On Thursday, August 6, 2020 at 9:50:24 PM UTC-4, Phil Hobbs wrote:> On 2020-08-06 19:01, Chris wrote: > > On 07/08/20 23:20, Rick C wrote: > > > >> I'm also concerned about the board layout. The board is not at all > >> dense but there are several > > > > parts that need copper area for heat sinking. They are using a linear > > regulator to drop 12+ volts > > > > to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So the 5V > > regulator gets quite warm, > > > > 70°C!!! > > > > You should be concerned. The whole thing about safety critical kit is > > that it should be conservatively designed, with all parts running well > > within their ratings. If the hw bod is happy with a regulator running > > at 70c, he must be clueless. Sorry, but it needs to be said. > > > > If you care about your professional career and any litigation > > issues, you should either have enough influence on the project > > direction to stop design errors like that, or just walk away from it... > > > > Chris > > Hmm, interesting. Most linear regulators I'm familiar with are rated to > 125 C junction temperature, and some higher. What safety or regulatory > issues have you run into that were caused by running some part 55C below > its rated temperature?That was the case temp via an IR gun. So the die was hotter. I would not approve a design with a part running so hot. I believe the failure rate goes up quite quickly with temp. The real point is this is because the layout was not correct using thermal breaks around the thermal pad!!! Easy enough to fix lowering the temp probably by 30 °C. Did you not read the original post about this? -- Rick C. +++ Get 1,000 miles of free Supercharging +++ Tesla referral code - https://ts.la/richard11209
Group Design
Started by ●July 7, 2020
Reply by ●August 6, 20202020-08-06
Reply by ●August 6, 20202020-08-06
On 2020-08-06 22:12, Rick C wrote:> On Thursday, August 6, 2020 at 9:50:24 PM UTC-4, Phil Hobbs wrote: >> On 2020-08-06 19:01, Chris wrote: >>> On 07/08/20 23:20, Rick C wrote: >>> >>>> I'm also concerned about the board layout. The board is not at all >>>> dense but there are several >>> >>> parts that need copper area for heat sinking. They are using a linear >>> regulator to drop 12+ volts >>> >>> to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So the 5V >>> regulator gets quite warm, >>> >>> 70°C!!! >>> >>> You should be concerned. The whole thing about safety critical kit is >>> that it should be conservatively designed, with all parts running well >>> within their ratings. If the hw bod is happy with a regulator running >>> at 70c, he must be clueless. Sorry, but it needs to be said. >>> >>> If you care about your professional career and any litigation >>> issues, you should either have enough influence on the project >>> direction to stop design errors like that, or just walk away from it... >>> >>> Chris >> >> Hmm, interesting. Most linear regulators I'm familiar with are rated to >> 125 C junction temperature, and some higher. What safety or regulatory >> issues have you run into that were caused by running some part 55C below >> its rated temperature? > > That was the case temp via an IR gun. So the die was hotter. I would not approve a design with a part running so hot. I believe the failure rate goes up quite quickly with temp. The real point is this is because the layout was not correct using thermal breaks around the thermal pad!!! Easy enough to fix lowering the temp probably by 30 °C. > > Did you not read the original post about this? >I've designed a fair bit of stuff to work in ambients higher than that, from remote satcom stuff to downhole instruments running at 175C. The amateur vent stuff is all March 2020 at this point, so I'm interested in the broader context. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●August 6, 20202020-08-06
On Thursday, August 6, 2020 at 10:27:27 PM UTC-4, Phil Hobbs wrote:> On 2020-08-06 22:12, Rick C wrote: > > On Thursday, August 6, 2020 at 9:50:24 PM UTC-4, Phil Hobbs wrote: > >> On 2020-08-06 19:01, Chris wrote: > >>> On 07/08/20 23:20, Rick C wrote: > >>> > >>>> I'm also concerned about the board layout. The board is not at all > >>>> dense but there are several > >>> > >>> parts that need copper area for heat sinking. They are using a linear > >>> regulator to drop 12+ volts > >>> > >>> to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So the 5V > >>> regulator gets quite warm, > >>> > >>> 70°C!!! > >>> > >>> You should be concerned. The whole thing about safety critical kit is > >>> that it should be conservatively designed, with all parts running well > >>> within their ratings. If the hw bod is happy with a regulator running > >>> at 70c, he must be clueless. Sorry, but it needs to be said. > >>> > >>> If you care about your professional career and any litigation > >>> issues, you should either have enough