SiC Power Switches; the New Weapon of Choice in the Quest for More and Better

  • Jul 05, 2018
  • UnitedSiC

By Christopher Rocneanu, Director of Sales EMEA & NA at UnitedSiC

Switched-mode topologies have come to dominate today’s power conversion scene, offering greater efficiency, reduced heat dissipation and smaller circuit size than traditional linear principles. These are compelling advantages in all but a few applications.

Still, there is room for improvement, as designers seek to drive efficiency even further above 90% while continuing to shrink power supply dimensions. However, the restrictive figures of merit (FOM) governing conventional silicon power MOSFETs and IGBTs have slowed the rate of progress. In addition, dealing with switching noise is a perennial challenge made tougher at the higher switching frequencies employed to reduce the sizes of filtering components.

Fortunately, there is a solution. Wide bandgap power semiconductors are now delivering on the promises they have offered during their development. Silicon Carbide (SiC) devices, in particular, are increasingly commercially competitive, especially bearing in mind that efficiency gains can allow the total number of power switches to be lower, while also reducing reliance on heatsinking, so allowing overall BOM cost to be offset.

Improvements in overall cost effectiveness now allows designers to start reaping the benefits of wide bandgap semiconductors: achieving those sought-after efficiency gains, shrinking circuit size even more, and designing-out switched-mode noise challenges.

SiC power transistors not only benefit from lower RDS(ON) and higher voltage in relation to die size – enabling greater current handling and breakdown safety margin in industry-standard package sizes – but also exhibit significantly cleaner and faster switching thanks to the absence of tail current. By enabling higher switching frequencies, smaller filter components, and lower electrical noise, SiC really does break the old silicon trade-offs and offer a “new deal” to power designers.

Real World Example

Deep Sea Electronics (DSE) has used SiC power transistors from UnitedSiC to achieve extremely high efficiency in its latest generation of backup-battery chargers (figure 1). These advanced components have also helped streamline design and ease compliance with electromagnetic compatibility (EMC) regulations.

DSE is a UK-based technology company with roots in commercial diving technology and a 40-year history in advanced power management. Its chargers are deployed where performance, quality and utter dependability are pre-requisites: in equipment such as fire safety or security systems, genset management, marine power, transportation and emergency vehicles.

Figure 1. DSE’s new 450W, 15A-output battery charger, featuring SiC cascodes for increased efficiency, lower noise and lower BOM.

DSE serves customers worldwide, so the chargers have a universal AC input voltage that is boosted to 400V DC by a built-in switched-mode power supply (SMPS). In the latest 450W charger, designers wanted to use a higher switching frequency of 70kHz for the SMPS, to increase efficiency over and above that of previous models while also allowing smaller external components. Although conventional silicon MOSFETs can handle these frequencies, the amount of switching noise produced would require extra suppression to meet EMC regulations. In addition, the first MOSFETs tested by DSE’s engineers needed a large snubber circuit as well as large heatsinks, adding to both size and BOM cost.

To improve on the 94% efficiency demonstrated by these first silicon-based prototypes, and tackle the noise, size and BOM-cost challenges, the team replaced the MOSFETs with UnitedSiC’s SiC cascodes.

The SiC cascode combines a SiC high-voltage JFET as the main current-carrying device, with a low-voltage conventional MOSFET to hold the SiC device OFF when the control signal is low and turn it ON when the control goes high. In this way, the cascode allows power designers to benefit from the inherently high efficiency of the normally-ON SiC JFET, while using conventional MOSFET gate-control voltages and driver circuitry.

The superior SiC switching characteristics – faster and with less noise – yielded an immediate increase in efficiency to 96%, and eliminated any need for a heatsink, while also simplifying EMC. The size and cost of the passive components, and the cost of the PCB, were reduced.

So what’s the key to this improvement in performance, unleashed simply by changing from silicon to SiC? One of the most influential parameters is the device output capacitance, Coss. The cascode’s Coss is much lower than that of a comparable MOSFET, so the output can change more quickly in response to the gate signal. The MOSFET’s higher Coss slows the output dV/dt; in fact, DSE’s engineers were obliged to slow the switching signals using a gate resistor, to allow the MOSFET’s output to keep up. In contrast, they were able to drive the SiC devices faster without any issue.

Moreover, Coss combines with on-resistance, RDS(ON), to define a critical FOM for power switches. The UJC0650K SiC cascode chosen for DSE’s battery charger has typical RDS(ON) in the range of 34-45mΩ, whereas a comparable silicon MOSFET could easily have RDS(ON) of 80mΩ. In addition, the temperature coefficient of RDS(ON) is much more stable in SiC devices: the UJC0650K demonstrated about 30% better stability than the MOSFETs DSE had evaluated. It is more stable than comparable gallium nitride (GaN) devices too, and – unlike GaN alternatives – SiC cascodes are readily sourced, more competitively priced, and available in industry-standard power packages.

DSE’s next-generation high-efficiency, low-noise battery chargers are now in production. The design team is now considering designing-in the latest surface-mount versions of UnitedSiC’s SiC cascodes for all new DSE battery chargers in the future.