Premium Power Supplies
Dealing With SiC MOSFETs in Power Design
SiC MOSFETs for State of the Art Design
At Premium we were pioneers 15 years ago when we first introduced SiC technology in our power solutions with the first generations of 1200V schottky diodes. Nowadays, the use of SiC diodes is part of a large number of our standard and customized solutions.
Once again, we include the use of SiC MOSFEF in our designs, providing great advantages in applications of over 600V. In comparison, SiC vs. Si MOSFETS offer:
- A drastic reduction in the specified ON-Resistance, especially for applications over 900V
- Less variation of ON-Resistance with temperature
- A large reduction in internal capacities
With these characteristics it is possible to achieve, compared with the use of 1200V IGBTs:
- Up to 30% lower losses
- 2-3 x faster switching speed
- 30% fewer components
- Lower overall system cost
On the other hand, the use of SiC MOSFETS requires an extra dose of RDI knowledge in order to overcome challenges that its use entails.
New Encapsulations for SiC MOSFETs
These devices can withstand high power densities thanks to their combination of low conduction and switching losses, high operating junction temperatures, and fast switching speeds.
High power densities are attractive for smaller power control and conversion circuits but cannot be achieved solely by choosing a semiconductor material. It is necessary for these devices to be integrated in packages with low thermal resistance, such as TO-247. Unfortunately TO-247 package connections often have high inductances, which can limit switching speeds. When switching a MOSFET at high frequency, the connection from the source to the device is a common point for the gate driver voltage and the source drain current.
Due to the common LS1 inductance, changes in current will affect the gate voltage in proportion to the LS1 and the current change rate. When the gate is turned off, the voltage appearing on the inductance acts to maintain the gate voltage for a longer time, slowing down the current drop through the device. When the MOSFET turns on, the voltage induced in the L coil acts to slow down the rise in current. This effect is critical in SiC transistors, as they can switch tens of amps in just a few nanoseconds. This effect causes a switching loss increase.
To solve this effect, the manufacturers of these devices have developed new packages with a Kelvin connection to ensure that the return of the device’s gate is as close as possible to the connection of the source in the MOSFET itself.
