Analysis of the effect of short circuit protection circuit on inverter

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 high level and is isolated to pin 5 of IC3B through D7. Pin 4 also serves as battery undervoltage 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 relatively short. 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.

This article mainly introduces the role of short-circuit protection for inverters and emphasizes the importance of short-circuit protection circuits in inverter design. I hope you can gain something after reading this article.