All SiC switching fits 10kW converter into five litre box

Rohm Semi and German lab Fraunhofer ISE have used SiC (silicon carbide) transistors to squeeze a 10kW power converter into 23 x 21 x 10cm and 4,5kg.
For better efficiency, the topology chosen is mixed voltage neutral-point clamped (MNPC, see diagram) – also known as ‘neutral point piloted’ (NPP) and ‘T-type’, according to the team – which has 12 transistors compared with only six needed for a three-phase bridge consisting of three half-bridges.
All mosfets and Schottkys are silicon carbide, made by Rohm and packaged in modules by Vincotech.
According to Rohm, compared with Si transistors, SiC allows switching frequency to be raised 3-9x – to 100kHz in this case – shrinking the passive components, but also increasing design challenges.
“As the high switching speed does also have an impact on the gate drive circuits the applied driver should provide a high immunity against common mode noise,” said the firm. “The high di/dt and du/dt during switching requires careful design of all switching loops and nodes. Small and carefully designed switching paths are a premise for a good SiC layout and are much more required for SiC than for Si systems.”
BM60015FV gate drivers were used, which come in a 3.5 x 10 x 1.8mm package with 3.75kV (VDE 0884-10 reinforced).
Using multi-mosfet modules – one per MNPC leg – reduces voltage spikes by keeping stray inductance down compared with discrete TO-247 and TO-220 packaging – stray inductance is 5nH (+/-dc vs neutral). The modules have an aluminium oxide base and offer 0.7K/W thermal resistance from chip to heatsink.
Input for the converter is 700-1,000Vand output is 400 at around 15Arms.
One reason for choosing MNPC topology is that not all components need to be rated for the input voltage, allowing some lower voltage types to be used with their inherent lower on-resistance.
In this case, for a 400V grid each of the six limbs can be made from two 650V devices (S2206 120mΩ mosfet, S6206 12A Schottky) and two 1,200V devices (S2301 80mΩ, S6301 5A Schottky).
These are second-generation SiC mosfets, with Rohm predicting improvements in efficiency if the design is altered for its recently introduced third generation (double trench gate) devices.

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