“RS485 is used for communication between equipment and computer or other equipment. In product application, its wiring is mostly mixed with power supply, power signal, etc., and there is an EMC hidden danger.
“
Schematic
NO.1
1. RS485 interface 6KV lightning protection circuit design
Figure 1 RS485 interface lightning protection circuit
Interface circuit design overview:
RS485 is used for communication between equipment and computer or other equipment. In product application, its wiring is mostly mixed with power supply, power signal, etc., and there is an EMC hidden danger.
From the principle of EMC, this scheme has carried out the design of related suppression of interference and anti-sensitivity, and solved the EMC problem from the design level.
2. Circuit EMC design description:
(1) Key points of circuit filter design:
L1 is a common mode inductance. The common mode inductance can attenuate the common mode interference, suppress the internal interference of the board and external interference, improve the anti-interference ability of the product, and also 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 and filter external interference at the same time; the capacitance value selection range is 22PF~1000pF, and the typical value is 100pF; if the signal The line-to-metal shell has insulation withstand voltage requirements, so the two filter capacitors of the differential line-to-ground need to consider the withstand voltage;
Consider reducing or removing the value of the filter capacitor when there are multiple nodes on the circuit. C3 is the jumper capacitor between the interface ground and the digital ground, the typical value is 1000pF, the capacitance value of C3 can be adjusted according to the test situation;
(2) Key points of circuit lightning protection design:
In order to meet the lightning protection test requirements of IEC61000-4-5 or GB17626.5 standard, common mode 6KV, differential mode 2KV, D4 is a three-terminal gas discharge tube to form the first-level protection circuit, which is used to suppress common mode and differential mode on the line. Mode surge interference to prevent interference from affecting the next-level circuit through the signal line;
The nominal voltage VBRW of the gas discharge tube is required to be greater than 13V, and the peak current IPP is required to be greater than or equal to 143A;
The peak power WPP requirement is greater than or equal to 1859W;
PTC1 and PTC2 are thermistors to form the second-level protection circuit, and the typical value is 10Ω/2W;
In order to ensure the smooth conduction of the gas discharge tube, it is necessary to increase this resistance to divide the voltage to ensure that most of the energy is removed through the gas discharge tube;
D1~D3 are TSS tubes (semiconductor discharge tubes) to form the third-level protection circuit. The nominal voltage VBRW of the TSS tube is required to be greater than 8V, the peak current IPP requirement is greater than or equal to 143A; the peak power WPP requirement is greater than or equal to 1144W;
3. Interface circuit design remarks:
If the device has a metal casing and the single board can independently divide the interface ground, the metal casing is directly electrically connected to the interface ground, and the single board ground is connected to the interface ground through a 1000pF capacitor;
If the device is a non-metallic shell, the interface ground PGND is directly electrically connected to the single-board digital ground GND.
PCB Design
NO.2
1. RS485 interface circuit layout
Figure 1 RS485 interface filtering and protection circuit layout
Program features:
(1) The protective devices and filter components should be placed close to the interface and should be placed in a compact and tidy manner. According to the rule of protection first and filtering later, the wiring should be avoided as far as possible;
(2) The common mode inductance and the jumping capacitor should be placed in the isolation belt.
case analysis:
(1) The interface and the interface filter protection circuit cannot be routed and high-speed or sensitive devices cannot be placed;
(2) The projection layer under the isolation belt should be hollowed out, and wiring is prohibited.
2. RS485 interface circuit design
Program features:
(1) In order to suppress the external conduction and radiation of the internal board noise through the RS485 interface, and to enhance the board’s anti-interference ability to external interference, a filter element is added at the RS485 interface for suppression. interface;
(2) Capacitors can be selectively added to the isolation band as the connection between the two grounds. The recommended value of capacitors C4 and C5 is 1000pF. The common mode inductor CM and capacitor filter are connected in series on the signal line, and GDT and The TVS tube is protected; 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 shown in the figure.
case analysis:
(1) When there is a circuit with poor compatibility or incompatibility between the interface and the board, it is necessary to perform “ground separation” processing between the interface and the board, that is, according to different port voltages, level signals and transmission rates to set the ground wire respectively. “Ground separation” can prevent the superposition of the return signals of incompatible circuits and prevent the impedance coupling of the common ground wire;
(2) The phenomenon of “ground separation” will cause the impedance of the return signal to become larger when it crosses the isolation strip, which will cause a great EMC risk. Therefore, a capacitor is used between the isolation strips to provide a return path for the signal.
