Post by thread:[EE] FET switch voltage drop

I was having a conversation with the chief design engineer at my place of work yesterday. He was saying he'd have to install an extra 3.3V voltage regulator in our design because I'd just discovered that we needed a second switched 3.3V supply. I suggested we used a high-side FET switch instead - a suggestion he dismissed by saying the voltage drop across the FET was around 0.6V which would put the supply well out of spec. This man is a chief hardware designer of 40 years experience. I'm just a computer scientist who does electronics for a hobby. Am I crazy or is he completely wrong on this one? I'd be expecting a voltage drop in the millivolts at the low currents we're talking about - not 0.6V. Can someone set me straight here? Cheers, Zik

On 3/28/07, Zik Saleeba <spam_OUTzikTakeThisOuTzikzak.net> wrote: > > I was having a conversation with the chief design engineer at my place > of work yesterday. He was saying he'd have to install an extra 3.3V > voltage regulator in our design because I'd just discovered that we > needed a second switched 3.3V supply. I suggested we used a high-side > FET switch instead - a suggestion he dismissed by saying the voltage > drop across the FET was around 0.6V which would put the supply well > out of spec. Probably rather high, for a low RDSon fet at moderate current, that is completely turned on. He was thinking bipolar.

On Thu, 2007-03-29 at 12:03 +1000, Zik Saleeba wrote: > I was having a conversation with the chief design engineer at my place > of work yesterday. He was saying he'd have to install an extra 3.3V > voltage regulator in our design because I'd just discovered that we > needed a second switched 3.3V supply. I suggested we used a high-side > FET switch instead - a suggestion he dismissed by saying the voltage > drop across the FET was around 0.6V which would put the supply well > out of spec. > > This man is a chief hardware designer of 40 years experience. I'm just > a computer scientist who does electronics for a hobby. Am I crazy or > is he completely wrong on this one? I'd be expecting a voltage drop in > the millivolts at the low currents we're talking about - not 0.6V. Can > someone set me straight here? Assuming you are driving the gate high enough, the FET should appear as a low value resistor, so any voltage drop will be directly related to the current going through it. Assuming a very low current the voltage drop will be very small. The engineer was probably thinking BJTs. I wouldn't bother debating, get a spice program and show him what the situation would really be. TTYL

Thanks guys. I can still claim not to have lost my mind then :) Yes, I think he was thinking bipolar initially and then was probably too embarrassed to admit his mistake. Cheers, Zik On 3/29/07, Herbert Graf <.....mailinglist3KILLspam@spam@farcite.net> wrote: {Quote hidden}> On Thu, 2007-03-29 at 12:03 +1000, Zik Saleeba wrote: > > I was having a conversation with the chief design engineer at my place > > of work yesterday. He was saying he'd have to install an extra 3.3V > > voltage regulator in our design because I'd just discovered that we > > needed a second switched 3.3V supply. I suggested we used a high-side > > FET switch instead - a suggestion he dismissed by saying the voltage > > drop across the FET was around 0.6V which would put the supply well > > out of spec. > > > > This man is a chief hardware designer of 40 years experience. I'm just > > a computer scientist who does electronics for a hobby. Am I crazy or > > is he completely wrong on this one? I'd be expecting a voltage drop in > > the millivolts at the low currents we're talking about - not 0.6V. Can > > someone set me straight here? > > Assuming you are driving the gate high enough, the FET should appear as > a low value resistor, so any voltage drop will be directly related to > the current going through it. Assuming a very low current the voltage > drop will be very small. > > The engineer was probably thinking BJTs. I wouldn't bother debating, get > a spice program and show him what the situation would really be. TTYL > > -

