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hi and welcome back to uh power
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electronics lectures
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and today we are going to cover fairmell
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consideration
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this is one of the important topics
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sometimes they are ignored
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in the curriculum of power electronics
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or sometimes delayed too much
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so the learners actually don't feel the
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importance of this to be so that's why i
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uh
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maybe this time organize it to be
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introduced earlier before even
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we talk about switches because later on
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when we talk about switches and
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participations
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and the advantage at this advantage that
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thermal consideration will be
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also considered during explaining these
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switches so today we will consider
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talking about this important topic
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one of our previous videos we have
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discussed the linear operation and the
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switch operation
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uh for for example for transistor and
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both
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uh have been used to step down the
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voltage from high voltage to lower
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voltage
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and but both of them they dissipated
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different values
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of power and for example for the linear
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we have seen something very high
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for the switch we have seen something
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very low and when
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we use the artist wise it's not zero but
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still some significant value and
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regardless of high and low this power
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distribution will be
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translated to heat finally and
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if i didn't consider good design
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for the heat i will came up to the
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maximum
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and finally i might damage my transistor
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so
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every mosfet or transistor has a data
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sheet and you have something called
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absolute
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maximum ratings and we let's look at the
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continuous
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drain current here we have two values
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and these two values at
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different temperatures so i can use this
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mosfet for example to pass or to flow
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eight ampere if i maintain the
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temperature of the case
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at 25 degree but if the temperature of
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the case
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raised to 100 degree i can't do
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eight and there again i have to reduce
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it or the maximum is reduced to five
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so see this reduction is from eight
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ampere to five ampere it's not
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small reduction it's very significant
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reduction and this link this with the
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power dissipation
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the maximum power dissipation at 25
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degree
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is 125 watt but
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if the temperature is increased so this
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one is not anymore working so
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we have to consider this mosfet maybe to
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be 100 watt or even
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80 watt and if i keep pushing that
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transistor to pass the same voltage and
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current
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i will end up damaging my mosfet
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so let's link this figure here with this
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figure which is called
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linear d rating factor that linear d
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rating factor it says it's
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one what pair cell is used where degrees
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celsius
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that means for each one degree series is
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increase
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above the ambient i'm i will lose one
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watt of the capability of
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dissipating power that means if i have
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10 degree above the ambient and instead
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of 25
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i have now my mosfet has a temperature
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of 35
