(Nevermind) Is there any simple arrangement that simulates a HexFET - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 15, 2023, 10:01:05 pm
content: Since HexFets are quite hard to come by apart for shady stock on ebay, Is there some simple arragement which simulates the load charectaristics of hex fets with conventional BJT's/Fets?I want to mess with those linear thingies in audio amp designs.

author: Benta
author_rank: Super Contributor
timestamp: August 15, 2023, 10:21:49 pm
content: Nothing special about HexFETs, it's just a marketing name for normal planar MOSFETs. Are you thinking about something else, like VFETs?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 15, 2023, 10:33:27 pm
content: Did you notice the log-log axes...Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 15, 2023, 10:39:47 pm
content: Are you sure you're not talking about (LDMOS) (Laterally Diffused) power MOSFETs? I've not seen the audio variety for years. Modern LTMOS are used for RF.HEXFET is a trademark of International Rectifier now Infineon Technologies.

author: Benta
author_rank: Super Contributor
timestamp: August 15, 2023, 10:55:38 pm
content: I still think the log-log point wasn't fully appreciated.

author: Benta
author_rank: Super Contributor
timestamp: August 15, 2023, 11:00:10 pm
content: Quote from: ELS122 on August 15, 2023, 10:49:12 pmAll companies that no longer exist in the semiconductor world.TMOS: standard planar MOSFETHEXFET; standard planar MOSFET.TrenchMOS: standad planar MOSFET.All just marketing names.

author: wraper
author_rank: Unknown
timestamp: August 15, 2023, 11:07:20 pm
content: Quote from: ELS122 on August 15, 2023, 10:39:32 pmThose are graphs totally not comparable visually.Here you go IRF540 from Vishay with no fancy trademark nameshttps://www.vishay.com/docs/91021/irf540.pdf

author: BrianHG
author_rank: Super Contributor
timestamp: August 16, 2023, 12:34:12 am
content: For audio, careful about the HexFETs, or any low voltage VGS logic level mosfets. They are designed to switch, not operate in the linear region as the sharp low voltage turn-on, beginning at around 2.5v, doesn't evenly turn on all of the silicon mosfet die structure at the same time, hence the Safe Operating Area / SOA is actually worse than older mosfet with a higher voltage VGS as the larger slowly switch on voltage of the mosfet's cells mostly begin to conduct more evenly throughout the entire silicon die.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2023, 01:37:39 am
content: Quote from: BrianHG on August 16, 2023, 12:34:12 amDoesn't matter, the SOA is what it says it is.I've tested IRF740PbF for example, at voltage, and found it free from 2nd breakdown at DC. Most datasheets unfortunately do not show DC SOA (though older datasheets may have had them). I wouldn't suggest using them if you need design guarantees, but I would recommend testing them for DC SOA at your required operating conditions if you are willing to do a qualification test / process.Shockingly, modern (SuperJunction) types often have full (DC) SOA, despite higher power density than ever; I don't know what it is that actually makes it work, but I'm not complaining.Note that many also do have 2nd breakdown, sometimes quite severe (I ~ 1/V^2 or worse). There's no generalization, just look at the datasheet and see what it says. If it doesn't have the SOA you need (time or power), keep shopping. Simple as that.I've also seen some people harp on about "Spirito effect", with a partially-formed argument based on gm tempco -- but transistors don't automatically fail from positive tempco. There's more to it than that. And the only way to show that, is to plot the SOA proper.Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2023, 08:04:26 am
content: Quote from: ELS122 on August 15, 2023, 10:49:12 pmNote many parts which are obsolete are still available in the PBF versions, which simply means lead free.https://www.mouser.co.uk/ProductDetail/Vishay-Semiconductors/IRF540PBF?qs=%2FRKvNCQzLu3BWqXFNIPTkQ%3D%3D&gclid=EAIaIQobChMI7KO6itPggAMVzO7tCh1q7welEAAYASAAEgKHVvD_BwEQuote from: ELS122 on August 16, 2023, 01:51:21 amhttps://polonai.se/scrapbook/ta-4650/index.html

author: wraper
author_rank: Unknown
timestamp: August 16, 2023, 12:29:35 pm
content: Quote from: T3sl4co1l on August 16, 2023, 01:37:39 amWithout any info I would guess improved stability of the process or maybe that vertically stacked junctions have much better heat exchange between them than horizontally arranged structures and randomization during production of multiple vertical layers makes the die more uniform horizontally. After all the issue with operation in linear region is that those tiny MOSFETs the die consists of are not made exactly equal due to the process variation and conduct/heat differently in linear region.

author: CaptDon
author_rank: Super Contributor
timestamp: August 16, 2023, 01:56:24 pm
content: How is it the HEXFET curves start at apparently negative 10 volts? 10 to the minus 1, through 10 to the null, then upwards through the positive voltages (10 to the 1st power etc.) Are those perpendicular lines the body diode curves? I am unfamiliar with curves in the negative voltage region for an N-Channel FET. I usually only see Tr speed specs and one representative diode voltage at some fixed current level.

author: magic
author_rank: Super Contributor
timestamp: August 16, 2023, 02:54:25 pm
content: Quote from: ELS122 on August 15, 2023, 10:31:04 pmI think you can reproduce this kind of curve with a power BJT plus a resistor in series with the collector.The "linear" segments are RDS(on). For amplifier output stages it means that as the drain gets close to the source, there is a point where drain current almost stops increasing with gate voltage and that this point is proportional to drain current. The amplifier clips before touching the rail and the more it is loaded, the sooner it clips.This is so-calledQuote from: wraper on August 16, 2023, 12:29:35 pmwhich is normally used for switching and rarely by linear circuits, which prefer to operate in the "flat" channel saturation region, where drain current is effectively controlled by gate voltage.Quote from: CaptDon on August 16, 2023, 01:56:24 pm10-1is simply 0.1.

author: Benta
author_rank: Super Contributor
timestamp: August 16, 2023, 06:29:38 pm
content: Quote from: ELS122 on August 16, 2023, 01:51:21 amThe VFET (aka SIT) prices are up there because they're made from unobtanium. They haven't been produced since 40 years, and practically all NOS has been claimed fully by HiFi enthusiasts.Only Sony and Yamaha produced VFETs, mainly for their own use or subsequent repairs.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 17, 2023, 07:56:37 am
content: Slap a massive cap in the gate.... oh yeah, big 'ol HEXERIE!