On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote:> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: > > > Hello, > > > > I've discovered logic-level mosfets, and am using an Arduino to drive a > > small 12VDC (about an amp or so) pump motor with PWM, using an IRLZ34N > > as the power mosfet. > > > > Would a mosfet-driver be recommended to switch that mosfet on and off > > more quickly? Or is a logic-level mosfet already designed to > > efficiently turn on and off with the weak logic level signal? > > Given that you are also asking about BJT vs. FET, note that in order to > switch at it's fastest, a BJT needs a base driver that can push charge in > quickly and actively pull it out quickly. The difference in recovery > from saturation when you pull the base below ground vs. when you just > pull it _to_ ground can be dramatic. > > And don't use a grotty old TIP31 if you want speed -- Zetex developed > some pretty impressive modern BJTs before they got bought by Diodes, > Inc., and turned into a cash cow. Those transistors are still sold, and > have some pretty impressive HFE_sat and VCE_sat numbers. > > -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.com > > I'm looking for work -- see my website!This is an interesting article. It does mention pulling the base to -1V. Is this the article you were referring to? https://www.diodes.com/assets/App-Note-Files/zetex/an14.pdf Also noteworthy: "This isn't to exclude MOSFET based designs (some IC vendors have specified MOS as this suits their technology) but in terms of equivalent on-resistance and silicon efficiency, the low voltage bipolar device has no equal. For example, the ZETEX ZTX849 E-Line (TO-92 compatible) transistor exhibits a RCE(sat) of 36mΩ. This can only be matched by a much larger (and expensive) MOSFET die, only available in TO-220, D-Pak, and similar larger packages." Michael
mosfet driver needed for logic-level mosfets?
Started by ●May 15, 2017
Reply by ●May 16, 20172017-05-16
Reply by ●May 16, 20172017-05-16
On Tue, 16 May 2017 11:56:55 -0700, mrdarrett wrote:> On Tuesday, May 16, 2017 at 11:01:05 AM UTC-7, Tim Wescott wrote: > > ~kapow~ > > >> >>> >Ok, thanks for the information. You've given me lots to think >> >>> >about. >> >>> > >> >>> >For higher-speed switching of BJTs, you recommend yanking the base >> >>> >to *below* ground, then? >> >>> >> >>> Yes. But not too far... ~3V maximum to avoid VBE breakdown. >> >> >> >> >> >>Oh cool. Thanks. >> >> >> >>Michael >> > >> > circuit is something called the Baker clamp >> > >> > https://en.wikipedia.org/wiki/Baker_clamp >> >> That's not at all what I was referring to. Zetex (or somebody) figured >> out that you could go ahead and let the transistor saturate, but still >> get significantly faster turn-off if you actively drew the carriers out >> of the base. >> >> -- >> >> Tim Wescott Wescott Design Services http://www.wescottdesign.com >> >> I'm looking for work -- see my website! > > > Is the app note somewhere in here? It's not exactly organized by Zetex, > but maybe you remember parts of the name of the app note..? > > https://www.diodes.com/design/support/application-notes/ > > It's too bad that companies don't always keep the old website when they > get bought out... > > MichaelNot there, but here: http://www.euedia.tuiasi.ro/lab_ep/ep_files/Lab_no_4_c1.pdf and here: http://nptel.ac.in/courses/108105066/PDF/L-3(DK)(PE)%20((EE)NPTEL).pdf and here: <https://electronics.stackexchange.com/questions/23349/how-do-i-make-the- turn-off-and-turn-on-time-equal-in-a-npn-transistor> which references this excerpt from a book: http://wps.prenhall.com/chet_paynter_introduct_6/0,5779,426359-,00.html This all came by searching on "speed up bjt switching with negative base drive" -- there are probably other fortuitous combinations if you wish to exercise your Google-Fu. -- www.wescottdesign.com
