Driving and Protecting SiC MOSFETs: Specs and Standards

More than ten years ago, we started seeing increasing silicon carbide (SiC) metal oxide semiconductor field effect transistors (MOSFETs) activities and a splash of product launches from key power semiconductor suppliers like Cree and Infineon. At the same time, many new suppliers were trying to dethrone the incumbent silicon and change the gameplay in their own ways. However, this is a mammoth task as insulated-gate bipolar transistors (IGBTs), which have been around for more than 40 years, are too entrenched in every power electronics engineer’s design.

Although SiC MOSFETs can bring forth many benefits, it took more than ten years for the suppliers to consolidate and align SiC MOSFET specifications and standards. These include the definitions of driving and protecting the SiC MOSFET. For example, there was the normally “ON” SiC Junction-Gate Field Effect Transistor (JFET) that will need a negative gate voltage to turn it off. There was also the more acceptable normally “OFF” switch, but it will require very high gate voltage of 20 V to ensure low conduction loss. Then, engineers had to redesign their power supply, which has been optimized for the IGBT at 15 V gate-emitter voltage (V_GE). This is just one of the problems, not to mention other challenges like high-speed operation and dv/dt noise when SiC MOSFETs switch faster.

Today, most of the disparities have been aligned with how we will drive IGBTs. Most importantly, gate drive technologies have also improved tremendously to catch up and enable the adoption of SiC MOSFETs. Broadcom newly released a 10 A gate drive optocoupler, the ACPL-355JC. It is able to fulfill the demanding requirements of driving and protecting SiC MOSFETs.

At the same time, most of the major suppliers in the power semiconductor industry are ramping up their SiC MOSFET production with packages and pinouts that can replace existing IGBTs easily. This drives the costs of SiC MOSFETs to a very competitive level, which is probably the most important factor that makes SiC MOSFET adoption take off.

 

The Standardization of SiC MOSFET Specifications

Broadcom gate driver optocoupler has evolved to meet the demand of SiC MOSFETs. Similarly, SiC MOSFETs have also evolved to be easily driven and protected by gate drivers. This section highlights some of the important changes in specifications which enable the growing adoption of SiC MOSFETs.

 

Figure 1. Infineon EasyDUAL 1B SiC MOSFET Module Driver Board. Image used courtesy of Bodo’s Power Systems [PDF]

 

The first specification would be gate-source voltage, V_GS. Over the years, the optimum V_GS for SiC MOSFET operations has reduced from 20 V to 18 V and finally settled to the same level as the V_GE of IGBTs, at 15 V. This made the definition of our gate driver power supply and under voltage lockout (UVLO) threshold more definite.

The ACPL-355JC gate drive optocoupler has a wide supply range from 0 to 30  V, which makes it very versatile for either unipolar gate driving or bipolar gate driving. These ensure the SiC MOSFETs are firmly switched on or off. The ACPL-355JC’s UVLO is set to 13 V, which is suitable to drive most of the latest SiC MOSFETs’ gates, which are designed to operate at 15 V V_GS.

The second specification is the total gate charge, Q_G. Q_G of SiC MOSFETs is more than 2 times smaller than their equivalent IGBT counterparts. This allows SiC MOSFETs to switch very quickly, reducing the switching losses and increasing the operating frequency. A lower Q_G also implies a lower gate current requirement, which helps to eliminate an additional current buffer stage. The ACPL-355JC has a 10 A peak driving current that can help overcome the input capacitance and charge up the SiC MOSFET’s gate quickly. This optimizes the potential of the SiC MOSFET and improves the overall system efficiency.

The third specification is the slew rate or dv/dt, which measures how fast the SiC MOSFET switches from zero to the BUS voltage within the shortest time. Although fast switching is critical to reducing switching losses, the high dv/dt generated can be a nuisance and cause noise to the SiC MOSFET control. The ability of the gate driver to reject the dv/dt noise is specified by the common mode transient immunity (CMTI). SiC MOSFETs are capable to switch 100 V/ ns, and the ACPL-355JC has a CMTI rating to guarantee a noise immunity of more than 100 kV/μs.

