As we all know, the overcurrent short circuit protection circuit of the inverter is very important in the safety of the inverter. If there is no overcurrent short circuit protection, the inverter is likely to be burnt due to overcurrent short circuit.
Let's first analyze the characteristics of the load. Most of the loads in real life are impact loads, such as incandescent bulbs. In cold state, the resistance is much lower than when lighting, such as computer, TV, etc. The input AC power is rectified and filtered with a relatively large capacitor, so the inrush current is relatively large. There is also a motor inductive load such as a refrigerator. The motor needs to generate a relatively large torque from the static to the normal rotation, and the starting current is also relatively large.
If our inverter can only set a rated output power that can work for a long time, the load with a starting power greater than this rated output power cannot be started. This requires the inverter to be equipped according to the starting power. It is a waste. In practice, we design two protection points, rated power and peak power when designing an overcurrent short circuit protection circuit. The general peak power is set to 2-3 times the rated power. The rated power in time is not protected by long-term operation, and the peak power is generally only maintained for a few seconds to protect. The following is an example of an overcurrent short circuit protection circuit designed by myself:
R5 is the high-voltage current sampling resistor of the full-bridge high-voltage inverter MOS tube source. We can understand that the magnitude of the high-voltage current basically determines the output power, so we use R5 to detect the high-voltage current. In the figure, the two comparator units of the LM339 are used for overcurrent and short circuit detection, respectively.
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First look at the overcurrent protection circuit composed of IC3D and its peripheral components. The reference pin of IC3D is set to a reference voltage. The voltage is determined by R33, VR4, R56 and R54. U8=5*(R33+VR4)/( R33 +VR4 +R56+R54). When the voltage on R5 passes R24, the delay of C17 exceeds the voltage of 8 pin 14 The output of the high pin is isolated to the 5 pin of IC3B through D7. 4 feet also do battery undervoltage protection. When normal, the voltage of pin 5 is lower than 4 pin. After overcurrent, the voltage of pin 5 is higher than 4 pin. The pin 2 outputs high level to control the high voltage MOS of the latter stage. Of course, it can also be controlled. The MOS of the stage is turned off together. The role of D8 is to over-current short-circuit or battery under-voltage after positive feedback lock 2 is high.
Look at the short circuit protection circuit composed of IC3C. The principle is similar to the overcurrent protection, but the delay time is short, the capacity of C19 is very small, and the speed of LM339 is very fast, so that the short circuit protection can be turned off within a few microseconds. , effectively protect the safety of high voltage MOS tubes. Incidentally, the short-circuit protection point is designed according to parameters such as the ID of the MOS transistor, the safety area, and the stray resistance of the circuit. Generally speaking, the current is within the ID, and the action time is safe within 30 microseconds.
4. Drive and short circuit protection of IGBT:
As a new type of power device, IGBT has the advantages of high voltage and current capacity, and the switching speed is much higher than that of the bipolar transistor and slightly lower than that of the MOS transistor. Therefore, it is widely used in various power supply fields. It is also widely used in the device.
The shortcoming of IGBT is that the collector current has a long tailing--the turn-off time is relatively long, so it is generally necessary to add a negative voltage to turn off the turn-off when turning off; second, the ability to resist DI/DT is relatively poor. If the MOS transistor is quickly turned off at a large short-circuit current like a protection MOS transistor, it may cause a high DI/DT at the collector, causing UCE to induce a high voltage due to the influence of the stray inductance of the pin and the loop. And damaged.
The short-circuit protection of the IGBT is generally achieved by detecting the saturation voltage drop of the CE pole. When the collector current is large or short-circuited, the IGBT exits the saturation region and enters the amplification region. As mentioned above, we can't turn off the IGBT directly and quickly. We can reduce the gate voltage to reduce the current of the collector to prolong the protection time and reduce the DI/DT of the collector. If you do not reduce the gate voltage to reduce the current of the collector, the short-circuit withstand of the IGBT with a saturation voltage drop of 2V or less is only 5μS; the short-circuit withstand of the 3V saturation voltage drop is about 10-15μS, 4-5 V saturation. The short-circuit tolerance of the voltage drop IGBT is approximately 30μS.
Another point, the gate voltage can not be too fast, generally controlled at about 2μS, that is, in order to reduce the collector current from a large short-circuit current to 1.2-1.5 times the overload protection, it is generally controlled at about 2μS. Can not be too fast, if the short circuit disappears within the delay of overload protection, it can be automatically restored. If it still maintains the overload protection current, the IGBT is turned off by the overload protection circuit.
Therefore, the short circuit protection of IGBT is generally matched with overload protection. The following is an application circuit diagram of TLP250 to increase the drive and short circuit protection of slow falling gate voltage:
When the circuit in the above figure works normally, the potential of the negative terminal of ZD1 is insufficient for ZD1 to turn on due to the conduction of D2, Q1 is cut off; the negative terminal of D1 is high, so Q3 is also cut off. C1 is not charged and the potential at both ends is 0. After the IGBT Q3 is short-circuited, it exits the saturation state, the collector potential rises rapidly, and D2 turns off from conduction. When the drive signal is high, ZD1 is broken down, and C2 can make Q1 open for a short period of time, so that Q3 can be turned on for a short period of time, which avoids the protection of the protection circuit during normal operation. After ZD1 is broken down, Q1 is turned on after a short period of time due to the existence of C2. C1 starts to charge through R4 and Q1, and the negative potential of D1 begins to decrease. When the negative potential of D1 begins to drop to D1 and Q3be When the junction voltage drops, Q3 starts to conduct, Q2, Q4 base potential begins to decrease, and Q3's gate voltage also begins to decrease. When C1 is charged to the breakdown voltage of ZD2, ZD2 is broken down, C1 stops charging, the process of falling gate voltage is also ended, and the gate voltage is clamped at a fixed level. The collector current of Q3 is also reduced to a fixed level.
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