Switching Losses in Full-SiC Power Modules

A full-SiC power module offers performance deriving from SiC that is superior to that of conventional power modules.
This time, we will explain the switching loss which can be a significant problem in the conventional power modules.

Switching Losses in Full-SiC Power Modules

Full-SiC power modules have two important advantages over conventional IGBT modules: 1) the ability to dramatically reduce switching losses, and 2) overall loss reduction that becomes more significant as the switching frequency rises.

The diagrams below compare a 1200 V/300 A full-SiC power module BSM300D12P2E001 with an equivalent IGBT. The graph on the left is a comparison based on data sheet specifications; Eon is the switch-on loss, Eoff is the switch-off loss, and Err is the recovery loss. Eon and Eoff are both reduced dramatically, and because almost no Irr current flows, Err is negligible. As a result, the overall switching loss is reduced by 77%. This is the first of the above-described advantages.

On the right are shown totals for the switching loss and conduction loss at switching frequencies of 5 kHz and 30 kHz in loss simulations for the case of a PWM inverter. Compared with an IGBT module, total losses are reduced by roughly 22% at 5 kHz. The orange areas represent switching losses; nearly all of the reduction in losses is due to reduced switching losses. At 30 kHz, the switching loss for the IGBT is greatly increased; this is a well-known issue with high-speed switching in IGBTs. The full-SiC power module also exhibits an increase in switching loss, but the proportion is smaller than the relative increase for the IGBT module. As a result, we see that at 30 kHz the total loss can be reduced by about 60%. This is the second of the above-mentioned advantages.

As one would expect, this small switching loss is due to the characteristics of the SiC elements that constitute the full-SiC module. In the sections on SiC MOSFETs and SiC Schottky barrier diodes, there are numerous articles comparing these products with equivalent Si components, and these should be referenced as necessary.

Switching Losses Further Lowered through Use of Third-Generation SiC Trench MOSFETs

ROHM is leading the industry in the mass production of trench-structure SiC MOSFETs. SiC power modules already use these trench MOSFETs, and switching losses are reduced even further compared with conventional SiC power modules.

This graph compares the switching losses, based on data sheet specifications, of 1200 V/180 A power modules: an IGBT module, the full-SiC power module BSM180D12P2C101 using second-generation DMOS-structure SiC MOSFETs, and the BSM180D12P3C007 power module using third-generation trench MOSFETs.

Compared with the IGBT module, the second-generation module achieves a roughly 60% reduction in switching losses, and the third-generation module reduces losses by an additional 42% over the second-generation product, achieving a switching loss reduction of about 77% compared with the IGBT module.

In this way, full-SiC power modules are capable of dramatic reductions in switching losses compared with equivalent IGBT modules, and the higher the switching frequency, the greater is the difference in losses relative to IGBT modules. This means that full-SiC power modules are capable of high-speed switching while sharply reducing losses, in contrast with IGBT modules, which are ill-suited to high-speed switching operation.