Reply by ●May 16, 20172017-05-16
On Tue, 16 May 2017 12:42:58 -0700, mrdarrett wrote:> On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote: >> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: >> >> > Hello, >> > >> > I've discovered logic-level mosfets, and am using an Arduino to drive >> > a small 12VDC (about an amp or so) pump motor with PWM, using an >> > IRLZ34N as the power mosfet. >> > >> > Would a mosfet-driver be recommended to switch that mosfet on and off >> > more quickly? Or is a logic-level mosfet already designed to >> > efficiently turn on and off with the weak logic level signal? >> >> Given that you are also asking about BJT vs. FET, note that in order to >> switch at it's fastest, a BJT needs a base driver that can push charge >> in quickly and actively pull it out quickly. The difference in >> recovery from saturation when you pull the base below ground vs. when >> you just pull it _to_ ground can be dramatic. >> >> And don't use a grotty old TIP31 if you want speed -- Zetex developed >> some pretty impressive modern BJTs before they got bought by Diodes, >> Inc., and turned into a cash cow. Those transistors are still sold, >> and have some pretty impressive HFE_sat and VCE_sat numbers. >> >> -- >> >> Tim Wescott Wescott Design Services http://www.wescottdesign.com >> >> I'm looking for work -- see my website! > > > This is an interesting article. It does mention pulling the base to > -1V. Is this the article you were referring to? > > https://www.diodes.com/assets/App-Note-Files/zetex/an14.pdf > > Also noteworthy: > > "This isn't to exclude MOSFET based designs (some IC vendors have > specified MOS as this suits their technology) but in terms of equivalent > on-resistance and silicon efficiency, the low voltage bipolar device has > no equal. For example, the ZETEX ZTX849 E-Line (TO-92 compatible) > transistor exhibits a RCE(sat) of 36mΩ. This can only be matched by a > much larger (and expensive) MOSFET die, only available in TO-220, D-Pak, > and similar larger packages." > > MichaelI think there was more than that, but dangit, I can't remember. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●May 16, 20172017-05-16
On Tue, 16 May 2017 13:00:57 -0500, Tim Wescott <seemywebsite@myfooter.really> wrote:>On Tue, 16 May 2017 05:01:34 -0400, default wrote: > >> On Mon, 15 May 2017 12:02:39 -0700 (PDT), mrdarrett@gmail.com wrote: >> >>>On Monday, May 15, 2017 at 11:58:39 AM UTC-7, Jim Thompson wrote: >>>> On Mon, 15 May 2017 11:33:07 -0700 (PDT), mrdarrett@gmail.com wrote: >>>> >>>> >On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote: >>>> >> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: >>>> >> >>>> >> > Hello, >>>> >> > >>>> >> > I've discovered logic-level mosfets, and am using an Arduino to >>>> >> > drive a small 12VDC (about an amp or so) pump motor with PWM, >>>> >> > using an IRLZ34N as the power mosfet. >>>> >> > >>>> >> > Would a mosfet-driver be recommended to switch that mosfet on and >>>> >> > off more quickly? Or is a logic-level mosfet already designed to >>>> >> > efficiently turn on and off with the weak logic level signal? >>>> >> >>>> >> Given that you are also asking about BJT vs. FET, note that in >>>> >> order to switch at it's fastest, a BJT needs a base driver that can >>>> >> push charge in quickly and actively pull it out quickly. The >>>> >> difference in recovery from saturation when you pull the base below >>>> >> ground vs. when you just pull it _to_ ground can be dramatic. >>>> >> >>>> >> And don't use a grotty old TIP31 if you want speed -- Zetex >>>> >> developed some pretty impressive