The Industry’s First Mass-Produced SiC MOS Module Without a Schottky Diode

In March of 2012 ROHM was the first supplier in the industry to mass produce 1200V/100A ‘Full SiC’ modules that utilize SiC for all power semiconductor elements. The goal was to balance the need for larger current handling capability with a smaller form factor for industrial equipment and other applications. Normally, increasing current rating entails integrating more MOSFET elements and similar measures. However, this requires diode rectification, which makes it extremely difficult to reduce or even maintain device size.

In response, ROHM has successfully developed an SiC power module utilizing 2nd generation SiC MOSFET technology that minimizes conduction degradation of the body diode, eliminating the need for a diode rectification. This makes it possible to increase the mounting area for higher current handling capability while maintaining the same compact form factor.

In addition, by improving processes and device structures related to crystal defects ROHM was able to overcome all problems related to reliability, including the body diode. The result is 50% less loss compared with conventional Si IGBTs used in general-purpose inverters. This decreased loss, along with an operating frequency greater than 50kHz, ensures compatibility with smaller peripheral components for greater end-product miniaturization.

Key Features

1. Switching characteristics maintained using a simple MOS structure
for minimal switching loss with no tail current
Switching characteristics are maintained even without an SBD. And unlike Si IGBTs, no tail current is generated, reducing loss by more than 50% for greater energy savings. In addition, switching frequencies over 50kHz are supported (impossible with Si IGBTs), contributing to smaller, lighter peripheral devices.

2. Reverse conduction possible for high efficiency synchronous rectification
General-purpose Si IGBT devices are not capable of conduction in the reverse direction. In contrast, the body diode in ROHM’s SiC MOSFET always conducts in reverse. Also, depending on the Gate signal input the MOSFET can operate in either direction for lower ON resistance vs. using just the diode. These reverse direction conduction characteristics allow high efficiency synchronous rectification in the 1000+V range – higher than diode rectification.

3. Body diode conduction degradation eliminated, enabling conduction over 1000 hours with no performance loss
Clarifying the mechanism by which defects are spread based on body diode conduction makes it possible to minimize the primary factors through process and device construction.With general-purpose products the ON resistance increases significantly after 20 hours. In contrast, ROHM’s new module ensures no ON resistance increases – even after more than 1000 hours.

Terminology

・Body diode
A parasitic diode formed on the MOSFET structure. During inverter operation current flows through this diode, reducing VF – requiring fast recovery characteristics.

・Tail current
Transient current that flows when an IGBT is turned OFF, generated due to the accumulation time of injected holes. During this period the drain voltage is high, increasing switching loss.

・IGBT (Insulated Gate Bipolar Transistor)
A type of power transistor that provides lower ON resistance through current flow using both electrons and minority carriers (holes). However, a major drawback is that high-speed operation is not possible due to the accumulation charge of injected holes, resulting in significant switching loss.

・Forward voltage
The voltage value generated when current flows through the diode in the forward direction. The lower the value the less the power consumption.

・ON resistance
The resistance value when the power element is in operation. This is considered to be the most important parameter for a power MOSFET. The lower the value the better the performance.