Why am I having so Much Trouble Choosing a Mosfet? - Page 1
Forum section: electronics / beginners

author: neppo1345
author_rank: Newbie
timestamp: June 08, 2018, 04:44:20 pm
content: Long time lurker, first time posting. Bear with meTrying to figure out circuit that will let me turn on/off a 5v 250mA load using a microcontroller with 3.3v logic. I'd like everything to be fairly compact, SO-8 size mosfet or smaller...but still solderable by hand.After some research I determined the easiest way to do this is probably using an N Channel Mosfet/low side switching. Ideally it would be high-side switching (to keep a common ground reference between the load and other components) with a P-channel, but then I would need to pick out a mosfet AND a transistor...and I was having enough trouble picking out a mosfet by itself....so I gave up and compromised on the low side.So.. did a bunch or research, realized I needed to keep an eye on thermal requirements no matter what the 'max' current was advertised on the datasheet.Looked at the IRF6201...2.10 mOhm @ VGS = 2.5V so that's what... (.250A^2)*(.002Ohm) = .125mW IRF6201 datasheet shows Pd @70c of 1.6W. Built it up, it works...but the mosfet gets screaming hot.So I go back to the datasheet to figure out WTF I did wrong and stumble upon Figure 8 showing 'Maximum Safe Operating Area' and find that my current/voltage combination is way out of the safe range for DC.Now that I know the datasheets have these figured I'm trying to find something based on other datasheets and their 'safe operating area' figures and for the life of me can't find a small mosfet that will fit my bill.Am I going about this the right way and just falling into a trap of impossible requirements (I didn't think 250mA at 5v sounded too crazy)? Am I just asking too much for an SO-8 package?If not can someone nudge me in the right direction to help me figure out what I'm doing wrong?Thanks,-Tim

author: 2N3055
author_rank: Super Contributor
timestamp: June 08, 2018, 07:16:16 pm
content: Measure voltage drop on the mosfet...

author: capt bullshot
author_rank: Super Contributor
timestamp: June 08, 2018, 07:34:30 pm
content: From your description, the MOSFET cannot get warm at all, since you intend to use it as a switch, and you wouldn't exceed SOA since you have either 5V or 0.25A (but not at the same time) across the MOSFET.Something must be wrong with your circuit.

author: mbless
author_rank: Regular Contributor
timestamp: June 08, 2018, 07:35:24 pm
content: Quote from: 2N3055 on June 08, 2018, 07:16:16 pmAlso what do you measure for the gate voltage? Is there a pull-down resistor on the gate?

author: Dukov Ahzrukhal
author_rank: Regular Contributor
timestamp: June 08, 2018, 08:56:01 pm
content: Could it be that 3.3v is not enough to fully turn on the MOSFET? You could try disconnecting it from the microcontroller and applying 5v to the gate to see if it still runs too hot.

author: 2N3055
author_rank: Super Contributor
timestamp: June 08, 2018, 10:41:01 pm
content: Quote from: Dukov Ahzrukhal on June 08, 2018, 08:56:01 pmThat is what I presume, that mosfet didn't fully open.. That's why you need to measure drain-source voltage...

author: cellularmitosis
author_rank: Unknown
timestamp: June 08, 2018, 10:41:33 pm
content: Quote from: capt bullshot on June 08, 2018, 07:34:30 pmThis MOSFET appears to have a very unusual safe operating area. It appears it isn't rated for DC above 0.2V.. I misunderstood how SOA works.

author: neppo1345
author_rank: Newbie
timestamp: June 09, 2018, 04:55:34 am
content: Yeah, I think I just did a really poor job picking a mosfet the first time around. I just assumed if the power dissipation was acceptable for a given Vgs it would be good-to-go...but I guess not.I'm going back out to try and find one that will let me run DC currents at 5v.Thanks everyone for the suggestions.=!

author: JS
author_rank: Frequent Contributor
timestamp: June 09, 2018, 08:32:21 am
content: To ease my mosfet choise dive by a µC and 6A load on the other side I used an irf540 and an extra pin in the µC to build a charge pump, so I was able to drive the mosfet with ~6V and be done with it.This was with an attiny, I just needed the mosfet, a few buttons and a display 2, 2 for the mosfet, the extra one for the buttons with a resistive devider into an analog pinJS

author: exe
author_rank: Unknown
timestamp: June 09, 2018, 01:09:33 pm
content: Quote from: cellularmitosis on June 08, 2018, 10:41:33 pmMy interpretation is they mean "this mosfet is a switch that must be fully open or fully closed". That's why when it conducts 10A the voltage drop should be so low (0.1V): 10A*0.1V = 1W, which is fine. So, nothing wrong with SOA, imho.I suspect gate voltage is too low or transistor is fake / defective.

