9 Replies Latest reply on Sep 14, 2021 3:34 AM by lui_gough

    For FET-sake, I keep switching and cannot find a suitable MOSFET (or BJT)


      I came across this lovely term on Twitter the other day for tablet and laptop battery packs that had seen better days (usually when continuously powered/charged). It's #spicy pillow. LOL.




      Anyway, this reminded me of a little project I had in mind (as shown in schema below).


      As a electronics novice, this appears to be a borderline case as I know that transistors tend to work well for mA switches while MOSFETs tend to be targeted at higher voltages (and say +3A current).


      So now struggling to choose a suitable component - I'm particularly interested at the 1.5A option as that tends to be the limit for many usb charger adapters:



      Having done some prelim research it looks like FET's are the way to go but when working through my design criteria I'm quickly getting bogged down in the detail... In this case I'm not coming up with options as I can see that I need to minimise my resistance losses to reduce voltage drop etc... I'm also looking for a low cost <1$ SMD option.


      Hence I thought to open this little design challenge to the members as I had to chuckle at alui_gough status update I found when doing my e14 search...


      Often we're lazy, choosing to use the same parts over and over. I'm guilty of this when it comes to...



      So, any old favourite FETs you have which could well work here?

        • Re: For FET-sake, I keep switching and cannot find a suitable MOSFET (or BJT)

          Absolutely loved the title of your post! You have to start writing for British tabloids.


          I missed the meaning of the hashtag. I have little exposure to the sun or social media from under the bridge I only venture out for the goats that attempt to cross. I figured I write something to see if you get the reference.


          I read your post and will start by saying, I don't know if the module will work for your application but I have had such great success with them, I buy them by the gross. They exceed your price limit. I paid $1.27 CAN not included overhead. The design of the module matches what you napkined in your post. Other than soldering on the input pins they are good to go from the package.



          2 of 2 people found this helpful
          • Re: For FET-sake, I keep switching and cannot find a suitable MOSFET (or BJT)

            Don't have favourites.

            Always use a part for a reason (usually many reasons). Having a reel of 999 of a part is a good reason if it will work OK !


            In your case with 3.3V drive, 5V power supply, a load with possibly a big switch on surge, consider:


            Max drain source voltage rating > 10V

            Max gate source voltage for 5A < 3V

            Gate source resistance @ 5A < 0.05R (would give 312mW @ 2.5A


            Putting more or less these parameters into the Farnell search engine I got to the STS6NF20VSTS6NF20V in SO8 package at £0.635 (10 off).

            It's very close but may be a little off on 5A on resistance with 3V gate drive.

            There are similar parts in much smaller packages but I prefer the SO8 as it can take a bit of power.



            5 of 5 people found this helpful
            • Re: For FET-sake, I keep switching and cannot find a suitable MOSFET (or BJT)

              I'm sorry BigG I read the title of your post again and just starting to laugh. It is good to this want-a-be writer on so many levels.

              • Re: For FET-sake, I keep switching and cannot find a suitable MOSFET (or BJT)

                Your choice of driving the MOSFET with 3.3V logic is definitely a big constraining factor, as it cannot be guaranteed the full 3.3V is available. The MOSFET threshold voltage (Vth) needs to be as low as possible to ensure it turns on well at the high logic output voltage. Choosing an N-channel MOSFET in this configuration is wise, as they are more efficient (generally speaking) and you would want the lowest Rds(on) that you can get for the price to reduce voltage loss and heating. As you're not rapidly switching on and off, the gate capacitance figures are perhaps less important.


                Because you have a preference for SMD, these are probably the two that I thought of at first (in-stock and relatively low-cost) -

                - Vishay Siliconix SI4160DY-T1-GE3 (SOIC) - https://www.newark.com/vishay/si4160dy-t1-ge3/transistor-polarity-n-channel/dp/15AC0296

                - Vishay Siliconix SiRA14DP-T1-GE3 (PowerPAK SO-8) - https://www.newark.com/vishay/sira14dp-t1-ge3/mosfet-n-ch-30v-ppak-so8/dp/63W4125


                Both have very low threshold voltages of ~ 1.1V and rated about 8.5mOhm at Vgs = 4.5V. This makes it more suitable for 3.3V driving, but you will suffer much higher resistances than the headline figure because you are not on the "good" side of the knee:

                Ideally, 5V drive would be best with these MOSFETs but that's not to say they won't work.


                I would say michaelkellett's suggestion of the STS6NF20VSTS6NF20V is definitely a safer bet with Vth = 0.6V and has Rds = 45mR at 2.7V drive, but I suspect that if you can keep the drive voltage around or above that level, the Vishay components may have slightly better effective Rds and more robustness. The ultra-low threshold does come with a bit higher price tag.


                But then I came across these that might also deserve some consideration (mainly for cost)

                - NXP PCMA14UNYL (DSN1010) - 11V, 14A, 13.2mR at Vgs=4.5V, Vth=0.6V - https://www.newark.com/nexperia/pmca14unyl/mosfet-n-ch-11v-14a-dsn1010-rohs/dp/84AH8275

                - On Semiconductor NTLJS4114NT1G (WDFN) - 30V, 7.8A, 20.3mR at Vgs=4.5V, Vth=0.55V - https://www.newark.com/on-semiconductor/ntljs4114nt1g/n-channel-mosfet-30v-7-8a-wdfn6/dp/10N9582


                The NXP PCMA14UNYL comes in a bit of an odd package I haven't seen before, but the characteristics are very nice for your needs ...


                The On Semiconductor NTLJS4114NT1G seems to be another less-common (2mmx2mm) package, but it seems to be on clearance right now at US$0.167ea ...


                Of course, take my suggestions with a grain of salt ... I just wanted to do some "virtual" shopping without actually buying anything .


                - Gough

                  • Re: For FET-sake, I keep switching and cannot find a suitable MOSFET (or BJT)

                    Thanks Gough. Your description of how you come up with your choices is really helpful as this is where I've been struggling to get grips between theory and the practical. Spoilt for choice now.

                      • Re: For FET-sake, I keep switching and cannot find a suitable MOSFET (or BJT)

                        It is a shame that not more datasheets have a Rds vs Vgs graph because that is often most useful for this kind of application. That's part of the reason I run my tests ... just to know that given a particular Vgs drive from a microcontroller, what kind of Rds(on) I can expect, because with that information and knowing the current of your load, you can calculate the power dissipated by the MOSFET (got to keep the junction temperature below limits given the thermal resistance) and the voltage dropped across it (to keep the load happy, primarily).


                        But on the whole, if you do have the graph, you really want to be on the "flat" side of the knee and not on the "steep" handle-side of the hockey stick as that would imply potentially massive changes in actually experienced Rds depending on part-to-part and temperature variations. If you don't have the graph, there's always the Id vs Vgs graph (that is more often available in the datasheet) and ultimately, you want to make sure that you're not "capped" for Id at the given drive voltage as that would suggest the MOSFET could potentially act as a current limiter and dissipate quite a bit of power (even if the load doesn't reach that level).


                        For high-speed switching, low Rds losses usually come with higher gate charge and capacitance which means slower switching speeds or more critical driver requirements. So choosing a higher Rds MOSFET may make sense for high PWM frequencies or where you are constrained with your driving set-up (e.g. no dedicated MOSFET driver chips, high source impedance).


                        - Gough

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