History has a habit of repeating itself, or so the often-quoted phrase goes. Sometimes our human fallibility does indeed lead us to repeat mistakes from the past, while on other occasions, a concept or technique leads us back to an approach that was once of value but ended up consigned to textbook history. The electronics industry is no stranger to picking up ideas from the past, dusting them off and giving them a new lease of life or incorporating them into a new technology that, initially, wasn’t ready for widespread adoption.
It is perhaps no surprise that silicon device manufacturers, grappling with using wide band-gap process technologies such as silicon carbide (SiC) and gallium nitride (GaN) in MOSFETs, turned to a technique first used in valve voltage stabilizers in the late 1930s. Termed ‘cascode’, a phrase believed to stem from a cascading triode arrangement using two valves, the technique was first suggested in a technical paper by two engineering professors, F.V. Hunt and R.W. Hickman, while they were working at Harvard University in 1938.
The current and recurring theme of increasing power conversion efficiency has been instrumental in driving power supply switching component manufacturers to investigate methods of harnessing the high-frequency switching and high operating temperature characteristics that wide band-gap semiconductor technologies offer. In short, wide band-gap devices such as SiC MOSFETs and SiC JFETs have been a component of choice in many low- and medium-power switching applications, however they have not been so widely adopted in high power designs until now.
UnitedSiC, for example, is one such device manufacturer that has used this approach in developing a range of SiC devices that incorporate a high-performance JFET and a MOSFET configured in a cascode arrangement in a single TO-247 package. These are aimed at a broad range of power solutions such as IGBT gate drives, PV inverters and electric vehicle charging.
So often a new component, while addressing operating limitations or characteristics that have become increasingly difficult to work with, ends up being suitable only for completely new designs. However, that is not the case with wide band-gap SiC cascode JFET devices, for example. For many applications, it is possible for them to replace existing components in a design, significantly extending the product life cycle and providing solution manufacturers with the opportunity to upgrade existing equipment in cases where a complete redesign is neither feasible nor economical.
Of course, while the potential to upgrade existing equipment extends a product’s sales during the ‘cash cow’ maturity phase, the even bigger opportunity comes from the new applications that SiC cascode devices can make possible. In that respect, any application where a high operating temperature capability and a fast switching rate are coupled with the need to minimise switching losses is a candidate for using wide band-gap cascode devices. Military and many industrial designs certainly fall into that category, as do high-power bridge circuits such as those used in welding, motor drives and class D audio amplifiers. Typically, the major design challenge when constructing high-power, fast-switching circuitry is managing the transient response and ensuring that the switching devices are robust enough to deal with them. In that context, SiC cascode JFETs have transient response and thermal characteristics, making them eminently suitable for use in high-power designs.
So, thanks to the valve circuit innovations, the cascode arrangement is accelerating the use of wide band-gap SiC and GaN semiconductor process technologies in several application areas, electric vehicles being one. As a key focus area across the whole electronics supply chain, electric vehicles and their associated control systems are typical applications that will benefit from WBG developments, and the use of cascodes brings commercial product success one step nearer.
Download the whitepaper below for more information on SiC cascodes, how they operate and typical applications.
Guest blog written by Anup Bhalla, VP Engineering at UnitedSiC.