author: mbless
author_rank: Regular Contributor
timestamp: June 09, 2018, 02:43:45 pm
content: Quote from: cellularmitosis on June 08, 2018, 10:41:33 pmThat's not how SOA works; it's the VDS when the mosfet is on. Given RDS ~= 2mOhm, VDS ought to be 0.5mV at 250mA! That's easily inside the DC SOA.Again, this is why we are telling OP to measure VDS. OP also doesn't say if the MOSFET is PWM'd or not. The gate charge is rather large, so fast PWM with a large gate resistor could result in a lot of time in the linear region.

author: neppo1345
author_rank: Newbie
timestamp: June 09, 2018, 03:54:49 pm
content: I checked gate voltage at 3.4V.Voltage drop across the transistor is checking at 0.5mVTransistor is an infineon and came from DigiKey (as reputable of a source as I know, so I hope its not fake).And hold on...the Vds used in the SOA is the voltage drop across the transistor? Holy crap, I had no idea I was reading those charts wrong. Why the heck do all the datasheets just straight up call it drain to source voltage?I'll solder up another board...maybe I damaged this mosfet while handling it prior to assembly.I really appreciate the help, looks like I may have had some major misunderstandings.

author: mbless
author_rank: Regular Contributor
timestamp: June 09, 2018, 05:20:42 pm
content: Quote from: neppo1345 on June 09, 2018, 03:54:49 pmIt's all the same thing. Vds is the voltage difference between the drain and source which is the voltage drop across the transistor (in the current path). The SOA uses the actual operating conditions: off and on. When off Vds = 5V and Id = 0A. When on Vds = 0.5mV and Id = 250mA. You can't combine the off Vds and the on Id together.

author: sleemanj
author_rank: Super Contributor
timestamp: June 10, 2018, 12:19:55 am
content: Quote from: neppo1345 on June 09, 2018, 03:54:49 pmAt 250mA load through it? Sounds about right then at 2mR Rds (0.0005V / 0.25A). Dissipation at that should be miniscule.Unless you meant 0.5V instead of 0.5mV - in which case I'd suggest you checked the pinout.

author: Teledog
author_rank: Regular Contributor
timestamp: June 10, 2018, 12:48:07 am
content: 2n7000 cheap like borschtMaybe put a 1-10k resistor on the gate &/or a 10k from gate to source if there's any floating logic issues

author: neppo1345
author_rank: Newbie
timestamp: June 10, 2018, 05:41:14 am
content: DOH!, okay yes, that makes perfect sense!For some reason I was failing to make the connection that the voltage differential isonly5V when the current is 0A and when the mosfet is 'on' the Vds will be a product of the Rds for the given gate voltage.

author: KL27x
author_rank: Super Contributor
timestamp: June 11, 2018, 06:48:41 pm
content: After sourcing FETs a few times, you develop a frame of reference.Just looking at max Id (27A) and max Vds (20V), your FET sucks for what you want (which is to drive a very modest current of 250mA at a very modest 5V). To directly drive a FET from a 3.3V micro pin, you need a sensitive FET. Max Id is going to be around 300mA to 3A and max Vds is going to be 12V.The thing you really want to look at is the Rds over Vgs graph. But that's not a searchable parameter in a database. You can search for things like "logic level" and whatnot, but that is an arbitrary and limiting term, and it might exclude some FETs which are better/cheaper but which weren't labelled as such in search terms.Depending on how fast you want to switch it, gate charge/capacitance can also come into play.The bigger the max Ids doesn't mean you have more margin of capacity/safety. To use a "more powerful" FET, you generally need more gate voltage and charge. It would be nice if we could pay a bit more to get a FET that does "everything," but that's not how it works. Most FETs are good FETs (at something). There are very few for sale that are simply inferior at anything.Depending on your needs and micro, you could potentially use PFET directly if your micro has an open drain output pin on it. Technically, it's not the fact that it is open drain, but that open drain output specifically will not have an ESD diode to Vdd, because they are specifically made to interface with higher voltage. (If not for ESD diode, you could drive PFET directly with any I/O pin by switching between digital low/ground and high Z).