Raveninghorde wrote: > Just been asked to look at a Li Ion protection circuit capable of 90A > continuous. In a working area 6" x 6".
> Even if I can get parallel FETS to give a milliohm on resistance > that's still 8 Watts dissipation and for a back to back pair it's 16 > watts.
> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms > is 28 Watts.
> I guess the power wires would have to be 6AWG, 16mm2. And how do you > connect them to a pcb.
> I don't think the customers engineers have thought this through.
You can get mosfets with 80A capability with ~2mOhm quite easily. (and I'm sure you can find others with even better)
Parallel 10 of these and you have sub-mOhm resistance. Since you don't seem to be using this for PWM you shouldn't have to worry about switching syncronicity. Hence the limiting factor is gate drive. Since you don't care about switching speed(within reason) it shouldn't be difficult. (I assume by protection you just want a switch to disconnect the circuit from power)
So it's not going to be the mosfets that are the problem(except, of course, it requires board area). You will need to use at least 4oz copper(thats the highest I've seen) I'd imagine else your traces will need to be quite thick.
For example, a going from 0.5 to 4oz would allow you to reduce your trace size by a factor of 8(or slightly more). So, if your the power, hypothetically, required 8 in thick traces @ 0.5oz then it could be reduced to 1 @ 4oz. Pretty significant.
I think your board, at 4oz, has a total resistance of about 20uOhms. If you route it properly then it shouldn't be an issue. That is about 0.2W total dissipation if 90A were to run through the solid copper.
I think the goal would be to divide the board in such a way that you can parallel the most number of mosfets and limit the gaps. One easy way, would be to simply divide the board into 6 "slots" analogous to 6 wires in parallel:
+-+-+-+ | | | | M M M M... | | | | +-+-+-+
The traces though are quite large with the gaps between different branches being minimal.
Considering the mosfets are actually quite small you could probably do 5 or 6 in a 6x2 board without to many issues. (I assume it is more than one layer so you can route the gates to another layer)
Of course some calculation may be wrong but it doens't seem implausable at all just based on these estimates.
Raveninghorde wrote: > On Tue, 03 Nov 2009 09:34:00 -0600, John Fields > <jfie...@austininstruments.com> wrote:
>> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde >> <raveninghorde@invalid> wrote:
>>> Just been asked to look at a Li Ion protection circuit capable of 90A >>> continuous. In a working area 6" x 6".
>>> Even if I can get parallel FETS to give a milliohm on resistance >>> that's still 8 Watts dissipation and for a back to back pair it's 16 >>> watts.
>>> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >>> is 28 Watts.
>>> I guess the power wires would have to be 6AWG, 16mm2. And how do you >>> connect them to a pcb.
>>> I don't think the customers engineers have thought this through. >> --- >> Is this going to be a standalone board and do you get to choose the >> copper thickness and I/O scheme?
>> JF
> It is a board to go inside a battery and has the protection and fuel > gauge electronics. At this stage I have a free hand within the space > constraints.
I would e very wary of using the pcb for this. Thicker copper = more expensive pcb, when it may be cheaper to use formed copper or ali bussbars between the device and edge of the board, mounted to pcb. Gets rid of the conductor heat problem as well.
Many high current psu designs use this sort of thing, bussbars coming out of the end of the case...
> Raveninghorde wrote: > > On Tue, 03 Nov 2009 09:34:00 -0600, John Fields > > <jfie...@austininstruments.com> wrote:
> >> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde > >> <raveninghorde@invalid> wrote:
> >>> Just been asked to look at a Li Ion protection circuit capable of 90A > >>> continuous. In a working area 6" x 6".
> >>> Even if I can get parallel FETS to give a milliohm on resistance > >>> that's still 8 Watts dissipation and for a back to back pair it's 16 > >>> watts.
> >>> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms > >>> is 28 Watts.
> >>> I guess the power wires would have to be 6AWG, 16mm2. And how do you > >>> connect them to a pcb.
> >>> I don't think the customers engineers have thought this through. > >> --- > >> Is this going to be a standalone board and do you get to choose the > >> copper thickness and I/O scheme?
> >> JF
> > It is a board to go inside a battery and has the protection and fuel > > gauge electronics. At this stage I have a free hand within the space > > constraints.
> I would e very wary of using the pcb for this. Thicker copper = more > expensive pcb, when it may be cheaper to use formed copper or ali > bussbars between the device and edge of the board, mounted to pcb. Gets > rid of the conductor heat problem as well.
> Many high current psu designs use this sort of thing, bussbars coming > out of the end of the case...