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i will lose 10 watt from my maximum
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power dissipation for example 115
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instead of 125
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and if i'm working at different
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temperatures i will lose
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a proportional percentage of this one so
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that's why
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the temperature actually present
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for me or produce for me a new mosfet
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with different ratings that i should
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consider
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and if i didn't i will damage my mosfet
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and the thing that i need to look at and
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maintain
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always is called the junction
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temperature we have here the junction
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temperature it says
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it's up to 150 degree that means my
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junction temperature which is
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the inside my transistor
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if it exceeds the temperature which is
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the in this table
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that means i will damage my mosfet so
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this is
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my goal and objective to keep this
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junction
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temperature to be less than 150 and to
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maintain
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also good margin i have to keep it below
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100
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degrees celsius for example and again
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from the same data sheet irf 840
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i have seen the current now
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it was 8 ampere if we consider 25 degree
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case temperature but if the temperature
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now start
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increasing and this is natural because
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it will increase if we draw some current
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and that will be translated to power
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dissipation and then heat
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so if for example i am now at hundred
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degree
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this will come up at five ampere so my
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uh transistor now is changing and the
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current
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that was able to to flow
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is not anymore working and the same for
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example
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something called power d rating curve
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and some that is each
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data sheets provide this curve and one
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of the
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mosfets is irfp 240. that curve tell us
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if you are at low temperatures here okay
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you can use this mosfet to dissipate 150
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watt
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but for example if your temperature is
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increasing because you didn't
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um design your thermal anticipation
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well and you didn't consider me for
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example a cooling element or a fan
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or a heatsink so this will increase
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maybe for example 100 degree 100 degree
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i will come up with
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60 watt dissipation maximum that means i
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if i if i kept my voltage and current
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the same
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for all the period while that
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temperature is increasing
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i will definitely lose my lose my mosfet
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because of
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the d rating factor that's shown in this
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curve
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so all this introduction is to emphasize
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the importance of thermal
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design and consideration that we have to
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take into account to make our mosfet
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work in any different
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uh environment or ambient temperatures
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so the primary goal of our design
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thermal design is to limit the junction
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temperature and you might ask what is
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the junction temperature
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look at the mos mosfet some mosfets
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these are two mosfets and these are two
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bgts
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for example and we have something inside
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them
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called a dye and that dye is the
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silicone piece
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okay um or different semiconductor
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a material and inside it there is a
