USB interface has the characteristics of fast transmission speed, supporting hot plug and connecting multiple devices. At present, it has been widely used in all kinds of computers and consumer products.

1、 The USB interface faces the problem of electromagnetic compatibility

Due to the high operating speed of USB interface, it is easy to exceed the standard of external high-frequency radiation through USB connecting cable. At the same time, it is easy to be affected by instantaneous voltage impact and electrostatic interference due to hot plug. Therefore, when designing the product interface, we need to focus on the EMC design from the aspects of interface filter design, protection design, PCB design and structural cable.

The EMC solution in this paper mainly combines the characteristics of USB 2.0 interface circuit and starts from the interface circuit of product schematic diagram to provide a specific EMC design scheme that meets the actual design requirements of the product, so that the product can meet the requirements of EMC standards and specifications, obtain good EMC quality and improve the reliability of the product.

2、 USB interface standard requirements

Typical consumer products with USB interface need to meet relevant electromagnetic compatibility requirements. The requirements of electromagnetic compatibility items related to USB are as follows. Other requirements are different for military products, automotive electronics and railway electronics. Please refer to relevant electromagnetic compatibility standards for details.

How to solve the electromagnetic compatibility problem of USB interface

3、 Schematic EMC Design:

How to solve the electromagnetic compatibility problem of USB interface

4、 Key points of schematic design:

4.1 key points of filter design:

L1 is the common mode filter inductance, which is used to filter the common mode interference on the differential signal;

L2 is a filter magnetic bead, which is used to filter the interference on the power supply;

C3 and C4 are power filter capacitors to filter out interference on the power supply;

C1 and C2 are reserved designs. Pay attention to the minimum capacitance. If the signal transmission is actually affected, it can not be welded.

4.2 key points of protection design:

D1, D2 and D3 form a USB interface protection circuit, which can quickly discharge electrostatic interference and avoid electrostatic interference to the internal circuit.

C5 and C6 are the jumper capacitance between interface ground and digital ground, the typical value is 1000pf, and the withstand voltage is required to be more than 2KV.

4.3 special requirements:

4.3 R1 and R2 current limiting resistors. The coupling between the differential lines will affect the external impedance of the signal line. This resistor can be used to achieve the best matching of the terminal, which can be adjusted according to the actual situation.

4.4 device selection requirements:

L1 is a common mode inductor. The impedance of the common mode inductor is selected from 60 Ω / 100MHz to 120 Ω / 100MHz, and the typical value is 90 Ω / 100MHz

L2 adopts magnetic beads, the impedance range of magnetic beads is 100 Ω / 100MHz ~ 1000 Ω / 100MHz, and the typical value is 600 Ω / 100MHz; When selecting magnetic beads, the flux shall meet the requirements of circuit current. It is recommended to use magnetic beads for power supply

The difference between C3 and C4 capacitors is 100 times, and the typical values are 1000pf and 0.1uF; The small capacitor is used to filter the high-frequency interference on the power supply, and the large capacitor is used to filter the ripple interference on the power supply line;

TVs is selected for D1, D2 and D3, and the reverse shutdown voltage of TVs is 5V. The junction capacitance of TVs tube has a certain influence on the signal transmission frequency. The junction capacitance of TVs of USB2.0 is less than 5pf;

C5 and C6 are the jumper capacitance between interface ground and digital ground, the typical value is 1000pf, and the withstand voltage is required to be more than 2KV.

4.5 list of recommendations for selection of relevant EMC devices

How to solve the electromagnetic compatibility problem of USB interface

5、 PCB design description

5.1 key points of layout design

Components shall be arranged according to signal flow direction;

The protective device shall be placed as close to the interface as possible to ensure the minimum lead inductance, so as to ensure the normal protective action of the protective device.

The chip shall be placed on the signal layer closest to the stratum and close to the USB socket as far as possible to shorten the routing distance of the differential line.

5.2 key points of wiring design

No other signal lines can be taken under the common mode inductor.

If the USB interface chip needs series terminal resistors or D-line pull-up resistors, be sure to place these resistors as close to the chip as possible.

The USB differential signal line is arranged in the signal layer closest to the formation.

Maintain the integrity of the stratum at the lower end of the USB differential line. If the stratum at the lower end of the differential line is divided, it will cause the discontinuity of the impedance of the differential line and increase the impact of external noise on the differential line.

During the wiring process of USB differential line, vias should be avoided on the differential line, which will cause impedance mismatch of the differential line.

Ensure the consistency of the line spacing of the differential line in the routing process. If the spacing of the differential line changes in the routing process, it will cause the discontinuity of the impedance of the differential line.

In the process of drawing the difference line, use 45 ° angle or circular arc angle to replace 90 ° angle, and try not to take other signal lines within 150 mils around the difference line, especially the digital signal lines with steep edges. Pay more attention that their routing can not affect the USB difference line.

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