modern BJTs before they got bought >>>> >> by Diodes, Inc., and turned into a cash cow. Those transistors are >>>> >> still sold, and have some pretty impressive HFE_sat and VCE_sat >>>> >> numbers. >>>> >> >>>> >> -- >>>> >> >>>> >> Tim Wescott Wescott Design Services http://www.wescottdesign.com >>>> >> >>>> >> I'm looking for work -- see my website! >>>> > >>>> > >>>> >Ok, thanks for the information. You've given me lots to think about. >>>> > >>>> >For higher-speed switching of BJTs, you recommend yanking the base to >>>> >*below* ground, then? >>>> >>>> Yes. But not too far... ~3V maximum to avoid VBE breakdown. >>> >>> >>>Oh cool. Thanks. >>> >>>Michael >> >> circuit is something called the Baker clamp >> >> https://en.wikipedia.org/wiki/Baker_clamp > >That's not at all what I was referring to. Zetex (or somebody) figured >out that you could go ahead and let the transistor saturate, but still >get significantly faster turn-off if you actively drew the carriers out >of the base.Fred Baker was doing that. We had these plasma panels that had a complex waveform and had to refresh the things at 15KHZ horizontal rate just like NTSC TV (back in the 70's) ~1/2 million pixals... X-Y diode steered addressing lines, I forget how fast the transistors were working but it was way beyond anything that this guy's PWM might encounter. One beauty of the panels (that we took advantage of) was that it didn't require memory, light up a pixal on the screen and it stayed there until you changed it, due to some magic in the power supply - some kind of stepped waveform. My memory isn't up to describing the whole system...
Reply by ●May 16, 20172017-05-16
On 5/16/2017 12:46 PM, Jim Thompson wrote:> On Tue, 16 May 2017 09:31:14 -0700 (PDT), mrdarrett@gmail.com wrote: > >> On Tuesday, May 16, 2017 at 2:01:36 AM UTC-7, default wrote: >>> On Mon, 15 May 2017 12:02:39 -0700 (PDT), mrdarrett@gmail.com wrote: >>> >>>> On Monday, May 15, 2017 at 11:58:39 AM UTC-7, Jim Thompson wrote: >>>>> On Mon, 15 May 2017 11:33:07 -0700 (PDT), mrdarrett@gmail.com wrote: >>>>> >>>>>> On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote: >>>>>>> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: >>>>>>> >>>>>>>> Hello, >>>>>>>> >>>>>>>> I've discovered logic-level mosfets, and am using an Arduino to drive a >>>>>>>> small 12VDC (about an amp or so) pump motor with PWM, using an IRLZ34N >>>>>>>> as the power mosfet. >>>>>>>> >>>>>>>> Would a mosfet-driver be recommended to switch that mosfet on and off >>>>>>>> more quickly? Or is a logic-level mosfet already designed to >>>>>>>> efficiently turn on and off with the weak logic level signal? >>>>>>> >>>>>>> Given that you are also asking about BJT vs. FET, note that in order to >>>>>>> switch at it's fastest, a BJT needs a base driver that can push charge in >>>>>>> quickly and actively pull it out quickly. The difference in recovery >>>>>>> from saturation when you pull the base below ground vs. when you just >>>>>>> pull it _to_ ground can be dramatic. >>>>>>> >>>>>>> And don't use a grotty old TIP31 if you want speed -- Zetex developed >>>>>>> some pretty impressive modern BJTs before they got bought by Diodes, >>>>>>> Inc., and turned into a cash cow. Those transistors are still sold, and >>>>>>> have some pretty impressive HFE_sat and VCE_sat numbers. >>>>>>> >>>>>>> -- >>>>>>> >>>>>>> Tim Wescott >>>>>>> Wescott Design Services >>>>>>> http://www.wescottdesign.com >>>>>>> >>>>>>> I'm looking for work -- see my website! >>>>>> >>>>>> >>>>>> Ok, thanks for the information. You've given me lots to think about. >>>>>> >>>>>> For higher-speed switching of BJTs, you recommend yanking the base to *below* ground, then? >>>>> >>>>> Yes. But not too far... ~3V maximum to