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junction
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okay p and junction p and p junction or
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or and channels or and
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any junction that uh will be uh
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controlled
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to uh past the um the the current
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and inside this die the junction we have
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to keep that junction
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below the 150 as we have seen in one of
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the data sheets
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so we have different shapes and
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different maybe um
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texture for them but all of these are
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embedded inside and we
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can't measure the temperature of that
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junction so the direct measurement of
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the junction is difficult because its
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package
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blocks access to that junction and
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sometimes we have to measure
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this uh this temperature of that
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junction
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but indirectly using the case
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or the body uh
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exposed metal just to know what is the
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temperature of the inside
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which is the junction and to do that we
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have to know the characteristics
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of the thermal components between that
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junction and between that case
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okay so this is the goal and we have now
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to understand
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how the heat transfer from this junction
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to the ambient okay and
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after that we do our calculation and
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really understand
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what is required to make a proper design
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to start we have
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a transistor here and that's the die
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with the junction
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and we have here the die that die with a
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junction
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will take the power electrical power
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dissipated
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which is um produced by multiplication
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of current and voltage
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and then translate that or convert it to
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heat
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that heat now will be
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flowing through the body of that dye
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and going to the case and that should be
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designed by the manufacturer
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to make a direct flow or the easy way
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for the heat to go from the dye to the
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case if it's kept inside it will be
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it will exceed the maximum temperature
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very quickly
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and damage the junction so that's why we
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have here the temperature of the
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junction
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and that dye is is sitting on the
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uh case and that case also by the time
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while while this developing higher and
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higher current
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higher and higher temperature that also
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the case will develop
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a new temperature but less than the
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uh junction now if that is a hundred
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degrees celsius
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the case will be less than 100 degrees
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celsius
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what controls how much less
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temperature it's called there's a new
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component here between them
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it's called the thermal resistance and
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the thermal resistance works exactly
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like the
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electrical resistance because the
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thermal resistance here
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it blocks the temperature to go from the
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die
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or the junction to the case and if that
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resistance is very very very small that
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means the way of the temperature to
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to go to the case is very easy and all
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will be
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uh fine to go without any obstacles in
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the way
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and the temperature of the case will be
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very close to the temperature of the
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junction but if the resistance here
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between the dye
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and the case very very high that means
