On Wed, 27 Jan 2016 07:23:41 -0800, Winfield Hill wrote:> Winfield Hill wrote... >> >> Q. Relative power dissipation capability of TO-92 transistor packages >> (viz, Zetex E-line parts) vs. sot-23 packages. More on this in my next >> post, first some background. > [ snip ] > > I mentioned before my worry about replacing a set of TO-92 transistors > with sot-23 parts. > Not only do the sot-23 parts usually have a lower Pdiss rating, they > dissipate their heat into the PCB (unlike TO-92 into air**), so a set > of them in close proximity would raise the PCB ambient, further > reducing their real power-dissipation handling capability. > > Specifically I've been using Zetex' ZTX450 and '550 E-line > complementary transistors, which are rated at 1-watt for Tamb=25°C. > Their corresponding sot-23 types might be FMMT619, '624, or '722, all > rated at 625mW. > > The datasheets have a note: For a device surface mounted on 25mm X 25mm > FR4 PCB with high coverage of single sided 1 oz copper. > > Can anyone give me some guidance on further derating for closely-spaced > sot-23 parts? > > BTW, going to packages like sot-223 could mean an even larger PCB > footprint. > > ** I know that TO-92 transistors dissipate some heat into the PCB, so > I'm limiting their dissipation to under 500mW.Hmm. A SOT-23 with a big collector tab would be nice here. Look at their SOT-89 parts. They list them as 1W dissipation, and 18.3 C/ W junction to leads -- that's about 1/10th the thermal resistance to copper as their SOT-23 parts, and a SOT-89 shouldn't take up much more space than a TO-92. My earlier comment about getting heat out of the circuit board itself still applies. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
TO-92 vs sot-23 transistors for power
Started by ●January 27, 2016
Reply by ●January 27, 20162016-01-27
Reply by ●January 27, 20162016-01-27
Jan Panteltje wrote...> >On a sunny day (27 Jan 2016 08:19:21 -0800) it happened Winfield Hill ><hill@rowland.harvard.edu> wrote in <n8aqm901d2f@drn.newsguy.com>: > >>George Herold wrote... >>> >>>> One of my design interests is high-frequency, >>>> high-voltage power amplifiers. For example, >>>> I'm doing a 45V, 5A peak, DC-10MHz amplifier. >>>> My design uses 20 TO-220F power transistors, >>>> mounted on a 200W fan-cooled heat-sink plate. >>>> They're mounted under the PCB in this photo. >>>> https://www.dropbox.com/s/k4k14zkaq0ewxa9/AMP-70A-2_proto_01.jpg >>>> https://www.dropbox.com/s/u6di0gb4xgvj3xv/AMP-70A_3D.jpg >> >>> I assume the amp is class A? >> >> Yes. Here's a schematic of a slowish version. >> https://www.dropbox.com/s/xwy4l5audcozylw/AMP-70A-2_sch_.pdf > > Strange, seems to be 2 completely identical amplifiers > driven by 1 opamp, but one negative and the other > positive, each a carbon copy of half the old audio amp.The push-pull configuration yields twice the slew rate at a given quiescent-current level. I'm ultimately aiming for about 3kV/us slewing. fmax full-swing = S/2pi A. If A (amplitude) = 40V, then 3kV/us yields full-power to 12 MHz.> Could that not be done like the old audio amp a > lot simpler? I remember doing a spice simulation > for something like that to 1 MHz or was it more, > just like an audio amp. > > Or is there a reason for all the > complicatiatitiated stuff?Some is to keep the TO-92 dissipation under control, other stuff is for thermal matching. In the next pass, add protective circuitry. -- Thanks, - Win
Reply by ●January 27, 20162016-01-27
On Wed, 27 Jan 2016 07:05:21 -0800, Winfield Hill wrote:> Q. Relative power dissipation capability > of TO-92 transistor packages (viz, Zetex E-line parts) vs. sot-23 > packages. More on this in my next post, first some background. > > One of my design interests is high-frequency, > high-voltage power amplifiers. For example, > I'm doing a 45V, 5A peak, DC-10MHz amplifier. > My design uses 20 TO-220F power transistors, > mounted on a 200W fan-cooled heat-sink plate. > They're mounted under the PCB in this photo. > https://www.dropbox.com/s/k4k14zkaq0ewxa9/AMP-70A-2_proto_01.jpg > https://www.dropbox.com/s/u6di0gb4xgvj3xv/AMP-70A_3D.jpg > > The input-stage driving circuitry uses 10W, > a significant part of which is dissipated by ten TO-92 E-line > transistors, lower left. > > Many of the parts are large: TO-220 trannies, > low-inductance power resistors, electrolytics, > converter modules, etc., and not well suited to surface-mount versions. > So I decided to forego most SMT parts on the layout, but I'm > rethinking that decision for my next pass. > > However I'm worried about the viability of crowding a bunch of > power-dissipating SOT-23 and 1210 parts together. They transfer heat > to the PCB, unlike through-hole parts, which are better at getting > their heat into the air. > So I'm worried about a hotspot and a reduced dissipation capability > from datasheet values. > > Please reply here if you want to discuss the amplifier, otherwise > advance to my next post to discuss the TO-92 vs SOT-23 issues.It appears from the labels on the I/O that this is a one-channel amplifier -- is that true? Are you then using 5 each input transistors in push-pull? Why not two bigger ones? -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●January 27, 20162016-01-27