influence on the project > >>> direction to stop design errors like that, or just walk away from it... > >>> > >>> Chris > >> > >> Hmm, interesting. Most linear regulators I'm familiar with are rated to > >> 125 C junction temperature, and some higher. What safety or regulatory > >> issues have you run into that were caused by running some part 55C below > >> its rated temperature? > > > > That was the case temp via an IR gun. So the die was hotter. I would not approve a design with a part running so hot. I believe the failure rate goes up quite quickly with temp. The real point is this is because the layout was not correct using thermal breaks around the thermal pad!!! Easy enough to fix lowering the temp probably by 30 °C. > > > > Did you not read the original post about this? > > > > I've designed a fair bit of stuff to work in ambients higher than that, > from remote satcom stuff to downhole instruments running at 175C.So??? Higher temps means shorter life. Is that a false statement? There is also the issue of heating up things adjacent to the hot part and at 70 °C there is risk of burning someone not used to board getting that hot. In other words, there is no reason to have such an abortion for a design that it gets so hot. Why is this an issue for you? You are not making any sense at all.> The amateur vent stuff is all March 2020 at this point, so I'm > interested in the broader context.What broader context??? -- Rick C. ---- Get 1,000 miles of free Supercharging ---- Tesla referral code - https://ts.la/richard11209
Reply by ●August 7, 20202020-08-07
On 2020-08-06 22:36, Rick C wrote:> On Thursday, August 6, 2020 at 10:27:27 PM UTC-4, Phil Hobbs wrote: >> On 2020-08-06 22:12, Rick C wrote: >>> On Thursday, August 6, 2020 at 9:50:24 PM UTC-4, Phil Hobbs >>> wrote: >>>> On 2020-08-06 19:01, Chris wrote: >>>>> On 07/08/20 23:20, Rick C wrote: >>>>> >>>>>> I'm also concerned about the board layout. The board is >>>>>> not at all dense but there are several >>>>> >>>>> parts that need copper area for heat sinking. They are using >>>>> a linear regulator to drop 12+ volts >>>>> >>>>> to 5V and 3.3V. The back lights for the LCDs draw 300 mA! >>>>> So the 5V regulator gets quite warm, >>>>> >>>>> 70°C!!! >>>>> >>>>> You should be concerned. The whole thing about safety >>>>> critical kit is that it should be conservatively designed, >>>>> with all parts running well within their ratings. If the hw >>>>> bod is happy with a regulator running at 70c, he must be >>>>> clueless. Sorry, but it needs to be said. >>>>> >>>>> If you care about your professional career and any >>>>> litigation issues, you should either have enough influence on >>>>> the project direction to stop design errors like that, or >>>>> just walk away from it... >>>>> >>>>> Chris >>>> >>>> Hmm, interesting. Most linear regulators I'm familiar with are >>>> rated to 125 C junction temperature, and some higher. What >>>> safety or regulatory issues have you run into that were caused >>>> by running some part 55C below its rated temperature? >>> >>> That was the case temp via an IR gun. So the die was hotter. I >>> would not approve a design with a part running so hot. I believe >>> the failure rate goes up quite quickly with temp. The real point >>> is this is because the layout was not correct using thermal >>> breaks around the thermal pad!!! Easy enough to fix lowering the >>> temp probably by 30 °C. >>> >>> Did you not read the original post about this? >>> >> >> I've designed a fair bit of stuff to work in ambients higher than >> that, from remote satcom stuff to downhole instruments running at >> 175C. > > So??? Higher temps means shorter life. Is that a false statement? > > There is also the issue of heating up things adjacent to the hot part > and at 70 °C there is risk of burning someone not used to board > getting that hot. > > In other words, there is no reason to have such an abortion for a > design that it gets so hot. Why is this an issue for you? You are > not making any sense at all.Okay, so you've got zilch. No worries. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●August 7, 20202020-08-07
Rick C <gnuarm.deletethisbit@gmail.com> writes:> Yeah, we are nearing the end of this road.It's ok, a group of Afghan schoolgirls is on top of it and seems to be doing a more serious job: https://www.reuters.com/article/us-health-coronavirus-afghanistan-ventil/afghan-all-girls-robotics-team-designs-low-cost-ventilator-to-treat-coronavirus-patients-idUSKCN24L0WO
Reply by ●August 7, 20202020-08-07