> Regards,
> Chris
I've seen solid copper bus bars, ~6x12mm cross section, sweated to PCBs. Worked great. High production? Commercial, solderable, bus bars might be the thing.
Raveninghorde wrote: > Just been asked to look at a Li Ion protection circuit capable of 90A > continuous. In a working area 6" x 6".
> Even if I can get parallel FETS to give a milliohm on resistance > that's still 8 Watts dissipation and for a back to back pair it's 16 > watts.
> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms > is 28 Watts.
> I guess the power wires would have to be 6AWG, 16mm2. And how do you > connect them to a pcb.
> I don't think the customers engineers have thought this through.
It can be done. On the very first design in my career the system was fed 5V at a whopping 100 amps. Good old 74AS, tons of them. At some point when they figured out that I know a thing or two about noise I became the official master of ceremonies for the motherboard (where the 100 amps when into).
You'll end up spending considerable time calculating planes, vias, thermal reliefs and (very important) the contact areas to the outside world. Then reflow temperature profiles and such because often you can't afford the amount of thermal relief that you are used to.
Don't try this with a 1oz copper four-layer, I've got some scary photo of boards where people did :-)
<raveninghorde@invalid> wrote: >Just been asked to look at a Li Ion protection circuit capable of 90A >continuous. In a working area 6" x 6".
>Even if I can get parallel FETS to give a milliohm on resistance >that's still 8 Watts dissipation and for a back to back pair it's 16 >watts.
>The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >is 28 Watts.
>I guess the power wires would have to be 6AWG, 16mm2. And how do you >connect them to a pcb.
>I don't think the customers engineers have thought this through.
Use enough fets in parallel to keep the dissipation down. Multiple fets scatter the heat, too.
Fastons make nice high-current connectors, females soldered on the board, males crimped on wires. If you go for, say, 15 or 20 amps per faston, and use separate wires for each (instead of one #6) you'll get good current sharing. 90 amps in one place can have current crowding problems.
DO NOT bolt lugs to the pcb. The FR4 will cold flow and the thing may catch fire.
1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
John Larkin wrote: > On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde > <raveninghorde@invalid> wrote:
>> Just been asked to look at a Li Ion protection circuit capable of 90A >> continuous. In a working area 6" x 6".
>> Even if I can get parallel FETS to give a milliohm on resistance >> that's still 8 Watts dissipation and for a back to back pair it's 16 >> watts.
>> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >> is 28 Watts.
>> I guess the power wires would have to be 6AWG, 16mm2. And how do you >> connect them to a pcb.
>> I don't think the customers engineers have thought this through.
> Use enough fets in parallel to keep the dissipation down. Multiple > fets scatter the heat, too.
> Fastons make nice high-current connectors, females soldered on the > board, males crimped on wires. If you go for, say, 15 or 20 amps per > faston, and use separate wires for each (instead of one #6) you'll get > good current sharing. 90 amps in one place can have current crowding > problems.
> DO NOT bolt lugs to the pcb. The FR4 will cold flow and the thing may > catch fire.
That's what we did. 100 amps. The trick is to plate correctly and make the connection compliant so the pressure is kept up.
> 1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
I've done some 2oz designs. Got another one coming up soon.
Joerg wrote: > John Larkin wrote: >> Use enough fets in parallel to keep the dissipation down. Multiple >> fets scatter the heat, too.
>> Fastons make nice high-current connectors, females soldered on the >> board, males crimped on wires. If you go for, say, 15 or 20 amps per >> faston, and use separate wires for each (instead of one #6) you'll get >> good current sharing. 90 amps in one place can have current crowding >> problems.
>> DO NOT bolt lugs to the pcb. The FR4 will cold flow and the thing may >> catch fire.
> That's what we did. 100 amps. The trick is to plate correctly and make > the connection compliant so the pressure is kept up.
>> 1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
> I've done some 2oz designs. Got another one coming up soon.
> [...]
Much smarter solution than my brute force busbar approach as well :-).
ChrisQ wrote: > Joerg wrote: >> John Larkin wrote:
>>> Use enough fets in parallel to keep the dissipation down. Multiple >>> fets scatter the heat, too.
>>> Fastons make nice high-current connectors, females soldered on the >>> board, males crimped on wires. If you go for, say, 15 or 20 amps per >>> faston, and use separate wires for each (instead of one #6) you'll get >>> good current sharing. 90 amps in one place can have current crowding >>> problems.
>>> DO NOT bolt lugs to the pcb. The FR4 will cold flow and the thing may >>> catch fire.
>> That's what we did. 100 amps. The trick is to plate correctly and make >> the connection compliant so the pressure is kept up.
