Can I add capcitor parallel to the DS of sic mosfet?

qwang_777

Hi, in the condition of zvs can be fullfilled ,like used in psfb, can I put a small capacitor paralleled to mosfet DS? This is to reduce the mosfet turn off loss and will not increase the turn on loss because zvs can still be fullfilled. Then another question, this cap must be a high frequency capacitor, and for sic has a high dv/dt like 50V/ns, what kind of cap could be chosen? Thanks.

TBhatia

Hello, thank you for your message. A member of our technical team is looking into your questions and we will respond shortly. 

Arka

Hi qwang_77,

In order to achieve ZVS, it is better if you can use the output parasitic capacitance of the MOSFET and design the converter accordingly instead of additional small capacitor parallel to MOSFET. An extra capacitor in parallel with the MOSFET will need more energy to discharge therefore a larger inductor or leakage is required for PSFB, or you need a smaller magnetizing inductance is needed which will increase conduction loss. Normally we don’t recommend adding an extra capacitor.

It is recommended to check how much turn-off loss can be saved with the extra capacitor. You can either do simulation or testing to get a rough number. In case, if you would like to go with additional capacitor, a ceramic capacitor with a suitable voltage rating should be chosen. Thanks.

qwang_777

Hi Arka,

It's grateful for your reply.

I mean, if the application is high voltage and high current like 600A, and I only pay attention to the heavy load loss, the mosfet turn on zvs is easy to achieve with little leakage inductance, in this condition, should I add extra capacitor to ds of mosfet to reduce turn off loss?

Besides ceramic capacitor, is CBB can also be used?

Thanks.

Arka

Hi qwang_777

We would recommend you to analyse the losses in this particular scenario. If adding capacitor in parallel to DS results in total reduction of overall losses of the converter (or if you can shift these losses from the semiconductor device to another part of the converter and achieve high power densities), then adding extra capacitor might makes sense. Besides ceramic, film capacitors of suitable rating can be used.

Thanks and Regards

TBhatia

Hi, I hope that this answered your question. I will close this discussion for now but if you have a follow up question, please "Start a New Discussion" and we would be glad to support you further.