PCB for 90 Amp continuous
Raveninghorde
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.
Jon Slaughter
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.
Jon Slaughter
John Fields
---
Is this going to be a standalone board and do you get to choose the
copper thickness and I/O scheme?
JF
Raveninghorde
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.
ChrisQ
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
dagmarg...@yahoo.com
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.
--
Cheers,
James Arthur
Joerg
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 :-)
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
John Larkin
<raveninghorde@invalid> 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.
1 oz copper is about 500 uohms per square, if you can get actual 1 oz.
Sounds OK with a little care.
John
Joerg
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.
[...]
ChrisQ
> 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 :-).
Regards,
Chris
Joerg
But yours looks more high-tech :-)
John Larkin
wrote:
It's only 90 amps!
John
dagmarg...@yahoo.com
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.
--
Cheers,
James Arthur
John Larkin
wrote:
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.
John
Joerg
[...]
>>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).
dagmarg...@yahoo.com
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Wed, 4 Nov 2009 17:03:06 -0800 (PST), dagmargoodb...@yahoo.com
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.
--
Cheers,
James Arthur
legg
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.
RL
Joerg
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.
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.
John
Joerg
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:
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
Joerg
> On Thu, 05 Nov 2009 09:01:24 -0800, Joerg <inv...@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:
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
Joerg
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.
ChrisQ
>
> 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.
>
So pack it more carefully :-). If you had to use the uk parcelforce, you
would *expect* it to be thrown around. Anything at all fragile needs at
least 1.5-2" of bubblewrap around every edge. then in a stiff card box.
The avionics people pack things the best ime. A 3" dia / 6" long gyro
foam packed in a foot and a half square box at least...
Regards,
Chris
ehsjr
> John Larkin wrote:
<snip>
>
>
>> 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.
Sometimes they just have to see the warm body on site, to get
the communications started/understood/implemented. The worst
case of that which happend to me: I told them what to do over the
phone. Net cost to client: one phone call. Nope, we need you
here. Ok, round trip to Montreal, hotel, meals, rental car, fee.
Result: all ok, same as it would have been for free if they did
what I told 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."
That gets two smileys! :-) :-)
Ed
>
JosephKK
<Jon_Sl...@Hotmail.com> wrote:
>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.
No, the reduction level is less than the thickness increase as the
potential radiating area goes down. Do try to remember the
thermodynamics to help with your estimates.
>
>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
That's what I thought.
I tried several online trackwidth calculators and they all decrease
track width in proportion to the increase in copper thickness.
JosephKK
Glad to confirm your intuition / experience.
John Larkin
>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.
I just got an email from the prez of IHI, about their 100 amp PCB
lugs:
http://lugsdirect.com/PCBsolderable-lugs.htm
John
Tim Williams
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> I just got an email from the prez of IHI, about their 100 amp PCB
> lugs:
>
> http://lugsdirect.com/PCBsolderable-lugs.htm
>
> John
Wow. Isn't #6 stiffer than even 0.187" thick PCB? Surely they must
require "welding cable" #6.
And people think Tek's TDS's have flimsy connectors...
Tim
Raveninghorde
Thanks for the link. Very useful.
Ruth
>
> - Show quoted text -
Hi, The techy (Chas Ridley) says the bending torque for #6 coarse
stranded copper wire (known as semi rigid) is only 10 in-lbs at the
lug so nothing near the torque needed to tighten a typical screw on a
mechanical connector which is more like 30-35 for a #6. Thanks for the
link by the way - we send out samples if you email us and provide your
UPS number. If you want to see what the B2A-PCB #6 PCB connector looks
like in a product see the ProStar product from Morningstar. They have
been using this high amp PCB lug for many years with great success in
fact replaced wire binding screws on other products as folks like the
easy attach and detatch so much. Glad to help anyone with high amp
wave solderable PCB connector applications up to 115 amps (#2
wire).
Ruth sales"AT"LugsDirect.com or lugsdirect.com"AT"gmail.com
Ruth
> link by the way - we send out samples if you email us and provide your
> UPS number. If you want to see what the B2A-PCB#6PCBconnectorlooks
> like in a product see the ProStar product from Morningstar. They have
> fact replaced wire binding screws on other products as folks like the
> easy attach and detatch so much. Glad to help anyone with high amp
> wire).
> Ruth sales"AT"LugsDirect.com or lugsdirect.com"AT"gmail.com- Hide quoted text -
>
> - Show quoted text -
I got some more info from Chas - he says all the high torque forces
are in line with the plane of the board (whatever that means) so it is
no problem for bending the board. The AWG #6 is tested to go up to 35
in-lbs screw tightening torque as mounted and the AWG #2 PCB connector
is tested up to 50 in-lbs
These IHI high amp PCB Connectors are currently shipped out to Asia,
Europe and the USA with usage (all styes) projected to reach 1 million
in 2010 so something must be good about these.
Thanks Ruth sales"AT"LugsDirect.com, LugsDirect.com"AT"gmail.com
Joerg
Nice. The traces to them look a bit skinny on their example though:
http://lugsdirect.com/images/IHICatalog%20PAGE13---3-11-05.pdf
Ruth
>
> - Show quoted text -
Yes you are right Joerg - the thin traces are used for only for
voltage sensing wires as they have a pretty sophisticated battery
charging algorythm. It is easier to use the same high amp PCB
connector for the signal wires too as they will also take a wire #14
or smaller. The wide traces are taking 30 amps in their model 30. To
get to 90 amps on one wire it would take the larger B2C-PCB (AWG#2)
and adequate sized traces and air. The comments about spreading the
current density around makes sense too - splitting 90 into 2 x 45 amps
would enable lower foil and trace sizes overall. We do supply the
copper staple separately as it can be used as a premade wave
solderable current shunt (also as wisely mentioned above). It is
commom for these lugs to be used in "shunt pairs" to loop the current
through as well as pass power to and from the PCB components. For
looping type wiring it is harder to control what currrent customers
put through there so it makes sense to have more than the traces. For
multiple small wires (AWG12) LugsDirect has a mini one at:
http://www.lugsdirect.com/images/IHICatalog%20PAGE10---3-11-05.pdf
which is UL recognized for #10-16 copper wire.
Sales"AT"LugsDirect.com.