Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Balticnoiseforge on January 20, 2025, 02:20:50 am

Title: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: Balticnoiseforge on January 20, 2025, 02:20:50 am
Been trying to find a mosfet for VVR power scaling for a ~10w SE amplifier.

But having issue finding mosfets that is currently avaible at mouser etc that would suit the use now that 2sk2968 is pretty much obsolete, even BUZ90.

There is IRFBC40 which is quite similar to BUZ90 - but not 100% and not suitable for scaling anode of power tube even if it should work for scaling only G2.

So iam asking if anyone is aware of a mosfet for it? That has datasheet with SOA graph and also dc curve?
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: glass54 on January 20, 2025, 03:08:17 am
Hi Bnf,
Consider the IRFPE50PBF Rated 800V, 7.8A, 190W TO247-3
https://au.mouser.com/ProductDetail/Vishay-Semiconductors/IRFPE50PBF?qs=cvaI6ThkwxuUgn1WDhVetA%3D%3D (https://au.mouser.com/ProductDetail/Vishay-Semiconductors/IRFPE50PBF?qs=cvaI6ThkwxuUgn1WDhVetA%3D%3D)
Also lower rated but could work IRF840. Rated 500V, 8A, 125W TO220-AB. Very Economical  :laugh:
https://au.element14.com/vishay/irf840apbf/mosfet-n-500v-8a-to-220/dp/8648557 (https://au.element14.com/vishay/irf840apbf/mosfet-n-500v-8a-to-220/dp/8648557)
Wish you success  :icon_biggrin:
Regards
Mirek
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: Balticnoiseforge on January 20, 2025, 04:01:17 am
None of those has DC operation graph in their drain to source :/
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: glass54 on January 20, 2025, 04:39:00 am
Have a look at link:
https://www.alldatasheet.com/datasheet-pdf/pdf/251475/VISHAY/IRFPE50PBF.html (https://www.alldatasheet.com/datasheet-pdf/pdf/251475/VISHAY/IRFPE50PBF.html)
also see attached extract  :laugh:
Regards
Mirek
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: Balticnoiseforge on January 20, 2025, 12:27:38 pm
Iam not going to pretend i know that much of mosfet, but quite rude to laugh while linking datasheets without safe operation for dc.
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: jansch on January 20, 2025, 02:22:49 pm
The DC operation limit is UDS * IDS <= Pmax, always. The other limits represent the influence of thermal capacity of the chip and the case. In real application, your Pmax will not be the one found in the datasheet, but derated by the total thermal resistance of your radiator.
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: Balticnoiseforge on January 21, 2025, 03:30:36 am
As i have understood it is if the mosfet has a datasheets with SOA graph of only ms, us it is mainly intended for switching.

While the older mosfets that would suit a VVR better also has a dc graph, such as for the 2sk2968 which shows DC voltage and mA line, which those mentioned lacks.

( 2sk2968 graph for contexts )

But maybe iam misunderstanding the mosfet linked above?
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: Balticnoiseforge on January 22, 2025, 01:05:31 pm
Found one that is currently avaible.
FQA8N100NC that still has a specified DC curve in the datasheet, should work for smaller amplifiers - for anyone else that is interested in VVRs.
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: Merlin on January 24, 2025, 03:37:04 am
A "10W" SE amp is gonna idle at around 25W. The MOSFET will have to dissipate up to a quarter of this at the half-voltage setting, so <7W. Pretty much any MOSFET will work for this. IRFBC40 should be fine.
Title: Re: Non obsolete VVR Mosfet with SOA graph and DC curve?
Post by: Merlin on January 24, 2025, 08:40:44 am
Quote from: Balticnoiseforge on January 21, 2025, 03:30:36 am
As i have understood it is if the mosfet has a datasheets with SOA graph of only ms, us it is mainly intended for switching.
But maybe iam misunderstanding the mosfet linked above?

Datasheets often don't bother to show the DC SOA because you already know what that is from the maximum voltage, continuous current, and continuous dissipation ratings.