SCR | Structure | Modes of Operation | Static V-I Characteristics | Power Electronics | Lecture 12

0:00 [Music] hello everyone welcome to power electronics lecture series in today's video we're going to take a look at acr the structure of scr and modes of operation of scr and finally we will be arriving at the static va characteristics of scr the link for the material is provided in the description and you can download it for your reference so let's get started acr at the first place stands for silicon control rectifier and there is a specific purpose for giving this name silicon is the material that is used

0:47 while manufacturing this device and that is pretty simple and straightforward whereas the word control is used for a specific purpose if you carefully look at the symbol of nacr it has three terminals anode cathode and gate whereas if you compare the symbol with a normal diode it is clearly understood that this is the additional terminal that is used and it is called as gate so since gate terminal is used and this terminal is actually used for controlling this device and that is why the word controlled is there in the abbreviation of scr and finally since it is used in rectifier applications or you can also say it is popularly used in high voltage dc applications and that is

1:35 why the name silicon control rectifier is given to it it is also referred by another name called as thyristor where this device belongs to the family of thyristor you can also call this as thyristor some other devices which belong to the family of thyristors are gate turn off thyristors and several other devices have been recently invented as well the first ever prototype of thyristor was introduced in the year 1957 so they wanted to make some modifications with the diode and transistor after analyzing those properties and they came up with a device that will be able to control so that is why k terminal was initially introduced now let us look at the structure the

2:21 basic level structure of scr and if you carefully observe it has three terminals anode cathode and gate in addition to this it contains four layers alternatively stacked together in this particular fashion p n p n and it contains three junctions j1 j2 and j3 so as a conclusion what can we say scr is a four layer three junction three terminal device very very important observation the gate terminal is usually placed closer to the cathode if you carefully observe it will be placed closer to the cathode and there is a specific reason as well so you not want the length of the wire to be very larger

3:06 when you are actually giving the supply to this and that is why the structure is arranged in such a way that the gate terminal is closer to the cathode terminal one of the major and important points that you need to remember if you understand this point the entire working of scr will be very simple and straightforward what is the condition for turning on the scr junction j1 j2 and j3 must be forward biased so when junction j1 j2 j3 is forward biased current will start flowing from cathode to anode and consequently the device will start conducting so basically this should act as short circuit this should act as short circuit and this layer or this junction also should act as short circuit that means all of them

3:51 should be forward passed and if this condition is satisfied then the scr will be turned on very very important point keep a note of this very very important now if you compare it with a normal diode what is the difference that is a common question that you might be having just like a diode acr is also a unidirectional device that means it does not allow the flow of current in both the direction it only allows the current to flow in one direction so usually the conventional flow of current will always be in the opposite direction that we have assumed centered so it will be from cathode to anode isn't it in addition to this scr has one more property where it will be able to block the current in the forward direction as

4:37 well with the help of gate terminal obviously that is the reason why we are going for this device isn't it so when the current is flowing in the forward direction with the help of gate terminal we will be able to block that current to flow and that is why this is used in a lot of applications where control is required so very very basic points that you need to make a note of and this will be provided in the material so you don't have to make uh all of these points but just be aware that these are very important points now let us look at the modes of operation so there are three different modes and based on the modes of operation you will be able to arrive at the static va characteristics and that is why this is very important

5:22 please pay attention towards this concept so if you carefully observe i have taken the structure again anode gate and cathode so the same structure and i have given the supply for anode and cathode in such a way that negative terminal is connected to anode and positive terminal is connected to cathode so junction j1 j2 and j3 is here now one important point is i am not giving the gate connection at all that means gate will be open and i am not going to give any supply to the k terminal very very important point that you have to make a note of so gate is not connected at this point in time now what happens to the working of this particular device it is pretty simple and straightforward

6:08 that is when negative terminal is connected to the anode that is basically the p type material obviously junction j1 will be reversed by us isn't it similarly positive terminal is connected to the n-type material and junction j3 will also be reversed biased whereas junction j2 will be forward biased because n is connected through p to the negative terminal and p is connected through n to the positive terminal as a result junction j2 is forward-passed so observation is that junction j1 and j3 will be reversed by s and junction j2 is forward biased so at this point in time what can we say the device is not conducting isn't it so acr does not conduct and acts as open

