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Cree, Inc. has gained the distinction of producing the industry's first fully-qualified, commercial silicon carbide (SiC) power MOSFET (Fig. 1). The company's SiC power MOSFET is the end result of many years devoted to materials research, process development and device design.
The SiC DMOSFET, designated CMF20120D, allows blocking voltages up to 1200V. Its consistency of performance characteristics across operating conditions, along with a true enhancement mode MOSFET architecture (normally-off), makes it well-suited for power electronics switching circuits. Compared with available silicon MOSFET or IGBT devices of similar ratings, the CMF20120D has the lowest gate drive energy (QG <100nC) across the recommended input voltage range. Plus, minimized conduction losses produce a forward drop (VF) of <2V at 20A.
The SiC MOSFET reduces switches losses compared with silicon MOSFETs and IGBTs. One reason is that the high voltage SiC MOSFET does not have the tail current losses found with IGBTs. In addition, the SiC MOSFET's high current density and small die size results in lower capacitance than with silicon MOSFETs. Fig. 2 compares the switching losses of IGBTs and silicon MOSFETs with those of the SiC MOSFETs.
This SiC MOSFET offers advantages over conventional silicon devices, enabling high system efficiency and/or reduced system size, weight and cost through its higher frequency operation. Compared to the best silicon IGBTs, the SiC device will improve system efficiency up to 2% and operate at 2-5 times the switching frequencies. Higher component efficiency also results in lower operating temperatures.
Combining these lower operating temperatures with the CMF20120Dís ultra-low leakage current (<1µA) can add significantly to system reliability. Fig. 3 compares the leakage current for silicon MOSFETs and IGBTs. SiC's wide band-gap ensures minimal leakage even at elevated reverse voltages and temperatures.
Although this SiC MOSFET has removed the upper voltage limit of silicon MOSFETs there are some differences in characteristics when compared to what is usually expected with high voltage silicon MOSFETs. These differences need to be carefully addressed to get maximum benefit from the SiC MOSFET. In general, the SiC MOSFET is a superior switch compared with its silicon counterparts, but it should not be considered as a direct drop-in replacement in existing applications.
Among the SiC MOSFET advantages over silicon devices is an RDS(ON) improvement. As shown in Fig. 4, SiC MOSFETís RDS(ON) increases only 20% over operating temperature compared with over 250% for 1200V silicon MOSFETs. The flatness of the SiC MOSFET RDS(ON) curve eases the design of high efficiency applications. It also ensures reliable system thermal performance. In addition, the SiC MOSFET's positive temperature coefficient allows easy paralleling to obtain higher operating currents.
DRIVING THE SIC MOSFET
There are two key characteristics that need to be kept in mind when applying the SiC MOSFETs:
Gate drive pulse fidelity
Modest transconductance requires a VGS of 20 V to optimize performance. This can be seen in the output and transfer characteristics shown in Figs. 5, 6, and 7. This also affects the transition where the device behaves as a voltage controlled resistance to where it behaves as a voltage controlled current source as a function of VDS. The result is that the transition occurs over higher values of VDS than are usually experienced with Si MOSFETs and IGBTs. This might affect the operation of anti-desaturation circuits, especially if the circuit takes advantage of the device entering the constant current region at low values of forward voltage.
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The modest transconductance needs to be carefully considered in the design of the gate drive circuit. The first obvious requirement is that the gate be capable of a >22 V (+20 V to -2V) swing. The recommended on-state VGS is +20 V and the recommended off-state VGS is between -2 V to -5 V. Even though the gate voltage swing is higher than the typical silicon MOSFETs and IGBTs, the total gate charge of the SiC MOSFET is considerably lower. In fact, the product of gate voltage swing and gate charge for the SiC MOSFET is lower than comparable silicon devices. The gate voltage must have a fast dV/dt to achieve fast switching times, which indicates the necessity of a very low impedance driver.
