In the planning and production process of the inverter, over-current and short-circuit maintenance is a very important link, which largely determines the safety of the inverter in practical use. If the over-current and short-circuit maintenance circuit fails, the reverse The inverter will most likely be burned, so the short-circuit maintenance circuit has a great effect on the inverter.
If you want to quickly understand the over-current and short-circuit maintenance circuit near the inverter, you must first analyze the load characteristics. Most of the loads in real life are impact loads, such as incandescent bulbs. The resistance in the cold state is much lower than when it is lit. For rectifying loads such as computers and televisions, the input AC power needs to be rectified. Compared with large capacitor filtering, the impact current is relatively large. There are also rational loads such as refrigerators. The motor needs to use electricity to generate a relatively large torque from a standstill to a normal rolling, so the starting current is also relatively large.
When the additional 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 operating output power. At this moment, it is necessary to equip the inverter according to the starting power, which is obviously a waste. In practice, when we plan the over-current short-circuit maintenance circuit, we will plan two maintenance points, additional power and peak power. Usually the peak power is set to 2-3 times the additional power. The extra power at all times cannot be maintained for long-term operations, and the peak power is usually maintained only for a few seconds. The following figure 1 is the planned over-current short-circuit maintenance circuit, and take this figure as an example to explain it as follows:
R5 is the high-voltage current sampling resistance of 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 check the size of the high-voltage current. The two comparator units of LM339 in the picture are used separately for over-current and short-circuit inspection.
First look at the overcurrent maintenance circuit composed of IC3D and its peripheral components. The 8 pins of IC3D set a reference voltage, and the value U8=5*(R33+VR4)/(R33 is determined by the partial pressure of R33, VR4, R56, and R54. +VR4+R56+R54). When the voltage on R5 is delayed by R24 and C17, it surpasses the voltage of pin 8 and pin 14 and outputs a high level, which is blocked by D7 to pin 5 of IC3B. Pin 4 is also used for battery under-voltage maintenance. Normally, the voltage of pin 5 is lower than pin 4, and the voltage of pin 5 is higher than pin 4 after over-current. Pin 2 outputs high level to control the high-voltage MOS of the rear stage to turn off. Of course, it can also be controlled before. The level of MOS is turned off together. The effect of D8 is that after over-current short-circuit or battery under-voltage, the positive feedback determines that pin 2 is high.
The short-circuit maintenance point should be planned 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 for the current to be within the ID and the action time to be within 30 microseconds. Looking at the short-circuit maintenance circuit composed of IC3C, the principle is similar to that of over-current maintenance. Only the delay time is relatively short. The capacity of C19 is small, and the speed of LM339 is very fast. It can complete short-circuit maintenance and shut down within a few microseconds. , Effectively maintain the safety of the high-voltage MOS tube.
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