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hi this continuation for power
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electronics lectures and today we are
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going to continue
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talking about transistors in the
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previous video we have
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spoken about bgt transistors
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and what are the differences between um
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the normal transistors and the power
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transistors
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and then we can throw some more details
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and today we are going to
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discuss the mosfet transistors
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especially the power
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and mosfet transistors so as we know
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from electronics fundamentals that the
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mosfet in general they are voltage
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controlled devices we don’t need to push
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current
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into the gate to make it on like the vgt
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transistors
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so we have three terminals for the
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mosfets
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which is the gate and the drain and the
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source
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and we have two types which are
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n-channel and p-channel mosfet
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and from the background i think we know
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that the mosfet is available as
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a in depletion type or enhancement type
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but the most popular one and common now
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is the enhancement type so that’s why
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i’m going to focus on enhancement type
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mosfets so the relation between
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the vgs which is the voltage between the
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gate and source
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and the current look like this okay we
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have this curve
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which describing the voltage i’m
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applying
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across the gate and source okay
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and the relation with the current going
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through that
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mosfet between drain and source so
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it’s different from the bgt transistor
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where i need
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to push current to make current between
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collector and emitter we have here to
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apply some
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voltage between gate and source okay to
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make
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current uh passing through the
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transistor from the drain to the source
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and there is very important point which
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is
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the voltage i’m applying is between the
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gate and the source
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not the gate and the ground because we
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sometimes have some circuits
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the ground is far away from the source
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and maybe you have your load or another
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a component between the source and
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between the ground so when you apply
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your voltage you have to make sure that
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you are applying between
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the gate and the source and the voltage
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between the gate and
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the source has something threshold and
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that threshold is
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mentioned in the data sheet and it’s uh
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it depends on the structure and the type
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of the mosfet and the power
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and current ratings so we have in this
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example definition is about
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2.2 voltage or 2.5 voltage and after
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that
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now raising the vgs the current go more
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and more and more and more
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the relation between the vgs and the
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current
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is determined by the this slope which is
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there the
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rds on reciprocal so the relation
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between that current and that voltage
