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| John Devereux... |
 Posted: Wed Oct 21, 2009 10:06 am |
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Guest
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dagmargoodboat at (no spam) yahoo.com writes:
[quote]On Oct 21, 8:52 am, dagmargoodb... at (no spam) yahoo.com wrote:
Yes, I've seen SEPIC inductors sold by one of the major mfrs as such;
can't recall who.
Here's one:
http://www.coilcraft.com/lpr4012.cfm
Price? Dunno.
[/quote]
Neither do Digikey, Mouser, Farnell or findchips.com :(
--
John Devereux |
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| John Devereux... |
| Posted: Wed Oct 21, 2009 10:13 am |
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Guest
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Joerg <invalid at (no spam) invalid.invalid> writes:
[quote]John Devereux wrote:
Joerg <invalid at (no spam) invalid.invalid> writes:
[...]
Another option would be to use a CD40106 or something similar as a
Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can
drive a little FET, a simple logic level device like a 2N7002 as long
as doesn't cost much. Pipe Vref out of your MCU (hoping it has that
...) and use a cheap opamp to pull the Schmitt oscillator input "to
the side" when the target voltage is reached. That reduces the duty
cycle as much as needed to maintain regulation, pretty much like the
throttle on a gasoline-powered generator. If the target voltage
doesn't have to be very precise you could also use a NPN plus zener
for that, without a reference source. Probably a TL431-type device
would work as well and those are quite cheap, in the penny range.
Hi Jeorg,
Are you talking about a SEPIC/flyback arrangement here? I.e., 2N7001
drives a transformer? How do you stop the opamp railing during startup
or a load step? The schmitt osc would stop and .. how does it go
... Phut! :)
If you use an opamp a simple trick to avoid this is a diode. That way
the enchilada can pull the Schmitt to one side but cannot push it to
the other. If you are really brazen use a comparator with O/C -> no
diode, saves 1-2 cents. If you use a transistor or TL431 the problem
goes away on its own, sans diode.
[/quote]
Of course, neat. Finally the famed Jeorgian discrete SMPS is starting to
take shape :)
[quote]Now, set the max duty cycle to
whatever worst case demand you calculated, at the minimum expected
input voltage, plus 20% margin or so.
Also of course by the time he's actually bought a SEPIC transformer it
could all approach his 1.8 Euros. Or are there some super-cheap ones
now?
Yes, but only when you buy in Asia.
[/quote]
Anyone know how to do this for smaller quantities (<~1k, say?). I've got
plenty of Asian suppliers trying to get our PCB business, but nobody
trying to sell me inductors  .
[quote]For some reason you can't get them much under a buck inside the US
even though I could imagine those are also made in China. Coilcraft is
typically the best deal for coupled SEPIC coils if it has to be
domestic.
Using two individual inductors gets you around the problem.
[/quote]
--
John Devereux |
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| Joerg... |
| Posted: Wed Oct 21, 2009 10:49 am |
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Guest
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John Devereux wrote:
[quote]Joerg <invalid at (no spam) invalid.invalid> writes:
John Devereux wrote:
Joerg <invalid at (no spam) invalid.invalid> writes:
[...]
Another option would be to use a CD40106 or something similar as a
Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can
drive a little FET, a simple logic level device like a 2N7002 as long
as doesn't cost much. Pipe Vref out of your MCU (hoping it has that
...) and use a cheap opamp to pull the Schmitt oscillator input "to
the side" when the target voltage is reached. That reduces the duty
cycle as much as needed to maintain regulation, pretty much like the
throttle on a gasoline-powered generator. If the target voltage
doesn't have to be very precise you could also use a NPN plus zener
for that, without a reference source. Probably a TL431-type device
would work as well and those are quite cheap, in the penny range.
Hi Jeorg,
Are you talking about a SEPIC/flyback arrangement here? I.e., 2N7001
drives a transformer? How do you stop the opamp railing during startup
or a load step? The schmitt osc would stop and .. how does it go
... Phut! :)
If you use an opamp a simple trick to avoid this is a diode. That way
the enchilada can pull the Schmitt to one side but cannot push it to
the other. If you are really brazen use a comparator with O/C -> no
diode, saves 1-2 cents. If you use a transistor or TL431 the problem
goes away on its own, sans diode.