avoid VBE breakdown. >>>> >>>> >>>> Oh cool. Thanks. >>>> >>>> Michael >>> >>> circuit is something called the Baker clamp >>> >>> https://en.wikipedia.org/wiki/Baker_clamp >> >> >> Impressive! Thanks! >> >> But by avoiding saturation, Vce,sat is not minimized, is it..? >> >> Michael > > A Baker clamp minimizes recovery time by preventing hard saturation > and charge storage... "Vsat" is now typically equal to Vbe. > > But, as shown by "default", it also prevents quickly sucking charge > out of the base... more complex arrangements are required.Or something simpler. Isn't that what the Schottky transistor does? It uses a single Schottky diode between the base and collector. Since the Schottky diode forward voltage is lower than the Vbe voltage it limits the collector voltage and keeps it out of saturation. But now there is no diode blocking the current path to turn off the base and drain the charge. -- Rick C
Reply by ●May 16, 20172017-05-16
On 5/16/2017 2:00 PM, Tim Wescott wrote:> On Tue, 16 May 2017 05:01:34 -0400, default wrote: > >> On Mon, 15 May 2017 12:02:39 -0700 (PDT), mrdarrett@gmail.com wrote: >> >>> On Monday, May 15, 2017 at 11:58:39 AM UTC-7, Jim Thompson wrote: >>>> On Mon, 15 May 2017 11:33:07 -0700 (PDT), mrdarrett@gmail.com wrote: >>>> >>>>> On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote: >>>>>> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: >>>>>> >>>>>>> Hello, >>>>>>> >>>>>>> I've discovered logic-level mosfets, and am using an Arduino to >>>>>>> drive a small 12VDC (about an amp or so) pump motor with PWM, >>>>>>> using an IRLZ34N as the power mosfet. >>>>>>> >>>>>>> Would a mosfet-driver be recommended to switch that mosfet on and >>>>>>> off more quickly? Or is a logic-level mosfet already designed to >>>>>>> efficiently turn on and off with the weak logic level signal? >>>>>> >>>>>> Given that you are also asking about BJT vs. FET, note that in >>>>>> order to switch at it's fastest, a BJT needs a base driver that can >>>>>> push charge in quickly and actively pull it out quickly. The >>>>>> difference in recovery from saturation when you pull the base below >>>>>> ground vs. when you just pull it _to_ ground can be dramatic. >>>>>> >>>>>> And don't use a grotty old TIP31 if you want speed -- Zetex >>>>>> developed some pretty impressive modern BJTs before they got bought >>>>>> by Diodes, Inc., and turned into a cash cow. Those transistors are >>>>>> still sold, and have some pretty impressive HFE_sat and VCE_sat >>>>>> numbers. >>>>>> >>>>>> -- >>>>>> >>>>>> Tim Wescott Wescott Design Services http://www.wescottdesign.com >>>>>> >>>>>> I'm looking for work -- see my website! >>>>> >>>>> >>>>> Ok, thanks for the information. You've given me lots to think about. >>>>> >>>>> For higher-speed switching of BJTs, you recommend yanking the base to >>>>> *below* ground, then? >>>> >>>> Yes. But not too far... ~3V maximum to avoid VBE breakdown. >>> >>> >>> Oh cool. Thanks. >>> >>> Michael >> >> circuit is something called the Baker clamp >> >> https://en.wikipedia.org/wiki/Baker_clamp > > That's not at all what I was referring to. Zetex (or somebody) figured > out that you could go ahead and let the transistor saturate, but still > get significantly faster turn-off if you actively drew the carriers out > of the base.That sounds similar to what they did in WWII radar sets where they drove the grid positive with respect to the cathode to drive more current to the plate. The grid would conduct significant current, but since they were generating pulses with a relatively low duty cycle the grid didn't melt. -- Rick C
Reply by ●May 16, 20172017-05-16
