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.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, > - WinWhat I've done with SO8 packages I didn't want spaced was use thermal insulating pads and heatsinks. Aavid makes these BGA heatsinks with springs to push down on the board. You then need to use the power drops of each transistor and the junction to package thermal resistances to work out hot the junction is going to be. You can also mount heatsinks on the copper clad underneath the board if you got the space.
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
On 27 Jan 2016 07:23:41 -0800, Winfield Hill <hill@rowland.harvard.edu> 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.The Zetex E-Line parts did not use conventional epoxy for their encapsulant, and derated based on 200degC junction temperature to achieve the 1W rating in free air. Rthjc 175degC/W is achieved through actual package material superior thermal conductivity. These parts can be profitably heatsunk. This is one reason why the generic substitutes in conventional epoxy have a lower free air listing, as derated based on 150degC Tj max at 200degC/W or ~625mW. The SOT23 or SOT223 bodies are similarly restricted to the same 150degC Tj max by the epoxy encapsulant. In an unimproved redesign, your first questions should be what junction temperatures are actually present in the current product, and what power dissipation and ambient temperatures are currently handled. A reduced Tj max means more area will be required for the same reliability margins, regardless of package size changes. RL
Reply by ●January 27, 20162016-01-27
On 27 Jan 2016 07:23:41 -0800, Winfield Hill <hill@rowland.harvard.edu> 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.Wait, that was supposed to be another thread! Do you have a ground plane? That, and power pours, spreads heat pretty well laterally. Vias from parts to power pours can radically reduce theta. A SOT89 takes about as much board area as a TO92 but can dissipate more power, a couple watts, and is easier for assembly.> > 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?1 oz copper (if you can really get it) is about 70 K/w per square, which can be used to very roughly eyeball heat spreading. We played with Sonnet Lite for a while and it looks like it can do 2.5D modeling of the electrical or thermal resistance of arbitrary shapes, but I'm doubtful that it's worth the effort, with Dremels so common.> > 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.TO92 types are good if you plan to have air flow (a fan) over the surface of the board. Not so good if you want the heat to flow through the leads into the board. (ROT: 150,000 K/w per ohm of copper) I sometimes add a gap-pad between the PCB and a baseplate or box. Heat from the surfmount parts goes into the board, gets spread by the planes, and is conducted by the pad to the base. https://dl.dropboxusercontent.com/u/53724080/Thermal/B-stack_2.JPG https://dl.dropboxusercontent.com/u/53724080/Thermal/Gap_Pad.jpg Aluminum or brass spacers can move heat to the baseplate, too. Or blocks of aluminum. -- John Larkin Highland Technology, Inc lunatic fringe electronics
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.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 looks like it might be difficult to align those bottomside TO220s and tighten the screws, especially if there are insulators and grease involved. We had a similar problem recently, and decided to use lots of D2PAKS on top, or cut rectangular holes in the board for TO247s so they could be easily screwed or clamped to the cold plate and "surface mount" (lap solder the leads) to the board. That job is still unresolved. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 27, 20162016-01-27
On 27 Jan 2016 08:19:21 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>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_.pdfWhat's the circle with JL inside? -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 27, 20162016-01-27
John Larkin 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 >> >> Yes. Here's a schematic of a slowish version. >> https://www.dropbox.com/s/xwy4l5audcozylw/AMP-70A-2_sch_.pdf > > What's the circle with JL inside?A John Larkin copyright notice? Dunno, where is it? -- Thanks, - Win
Reply by ●January 27, 20162016-01-27
Winfield Hill wrote...> 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.There are twelve 10-ohm 1/4 W carbon comps, for power handling, as mentioned. But the standout 27-ohm 5% resistors are actually TE Connectivity ROX3S 3-watt metal oxide film types, with very low self inductance.>> 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
Reply by ●January 27, 20162016-01-27
On 27 Jan 2016 09:25:23 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>John Larkin 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 >>> >>> Yes. Here's a schematic of a slowish version. >>> https://www.dropbox.com/s/xwy4l5audcozylw/AMP-70A-2_sch_.pdf >> >> What's the circle with JL inside? > > A John Larkin copyright notice? Dunno, where is it?I see it to the right of your Dropbox schematic. Maybe Dropbox is goading me to add a comment. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 27, 20162016-01-27
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 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_.pdfStrange, 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. 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?> >-- > Thanks, > - Win >
Reply by ●January 27, 20162016-01-27
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.Look at the "Thermal characteristics" table on page 2. The thermal resistance, junction to ambient, is 200 C/W with that inch-square copper, while the thermal resistance, junction to leads, is 194 C/W. So the bulk of the thermal resistance is junction to lead, with a tiny bit left over for the actual pad. Making the bone-headed -- and probably erroneous -- assumption that the copper pour is all at one temperature, ten devices on that same pad would have an aggregate thermal conductivity of 25.4 degrees C/W, with 19.4 C/W of that from chip to copper, and the same ol' 6 C/W from copper to ambient. I'm sure that there are people out there who are much better at this stuff than I am, but I would consider making the board with components only on the top side, putting lots of thermal vias to big copper pours on the back side, and gluing it to a heat sink (or heat spreader) with thermally conductive glue. You don't have to fly in the dark on this -- I know there's white papers out there that detail the methods to calculate the thermal resistance of vias, and -- if you don't want to use FEM calculators -- estimate the thermal resistance to ambient of copper pours to ambient. If your collectors have RF on them then you'll have all sorts of interesting capacitance-to-ground issues to deal with, though. I'm sure there's ways around that problem, too, possibly involving alumina or beryllium oxide. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