On 08/07/20 02:50, Phil Hobbs wrote:> On 2020-08-06 19:01, Chris wrote: >> On 07/08/20 23:20, Rick C wrote: >> >>> I'm also concerned about the board layout. The board is not at all >>> dense but there are several >> >> parts that need copper area for heat sinking. They are using a linear >> regulator to drop 12+ volts >> >> to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So the 5V >> regulator gets quite warm, >> >> 70°C!!! >> >> You should be concerned. The whole thing about safety critical kit is >> that it should be conservatively designed, with all parts running well >> within their ratings. If the hw bod is happy with a regulator running >> at 70c, he must be clueless. Sorry, but it needs to be said. >> >> If you care about your professional career and any litigation >> issues, you should either have enough influence on the project >> direction to stop design errors like that, or just walk away from it... >> >> Chris > > Hmm, interesting. Most linear regulators I'm familiar with are rated to > 125 C junction temperature, and some higher. What safety or regulatory > issues have you run into that were caused by running some part 55C below > its rated temperature? > > Cheers > > Phil Hobbs >That's not the point and reliability is affected by temperature. It may not matter for a short lived down hole item where failure has no safety implications, but not good enough otherwise. The constant temperature cycling will affect wire bonds and reliability. Cheap tacky design, using a linear regulator to go from 12v to 5, then 3.3, considerable current, where a point of use switcher would be a far more efficient and probably more reliable solution as well Wasted power needs a bigger backup battery etc... Chris
Reply by ●August 7, 20202020-08-07
On 2020-08-07 07:30, Chris wrote:> On 08/07/20 02:50, Phil Hobbs wrote: >> On 2020-08-06 19:01, Chris wrote: >>> On 07/08/20 23:20, Rick C wrote: >>> >>>> I'm also concerned about the board layout. The board is not at >>>> all dense but there are several >>> >>> parts that need copper area for heat sinking. They are using a >>> linear regulator to drop 12+ volts >>> >>> to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So >>> the 5V regulator gets quite warm, >>> >>> 70°C!!! >>> >>> You should be concerned. The whole thing about safety critical >>> kit is that it should be conservatively designed, with all parts >>> running well within their ratings. If the hw bod is happy with a >>> regulator running at 70c, he must be clueless. Sorry, but it >>> needs to be said. >>> >>> If you care about your professional career and any litigation >>> issues, you should either have enough influence on the project >>> direction to stop design errors like that, or just walk away from >>> it... >>> >>> Chris >> >> Hmm, interesting. Most linear regulators I'm familiar with are >> rated to 125 C junction temperature, and some higher. What safety >> or regulatory issues have you run into that were caused by running >> some part 55C below its rated temperature? >> >> Cheers >> >> Phil Hobbs >> > > That's not the pointNot yours, I gather, but it is mine. I'm not being argumentative, I just want to know if there's good reliability data out there that I should know about. Opinions can be useful too, if they're at least based on detailed anecdotes.> and reliability is affected by temperature. It > may not matter for a short lived down hole item where failure has no > safety implications, but not good enough otherwise. The constant > temperature cycling will affect wire bonds and reliability.So we're told by MIL-HDBK-217, based on a famously flawed methodology (removing parts always improves reliability), invalid statistical assumptions (Arrhenius scaling of failure rates over large temperature ranges), and data from the 1960s iirc. Is there actual data to support that with modern components?> Cheap tacky design, using a linear regulator to go from 12v to 5, > then 3.3, considerable current, where a point of use switcher would > be a far more efficient and probably more reliable solution as well > Wasted power needs a bigger backup battery etc...I know it isn't what the cool kids do, but that doesn't make it unreliable or cause it to fail safety inspections AFAICT. Medical may be different, but it would pass UL with no issues. What regulations nwould you expect it to fall foul of? Cheers Phil Hobbs>-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●August 7, 20202020-08-07
