Short circuit of IGBT in the system and its protection

Short circuit of IGBT in the system and its protection

summary

This paper mainly briefly discusses several commonly used short-circuit protection of IGBT in low-voltage small and medium power system, and mainly introduces the way of using Vce detection.The working principle of short circuit protection and the rationality of protection circuit design are briefly explained.

Introduction of the short-circuit protection scheme

In the common short-circuit protection design, the current sampling mainly has three ways, respectively: N-BUS (zero

Line) shunt resistance detection, output Hall detection, Vce voltage detection.

graph 1

The voltage signal is used by connecting a small resistance for determining the short circuit in the bus circuit.This way has high accuracy and sensitivity, and can protect the short circuit to the ground, the disadvantage is only suitable for small power machines, high current to the resistance power requirement is too high.

As Hall's response time continues to improve, it not only has the function of converting the current size, but also protects the short-circuit current through the hardware circuit.Due to the response time problems of the Hall detection, the reliability was relatively lower than the other two methods.The Hall sensor is not available because it is installed on the output end

Method protects the upper and lower pipe through.

The photocoupled protection IGBT is achieved by detecting the C-E voltage, and according to the relationship between V ce and Ic, when Ic rises rapidly, Vce follows up.When the Vce value rises to the protection point voltage, the optical coupling will turn off itself and send the error signal to the DSP, the whole process is generally between 5 and 10us.Because such protection is very highly sensitive and has poor accuracy, it is only suitable for short-circuit protection.Figure 2 shows the diagram of Vce and Ic of GD200HFL120C2S. With the rise of Vce, the increase of IC is increasing. The Ic at + 7V actually far exceeds the short-circuit current of the module.When doing dynamic short circuit test, L, Vg, tr, tf and other parameters have strict and stable control, and the current is generally controlled at 8-10 times Ic, as shown in Figure 3. However, the short circuit is tested on the system, and the current will often rise higher due to the switch characteristics, the circuit load and the interference.

Common short-circuit protection to drive optical coupling

(I) PC929

PC929 is a common drive optical coupling in the inverter industry, with short circuit protection function (PC923 without protection). Because its output peak current is only 0.4A, driving high power IGBT needs to be amplified at the rear end to drive IGBT.How much current of the module that the P C 929 can drive depends on the choice of the tube. As long as the PC929 can push the tube and the tube can push the IGBT, then it can be achieved.

Figure 3 shows the PC929 internal protection circuit:

1, When the IGBT is turned off, the 9-foot C voltage is pulled to zero.

2. When IGBT is turned on, Vcc charges the Cp through Rc, with the charging voltage exceeding + 7V, and O2 transmission

With the soft turn-off, FS sends the error signal CPU at the same time, FS is effective at the low level, and the charging speed of Cp is determined by Rc and Cp.

3. When IGBT is turned off, C is quickly pulled down, and the down speed is much greater than IGBT off speed.

Many people reflect that P C 929 is prone to misaction, but the mechanism of misaction is not very clear. Some people think that the protection time is too short, while others think that THE pressure drop of IGBT is too large.Then let's discuss the reason why P C 929 misacts, which is very important for the protection circuit design of IGBT.

In theory, the larger the Vce (sat), the faster the IGBT reaches the protection voltage + 7V in the linear zone, which is correct, but is not the cause of misaction.The value of Ic corresponding to the saturation pressure drop of + 7V is far greater than the saturation pressure drop of the maximum overload current, while the pressure drop of the general chip is only below 1V, and this gap will not cause misaction in the non-short circuit state.

When IGBT is opened normally, the main reason for misprotection error is the time of V c e falls when IGBT opens, and the charging time of C p. See Figure 4.

The power supply Vcc charges the Cp through the Rc, with the charging voltage of Ucp.

If Vce goes down along a path, then Vce has dropped below + 7V before Ucp reaches + 7V, then the 9-foot voltage does not appear above + 7V.

If Vce drops down the path c, then Vce is still above + 7V when Ucp reaches + 7V, then the 9-foot voltage detection will appear above + 7V, jump short circuit protection.

Conclusion: In order to avoid opening error action, you can extend the charging time or opening speed a little faster.

(2) 316J

The 316J is also widely used for IGBT driven and optical coupling with Vce detection. The biggest difference from the PC929 is that the 316J is able to directly drive the 150A module without the need for tubes.In terms of protection mechanism, it is also very similar to PC929. See Figure 5: when IGBT is turned off, DESAT (14) is pulled to the ground through a high-speed MO SFET, MOSFET is turned off after IGBT is opened, and 14 feet are charged through internal current source and capacitor, and voltage increases more than + 7V and 316J protects.PC929 is charging the capacitor with Vcc through resistance; 316J is directly charging the capacitor through the internal current source. Because the capacitor is charged with a constant current source, the charging time can also be calculated more accurately:

t=CV / I, select C=100p

t =100p *7V /250u A =2.8u s

This means that the Vce must drop below + 7V within 2.8us, or it will be delayed.

graph 6

Introduction of the short-circuit protection experiment

Short circuit protection can be divided into alternate short circuit according to the short circuit form, relative short circuit.But no matter what kind of short circuit, to have a current flow, it has to constitute a loop, so in the design of short circuit protection can be detected in any position of the circuit, of course, the effect is not the same.We generally chose to detect the V c e

Voltage, because it is relatively effective and reliable.

In the inverter industry phase short circuit protection test, there are first short circuit after operation and first run after short circuit.The short circuit condition of the former is relatively single, the output has been short circuit, when the open signal comes, the current begins to rise; the latter condition is more complex, when the system has been running, the short circuit position can be at any point in the working cycle, so the waveform of each short circuit is also very different.So, which protective current is greater?After testing on the system, we found that the short circuit during operation, the current can rush higher, because the short circuit when IGBT opens, Vg is raised larger, and the I c in the linear region is mainly affected by Vg.

I r es =C r es *dv /d t

△ Vg=Ires*(Rg+Ri nt)

I c =K (V g -V t h )2

Figure 7 shows the short-circuit waveform measured on the dynamic tester. We find that after the I sc rises steadily, it is limited by the chip itself.The gate voltage Vg was not greatly disturbed during the whole process.

Figure 8 shows the short-circuit waveform of the 1200V / 50A module on the frequency converter.

- -t 1: dv / d t continuously affects V g; I sc is in the rise, the slope is determined by the parasitic load inductor L, Isc=K (Vg-Vth) 2

- -t2: dv / dt stop affects Vg, Vg decreases, and Isc decreases with Vg.

- -t3: Vg is stable and Isc is stable.

- -t4: IGBT off, Isc reduced, Vce=Vdc + di / dt * Lbus, so the voltage overshoot.