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is determined by the rds on which is
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also mentioned
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in the data sheet so mosfet can be
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easily modeled as a resistance
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rds on between the drain and the source
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so it’s capable of
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bi-directional current flow and this is
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very important point
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so the mosfet by this way okay it can
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pass the current from the drain to the
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source and
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from the source to the drain it’s
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different from the bgt where we have
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just from the cooling 30 emitter if it’s
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np and transistor
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it doesn’t uh allow the current to flow
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from the emitter
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to the collector and this is good
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feature for the mosfet in some circuits
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like
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inverters so now we have n channel and p
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channel but
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which one is more popular in our
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circuits is the n channel and the reason
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behind it is
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is it cheaper uh compared with the p
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channel with the same ratings and also
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it has
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very low uh rds on resistance
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uh during the on time compared with a
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pitch and a p
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channel sometimes has uh three times or
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four times
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are the s on if we compare it with the n
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channel
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so that’s why they don’t prefer to use p
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channel
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in the circuits it has one or two
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advantage when you drive it in high side
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which we will
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discuss later but the ant channel is
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more popular to use
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and if we look at the structure of the
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mosfet again
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we have a little bit different structure
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like the
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diodes and transistors they have a
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different regions with different
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turbine uh percentage here so we have
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here
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and positive and then pre and then n
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negative
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and then n possessive and we have here
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the source
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okay and the gate and here we have the
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drain
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okay but this region here which is
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s i o two it’s like a direct lick region
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like insulator that’s why we don’t have
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a
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an electric contact between the gate and
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the other pn junction
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inside the mosfet and if you look deeply
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into this structure
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you have n p and n
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you have also p and n what does it mean
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it means that we have inside the mosfet
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we have
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other structures for other components
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like
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pgt transistor and like diodes
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so that’s why it this structure has
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inherently and has parasitic components
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and one of the main parasitic components
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is that deut
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we have look at the this junction we
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have p
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and n negative and and positive what
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does this
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remind you i think this is the same
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structure as the power div
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okay so we have inside the mosfet
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parasitic diode
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and that there is a is is reversed there
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so we have the symbol sometimes