On Friday, August 7, 2020 at 7:30:44 AM UTC-4, Chris wrote:> On 08/07/20 02:50, Phil Hobbs wrote: > > On 2020-08-06 19:01, Chris wrote: > >> On 07/08/20 23:20, Rick C wrote: > >> > >>> I'm also concerned about the board layout. The board is not at all > >>> dense but there are several > >> > >> parts that need copper area for heat sinking. They are using a linear > >> regulator to drop 12+ volts > >> > >> to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So the 5V > >> regulator gets quite warm, > >> > >> 70°C!!! > >> > >> You should be concerned. The whole thing about safety critical kit is > >> that it should be conservatively designed, with all parts running well > >> within their ratings. If the hw bod is happy with a regulator running > >> at 70c, he must be clueless. Sorry, but it needs to be said. > >> > >> If you care about your professional career and any litigation > >> issues, you should either have enough influence on the project > >> direction to stop design errors like that, or just walk away from it... > >> > >> Chris > > > > Hmm, interesting. Most linear regulators I'm familiar with are rated to > > 125 C junction temperature, and some higher. What safety or regulatory > > issues have you run into that were caused by running some part 55C below > > its rated temperature? > > > > Cheers > > > > Phil Hobbs > > > > That's not the point and reliability is affected by temperature. It may > not matter for a short lived down hole item where failure has no safety > implications, but not good enough otherwise. The constant temperature > cycling will affect wire bonds and reliability. Cheap tacky design, > using a linear regulator to go from 12v to 5, then 3.3, considerable > current, where a point of use switcher would be a far more efficient and > probably more reliable solution as well Wasted power needs a bigger > backup battery etc...Efficiency may be personally important to an engineer who wants to overdesign everything, but this device has a 2 amp motor running off the 15 volts and it is all line powered, so no need to squeeze every last joule out of the design. We are measuring some low level signals that could easily be disrupted by the switching currents. So adding that type of a noise source would only be done if actually required. Oddly enough most of the power used in the electronics other than the motor are in the LED back lights for the low power LCD displays. I have thought of using a switcher for them, but even at the 250 mA power level it's just not worth the hassle of dealing with the noise. Hmmm... maybe that is a bigger impact than anyone here is thinking. The display is on all the time. The motor only runs at high power about half the time. So it's really more like 1 amp. Now the ratio is more like 4:1. We don't have good data on the motor current, so no one has paid attention to the battery time. There are so many holes in this net. lol BTW, does anyone understand what Phil Hobbs is going on about??? Why does he talk in code? -- Rick C. ---+ Get 1,000 miles of free Supercharging ---+ Tesla referral code - https://ts.la/richard11209
Reply by ●August 7, 20202020-08-07
On 8/7/2020 14:30, Chris wrote:> On 08/07/20 02:50, Phil Hobbs wrote: >> On 2020-08-06 19:01, Chris wrote: >>> On 07/08/20 23:20, Rick C wrote: >>> >>>> I'm also concerned about the board layout. The board is not at all >>>> dense but there are several >>> >>> parts that need copper area for heat sinking. They are using a linear >>> regulator to drop 12+ volts >>> >>> to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So the 5V >>> regulator gets quite warm, >>> >>> 70°C!!! >>> >>> You should be concerned. The whole thing about safety critical kit is >>> that it should be conservatively designed, with all parts running well >>> within their ratings. If the hw bod is happy with a regulator running >>> at 70c, he must be clueless. Sorry, but it needs to be said. >>> >>> If you care about your professional career and any litigation >>> issues, you should either have enough influence on the project >>> direction to stop design errors like that, or just walk away from it... >>> >>> Chris >> >> Hmm, interesting. Most linear regulators I'm familiar with are rated to >> 125 C junction temperature, and some higher. What safety or regulatory >> issues have you run into that were caused by running some part 55C below >> its rated temperature? >> >> Cheers >> >> Phil Hobbs >> > > That's not the point and reliability is affected by temperature. It may > not matter for a short lived down hole item where failure has no safety > implications, but not good enough otherwise. The constant temperature > cycling will affect wire bonds and reliability. Cheap tacky design, > using a linear regulator to go from 12v to 5, then 3.3, considerable > current, where a point of use switcher would be a far more efficient and > probably more reliable solution as well Wasted power needs a bigger > backup battery etc... > > Chris > >A 7805 at 70C is not too hot, nothing like that. I'd be worried of any nearby electrolytic capacitors, I have seen these fail if put too close to some hot part. Once on a product of mine - though the board was "too hot" only for one cap of a particular series, other units have worked for well over 5 years 24/7. And once I repaired our TV set (some 15+ years ago, a CRT one), it had an electrolytic cap next to a "red" hot 7805. I have *never* seen a 78xx regulator fail because of overheating and I have designed in quite a few. One can have them run hot within reason, just keep in mind that they have temperature protection and if you overheat one it will protect itself by more or less cutting off the power at its output. Another thing to keep in mind is to not forget to put a diode in parallel if the output voltage is > say 5-6V; they do die if the input goes 0V or so with the output capacitor charged. Dimiter ====================================================== Dimiter Popoff, TGI http://www.tgi-sci.com ====================================================== http://www.flickr.com/photos/didi_tgi/
