MOSFET 'Vdsx' parameter - smaller than Vdss - Page 1
Forum section: electronics / projects

author: tom66
author_rank: Super Contributor
timestamp: February 24, 2025, 10:53:38 am
content: I recently designed a Toshiba SSM6P812R into a product. This dual p-MOSFET works absolutely fine in the product. However, upon closely reading the datasheet, I found an interesting parameter. The MOSFET is rated for 'Vdss', which is with the gate-source shorted, andseparately, 'Vdsx', which is for the gate-source reverse biased. And Vdsx is only 15V, whereas Vdss is 20V. This suggests the FET's breakdown voltage is worse when the gate is reverse biased.I checked a few other datasheets from other manufacturers, and could not find other manufacturer's datasheets with this parameter, only a few others from Toshiba. I cannot even find a mention of 'Vdsx' outside of Toshiba's own website. Is this a characteristic of only some FETs? From a semiconductor physics point of view, it makes some sense that the drain-gate junction may have a voltage breakdown limit, but if that is what defines this parameter, why not specify a drain-gate breakdown voltage?https://toshiba.semicon-storage.com/info/SSM6K208FE_datasheet_en_20140301.pdf?did=11044&prodName=SSM6K208FEhttps://toshiba.semicon-storage.com/info/application_note_en_20180726_AKX00062.pdf?did=13414(page 4).I am interested if anyone else knows where this comes from and if it impacts other manufacturers, or if Toshiba are just the only ones honest about it, or if it's just a quirk of some of their small FETs. Could be important for applications that might rely on driving the gate above the source to simplify the driver circuit design, I've definitely done that in the past.Edit: corrected broken links

author: temperance
author_rank: Super Contributor
timestamp: February 24, 2025, 11:57:57 am
content: Interesting, in an other app note by Toshiba I found the following:The withstand voltage is limited by VDSSexcept for trench MOSFETs operating in a reverse gate bias condition (during which the withstand voltage is restricted by VDSX).https://toshiba.semicon-storage.com/info/application_note_en_20180726_AKX00061.pdf?did=13413Table 1.2If you look up data sheets for Trench FET's from other manufacturers like ON, you will only find VDSSratings for Vgsis 0 V. Out if interest I tried finding some old app notes from Fairchild. But I couldn't find much.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 24, 2025, 09:56:21 pm
content: Quote from: tom66 on February 24, 2025, 10:53:38 amI guess it makes sense, as the gate-drain oxide is the region of stress.If you pull them in opposite directions, the total voltage is what matters.

author: tom66
author_rank: Super Contributor
timestamp: February 24, 2025, 10:59:59 pm
content: Quote from: PCB.Wiz on February 24, 2025, 09:56:21 pmIt makes sense to me to have a specification that is gate-drain voltage, for instance, for the N-channel part I linked it is implied that the gate-drain voltage is equal to the drain-source voltage limit by the values given. So they could specify Vgd is +/-30V, and the requirement is both Vgs and Vgd are kept within the part's limits to avoid damage.It seems pretty weird to specify a drain-source voltage limit that's dependent upon the gate voltage, it's almost as if there's a second order effect at play, but I can't find any references to this. It's the first FET I've seen with this specified in any form.