On Tue, 16 May 2017 21:27:43 -0400, rickman wrote:> On 5/16/2017 2:00 PM, Tim Wescott wrote: >> On Tue, 16 May 2017 05:01:34 -0400, default wrote: >> >>> On Mon, 15 May 2017 12:02:39 -0700 (PDT), mrdarrett@gmail.com wrote: >>> >>>> On Monday, May 15, 2017 at 11:58:39 AM UTC-7, Jim Thompson wrote: >>>>> On Mon, 15 May 2017 11:33:07 -0700 (PDT), mrdarrett@gmail.com wrote: >>>>> >>>>>> On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote: >>>>>>> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: >>>>>>> >>>>>>>> Hello, >>>>>>>> >>>>>>>> I've discovered logic-level mosfets, and am using an Arduino to >>>>>>>> drive a small 12VDC (about an amp or so) pump motor with PWM, >>>>>>>> using an IRLZ34N as the power mosfet. >>>>>>>> >>>>>>>> Would a mosfet-driver be recommended to switch that mosfet on and >>>>>>>> off more quickly? Or is a logic-level mosfet already designed to >>>>>>>> efficiently turn on and off with the weak logic level signal? >>>>>>> >>>>>>> Given that you are also asking about BJT vs. FET, note that in >>>>>>> order to switch at it's fastest, a BJT needs a base driver that >>>>>>> can push charge in quickly and actively pull it out quickly. The >>>>>>> difference in recovery from saturation when you pull the base >>>>>>> below ground vs. when you just pull it _to_ ground can be >>>>>>> dramatic. >>>>>>> >>>>>>> And don't use a grotty old TIP31 if you want speed -- Zetex >>>>>>> developed some pretty impressive modern BJTs before they got >>>>>>> bought by Diodes, Inc., and turned into a cash cow. Those >>>>>>> transistors are still sold, and have some pretty impressive >>>>>>> HFE_sat and VCE_sat numbers. >>>>>>> >>>>>>> -- >>>>>>> >>>>>>> Tim Wescott Wescott Design Services http://www.wescottdesign.com >>>>>>> >>>>>>> I'm looking for work -- see my website! >>>>>> >>>>>> >>>>>> Ok, thanks for the information. You've given me lots to think >>>>>> about. >>>>>> >>>>>> For higher-speed switching of BJTs, you recommend yanking the base >>>>>> to *below* ground, then? >>>>> >>>>> Yes. But not too far... ~3V maximum to avoid VBE breakdown. >>>> >>>> >>>> Oh cool. Thanks. >>>> >>>> Michael >>> >>> circuit is something called the Baker clamp >>> >>> https://en.wikipedia.org/wiki/Baker_clamp >> >> That's not at all what I was referring to. Zetex (or somebody) figured >> out that you could go ahead and let the transistor saturate, but still >> get significantly faster turn-off if you actively drew the carriers out >> of the base. > > That sounds similar to what they did in WWII radar sets where they drove > the grid positive with respect to the cathode to drive more current to > the plate. The grid would conduct significant current, but since they > were generating pulses with a relatively low duty cycle the grid didn't > melt.There are class-B amplifier tubes that are designed for that even with continuous-wave or audio service -- but they need to have grids that are capable of withstanding (and dissipating, I think) the extra heat. -- www.wescottdesign.com
Reply by ●May 17, 20172017-05-17
On Tue, 16 May 2017 12:42:58 -0700 (PDT), mrdarrett@gmail.com wrote:>On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote: >> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: >> >> > Hello, >> > >> > I've discovered logic-level mosfets, and am using an Arduino to drive a >> > small 12VDC (about an amp or so) pump motor with PWM, using an IRLZ34N >> > as the power mosfet. >> > >> > Would a mosfet-driver be recommended to switch that mosfet on and off >> > more quickly? Or is a logic-level mosfet already designed to >> > efficiently turn on and off with the weak logic level signal? >> >> Given that you are also asking about BJT vs. FET, note that in order to >> switch at it's fastest, a BJT needs a base driver that can push charge in >> quickly and actively pull it out quickly. The