a laptop to an electric bus. It is expected that this technology will become very commonplace in many applications over the coming years.
Expected price trends via acceleration of the EV market
Wide bandgap technology currently has a price premium, due to it being such a new technology with a limited number of suppliers. However, as time goes by and there is more availability, price is expected to drop.
It is expected that rising recent interest in electric vehicles (EVs) is likely to accelerate this process. Vehicle manufacturers who use the technology will enjoy noticeable improvements, with the cost savings from reduced weight and greater efficiency easily off-setting the cost of adding the new technology.
Lower cost applications will see a slower transition, since if they use current technology, it will take far longer for price parity to be reached.
In the next 5 years, it is expected that the higher cost applications will create a trickledown effect, with manufacturing costs lowering, reducing volume and cost disparity. As the technology becomes adopted more frequently, the cost per demand will drop as demand grows.
Which is better? SiC or GaN?
In brief, the emerging GaN market has products aimed at lower voltage applications (100V – 650V DC rating) and is ideally suited for single phase power converters. SiC devices are, in general, aimed at applications with a 3-phase supply (1200V-1600V DC rating). However, SiC is easier to incorporate into existing technology and does not require as significant a modification to existing designs. Even when operating at a lower voltage, a 1200V SiC MOSFET will offer significant improvement when compared to a silicon MOSFET of similar rating.
When to use SiC: If you have an existing product powered by an 110V-220V AC supply and you want to make it work more efficiently, or have a reduced volume, SiC can be adapted into the design. This is useful if you don’t wish (or have the resources) to start a redesign from scratch.
When to use GaN: This technology performs far better for low voltage applications, but requires a far more tailor-made design. The rewards for starting a redesign with GaN can result in further significant improvements in efficiency and power density.
One huge pitfall to avoid
EMC (electromagnetic compatibility) can be a real issue for an effective product development process, if it is not correctly implemented with SiC or GaN. As soon as you increase the switching frequency, the switching frequency harmonics have the potential to cause significant EMC issues. While SiC and GaN may work well in a research setting, when they are let loose on commercial products, without thorough testing, this can lead to real problems.
Why is getting EMC right so important
While EMC may have minimal effect on user experience, in terms of a buzzing sound or a flicker on the TV, the main problem with ignoring EMC in your product designs is compliance.
In order to meet CE and UKCA marking requirements, EMC must be addressed. This is to stop your product interfering with emergency radio broadcasts or healthcare devices, such as pacemakers.
You could be left having to look at a major redesign if you don’t consider EMC early on – so it should be front of mind for every product designer, as every aspect has to be specified to account for it. In short, EMC must be considered all the way through the product development process before it creates significant issues – but luckily, Ignys engineers have plenty of experience helping customers reduce EMC and can help navigate EMC requirements for product designs.
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