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is like this so we have this body that
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is it drawn
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from the source to the drain like that
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okay so that one
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is not intentionally made but it is a
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parasitic one we have also another
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structure here as well which is
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n p n what is np and
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it’s a transistor but that transistor
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has been cancelled
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when they short this pn junction by a
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metal
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okay so that one here is shorted
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and they cancel that a npn transistor
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and the one that is kept is the uh
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p and junction here which is uh a power
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diode and this picture here shows that
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terminals of
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an example a mosfet we have a gate drain
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and source and we have that metal
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which is the body one is drain terminal
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as well
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and we have a very important question
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which is is there any problem with this
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plastic
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diode inside the mosfet the main problem
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is
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it has very long reverse recovery time
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that means if we have a circuit that we
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need to switch
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off the mosfet that diode here
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try to pass the current for a short time
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which
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is the reverse recovery time through the
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mosfet
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through the data inside the mosfet and
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that sometimes
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makes some problem for some structures
00:07:25.599
so that’s why we will study this
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maybe in in later circuits but keep in
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mind that
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this has a long reversible time and
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and sometimes to deal with this they
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they have to provide another day
00:07:39.360
externally
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with lower voltage and higher speed just
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to make
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a replacement of that build and now as
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before we have to discuss the
00:07:48.479
switching power mosfets and because of
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the structure of the mosfet and there
00:07:53.360
are some insulations
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and some dielectric we make also
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parasitic capacitors
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so for example we have a capacit small
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capacitance between the drain
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and the gate and between the gate and
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the source and also between the drain
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and the source okay and when we switch
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on
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and off this mosfet here we have to
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just put in mind that these
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voltages and these currents pushed into
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the mosfet
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they have to charge these capacitance
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during the switching time
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okay and that makes sometimes problems
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to the most with and how we drive it and
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that’s why we have to
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select some drivers uh carefully to
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satisfy the mosfets and the capacitance
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appearing at the terminals of these
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mosfets
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and now for example we have here a
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switching
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diagram we have off time and then we
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applied some voltage here
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at the gate and that voltage we
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mentioned before that there is
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insulation here and there is no current
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will go to the
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gate yes there is no current will go to
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the structure there
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but because of the capacitance there is
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current will go through the
