How to replicate the improvements of next-generation gate drive optocouplers to drive and protect SiC MOSFETs

This white paper provides an overview of SiC MOSFET gate drive optocouplers. It discusses the advantages of SiC MOSFETs capable of operating at high voltages, high frequencies, and high temperatures. It also discussed how SiC MOSFETs can increase overall system efficiency by more than 10% and have higher switching capabilities, which can reduce overall system size and cost.

This white paper provides an overview of SiC MOSFET gate drive optocouplers. It discusses the advantages of SiC MOSFETs capable of operating at high voltages, high frequencies, and high temperatures. It also discussed how SiC MOSFETs can increase overall system efficiency by more than 10% and have higher switching capabilities, which can reduce overall system size and cost.

introduce

Silicon carbide (SiC) power semiconductors are rapidly entering the commercial market and have many advantages over traditional silicon-based power semiconductors. SiC MOSFET can increase the overall system efficiency by more than 10%, and higher switching capabilities can reduce the overall system size and cost. Technical advantages and lower costs have enabled SiC power semiconductors to be rapidly adopted in applications such as industrial motor control, induction heating and industrial power supplies, and renewable energy.

Avago Technologies’ gate drive optocouplers are widely used to drive silicon-based semiconductors such as IGBTs and power MOSFETs. Optocouplers are used to provide enhanced electrical insulation between the control circuit and high voltage and power semiconductors. The ability to suppress high common mode noise (CMR) will prevent incorrect driving of power semiconductors during high-frequency switching. This article will discuss how next-generation gate drive optocouplers can be used to protect and drive SiC MOSFETs.

Advantages of SiC MOSFET

Silicon carbide is a wide band gap (3.2 eV) compound composed of silicon and carbon. In addition to being able to work at high voltage, frequency and temperature, wide band gap SiC has an on-resistance and gate charge that are an order of magnitude lower than silicon materials. In the evaluation conducted by CREE, the second-generation 1200 V/20 A SiC MOSFET was compared with the silicon high-speed 1200 V/40 A H3 IGBT using a 10 KW hard-switching interleaved Boost DC/DC converter. The results show that even if the switching frequency is five times the original, the SiC solution can still achieve a maximum efficiency of 99.3% at 100 KHz, which reduces losses by 18% compared to the best efficiency of the IGBT solution at 20 KHz.

The C2M MOSFET series recently released by CREE provides engineers with a wide range of competitively priced 1200 V and 1700 V SiC MOSFETs suitable for various applications. Cree can significantly reduce costs while providing improved switching performance and lower Rds(on). Increasing the switching frequency can significantly reduce the size of the Inductor. Lower conduction and switching losses allow engineers to reduce the size of the heat sink, or potentially remove the fan and switch to passive cooling solutions. Smaller inductors and heat sinks can significantly reduce system size. Although the cost of SiC semiconductor is higher than that of Si, the BOM cost of the entire system can be 20% lower than that of Si technology.

SiC MOSFET market and adoption

SiC technology is now recognized as a reliable substitute for silicon. More than 30 companies around the world have established SiC technology manufacturing capabilities and have carried out related commercial and promotional activities. Many power module and power inverter manufacturers have included SiC in their roadmaps for future products.

Solar inverter manufacturers and server power supplies were the first companies to adopt SiC semiconductors because efficiency is critical to their technology rankings. In 2013, European top solar inverter manufacturers REFU, SMA and Delta announced a new model with SiC inside. Compared with the equivalent IGBT inverter, “Photon” magazine evaluated the SMA SiC inverter, the efficiency was increased from 98% to 99%, and the physical weight of the inverter was reduced by 30%. With the improvement of availability, the increase of voltage types and the reduction of costs, the output of SiC semiconductors will gradually increase, which will lead to more adoption in applications such as motor drive, railway traction industry UPS and even hybrid vehicles.

Avago Technologies’ gate drive optocouplers have been widely used to drive silicon-based semiconductors such as IGBTs. This article will discuss how to replicate the improvements of next-generation gate drive optocouplers to drive and protect SiC MOSFETs.

SiC MOSFET gate drive optocoupler

Avago Technologies has worked closely with SiC market leader CREE Inc to determine the appropriate gate drive optocoupler for SiC MOSFET operation. We evaluated the ACPL-W346 and ACPL-339J gate drive optocouplers with CREE C2M SiC MOSFETs using a 100 kHz 8A SEPIC DC-DC converter. The gate drive capability of these two optocouplers meets the 98% efficiency requirement of CREE, as shown in Figure 1.

How to replicate the improvements of next-generation gate drive optocouplers to drive and protect SiC MOSFETs
Use Avago gate drive photocoupler and CREE C2M0080120D SiC MOSFET to achieve high efficiency

In order to match the low switching loss of CREE SiC MOSFET, the gate driver must be able to provide high output current and voltage at a fast slew rate to overcome the gate capacitance of the SiC MOSFET. The oscilloscope capture in Figure 2 shows that ACPL-W346 has a fast rise and fall time signal curve of 20 V at the gate of the SiC MOSFET, which is necessary for effectively switching the SiC MOSFET.

How to replicate the improvements of next-generation gate drive optocouplers to drive and protect SiC MOSFETs
ACPL-W346 SiC MOSFET gate signal curve

ACPL-W346 is a basic gate driver optocoupler, used to isolate and drive SiC MOSFETs operating at high DC bus voltages. It has rail-to-rail output with a maximum output current of 2.5A.

The unique feature of ACPL-W346 is speed, and it is the fastest in its class. The maximum propagation delay is 120 ns, and the typical rise and fall times are about 10 ns.

A high CMR and a common-mode rejection ratio of 50 kV/µs are required to isolate the high transient noise during high-frequency operation from the signal that causes the false output. When ACPL-W346 is used in conjunction with a bipolar current buffer stage, it can provide fast switching high voltage and high drive current to efficiently and reliably turn on and turn off the SiC MOSFET. Compared with earlier reference designs that used dedicated MOSFET drivers and proprietary circuits, ACPLW346 and a general off-the-shelf bipolar current buffer provide a cheaper and easy-to-implement gate drive solution.

How to replicate the improvements of next-generation gate drive optocouplers to drive and protect SiC MOSFETs
ACPL-W346 and CREE SiC MOSFET reference design

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