"In recent days, I have devoted myself to replacing the rear stage of the inverter, that is, the familiar H-bridge circuit, with IGBT tubes to deeply understand the relevant characteristics.
As we all know, IGBT single transistor is quite fragile. IGBT single transistor with the same current capacity is much more fragile than MOSFET with the same current capacity. In other words, there is no problem with MOSFET in inverter H bridge, but IGBT may explode when it is powered on and loaded. Many people probably have a deep understanding of this. At that time, I saw that the man who made the fish machine used fgh25n120and, which reflected that it was easy to burn, but I didn't think so at that time.
It was only when I had to use IGBT at work that I found that I was wrong. At the beginning, I naively thought that an irfp460, 20A / 500V MOSFET would not explode if I used an sgh40n60ufd40a / 600V IGBT. The actual situation is that after loading, I suddenly added and removed the load, and it exploded several times, I thought the circuit was not welded well, and then the same was replaced and blew up. In this way, a lot of IGBT was wasted.
Later, I found some rules, that is, the use of peak current protection measures can prevent IGBT from exploding. Next, I will say these things in detail. If you don't say well, please forgive me. This post will be updated slowly, and I hope experts can put forward more opinions.
We will solve this problem in several parts:
1. Drive circuit;
2. Current collection;
3. Protection mechanism;
1、 Driving circuit
The IGBT used this time is IXYS, ixgh48n60b3d1. The detailed specifications are as follows: ixgh48n60b3d1
The driving circuit is as follows:
This is a very typical application circuit, which can be used for IGBT or MOSFET, but there are some differences.
1. There is a negative pressure generation circuit,
2. Isolate the drive,
3. Separate power supply.
First of all, let's take a general look. This circuit has no protection and is 100% used in inverter, but we can understand the essence of this circuit.
Let's focus first:
1: The driving resistance R2 is very important in the driving. In the figure, D1 can discharge the CGE of IGBT quickly when it is closed. In fact, depending on the need, D1 can also be omitted, or a resistance can be connected in series in the D1 circuit as the gate resistance when 0ff is closed.
Here are some waveform photos. The actual situation of the upper and lower IGBT grids when different grid resistances and high voltage HV + 400V work together.
The figure above shows the grid waveform between the upper and lower tubes when the negative pressure is cancelled, and the grid resistance is at 10R.
The figure above is to measure the g-pole waveform of 2 tubes without dc400v, and the figure below is the grid waveform of 2 tubes under dc400v.
Why does the second figure have a peak. This should start with the internal situation of IGBT. In short, there is a parasitic capacitor on the Ge of IGBT, which forms a water pool together with another parasitic capacitor of CGC, that is QG. In fact, this is also very similar to MOSFET.
So let's see why adding 400V will produce a peak on class G on the lower tube. Borrow flowers to offer Buddha, grab a picture to illustrate:
As shown in the above figure, when the upper tube is opened, it is closed at this time. Because the concept of DV / DT is introduced at this time when the upper tube is opened. This is more abstract. Ignore it first. In simple and popular terms, when the upper tube is opened, the upper tube is equivalent to a straight through, and + dc400v voltage is immediately added to the level C of the lower tube, Such a high voltage immediately generates an induced current from the parasitic capacitance of IGBT. The induced current is calculated by the formula in the figure above. Under the joint action of RG resistance and driving internal resistance, this current forms a peak voltage on the grid of the lower tube, as shown in the screenshot of the oscilloscope above. So far, the concept of Miller capacitor has not been introduced. After understanding these, it is much easier to understand what Miller capacitor is and what impact it has on the circuit.
This peak has many disadvantages. It can be seen from the screenshot of the oscilloscope above that at the peak moment, the lower tube has actually reached the voltage of 7V, that is to say, during this time period of the peak, the upper and lower tubes are connected together. The conduction time of the lower tube is short, but due to the time relationship of ton, the current will not be too large. The pipe will not explode, but it will generate heat. With the greater transmission power, this situation will become more serious and greatly affect the efficiency.
It was supposed to send out the waveform photos after adding negative pressure. Negative pressure can make this peak within a safe level range. The oscilloscope needs a USB flash disk to export the bitmap. It's so clear. I'm lazy today and didn't touch the instrument. I'll make it up later.
2、 Current acquisition circuit
When it comes to this step, it is not far from protection. My experience is that the current acquisition speed should be fast, so as to quickly tell the following circuit in case of overcurrent or short circuit. There is a problem here. Let IGBT shut down quickly and safely.