Of course, neat. Finally the famed Jeorgian discrete SMPS is starting to
take shape :)
[/quote]
My first one took shape about 15 years ago, tons of them in use by
consumers right now, still coming off the conveyor belt (in China). I
don't know if it will outlast the VW Beetle in production but it might.
It has already survived the client company's president :-(
Back then I learned a valuable lesson: Inductor prices in the western
world must be taken with a grain of salt. A custom mfg place in Taiwan
outbid a catalog (!) part from a western manufacturer, big time. And we
even got an EMI-savvy toroid for our money versus the cheap open cores
from mainstream distributors. Of course it does help not to have a
middleman in the game.
[quote]Now, set the max duty cycle to
whatever worst case demand you calculated, at the minimum expected
input voltage, plus 20% margin or so.
Also of course by the time he's actually bought a SEPIC transformer it
could all approach his 1.8 Euros. Or are there some super-cheap ones
now?
Yes, but only when you buy in Asia.
Anyone know how to do this for smaller quantities (<~1k, say?). I've got
plenty of Asian suppliers trying to get our PCB business, but nobody
trying to sell me inductors .
[/quote]
<1k? No, I don't think they'd even talk to you. However, Asia is all
about one thing: Connections. Ask the PCB stuffing places what sort of
deals they can get locally. They might be able to swing it, with a
supplier they already buy lots of other stuff from.
If you have reputable places there that do <1k qty board runs with
decent quality let me (and others here) know. I bet lots of us are
interested.
[...]
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM. |
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| Joerg... |
| Posted: Wed Oct 21, 2009 10:52 am |
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Guest
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John Devereux wrote:
[quote]dagmargoodboat at (no spam) yahoo.com writes:
On Oct 21, 8:52 am, dagmargoodb... at (no spam) yahoo.com wrote:
Yes, I've seen SEPIC inductors sold by one of the major mfrs as such;
can't recall who.
Here's one:
http://www.coilcraft.com/lpr4012.cfm
Price? Dunno.
Neither do Digikey, Mouser, Farnell or findchips.com :(
[/quote]
With Coilcraft you have to become a "member". Sign up and then you get
instant pricing access on their site. If you don't want to then request
a quote, comes within minutes via email. Their pricing is actually quite
favorable compared to other domestic suppliers, maybe because they do a
lot of automotive.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM. |
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| John Devereux... |
| Posted: Wed Oct 21, 2009 11:15 am |
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Guest
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Joerg <invalid at (no spam) invalid.invalid> writes:
[quote]John Devereux wrote:
Joerg <invalid at (no spam) invalid.invalid> writes:
John Devereux wrote:
[/quote]
[...]
[quote]Of course, neat. Finally the famed Jeorgian discrete SMPS is starting to
take shape :)
My first one took shape about 15 years ago, tons of them in use by
consumers right now, still coming off the conveyor belt (in China). I
don't know if it will outlast the VW Beetle in production but it
might. It has already survived the client company's president :-(
Back then I learned a valuable lesson: Inductor prices in the western
world must be taken with a grain of salt. A custom mfg place in Taiwan
outbid a catalog (!) part from a western manufacturer, big time. And
we even got an EMI-savvy toroid for our money versus the cheap open
cores from mainstream distributors. Of course it does help not to have
a middleman in the game.
[/quote]
[...]
[quote]Anyone know how to do this for smaller quantities (<~1k, say?). I've got
plenty of Asian suppliers trying to get our PCB business, but nobody
trying to sell me inductors .
1k? No, I don't think they'd even talk to you.
[/quote]
I should have said "single reel" since there are typically more than
that on a reel. I guess we could probably make a "strategic" buy of ~5k
if needed. If they're *really* cheap 10k! :)
[quote]However, Asia is all about one thing: Connections. Ask the PCB
stuffing places what sort of deals they can get locally. They might be
able to swing it, with a supplier they already buy lots of other stuff
from.
If you have reputable places there that do <1k qty board runs with
decent quality let me (and others here) know. I bet lots of us are
interested.
[/quote]
Sorry, I meant bare PCB not stuffed. They advertise stuffing too - down
to any quantity - but they said once they buy non-free issued parts from
Digikey... :)
I just thought it strange that I can get small qtys of PCBs at 1/2 the
price of local suppliers, but nobody does the same for parts. (Which
ought to be much easier to supply).
[quote][...]
