Introduction

When changing the power elements used in converters and inverter power supplies from IGBTs to SiC-MOSFETs, why not also change the gate drivers to high-CMTI products at the same time?
This article explains why you should switch to a high CMTI product.

Purpose of changing power elements

Because switching losses can be reduced, increasing the switching frequency compared to the current configuration offers advantages such as miniaturization of the power supply size.

Points to note when making changes

Generally, IGBTs and SiC-MOSFETs have different gate characteristics, so attention must be paid to drive voltage, drive current, UVLO, and protection/auxiliary functions such as Desat and Clamp. Furthermore, IGBTs and SiC-MOSFETs also differ in their dV/dt characteristics.

example)
​dV/dt at turn-on @ 25℃
IGBTsare approximately 2.5V/ns
SiC-MOSFETs have a voltage of approximately 10V/ns.

​ ​dV/dt at turn-off at 25℃
IGBTs are approximately 7V/ns
•SiC-MOSFETs have a voltage of approximately 13.5V/ns.

As can be seen from the waveform in the figure below, the dV/dt of SiC-MOSFETs is much faster (larger) than that of IGBTs.

Since the dV/dt of both IGBTs and SiC-MOSFETs changes depending on the gate resistance, if the gate resistance of a SiC-MOSFET is increased, it is possible that it will become as slow (small) as an IGBT.
However, in that case, it becomes difficult to increase the switching frequency, and the benefits of changing from IGBT to SiC-MOSFET are diminished.

Therefore, to use SiC-MOSFETs at high frequencies, it is necessary to use them with a fast (large) dV/dt and change the gate driver to one that has the optimal CMTI capability to match the dV/dt. (CMTI: Common Mode Transient Immunity)

If CMTI is deficient

As shown in the upper right diagram, if the gate driver's CMTI is insufficient, it will remain on even though an OFF instruction has been issued, and in a bridge circuit like the one shown on the left, both the high-side and low-side switches will be turned on.

Calculation of dV/dt and CMTI when using SiC-MOSFETs

Example: If 10%-90% of a 400V bus voltage is 320V, and the rise and fall times are 10ns,
         dV/dt is 32V/ns(typ) = 32kV/us(typ), so the required CMTI is 32V/ns(typ) or higher.

In the cases described above, we recommend switching to a gate driver product with a CMTI of at least 150V/ns, taking into account variations in gate drivers and external system factors.
(It's not that SiC-MOSFETs necessarily require 150V/ns or more; it depends on the dV/dt conditions used.)

lastly

ON Semiconductor's gate driver product, NCP51752, has a CMTI of 200V/ns.
Please also refer to the NCP51752 datasheet below.
​NCP51752 datasheet

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