How should this circuit be implemented? For the inverter circuit, we can directly use resistance sampling or VCE tube voltage drop detection. There have been many discussions on tube voltage drop detection in this forum, but none of them can really be used, actually applied and tested (except for special driver chip). This is because the parameters of each practical application are very different. For example, IGBT parameters are different, and many parameters need to be adjusted, which requires some experience.
We can start with the simplest way, use the resistance to detect the current. When the short circuit comes, a voltage drop can be generated on the resistance. Compare the comparator with the voltage to obtain whether there is an overcurrent or short circuit signal.
Just use this diagram, because the principle is very simple. For a comparative function, it will be very easy for everyone to implement, and there are not many parameters to adjust.
The above figure shows the current of sampling H-bridge to ground. For example, if the IGBT is 40a, we can take about twice the peak current, that is, 80A. Corresponding to the above figure, RS is 0.01r. If more than 80A pulse current flows in, a voltage of 0.01r * 80A = 0.8V will be generated on the resistance. After R11 and C11 blanking, this voltage will be sent to the + terminal of the comparator and compared with the reference voltage from the - terminal, The reference resistance at the - end in the figure is not set correctly. In practice, please calculate separately. In this example, the partial voltage of 5.1k and 1K resistors can be changed to about 0.81v to the - end. At this time, if the voltage on the sampling resistance Rs exceeds 0.8V, the comparator will turn over immediately and output SD 5V level to the external circuit. This changed level signal is the signal we need to use next to check whether there is short circuit or overcurrent.
With this signal, how can we turn off IGBT? We can see whether the situation adopts soft closing or direct hard closing.
Soft closing can effectively prevent the voltage rise of the circuit at the moment of closing. The closing characteristic is very soft and gentle, which is very suitable for high-voltage and high-power drive circuits.
If hard closing is adopted, it may cause voltage overshoot on the high-voltage DC. For example, the high voltage of dc400v in the first figure may become DC600V in an instant. At that time, when I read the records on some materials, it is very difficult to understand: if it is closed, does the high voltage rise by itself? The actual situation is real.
If it is difficult for you to understand, you can do an experiment. If you have a water tower at home, it is most clear that the water tower is high upstairs and the faucet is on the first floor. Turn on the faucet and the water stays. Then turn off the faucet at a very fast speed. You will hear the sound of the water pipe and even the water pipe will vibrate (I don't know if you are right, please correct it, I have to thank my teacher for introducing the example of faucet here. When I was reading, I saw that we were too stupid. I made an analogy when talking about the characteristic principle of triode. Here I want to thank him). The same is true for the principle of IGBT in bridge circuit. When the IGBT is seriously short circuited, if you turn off the IGBT immediately, it will only cause the induced voltage of overshoot on the bus (as for why overshoot occurs, you can check the relevant data, many data have said), and the pipe can resist the past. For example, you have a very good absorption capacitor in parallel on the DC high-voltage bus, multiple absorption circuits, etc
In particular, when the tube is closed, it will fail, and it is useless to close it. IGBT will still be broken down and short circuited by overshoot voltage, and this short circuit can not be recovered, which will immediately damage a lot of circuits. Sometimes this phenomenon can be caused without overvoltage. The specific model of the failure principle is unknown, but it can be imagined that the failure may be caused by the joint failure of other parasitic capacitors related to the tube and Miller capacitors, or the engine effect of IGBT has occurred when overcurrent and short-circuit signals occur. Even if it is turned off, it cannot be turned off.
There is also a third method, which is called secondary shutdown. This method is simply to detect the short circuit and overcurrent signal. At this time, the PWM pulse does not intend to soft shutdown or direct shutdown, but immediately reduce the VGE drive pulse voltage corresponding to this time to about 8V, so as to judge whether it is still in the overcurrent or short circuit area. If so, Continue to use this 8V drive until the set time, for example, multiple us will be turned off immediately. If so, PWM will return to normal. This way may not be seen much, so we don't do in-depth research.
After understanding these, we can adopt the specific closing methods according to the situation. I think in 2kW level, within dc380v, direct hard closing can meet the requirements. 600V IGBT can be used only by paralleling a capacitor with good absorption characteristics on the H bridge.
The key point is to detect quickly and close quickly after detection. Only when it is fast, IGBT will not burn.
Original title: dry goods | detailed explanation of inverter H-bridge IGBT single tube drive + protection
The source of the article: WeChat official account: Wang Cai chip, welcome to add attention! Please indicate the source of the article.
Responsible editor: HAQ, read the full text, original title: dry goods | detailed explanation of inverter H-bridge IGBT single tube drive + protection
Source: [micro signal: WC]_ YSJ, WeChat official account: Guangdong electronics and Information Industry Association] welcome to add attention! Please indicate the source of the article“
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