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From: Grant on 16 Jul 2010 18:35
On Fri, 16 Jul 2010 09:03:11 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Fri, 16 Jul 2010 10:44:40 -0500, "Tim Williams"
><tmoranwms(a)charter.net> wrote:
>
>>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:86t046tr9cu6p5n0f0c658sh1517m3p8so(a)4ax.com...
>>> A small part like this doesn't conduct heat into pcb pours very well.
>>> If you stick a part to a relatively thin thermally conductive sheet,
>>> theta goes up as the part footprint area goes down [1].
>><snip>
>>> [1] anybody know the exact relation?
>>
>>Easy to approximate. Assume a circular footprint (cf. spherical chicken). Assume heat dissipation at the center is zero (fair for an infinnitessimal segment, blatantly false for an infinite number of them). If heat diffuses through copper out to infinity, temperature drops inversely with distance (because cross sectional area increases linearly). It looks like a point charge in space, and the temperature is defined by Gauss' law.
>>
>>Of course, heat diffuses through two or three means, with strange temperature-dependent coefficients besides. So it's not at all true that, the device itself, and the little bit of copper surrounding it that doesn't have a quite circular temperature profile, isn't dissipating any power. In fact, it could be dissipating a considerable amount of power. If the heat source were an infinnitessimal point, it would have infinite temperature, and therefore radiate infinite power density (power can still be finite, since the area is infinnitessimal).
>>
>>However, it is true that heat diffuses out, one way or another, so maybe the power dissipation is just a little higher in the center, and spreads out in a slightly-steeper-than-inverse relationship, eventually going to zero at infinity all the same. The trouble is deriving the exponent and coefficient of that power law.
>>
>>Tim
>
>Nice rant, but still no answer.
>
>Given a perfectly thermally conductive puck attached to an infinite
>sheet of thin [1] finite-thermal-conductivity material, and assuming
>conduction cooling only, what is the relationship of puck theta to
>puck diameter?
>
>This is relevant to situations where you have a choice of, say, SOT89
>versus DPAK versus D2PAK and you're heatsinking to copper foil.

PC with Core2 series CPU is running >100A regulator 3, 4, or 6 phase
around the CPU area, lots of cooling air flow. But they're not trying
to get that current off board, just direct it through a couple in^^2
of CPU chip.

OP needs controlled bridge to regulate the charger?

Grant.
>
>John
>
>[1] thin relative to puck diameter
From: Muzaffer Kal on 17 Jul 2010 22:42
On Fri, 16 Jul 2010 04:57:11 -0500, "George Jefferson"
<phreon111(a)gmail.com> wrote:

>I want to replace a CT full wave rectifier for a 200A+ battery charger with
>a active rectifying system.
>
>http://www.nxp.com/documents/data_sheet/PSMN2R5-30YL.pdf
>
>Are some 100A 80W logic level fets which means I can parallel 3 or 4 of them
>and increase the efficiency of the charger(there are other reasons for doing
>it too).
>
>I'll be driving the mosfets using a driver circuit instead of a uC of
>course(just to reducing cross conduction problems).
>
>In any case the real issue I have is how to do this on a pcb, if it is even
>possible. Not only are the leads to the packages somewhat small I can't
>think of any way how this can be done on a pcb(30oz pcb's?) or large solder
>traces in any decent fashion.
>
>Are thee any tricks to defeat mother nature or will I have to design some
>special module that uses a copper bus bar like thing with the mosfets
>soldered directly to it?

2 ounce copper should give you ~250 uohm per square so a two layered
board 2 ounce board would dissipate <5W over the etch which is
certainly acceptable. I think your problem is more basic than that. If
you look at Figure 3 of the datasheet you refered to, you would notice
that at 10V Vds, DC current carrying capacity of the device is less
than 5A!
--
Muzaffer Kal

DSPIA INC.
ASIC/FPGA Design Services

http://www.dspia.com
From: Spehro Pefhany on 17 Jul 2010 22:52
On Sat, 17 Jul 2010 19:42:43 -0700, the renowned Muzaffer Kal
<kal(a)dspia.com> wrote:

>On Fri, 16 Jul 2010 04:57:11 -0500, "George Jefferson"
><phreon111(a)gmail.com> wrote:
>
>>I want to replace a CT full wave rectifier for a 200A+ battery charger with
>>a active rectifying system.
>>
>>http://www.nxp.com/documents/data_sheet/PSMN2R5-30YL.pdf
>>
>>Are some 100A 80W logic level fets which means I can parallel 3 or 4 of them
>>and increase the efficiency of the charger(there are other reasons for doing
>>it too).
>>
>>I'll be driving the mosfets using a driver circuit instead of a uC of
>>course(just to reducing cross conduction problems).
>>
>>In any case the real issue I have is how to do this on a pcb, if it is even
>>possible. Not only are the leads to the packages somewhat small I can't
>>think of any way how this can be done on a pcb(30oz pcb's?) or large solder
>>traces in any decent fashion.
>>
>>Are thee any tricks to defeat mother nature or will I have to design some
>>special module that uses a copper bus bar like thing with the mosfets
>>soldered directly to it?
>
>2 ounce copper should give you ~250 uohm per square so a two layered
>board 2 ounce board would dissipate <5W over the etch which is
>certainly acceptable. I think your problem is more basic than that. If
>you look at Figure 3 of the datasheet you refered to, you would notice
>that at 10V Vds, DC current carrying capacity of the device is less
>than 5A!

