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. -- Thanks, - Win
TO-92 vs sot-23 transistors for power
Started by ●January 27, 2016
Reply by ●January 27, 20162016-01-27
On 27 Jan 2016 07:05:21 -0800, Winfield Hill <hill@rowland.harvard.edu> 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.jpgBlue boards work much better than green boards.> > 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.Are the outputs bips or mosfets? Or something exotic like GaN? -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 27, 20162016-01-27
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. -- Thanks, - Win
Reply by ●January 27, 20162016-01-27
John Larkin wrote...> > Blue boards work much better than green boards.Really?> Are the outputs bips or mosfets? Or something > exotic like GaN?They're low-capacitance video transistors, etc. -- Thanks, - Win
Reply by ●January 27, 20162016-01-27
On Wednesday, January 27, 2016 at 10:05:36 AM UTC-5, 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.jpgI spy a bunch of carbon comp resistors. (Can you say why?) I assume the amp is class A? George H.> 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. > > > -- > Thanks, > - Win
Reply by ●January 27, 20162016-01-27
On Wednesday, January 27, 2016 at 10:23:48 AM UTC-5, 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?Never did it... my first pass guess would be to take the board area you have divide that by the number of sot-23's and get an area per device... then scale the power down by the area ratio. George H.> > 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. > > > -- > Thanks, > - Win
Reply by ●January 27, 20162016-01-27
On 1/27/2016 10:23 AM, 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.I don't think the single fact of using SOT-23 devices crowded together means you will be able to dissipate less heat. When coupling into the air you need to consider how the air will be cooled. When coupling into the PCB the construction of the PCB is important as well as how the PCB will be cooled. Thermal impacts are a system design issue, not just a transistor package selection issue. Is this in an enclosure? Is the PCB cooled by the air alone or will there be significant conduction away from the PCB? Is this a single sided PCB, two sided, multi-layer? Can you add power planes to the board to conduct heat away from the hot parts? The board layout I see looks like it can easily support heat conduction away from SOT-23s depending on what the rest of the parts are doing. -- Rick
Reply by ●January 27, 20162016-01-27
On a sunny day (27 Jan 2016 07:05:21 -0800) it happened Winfield Hill <hill@rowland.harvard.edu> wrote in <n8ambh015ao@drn.newsguy.com>:> https://www.dropbox.com/s/k4k14zkaq0ewxa9/AMP-70A-2_proto_01.jpg > https://www.dropbox.com/s/u6di0gb4xgvj3xv/AMP-70A_3D.jpgIt is full of holes!!! No SMDs?
Reply by ●January 27, 20162016-01-27
Jan Panteltje wrote...> > Winfield Hill wrote: > >> https://www.dropbox.com/s/k4k14zkaq0ewxa9/AMP-70A-2_proto_01.jpg >> https://www.dropbox.com/s/u6di0gb4xgvj3xv/AMP-70A_3D.jpg > > It is full of holes!!!Yes, 20 big holes for the a screwdriver for the TO-220F screws, and more big holes for air flow for some of the power resistors.> No SMDs?There are some on the bottom. -- Thanks, - Win
Reply by ●January 27, 20162016-01-27
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 spy a bunch of carbon comp resistors. (Can you say why?)Low inductance and transient power-handling.> 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 -- Thanks, - Win