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that junction temperature will be kept
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inside
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and the difference between them will be
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also very high
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this is exactly like the electrical
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component
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and this resistance here is is called
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our theta okay thermal resistance and
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because
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our theta this one is between the
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junction and the case
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we call it r theta j c or
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our theta junction to the case okay so
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now look at the junction which is this
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inside
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here we can't actually see it but here
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we have the case
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okay and the case some part of the case
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is exposed to the ambient
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that means some maybe the front side
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here
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is can deliver something richer to the
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ambient and radiate it
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so that's why we have ambient
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temperature here
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and we have something called the thermal
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resistance between the case
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and the ambient and we call it our theta
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case to the ambient
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okay but because that
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case okay can also be attached to the
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heatsink
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if we choosing to install heatsink here
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we have
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another uh choice to make the heatsink
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here
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and there is another component between
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the case and the heatsink which is
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another thermal resistance
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that resistance is called case to the
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heatsink okay
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if we made a good contact between that
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case and the heatsink
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and the the surface is finished very
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well to make a very good
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thermal contact for example we added
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some grease
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or some thermal sheets between these two
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surfaces
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this will increase the surface area
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that can really radiate the heat and
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that
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will make this uh resistance very very
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low
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and that temperature developed in the
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case will also go
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and dissipated also by the heatsink
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because the heatsink now will start
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acquiring this temperature and this heat
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and and raise the
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it's simply above their fins here will
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dissipate it to the ambient
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so this is actually what we have and
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and maybe the last point here we have
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another
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uh resistance which is from the heatsink
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to the ambient okay so now the journey
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from the junction to the ambient okay
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it's also from the junction
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through the junction to case thermal
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resistance
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to case uh to the um
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case heatsink thermal resistance and
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then heatsink to ambient thermal
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resistance
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this is the easy way to flow we have
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another way here
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okay but that resistance is considered
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as very high and we know
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the electrical current if it finds
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two resistors it will go through the
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least
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uh uh resistance way uh in
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in more proportion than the higher
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resistance way okay
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and the same for the heat the heat will
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come here okay
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and we'll find two ways the way to the
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ambient and the way to the heatsink and
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because we have done here
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a good uh thermal
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a contact with very
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low resistance it will prefer to go to
