LE3_3b Diode - Schaltverhalten, Forward und Reverse Recovery

In addition to static freedom, show enthusiasts are also interested in dynamic behavior, i.e. the transition from the locked state to the connected state, here being of interest for the pin

diode

essentially two effects: firstly, the so-called switching effect. direct

recovery

when turning on and secondly we will see the

reverse

recovery

effect when turning off. First of all, look at this required recovery effect, that is, the switching process of a pin

diode

, as seen here. current in the diode, that is, the voltage here and the current there, idea and We want to see what it looks like dynamically when we look at these current curves.

The diode is turned on so that the current increases with a certain defined. slope Now here initially it will be 0 with the DJs off, then the current increases with a constant slope until we get the value in the on state, which would be this a switching process in the diode, here the current idea slowly increases to its maximum value in the zone with a slope following dt and then we would notice with a pin diode like this that it does not immediately assume a small voltage value at this point, but in the normal case the voltage is first increased. greater and after a certain time it would be reduced to the connection value, for example a word, or corresponds to the value we have in the connection state without it, here a maximum current value is produced with a voltage vfr value maximum voltage values in the so-called direct recovery effect and fades over time.

It is sometimes said that the diode showed such active switching behavior, that is, a short-term overvoltage. This? So here we have a power-on surge, which is caused by the low dose interlayer. That is, through this layer that we have on the diode, let's look at the doping profile here on the right side and we want to derive. or look at how the charge carriers develop over time, i.e. I have it here represented as columns more or less the doping profile of a pin diode, i.e. I was expecting once again a lot of holes on the other side a lot of electrons in the advantage as free freight carriers between this intensive layer that we are here now the quote price is a lot and now we want We think about what happens when we turn on, actually when we turn on there will be holes of the p + region in On the one hand , these are low through the third rhein d layer and vice versa, electrons from the m + layer and will actually increase the conductivity here in this intermediate zone.

This will not happen suddenly, but little by little we will come to terms with each other in the first state, perhaps over time, then the visions will have a progression and at some point we will get progressions such that as time goes by, the number of charge carriers increases here, which means here This is now, in principle, the density of charge carriers in this zone 1. With increasing time, the number of charge carriers increases with increasing charge carriers, but therefore the conductivity will also increase and therefore the resistance in this layer will decrease, that is, this surge that arises.

This arises because the low dose in the shell initially is still high around me at power-up and only through fusion do the charge carriers have to ignite at power-up and only when there are enough charge carriers in it does the shell not It will be enough for me. The voltage drop here can decrease again. This is the so-called direct recovery effect. Sometimes the time period for this recovery effect is also defined, which should be checked on the manufacturer's data sheet to see how long it is. , which means you have time for direct recovery time, which describes how long the time period is at this point until the surge has subsided.

There is a definition of how much percentage it has decreased from the final value, for example up to 10% above. the stationary value, either 20, a little depending on the manufacturer and also the second value with which it is characterized is this maximum value of this voltage. In addition to this

forward

recovery effect, there is a second effect called the

reverse

recovery effect. and this is often the most important effect with these power diodes. This essentially describes the shutdown process and can be traced back to similar causes, as in this case the recovery effect on the site has become what we will see now, so here again.

We will start from a diode with a voltage and a current, that is, here as the voltage if the current idea and now we are looking at the case that this diode turns off, that is what we do here Once again we show the course of current and voltage on the diode during the shutdown phase. At this point we again show the current in red and the voltage in blue again and then we want to take a look at, okay, how this is going to behave now when we turn off the diode for the first time.

The off diode will be turned on beforehand so initially it will carry a maximum current that it has when turned on and then at some point the diode will turn off when turning on another switch. In fact, the current will not be able to do it. decrease suddenly, but slowly, that's what they still call you here get a slope with which the current decreases and now show that with the reverse recovery effect the diode does not stop conducting, the current becomes zero, but the current will flow briefly In the reverse direction, a negative current then decays to zero, that is, this course because the current flows in the reverse direction, this effect is called reverse recovery effect, which means that when the diode turns off, a current will flow briefly. in the reverse direction and what will also happen to the diode that was previously born in the flow direction is a small and low voltage in the flow direction.

It is only during this reverse current spike that a voltage can begin to be absorbed, i.e. to take control of the voltage. It normally oscillates a little bit and then reaches a stationary final value here at this point and this final value becomes the permanent presence of the funny voltage which is now one of these, the ode to which we. Now we have the course of the tension here and the one we have there that we can now determine as a phenomenon. Now there are some parameters that can be used to characterize this course. The most important or One of the most important is this reverse current peak. , which is called mgm, is the maximum reverse recovery, that is, the maximum value of the peak reverse current that we have here.

Then we define a time period here, that is, the time period that describes how long this process takes, which is actually here. is the time period trr in German for delay time or in English at this point the reverse recovery time is short in Germany in spa delay time and another important size is the so-called recovery charge, the amount of charge you must be removed from the diode during this peak reverse current that the area under the current of this curve is shown shaded here qr the so-called reverse charge or blocking delay charge in German these are the essential variables.

In fact, at this point the backscatter peak is critical. many applications because it does not directly charge the diode. But turning on the component will charge it, we will see later what this effect depends on? Does this effect depend on how strong the current change is here at the moment? Typically, each current decreases the greater this gradient, the greater the reverse current also becomes, and at this point, the greater the current when turned on, the longer the reverse current time will be. Now there are some others. definitions You can now enter a backup time and a storage time.

Then we will see it in the script. Now at this point just a brief explanation of why this is the effect here. So here we have a reverse current spike. and this is caused by the charge stored in this intrinsic zone in the middle layer and we want to take a quick look at it now. Like before, we had this quote profile up here, we want to see exactly the same thing again. So I'm going to imagine it here again how we can explain it. Here again the doping is a large sp doped area, then these inner layers and then the high ends here the highly doped area and now the following has happened. be on, that means if we draw the charge carrier concentration here like this then at the first moment we will be in front here if we do that at time 1 we will be here At this point in time 1 we want to have a very charge carrier big.

Now we want to turn off the diode. Now we have to remove the excess charge carriers from the diode because as long as the charge is present the diode will be conductive which means what will happen over time. it will be these charge carriers here that decrease, that is, here in state 2, for example, the charge carrier density has decreased, that could now be somewhere here, then the charge carrier density continues to decrease from the third point in time and Only when a small space charge zone can form here, i.e. when such dense charge carriers have decreased to such an extent that the two highly doped zones are no longer connected to each other, the diode can absorb a voltage, that is, now, for example, time 3, that would be this moment where the voltage on the diode increases, that is, if we represent it here, we are this.

Only then does the voltage increase until then it remains conductive because both are highly doped. Through these charge carriers in the middle layer, the edge zones are conductively connected to each other. As the time during shutdown increases, the charge carriers now decrease and at this point it is only when the density of the charge carriers is so small that I can do it slowly. extended in a space, the diode can absorb a strong voltage, the remaining charge carriers that are still stored here in this area due to this voltage jump must be reduced by recombination and now it is as if this effect severely limits the use of diodes, which means that the jews in power electronics are actually selected according to reverse recovery behavior, which means that we choose the diodes for the best possible properties, which now means the best possible properties, ideally, which Now it means that these times trr tend to zero and that the charge qr tends to zero, that would be the ideal optimization criterion so to speak, I would like to say that the truth is that a diode is good and has a very good switching behavior . is optimal if the recovery can this lock delay approaches zero and if this If the lock delay load approaches zero at this point, then we can say that it has optimal switching behavior