[/quote]
--
John Devereux |
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| Joerg... |
| Posted: Wed Oct 21, 2009 11:34 am |
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Guest
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John Devereux wrote:
[quote]Joerg <invalid at (no spam) invalid.invalid> writes:
John Devereux wrote:
Joerg <invalid at (no spam) invalid.invalid> writes:
John Devereux wrote:
[...]
Of course, neat. Finally the famed Jeorgian discrete SMPS is starting to
take shape :)
My first one took shape about 15 years ago, tons of them in use by
consumers right now, still coming off the conveyor belt (in China). I
don't know if it will outlast the VW Beetle in production but it
might. It has already survived the client company's president :-(
Back then I learned a valuable lesson: Inductor prices in the western
world must be taken with a grain of salt. A custom mfg place in Taiwan
outbid a catalog (!) part from a western manufacturer, big time. And
we even got an EMI-savvy toroid for our money versus the cheap open
cores from mainstream distributors. Of course it does help not to have
a middleman in the game.
[...]
Anyone know how to do this for smaller quantities (<~1k, say?). I've got
plenty of Asian suppliers trying to get our PCB business, but nobody
trying to sell me inductors .
1k? No, I don't think they'd even talk to you.
I should have said "single reel" since there are typically more than
that on a reel. I guess we could probably make a "strategic" buy of ~5k
if needed. If they're *really* cheap 10k! :)
[/quote]
You could talk to companies like this:
http://www.xfmrs.com/
But since 10k is a small qty mostly you'll have to start here:
http://www.globalsources.com/gsol/I/Power-inductor-manufacturers/b/2000000003844/3000000184262/22705.htm
[quote]However, Asia is all about one thing: Connections. Ask the PCB
stuffing places what sort of deals they can get locally. They might be
able to swing it, with a supplier they already buy lots of other stuff
from.
If you have reputable places there that do <1k qty board runs with
decent quality let me (and others here) know. I bet lots of us are
interested.
Sorry, I meant bare PCB not stuffed. They advertise stuffing too - down
to any quantity - but they said once they buy non-free issued parts from
Digikey... :)
[/quote]
That won't give you much of an upside. I recently had a proto-run fabbed
in the US (Aurora, Colorado), full turn-key, and was pleasantly surprised:
http://www.aapcb.com/
[quote]I just thought it strange that I can get small qtys of PCBs at 1/2 the
price of local suppliers, but nobody does the same for parts. (Which
ought to be much easier to supply).
[/quote]
They do, in places like Shenzen, if you let them purchase. But be
careful that they don't substitute, say, an electrolytic for a more
"economical" part.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM. |
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| John Devereux... |
| Posted: Wed Oct 21, 2009 12:48 pm |
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Guest
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Joerg <invalid at (no spam) invalid.invalid> writes:
[quote]John Devereux wrote:
[/quote]
[...]
[quote]
I should have said "single reel" since there are typically more than
that on a reel. I guess we could probably make a "strategic" buy of ~5k
if needed. If they're *really* cheap 10k! :)
You could talk to companies like this:
http://www.xfmrs.com/
But since 10k is a small qty mostly you'll have to start here:
http://www.globalsources.com/gsol/I/Power-inductor-manufacturers/b/2000000003844/3000000184262/22705.htm
[/quote]
[...]
[quote]Sorry, I meant bare PCB not stuffed. They advertise stuffing too - down
to any quantity - but they said once they buy non-free issued parts from
Digikey... :)
That won't give you much of an upside. I recently had a proto-run
fabbed in the US (Aurora, Colorado), full turn-key, and was pleasantly
surprised:
http://www.aapcb.com/
[/quote]
Thanks for the links; bookmarked. Will look into it further.
[quote]I just thought it strange that I can get small qtys of PCBs at 1/2 the
price of local suppliers, but nobody does the same for parts. (Which
ought to be much easier to supply).
They do, in places like Shenzen, if you let them purchase. But be
careful that they don't substitute, say, an electrolytic for a more
"economical" part.
[/quote]
--
John Devereux |
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| Nico Coesel... |
| Posted: Wed Oct 21, 2009 3:26 pm |
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Guest
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Joerg <invalid at (no spam) invalid.invalid> wrote:
[quote]Nico Coesel wrote:
"michael nikolaou" <michaelnikolaou_remove_me_ at (no spam) yahoo.com> wrote:
Hi to newsgroup
I'm making a mcu based device which i want to be very small and low cost
The design consumes 100..120 ma at (no spam) 3.3V
The problem is the installation that requires 12 or 24 volts to operate.