>>> 1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
>> I've done some 2oz designs. Got another one coming up soon.
>> [...]
> Much smarter solution than my brute force busbar approach as well :-).
>John Larkin wrote: >> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde >> <raveninghorde@invalid> wrote:
>>> Just been asked to look at a Li Ion protection circuit capable of 90A >>> continuous. In a working area 6" x 6".
>>> Even if I can get parallel FETS to give a milliohm on resistance >>> that's still 8 Watts dissipation and for a back to back pair it's 16 >>> watts.
>>> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >>> is 28 Watts.
>>> I guess the power wires would have to be 6AWG, 16mm2. And how do you >>> connect them to a pcb.
>>> I don't think the customers engineers have thought this through.
>> Use enough fets in parallel to keep the dissipation down. Multiple >> fets scatter the heat, too.
>> Fastons make nice high-current connectors, females soldered on the >> board, males crimped on wires. If you go for, say, 15 or 20 amps per >> faston, and use separate wires for each (instead of one #6) you'll get >> good current sharing. 90 amps in one place can have current crowding >> problems.
>> DO NOT bolt lugs to the pcb. The FR4 will cold flow and the thing may >> catch fire.
>That's what we did. 100 amps. The trick is to plate correctly and make >the connection compliant so the pressure is kept up.
>> 1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
>I've done some 2oz designs. Got another one coming up soon.
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde
> <raveninghorde@invalid> wrote: > >Just been asked to look at a Li Ion protection circuit capable of 90A > >continuous. In a working area 6" x 6".
> >Even if I can get parallel FETS to give a milliohm on resistance > >that's still 8 Watts dissipation and for a back to back pair it's 16 > >watts.
> >The spec is max board impedance of 35 milliohms. Even 3.5 milliohms > >is 28 Watts.
> >I guess the power wires would have to be 6AWG, 16mm2. And how do you > >connect them to a pcb.
> >I don't think the customers engineers have thought this through.
> Use enough fets in parallel to keep the dissipation down. Multiple > fets scatter the heat, too.
> Fastons make nice high-current connectors, females soldered on the > board, males crimped on wires. If you go for, say, 15 or 20 amps per > faston, and use separate wires for each (instead of one #6) you'll get > good current sharing. 90 amps in one place can have current crowding > problems.
> DO NOT bolt lugs to the pcb. The FR4 will cold flow and the thing may > catch fire.
> 1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
> Sounds OK with a little care.
> John
From basics, I calculate (surprise) 246uohms per cm for a 2cm wide 1oz trace, at 20c. So, that's 1W per cm^2 dissipation @ 90A. Top-and- bottom traces cut that to 250mW/cm^2, and wider traces, proportionally.
I can imagine a super-short current path, with i/o wires running virtually to the FETs--that'd be pretty decent.
Depends on whether there's airflow or not. I'd assumed a closed battery case before, but that may not be.
>On Nov 4, 3:54 pm, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde
>> <raveninghorde@invalid> wrote: >> >Just been asked to look at a Li Ion protection circuit capable of 90A >> >continuous. In a working area 6" x 6".
>> >Even if I can get parallel FETS to give a milliohm on resistance >> >that's still 8 Watts dissipation and for a back to back pair it's 16 >> >watts.
>> >The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >> >is 28 Watts.
>> >I guess the power wires would have to be 6AWG, 16mm2. And how do you >> >connect them to a pcb.
>> >I don't think the customers engineers have thought this through.
>> Use enough fets in parallel to keep the dissipation down. Multiple >> fets scatter the heat, too.
>> Fastons make nice high-current connectors, females soldered on the >> board, males crimped on wires. If you go for, say, 15 or 20 amps per >> faston, and use separate wires for each (instead of one #6) you'll get >> good current sharing. 90 amps in one place can have current crowding >> problems.
>> DO NOT bolt lugs to the pcb. The FR4 will cold flow and the thing may >> catch fire.
>> 1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
>> Sounds OK with a little care.
>> John
>From basics, I calculate (surprise) 246uohms per cm for a 2cm wide 1oz >trace, at 20c. So, that's 1W per cm^2 dissipation @ 90A. Top-and- >bottom traces cut that to 250mW/cm^2, and wider traces, >proportionally.
>I can imagine a super-short current path, with i/o wires running >virtually to the FETs--that'd be pretty decent.
Yes. A 20 amps per wire+connection, and one square of 1 oz copper per, that's just 0.2 watts per connection. A fraction of a square should be feasible.
>Depends on whether there's airflow or not. I'd assumed a closed >battery case before, but that may not be.
One problem with dumping 90 amps into a single spot will be current crowding. A small diameter contact will have very high current densities close into the contact. The bigger the diameter, the better. More low-current connections is yet better.