“RS485 is used for communication between equipment and computer or other equipment. In product application, its wiring is mostly mixed with power supply, power signal, etc., and there is an EMC hidden danger.
“
Schematic
NO.1
1. RS485 interface 6KV lightning protection circuit design
Figure 1 RS485 interface lightning protection circuit
Interface circuit design overview:
RS485 is used for communication between equipment and computer or other equipment. In product application, its wiring is mostly mixed with power supply, power signal, etc., and there is an EMC hidden danger.
From the principle of EMC, this scheme has carried out the design of related suppression of interference and anti-sensitivity, and solved the EMC problem from the design level.
2. Circuit EMC design description:
(1) Key points of circuit filter design:
L1 is a common mode inductance. The common mode inductance can attenuate the common mode interference, suppress the internal interference of the board and external interference, improve the anti-interference ability of the product, and also 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 and filter external interference at the same time; the capacitance value selection range is 22PF~1000pF, and the typical value is 100pF; if the signal The line-to-metal shell has insulation withstand voltage requirements, so the two filter capacitors of the differential line-to-ground need to consider the withstand voltage;
Consider reducing or removing the value of the filter capacitor when there are multiple nodes on the circuit. C3 is the jumper capacitor between the interface ground and the digital ground, the typical value is 1000pF, the capacitance value of C3 can be adjusted according to the test situation;
(2) Key points of circuit lightning protection design:
In order to meet the lightning protection test requirements of IEC61000-4-5 or GB17626.5 standard, common mode 6KV, differential mode 2KV, D4 is a three-terminal gas discharge tube to form the first-level protection circuit, which is used to suppress common mode and differential mode on the line. Mode surge interference to prevent interference from affecting the next-level circuit through the signal line;
The nominal voltage VBRW of the gas discharge tube is required to be greater than 13V, and the peak current IPP is required to be greater than or equal to 143A;
The peak power WPP requirement is greater than or equal to 1859W;
PTC1 and PTC2 are thermistors to form the second-level protection circuit, and the typical value is 10Ω/2W;
In order to ensure the smooth conduction of the gas discharge tube, it is necessary to increase this resistance to divide the voltage to ensure that most of the energy is removed through the gas discharge tube;
D1~D3 are TSS tubes (semiconductor discharge tubes) to form the third-level protection circuit. The nominal voltage VBRW of the TSS tube is required to be greater than 8V, the peak current IPP requirement is greater than or equal to 143A; the peak power WPP requirement is greater than or equal to 1144W;
3. Interface circuit design remarks:
If the device has a metal casing and the single board can independently divide the interface ground, the metal casing is directly electrically connected to the interface ground, and the single board ground is connected to the interface ground through a 1000pF capacitor;
If the device is a non-metallic shell, the interface ground PGND is directly electrically connected to the single-board digital ground GND.
PCB Design
NO.2
1. RS485 interface circuit layout
Figure 1 RS485 interface filtering and protection circuit layout
Program features:
(1) The protective devices and filter components should be placed close to the interface and should be placed in a compact and tidy manner. According to the rule of protection first and filtering later, the wiring should be avoided as far as possible;
(2) The common mode inductance and the jumping capacitor should be placed in the isolation belt.
case analysis:
(1) The interface and the interface filter protection circuit cannot be routed and high-speed or sensitive devices cannot be placed;
(2) The projection layer under the isolation belt should be hollowed out, and wiring is prohibited.
2. RS485 interface circuit design
Program features:
(1) In order to suppress the external conduction and radiation of the internal board noise through the RS485 interface, and to enhance the board’s anti-interference ability to external interference, a filter element is added at the RS485 interface for suppression. interface;
(2) Capacitors can be selectively added to the isolation band as the connection between the two grounds. The recommended value of capacitors C4 and C5 is 1000pF. The common mode inductor CM and capacitor filter are connected in series on the signal line, and GDT and The TVS tube is protected; 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 shown in the figure.
case analysis:
(1) When there is a circuit with poor compatibility or incompatibility between the interface and the board, it is necessary to perform “ground separation” processing between the interface and the board, that is, according to different port voltages, level signals and transmission rates to set the ground wire respectively. “Ground separation” can prevent the superposition of the return signals of incompatible circuits and prevent the impedance coupling of the common ground wire;
(2) The phenomenon of “ground separation” will cause the impedance of the return signal to become larger when it crosses the isolation strip, which will cause a great EMC risk. Therefore, a capacitor is used between the isolation strips to provide a return path for the signal.
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