The last specification is the short circuit withstand time (SCWT) of the SiC MOSFET. Silicon IGBTs, in general, have superior SCWTs to SiC MOSFETs. Hence, any short circuit fault current in the SiC MOSFET needs to be extinguished faster before the switch is destroyed. Typically, the rule of thumb is 1 to 3 μs for SiC MOSFETs, as compared to 5 to 10 μs for IGBTs. In terms of short circuit protection, the ACPL-355JC uses the same methodology as DESAT sensing of the IGBTs. The ACPL-355JC monitors the drain and source of the SiC MOSFET and triggers a soft shutdown when a high fault current increases the drain-source voltage. To address the difference in time, how fast the SiC MOSFET and IGBT need to be protected, the detection voltage, detection time, and shutdown time of the ACPL-355JC can be adjusted using external discreet components.

 

Driving and Protecting SiC MOSFET Modules in Standard Packages

This section will look into driving 1200  V SiC MOSFETs from two major suppliers in standard module packages for low and high currents.

 

Table 1. 1200 V SiC MOSFET Modules in Standard Packages for Different Current Classes

Supplier	Part Number	Package	Current Class
Infineon	FF11MR12W1M1	EasyDUAL™ 1B	100 A
Wolfspeed	WAB300M12BM3	62 mm	300 A

 

Figure 2. Infineon EasyDUAL 1B SiC MOSFET Module ACPL-355JC Gate Driving Circuitry. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 3. Infineon’s FF11MR12W1M1 SiC MOSFET Module Turn-on Switching Waveforms. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 4. Infineon’s FF11MR12W1M1 SiC MOSFET Module Turn-off Switching Waveforms. Image used courtesy of Bodo’s Power Systems [PDF]

 

Infineon FF11MR12W1M1 1200 V/100 A SiC MOSFET Module

Figure 1 shows the driver board, which features two gate drive optocouplers ACPL-355JC, for driving a SiC MOSFET module in EasyDUAL 1B package. The board has an integrated capacitor DC bus, isolated switch mode power supplies (SMPS) for the gate drivers, and access to pulse width modulated (PWM) inputs and short circuit fault signals.

 

Figure 5. Infineon’s FF11MR12W1M1 SiC MOSFET Module Turn-on Switching Energy Losses. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 6. Infineon’s FF11MR12W1M1 SiC MOSFET Module Turn-off Switching Energy Losses. Image used courtesy of Bodo’s Power Systems [PDF]

 

The isolated SMPS, which serves as the ACPL-355JC secondary side power supply, is designed for bipolar driving of the gate at +18 V and -3.4 V. This is recommended by the Infineon application note AN2018-09, for high-frequency switching.

The ACPL-355JC has two outputs, VOUTP and VOUTN, which are connected to 5 Ω gate resistances for positive and negative gating. The 5 Ω gate resistances are realized with two parallel resistors to increase the power dissipation capability. The resulting peak current is approximately 4 A, which is lower than ACPL-355JC’s peak limit of 10 A. In addition, the Schottky diode D8, placed between the gate and VOUTP pin, is used together with CLAMP function to shunt parasitic Miller current during the off cycle.

Using this driving circuit, the switching waveforms of Infineon’s FF11MR12W1M1 are measured using the double pulse test at 600 V V_{DC_BUS}. Figures 3 and Figure 4 show the turn-on and turn-off switching transients at different drain current levels I_DS.

The instantaneous power during switching and the resulting switching energy losses can be calculated as shown in Figures 5 and 6. Based on the switching energies of 1.8 mJ (E_on) and 0.6 mJ (E_off) at 100 A, the switching performance measured is on par with what is specified in Infineon’s datasheet.

 

Figure 7. Infineon’s FF11MR12W1M1 SiC MOSFET Module Overcurrent Protection. Image used courtesy of Bodo’s Power Systems [PDF]

 

In Figure 2, the ACPL-355JC and its short circuit and overcurrent protection circuit, made up of OC (Pin 14), Zener diode (D4), and high voltage blocking diodes (D5 and D6), is connected to the drain of the SiC MOSFET module. Using this connection, the ACPL-355JC senses if there is an increase in the V_DS over the SiC MOSFET in the event of a short circuit or overcurrent condition. And depending on the blanking time, which can be adjusted by C23 and R40, the speed of the high current fault detection can be adjusted. For example, the Infineon datasheet states that the short circuit duration should be kept under 2 µs to prevent the SiC MOSFET module from exceeding its package thermal dissipation.