Also most circuits will have the current coming into the contact from one direction, so the copper "on the other side" doesn't help... and current density is that much higher.
>>From basics, I calculate (surprise) 246uohms per cm for a 2cm wide 1oz >> trace, at 20c. So, that's 1W per cm^2 dissipation @ 90A. Top-and- >> bottom traces cut that to 250mW/cm^2, and wider traces, >> proportionally.
>> I can imagine a super-short current path, with i/o wires running >> virtually to the FETs--that'd be pretty decent.
> Yes. A 20 amps per wire+connection, and one square of 1 oz copper per, > that's just 0.2 watts per connection. A fraction of a square should be > feasible.
>> Depends on whether there's airflow or not. I'd assumed a closed >> battery case before, but that may not be.
> One problem with dumping 90 amps into a single spot will be current > crowding. A small diameter contact will have very high current > densities close into the contact. The bigger the diameter, the better. > More low-current connections is yet better.
> Also most circuits will have the current coming into the contact from > one direction, so the copper "on the other side" doesn't help... and > current density is that much higher.
> So multiple contacts help a whole lot.
Seriously, I'd go to at least 2oz on this. Or more. Copper ain't that expensive (yet).
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 4 Nov 2009 17:03:06 -0800 (PST), dagmargoodb...@yahoo.com > wrote:
> >On Nov 4, 3:54 pm, John Larkin > ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde
> >> <raveninghorde@invalid> wrote: > >> >Just been asked to look at a Li Ion protection circuit capable of 90A > >> >continuous. In a working area 6" x 6".
> >> >Even if I can get parallel FETS to give a milliohm on resistance > >> >that's still 8 Watts dissipation and for a back to back pair it's 16 > >> >watts.
> >> >The spec is max board impedance of 35 milliohms. Even 3.5 milliohms > >> >is 28 Watts.
> >> >I guess the power wires would have to be 6AWG, 16mm2. And how do you > >> >connect them to a pcb.
> >> >I don't think the customers engineers have thought this through.
> >> Use enough fets in parallel to keep the dissipation down. Multiple > >> fets scatter the heat, too.
> >> Fastons make nice high-current connectors, females soldered on the > >> board, males crimped on wires. If you go for, say, 15 or 20 amps per > >> faston, and use separate wires for each (instead of one #6) you'll get > >> good current sharing. 90 amps in one place can have current crowding > >> problems.
> >> DO NOT bolt lugs to the pcb. The FR4 will cold flow and the thing may > >> catch fire.
> >> 1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
> >> Sounds OK with a little care.
> >> John
> >From basics, I calculate (surprise) 246uohms per cm for a 2cm wide 1oz > >trace, at 20c. So, that's 1W per cm^2 dissipation @ 90A. Top-and- > >bottom traces cut that to 250mW/cm^2, and wider traces, > >proportionally.
> >I can imagine a super-short current path, with i/o wires running > >virtually to the FETs--that'd be pretty decent.
> Yes. A 20 amps per wire+connection, and one square of 1 oz copper per, > that's just 0.2 watts per connection. A fraction of a square should be > feasible.
> >Depends on whether there's airflow or not. I'd assumed a closed > >battery case before, but that may not be.
> One problem with dumping 90 amps into a single spot will be current > crowding. A small diameter contact will have very high current > densities close into the contact. The bigger the diameter, the better. > More low-current connections is yet better.
> Also most circuits will have the current coming into the contact from > one direction, so the copper "on the other side" doesn't help... and > current density is that much higher.
I assumed feed wires to all points (top & bottom), but your point is well taken--I was wrong to assume. Language is a b*tch, ain't it?
> So multiple contacts help a whole lot.
Yep. Fat traces too. I'd feel better with thicker copper.
<raveninghorde@invalid> wrote: >On Tue, 03 Nov 2009 09:34:00 -0600, John Fields ><jfie...@austininstruments.com> wrote:
>>On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde >><raveninghorde@invalid> wrote:
>>>Just been asked to look at a Li Ion protection circuit capable of 90A >>>continuous. In a working area 6" x 6".
>>>Even if I can get parallel FETS to give a milliohm on resistance >>>that's still 8 Watts dissipation and for a back to back pair it's 16 >>>watts.
>>>The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >>>is 28 Watts.
>>>I guess the power wires would have to be 6AWG, 16mm2. And how do you >>>connect them to a pcb.
>>>I don't think the customers engineers have thought this through.
>>--- >>Is this going to be a standalone board and do you get to choose the >>copper thickness and I/O scheme?