Pop quiz: which of the 7 deadly sins is your chief hooo-ha guilty of? Please don't have him work for me. Any good engineer knows he/she will make mistakes. How the unavoidable errors are handled and risk managed are as important as creative design abilities. A cheap PFET will do it easily, assuming we aren't talking amps here. Alternatively 3.3V regulators with enable pins are common and also pretty cheap. I've done it both ways. J Zik Saleeba wrote: > Yes, I think he was thinking bipolar initially and then was probably > too embarrassed to admit his mistake. > > > On 3/29/07, Herbert Graf <mailinglist3KILLspamfarcite.net> wrote: >> On Thu, 2007-03-29 at 12:03 +1000, Zik Saleeba wrote: >>> >>> This man is a chief hardware designer of 40 years experience. I'm just >>> a computer scientist who does electronics for a hobby. Am I crazy or

Which of the deadly sins is he guilty of? You know I got as far as considering deadly sin #1 - lust - and then I felt unaccountably ill. Workplace safety announcement: considering your workmates engaging in lust is not good for productivity. And it does your head in. Cheers, Zik On 3/29/07, Jesse Lackey <.....jsl-mlKILLspam.....celestialaudio.com> wrote: {Quote hidden}> Pop quiz: which of the 7 deadly sins is your chief hooo-ha guilty of? > > Please don't have him work for me. Any good engineer knows he/she will > make mistakes. How the unavoidable errors are handled and risk managed > are as important as creative design abilities. > > A cheap PFET will do it easily, assuming we aren't talking amps here. > Alternatively 3.3V regulators with enable pins are common and also > pretty cheap. I've done it both ways. > > J > > Zik Saleeba wrote: > > Yes, I think he was thinking bipolar initially and then was probably > > too embarrassed to admit his mistake. > > > > > > On 3/29/07, Herbert Graf <EraseMEmailinglist3spam_OUTTakeThisOuTfarcite.net> wrote: > >> On Thu, 2007-03-29 at 12:03 +1000, Zik Saleeba wrote: > >>> > >>> This man is a chief hardware designer of 40 years experience. I'm just > >>> a computer scientist who does electronics for a hobby. Am I crazy or > > -

> Yes, I think he was thinking bipolar initially and then was probably > too embarrassed to admit his mistake. It's not true for bipolar either. Vbe is about 0.6V but at low currents Vce saturation can be under 0.1V with ease. In fact, Vce can be under 10 mV if it must be make it so'd. This may require Ic to be >> Ic, but sometimes this is acceptable :-) ). eg I have used a bipolar in preference to a FET to switch an input voltage from a lead acid battery to a divider to an ADC input with saturation voltage of around 10 mV and base drive much larger than the small divider current. In that case the voltage drop matters and the styill small base current is irrelevant. Russell.

> I'd > be expecting a voltage drop in the millivolts at the low > currents we're talking about - not 0.6V. Can someone set me > straight here? How about setting us straight first - what current are you talking about? Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products docent Hogeschool van Utrecht: http://www.voti.nl/hvu

Always the detail man Wouter! I checked the docs - looks like it's 57mA. Cheers, Zik On 3/29/07, wouter van ooijen <wouterspam_OUTvoti.nl> wrote: {Quote hidden}> > I'd > > be expecting a voltage drop in the millivolts at the low > > currents we're talking about - not 0.6V. Can someone set me > > straight here? > > How about setting us straight first - what current are you talking > about? > > Wouter van Ooijen > > -- ------------------------------------------- > Van Ooijen Technische Informatica: http://www.voti.nl > consultancy, development, PICmicro products > docent Hogeschool van Utrecht: http://www.voti.nl/hvu > > > > -

> Always the detail man Wouter! It never hurts to verify. In some contexts 2A would be considered a small current... > I checked the docs - looks like it's 57mA. I recall you were switching 3.3V - so take care which FET you take ('default' FETs require ~ 8V to switch fully on) , but I think 1 Ohm RDSon (57 mV drop) should be no problem. Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products docent Hogeschool van Utrecht: http://www.voti.nl/hvu

>> Always the detail man Wouter! > >It never hurts to verify. In some contexts 2A would be considered >a small current... > >> I checked the docs - looks like it's 57mA. > >I recall you were switching 3.3V - so take care which FET you take >('default' FETs require ~ 8V to switch fully on) , but I think 1 Ohm >RDSon (57 mV drop) should be no problem. On the other hand selecting something from this table might be advantageous ... http://www.zetex.com/3.0/3-3-2b.asp?rid=21 especially the three in the lower block which have Vgs=1.8V figures.