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the
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to the heatsink and dissipated by the
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heatsink okay
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and the amount of heat will dissipate
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directly by that case it will be very
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tiny
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and we will see this actually from the
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data sheet where they specify this one
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is much much higher than these two
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and now after understanding the flow of
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the heat from the junction to the
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ambient
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and we mentioned many times the
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resistance resistance resistance and we
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know there is a resistance
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in the electrical also
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circuits so is that really related to
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electrical can we really
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simulate the heat flow here and we know
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the temperature of the junction or the
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case or heatsink
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by knowing the voltages for example yes
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we can really mimic that flow from the
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junction to the
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ambient by another topology like this
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one
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okay that g is using a current source
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that
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the value of that current source is is
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the same as power dissipated
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so the participated here is translated
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to heat
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but here we are translating it to
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current source okay
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and that current source will push some
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current through these resistances
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which are the exact resistances from the
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way
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uh that the heat flow here okay so we
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have here the junction
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the resistance from the junction to the
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case and then the resistance from the
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case to heatsink
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and then the resistance from the
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heatsink to the ambient and the final
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point here is the ambient
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uh temperature and because we consider
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sometimes the ambient temperature to be
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a constant
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for example 25 degree so that's why we
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have
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uh we have set here a voltage source
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that voltage source
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has a 25 volt and the volt here
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it it means that the cylinders degree so
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the temperature of the ambient
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here will be 25 uh series use
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degree celsius and if you look at the
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circuit here
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i think it's very very easy circuit
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because you have
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defined current okay and that current
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will go through this resistance
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and will develop some voltage okay that
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voltage
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it's the difference between that
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junction
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and that temperature case okay and will
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also flow
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through this resistor here and develop
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also another voltage
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and also we'll flow this uh through this
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one and develop another
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voltage and we know this ambient
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temperature we know that
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the the the current and the values of
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this resistance so all the differences
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here between these points and bridges
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will be also known so some of this
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resistance value will be taken
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from data sheet the power dissipated
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will be calculated by u
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as we have explained before and the
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ambient will be considered as
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as constant as 25 or other um
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temperature values according to what is
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given and that enables
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us to estimate the junction temperature
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that we can't actually measure
00:16:50
okay directly we can estimate our
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calculate the junction temperature
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by knowing all these resistances and the
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power dissipation