Figure 7 shows the overcurrent protection waveforms done with a loop inductance of 2.5 µH. The current surged almost 5 times above the rated current of 100 A before being brought down to 0 A quickly within 2 μs. The shutdown at V_GS 18 V is completed softly via the ACPL-355JC soft shut (SS) pin 13 to minimize the SiC MOSFET VDS overshoot that governs by V_DS = V_{DC_BUS} – L_par * di/dt. The soft shutdown lowers the negative di/dt, which causes the overshoot.

 

Figure 8. Wolfspeed 62 mm Half-Bridge SiC MOSFET Module Driver Board. Image used courtesy of Bodo’s Power Systems [PDF]

 

Wolfspeed WAB300M12BM3 1200 V/300 A SiC MOSFET Module

This driver board also features two ACPL-355JC for driving SiC MOSFETs in 62 mm housing. The board has an isolated SMPS for gate drivers and access to PWM inputs and short-circuit fault signals.

WAB300M12BM3 has a higher current rating than FF11MR12W1M1. As such, a larger peak gate current is required to overdrive Q_G of the SiC MOSFETs. The isolated SMPS is designed for bipolar driving of the gate at +15 V and -4 V, as recommended in the WAB300M12BM3 datasheet. To achieve a larger gate current, 2.95 Ω gate resistances are used for positive and negative gating. The 2.95  Ω gate resistances are realized with two parallel 5.9 Ω resistors to increase the power dissipation capacity. The resulting peak current is approximately 6 A, which is lower than ACPL-355JC’s peak limit of 10 A.

 

Figure 9. Wolfspeed 62 mm SiC MOSFET Module ACPL-355JC Gate Driving Circuitry. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 10. Wolfspeed’s WAB300M12BM3 SiC MOSFET Module Turn-on Switching Waveforms. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 11. Wolfspeed’s WAB300M12BM3 SiC MOSFET Module Turn-off Switching Waveforms. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 12. Wolfspeed’s WAB300M12BM3 SiC MOSFET Module Turn-on Switching Energy Losses. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 13. Wolfspeed’s WAB300M12BM3 SiC MOSFET Module Turn-off Switching Energy Losses. Image used courtesy of Bodo’s Power Systems [PDF]

 

Using this driving circuit, the switching waveforms of Wolfspeed’s WAB300M12BM3 are measured using the double pulse test at 600 V V_{DC_BUS}. Figures 10 and 11 show the turn-on and turn-off switching transient at different drain current levels I_DS.

The instantaneous power during switching and the resulting switching energy losses can be calculated as shown in Figures 12 and 13. Based on the switching energies of 5.8 mJ (E_on) and 5 mJ (E_off) at 300 A, the switching performance measured is on par with what is specified in Wolfspeed’s datasheet.

In Figure 9, the ACPL-355JC and its short circuit and overcurrent protection circuit, made out of OC (Pin 14), Zener diode (D2), and high voltage blocking diodes (D3 and D4), is connected to the drain of the SiC MOSFET. Using this connection, the ACPL-355JC senses if there is an increase in V_DS over the SiC MOSFET in the event of a short circuit or overcurrent condition. And depending on the blanking time, which can be adjusted by C2 and R13, the speed of the high current fault detection can be adjusted.

 

Figure 14. Wolfspeed’s WAB300M12BM3 SiC MOSFET Module Overcurrent Protection. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 14 shows the overcurrent protection waveforms done with a loop inductance of 1.5 µH. The current surged to 4 times above the rated current of 300 A before being brought down to 0 A quickly within 3 µs. The shutdown at V_GS 15 V is completed softly via the ACPL-355JC softshut (SS) pin 13 to minimize the SiC MOSFET VDS overshoot.

 

Conclusion

This article demonstrates the driving and protection of 1200 V SiC MOSFETs from two different suppliers with different current ratings and module packages. With the unification of SiC MOSFETs specifications and the ACPL-355JC’s versatility in terms of output gate voltage, driving current, and adjustable short circuit or overcurrent detection time, it is easy to drive and protect SiC MOSFETs now.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine