RS485 interface 6KV lightning protection circuit design and PCB layout

RS485 interface lightning protection circuit interface circuit design overview: RS485 is used for communication between equipment and computers or other equipment. In product applications, its wiring is mostly mixed with power supplies, power signals, etc., and there are hidden EMC hazards.

1. Schematic

1. RS485 interface 6KV lightning protection circuit design plan

RS485 interface lightning protection circuit interface circuit design overview: RS485 is used for communication between equipment and computers or other equipment. In product applications, its wiring is mostly mixed with power supplies, power signals, etc., and there are hidden EMC hazards.

RS485 interface 6KV lightning protection circuit design and PCB layout

Based on the EMC principle, this scheme has carried out related interference suppression and anti-sensitivity design, and solves the EMC problem from the design level.

2. Circuit EMC design description:

(1) Key points of circuit filtering design:

L1 is a common mode inductance. The common mode inductance can attenuate common mode interference, suppress internal interference and external interference on the board, improve the anti-interference ability of the product, and reduce the external radiation through the 429 signal line. , The common mode Inductor impedance selection range is 120Ω/100MHz ~ 2200Ω/100MHz, and the typical value is 1000Ω/100MHz; C1 and C2 are filter capacitors, which provide a low impedance return path for interference, which can effectively reduce the external common mode current at the same time It can filter external interference; the capacitance value selection range is 22PF~1000pF, and the typical value is 100pF; if the signal line has insulation withstand voltage requirements for the metal shell, then the two filter capacitors of the differential line to the ground need to consider the withstand voltage; when the circuit When there are multiple nodes, consider reducing or removing the value of the filter capacitor. C3 is the jumper capacitance between the interface ground and the digital ground, the typical value is 1000pF, and the capacitance value of C3 can be adjusted according to the test situation; (2) Circuit lightning protection design points:

In order to meet the IEC61000-4-5 or GB17626.5 standard, common mode 6KV, differential mode 2KV lightning protection test requirements, D4 is a three-terminal gas discharge tube composition level protection circuit, used to suppress common mode and differential mode waves on the line Surge interference to prevent interference from affecting the circuit through the signal line; the nominal voltage VBRW of the gas discharge tube must be greater than 13V, the peak current IPP must be greater than or equal to 143A; the peak power WPP must be greater than or equal to 1859W; PTC1 and PTC2 are thermistors forming the second level Protective circuit, the typical value is 10Ω/2W; in order to ensure the smooth conduction of the gas discharge tube, this resistance must be increased to divide the pressure of the bleeder amplifier energy to ensure that most of the energy is removed through the gas discharge tube; D1~D3 are TSS tubes (semiconductor discharge tube) constitutes the third-level protection circuit. The nominal voltage VBRW of the TSS tube is required to be greater than 8V, and the peak current IPP is required to be greater than or equal to 143A; the peak power WPP is required to be greater than or equal to 1144W; 3. Interface circuit design remarks:

If the device is in a metal enclosure and the board can be separated from the interface ground independently, then the metal enclosure and the interface ground are directly electrically connected, and the board ground and the interface ground are connected through a 1000pF capacitor; if the device is a non-metal enclosure, then the interface ground is PGND Direct electrical connection with the digital ground GND of the board.

2. PCB design

1. RS485 interface circuit layout diagram

RS485 interface filtering and protection circuit layout plan features: (1) The protection devices and filter parts should be placed close to the interface and they should be placed compactly and neatly. According to the rule of first protection and then filtering, try to avoid twists and turns when routing. Situation: (2) The common mode inductance and jumper capacitance should be placed in the isolation band.

Solution analysis: (1) No wiring and high-speed or sensitive devices can be placed around the interface and interface filter protection circuit; (2) The projection layer under the isolation belt should be hollowed out, and wiring is prohibited.

2. Features of RS485 interface circuit ground design scheme:

(1) In order to suppress internal board noise from conducting radiation to the outside through the RS485 interface, and to enhance the board’s immunity to external interference, a filter is added at the RS485 interface for suppression, and the size of the filter is defined as the boundary Interface ground; (2) Capacitors can be selectively added in the isolation band as a connection between the two grounds. The recommended value of capacitors C4 and C5 is 1000pF. GDT and TVS tubes are connected in parallel for protection; and all protection devices are placed close to the interface, and the common mode inductor CM is placed in the isolation band. The specific layout is as shown in the figure.

Solution analysis: (1) When there is a circuit with poor compatibility or incompatibility between the interface and the single board, it is necessary to “separate ground” between the interface and the single board, that is, according to different port voltages and level signals And the transmission rate to set the ground wire separately. “Separation of ground” can prevent the superimposition of the return signals of incompatible circuits and prevent the impedance coupling of the common ground wire; (2) The phenomenon of “separation of ground” will cause the return signal to increase in impedance when it crosses the isolation band, thus causing great EMC Risk, therefore, provide a return path for the signal through a capacitor between the isolation band.

The Links:   FF1400R17IP4 PN100RL1B060

Related Posts

Leave a Reply

Your email address will not be published. Required fields are marked *