>>JF
>It is a board to go inside a battery and has the protection and fuel >gauge electronics. At this stage I have a free hand within the space >constraints.
Just keep the actual current tracing low L/W ratio (on as many layers as possible) and the interconnect to battery/socket routing (formed metal?) multi-point, to reduce loss and spread heat.
Your 'back-to-back' fets will literally be just that - likely on opposite sides of the same real estate, if pass-through impedances can be kept low enough.. Logic and control lines have to work around the periphery of conducting paths, in the cracks and out of the way. Current sensing may be a major issue.
Regardless of what's on paper, your permissible losses are thermally limited. I also strongly suspect that the 90A is an intermittent or limit condition - not one for which normal rises for continuous operation will apply.
legg wrote: > On Tue, 03 Nov 2009 15:47:20 +0000, Raveninghorde > <raveninghorde@invalid> wrote:
>> On Tue, 03 Nov 2009 09:34:00 -0600, John Fields >> <jfie...@austininstruments.com> wrote:
>>> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde >>> <raveninghorde@invalid> wrote:
>>>> Just been asked to look at a Li Ion protection circuit capable of 90A >>>> continuous. In a working area 6" x 6".
>>>> Even if I can get parallel FETS to give a milliohm on resistance >>>> that's still 8 Watts dissipation and for a back to back pair it's 16 >>>> watts.
>>>> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >>>> is 28 Watts.
>>>> I guess the power wires would have to be 6AWG, 16mm2. And how do you >>>> connect them to a pcb.
>>>> I don't think the customers engineers have thought this through. >>> --- >>> Is this going to be a standalone board and do you get to choose the >>> copper thickness and I/O scheme?
>>> JF >> It is a board to go inside a battery and has the protection and fuel >> gauge electronics. At this stage I have a free hand within the space >> constraints.
> Just keep the actual current tracing low L/W ratio (on as many layers > as possible) and the interconnect to battery/socket routing (formed > metal?) multi-point, to reduce loss and spread heat.
> Your 'back-to-back' fets will literally be just that - likely on > opposite sides of the same real estate, if pass-through impedances can > be kept low enough.. Logic and control lines have to work around the > periphery of conducting paths, in the cracks and out of the way. > Current sensing may be a major issue.
> Regardless of what's on paper, your permissible losses are thermally > limited. I also strongly suspect that the 90A is an intermittent or > limit condition - not one for which normal rises for continuous > operation will apply.
When doing opposite structures mind the thermal reliefs that CAD programs put in by default. You probably don't want any for this purpose. Another trick is to stuff dummy parts in there so you get a through-hole soldered connection as well. But the wave-solder temp profile will cause some cussing among the production guys.
>legg wrote: >> On Tue, 03 Nov 2009 15:47:20 +0000, Raveninghorde >> <raveninghorde@invalid> wrote:
>>> On Tue, 03 Nov 2009 09:34:00 -0600, John Fields >>> <jfie...@austininstruments.com> wrote:
>>>> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde >>>> <raveninghorde@invalid> wrote:
>>>>> Just been asked to look at a Li Ion protection circuit capable of 90A >>>>> continuous. In a working area 6" x 6".
>>>>> Even if I can get parallel FETS to give a milliohm on resistance >>>>> that's still 8 Watts dissipation and for a back to back pair it's 16 >>>>> watts.
>>>>> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >>>>> is 28 Watts.
>>>>> I guess the power wires would have to be 6AWG, 16mm2. And how do you >>>>> connect them to a pcb.
>>>>> I don't think the customers engineers have thought this through. >>>> --- >>>> Is this going to be a standalone board and do you get to choose the >>>> copper thickness and I/O scheme?
>>>> JF >>> It is a board to go inside a battery and has the protection and fuel >>> gauge electronics. At this stage I have a free hand within the space >>> constraints.
>> Just keep the actual current tracing low L/W ratio (on as many layers >> as possible) and the interconnect to battery/socket routing (formed >> metal?) multi-point, to reduce loss and spread heat.
>> Your 'back-to-back' fets will literally be just that - likely on >> opposite sides of the same real estate, if pass-through impedances can >> be kept low enough.. Logic and control lines have to work around the >> periphery of conducting paths, in the cracks and out of the way. >> Current sensing may be a major issue.
>> Regardless of what's on paper, your permissible losses are thermally >> limited. I also strongly suspect that the 90A is an intermittent or >> limit condition - not one for which normal rises for continuous >> operation will apply.
>When doing opposite structures mind the thermal reliefs that CAD >programs put in by default. You probably don't want any for this >purpose. Another trick is to stuff dummy parts in there so you get a >through-hole soldered connection as well. But the wave-solder temp >profile will cause some cussing among the production guys.
