en-AU.vtt

WEBVTT
Kind: captions
Language: en

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English (AU) (Spoken) [Manually Transcribed Captions]
github.com/WizardTim/WizardTim-captions

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This is the ubiquitous TO-220
through-hole package that I'm sure we're all familiar with.

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This device in particular is the popular 1117 low-drop-out voltage regulator

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manufactured by ST. And the first thing you're probably

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going to calculate when using one of
these

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is the thermal performance and so the
data sheet here gives us these figures

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for the package thermal resistances. With
this here junction to ambient figure we

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can calculate the die temperature for a
given amount of power

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or conversely the power at which the
regulator reaches Tj max.

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So as you can see calculating the
thermal performance of this package when

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free standing
is very easy and the numbers you get are

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quite often very close to reality.
But of course the big advantage of using

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a TO-220 style device is the fact that
it can be bolted to an external heatsink

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which allows it to dissipate
significantly more than 2 watts

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or simply just operate at a lower
temperature and extend its life.

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So this here is a pretty standard
stamped copper heatsink from AAVID Thermology

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and looking at the data sheet for this

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we find
a nice little graph here which is quite

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common amongst heatsinks
although i wish manufacturers would use

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meters per second
not US customary feet per minute.

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But using this line here we can find how
hot our heatsink will get for a given

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amount of power dissipated with
convection cooling only

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or if we have forced airflow we can use
this other line to get a thermal

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resistance to ambient figure.
Doing the calculations again but instead

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for this configuration
we can easily show that this will reduce

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the die temperature
at 2 watts to significantly less than Tj max.

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So doing this kind of thermal design

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calculations with this TO-220
through-hole part is pretty easy and

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straightforward.
Instead you should be worrying about

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making sure you have the correct
mounting force

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or whether or not you're placing it next
to a wet electrolytic capacitor.

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But what about this same 1117
regulator but instead from

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Advanced Monolithic Systems and in this 
SOT-223 package

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how do we do the same calculations. Well
looking at the datasheet again we see

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thermal resistance numbers
but with this asterisk; thermal

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resistance depends on the size of copper
area connected to the case tab.

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Okay that makes sense seeing as the
tab is soldered to the PCB

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and it's not like there's a bolt hole
you can connect the heatsink to.

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And so if we look further we see that
they've given us a nice little table of

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thermal resistances
for several different sizes of copper

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areas and well this looks easy enough to
calculate

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and it actually is. However, the problem
comes when you actually come to apply

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this
let's look here at this PCB and well

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what here counts towards our copper area
what

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parts will actually provide useful heat
sinking

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I mean all this copper here in the
ground plate probably counts right?

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There's only these little tiny gaps
between the copper areas here between

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the power rail and the ground plane
and well I see regulators on PCBs like

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this all the time
that just use the minimum footprint size

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with the ground plane surrounding it
I mean the thermal resistance of the pcb

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can't be that bad right?
I mean look here we've got copper at 400

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watts per meter kelvin and
going down, oh, ohhhhhh...

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okay yeah this might be a problem.

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So
this here is a PCB I designed back in

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2016
I made it for fun to see if I could make

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my own multiplexed LED matrix
and I'm pretty proud of it I was even

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able to get the entire LED matrix on
one layer because I used the LED leads

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themselves as jumpers across the common anode lines

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useful little trick for you however
today we're interested in it because on

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the back of it we have the same
AMS1117 regulator with a nice

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little PCB heatsink.
Now at full brightness this regulator

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dissipates
less than a quarter of a watt so this

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heatsink is plenty big enough.
However I have here a spare PCB that

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I've repurposed as a thermal test board
and I've been

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abusing it just a little bit. By the way
if you've ever wondered what happens to

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a 1206
quarter watt resistor when you put

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several watts through it
for a little bit too long yeah don't do it.

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So we're going to use this
PCB

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to test our theory to see whether or not
these little gaps actually do anything

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currently I have the board under the
thermal camera now

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and I've got the output loaded down so
it will dissipate about one and a

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quarter watts
and when we turn it on we instantly see

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the temperature rise.

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Okay now that it's reached steady state
we can clearly see the outline of that

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copper area directly connected to the
package tab

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so unfortunately that means the
temperature drop across this little gap

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here is
rather significant so that's definitely

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going to have an effect on how much heat
is conducted into this ground plane and

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it's probably not as much as I would
have liked.

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And of course if we push this
device even further

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to dissipate several watts we can see
this little heatsink is never gonna save it.

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And so this brings us to the part where

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I screwed it all up with my thermal
design and

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roasted some LEDs. This here
is a modular LED panel I designed back

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in 2018
before I had a thermal camera and before

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I knew
these gaps were a big problem for SMT design.

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This thing is pretty simple it
takes 12 volts and steps it down to a

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constant current for the LEDs
with near zero flicker and the

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brightness is controlled with this
potentiometer.

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I actually use three of these panels on
a shared heatsink above me

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for filming and reference photography
and at

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10 watts per panel I get about a
1,000 lux here with them all at 50%

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which is more than enough. So overall I'm
happy with them and all of the files are

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on GitHub if you're interested
although you might not want to use them

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after I show you the problem it has.
If we look here at the LED side of the

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traces
we see the pads of the LEDs are

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connected to these big traces
and when designing it I paid extra

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special attention
to length matching these traces to

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equally drive the LEDs.
But I just assume these little gaps here

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between the traces in the ground plane
wouldn't be a big contributor to thermal

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problems
I even had plenty of vias to transfer

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heat to the other side of the board
but again here I have the same problem

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with traces and gaps in the ground plane.
I was assuming that this would be enough

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to transfer
heat out of the PCB and into the

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heatsink
and well I was kind of correct it

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doesn't instantly catch fire
but, I definitely could have done a lot

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better and not needed to actively cool
the heatsink

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if I had designed it just a little
different.

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So I have the LED side of the PCB

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now under the thermal
camera and

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turning it on yeah we see the gaps are
definitely a problem here as well.

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And now at steady state we can see this
thing has

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big problems running at 100%. Now remember
this thermal camera

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can only show us the temperature of the
top of the phosphor of this LED

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and that's at 112 °C that's
not the die temperature

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the die temperature is
definitely over Tj max which is 115 °C

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so this LED is definitely having a short life.

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And again we see the heat isn't

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conducted very well across this gap, heat
is still

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transferred but it's nowhere near as
good as it could be

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taking a look at the other side of the
PCB now

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and we can see the vias are doing a
great job at transferring heat through

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the PCB to the other side
however we have the same problem.

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And this side we're relying on the copper
area

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to spread out the heat so it can be
transferred through more surface area to

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the heatsink and it's definitely not
doing a good job at that.

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So I hope you've learned something
useful here so you don't have to go

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through the same mistakes i did to learn
this.

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So as always thanks for watching.     :)