Zik Saleeba wrote: >I was having a conversation with the chief design engineer at my place >of work yesterday. He was saying he'd have to install an extra 3.3V >voltage regulator in our design because I'd just discovered that we >needed a second switched 3.3V supply. I suggested we used a high-side >FET switch instead - a suggestion he dismissed by saying the voltage >drop across the FET was around 0.6V which would put the supply well >out of spec. > >This man is a chief hardware designer of 40 years experience. I'm just >a computer scientist who does electronics for a hobby. Am I crazy or >is he completely wrong on this one? I'd be expecting a voltage drop in >the millivolts at the low currents we're talking about - not 0.6V. Can >someone set me straight here? > >Cheers, >Zik > > When I think high side switch, I choose P-Channel devices. (Pulling the gate to ground to turn it on.) A random SOT-23 device with logic level and 1 amp. NDS352AP Do the math, the on resistance is less than an ohm, times the current, (60 ma.). Tom -- * | __O Thomas C. Sefranek @spam@WA1RHPKILLspamARRL.net |_-\<,_ Amateur Radio Operator: WA1RHP (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz hamradio.cmcorp.com/inventory/Inventory.html http://www.harvardrepeater.org

>2n7000@5v,50ma is 1.7Ohm Typical. The zetex ones I posted the link to have 0.075 ohm at 1.8V drive, 0.055 ohm at 2.5V drive and over 4A capable. Others at the top of the table give 0.04 ohm at 2.5V drive, and 0.02 ohm at 4.5V drive and 10A capable. All are 20V BVdss. >57ma through 1.7ohms produces a voltage drop of 0.0969V. 0.075 ohm gives 4.2mV 0.055 ohm gives 3.1mV 0.040 ohm gives 2.28mV 0.020 ohm gives 1.14mV

I saw these claims of unbelievable current and wondered if a FET in a TO220 case could really handle 80amps. I found this artical and found it useful for understanding that question. I think it is a good read if you have time. www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html FJ ======================================================= From: "Derward" <KILLspamwdmyrickKILLspamearthlink.net> Look at this one http://www.st.com/stonline/books/ascii/docs/11028.htmk . I have used this one(STP140NF55) and it has .008 ohms and will take 80 amps. They are in a 220 package. I buy them from Mouser at less than $2.00. Derward Myrick {Original Message removed}

I know it is overkill, but it does not cost much and very low on resistance. I use them in a product and they work great. Derward ----- Original Message ----- From: "Alan B. Pearce" <RemoveMEA.B.PearceTakeThisOuTrl.ac.uk> To: "Microcontroller discussion list - Public." <spamBeGonepiclistspamBeGonemit.edu> Sent: Thursday, March 29, 2007 9:07 AM Subject: Re: [EE] FET switch voltage drop > >will take 80 amps. > > Seems a bit of overkill for 57mA ... ;) > --

I agree with you, but I only use them at 2 amps. I was just quoting their specs. Derward ----- Original Message ----- From: "fred jones" <TakeThisOuTboattowEraseMEspam_OUThotmail.com> To: <RemoveMEpiclistTakeThisOuTmit.edu> Sent: Thursday, March 29, 2007 9:02 AM Subject: Re: [EE] FET switch voltage drop {Quote hidden}>I saw these claims of unbelievable current and wondered if a FET in a TO220 > case could really handle 80amps. I found this artical and found it useful > for understanding that question. I think it is a good read if you have > time. > > www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html > FJ > ======================================================= > From: "Derward" <wdmyrickEraseME.....earthlink.net> > > Look at this one http://www.st.com/stonline/books/ascii/docs/11028.htmk . > I have used this one(STP140NF55) and it has .008 ohms and will take 80 > amps. > They are in a 220 package. > I buy them from Mouser at less than $2.00. > > Derward Myrick > > > ----- Original Message ----- > From: "Alan B. Pearce" <EraseMEA.B.Pearcerl.ac.uk> > To: "Microcontroller discussion list - Public." <RemoveMEpiclistEraseMEEraseMEmit.edu> > Sent: Thursday, March 29, 2007 6:41 AM > Subject: Re: [EE] FET switch voltage drop > > >> >2n7000@5v,50ma is 1.7Ohm Typical. >> >>The zetex ones I posted the link to have 0.075 ohm at 1.8V drive, 0.055 >>ohm >>at 2.5V drive and over 4A capable. Others at the top of the table give >>0.04 >>ohm at 2.5V drive, and 0.02 ohm at 4.5V drive and 10A capable. All are 20V >>BVdss. >> >>>57ma through 1.7ohms produces a voltage drop of 0.0969V. >> >>0.075 ohm gives 4.2mV >>0.055 ohm gives 3.1mV >>0.040 ohm gives 2.28mV >>0.020 ohm gives 1.14mV >> >>-