difference in recovery >> from saturation when you pull the base below ground vs. when you just >> pull it _to_ ground can be dramatic. >> >> And don't use a grotty old TIP31 if you want speed -- Zetex developed >> some pretty impressive modern BJTs before they got bought by Diodes, >> Inc., and turned into a cash cow. Those transistors are still sold, and >> have some pretty impressive HFE_sat and VCE_sat numbers. >> >> -- >> >> Tim Wescott >> Wescott Design Services >> http://www.wescottdesign.com >> >> I'm looking for work -- see my website! > > >This is an interesting article. It does mention pulling the base to -1V. Is this the article you were referring to? > >https://www.diodes.com/assets/App-Note-Files/zetex/an14.pdf > >Also noteworthy: > >"This isn't to exclude MOSFET based designs (some IC vendors have specified MOS as this suits their technology) but in terms of equivalent on-resistance and silicon efficiency, the low voltage bipolar device has no equal. For example, the ZETEX ZTX849 E-Line (TO-92 compatible) transistor exhibits a RCE(sat) of 36m?. This can only be matched by a much larger (and expensive) MOSFET die, only >available in TO-220, D-Pak, and similar larger packages." > >MichaelThat's kinda old. Actually, very old. There are some milliohm mosfets around now. https://www.digikey.com/product-detail/en/alpha-omega-semiconductor-inc/AO3416/785-1011-2-ND/1855783 -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●May 17, 20172017-05-17
On Tuesday, May 16, 2017 at 8:30:26 PM UTC-7, John Larkin wrote:> On Tue, 16 May 2017 12:42:58 -0700 (PDT), mrdarrett@gmail.com wrote: > > >On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote: > >> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: > >> > >> > Hello, > >> > > >> > I've discovered logic-level mosfets, and am using an Arduino to drive a > >> > small 12VDC (about an amp or so) pump motor with PWM, using an IRLZ34N > >> > as the power mosfet. > >> > > >> > Would a mosfet-driver be recommended to switch that mosfet on and off > >> > more quickly? Or is a logic-level mosfet already designed to > >> > efficiently turn on and off with the weak logic level signal? > >> > >> Given that you are also asking about BJT vs. FET, note that in order to > >> switch at it's fastest, a BJT needs a base driver that can push charge in > >> quickly and actively pull it out quickly. The difference in recovery > >> from saturation when you pull the base below ground vs. when you just > >> pull it _to_ ground can be dramatic. > >> > >> And don't use a grotty old TIP31 if you want speed -- Zetex developed > >> some pretty impressive modern BJTs before they got bought by Diodes, > >> Inc., and turned into a cash cow. Those transistors are still sold, and > >> have some pretty impressive HFE_sat and VCE_sat numbers. > >> > >> -- > >> > >> Tim Wescott > >> Wescott Design Services > >> http://www.wescottdesign.com > >> > >> I'm looking for work -- see my website! > > > > > >This is an interesting article. It does mention pulling the base to -1V. Is this the article you were referring to? > > > >https://www.diodes.com/assets/App-Note-Files/zetex/an14.pdf > > > >Also noteworthy: > > > >"This isn't to exclude MOSFET based designs (some IC vendors have specified MOS as this suits their technology) but in terms of equivalent on-resistance and silicon efficiency, the low voltage bipolar device has no equal. For example, the ZETEX ZTX849 E-Line (TO-92 compatible) transistor exhibits a RCE(sat) of 36m?. This can only be matched by a much larger (and expensive) MOSFET die, only > >available in TO-220, D-Pak, and similar larger packages." > > > >Michael > > That's kinda old. Actually, very old. There are some milliohm mosfets > around now. > > https://www.digikey.com/product-detail/en/alpha-omega-semiconductor-inc/AO3416/785-1011-2-ND/1855783 > > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronicsOh nice! From the datasheet, Vgs,th,max = 1.1V. This should work fine as a logic-level mosfet, then? http://aosmd.com/res/data_sheets/AO3416.pdf In general, for PWM at 1-2A, 12-18VDC, are mosfets preferred (lower resistance, cheaper) than BJTs? Thanks, Michael