Except during switching transitions, when "on" the Vds of a full-wave
rectifier MOSFET can't really exceed minus one diode drop, nor would
it be very useful if it did.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff(a)interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
From: Muzaffer Kal on 17 Jul 2010 23:57
On Sat, 17 Jul 2010 22:52:12 -0400, Spehro Pefhany
<speffSNIP(a)interlogDOTyou.knowwhat> wrote:

>On Sat, 17 Jul 2010 19:42:43 -0700, the renowned Muzaffer Kal
><kal(a)dspia.com> wrote:
>
>>On Fri, 16 Jul 2010 04:57:11 -0500, "George Jefferson"
>><phreon111(a)gmail.com> wrote:
>>
>>>I want to replace a CT full wave rectifier for a 200A+ battery charger with
>>>a active rectifying system.
>>>
>>>http://www.nxp.com/documents/data_sheet/PSMN2R5-30YL.pdf
>>>
>>>Are some 100A 80W logic level fets which means I can parallel 3 or 4 of them
>>>and increase the efficiency of the charger(there are other reasons for doing
>>>it too).
>>>
>>>I'll be driving the mosfets using a driver circuit instead of a uC of
>>>course(just to reducing cross conduction problems).
>>>
>>>In any case the real issue I have is how to do this on a pcb, if it is even
>>>possible. Not only are the leads to the packages somewhat small I can't
>>>think of any way how this can be done on a pcb(30oz pcb's?) or large solder
>>>traces in any decent fashion.
>>>
>>>Are thee any tricks to defeat mother nature or will I have to design some
>>>special module that uses a copper bus bar like thing with the mosfets
>>>soldered directly to it?
>>
>>2 ounce copper should give you ~250 uohm per square so a two layered
>>board 2 ounce board would dissipate <5W over the etch which is
>>certainly acceptable. I think your problem is more basic than that. If
>>you look at Figure 3 of the datasheet you refered to, you would notice
>>that at 10V Vds, DC current carrying capacity of the device is less
>>than 5A!
>
>Except during switching transitions, when "on" the Vds of a full-wave
>rectifier MOSFET can't really exceed minus one diode drop, nor would
>it be very useful if it did.

Of course you're right. I should get near a computer during barbeque
saturdays. Vds(on) is actually ~450mV at 100A for 45W which is not
that easy to remove from that package. I think based on other parts
which I have used, one shouldn't dissipate more than ~10W even with a
large piece of copper around the package.
--
Muzaffer Kal

DSPIA INC.
ASIC/FPGA Design Services

http://www.dspia.com
From: JosephKK on 19 Jul 2010 06:19
On Sat, 17 Jul 2010 08:35:54 +1000, Grant <omg(a)grrr.id.au> wrote:

>On Fri, 16 Jul 2010 09:03:11 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Fri, 16 Jul 2010 10:44:40 -0500, "Tim Williams"
>><tmoranwms(a)charter.net> wrote:
>>
>>>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:86t046tr9cu6p5n0f0c658sh1517m3p8so(a)4ax.com...
>>>> A small part like this doesn't conduct heat into pcb pours very well.
>>>> If you stick a part to a relatively thin thermally conductive sheet,
>>>> theta goes up as the part footprint area goes down [1].
>>><snip>
>>>> [1] anybody know the exact relation?
>>>
>>>Easy to approximate. Assume a circular footprint (cf. spherical chicken). Assume heat dissipation at the center is zero (fair for an infinnitessimal segment, blatantly false for an infinite number of them). If heat diffuses through copper out to infinity, temperature drops inversely with distance (because cross sectional area increases linearly). It looks like a point charge in space, and the temperature is defined by Gauss' law.
>>>
>>>Of course, heat diffuses through two or three means, with strange temperature-dependent coefficients besides. So it's not at all true that, the device itself, and the little bit of copper surrounding it that doesn't have a quite circular temperature profile, isn't dissipating any power. In fact, it could be dissipating a considerable amount of power. If the heat source were an infinnitessimal point, it would have infinite temperature, and therefore radiate infinite power density (power can still be finite, since the area is infinnitessimal).
>>>
>>>However, it is true that heat diffuses out, one way or another, so maybe the power dissipation is just a little higher in the center, and spreads out in a slightly-steeper-than-inverse relationship, eventually going to zero at infinity all the same. The trouble is deriving the exponent and coefficient of that power law.
>>>
>>>Tim
>>
>>Nice rant, but still no answer.
>>
>>Given a perfectly thermally conductive puck attached to an infinite
>>sheet of thin [1] finite-thermal-conductivity material, and assuming
>>conduction cooling only, what is the relationship of puck theta to
>>puck diameter?
>>
>>This is relevant to situations where you have a choice of, say, SOT89
>>versus DPAK versus D2PAK and you're heatsinking to copper foil.
>
>PC with Core2 series CPU is running >100A regulator 3, 4, or 6 phase
>around the CPU area, lots of cooling air flow. But they're not trying
>to get that current off board, just direct it through a couple in^^2
>of CPU chip.

Really? If the chip requires only 65 Watts how do you get to 100 A
even at 1.2 V?

Of course with modern CPU chips that take far less power at idle these
issues become very fuzzy.

>
>OP needs controlled bridge to regulate the charger?
>
>Grant.
>>
>>John
>>
>>[1] thin relative to puck diameter
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