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How to simplify power supplies and save cost with SiC

  Knowledge

Silicon carbide technology can offer many advantages besides greater efficiency, enabling designers to increase power density, enhance reliability, and economise on the overall bill of materials, even in a relatively simple circuit such as a flyback converter

More Accessible Than You Might Think

Silicon carbide (SiC) power semiconductors are understood to enable greater efficiency in power-conversion applications where energy is extremely precious, such as solar generators and high-end electric vehicles.

In fact, they have many more advantages to offer, including increasing power density and reliability due to their increased temperature capability, simplifying circuit design, reducing reliance on external components, and allowing smaller and lower-cost passive components. We can see how these benefits can be unleashed in ordinary applications by comparing several designs for the flyback converter of an auxiliary power supply using both SiC and silicon technologies.

Recap: The Root of SiC's Advantage

In the form used for fabricating power semiconductors, SiC has a bandgap of 3.2 eV between the valence and conduction bands technology, which is about three times that of ordinary silicon. In addition, its dielectric breakdown field strength is about 10 times that of silicon. Together, these two characteristics endow SiC devices with superior properties, including faster switching, higher efficiency, greater stability over temperature, and higher maximum operating temperature. For equipment designers, this can relieve demand for thermal management without compromising device reliability.

Sic's higher breakdown field strength allows MOSFETs to be designed with a much thinner drift layer, resulting in lower on-resistance, RDS(ON) relative to the die area, for a given breakdown voltage. To achieve high breakdown voltage in ordinary silicon, the MOSFET has higher RDS(ON) leading to greater conduction losses. SiC also permits lower MOSFET gate charge, Qg, enabling faster switching with lower energy loss, concurrently with low RDS(ON) and high breakdown voltage.

Auxiliary Power Supply Design Challenges

Equipment such as solar inverters, industrial DC/DC converters, battery chargers, and others often contain an auxiliary power unit running off the main input to supply subsystems such as sensor modules, a display, and other control units or drivers (figure 1).

For simplicity a flyback converter is typically used. The main power switch must be able to withstand the worst-case drain-source voltage due to the reflected voltage from the secondary side, maximum turn-off overshoot, and DC input voltage (figure 2). The sum of these voltages can exceed 1300V.

A variety of design approaches can be considered to ensure the power transistor is able to withstand worst-case voltage applied across the drain and source terminals. Each has its own advantages and disadvantages.

One approach is to choose a power transistor that has high breakdown voltage, say 1500V. However, ordinary silicon high-voltage transistors come with relatively high on-resistance (RDS(ON)) thereby incurring undesirable conduction loss and heat dissipation. They also tend to have high gate charge (Qg), which causes high driving losses, as well as high leakage current, especially at high temperatures.

An alternative is to connect a pair of 800V silicon MOSFETs in series. This calls for more complex gate-driving circuitry, and a voltage-balancing circuit is also needed. In addition, both devices require a heatsink therefore adding to the space consumed.

Another solution is to use a two-switch flyback topology (figure 3), at the expense of greater circuit complexity. An isolated gate driver and power supply is needed to control the high-side switch and, again, a heatsink is required for each device.

Instead, a single SiC MOSFET such as the SCT2H12NZ, which has 1700V breakdown voltage and 3.7A current rating, can be used. This device combines high breakdown voltage with RDS(ON) ranging from ½ to 1/8 that of comparable 1500V silicon MOSFETs. In addition, Qg and input capacitance are greatly reduced, hence permitting higher switching frequency and so allowing for smaller external components. Moreover, SiC's ability to withstand higher operating temperature relieves heatsinking requirements.

By allowing a single-FET flyback circuit to achieve the required breakdown voltage with minimal conduction losses, and operate at higher switching frequency, turning to SiC can yield BOM savings that result in a more economical solution overall.

A dedicated flyback controller IC, the BD7682FJ, is featured for driving SiC MOSFETs. In addition to generating a gate-drive signal in the recommended 14V-22V range for SiC devices (typically about 18V), it incorporates a 14V Under-Voltage Lockout (UVLO) to avoid thermal problems as well as an output clamp to prevent over-voltage on the SiC gate. The controller implements quasi-resonant switching to minimise dynamic losses and achieve low noise, and features a burst mode to enhance light-load efficiency. Protection functions, such as soft-start, over-current limiter per cycle, over-voltage protection function, and overload protection are also built-in.

Performance Evaluation

Rohm has built a 100W auxiliary power supply evaluation board featuring the SCT2H12NZ and BD7682FJ, which is able to operate with input voltage from 210-480V AC or 300-900V DC.

Figure 4 shows the transistor VGS and VDS waveforms at light load (left), 50% load (centre) and nominal load (right). The light-load waveforms show how the controller waits several valleys before turning the MOSFET on, resulting in a lower operating frequency than the nominal 90-120kHz range. As the output power increases the delay time is reduced and operating frequency increases. At nominal power, turn on occurs in the first valley. Measurements taken across the load range have shown that efficiency rises to 88-92% at the nominal power output, for input voltages from 300-900V DC.

By creating this auxiliary power supply evaluation board, Rohm demonstrated that system-level cost savings can be achieved if the advantages of SiC devices are fully utilised.

Full Integration for Maximum Advantage

Rohm has now taken things a step further by creating the BM2SCQ121T-LBZ quasi-resonant AC/DC converter that combines a fully integrated 4A 1700V SiC MOSFET with the functionality of the BD7682FJ including UVLO, voltage clamping, and burst mode. Conveniently packaged in TO-220-6M, the converter makes it easier than ever to design with SiC and maximises savings in part-count and board real-estate.

Conclusion

By enabling a combination of high breakdown voltage rating with low RDS(ON), as well as high switching speed, low switching loss, and high temperature capability, silicon carbide MOSFETs enable designers to simplify circuit design and lower bill of materials costs in a wide range of applications including simple flyback converters.

A new, fully integrated, flyback converter IC containing gate-driving and control circuitry with a 1700V SiC MOSFET built in now encapsulates these advantages in an easy-to-use industry-standard power package.

Find components at www.rutronik24.com.

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