By Christopher Rocneanu, Technical Business Development at UnitedSiC and Firas Hijjawi, Deep Sea Electronics Power division team leader
Efficiency is a concept that applies to many things, from the way we tie our shoelaces to the flight paths of airliners. The pursuance of efficiency can form the foundation of a business, particularly one that is deeply involved with energy in all its forms.
Deep Sea Electronics (DSE) is UK based market leader in the design and manufacturing of generator control systems, battery chargers, automatic transfer switch controllers and control systems for off-highway vehicles and mobile machinery. For more than 40 years it has adopted new technologies focused on driving innovation in the relevant markets and to increase efficiency. There is a pioneering spirit at the heart of the company, and its philosophy of pursuing new opportunities and adopting new technologies is apparent in every product and development within the company.
Within the DSE Power Division, when the need to develop a new AC/DC switched-mode power supply (SMPS) with power factor correction (PFC) for battery charging applications arose, it was seen as an opportunity to find a more efficient solution to power conversion.
DSE battery chargers offer a wide (90 VAC to 305 VAC) input voltage, which is then converted to a DC output using an SMPS topology. The quest for efficiency dictated that the design and components used would need to switch at high frequencies and withstand the operating conditions with high reliability. Initially, a number of designs were evaluated and although they were capable of switching at high frequency they were electrically noisy and required significant heatsinking, a potential problem when designing to meet EMC regulations and achieving a compact design with the required heat dissipation. Even if these initial concept design issues could be resolved, the resulting efficiency of the product was around 94%, which is high but not as high as DSE or its market would like.
It was at this point that the design team decided to widen its scope and approached UnitedSiC to assess its range of silicon carbide (SiC) cascode devices, which brings together a SiC JFET with a conventional MOSFET in a package that, from a design point of view, is a drop-in replacement for a conventional MOSFET. The design team soon discovered that the cascode devices delivered several benefits.
Firstly, they are able to switch at high frequencies without the losses associated with MOSFETs, thanks to the speed at which they turn on and off, meaning no heatsinks were required. Their thermal performance is also better and, electrically, they emit much less noise, so EMC compliance was also simpler.
A key parameter in their switching efficiency is their output capacitance, Coss, which is much lower than that of the MOSFETs evaluated. Consequently, their rate of change (dv/dt) is much higher, which, in a MOSFET, can be a limitation. Often the dv/dt rating could easily be exceeded using a MOSFET, meaning it actually has to be slowed down using a gate resistor.
Another critical parameter is the on-resistance, or RDS(on). The part selected has a typical RDS(on) of 34 mΩ to 45 mΩ: a figure that can easily be around 80 mΩ in a MOSFET. Furthermore, the temperature coefficient of the RDS(on) for the SiCs is also much improved. Tests showed the MOSFETs could exhibit a three-fold increase in RDS(on) with temperature, while the SiC devices only increased around two-fold. This was also found to be better than the gallium nitride (GaN) devices considered.
In fact, the team discounted GaN on several counts: it is still a comparatively expensive technology but perhaps, more importantly, each device is effectively provided in a bespoke package, which makes second-sourcing an issue. SiC devices are more competitively priced and offered in an industry-standard package, and they are also becoming more commonly available.
By adopting the SiC solution the DSE team was able to reduce the size of the sintered passive components used too, which yielded further space savings for a compact design. Perhaps just as importantly as all these benefits, the efficiency of the PSU also went up, to 96% under the same operating conditions.
The new design is now in production and DSE expects to be manufacturing substantial numbers of the new SMPSs, forming a significant percentage of the company’s global battery charger market share.
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