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　　Analysis of the effect of short circuit protection circuit on inverter

　　In the design and manufacturing process of the inverter, over-current short-circuit protection is a very important link, which largely determines the safety of the inverter in actual use. If the over-current short-circuit protection circuit fails, then the inverter will The inverter will most likely burn out, so the short-circuit protection circuit plays a great role in the inverter.

　　If you want to quickly understand the overcurrent short-circuit protection circuit in the inverter, you must first conduct a comprehensive analysis of the load characteristics. Most of the loads in real life are impact loads, such as incandescent light bulbs. The resistance when cold is much lower than when it is lit. For rectifier loads such as computers and televisions, since the input alternating current needs to be rectified, a A relatively large capacitor is used for filtering, so the inrush current is relatively large. There are also inductive loads on motors such as refrigerators. The motor also needs electricity to generate relatively large torque from standstill to normal rotation, so the starting current is also relatively large.

　　When the rated output power is less than the starting power, it cannot be started. Of course, this only refers to the situation where the inverter can only set a long-term working output power. At this time, the inverter needs to be equipped according to the starting power, which is obviously a waste. In practice, when we design an overcurrent short-circuit protection circuit, we will design two protection points, rated power and peak power. Generally, the peak power is set to 2-3 times the rated power. In terms of time, the rated power will not be protected after working for a long time, and the peak power will generally only last for a few seconds before being protected. Let’s take the designed overcurrent short-circuit protection circuit as an example to explain:

　　R5 is the high-voltage current sampling resistor at the source of the full-bridge high-voltage inverter MOS tube. We can understand that the size of the high-voltage current basically determines the size of the output power, so we use R5 to detect the size of the high-voltage current. The two comparator units of LM339 in the picture are used for over-current and short-circuit detection respectively.

　　Let’s first look at the overcurrent protection circuit composed of IC3D and its peripheral components. Pin 8 of IC3D sets a reference voltage, and its value is determined by the voltage division of R33, VR4, R56, and R54. U8=5*(R33+VR4)/(R33 +VR4+R56+R54). When the voltage on R5 exceeds the voltage of pin 8 after being delayed by R24 and C17, pin 14 outputs a high level and is isolated to pin 5 of IC3B through D7. Pin 4 also serves as battery under-voltage protection. Normally, the voltage of pin 5 is lower than pin 4. After overcurrent, the voltage of pin 5 is higher than pin 4. Pin 2 outputs a high level to control the high-voltage MOS of the subsequent stage to turn off. Of course, it can also control the front stage. level MOS is turned off together. The function of D8 is to lock pin 2 to high level with positive feedback after overcurrent, short circuit or battery undervoltage.

　　The short-circuit protection point should be designed according to parameters such as the ID of the MOS tube, the safe area and the loop stray resistance. Generally speaking, it is relatively safe if the current is within ID and the action time is within 30 microseconds. Let’s look at the short-circuit protection circuit composed of IC3C. The principle is similar to that of over-current protection, except that the delay time is shorter. The capacity of C19 is very small, and the speed of LM339 is very fast. The short-circuit protection can be turned off within a few microseconds. , effectively protecting the safety of high-voltage MOS tubes.

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