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and also uh we can also estimate uh the
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the the
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temperatures and the case and also the
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hissing
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so let's now go another
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step and see how we really translate
00:17:12
that
00:17:13
circuit to something that we can use in
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any simulator
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and we can run some simulation exactly
00:17:19
like
00:17:20
calculating ohm's law so the thermal
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resistance is a component that
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has two points difference
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in temperature okay which is tj and tc
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and that tj junction and case the
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difference between
00:17:36
them will be directly proportional to
00:17:39
that
00:17:39
participated and after some experiments
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you will find that
00:17:44
the relation between the difference of
00:17:46
temperatures here and power dissipation
00:17:48
is controlled by the thermal resistance
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so now we have different uh temperatures
00:17:55
here
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the difference between them is
00:17:57
controlled by the power dissipation
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times the
00:18:00
thermal resistance by taking and
00:18:02
manipulating this relation
00:18:03
a little bit we will come up with this
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relation which is
00:18:07
the thermal resistance equal the
00:18:09
temperature difference between these two
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points
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divided by the power dissipation
00:18:15
and if we compare it with the ohms
00:18:18
though electrical ohms law
00:18:20
we find that electrical ohm's law okay
00:18:23
electrical resistance equal the
00:18:24
difference in voltage between that point
00:18:27
and that point
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divided by the current okay
00:18:31
but in the thermal the thermal
00:18:34
resistance equal the difference
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in temperature here okay between that
00:18:38
point and that point so the temperature
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difference is equal to the voltage
00:18:42
difference divided by the power
00:18:44
dissipation
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so that's why we represented the power
00:18:48
dissipation by a current source because
00:18:51
it mimics the electrical current source
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and now by taking this equation a little
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bit
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more we have the participation here
00:19:00
that's
00:19:01
considered as a current source and that
00:19:03
will go through the
00:19:04
uh junction to case thermal resistance
00:19:07
look at this one junction two case
00:19:10
thermal resistance
00:19:11
and after that we have two ways the
00:19:13
first way is
00:19:14
from the case to the ambient and the
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second way is
00:19:17
from the case to the heatsink so one
00:19:20
once we just came up to the temperature
00:19:22
of the case we have two ways
00:19:24
the case to the ambient and it reached
00:19:26
the ambient temperature here
00:19:28
and also the case to the heat sink and
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after going
00:19:31
from the case to the heatsink we have
00:19:33
also another one which is from the
00:19:35
heatsink to the
00:19:36
ambient okay and because this one was
00:19:39
very very very large so that's why i
00:19:42
uh ignored this in the first
00:19:46
graph but here i just want to consider
00:19:49
it because
00:19:50
i want to deliver my uh full view
00:19:53
okay so now we have current source and
00:19:56
we have voltage here
00:19:57
uh constant voltage okay that means that
00:19:59
makes this point
00:20:00
constant the temperature at any point
00:20:03
here
00:20:04
is the same voltage we calculate for
00:20:07
this
00:20:07
electrical circuit and the current
00:20:11
here the value of the current source
00:20:13
here okay is actually the same as the
00:20:15
power dissipated calculated electrically
00:20:18
and the temperature or the thermal uh
00:20:21
resistance here can be taken from the
00:20:24
data sheet or the
00:20:26
uh data sheet of the mosfet or the or
00:20:28
data sheet of the heatsink itself
00:20:31
and you will find some data sheets
00:20:35
calculated or presented as uh how many
00:20:38
degrees where what
00:20:39
or how many kelvins per watt if you find
00:20:42
this or find this
00:20:43
they are the same why because the the
00:20:46
increment
00:20:46
in kelvin pair while it's exactly the
00:20:49
same as
00:20:50
increment of degrees per watt so it's a
00:20:53
ratio
00:20:54
equals a ratio okay so that's why if you
00:20:56
if you found kelvin or degree
00:20:58
don't try to convert okay it's just take
00:21:01
it as it is because they are
00:21:03
equal and if you look at this model this
00:21:05
is the electrical model
00:21:06
and if you then throw some references
00:21:09
for example books or
00:21:12
papers you will find maybe a little bit
00:21:15
different models okay
00:21:17
one of them they use this uh
00:21:21
this current source and that was is
00:21:23
floating
00:21:24
but uh maybe the correct one is to
00:21:27
consider this
00:21:29
current source is is reference to the
00:21:31
ambient temperature because the
00:21:33
definition
00:21:34
of the power dissipation is
00:21:37
is the power dissipation that's
00:21:38
converted to heat to raise
00:21:41
the temperature of the junction above
00:21:43
the ambient okay
00:21:44
so that's why maybe this one is more
00:21:46
accurate and some
00:21:48
references they just attach to the
00:21:50
ground which is zero degree here okay
00:21:52
so if you consider this one or this one
00:21:54
or this one
00:21:55
the good news is they are bringing to
00:21:58
you the same answer so
00:22:00
don't worry about which one you find it
00:22:02
which one is correct or wrong no
00:22:04
all of them are right and uh correct and
00:22:07
they can consider
00:22:08
that they can be considered but maybe
00:22:10
the
00:22:11
this one and this one can be easily
00:22:14
drawn in
00:22:14
uh electrical simulator and the circuit
00:22:17
simulators
00:22:18
like lt spies and and can be used for a
00:22:21
calculation of these temperature
00:22:23
uh points here and maybe in in this
00:22:25
module i will consider this one
00:22:27
because it's easy uh for us just to draw
00:22:30
it and
00:22:31
just to bring all these uh
00:22:34
calculation very easily uh one important
00:22:37
point which is
00:22:38
all these models are steady state models
00:22:41
if you also gonna throw some references
00:22:43
you will find you will find
00:22:45
