“Full SiC” Power Modules Using ROHM’s Own SiC-MOSFET and SiC-SBD : Smaller Equipment Sizes through Reduced Switching Losses and Higher-Frequency Operation

ROHM has, for the first time in the world, begun mass production of a “full SiC” power module that uses SiC-MOSFETs and SiC-SBDs manufactured by the company itself. Faster switching and greatly reduced losses are possible compared with conventional Si-IGBT power modules. The latest modules adopt third-generation SiC-MOSFETs, to achieve still lower losses.

Construction of Full SiC Modules

The full SiC power modules that are currently in mass production include a 2-in-1 type, suitable for configuring a half-bridge circuit with a single module, and a chopper type that can be used to configure a step-up circuit using a single module. There are modules that use both SiC-MOSFETs and SiC-SBDs (Schottky barrier diodes), and modules that employ only SiC-MOSFETs.

Greatly Reduced Switching Losses Compared with Si-IGBT Power Modules

IGBT power modules that combine Si-IGBTs and FRDs (fast recovery diodes) are widely used as power modules that handle large currents. IGBT modules are plagued by large switching losses due to IGBT tail currents and FRD recovery currents, but “full SiC” modules using SiC-MOSFETs and SiC-SBDs reduce switching losses considerably.

The diagram on the right compares results for switching losses actually measured in the same environment for the BSM300D12P2E001 (1200 V/300 A), which is an SiC module using an SiC-MOSFET and an SiC-SBD, and for an IGBT+FRD module.

Eon is the loss while switching on (including reverse recovery losses), Eoff is the loss while off, and Err is the reverse recovery loss in the body diode.

An SiCMOSFET does not have a tail current during switching off like that of an IGBT, and so Eoff is much lower. And because there is almost no reverse recovery current, Err is far lower as well. Eon is also reduced by about 30%, so that the overall switching loss can be decreased by 77%.

Faster Switching than with IGBTs

Full SiC modules are capable of faster switching than IGBT modules. The following diagram shows simulated results for losses when a PWM inverter is driven at 5 kHz and 30 kHz. Because SiC modules are capable of fast switching, the results indicate that when operating at 30 kHz, switching losses can be cut by 60%. Or, the frequency can be increased sixfold without increasing losses.

Advantages Gained from Reduced Switching Losses and Faster Switching

By reducing switching losses, efficiency is improved and heat generation is cut. As a result, cooling equipment can be simplified. For example, heat sinks can be made smaller, and water cooling or forced air cooling can be replaced with natural air cooling. These lead in turn to smaller overall system sizes and reduced costs.

Higher operating frequencies through fast switching contribute to miniaturization of such peripheral components as reactors and capacitors. This is the same as for typical switching power supply circuits. Moreover, in SiC-SBDs there is no short-pulse reverse recovery phenomenon, and so PWM control is possible without being concerned about abnormal surge voltages when short pulses occur.

A major advantage of full SiC power modules is the possibility they afford for smaller inverter and power supply sizes while improving efficiency.