How to know the RDS when a VDS=VGS? - Page 1
Forum section: electronics / beginners

author: PLUS
author_rank: Newbie
timestamp: February 29, 2016, 09:58:38 pm
content: Dear ALL!!Regarding a 2N7002KQ-7 FET,I wondering for the RDS on resistance during a VDS = VGS condition.especially, less than 2.5V area I want to know for this.as I checked datasheet. they indicated two values only.(VGS = 10V & IDS = 0.5A, VGS = 5V & IDS = 0.05A)but, I want to know more things. such as below things. (Changed RDS of the threshold voltage range)| VDS | VGS | RDS || 1V | 1V | ? || 1.5V | 1.5V | ? || 2V | 2V | ? || 2.5V | 2.5V | ? |Please let me know to the calculation method or basic principle.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 29, 2016, 10:16:03 pm
content: The simple answer is, if it's not in the data sheet, then it's not guaranteed to have any particular value. Your best bet is probably to buy some, from different manufacturers and different batches to get some idea of the spread, and to measure the current experimentally. Be prepared for it to increase quite sharply as the device reaches Vgs = Vgs(threshold); if you get a meaningful answer at Vgs = Vds = 2.5V without the device smoking, I'll be surprised.As an aside, this sounds like the sort of thing that's sometimes done as a substitute for a 'real' resistor on a silicon die. A FET with its gate and drain shorted together takes up much less die area than a resistor, but can serve as (say) a pull-up on a digital signal. The fact that its V/I characteristic is highly non-linear doesn't matter in some cases.

author: biffster
author_rank: Contributor
timestamp: February 29, 2016, 10:33:32 pm
content: The problem is also that RDS(ON) can change rapidly with a small change in VGS and the gate threshold voltage - VGS(th) - changes with temperature so it's not like you can really plot a fixed relationship between VGS and RDS(ON) and expect it to stay the same when the FET is at different temperatures, when it's dissipating different amounts of heat for example. You can see these relationships in figures 3 and 6 in the data sheet.Because of this, when I use FETs I tend to design the circuit so that RDS is either controlled by a feedback loop that alters VGS to keep RDS where I want it or I design it so that it doesn't matter. Hope that helps.

author: KL27x
author_rank: Super Contributor
timestamp: February 29, 2016, 11:51:05 pm
content: I did not even know that VDS affected RDSON.I see charts that show a relationship with Id...Which may or may not be rated to Vds.edit... which is of course the same relationship to "Vds"... but is perhaps not what the OP meant. It seems to me he probably wants to know RDSon when the transistor base is being driven to the supply rails, at different supply voltages, from the looks of his question/chart.

author: c4757p
author_rank: Super Contributor
timestamp: March 01, 2016, 12:00:50 am
content: Quote from: KL27x on February 29, 2016, 11:51:05 pmIt definitely does:https://en.wikipedia.org/wiki/Channel_length_modulation

author: KL27x
author_rank: Super Contributor
timestamp: March 01, 2016, 12:07:09 am
content: QuoteThanks. Clear as day, now. Easy when it's spelled out like.(I only wish I could understand this)It's particularly confusing to me, because this equation for MOSFET "output resistance" (is same as RDSon?) seemingly doesn't take Vgs into consideration, at all.

author: c4757p
author_rank: Super Contributor
timestamp: March 01, 2016, 12:21:09 am
content: Hey, I never said it was explained well there

author: KL27x
author_rank: Super Contributor
timestamp: March 01, 2016, 12:35:40 am
content: Well, on the surface, it seems like OP's question is a good one. We would ALL want to know what the RDSON is for, say, any FET that is driven to supply rails, at certain levels of supply voltage, let's say.But in practice, Vds is usually not going to be equal Vsupply, except when the transistor is fully off. Vds will be the voltage drop across the FET in series with a load - and therefore, Vds is going to be much less than Vsupply when switched on in any typical FET application (usually on the order of 1% of supply voltage). Perhaps this is why RDSon is typically shown in a datsheet as a function of Id rather than Vds. Vds could be used, because current and voltage are two sides of the same coin, but it would be potentially easy to confuse Vds with current supply voltage (or with max Vds) in this context, and it's simply a confuscation from the more common purpose of wanting to know this information. (In fact, this is the mistake I made in my first response to this thread).Now that I think I'm reading this question correctly, I'm left wondering if this is not the intended question, at all. Vgs = Vds will pretty much happen only in one point, somewhere mid-switching, and it is sort of an arbitrary point where I fail to see the purpose of the curiosity. RDSon will be high at that point, and the situation will be unsustainable, anyway. The exact voltage where they will fleetingly match up will depend on the characteristics of the transistor, the supply voltage, and the load. And there will be only one point per set of variables where they cross... so the chart that the OP wrote doesn't make any sense, because who's to say that at Vgs = X you can even get Vds to also equal X (without changing load and/or supply voltage). Vds will be dependent on Vgs, Vsupply, load, and as someone else ponted out, temperature.Or I'm totally in the dark. This question is very confusing and potentially misleading.

author: biffster
author_rank: Contributor
timestamp: March 02, 2016, 10:20:41 pm
content: PLUS could you explain why you want to know? Do you have a project where you are connecting the drain and gate for some reason for example? It would help.

author: danadak
author_rank: Super Contributor
timestamp: March 02, 2016, 11:53:28 pm
content: The datasheet has a graph you can calc this from (typicals) -http://www.nxp.com/documents/data_sheet/2N7002.pdfRegards, Dana.

author: PLUS
author_rank: Newbie
timestamp: March 26, 2016, 05:23:08 am
content: Thank you for all answer.I want to know Id orRDS at a less than 3V VGS.?? LG-F510L ? Tapatalk?? ??

author: danadak
author_rank: Super Contributor
timestamp: March 26, 2016, 10:40:17 am
content: A good explanation here. The constants of process you can eliminate,or determine, from typical curve and some algebra.http://ecee.colorado.edu/~bart/book/book/chapter7/ch7_3.htmRegards, Dana.