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parasitic capacitance appearing between
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the terminals
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and practically we have to see some
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current going through there
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just to charge these capacitors because
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without
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charging these capacitors we are not
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going to pass
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the v threshold and turn on our mosfet
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so for example now we applying here um
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a pulse okay and we have that a turn
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on edge as it’s shown here what happens
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when we
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giving a turn on edge there’s current
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will go to
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started charging this mosfet which is
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the between the gate and the source
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and it’s like charging any capacitor the
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voltage will grow smoothly okay
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until we pass the something called v
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threshold and that v threshold with it’s
00:09:59.040
the value that
00:10:00.399
will the current will start passing
00:10:02.959
between the
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drain and source so when i reach this
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point now
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the drain will open a little bit
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and the current will start flowing
00:10:16.399
between the drain
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and the source and at that time the
00:10:20.720
voltage between
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and the source will now start to drop
00:10:24.399
okay
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so now at that threshold exactly we have
00:10:28.320
a drop in voltage between the drain and
00:10:30.399
the source and also flow of current
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so what happens later because the drain
00:10:36.320
now
00:10:36.800
start to uh to to
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drop the voltage between between it and
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between the source so
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for example we have hundred volt between
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the transfers in the off time
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but we in the on time process now the
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voltages
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start to drop drop the drop drop between
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the drain and the source as
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as we see here that means this terminal
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of that capacitor now
00:11:02.000
is going is going is going to be
00:11:05.360
at this point at that point eventually
00:11:07.760
so what happened next
00:11:09.839
actually we can simulate and model
00:11:12.880
what’s happening in this process here
00:11:15.920
by paralleling these capacitors that
00:11:18.800
means
00:11:19.279
instead of charging one capacitor during
00:11:22.320
the
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on time we will see another capacitor is
00:11:27.040
included
00:11:28.000
okay and that capacitor is not small is
00:11:30.320
also big
00:11:31.440
and that will make the charging
00:11:35.360
uh like like stable here okay and there
00:11:38.079
is no voltage will go
00:11:39.680
grow okay and that um
00:11:42.959
a flat edge is called the plateau okay
00:11:46.160
and that’s called because of the miller
00:11:48.480
capacitance and miller
00:11:50.240
effect okay so that capacitor here will
00:11:52.959
look like
00:11:53.760
big capacitor during this transition and
00:11:56.639
the
00:11:57.200
current needed to charge one capacitor
00:11:59.680
now we have to charge
00:12:01.680
two capacitors now okay so that’s why
00:12:04.240
now we
00:12:06.000
will see a turn on okay edge
00:12:09.040
and then a flat edge and then after
00:12:12.079
start discharging this capacitor from
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the previous
00:12:15.920
charge and then recharge it with a new
00:12:18.800
charge from the gate
00:12:20.320
so it spends some time and then we’ll
00:12:23.600
charge it again to the value that we
00:12:26.240
need okay
00:12:27.120
so the process of of switching on the
00:12:29.839
mosfet and the gate of the mosfet
00:12:31.600
has a three parts which is the v
00:12:34.560
threshold
00:12:35.440
and then the plateau and this edge now
00:12:38.079
is called the gate blatant
00:12:39.760
voltage and then completing the
00:12:43.279
the charging process or turning on the
00:12:45.920
mosfet
00:12:46.800
and this is the process for the for
00:12:50.320
switching on the mosfet and when we look
00:12:52.560
at the charges we
00:12:53.920
spending and we have to push into the
00:12:56.720
gate
00:12:57.440
to charge the phase capacitance and that
00:13:00.480
second capacitance also it has
00:13:02.720
this a waveform so this is the charge
00:13:05.839
axis
00:13:06.320
and this is the vgs axis so the voltage
00:13:10.000
will grow if i pushed more charges and
00:13:13.120
this is known
00:13:14.240
in the capacitance okay so we have zero
00:13:16.560
charge here okay
00:13:17.839
there is nothing pushed but once you
00:13:20.079
turn on you start pushing some charges
00:13:22.160
with
00:13:22.480
which is uh relevant to that current you
00:13:25.519
you are pushing and this charge now is
00:13:28.480
just responsible
00:13:29.920
to be charging this capacitor and later
00:13:33.519
on because
00:13:34.240
the turning on make that drain go and
00:13:37.519
drop a drop
00:13:38.240
to the source point here so we have to
00:13:41.760
a charge also that capacitance and that
00:13:45.040
also needs some time so that’s why we
00:13:48.079
have
00:13:48.480
the flat plateau uh edge here which is
00:13:51.920
miller plateau and then we complete the
00:13:54.560
charge so
00:13:55.519
that the charge is called the aqs
