Diode symbols in MOSFETs - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: May 25, 2018, 09:16:40 pm
content: Are they a common MOSFET and diode or zener diode combined, so 2 parts instead of 1 ? So in a SMPS it could act as the switcher when it's on, and then the clamping diode for resetting a transformer when it's off ?

author: james_s
author_rank: Super Contributor
timestamp: May 25, 2018, 09:32:56 pm
content: I believe you're referring to the body diode, which is actually a parasitic diode that exists due to the way the mosfet is constructed.Occasionally you will see a mosfet that has an internal protection zener diode on the gate, that is intentionally added to limit the Vgs.

author: lordvader88
author_rank: Frequent Contributor
timestamp: May 25, 2018, 10:19:34 pm
content: Ok some mosfets have internal protection added, so that's 2 in 1 as well. Some have a zenor symbol so does that mean if they are reversed biassed a predictable current (hopefully) would flow ?Do any 3 pin mosfets have it such that if on a N-ch modfet, if the source voltage is higher than the drain by a diode amount...then an internal diode would forward conduct ?Maybe a - +3 pin SMD chip does, I was reading some datasheet and IDR, maybe a combined mosfet/clamp diode.IDK if a 3 pin device can act as a typical mosfet with Vdd>Vss but as a diode when Vss>Vdd

author: bson
author_rank: Unknown
timestamp: May 26, 2018, 02:55:14 am
content: I don't know if a body zener is reliably usable for anything other than ESD and transient protection...

author: Kleinstein
author_rank: Super Contributor
timestamp: May 26, 2018, 10:13:08 am
content: All normal MOSFETs with 3 pins have an internal diode from source to drain. This is part of the construction, as source is connected to the substrate. Only special MOSFETs with a separate substrate pin can be without that diode. Sometimes the symbol includes that diode, but the diode is always there.The gate protection is often found, but not always. It is an extra feature to protect the gate from too much voltage, that could damage the FET.#Many mosfets are more or less capable to operate in avalanche mode, just like a higher voltage zener. So if the drain to source voltage gets too high, it will act like a zener diode. If the power is not to high it can survive this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2018, 10:16:22 am
content: Possibly only "FETky" parts are actually co-pack, or monolithic pairs.All others are parasitic body diode, as indicated by the arrow pointing into/out of the channel.The parallel diode is superfluous, because it's already indicated.Tim

author: digsys
author_rank: Unknown
timestamp: May 26, 2018, 10:42:51 am
content: Quote from: lordvader88The "reverse body" diode is extremely useful in many new application .. ie Ideal bridges, ideal auctioneering diode functions.This diode can usually carry as much current as the FET DS itself, sometimes more.Here's how it works - The FET is wired in circuit in reverse ! ie so that the body diode is in conduction. When a VI sensor detects voltage / current flowingacross this diode, it immediately "switches on" the FET -full conduction. At this point the Vloss drops from say 0.5-0.7V to 0.01-0.02, depending on RDSon.The ones I use in my 30A bridge are ~ 0.5mR egwww.pbase.com/digsys/image/153385878This use has become very popular over the last several years .. check terms like -synchronous rectification etc

author: David Hess
author_rank: Super Contributor
timestamp: May 26, 2018, 02:01:22 pm
content: Quote from: T3sl4co1l on May 26, 2018, 10:16:22 amI am not convinced that is what is being shown. There are two parasitic diodes from the substrate to the source and drain forming a parasitic bipolar transistor. The vertical MOSFET structure shorts out the parasitic base-emitter junction for all kinds of good reasons (1) but leaves the base-collector junction which forms the body diode.The symbol showing one diode seems to show the largely irrelevant (2) shorted base-emitter junction and I prefer the two diode symbol to make the base-collector junction explicit. In the past, that little substrate diode reminded me to watch out for operating conditions.(1) SCRs do the same thing with extra metalization for the same reason which is left off in sensitive gate SCRs.(2) Vertical MOSFETs have gotten better but that parasitic NPN did not used to be irrelevant; you had to watch the operating conditions to prevent activating it and destroying the device. Is it ever a concern anymore?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2018, 07:10:37 pm
content: Quote from: David Hess on May 26, 2018, 02:01:22 pmYeah, it's not too relevant anymore, literally speaking, since the reign of lateral DMOS passed decades ago. It would be more accurate moved around a bit, but reinterpreting the classic symbol for modern structures is perfectly fine in my book.QuoteI don't like the diode, because it implies it's external. An IGBT co-pack is truly external, and the symbol reflects this.Not that the rest of the IGBT symbol really means anything, either, sadly; the "insulated gate BJT" one is maybe more accurate, but less distinctive, and probably a bit disingenuous of an oversimplification, anyway.QuoteI think so -- avalanche breakdown is still very much a concern, and it activates the parasitic BJT. While MOSFETs are typically rated for near-die-cracking discharges*, it's a one-time thing, and relying on it outside of the rated range (under repetitive use or at excessive peak current) is destructive.*That is, Tj(max) limited. Please don't actually try to thermal shock your transistors...A good example is a switching converter with excess stray inductance (unclamped), which causes repetitive avalanche, at whatever the peak switching current is. The voltage after avalanche is not nearly zero (as it is in the datasheet test case), but the full supply voltage, maybe 50 to 80% of rated Vdss (depending on how tight the converter design is being pushed..). The current tail after avalanche decays slowly, dissipating tons more energy than E(inductor) implies. (If nothing else, the inductor's energy adds with the supply anyway, so that E(diss) > E(inductor) to begin with.)I don't actually know how fast avalanche charge clears; it's not like a full t_rr. Is it proportional to reverse-bias voltage? So if the body diode clears in, say, 800ns at 0.5V, is it 0.8ns at 500V? That would be fast enough not to worry. Purpose-made zener diodes tend to respond very quickly indeed (only a very small amount of excess discharge, visible at very low bias, where the noise voltage looks like a random sawtooth), but I don't have a feel for how much worse MOSFETs are in this mode of operation. (Proper BJTs are quite awful, but that's a side-effect of the 3-layer structure; I don't know of any diodes that avalanche in the same way. Possibly a very hard avalanche would activate the 3-layer structure in MOSFETs, but that would be difficult to achieve without destroying it somehow anyway, I suspect.)Tim