6:55 circuit now you might be asking a doubt as how do we say that scr is acting as open circuit so simple assumption or simple schematic representation of this can be drawn and you will understand from this so if you are considering this as switches like this and i'm connecting it to positive and negative like the battery over here and this is junction j1 it is open circuited junction g3 is also open circuited so you only have junction j2 which is acting as short circuited because it is forward passed so in this case the current will try to flow here but since j1 is acting as open circuit obviously no current will be flowing through this path and that is why acr does not conduct and access open circuit

7:42 i hope this point is clear however small amount of current flows due to the minority charge carriers and it is called as reverse leakage current this concept is quite similar to that of a diode isn't it because of the flow of or the conduction of minority charge carriers a small amount of current called as reverse leakage current will be flowing through the device if a reverse voltage applied exceeds beyond a certain value say vbo or reverse breakover voltage then the device will be permanently damaged that is if you are applying this voltage the reverse voltage vak now such that it is greater than vbo then the device will be permanently damaged so be very careful this is something that you need to take into

8:27 consideration when you are practically working on these devices so the device will be damaged and that is why we don't do this method or we don't apply v ak uh which a higher value of voltage so now the note is that suppose i apply gate voltage at this point in time when i apply gate voltage what will happen junction j3 will be followed by asked isn't it even though junction j3 is forward best it acts as short circuit now which was previously open circuit now it acts as short circuit but ser still will be turned off because j1 is open isn't it so that is why we don't have any purpose by applying gate terminal you are adding to additional power losses isn't it so be careful

9:14 while the device is operating in reverse blocking mode the gate should never be applied and that is why we do not apply the gate supply so this is very very important this is quite similar to the reverse biased operation of a normal diode as well now let us look at forward blocking mode i mentioned one point with the help of gate terminal we will be able to control the the forward current flowing through the device and that can be understood in forward blocking mode now i am considering the stream structure and now again gate will be open i am not applying any gate voltage at this point in time so we have three junctions junction j1 j2 and j3 and consequently we are applying

10:00 positive and negative basically i am reversing the polarity so when positive is connected to p type material and negative is connected to n type material obviously j1 and j3 will be forward biased isn't it but n is connected to the positive through p and p is connected to negative through n and that is why j2 will be reversed by us isn't it so what are the observations junction j1 and j3 will be forward biased and junction j2 is reversed passed so now at this point in time what can be observed is that let us draw the equivalent circuit again so this will give you a clear understanding so we have three junctions so when j1 and j3 are forward biased it

10:46 will be acting as short circuit in this particular fashion whereas when junction g2 is reverse biased it will be acting as open circuit this is j2 this is j1 and j3 now the current tries to flow through this direction it knows that j1 is short-circuited and flows till this point but since j2 is open circuited no current flows beyond this point and consequently scr will be in off condition in this mode as well so even if you are forward biasing or positive voltage is applied in this particular fashion acr still is in off state so scr does not conduct an access open circuit however a small current flows due to minority charge carriers and it is called as forward leakage current this phenomenon is quite similar to the

11:32 reverse blocking mode as well now one important point is that if the forward voltage applied exceeds beyond a certain point say vbo so if you are applying vak such that it is greater than vbo forward blocking voltage then the device will start conducting what happens is that more will be the charge carriers injected into the junction and the depletion region of junction g2 will be narrowed down so it will be very very very thin so once it gets thin avalanche breakdown of junction j2 takes place and it acts as short circuit and consequently if you carefully absorb over here since it was open circuit it becomes short circuit and consequently acr will start conducting so scr conducts when what happens

12:19 vak is greater than bbo but we don't prefer this because this is not the right way of triggering the acr because the current can exceed to dangerously high value when the anode to cathode voltage exceeds vbo and that is why we don't generally prefer this method so the observation is that gate is still open now in forward conduction mode what we will be doing is we will be applying the gate voltage so we have j1 and j2 and j3 isn't it so similarly we will be taking the same polarity of anode to cathode voltage positive and negative just like the way we actually triggered in forward blocking mode and if you