I don't want to use a switching regulator to make the voltage drop since its
to much
circuit involved and also not a zener since its to much heat involved .
Too much circuitry? Look at devices from TI like the TPS5410. Very
small and it will run at a wide variety of input voltages.
But mucho Dolares. I'd try using the MCU if possible if this is a high
volume product. But it'll require lots of nifty engineering. For low
volume, yeah, don't bother and use a chip. Then I'd use the MC34063
which costs under 20 cents. None of those high-faluting newfangled ritzy
ones 
[/quote]
Yes, but the decrease in costs of the chip is compensated by a large
inductor and capacitors.
--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
"If it doesn't fit, use a bigger hammer!"
-------------------------------------------------------------- |
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| Joerg... |
| Posted: Wed Oct 21, 2009 3:36 pm |
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Guest
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Nico Coesel wrote:
[quote]Joerg <invalid at (no spam) invalid.invalid> wrote:
Nico Coesel wrote:
"michael nikolaou" <michaelnikolaou_remove_me_ at (no spam) yahoo.com> wrote:
Hi to newsgroup
I'm making a mcu based device which i want to be very small and low cost
The design consumes 100..120 ma at (no spam) 3.3V
The problem is the installation that requires 12 or 24 volts to operate.
I don't want to use a switching regulator to make the voltage drop since its
to much
circuit involved and also not a zener since its to much heat involved .
Too much circuitry? Look at devices from TI like the TPS5410. Very
small and it will run at a wide variety of input voltages.
But mucho Dolares. I'd try using the MCU if possible if this is a high
volume product. But it'll require lots of nifty engineering. For low
volume, yeah, don't bother and use a chip. Then I'd use the MC34063
which costs under 20 cents. None of those high-faluting newfangled ritzy
ones ;-)
Yes, but the decrease in costs of the chip is compensated by a large
inductor and capacitors.
[/quote]
If you have the space those are cheap. Unfortunately Michael doesn't
have the space. But he also doesn't have the BOM budget for a modern
switcher chip :-(
IOW he is between a rock and a hard spot so the MCU or homebrew may be
his only options. I don't know if his ARM7 MCU has good timers left in
there. If not he's going to have to roll his own around a Schmitt inverter.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM. |
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| Jon Elson... |
| Posted: Wed Oct 21, 2009 10:54 pm |
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Guest
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michael nikolaou wrote:
[quote]Hi to newsgroup
I'm making a mcu based device which i want to be very small and low cost
The design consumes 100..120 ma at (no spam) 3.3V
The problem is the installation that requires 12 or 24 volts to operate.
Check out the LM2575 series of regulator. There[/quote]
should be a 3.3 V version
of it, I use the 5 V version in one of my
products. It needs a Schottky diode
and an inductor as the only additional parts other
than input and output
capacitors. No need for post regulators, it is
very clean.
Jon |
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| legg... |
| Posted: Thu Oct 22, 2009 8:02 am |
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Guest
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On Wed, 21 Oct 2009 10:40:10 +0300, "michael nikolaou"
<michaelnikolaou_remove_me_ at (no spam) yahoo.com> wrote:
[quote]I need to make clear so we avoid confusions
The device uses a 2.2" tft lcd . This thing has a backlight and no matter
what
draws 60 ma at (no spam) 3.3V .That i can't avoid . So with 60 ma Arm7 consumption
i end up to 120 ma.
[/quote]
As described, you intend this 'small' device to dissipate almost 4W
with a linear solution. You should assume a surface temperature rise
of one degree for every milliwatt dissipated by a square centimeter of
surface area for the package. You might want to recalculate
permissible losses on this basis and reappraise some basic design/cost
considerations that include your power source.
[quote]Now as i see whether i have a small switcher with huge inductor and
capacitor
so my board space is too large .
The other solution is super fast switcher that has everything small but
needs 2 euros
at least .
I imagine every engineer designing devices has faced this question .
I'm sure though that a simple cpu controlled switcher could cope with the
large dropout
from 24V to 4-5 volts and then a simple linear regulator could take you to
3.3V so you play
it safe.If things go wrong (cpu malfunction ) thermal shutdown from the
linear regulator would
reset the cpu and that would restart the system
As i see it a simple fet switch with cpu pwm control should satisfy that .
Any opinions or experience on that subject???
[/quote]
In any system with a self-regulated supply voltage, the first question
is 'Who's on first?'. Don't turn this into an Abott and Costello
routine.
