100A switch & pcb connection

dagmargoodboat · 2011-05-16T16:07:59+00:00

I'm contemplating a high-current d.c. switch that, among other things,would provide a soft-switching function (1-10mS? switching time).Rough specs: 24VDCOperating current: 15AFault current: 120AIn-rush surge: 500A x 1mSr.on (including connections): not to exceed 2 milliohms. max.This switch interfaces a load to a 2kW d.c. supply, lives in a box inan indoor instrument, with forced air. For safety, it has to be ableto withstand short-circuit of that supply.For this I'm figuring something like a bunch of FETs on heatsinksspread across FR-4 4 oz. copper (125 u-ohms / sq.), double-sided,paralleled traces on both sides. Connections to be made with crimpedring terminals bolted to a 1mm copper bus bar, itself bolted flat tothe pcb (to spread current and protect the pcb), spring washers tomaintain tension.If I can make the current path 2x4", that's 2 squares x (125 u-ohms /two traces), or only ~130u-ohms. Hmm. even 2oz Cu only makes aquarter micro-ohm...that's fine.Do the 100A pcb terminations sound reasonable?I've not switched big FETs that slowly. Do they scream? (They'llhave gate resistors, of course, but wiring inductance will besignificant.)--Cheers,James Arthur

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dagmargoodboat 15 years ago

=A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0/

/________

x/ /x x x x x|

/x x x x x |

/x_x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 / /

Yes, the bus bar is soldered to the PCB foil, which is held onto the fiberglass by glue, but it's quite a lot of glue.

Measuring, it looks like the bus bar has to be 15mm wide, minimum, to accommodate the ring terminal, so we're talking a 15mm x 50mm attachment.

Rather than hang the bus bar clear it might be better to keep it on top of the PCB but add a big clearance hole for the bolt. That's mechanically stronger and it keeps the hot electrical contacts from sticking out and begging for trouble.

Does this make you feel better?

bolt / washer crimp _ /_ / wire / _|_\/_|_/ ring terminal / ______ |__|--|__| /

------| |------ |--| ---. copper bus bar, soldered to FR-4 .'.'.'| |\ \ \ \|--| \ \| /

------|_____|------ |--| ---'_________________ FR-4 |@ @ @|--|@ @ @ @ @ @ @ @ @ @ @ | / |_@_@_|--|_@_@_@_@_@_@_@_@_@_@_@|_________/ /________ |x| |__|--|__| |x x x x x x x x x x x x x/ /x x x x x| | | /| |--| | | x x x x x x x x x x x x/ /x x x x x | |x|/ |_|--|_| |x_x_x_x_x_x_x_x_x_x_x_x/ /x_x_x_x_x_x| / '--' \ / / split nut . washer . . ||

-- Cheers, James Arthur

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George Herold 15 years ago

Yeah but just add some more area to the current path. A bit more mass helps you too... it will take longer to heat up.

Called PC screw terminals in my Keystone catalog, good to 30A.

George H.

=A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0 /

________

x x x x|

x_x_x_x_|

=A0 =A0 =A0 =A0 =A0 / /

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George Herold 15 years ago

R4

=A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0/

/ /________

x x/ /x x x x x|

/ /x x x x x |

x/ /x_x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 / /

d

n

=A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0/

________

/x x x x x|

x x x x x |

_x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 =A0 / /

=A0 =A0 =A0 .

=A0 =A0 =A0 =A0.

=A0 =A0 =A0 =A0 =A0 .

That looks nice! Do you worry about the bus bar coming loose as you torque the bolt? Perhaps this is too fancy, but you could machine a piece of copper to have some 'fingers' that stick dwon through holes in the pcb to 'hold the torque' if you get my drift.

Is this a one of, or hundreds, thousands of units? If you are getting some custom machine job check out Tellurium copper. You can put a thread in it no problem, "Machines like brass".

George H.

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Fred Bloggs 15 years ago

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dagmargoodboat 15 years ago

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FR4

=A0 =A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0/

__/ /________

x x x/ /x x x x x|

x/ /x x x x x |

x_x/ /x_x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 =A0 / /

xed

een

-4

=A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0/

/________

/ /x x x x x|

/x x x x x |

/x_x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 =A0 =A0 / /

=A0 =A0 =A0 .

=A0 =A0 =A0 =A0.

=A0 =A0 =A0 =A0 =A0 .

I'm not too concerned about torquing the bus bar off. It's going to be soldered to about 15x50mm worth of FR-4 pcb material, the screw's in the center, and the twisting moment won't be all that high. Washers buffer that too, isolating the parts from the tightening action.

That definitely would be sweet. Machining-wise one could also drill and press alignment pins into flat copper bar stock--that saves a lot of milling.

Hundreds a year.

Nice tip, thanks. Brass machines beautifully and copper's gummy, so that's a definite bonus.