We do "flood over" (no thermals) on lots of boards, for better heatsinking to planes or to reduce via inductance. Production doesn't mind. The reflow profile doesn't change, and a beefy Metcal can handle the hand-soldered thru-hole stuff.
John Larkin wrote: > On Thu, 05 Nov 2009 07:59:46 -0800, Joerg <inva...@invalid.invalid> > wrote:
>> legg wrote: >>> On Tue, 03 Nov 2009 15:47:20 +0000, Raveninghorde >>> <raveninghorde@invalid> wrote:
>>>> On Tue, 03 Nov 2009 09:34:00 -0600, John Fields >>>> <jfie...@austininstruments.com> wrote:
>>>>> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde >>>>> <raveninghorde@invalid> wrote:
>>>>>> Just been asked to look at a Li Ion protection circuit capable of 90A >>>>>> continuous. In a working area 6" x 6".
>>>>>> Even if I can get parallel FETS to give a milliohm on resistance >>>>>> that's still 8 Watts dissipation and for a back to back pair it's 16 >>>>>> watts.
>>>>>> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >>>>>> is 28 Watts.
>>>>>> I guess the power wires would have to be 6AWG, 16mm2. And how do you >>>>>> connect them to a pcb.
>>>>>> I don't think the customers engineers have thought this through. >>>>> --- >>>>> Is this going to be a standalone board and do you get to choose the >>>>> copper thickness and I/O scheme?
>>>>> JF >>>> It is a board to go inside a battery and has the protection and fuel >>>> gauge electronics. At this stage I have a free hand within the space >>>> constraints. >>> Just keep the actual current tracing low L/W ratio (on as many layers >>> as possible) and the interconnect to battery/socket routing (formed >>> metal?) multi-point, to reduce loss and spread heat.
>>> Your 'back-to-back' fets will literally be just that - likely on >>> opposite sides of the same real estate, if pass-through impedances can >>> be kept low enough.. Logic and control lines have to work around the >>> periphery of conducting paths, in the cracks and out of the way. >>> Current sensing may be a major issue.
>>> Regardless of what's on paper, your permissible losses are thermally >>> limited. I also strongly suspect that the 90A is an intermittent or >>> limit condition - not one for which normal rises for continuous >>> operation will apply.
>> When doing opposite structures mind the thermal reliefs that CAD >> programs put in by default. You probably don't want any for this >> purpose. Another trick is to stuff dummy parts in there so you get a >> through-hole soldered connection as well. But the wave-solder temp >> profile will cause some cussing among the production guys.
> We do "flood over" (no thermals) on lots of boards, for better > heatsinking to planes or to reduce via inductance. Production doesn't > mind. The reflow profile doesn't change, and a beefy Metcal can handle > the hand-soldered thru-hole stuff.
Your production guys probably don't bitch about it because you sometimes take them out for a nice Belgian beer from tap :-)
>John Larkin wrote: >> On Thu, 05 Nov 2009 07:59:46 -0800, Joerg <inva...@invalid.invalid> >> wrote:
>>> legg wrote: >>>> On Tue, 03 Nov 2009 15:47:20 +0000, Raveninghorde >>>> <raveninghorde@invalid> wrote:
>>>>> On Tue, 03 Nov 2009 09:34:00 -0600, John Fields >>>>> <jfie...@austininstruments.com> wrote:
>>>>>> On Tue, 03 Nov 2009 14:36:16 +0000, Raveninghorde >>>>>> <raveninghorde@invalid> wrote:
>>>>>>> Just been asked to look at a Li Ion protection circuit capable of 90A >>>>>>> continuous. In a working area 6" x 6".
>>>>>>> Even if I can get parallel FETS to give a milliohm on resistance >>>>>>> that's still 8 Watts dissipation and for a back to back pair it's 16 >>>>>>> watts.
>>>>>>> The spec is max board impedance of 35 milliohms. Even 3.5 milliohms >>>>>>> is 28 Watts.
>>>>>>> I guess the power wires would have to be 6AWG, 16mm2. And how do you >>>>>>> connect them to a pcb.
>>>>>>> I don't think the customers engineers have thought this through. >>>>>> --- >>>>>> Is this going to be a standalone board and do you get to choose the >>>>>> copper thickness and I/O scheme?
>>>>>> JF >>>>> It is a board to go inside a battery and has the protection and fuel >>>>> gauge electronics. At this stage I have a free hand within the space >>>>> constraints. >>>> Just keep the actual current tracing low L/W ratio (on as many layers >>>> as possible) and the interconnect to battery/socket routing (formed >>>> metal?) multi-point, to reduce loss and spread heat.
