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I am looking at the datasheet of a SiC MOSFET, which has a junction to case thermal resistance Rth(j-c) max of 1.41°C / W.

rth

The junction to ambient thermal resistance is not provided, as this type of power transistors are usually used with a heatsink.

Is there a way to estimate the Rth(j-a) ? I want to estimate the losses without a heatsink.

Thanks.

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Is there a way to estimate the Rth(j-a) ?

There are some hints on this wiki site about TO-220 packages and there is a useful paragraph that compares values including that of the TO-247-3 package (used in that rather feeble GeneSic MOSFET you are asking about): -

If more heat needs to be dissipated, devices in the also widely used TO-247 (or TO-3P) package can be selected. TO-3P has a typical junction-to-ambient (heatsink) thermal resistance of only about 40 °C/W

So, I think the answer you require is ~ 40°C/W

I want to estimate the losses without a heatsink

Well, the "losses" will be the same for a given current and voltage but, the temperature difference will be vastly changed.


Be aware that the G3R160MT12D device is rather poor on peak power dissipation (see figure 14 in its data sheet) compared to other 1,200 volt devices such as those from ON-SEMI. However, you may be considering this part not because it might appear to be attractive but, because it is available. Be very aware about its limitations

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  • \$\begingroup\$ Thank you for your answer. I am indeed facing shortage issues. We have difficulties sourcing the STW11NK100Z (which has a Rthja of 50°C/W) so I am looking for a second source. It is used in a CrM flyback converter. \$\endgroup\$
    – Ultra67
    Dec 16, 2021 at 12:08
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    \$\begingroup\$ @Ultra67 if you are asking the MOSFET to conduct for shorter periods than 100 microseconds, you can't expect it to handle power that's greater than that implied by the 100 microsecond curve. I fell foul of that with this specific device. Just saying. Pretty much all the GeneSic devices I've come across are really weak in this area. \$\endgroup\$
    – Andy aka
    Dec 16, 2021 at 12:13
  • \$\begingroup\$ I will be careful of this graph when looking for other MOSFETs. Did you find an equivalent of this specific device? \$\endgroup\$
    – Ultra67
    Dec 16, 2021 at 12:38
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    \$\begingroup\$ An equivalent is not what I was looking for!! I wanted a better device and I went for an ON_SEMI 1200 volt SiC MOSFET that had a SOA graph that fitted my needs. Can't remember the actual part number (it began with an "N") but, these are also becoming hard to obtain so I ordered a small stock of them to cover me. I think it was this one: uk.farnell.com/on-semiconductor/nvhl040n120sc1/… and Farnell have some available in early 2022. \$\endgroup\$
    – Andy aka
    Dec 16, 2021 at 12:49

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