Reply by ●May 17, 20172017-05-17
On Wed, 17 May 2017 09:40:57 -0700, mrdarrett wrote:> On Tuesday, May 16, 2017 at 8:30:26 PM UTC-7, John Larkin wrote: >> On Tue, 16 May 2017 12:42:58 -0700 (PDT), mrdarrett@gmail.com wrote: >> >> >On Monday, May 15, 2017 at 10:37:52 AM UTC-7, Tim Wescott wrote: >> >> On Mon, 15 May 2017 08:33:58 -0700, mrdarrett wrote: >> >> >> >> > Hello, >> >> > >> >> > I've discovered logic-level mosfets, and am using an Arduino to >> >> > drive a small 12VDC (about an amp or so) pump motor with PWM, >> >> > using an IRLZ34N as the power mosfet. >> >> > >> >> > Would a mosfet-driver be recommended to switch that mosfet on and >> >> > off more quickly? Or is a logic-level mosfet already designed to >> >> > efficiently turn on and off with the weak logic level signal? >> >> >> >> Given that you are also asking about BJT vs. FET, note that in order >> >> to switch at it's fastest, a BJT needs a base driver that can push >> >> charge in quickly and actively pull it out quickly. The difference >> >> in recovery from saturation when you pull the base below ground vs. >> >> when you just pull it _to_ ground can be dramatic. >> >> >> >> And don't use a grotty old TIP31 if you want speed -- Zetex >> >> developed some pretty impressive modern BJTs before they got bought >> >> by Diodes, Inc., and turned into a cash cow. Those transistors are >> >> still sold, and have some pretty impressive HFE_sat and VCE_sat >> >> numbers. >> >> >> >> -- >> >> >> >> Tim Wescott Wescott Design Services http://www.wescottdesign.com >> >> >> >> I'm looking for work -- see my website! >> > >> > >> >This is an interesting article. It does mention pulling the base to >> >-1V. Is this the article you were referring to? >> > >> >https://www.diodes.com/assets/App-Note-Files/zetex/an14.pdf >> > >> >Also noteworthy: >> > >> >"This isn't to exclude MOSFET based designs (some IC vendors have >> >specified MOS as this suits their technology) but in terms of >> >equivalent on-resistance and silicon efficiency, the low voltage >> >bipolar device has no equal. For example, the ZETEX ZTX849 E-Line >> >(TO-92 compatible) transistor exhibits a RCE(sat) of 36m?. This can >> >only be matched by a much larger (and expensive) MOSFET die, only >> >available in TO-220, D-Pak, and similar larger packages." >> > >> >Michael >> >> That's kinda old. Actually, very old. There are some milliohm mosfets >> around now. >> >> https://www.digikey.com/product-detail/en/alpha-omega-semiconductor-inc/AO3416/785-1011-2-ND/1855783>> >> >> >> -- >> >> John Larkin Highland Technology, Inc >> >> lunatic fringe electronics > > > Oh nice! > > From the datasheet, Vgs,th,max = 1.1V. This should work fine as a > logic-level mosfet, then? > > http://aosmd.com/res/data_sheets/AO3416.pdf > > In general, for PWM at 1-2A, 12-18VDC, are mosfets preferred (lower > resistance, cheaper) than BJTs?AFAIK FETs have pretty much taken over any space where BJTs were once useful as switching power devices. Once you get above the convenient operating voltage of FETs, you start seeing IGBTs and SCRs. There may be some fringe applications, and some older circuits that are still being shamelessly copied (photoflash units come to mind), but by and large I think that the BJT has had its day for this. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