>>>> Your 'back-to-back' fets will literally be just that - likely on >>>> opposite sides of the same real estate, if pass-through impedances can >>>> be kept low enough.. Logic and control lines have to work around the >>>> periphery of conducting paths, in the cracks and out of the way. >>>> Current sensing may be a major issue.
>>>> Regardless of what's on paper, your permissible losses are thermally >>>> limited. I also strongly suspect that the 90A is an intermittent or >>>> limit condition - not one for which normal rises for continuous >>>> operation will apply.
>>> When doing opposite structures mind the thermal reliefs that CAD >>> programs put in by default. You probably don't want any for this >>> purpose. Another trick is to stuff dummy parts in there so you get a >>> through-hole soldered connection as well. But the wave-solder temp >>> profile will cause some cussing among the production guys.
>> We do "flood over" (no thermals) on lots of boards, for better >> heatsinking to planes or to reduce via inductance. Production doesn't >> mind. The reflow profile doesn't change, and a beefy Metcal can handle >> the hand-soldered thru-hole stuff.
>Your production guys probably don't bitch about it because you sometimes >take them out for a nice Belgian beer from tap :-)
Not to mention the bonuses, 401K, free indoor parking, the cabin in Truckee, and the occasional barbeques. Happy people do better work.
We stay very close to our production and test people. We get their opinions on packaging and placement before we release new products and get feedback on existing ones. After all, they're the ones who make the money. Our biggest problem is to get them to complain to engineering when they spot a possible problem pattern, as opposed to working around it. When engineering messes up, we need to know it.
We had a vendor review on Tuesday, with a gigabuck instrument company, our biggest customer. Our quality rating in the last 4 quarters was 99, 98, 98, 98. Their QC manager said "there's nothing for me to say about that."
John Larkin wrote: > On Thu, 05 Nov 2009 09:01:24 -0800, Joerg <inva...@invalid.invalid> > wrote:
>> John Larkin wrote:
[...]
>>> We do "flood over" (no thermals) on lots of boards, for better >>> heatsinking to planes or to reduce via inductance. Production doesn't >>> mind. The reflow profile doesn't change, and a beefy Metcal can handle >>> the hand-soldered thru-hole stuff.
>> Your production guys probably don't bitch about it because you sometimes >> take them out for a nice Belgian beer from tap :-)
> Not to mention the bonuses, 401K, free indoor parking, the cabin in > Truckee, and the occasional barbeques. Happy people do better work.
Yep. One client of mine has already planned out the Christmas company dinner. They'll take them to the most fancy restaurant in the whole area, a place where you can easily rack up $100/person. But, no consultants :-(
> We stay very close to our production and test people. We get their > opinions on packaging and placement before we release new products and > get feedback on existing ones. After all, they're the ones who make > the money. Our biggest problem is to get them to complain to > engineering when they spot a possible problem pattern, as opposed to > working around it. When engineering messes up, we need to know it.
That's the way to go. I am always disappointed when engineers hint that they are different from the people "on the floor". They shouldn't be. A very long time ago I had a situation where I ended up talking to a line lead in production and (together) figuring out a solution. Turned out the engineers hadn't talked to production about a yield issue. Billed consulting hours: 4. Billed travel time to get there, at half rate: about 40. It was half way around the world. The business class air fare alone was north of $5k, they needed me there immediately.
> We had a vendor review on Tuesday, with a gigabuck instrument company, > our biggest customer. Our quality rating in the last 4 quarters was > 99, 98, 98, 98. Their QC manager said "there's nothing for me to say > about that."
I had a conversation with a client, asking about field failure rates after the redesign (it had been huge before). "Basically none, except cases such as where a truck rolled over it."
>John Larkin wrote: >> On Thu, 05 Nov 2009 09:01:24 -0800, Joerg <inva...@invalid.invalid> >> wrote:
>>> John Larkin wrote:
>[...]
>>>> We do "flood over" (no thermals) on lots of boards, for better >>>> heatsinking to planes or to reduce via inductance. Production doesn't >>>> mind. The reflow profile doesn't change, and a beefy Metcal can handle >>>> the hand-soldered thru-hole stuff.
>>> Your production guys probably don't bitch about it because you sometimes >>> take them out for a nice Belgian beer from tap :-)
>> Not to mention the bonuses, 401K, free indoor parking, the cabin in >> Truckee, and the occasional barbeques. Happy people do better work.
