The design of lightning surge circuits has become a key issue in instrument development

Lightning protection switch power circuit design scheme

Lightning rods on general buildings can only prevent direct lightning strikes, but induced lightning strikes and impulse voltages generated by strong electromagnetic fields can sneak into the room, endangering television, telephone, electronic instruments and other electrical equipment. In particular, solar control instruments, due to the special installation location of solar energy, their use stability has been the focus of most developers.

Instantaneous high-voltage lightning surge and signal system surge are important reasons for poor stability of the instrument. Induced lightning stroke is the main source of surge voltage of signal system. Electromagnetic interference (EMI), radio interference and electrostatic interference. Metal objects (such as telephone lines) affected by these interference signals will cause data code errors in transmission, affecting the accuracy and transmission rate of transmission. How to design lightning protection circuit has become a key issue in instrument research and development.

Analysis of lightning surge

The most common hazards of electronic equipment are not caused by direct lightning strikes, but by current surges in power supply and communication lines. On the one hand, the internal structure of electronic equipment is highly integrated (VLSI chip), which leads to the reduction of equipment withstand voltage and overcurrent resistance, and the reduction of lightning (including induced lightning and switching overvoltage surge) bearing capacity. On the other hand, due to the increase of signal source paths, the system is more vulnerable to the intrusion of lightning waves than before. The surge voltage can enter the computer equipment through the power line or signal line. We will discuss the following two aspects respectively:

1) Power surge

When the power system has a short circuit fault, the power surge is not only from lightning strikes. Large loads will generate power surge, and the power grid will extend for thousands of miles. The probability of lightning stroke and line surge is high. When lightning strikes occur hundreds of kilometers away from you, the lightning surge is transmitted at the speed of light through the power grid and attenuated through the substation. 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 the computer, but it has great damage to the semiconductor components inside the computer, just as the noise of the old sound is greater than that of the new sound, because the internal components are damaged. With the deepening of these damages, the computer gradually becomes more and more unstable, or may cause the loss of your important data.

The surge voltage (110) V) of the low-voltage distribution line (110) measured by GE in the United States in general families, restaurants, apartments and other places occurred between lines within 10000 hours (about one year and two months), and reached more than 800 times, of which more than 1000 times of such surge voltage may completely damage the electronic equipment at one time.

2) Signal system surge

Induced lightning stroke is the main source of surge voltage of signal system, including electromagnetic interference, radio interference and electrostatic interference. These interference signals affect metal objects (such as telephone lines), which will cause data code errors in transmission and affect transmission accuracy and transmission rate. Eliminating these interferences will improve the transmission status of the network.

Based on the above technical defects and conditions, a switching power supply circuit based on varistor and ceramic gas discharge tube is designed.

Design lightning surge circuit

In this paper, a single-phase parallel lightning surge resistance circuit based on varistor and ceramic gas discharge tube is designed and applied to the switching power supply of the instrument. The whole circuit includes lightning protection circuit and switching power supply circuit, wherein the lightning protection circuit is composed of three varistors and a ceramic gas discharge tube, with common mode and complete protection. The lightning protection instrument power circuit composed of the classic switching power supply circuit uses the piezoresistor in parallel to extend the service life. After the short circuit fault of the varistor, it will be separated from the switch power supply circuit and will not cause fire.

In order to achieve the above purpose, the design scheme is to apply a single-phase parallel lightning surge circuit of piezoresistor and ceramic gas discharge tube on the power supply of the instrument. It is mainly divided into lightning protection circuit and switching power supply circuit. The circuit is simple, and adopts compound symmetrical circuit, common mode, differential contact full protection, and can be connected regardless of L.N. The varistor RV1 is located in parallel at the front end of the patch rectifier module L.N, mainly to clamp the voltage between the L.N lines. The varistor RV0.RV2 is connected in series with the ceramic gas discharge tube FD after grounding. The series connection of RV0 and FD1 is mainly to induce the lightning surge current on the discharge line L, and the series connection of RV2 and FD1 is mainly the energy on the discharge V reference potential of the signal port 24. After the RV0.RV2 short circuit fault, the FD can be separated from the power circuit without causing fire.

In case of RV1 short circuit fault on the front end line connected in series with a winding resistance of RV, the winding resistance can act as a fuse to disconnect the short circuit circuit. The varistor is a voltage clamp protection device, and the selection of clamp voltage points, that is, the parameters of the varistor, are relatively important (selecting the ones with higher voltage and larger flow rate is safer, more durable, and lower failure rate); Select the shape, size, and packaging form according to the flow capacity requirements. This circuit uses 561k-10D in series with a ceramic gas discharge tube to extend its service life and ensure safety.

Select the flow capacity of ceramic gas discharge tube according to the required flow capacity, and the circuit is 3. RM470L － 7.5-L, with a flow rate of 5000A. The wire wound resistor R1 plays a current limiting and voltage dividing role; The patch rectification module plays the role of front-end rectification of the switching power supply circuit. C1 is the high-voltage filter capacitor, Y resistance is the decoupling capacitor R2, and capacitance C2 and VD2 form the TNY27 series of the switching power supply chip MOS absorption clamp circuit, protection chip and switching power supply chip PI company, TR1 is the high-frequency transformer, and VD3.C3 forms the high-frequency transformer secondary filter, U2.VD4.R3.R4.R5 constitutes the feedback circuit of the switching power supply circuit, which can stabilize the secondary output voltage of the transformer in the design value, and has achieved good results in practical use.

In the design of lightning surge, the single parallel lightning protection circuit based on varistor and ceramic gas discharge tube has gradually been favored by the majority of designers in the development of solar control instruments in recent years. The circuit designed in this paper fully conforms to the test standard of GB/T17626.5 for its preciseness. In practical use, the space of PCB can be vacated to provide developers with a design stage at will.