00:14:00.720
charge okay and this charge
00:14:04.399
is responsible to complete that charging
00:14:07.600
process
00:14:08.320
but with the g e d
00:14:11.920
charge okay capacitance here okay so
00:14:14.639
this is how it look like when we push
00:14:16.560
some charges and some currents okay
00:14:18.639
and that one is the completion of the
00:14:21.040
charging process
00:14:22.320
now where we find these capacitors value
00:14:26.399
and where we find
00:14:27.760
these charge values all of them are in
00:14:30.160
the data sheet
00:14:31.279
but if we got the qgs and the qgd this
00:14:34.959
is not
00:14:35.360
enough to to charge the the whole
00:14:37.680
capacitance because we have this part
00:14:39.600
to include this part they they have
00:14:41.760
defined for us in the data sheet
00:14:43.360
something called
00:14:44.880
the qg okay so the qg is the total
00:14:48.639
charge
00:14:49.279
you have to push to charge the phase
00:14:51.519
capacitance and also the second
00:14:53.120
capacitors
00:14:53.920
and push it more to arrive the
00:14:56.959
uh the turn on voltage okay so this is
00:15:00.160
the total capacitance that we care about
00:15:02.800
in our design and this waveform just
00:15:05.440
show to us
00:15:06.560
uh the current and and voltage waveform
00:15:09.440
with the vgs waveform as well
00:15:11.440
so during the charging here this is the
00:15:13.440
first time we
00:15:14.560
we we apply some pulses um
00:15:17.600
uh between the gate and the source
00:15:20.639
and there’s v threshold here where the
00:15:22.800
current start passing
00:15:24.240
flowing and also the uh voltage between
00:15:27.839
the drain and source
00:15:29.040
started dropping okay at that point we
00:15:31.759
have
00:15:33.120
another capacitance included uh within
00:15:36.480
the process
00:15:37.120
the charging time and this is the
00:15:39.120
plateau and we have here the current
00:15:41.759
is already gone to the maximum and the
00:15:44.800
voltage needs more
00:15:46.000
time to go to the lowest point here
00:15:48.800
until we
00:15:49.600
finish the plateau time here
00:15:52.639
and then the remaining of the process
00:15:55.920
time now will start to grow and grow
00:15:57.440
grow
00:15:57.920
until we finish the turning on the
00:16:00.320
mosfet
00:16:00.959
and again i think i mentioned that these
00:16:03.680
capacitance are mentioned in the data
00:16:05.360
sheet bob
00:16:06.160
they are not mentioned as cgs and cgd
00:16:10.160
and cds no they are mentioned in
00:16:12.560
different way
00:16:13.519
which is c i s s which is the input
00:16:15.920
capacitance
00:16:16.880
c o s s which is the outcome output
00:16:19.360
capacitance
00:16:20.240
and c r as us which is the reverse
00:16:22.720
transfer capacitance between
00:16:24.720
the the gate and the drain okay the gate
00:16:28.160
and the drain this one and this is an
00:16:30.399
example of
00:16:31.360
how the datasheet mentioned these so all
00:16:34.079
these capacitors here
00:16:35.279
and also it mentions that the qg
00:16:38.639
and also qgs and the qgd qgs and qgd
00:16:42.720
is not enough to determine the total
00:16:45.279
charge i need to turn on completely the
00:16:47.440
mosfets that’s why
00:16:48.639
they have included qg and for example
00:16:51.040
for this mosfet we have it as
00:16:53.040
63 nano column okay so we have here a
00:16:56.720
question which is important question
00:16:58.959
look at these capacitors here and mainly
00:17:00.959
the ciss the input capacitance i want to
00:17:04.160
charge
00:17:04.720
to turn in my mosfet i think here it
00:17:07.199
mentions
00:17:07.760
1 and 300 peak of arad
00:17:11.199
it’s very very tiny so why
00:17:14.720
we have concerns about the charging this
00:17:17.439
tiny
00:17:18.240
capacitors the concern because if we are
00:17:21.839
looking
00:17:22.480
at a turning on this mosfet
00:17:25.760
in very short time like 100 nanoseconds
00:17:28.640
so look at this
00:17:29.760
example now we have for example
00:17:32.960
uh capacitance between a gate and the
00:17:36.000
drain so this one
00:17:38.000
okay we will forget this one just to
00:17:40.080
make example simple
00:17:41.440
okay so this one now is about 500
00:17:44.160
picofarad for example
00:17:45.679
and we have the turn on time is 100
00:17:48.480
nanosecond
00:17:49.360
so i want to go through all that process
00:17:52.000
that i showed you
00:17:53.120
within just 100 nanoseconds how i
00:17:55.760
achieved that is it simple
00:17:57.360
or no so because the the time is very
00:18:01.360
short
00:18:02.240
even if the capacitance is very small i
00:18:05.360
need
00:18:05.919
huge current to push during this short
00:18:08.880
time
00:18:09.679
to turn on or to charge this capacitance
00:18:13.120
okay
00:18:13.679
and for example if we are looking at
00:18:15.679
this capacitor here
00:18:17.120
okay and we have these values as 400 and
00:18:20.160
i want to drive my mosfet
00:18:22.000
by 20 volt so now i am buying here 20
00:18:25.760
volt
00:18:26.480
okay and that started charging that
00:18:28.559
capacitor
00:18:30.080
and the voltage now dropped between the
00:18:32.559
call the drain and the source
00:18:34.640
making this point close now closer
00:18:37.520
closer closer to zero
00:18:38.880
it was 400 at this terminal okay
00:18:42.400
and finally this terminal will be zero
00:18:45.440
and this terminal should be 20. so what
00:18:48.320
is the difference in this process
00:18:50.160
that point was 400 and then it will be
00:18:53.919
zero so i have to discharge the 400 and
00:18:57.039
then
00:18:57.440
charge it in the reverse polarity
00:19:01.120
20 volts here okay coming from the gate
00:19:03.760
so
00:19:04.160
look at this complicated process here so
00:19:06.480
what is the difference between
00:19:08.559
the uh the first position
00:19:12.080
where we have 400 here and then the
00:19:14.880
final decision where we have a 20 volt
00:19:17.200
there and zero there
00:19:18.559
okay so the difference in this process
00:19:21.679
where we are tearing on our mosfet the
00:19:24.080
change in the gate to drain
00:19:25.840
voltage is 420 why 420 because we have
00:19:30.480
to finish this 400 first to make it zero
00:19:33.440
and then we charge it
00:19:34.480
from that terminal 20 so we have to
00:19:37.760
deplete the 400 and push another
00:19:40.960
to have a 20 volt so now this is the
00:19:43.760
change in voltage
00:19:45.200
and this is the change in current within
00:19:47.440
the 100 nanosecond
00:19:49.039
so that that current i need to make
00:19:52.320
this change equals the capacitance
00:19:55.760
multiplied by the difference in voltage
00:19:59.280
divided by difference in current
00:20:00.960
and in time and that equals 500
00:20:03.360
picofarad
00:20:04.400
and the difference in voltage is 420