RL |
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| Phil Hobbs... |
| Posted: Thu Oct 22, 2009 8:06 am |
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Guest
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legg wrote:
[quote]On Wed, 21 Oct 2009 10:40:10 +0300, "michael nikolaou"
michaelnikolaou_remove_me_ at (no spam) yahoo.com> wrote:
I need to make clear so we avoid confusions
The device uses a 2.2" tft lcd . This thing has a backlight and no matter
what
draws 60 ma at (no spam) 3.3V .That i can't avoid . So with 60 ma Arm7 consumption
i end up to 120 ma.
As described, you intend this 'small' device to dissipate almost 4W
with a linear solution. You should assume a surface temperature rise
of one degree for every milliwatt dissipated by a square centimeter of
surface area for the package. You might want to recalculate
permissible losses on this basis and reappraise some basic design/cost
considerations that include your power source.
Now as i see whether i have a small switcher with huge inductor and
capacitor
so my board space is too large .
The other solution is super fast switcher that has everything small but
needs 2 euros
at least .
I imagine every engineer designing devices has faced this question .
I'm sure though that a simple cpu controlled switcher could cope with the
large dropout
from 24V to 4-5 volts and then a simple linear regulator could take you to
3.3V so you play
it safe.If things go wrong (cpu malfunction ) thermal shutdown from the
linear regulator would
reset the cpu and that would restart the system
As i see it a simple fet switch with cpu pwm control should satisfy that .
Any opinions or experience on that subject???
In any system with a self-regulated supply voltage, the first question
is 'Who's on first?'. Don't turn this into an Abott and Costello
routine.
RL
[/quote]
1000 kelvin per watt in 1 cm**2? That's pretty pessimistic. Even
SOT-23s do better than that.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net |
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| legg... |
| Posted: Thu Oct 22, 2009 8:41 am |
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Guest
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On Thu, 22 Oct 2009 10:06:35 -0400, Phil Hobbs
<pcdhSpamMeSenseless at (no spam) electrooptical.net> wrote:
[quote]legg wrote:
On Wed, 21 Oct 2009 10:40:10 +0300, "michael nikolaou"
michaelnikolaou_remove_me_ at (no spam) yahoo.com> wrote:
I need to make clear so we avoid confusions
The device uses a 2.2" tft lcd . This thing has a backlight and no matter
what
draws 60 ma at (no spam) 3.3V .That i can't avoid . So with 60 ma Arm7 consumption
i end up to 120 ma.
As described, you intend this 'small' device to dissipate almost 4W
with a linear solution. You should assume a surface temperature rise
of one degree for every milliwatt dissipated by a square centimeter of
surface area for the package. You might want to recalculate
permissible losses on this basis and reappraise some basic design/cost
considerations that include your power source.
Now as i see whether i have a small switcher with huge inductor and
capacitor
so my board space is too large .
The other solution is super fast switcher that has everything small but
needs 2 euros
at least .
I imagine every engineer designing devices has faced this question .
I'm sure though that a simple cpu controlled switcher could cope with the
large dropout
from 24V to 4-5 volts and then a simple linear regulator could take you to
3.3V so you play
it safe.If things go wrong (cpu malfunction ) thermal shutdown from the
linear regulator would
reset the cpu and that would restart the system
As i see it a simple fet switch with cpu pwm control should satisfy that .
Any opinions or experience on that subject???
In any system with a self-regulated supply voltage, the first question
is 'Who's on first?'. Don't turn this into an Abott and Costello
routine.
RL
1000 kelvin per watt in 1 cm**2? That's pretty pessimistic. Even
SOT-23s do better than that.
Cheers
Phil Hobbs
[/quote]
An SOT23 is not, in itself, the package that claims the Rthja rating;
it counts on conduction through the leadframe to larger external
surface areas (standard coupon patterns) and produces junction/surface
limits greatly exceeding those permissible to commercial external
surface touch or hold limits.
This simple rule of thumb for average surface temperature rise works
at room temperature for packages between matchbook and breadbox sizes,
without forced convection, to an accuracy of 20%.
This is not manufacturer's published bumph; it is not optimism or
pessimism; it is the result of many physical demonstrations over the
years, repeated usually for the benefit of midding-to-highly educated
and highly over-paid doubting Thomases.
