Design Scheme of Lightning Protection Circuits for Switching Power Supplies
Lightning surge analysis
The most common harm to electronic devices is not caused by direct lightning strikes, but by current surges induced in power and communication lines during lightning strikes. On the one hand, due to the highly integrated internal structure of electronic devices (VLSI chips), the level of voltage and overcurrent resistance of the equipment has decreased, and the ability to withstand lightning (including induced lightning and operational overvoltage surges) has also decreased. On the other hand, due to the increase in signal source paths, the system is more susceptible to lightning wave intrusion than before. Surge voltage can enter computer equipment through power lines or signal lines, and we will discuss these two aspects separately:
1) Power surge
Power surges are not only caused by lightning strikes, but also by short circuit faults and heavy load switching in the power system. The power grid stretches for thousands of miles, and the probability of both lightning strikes and line surges is high. When a lightning strike occurs hundreds of kilometers away from you, the lightning surge is transmitted at the speed of light through the power grid, attenuated by substations, etc. When it reaches your computer, there may still be thousands of volts. This high voltage is very short, only tens to hundreds of microseconds, or not enough to burn out the computer. However, it causes great damage to the semiconductor components inside the computer, just like the noise of old speakers is greater than that of new ones because the internal components are damaged. As these damages deepen, the computer gradually becomes more unstable or may cause the loss of important data.
GE Corporation in the United States has determined that low-voltage distribution lines (110V) in general households, restaurants, apartments, and other areas have experienced more than 800 surge voltages exceeding the original operating voltage within 10000 hours (approximately one year and two months), with over 300 exceeding 1000V. Such surge voltage could potentially damage electronic devices in one go.
2) Signal system surge
The main sources of surge voltage in signal systems are induced lightning strikes, electromagnetic interference, radio interference, and electrostatic interference. Metal objects (such as telephone lines) are affected by these interference signals, which can cause errors in the transmitted data, affecting the accuracy and transmission rate of the transmission. Eliminating these interferences will improve the transmission status of the network.
Based on the above technical defects and conditions, this article designs a single-phase parallel anti lightning surge switching power supply circuit based on varistors and ceramic gas discharge tubes according to actual use.