13:05 carefully observe in the previous case we studied that junction j1 and j3 was forward biased whereas junction j2 is also forward biased because gate voltage is applied when i am playing positive to the p type material the breakdown of this junction will take place eventually and j2 will be far advanced so basically j1 j2 j3 is forward biased isn't it so when all of these three are forward-based as i mentioned earlier according to the condition of acr to turn on all these junctions should be forward biased and that is achieved and at this mode we will be seeing that the scr starts conducting so with the help of gate terminal we are able to turn on the scr isn't it so very very important point that you have to make a note of

13:50 through gate we are able to achieve the turn off of device so what is the observation we are able to control this device with the help of gate terminal that is the important conclusion that you need to make so once acr starts conducting one important point that you need to remember is gate has no control over the ser so we remove the gate supply once the acr is turned on so we don't have the control over the device once acr turns on now however before removing the gate supply care must be taken such that acr has reached a minimum value of current called as latching current so this is a very important concept called as latching current and holding current which we will be

14:35 going through so latching current is the minimum value of current that needs to be reached for the acr to be turned on so latching current is basically associated with turn-on process and it is the minimum current required for acr to turn on for example when the device was in a forward blocking mode for example and you're applying gate voltage jt j2 is forward biased and consequently starts conducting and the minimum current that is required for the acr to turn on will be the latching current i hope this point is clear now you might have a question as how do we turn off this particular device because i mentioned git has no control it has control only during the turn on process so how do we turn this off

15:20 so this can be turned off by applying a reverse voltage so it is positive and negative whereas if we apply negative and positive then obviously we will be able to apply some amount of reverse voltage to this device and in that case what will happen when we apply the reverse voltage anode current that is i a k will start reducing below a minimum value of current called as holding current so once it reaches below holding current the device will be turned off so holding current is basically associated with turn off process so very very important point a lot of people will get confused between latching current and holding current so latching current is associated during turn on whereas holding current is associated during turn off process so one more important point is latching

16:06 current is always greater than holding current so we will be looking at the characteristics as well but this is one of the most important observation from experimental analysis that was obtained for the ser now how do we analyze the static vi characteristics of scr static characteristics is basically a plot of anode to cathode that is anode current with respect to anode to cathode voltage that is applied now we will be starting to analyze this particular characteristic step by step just like the way we analyze the modes of operation and with the help of modes of operation we will be able to easily understand the static characteristics initially what happens whenever we are

16:53 applying some amount of anode to cathode voltage small amount of anode current starts flowing and the changes in anode to cathode current will be very very very small so this mode or you can also say very small amount of forward leakage current flows so basically this region we will be calling it as forward leakage current or you can say the corresponding mode as forward blocking mode because acr is still not turned on so this region is called as forward blocking mode and correspondingly the current that is flowing is forward leakage current so very very important point so once this is done what we will be doing is we will be increasing the vak value

17:38 so once you increase the value of vak what happens the current rapidly increases beyond a certain point so here you will be seeing the transition taking place so from here the curves drastically changes to this point and starting at this point the current iak rapidly increases from this point to this point particular point is called as the latching current the minimum current that is required for the acr to turn on and this corresponding mode is called as forward conduction mode you can represent it as fcm now below latching current minimum current to turn this off is basically the holding current that is ih so that is why i mentioned il

18:24 is always greater than ih i hope this point is clear so these are basically the different values of gate current over here it can be ig one some amount of gate small current and once you increase gate supply ig2 will be much greater and ig3 will be much more greater and beyond some point the device will start conducting as you increase the value of gate voltage what will happen the ig current also increases and once ig current starts increasing the device will move from forward blocking mode to forward conduction mode now in the reverse region the characteristics will be quite similar to that of a normal diode where it goes on increasing and below or beyond a certain value say we break down voltage vbr or vbo in the

19:09 reverse region what will happen the current abruptly increased to a very large value and the device will be damaged so this region is called as reverse blocking mode or reverse blocking region so this is the static va characteristics of scr so from the operation or modes of operation of scr we are able to arrive at the static va characteristics of acr i hope this point is clear and you were able to understand this in a much better way in case you have any questions feel free to reach out by typing in your questions in the comment section below thanks for watching please do keep supporting thank you