some references adding some capacitors
00:22:48
across these resistors because they want
00:22:51
to say the grow
00:22:53
okay over time of that temperature
00:22:56
but these models are giving us the final
00:22:59
temperature they don't actually
00:23:01
uh introduce us or show to us the
00:23:04
growth of the temperature over time like
00:23:06
the capacitor
00:23:08
charging okay so that's why we will
00:23:10
consider this just for design
00:23:12
we want to know what's the final value
00:23:14
to consider it if we want to
00:23:17
use heatsink or a fan or just uh we are
00:23:20
fine with it okay
00:23:21
so we don't or we we are not interested
00:23:23
here in the transient
00:23:25
and if you're gonna throw many data
00:23:27
sheets and for example rf
00:23:29
840 just to know where where are the
00:23:32
uh these thermal resistance uh
00:23:36
may be included so we have some some
00:23:38
parts of the tables they call
00:23:40
them resistance ratings and one of them
00:23:42
is called
00:23:43
junction to ambient that means it's the
00:23:46
r theta junction to ambient
00:23:49
okay from junction to the ambient does
00:23:52
it consider
00:23:54
the heatsink or no the answer is
00:23:57
no the junction to ambient means this
00:24:00
resistor
00:24:01
plus this resistor they are all
00:24:03
contained now
00:24:04
together all combined together as one
00:24:07
resistor
00:24:08
because they consider that you will not
00:24:10
use the heatsink so
00:24:12
all this part now is not there okay
00:24:15
for the first item here that means this
00:24:18
resistor plus this resistor
00:24:20
is 62 series per watt
00:24:24
okay and that value is taken
00:24:27
if you are considering your calculation
00:24:30
without heatsink you want to know
00:24:32
if you didn't use the heatsink how much
00:24:34
the
00:24:35
junction temperature okay so it's easy
00:24:37
you know the participation
00:24:39
you know this value plus this value is
00:24:41
62
00:24:42
and you know the ambient so it's easy to
00:24:44
calculate the junction
00:24:46
temperature and you will finally
00:24:49
decide whether it's more than 150 or no
00:24:52
but
00:24:52
now we have also different values this
00:24:55
value is called case to heatsink
00:24:58
so i think we mean this one okay
00:25:01
case to heat sink and it says here we
00:25:04
have a flat
00:25:04
surface and the greased that means it
00:25:07
look like this
00:25:08
okay so it's a flat surface and the
00:25:11
grease that means we have
00:25:12
some material that make a good and
00:25:14
perfect contact
00:25:16
between the two surfaces because we have
00:25:18
some defects and that
00:25:19
grease will will fill these defects and
00:25:22
make a good thermal contact
00:25:24
and this resistor here if we've done
00:25:27
this
00:25:28
we will bring the resistance to be
00:25:31
about 0.5 cylinders per watt which is
00:25:34
very very low
00:25:35
okay so this resistance is very low
00:25:37
sometimes we ignore it
00:25:38
if we made consider the thermal
00:25:44
greasing or or or sheet
00:25:47
okay maximum junction to case now we
00:25:49
have junction to case this one
00:25:52
this one is considered also alone if you
00:25:55
are considering a heat
00:25:56
um sink later on and that one is one
00:26:00
silicon cylinders per watt okay so the
00:26:03
the other two here are considering if i
00:26:06
want to use
00:26:07
the uh the heatsink okay or the first
00:26:10
one no
00:26:11
it just goes to the ambient uh
00:26:14
directly what about the heatsink the
00:26:16
heatsink
00:26:18
thermal resistant that one is brought
00:26:21
from the
00:26:24
data sheet of the heatsink itself so we
00:26:27
don't
00:26:28
we don't actually get it from the data
00:26:30
sheet of the mosfet and here is another
00:26:32
example which is
00:26:33
bgt transistor bd139
00:26:37
and we have now thermal resistance from
00:26:39
junction to case
00:26:40
junction to case this is the resistance
00:26:43
uh
00:26:44
value which is 10 series per watt and it
00:26:47
gives us the resistance between junction
00:26:49
to ambient
00:26:50
anything junction to ambient doesn't
00:26:52
consider the
00:26:53
uh heatsink so junction to ambient this
00:26:56
one and that one
00:26:58
is a hundred okay so now you can really
00:27:01
calculate the junction or to k
00:27:05
the case to the ambient as 90 why
00:27:08
because the junction to the case
00:27:10
is 10 and junction to ambient is 100
00:27:14
so the case to ambient should be 90 and
00:27:17
that
00:27:18
reveals that it's very very high that's
00:27:20
why we usually ignore it
00:27:22
but again you have to consider this
00:27:24
value if you are
00:27:25
using a heatsink and you have to
00:27:27
consider this value if you are not using
00:27:29
the heatsink
00:27:31
and another a third example we have this
00:27:34
mosfet as well and that mosfet has
00:27:37
three packages this is the first package
00:27:41
which is to263 or d-square pack
00:27:44
this is the second package which is 220
00:27:47
this is the shape of the package it's
00:27:49
it's uh this is the name of that shape
00:27:52
and we have a third package which is
00:27:54
t0247
00:27:55
what's the point here the point is the
00:27:57
data sheet is giving us different
00:28:00
thermal resistance for each package and
00:28:02
you have to consider
00:28:04
not just the mosfet itself you have to
00:28:05
consider also what package
00:28:07
you will get uh from the manufacturer
00:28:10
okay
00:28:10
so for example let's see the first one
00:28:12
thermal resistance
00:28:14
junction to case okay so from the
00:28:17
junction to the case
00:28:19
junction to the case junction to the
00:28:21
case okay
00:28:22
it's the same for all three because it's
00:28:24
very obvious that the structure is the
00:28:26
same
00:28:27
and the case may be the case uh is
00:28:30
optimized to also to deliver
00:28:32
the the the same resistance okay
00:28:35
between the junction and that case so
00:28:38
the value
00:28:39
of that resistance is 0.83
00:28:42
now look at the second one it's
00:28:44
resistance
00:28:45
junction to the bcb this is a new one
00:28:49
but it gives us just value for the first
00:28:51
package
00:28:52
not the second and the third why because
00:28:55
the first package
00:28:56
is considered to be soldered on the pcb
00:28:59
itself
00:29:00
and the pcb will be like like a heat
00:29:03
sink okay
00:29:04
so that's why it's like a smd
00:29:08
component and will be sorted on the
00:29:10
surface of that
00:29:11
pcb and it says here the junction to the
00:29:14
pcb
00:29:15
junction to the pcb through the case it
00:29:18
will be
00:29:19
30 okay and you have to maybe calculate
00:29:22
also the thermal resistance
00:29:24
of that pcb by some other ways
00:29:27
to get the final temperature flow
00:29:30
okay but we will not consider this now
00:29:33
we will consider that junction to
00:29:34
ambient
00:29:35
look at that junction to emit so if we
00:29:37
are going to the ambient that means we
00:29:39
are not considering heatsink
00:29:40
so from junction to the case to the
00:29:43
ambien directly without heat sink okay
00:29:45
and those two we have 62.5 for this one
00:29:50
and we have 50 which is lower for this
00:29:52
one
00:29:53
lower is bitter okay and that's that's
00:29:56
obvious because this one is bigger that
00:29:57
means bigger surface
00:29:59
more radiation and maybe it's optimized
00:30:02
more for for uh for dissipating that
00:30:05
heat okay
00:30:07
so i want just to highlight some
00:30:09