-- Cheers, James Arthur

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dagmargoodboat 15 years ago

GREAT resource. Thanks.

-- Cheers, James Arthur

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John Larkin 15 years ago

There are commercial PCB-mount bus bars that look like

---------------------------------- | | | | - ------ ------ ----- ------ - || || || || || || || || || ||

They stand up on the PCB and solder into thru-holes. Drill a hole on the flat part and bolt the cable to that. Preferebly near the middle.

John

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Rich Grise 15 years ago

Unless you're pinching fractions of a penny, I'd speck the thickest copper you can get. I was doing just this one time, and calling board vendors, it was like pulling teeth to get them to tell me what my options are as far as copper thickness. (I was new to speccing FR4 plated material at the time.) He was like, "Oh, just spec what you want, and we'll do it." I tried to 'splain to the guy, I'm kinda new at this; I don't wanna order something that's gonna cost me an arm and a leg, if there's a stock size that's 1/10 the cost but will meet my needs." Finally, he said, "Oh, OK. Six ounce."

Have Fun! Rich

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Rich Grise 15 years ago

Yes, lots, as a matter of fact. :-) But isn't there a cadre who say that the FR-4 will flow and you'll lose contact pressure and all that? Although, the split washer should provide pressure with a few mils of compliance for shrinkage. But I'd use a flat washer between the board and the split washer, NOT soldered down. (BTDT) :-)

Thanks! Rich

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Grant 15 years ago

FR-4

/

I'd still bolt the busbar to PCB, for mechanical strength, one decent bump on the cable will lift that copper busbar and the copper underneath off the CPB material. Very nice artwork ;)

Grant.

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dagmargoodboat 15 years ago

en

-4

=A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0/

/________

/ /x x x x x|

/x x x x x |

/x_x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 =A0 =A0 / /

=A0 =A0 =A0 .

=A0 =A0 =A0 =A0.

=A0 =A0 =A0 =A0 =A0 .

Look carefully and you'll see the ring is bolted directly to the copper bus bar. FR-4 is not clamped by the bolt--that's either the bus bar or the ring terminal you're looking at.

Flat washer under the split washer? Sure, I s'pose. It doesn't matter much if the bus bar gets scuffed.

-- Cheers, James Arthur

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dagmargoodboat 15 years ago

4

=A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0/

/________

x x x x x|

x x x x |

x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 =A0 / /

=A0 =A0 =A0 .

=A0 =A0 =A0 =A0.

=A0 =A0 =A0 =A0 =A0.

A couple bolts wouldn't hurt, to be sure.

Not perfectly clear though; it could be improved. Maybe an exploded diagram, and an ASCII animation?

-- Cheers, James Arthur

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George Herold 15 years ago

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o FR4

=A0 =A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0/

___/ /________

x x x/ /x x x x x|

x x/ /x x x x x |

_x_x/ /x_x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 =A0 / /

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R-4

=A0 =A0 =A0 FR-4

=A0 =A0 =A0 =A0 =A0 =A0 =A0/

/ /________

x/ /x x x x x|

/ /x x x x x |

/x_x_x_x_x_x|

=A0 =A0 =A0 =A0 =A0 =A0 =A0 / /

=A0 =A0 =A0 =A0 .

=A0 =A0 =A0 =A0.

=A0 =A0 =A0 =A0 =A0 =A0 .

That sounds perfect. So much cheaper to find a commercial product.

(The maximum current I've done on a pcb is 3A.) George H.

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Winston 15 years ago

(...)

I read somewhere that solder has ~8.8 X the electrical resistance of copper...

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--Winston

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Winston 15 years ago

Looka this!

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--Winston

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dagmargoodboat 15 years ago

Ahhhh, copper. Where would we be without it?

-- Cheers, James Arthur

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dagmargoodboat 15 years ago

It's horrible, but it's awfully handy. Thin films, of course, are no problem at all.

-- Cheers, James Arthur

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Winston 15 years ago

(...)

Indeed.

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Wouldn't it be nifty if you could copper plate over your soldered bus bar to establish a low resistance parallel path?

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--Winston

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dagmargoodboat 15 years ago

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Not much need to really. If we take your reference for the resistivity of solder @ 150e-9 ohm-meters, and assume a crazy thick solder film equal to the thickness of 1oz pcb copper, for my 50x15mm bus bar attachment we get:

R =3D pL/A

where p=3D150e-9 ohm-meters L =3D 35u meters A =3D (.05 x .015) meters^2

Crunching the numbers, attaching the bus bar by soldering adds R =3D 7 nanoohms to the total resistance. So, that's not a big deal at all.

-- Cheers, James Arthur

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Winston 15 years ago

(...)

That'll be what I learned today.

Thanks!

--Winston

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100A switch & pcb connection

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