>I saw these claims of unbelievable current and wondered if a >FET in a TO220 case could really handle 80amps. I found this >artical and found it useful for understanding that question. >I think it is a good read if you have time. ... Certainly a good read. I have been using some chips where the current rating is specifically defined as "package limited" and have versions of the chip in 2 packages, one TO-39, and the other SMD0.5, with the latter having a significantly higher current rating. The distributor assures me they have the same die inside. For many of these modern power switching FET chips, the current limitation seems to be the package lead resistance.

Here is a part I've used in the past, pretty cheap too. Depending on how much current he needs at 3.3V, a LDO could be as cheap but then add the caps in.... MIC94062 logic level drive, internal pup (I think..?) and is rated up to 2A --------------------------------- 8:00? 8:25? 8:40? Find a flick in no time with theYahoo! Search movie showtime shortcut.

At 12:53 PM 3/30/2007, Rikard Bosnjakovic wrote: >On 3/29/07, fred jones <EraseMEboattowspamspamBeGonehotmail.com> wrote: > > > http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html > >Argh. Why would anyone writing a technical article use the font Comic Sans? I dunno what the problem is. I find it easy to read, easy on the eyes. In other words, I thought it looked just fine. What are your objections to that specific font? dwayne -- Dwayne Reid <RemoveMEdwaynerKILLspamplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 22 years of Engineering Innovation (1984 - 2006) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email.

On Thu, 05 Apr 2007 11:41:57 -0600, Dwayne Reid wrote: >At 12:53 PM 3/30/2007, Rikard Bosnjakovic wrote: >>On 3/29/07, fred jones wrote: >> >> > www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html >> >>Argh. Why would anyone writing a technical article use the font Comic Sans? > >I dunno what the problem is. I find it easy to read, easy on the eyes. > >In other words, I thought it looked just fine. > >What are your objections to that specific font? > I find it to be a very readable font as well, especially for reading on the web. Most of the objections to it that I've heard seem to be relate more to the name of the font rather its appearance. Unfortunately, the use of the word "comic" implies a notion of non-professionalism to many people. Regards, Bob

Bob Barr <bbarr <at> california.com> writes: > I find it to be a very readable font as well, especially for reading > on the web. Imho if one reads a lot (like 10+ hours a day every day), one becomes very picky about fonts, to the point of using the browser's style switch to kill any funny fonts and substitute one's preferred one whenever possible. The Comic font is ok but it has just a little too much 'decoration' for easy quick reading. Most people who need to read a lot strongly prefer something like Arial Sans or Schumacher Clean or Adobe Helvetica. All those little extra decorations add up to the 'workload' and eventually imho one tires faster because of them. They are ok for titles or inscriptions on diagrams or advrtising, but for content, something simple and smooth is best. just an opinion from someone who reads a little more than most on the www, Peter P.

Bob Barr wrote: >>> > http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html >>> Argh. Why would anyone writing a technical article use the font Comic >>> Sans? >> What are your objections to that specific font? > Most of the objections to it that I've heard seem to be relate more to > the name of the font rather its appearance. And probably to the fact that it's a standard Windows font :) Gerhard