differences between data sheets and how
00:30:11
we read
00:30:12
these values from these data sheets and
00:30:14
finally is the heatsink and if you
00:30:16
consider hitting in your design
00:30:18
i think you will find various shapes
00:30:20
ratings and different
00:30:23
series among the manufacturers
00:30:26
and to be honest we can't actually bring
00:30:28
them all in one slide
00:30:30
but maybe some of them are very popular
00:30:33
like the e-series
00:30:34
okay where you have to fit the
00:30:38
to220 mosfet here and by some bolt and
00:30:41
also you can
00:30:42
grease it and then you consider it as a
00:30:45
heatsink for your mosfet
00:30:47
we have different shapes i will not
00:30:48
actually talk about them because
00:30:50
the everyone has has has its own um
00:30:53
pros and cons and uh we really it's not
00:30:56
it's not it's not an easy topic to cover
00:30:59
so we will consider one example which is
00:31:01
the e-series and that one if you look at
00:31:04
the
00:31:04
data sheet for one of them okay it will
00:31:08
it will tell you what is the thermal
00:31:10
resistance to be
00:31:12
to be considered a new calculation and
00:31:14
the thermal resistance here
00:31:16
for the first one it has different types
00:31:19
and different sizes as well
00:31:21
okay and they bring them here for us
00:31:24
okay
00:31:25
so the first one is different from the
00:31:27
second and the third series
00:31:28
is by the size height okay and the v
00:31:32
and a we have t okay we have v and the
00:31:35
v and the v and a it means the finish
00:31:38
the
00:31:38
the surface finish and if it's grease or
00:31:41
not grease or
00:31:42
other things okay but forget this we
00:31:45
are focusing now on the thermal
00:31:46
resistance for the first one it's about
00:31:49
11.4
00:31:51
okay and the second is 9.5
00:31:54
9 7.5 7.4 6.2
00:31:58
and those thermal resistances
00:32:02
have been calculated if the power
00:32:05
dissipated
00:32:06
is about 10 watt okay so
00:32:09
these values are specified at the tin
00:32:12
watt for example
00:32:13
but if i'm not considering tin what will
00:32:16
i find something that helped me
00:32:17
the specified thermal resistance for
00:32:20
that
00:32:21
uh participation which is different from
00:32:23
tinua and there is something here called
00:32:26
natural conviction what the meaning of
00:32:29
that
00:32:29
if you have gone to this heatsink data
00:32:33
sheet you will find something like this
00:32:35
and this is very common between the
00:32:36
heatsink
00:32:37
you have four axes two horizontal and
00:32:41
two vertical
00:32:43
the left side with the bottom side axis
00:32:46
that one and that one they are two
00:32:48
together and you might find these arrows
00:32:50
here that means
00:32:51
these three curves here okay are
00:32:55
read by this co this axis and that axis
00:32:58
okay and those three are considered if
00:33:01
you are considering natural cooling
00:33:03
what we mean by natural cooling that
00:33:05
means we don't use a fan
00:33:07
or any other cooling element okay so
00:33:10
it's just the heat sink
00:33:11
uh with contact to the natural ambient
00:33:15
okay
00:33:15
so that's why it's called natural
00:33:17
cooling and if we want to read
00:33:19
the natural cooling case we have to
00:33:21
consider these curves
00:33:23
and these axes for example
00:33:27
if i have 5 watt and i'm using
00:33:30
the second heat sink second heat sink
00:33:34
means the
00:33:35
middle curve and five word dissipation
00:33:38
means here if you gone like that and
00:33:42
there
00:33:42
you will find 40 40 watt it's
00:33:45
40 a degree difference
00:33:49
from uh from the ambient okay so the
00:33:52
temperature rise will be
00:33:54
40 and if you divide 40 by five
00:33:57
you will come up with eight it uh
00:34:00
silly zeus where what this is the value
00:34:03
of your resistance
00:34:05
uh or thermal resistance okay
00:34:08
now if you have more or less power
00:34:11
dissipation
00:34:12
you can also calculate that um in a
00:34:14
similar way
00:34:16
but if we have a fan or we want to use a
00:34:20
fan and we want to know if the thermal
00:34:22
resistance is different
00:34:23
we can consider the other three curves
00:34:26
here okay
00:34:27
this three here can be read by the right
00:34:30
axis this one
00:34:31
and the top x is that one the tube axis
00:34:34
is that
00:34:34
air velocity if you have a fan you can
00:34:37
sit it on some velocity
00:34:39
and you can use some sensors to see
00:34:43
what is the air velocity for example if
00:34:45
i have 500
00:34:47
feet per minute okay that 500 will go to
00:34:50
the same heatsink which is the middle
00:34:52
one
00:34:53
and then this one okay and go to the
00:34:55
right
00:34:56
and i find the value of the heat of the
00:34:59
thermal resistance
00:35:00
is four it was before eight without a
00:35:03
fan and now it's
00:35:04
four with a fan that means my fan
00:35:07
reduces the thermal resistance
00:35:09
and makes the heat sink dissipate and
00:35:12
makes the
00:35:13
transistor uh see the heatsink as
00:35:17
easy source easy sink for its
00:35:19
temperature and that will dissipate
00:35:21
more heat okay so these are very very
00:35:24
important to consider
00:35:25
if you are looking for heatsinks and
00:35:28
finally just a quick information about
00:35:30
the
00:35:31
heatsink sometimes you might consider
00:35:33
more
00:35:34
fins for the heatsink means more
00:35:36
radiation
00:35:37
and less thermal resistance which is not
00:35:40
actually
00:35:41
accurate because this figure now
00:35:44
shows to us uh the inside thin spacing
00:35:48
so if i have
00:35:49
many fins and they are very very close
00:35:52
to each other
00:35:53
the uh the thermal resistance start to
00:35:56
increase
00:35:57
by decreasing the distance uh of of
00:36:00
these fins okay
00:36:01
that increase increasing and that's due
00:36:03
to that closer proximity
00:36:05
of adjacent fence and uh there is no
00:36:07
flow or proper flow
00:36:09
will go between these fins and if you
00:36:12
start
00:36:12
increasing the distance between these
00:36:14
fins you will
00:36:16
bring the uh the the thermal resistance
00:36:19
to an optimum value but
00:36:21
more increasing you will end up with
00:36:24
less surface because if you're rid of
00:36:26
that one and rid of that one
00:36:28
and you have two uh
00:36:31
good spacing or highly spaced space fans
00:36:35
you will reduce the
00:36:38
surface that's exposed to the air and
00:36:41
this
00:36:41
is changing the heat with it so this is
00:36:44
the point of this figure it's not your
00:36:45
job to
00:36:46
look at the spacing for the fence but
00:36:50
maybe it's just a correction of some
00:36:52
concepts here okay and the
00:36:54
optimization of any heatsink is done by
00:36:56
the manufacturer
00:36:58
and also they finally bring to you the
00:37:01
curves that we have seen in the previous
00:37:03
slide to decide what is the thermal
00:37:05
resistance
00:37:06
for any power dissipation or uh for any
00:37:09
air velocity
00:37:10
and that brings me to the end of this
00:37:12
slide
00:37:13
uh before covering some examples next
00:37:16
time in the next video
00:37:17
uh for how we use that electrical model
00:37:20
to
00:37:20
calculate the junction temperature and
00:37:22
case temperature or any other things or
00:37:25
use it for the design as well and to
00:37:27
decide whether we need heatsink or a fan
00:37:30
or no so see you in the coming video
00:37:33
thank you