00:20:07.520
within within time of 100 nanosecond and
00:20:10.640
at that equals
00:20:11.919
2.1 ampere so i have to push
00:20:15.679
in 100 nanoseconds 2.1 ampere is it easy
00:20:19.039
it’s not very easy okay that’s why we
00:20:22.159
have to use get drivers to do this
00:20:24.320
okay we can’t uh drive this mosfet
00:20:28.720
by applying zero five zero five from
00:20:31.919
microcontroller
00:20:33.360
from arduino from any microcontroller
00:20:35.919
stm microcontrollers
00:20:37.600
because it’s not a capable of pushing
00:20:40.640
that currently
00:20:42.159
during this time this is first reason
00:20:44.559
second reason
00:20:45.520
because it doesn’t have the voltage
00:20:47.760
sometime because some mosfets
00:20:49.600
they need to trigger on maybe at
00:20:52.799
15 volt and the microturns they don’t
00:20:55.679
have that voltage so that’s why
00:20:57.440
we have to use the gate drivers and if
00:21:00.000
we use that
00:21:00.880
charge information from the data sheet
00:21:03.280
and for example we have a qg here
00:21:05.520
equals 100 nano columns so we didn’t use
00:21:07.600
the capacitance we use the
00:21:09.440
qg and that qg i have to
00:21:13.120
to to deliver this is charged to the
00:21:16.320
mosfet okay within 100 nanosecond and
00:21:19.360
how i
00:21:20.159
also calculate the current the current
00:21:22.159
equals this charge
00:21:23.440
over time and we have 100 nano coulomb
00:21:26.320
divided by 100 nanosecond and that
00:21:28.080
equals
00:21:28.880
one ampere so i have to push when i’m
00:21:30.960
there again our microcontrollers can’t
00:21:33.200
do that easily
00:21:34.320
and that’s why we have to use get driver
00:21:36.720
i will show you now just
00:21:38.240
one example of without and with the gate
00:21:41.120
driver
00:21:41.760
what’s the situation and how that
00:21:43.600
switching look like
00:21:45.039
this is the vds waveform the voltage
00:21:47.440
between the drain and source
00:21:48.880
and this is the voltage between gate and
00:21:51.520
source
00:21:52.080
the voltage between gate and source is
00:21:54.400
developing like this
00:21:55.520
this is the first part during uh
00:21:58.799
charging the uh capacitance between the
00:22:01.600
the gate and source
00:22:02.799
and this is the plateau and then
00:22:05.280
complete the charging process
00:22:07.120
and that takes time that means it it
00:22:10.080
dissipates more power
00:22:12.000
during turning on that means increasing
00:22:14.720
the switching losses
00:22:16.159
okay but if we use the driver and good
00:22:19.600
design driver i think we can shorten
00:22:22.240
this
00:22:22.880
period to very small period and also
00:22:26.000
make the charging process faster
00:22:29.039
making the switching losses lower so
00:22:31.520
this is
00:22:32.080
with a gate driver and how it looked
00:22:34.799
like
00:22:35.120
and the final word about mosfets i think
00:22:38.799
we have other switches like igbts and i
00:22:41.280
will not talk about igbts because they
00:22:43.520
are very
00:22:44.400
close to the properties of uh driving
00:22:47.520
a mosfet and also it has some terminals
00:22:49.840
here shared with
00:22:51.280
bipolar transistors so that’s why
00:22:53.440
talking about igbt will not be very
00:22:55.360
different
00:22:56.000
if we just know the information about
00:22:58.320
mosfets and
00:22:59.520
by board transfer transistors so the
00:23:02.080
igbt
00:23:02.799
look like this it has collector and
00:23:04.559
emitter and they took
00:23:06.480
the input side here from the mosfets
00:23:08.799
which is the gate because
00:23:10.159
we don’t need to push current we have
00:23:12.240
just to apply some voltage
00:23:13.679
and it’s easier to drive than the
00:23:16.159
bipolar
00:23:17.600
transistors so as a comparison very
00:23:20.240
quick comparison
00:23:21.360
we have the speed for the bipolar
00:23:24.080
transistors
00:23:25.120
is slow okay for power
00:23:28.240
applications and for the mosfet it’s
00:23:31.360
fast and for the
00:23:32.640
igbt is slower than the mosfet but
00:23:35.840
faster than
00:23:37.039
the bybora transistors for the drive
00:23:39.679
power we have to drive it
00:23:41.279
very highly because as we mentioned in
00:23:43.279
some examples before that we have to
00:23:44.880
push
00:23:45.279
high current sometime and it has some
00:23:47.279
complication
00:23:48.320
but this one is is is no compared with
00:23:51.279
that one and this one is low because we
00:23:53.200
have
00:23:53.600
gate okay uh and that gate doesn’t need
00:23:56.880
a constant and the steady current to be
00:23:59.360
pushed toward the
00:24:00.799
uh the gate we just want to push high
00:24:03.360
current
00:24:04.000
just to charge the capacitors and that’s
00:24:06.000
it okay
00:24:07.200
for the large current capability it’s
00:24:09.360
very excellent so that’s why we have
00:24:11.440
between the emitter and collector we can
00:24:13.440
apply very high voltage
00:24:14.880
but for the mosfets for the silicon
00:24:17.840
mosfets yeah we don’t have that high
00:24:19.679
capability but
00:24:20.880
for the silicon carbide mosfets we have
00:24:23.840
we have up to
00:24:25.120
one thousand two hundred and one
00:24:27.520
thousand seven hundred as well
00:24:29.039
and maybe will be bigger and bigger in
00:24:30.559
the future as well so
00:24:32.240
silicon mosfets they are not capable of
00:24:34.720
blocking higher voltage and higher
00:24:36.559
current but for silicon carbide mosfet
00:24:38.960
yes it does okay and for the igbt yes
00:24:42.559
it’s
00:24:42.880
it has high voltage and current
00:24:45.679
properties and very excellent capability
00:24:47.919
so now according of what i uh see in the
00:24:51.279
application of
00:24:52.320
and and circuits of power electrons they
00:24:54.240
mainly use mosfets now
00:24:56.000
and by providing the the new material
00:24:58.880
silicon carbide
00:25:00.000
mosfets and thermospheres they
00:25:03.120
they make the usage of mosfets more
00:25:07.120
popular
00:25:07.919
and maybe a visible more if we compared
00:25:11.360
it with
00:25:11.760
lgbts and bipolar transistors so main
00:25:14.720
focus if you want just
00:25:16.480
look at the resources okay talking about
00:25:19.120
mosfets more
00:25:20.159
and make yourself knowledgeable about
00:25:22.400
what is the trends and what is coming
00:25:24.480
for as a new technology
00:25:26.400
to use it in your applications and to
00:25:28.640
make the reduction of
00:25:30.799
switching losses conduction lessons
00:25:32.720
worth for your new applications
00:25:34.880
that’s it for this part of the lecture
00:25:36.720
and see you in the coming lecture thank
00:25:38.240
you very much BU YAZUYU BANA TÜRKÇE ÖZETLE VE İÇERİSİNDE BULUNAN teknik bilgileri  BAANA SEBEPLERİYLE AÇIKLA