Tim Wescott wrote...> >On Wed, 27 Jan 2016 07:05:21 -0800, Winfield Hill wrote: > >> Q. Relative power dissipation capability >> of TO-92 transistor packages (viz, Zetex >> E-line parts) vs. sot-23 packages. More >> on this in my next post, first some background. >> >> One of my design interests is high-frequency, >> high-voltage power amplifiers. For example, >> I'm doing a 45V, 5A peak, DC-10MHz amplifier. >> My design uses 20 TO-220F power transistors, >> mounted on a 200W fan-cooled heat-sink plate. >> They're mounted under the PCB in this photo. >> https://www.dropbox.com/s/k4k14zkaq0ewxa9/AMP-70A-2_proto_01.jpg >> https://www.dropbox.com/s/u6di0gb4xgvj3xv/AMP-70A_3D.jpg >> >> The input-stage driving circuitry uses 10W, >> a significant part of which is dissipated >> by ten TO-92 E-line transistors, lower left. >> [ snip ]> It appears from the labels on the I/O that this > is a one-channel amplifier -- is that true?Yes, of course.> Are you then using 5 each input transistors > in push-pull? Why not two bigger ones?After the complementary input EF, there are two each common-emitter transistor pairs, biased up with current sources + resistors. A pair is used to lower the dissipation, so the Vbe's won't thermally get out of hand. I haven't been able to find slightly larger parts, e.g., 2x or 3x larger. We're always looking to keep the Cob / Pd ratio low. -- Thanks, - Win
Reply by ●January 27, 20162016-01-27
Winfield Hill wrote:> I mentioned before my worry about replacing > a set of TO-92 transistors with sot-23 parts.What you didn't mention, and what so far nobody asked, is why you want to go SMD at all. I don't see potential for a significant reduction of PCB footprint if that's what you're after. robert
Reply by ●January 27, 20162016-01-27
On 27 Jan 2016 21:13:22 GMT, Robert Latest <boblatest@yahoo.com> wrote:>Winfield Hill wrote: > >> I mentioned before my worry about replacing >> a set of TO-92 transistors with sot-23 parts. > >What you didn't mention, and what so far nobody asked, is why you want to go >SMD at all. I don't see potential for a significant reduction of PCB >footprint if that's what you're after. > >robertIt's usually done to allow using a pick-and-place/reflow process for assembly. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●January 27, 20162016-01-27
On Wed, 27 Jan 2016 13:18:06 -0600, Tim Wescott <seemywebsite@myfooter.really> wrote:>On Wed, 27 Jan 2016 07:23:41 -0800, Winfield Hill wrote: > >> Winfield Hill wrote... >>> >>> Q. Relative power dissipation capability of TO-92 transistor packages >>> (viz, Zetex E-line parts) vs. sot-23 packages. More on this in my next >>> post, first some background. >> [ snip ] >> >> I mentioned before my worry about replacing a set of TO-92 transistors >> with sot-23 parts. >> Not only do the sot-23 parts usually have a lower Pdiss rating, they >> dissipate their heat into the PCB (unlike TO-92 into air**), so a set >> of them in close proximity would raise the PCB ambient, further >> reducing their real power-dissipation handling capability. >> >> Specifically I've been using Zetex' ZTX450 and '550 E-line >> complementary transistors, which are rated at 1-watt for Tamb=25�C. >> Their corresponding sot-23 types might be FMMT619, '624, or '722, all >> rated at 625mW. >> >> The datasheets have a note: For a device surface mounted on 25mm X 25mm >> FR4 PCB with high coverage of single sided 1 oz copper. >> >> Can anyone give me some guidance on further derating for closely-spaced >> sot-23 parts? >> >> BTW, going to packages like sot-223 could mean an even larger PCB >> footprint. >> >> ** I know that TO-92 transistors dissipate some heat into the PCB, so >> I'm limiting their dissipation to under 500mW. > >Hmm. A SOT-23 with a big collector tab would be nice here. > >Look at their SOT-89 parts. They list them as 1W dissipation, and 18.3 C/ >W junction to leads -- that's about 1/10th the thermal resistance to >copper as their SOT-23 parts, and a SOT-89 shouldn't take up much more >space than a TO-92. > >My earlier comment about getting heat out of the circuit board itself >still applies.I decided that I could dissipate 3 watts from one of those Supertex SOT89 depletion fets. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●January 27, 20162016-01-27
John Larkin wrote...> Tim Wescott wrote: > >> Hmm. A SOT-23 with a big collector tab would be nice here. >> >> Look at their SOT-89 parts. ... > > I decided that I could dissipate 3 watts from > one of those Supertex SOT89 depletion fets.FETs, yes. Some sot-23 MOSFETs do well, rated over 1 watt. -- Thanks, - Win
Reply by ●January 27, 20162016-01-27
John Larkin wrote...> Robert Latest wrote: >> Winfield Hill wrote: >> >>> I mentioned before my worry about replacing >>> a set of TO-92 transistors with sot-23 parts. >> >> What you didn't mention, and what so far >> nobody asked, is why you want to go SMD >> at all. I don't see potential for a >> significant reduction of PCB footprint >> if that's what you're after.SMD resistors take far less space than 1/4-watt axial-lead parts. But mixing 0805 in among TO-92 makes hand assembly a pain. Solder the 0805s first, yes. But when coming back to change the value, it's hard to get to with the iron tip. -- Thanks, - Win
Reply by ●January 27, 20162016-01-27
On 27 Jan 2016 13:38:34 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>John Larkin wrote... >> Tim Wescott wrote: >> >>> Hmm. A SOT-23 with a big collector tab would be nice here. >>> >>> Look at their SOT-89 parts. ... >> >> I decided that I could dissipate 3 watts from >> one of those Supertex SOT89 depletion fets. > > FETs, yes. Some sot-23 MOSFETs do well, > rated over 1 watt.Bipolar chips are too damned small. Which is why c is low. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