It is also demonstrated in the free magnetics and capacitor
application software provided by mfrs such as Siemens, Magnetics Inc
and Cornell Dubilier - with pedigrees that reach back into the ancient
past of (gasp) print literature. Crunch the numbers back yourself on
whatever packages you currently are familiar with. Any software
package (no matter how expensive) has been mis-programmed or
mis-applied, if it predicts otherwise. Ball park compliance is a good
two-second paper and pencil check of the integrity of a convective
simulation.
RL |
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| John Devereux... |
| Posted: Thu Oct 22, 2009 8:58 am |
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Guest
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legg <legg at (no spam) nospam.magma.ca> writes:
[quote]On Thu, 22 Oct 2009 10:06:35 -0400, Phil Hobbs
pcdhSpamMeSenseless at (no spam) electrooptical.net> wrote:
legg wrote:
[/quote]
[...]
[quote]As described, you intend this 'small' device to dissipate almost 4W
with a linear solution. You should assume a surface temperature rise
of one degree for every milliwatt dissipated by a square centimeter of
surface area for the package. You might want to recalculate
permissible losses on this basis and reappraise some basic design/cost
considerations that include your power source.
[/quote]
[...]
[quote]
1000 kelvin per watt in 1 cm**2? That's pretty pessimistic. Even
SOT-23s do better than that.
An SOT23 is not, in itself, the package that claims the Rthja rating;
it counts on conduction through the leadframe to larger external
surface areas (standard coupon patterns) and produces junction/surface
limits greatly exceeding those permissible to commercial external
surface touch or hold limits.
This simple rule of thumb for average surface temperature rise works
at room temperature for packages between matchbook and breadbox sizes,
without forced convection, to an accuracy of 20%.
This is not manufacturer's published bumph; it is not optimism or
pessimism; it is the result of many physical demonstrations over the
years, repeated usually for the benefit of midding-to-highly educated
and highly over-paid doubting Thomases.
It is also demonstrated in the free magnetics and capacitor
application software provided by mfrs such as Siemens, Magnetics Inc
and Cornell Dubilier - with pedigrees that reach back into the ancient
past of (gasp) print literature. Crunch the numbers back yourself on
whatever packages you currently are familiar with. Any software
package (no matter how expensive) has been mis-programmed or
mis-applied, if it predicts otherwise. Ball park compliance is a good
two-second paper and pencil check of the integrity of a convective
simulation.
RL
[/quote]
That's an interesting and useful "rule of thumb", thanks.
--
John Devereux |
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| John Devereux... |
| Posted: Thu Oct 22, 2009 11:49 am |
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Guest
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legg <legg at (no spam) nospam.magma.ca> writes:
[quote]On Thu, 22 Oct 2009 10:06:35 -0400, Phil Hobbs
pcdhSpamMeSenseless at (no spam) electrooptical.net> wrote:
As described, you intend this 'small' device to dissipate almost 4W
with a linear solution. You should assume a surface temperature rise
of one degree for every milliwatt dissipated by a square centimeter of
surface area for the package. You might want to recalculate
permissible losses on this basis and reappraise some basic design/cost
considerations that include your power source.
[/quote]
[...]
[quote]
1000 kelvin per watt in 1 cm**2? That's pretty pessimistic. Even
SOT-23s do better than that.
Cheers
Phil Hobbs
An SOT23 is not, in itself, the package that claims the Rthja rating;
it counts on conduction through the leadframe to larger external
surface areas (standard coupon patterns) and produces junction/surface
limits greatly exceeding those permissible to commercial external
surface touch or hold limits.
This simple rule of thumb for average surface temperature rise works
at room temperature for packages between matchbook and breadbox sizes,
without forced convection, to an accuracy of 20%.
This is not manufacturer's published bumph; it is not optimism or
pessimism; it is the result of many physical demonstrations over the
years, repeated usually for the benefit of midding-to-highly educated
and highly over-paid doubting Thomases.
It is also demonstrated in the free magnetics and capacitor
application software provided by mfrs such as Siemens, Magnetics Inc
and Cornell Dubilier - with pedigrees that reach back into the ancient
past of (gasp) print literature. Crunch the numbers back yourself on
whatever packages you currently are familiar with. Any software
package (no matter how expensive) has been mis-programmed or
mis-applied, if it predicts otherwise. Ball park compliance is a good
two-second paper and pencil check of the integrity of a convective
simulation.
[/quote]
That's an interesting and useful "rule of thumb", thanks.
--
John Devereux |
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