>Yep. One client of mine has already planned out the Christmas company >dinner. They'll take them to the most fancy restaurant in the whole >area, a place where you can easily rack up $100/person. But, no >consultants :-(
>> We stay very close to our production and test people. We get their >> opinions on packaging and placement before we release new products and >> get feedback on existing ones. After all, they're the ones who make >> the money. Our biggest problem is to get them to complain to >> engineering when they spot a possible problem pattern, as opposed to >> working around it. When engineering messes up, we need to know it.
>That's the way to go. I am always disappointed when engineers hint that >they are different from the people "on the floor". They shouldn't be. A >very long time ago I had a situation where I ended up talking to a line >lead in production and (together) figuring out a solution. Turned out >the engineers hadn't talked to production about a yield issue. Billed >consulting hours: 4. Billed travel time to get there, at half rate: >about 40. It was half way around the world. The business class air fare >alone was north of $5k, they needed me there immediately.
I bet you wind up acting as liaison between people in the same company, maybe the same building. We wind up doing that sometimes, too. We have one customer whose people never copy one another on emails, so we have to do it for them.
>> We had a vendor review on Tuesday, with a gigabuck instrument company, >> our biggest customer. Our quality rating in the last 4 quarters was >> 99, 98, 98, 98. Their QC manager said "there's nothing for me to say >> about that."
>I had a conversation with a client, asking about field failure rates >after the redesign (it had been huge before). "Basically none, except >cases such as where a truck rolled over it."
Some of our stuff, especially the big gradient amps, get dropped and bent. And some come back with nothing apparently wrong. Maybe half of our returns are actual failures.
John Larkin wrote: > On Thu, 05 Nov 2009 10:43:25 -0800, Joerg <inva...@invalid.invalid> > wrote:
>> John Larkin wrote: >>> On Thu, 05 Nov 2009 09:01:24 -0800, Joerg <inva...@invalid.invalid> >>> wrote:
>>>> John Larkin wrote: >> [...]
>>>>> We do "flood over" (no thermals) on lots of boards, for better >>>>> heatsinking to planes or to reduce via inductance. Production doesn't >>>>> mind. The reflow profile doesn't change, and a beefy Metcal can handle >>>>> the hand-soldered thru-hole stuff.
>>>> Your production guys probably don't bitch about it because you sometimes >>>> take them out for a nice Belgian beer from tap :-) >>> Not to mention the bonuses, 401K, free indoor parking, the cabin in >>> Truckee, and the occasional barbeques. Happy people do better work.
>> Yep. One client of mine has already planned out the Christmas company >> dinner. They'll take them to the most fancy restaurant in the whole >> area, a place where you can easily rack up $100/person. But, no >> consultants :-(
>>> We stay very close to our production and test people. We get their >>> opinions on packaging and placement before we release new products and >>> get feedback on existing ones. After all, they're the ones who make >>> the money. Our biggest problem is to get them to complain to >>> engineering when they spot a possible problem pattern, as opposed to >>> working around it. When engineering messes up, we need to know it.
>> That's the way to go. I am always disappointed when engineers hint that >> they are different from the people "on the floor". They shouldn't be. A >> very long time ago I had a situation where I ended up talking to a line >> lead in production and (together) figuring out a solution. Turned out >> the engineers hadn't talked to production about a yield issue. Billed >> consulting hours: 4. Billed travel time to get there, at half rate: >> about 40. It was half way around the world. The business class air fare >> alone was north of $5k, they needed me there immediately.
> I bet you wind up acting as liaison between people in the same > company, maybe the same building. We wind up doing that sometimes, > too. We have one customer whose people never copy one another on > emails, so we have to do it for them.
Oh yeah. Sometimes it goes farther, once I was more in the role of a lay caregiver which I usually only do for our church. A client engineer went through some serious personal grief and needed this. Of course that part was zero-Dollar work.
>>> We had a vendor review on Tuesday, with a gigabuck instrument company, >>> our biggest customer. Our quality rating in the last 4 quarters was >>> 99, 98, 98, 98. Their QC manager said "there's nothing for me to say >>> about that."
>> I had a conversation with a client, asking about field failure rates >> after the redesign (it had been huge before). "Basically none, except >> cases such as where a truck rolled over it."
> Some of our stuff, especially the big gradient amps, get dropped and > bent. And some come back with nothing apparently wrong. > Maybe half of our returns are actual failures.
My first design after getting the degree was part of an ultrasound machine. Sent a unit from the first run to England. Came back, supposedly DOA. Department head was fuming. When we uncrated the returning unit a minor question arose: "Err, why is a quarter of its chassis base missing?" Looked like a Land Rover had crashed into it. Turns out it had been unloaded from a Boeing and instead of traveling down on the rubber belt it fell straight onto the tarmac.
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