Reply by ●August 7, 20202020-08-07
On Friday, August 7, 2020 at 1:42:37 PM UTC-4, Dimiter wrote:> On 8/7/2020 14:30, Chris wrote: > > On 08/07/20 02:50, Phil Hobbs wrote: > >> On 2020-08-06 19:01, Chris wrote: > >>> On 07/08/20 23:20, Rick C wrote: > >>> > >>>> I'm also concerned about the board layout. The board is not at all > >>>> dense but there are several > >>> > >>> parts that need copper area for heat sinking. They are using a linear > >>> regulator to drop 12+ volts > >>> > >>> to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So the 5V > >>> regulator gets quite warm, > >>> > >>> 70°C!!! > >>> > >>> You should be concerned. The whole thing about safety critical kit is > >>> that it should be conservatively designed, with all parts running well > >>> within their ratings. If the hw bod is happy with a regulator running > >>> at 70c, he must be clueless. Sorry, but it needs to be said. > >>> > >>> If you care about your professional career and any litigation > >>> issues, you should either have enough influence on the project > >>> direction to stop design errors like that, or just walk away from it... > >>> > >>> Chris > >> > >> Hmm, interesting. Most linear regulators I'm familiar with are rated to > >> 125 C junction temperature, and some higher. What safety or regulatory > >> issues have you run into that were caused by running some part 55C below > >> its rated temperature? > >> > >> Cheers > >> > >> Phil Hobbs > >> > > > > That's not the point and reliability is affected by temperature. It may > > not matter for a short lived down hole item where failure has no safety > > implications, but not good enough otherwise. The constant temperature > > cycling will affect wire bonds and reliability. Cheap tacky design, > > using a linear regulator to go from 12v to 5, then 3.3, considerable > > current, where a point of use switcher would be a far more efficient and > > probably more reliable solution as well Wasted power needs a bigger > > backup battery etc... > > > > Chris > > > > > > A 7805 at 70C is not too hot, nothing like that. I'd be worried of any > nearby electrolytic capacitors, I have seen these fail if put too close > to some hot part. Once on a product of mine - though the board was "too > hot" only for one cap of a particular series, other units have worked > for well over 5 years 24/7. And once I repaired our TV set (some 15+ > years ago, a CRT one), it had an electrolytic cap next to a "red" hot > 7805. > I have *never* seen a 78xx regulator fail because of overheating > and I have designed in quite a few. One can have them run hot > within reason, just keep in mind that they have temperature protection > and if you overheat one it will protect itself by more or less > cutting off the power at its output. > Another thing to keep in mind is to not forget to put a diode > in parallel if the output voltage is > say 5-6V; they do die > if the input goes 0V or so with the output capacitor charged.No, a 7805 isn't going to burn up when it is too hot because they shut off when overheating which is what this one was doing. That is very inconvenient when working on a circuit. I don't think the user would appreciate it either... especially when it is being used on a patient. I've actually become very demoralized about this project. Not only is it being designed not unlike a hobby project (aside from my circuits, lol) it seems the project lead is getting tired of working on this and now that we are starting to generate a requirement document, massive corners are being cut. Yesterday he literally justified not extending the requirements to the board design because it would take too much time. I've been thinking he was talking about schedule when he talked about time before. But now I realize he is talking about HIS time and others on the project. Their commitment is wearing a bit thin I guess. It's just that I've been pushing for defining clear, unambiguous requirements for a few weeks now and I thought we were going to have that. Yesterday the project lead and the requirements lead are saying it doesn't need to be rigorous because whoever takes this over to manufacture will have to redo it all anyway. So the bottom line is with no requirements, that means we are already done! -- Rick C. --+- Get 1,000 miles of free Supercharging --+- Tesla referral code - https://ts.la/richard11209
