Anup Bhalla, VP Engineering at UnitedSiC
SiC MOSFETs can deliver impressive performance and efficiency gains in power-conversion circuits, but a clean-sheet design is needed for best results. SiC cascodes offer a drop-in replacement bringing instant access to many of the advantages of wide-bandgap devices.
Drop In, Turn On
Silicon carbide diodes, and MOSFETs in 600 V or 1200 V ratings, are now readily available in the market and enable power-systems designers to leverage the advantages of wide-bandgap (WBG) semiconductors in power-conversion circuits like H-bridges to increase energy efficiency, temperature capability and power density. To maximize the benefits of these devices, the circuit ideally should be designed from the ground up, using optimized gate-drive circuitry to provide the asymmetrical turn-on/turn-off voltages required by SiC FETs, and smaller magnetic components made possible by SiC’s high-frequency capability.
Such a clean-sheet approach is practicable when designing advanced new equipment such as inverters for electric vehicles and power conditioning for wind or solar microgenerators, where ultimate performance is critical for success. However, there are numerous established applications that could benefit from valuable performance and efficiency improvements if upgrading to WBG devices such as SiC FETs could be made faster, less complicated and cost-neutral. Ideally, the industry needs drop-in replacements for existing silicon devices, to allow products to be upgraded at a suitable juncture in the model’s history. This could be when a significantly revised version is to be launched in the market, or when a current component becomes obsolete.
Directly replacing a silicon MOSFET with a comparable SiC MOSFET is not possible without additional revisions to the circuit, since SiC requires different gate-drive voltages. In addition, there are complications when current needs to flow in reverse, such as in hard-switched half-bridge or ‘totem-pole’ PFC circuits, because the SiC MOSFET body diode has a large VF of about 4 V that causes high dissipation. This could be overcome using a parallel SiC diode, which has low VF but brings space and cost implications.
As a potential direct replacement, a SiC JFET can avoid these issues but has one major disadvantage: it is a normally-ON device, whereas the silicon MOSFET is, of course, normally OFF. Normally-ON behavior has merits in an application such as a circuit breaker, but is not preferred in power conversion.
Cascode: A Convenient Solution
Connecting a SiC JFET with a traditional silicon MOSFET as a cascode circuit can overcome this challenge at the same time as boosting efficiency, by combining the normally-OFF behavior and low-VF body diode of the silicon MOSFET with the low RDS(ON) of the WBG device. The control signal from the gate driver is applied to the gate of the MOSFET. When this a positive voltage, the MOSFET turns ON and so turns the SiC JFET ON by short-circuiting the gate to source. When the MOSFET is turned OFF, its drain voltage rises. When it reaches about +7 V, the SiC JFET gate becomes 7 V more negative than its source, which is enough to turn the JFET OFF.
Because the gate-drive signal is applied to the MOSFET, the cascode can be controlled using a standard gate driver. The voltage is non-critical, and typically can be up to ±25 V. Moreover, as a low-voltage device, the MOSFET has much lower RDS(ON) than the SiC JFET. In addition, the MOSFET body diode has fast recovery as well as low VF, and the overall combination has excellent short-circuit performance and avalanche robustness. Because most of the power is dissipated in the JFET die, the temperature capability is defined by that of the SiC technology. Theoretically, this is 250°C, although package limitations impose a practical maximum of 175-200°C.
A co-packaged MOSFET/SiC JFET cascode gives power designers a three-terminal device that combines the desirable characteristics of a high-performance silicon MOSFET with the faster switching performance, increased energy efficiency and greater temperature capability of SiC.
Practical Direct Replacement
SiC cascodes in standard power packages like TO-247 can directly replace silicon MOSFETs without requiring significant board redesign. Such devices are in the market now, in 650 V and 1200 V ratings, with current ratings up to 85 A.
With only minor re-optimization of external resistors, they can be controlled using standard IGBT or silicon MOSFET gate-drive circuitry. Although the faster turn-on/turn-off transition times, due to SiC technology, demand greater attention to EMI, this can be handled by controlling dV/dt and di/dt using suitable gate resistor values. On the other hand, gate drive power is significantly lower, which enhances both reliability and efficiency.
Micropower Groupof Sweden has successfully designed SiC cascodes into a 3-phase, 8 kW charger (figure 2) for materials-handling applications, to replace standard silicon MOSFETs. The demand to redesign came about when the MOSFET supplier suddenly issued a last-time-buy notice indicating the devices were being made obsolete. With market demand for the chargers running at 7000 units per year, and no time to substantially redesign the system around new technology such as SiC MOSFETs, the engineering team sought a suitable drop-in replacement part.
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Figure 2. Micropower Group’s Access 100 3-phase 8 kW charger
The original design contains 12 silicon MOSFETs in phase-shifted full-bridge connection. Traditional planar MOSFETs were the first replacements to be considered, but their inherently lower current density would require multiple devices in parallel with a consequent increase in BOM costs. The team also evaluated super-junction transistors, although unexplained failures of prototypes undermined confidence in the robustness of the body diodes. Replacing the MOSFETs with IGBTs would prevent the chargers meeting California Energy Commission (CEC) specifications for minimum efficiency.
When evaluating SiC cascodes, Micropower’s engineers found that they could use existing ±13 V gate-drive voltages, and benefit from greater than 66% reduction in RDS(ON) per device. In addition to adjusting the gate-drive resistor values, only a small change in driver dead-time was required. Among other minor changes, the team took the opportunity to use smaller, lower-cost snubbers, and added two Y capacitors to meet EMC standards.
Moreover, the devices – from UnitedSiC. – had 250 V higher voltage rating than the original MOSFETs and, unlike the super-junction transistors, there were no doubts about body diode ruggedness. The overall cost of the solution was no greater than that of the original MOSFET-based circuit.
Tests on the first prototypes showed an immediate efficiency improvement of 1% at operating load levels (figure 3). In an 8 kW application like this, such an apparently small gain represents a saving of some 750 kWh over five years of ownership. Light-load efficiency leapt by almost 10%.
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Figure 3. Efficiency improvements – SiC cascode vs Si-MOSFET
Detailed analysis of switch behavior, radiated and conducted EMI, thermal cycling over 6 months, and repeated exposure (20,000 times) to voltage surges, output short-circuit/load disconnection and ‘bouncing phase error’ revealed no failures. The system also demonstrated a safe response to thermal stress from cooling-fan failure.
The entire project was completed within 12 months from start to production, and the redesigned chargers now deliver better performance than the superseded models at the same cost.
Conclusion: Drop-In Upgrade to WBG Advantages
SiC cascodes are ready now, to drop into existing power-conversion applications, delivering extra efficiency at no extra cost compared to IGBTs and silicon MOSFETs. Offering greater ruggedness and power density than alternatives such as planar or super-junction MOSFETs, and with flexible gate-drive requirements, they enable fast and easy access to the advantages of wide-bandgap semiconductors in high-power circuits.
United Silicon Carbide