Gate drive pulse fidelity must be carefully controlled. The nominal threshold voltage is 2.5V and the device is not fully on (dVDS/dt≈0) until the VGS is above 16V. This is a noticeably wider range than what is typically experienced with silicon MOSFETs and IGBTs. The net result of this is that the SiC MOSFET has a somewhat lower ‘noise margin’. Any excessive ringing that is present on the gate drive signal could cause unintentional turn-on or partial turn-off of the device. The gate resistance should be carefully selected to ensure that the gate drive pulse is adequately dampened. To a first order, the gate circuit can be approximated as a simple series RLC circuit driven by a voltage pulse as shown in Fig. 8.
As shown, minimizing LLOOP minimizes the value of RLOOP needed for critical damping. Minimizing LLOOP also minimizes the rise/fall time. Therefore, it is strongly recommended that the gate drive be located as close to the SiC MOSFET as possible to minimize LLOOP.
The internal gate resistance of the SiC MOSFET is 5Ω. An external resistance of 6.8Ω was used to characterize this device. Lower values of external gate resistance can be used so long as the gate fidelity is maintained. In the event that no external gate resistance is used, it is suggested that the gate current be checked to indirectly verify that there is no ringing present in the gate circuit. This can be accomplished with a very small current transformer.
A recommended setup is a two-stage current transformer as shown in Fig. 9. The two-stage current transformer first stage consists of 10 turns of AWG30 wire on a small high permeability core. A Ferroxcube 3E27 material is recommended. The second stage is a small wide bandwidth current transformer such as the Tektronix CT-2. Finally, a separate source return should be used for the gate drive, as shown in Fig. 10a and b.
CHOOSE GATE DRIVER CAREFULLY
5Careful consideration must be given to the selection of a gate driver. A typical application error is selection of a gate driver that has adequate swing, without careful consideration of output resistance and current drive capability. Therefore, an appropriate gate driver must have these characteristics:
High peak current capability
Low output resistance
Adequate voltage swing
A significant benefit of the SiC MOSFET is the elimination of the tail current observed in silicon IGBTs. However, it is very important to note that the current tail does provide a certain degree of parasitic dampening during turn-off. Additional ringing and overshoot is typically observed when silicon IGBTs are replaced with SiC MOSFETs. The additional voltage overshoot can be high enough to destroy the device. Therefore, it is critical to manage the output interconnection parasitics (and snubbing circuitry ) to keep the ringing and overshoot from becoming a problem.
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COMPONENT COST VS. SYSTEM PERFORMANCE
Obviously, because it is a new technology, the SiC MOSFET will have a higher component price tag than it's silicon counterpart. However, overall performance characteristics of a power conversion system using the SiC MOSFET and SiC Schottky diodes can be superior to a traditional all-silicon system.
SiC devices allow power circuits to operate at higher switching speeds, which reduces the cost of magnetics as well as system size and weight. In addition, thermal management considerations such as heat sinking and cooling are less stringent, because these systems are more thermally efficient.
A cost tradeoff based on component price alone does not provide a realistic comparison of long-term operating costs. SiC MOSFET prices will eventually go down because Cree has been using 4-in. wafers and will eventually go to 6-in. wafers, which offer greater economies of scale.
Cree intends to produce SiC MOSFETs in an N-channel configuration rated higher than 1,200V, in a variety of current ratings.
SILICON CARBIDE: THE NEW MOSFET IN TOWN
SILICON CARBIDE (SIC) possesses many favorable properties making it useful for high-temperature, high-frequency and high-power applications, including:
High thermal conductivity
High breakdown electric field strength (about 10X of Si),
High saturated drift velocity (higher than GaAs)
High thermal stability
These properties allow a high power device to block several kilovolts in the blocking mode and conduct high currents in the conducting mode. Typical switching devices with these characteristics are conventional silicon MOSFETs and IGBTs.
A major advantage of SiC-based switching devices is operation in hostile environments (600°C) where conventional silicon-based electronics cannot function. Silicon carbide's ability to function in high temperature, high power, and high radiation conditions will enable large performance enhancements to a wide variety of systems and applications. For example, SiC's high-temperature high-power capabilities can benefit aircraft, automotive, communications, power, and spacecraft applications.
While producing SiC commercially for MOSFETs has proven to be a daunting task, Cree has overcome the problems of producing MOSFET-grade SiC.
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