Shulou(Shulou.com)06/01 Report--

This article to share with you is about USB in the electromagnetic compatibility EMC design to pay attention to what problems, Xiaobian think quite practical, so share to everyone to learn, I hope you can read this article after some harvest, not much to say, follow Xiaobian to see it.

Usb interface has the characteristics of fast transmission speed, hot plug support and connection of multiple devices, and has been widely used in various types of computers and consumer products.

1,usb interface faces electromagnetic compatibility problems

Due to the high speed of usb interface, it is easy to exceed the standard of external high-frequency radiation through usb connection cable. At the same time, due to hot plug, it is easy to be subjected to instantaneous voltage shock and electrostatic interference. Therefore, when designing product interfaces, we need to focus on electromagnetic compatibility design from interface filter design, protection design, PCB design, and structural cables.

EMC solutions mainly combine the characteristics of USB2.0 interface circuit, starting from the interface circuit of the product schematic diagram, to provide specific emc design solutions that meet the actual design requirements of the product, so that the product can meet the electromagnetic compatibility standards and specifications, obtain good emc quality, and improve product reliability.

2,USB interface standard requirements

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

3, schematic emc design

4. Schematic diagram design key points description

4.1 Filter design points:

L1 is a common-mode filter inductor for filtering common-mode interference on differential signals;

L2 is a filter bead, which is used to filter out 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 design, pay attention to the minimum capacitance, such as the actual impact on signal transmission, can not be welded.

4.2 Protection design points:

D1, D2 and D3 constitute USB interface protection circuit, which can quickly discharge electrostatic interference and avoid internal circuit from electrostatic interference.

C5 and C6 are the jumper capacitors between the interface ground and the digital ground. The typical value is 1000pF, and the withstand voltage is required to reach more than 2KV.

4.3 Special requirements:

4.3 R1 and R2 are current-limiting resistors. Coupling between differential lines will affect the external impedance of signal lines. This resistor can be used to achieve optimal matching of terminals. It can be adjusted according to actual conditions when used.

4.4 Device selection requirements:

L1 is common mode inductance, common mode inductance impedance selection range is 60Ω/100MHz~120Ω/100MHz, typical value is 90Ω/100MHz

L2 magnetic beads, magnetic beads impedance range of 100Ω/100MHz~1000Ω/100MHz, typical value of 600Ω/100MHz ; magnetic beads in the selection of flux should meet the requirements of circuit current, magnetic beads recommended for power supply

C3, C4 two capacitors in the value of the difference of 100 times, typical values of 1000pF, 0.1uF; small capacitor used to filter out high-frequency interference on the power supply, large capacitor used to filter out ripple interference on the power line;

D1, D2, D3 select TVS, TVS reverse turn-off voltage is 5V. The junction capacitance of TVS tube has certain influence on signal transmission frequency. The junction capacitance of TVS tube of usb2.0 is less than 5pF.

C5 and C6 are the jumper capacitors between the interface ground and the digital ground. The typical value is 1000pF, and the withstand voltage is required to reach more than 2KV.

5,PCB design description

5.1 Layout design points

The layout of components should be arranged according to the signal flow direction;

The protective device should be placed as close to the interface as possible to ensure that the lead inductance is minimized to ensure that the protective device can normally perform protective action.

The chip should be placed on the signal layer closest to the stratum and as close as possible to the usb socket to shorten the differential line routing distance.

5.2 Cabling design essentials

No other signal lines can be routed below the common-mode inductance.

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

Route usb differential signal lines to the signal layer closest to the stratum.

To maintain the integrity of the formation at the lower end of the usb differential line, if the formation at the lower end of the differential line is divided, it will cause discontinuity in the impedance of the differential line and increase the influence of external noise on the differential line.

In the routing process of usb differential lines, it is necessary to avoid placing vias on differential lines, which will cause impedance mismatch of differential lines.

Ensure the consistency of the line spacing of the differential lines in the routing process. If the spacing of the differential lines changes in the routing process, the impedance of the differential lines will be discontinuous.

In the process of drawing differential lines, use a 45° bend angle or a circular arc bend angle instead of a 90° bend angle, and try not to walk other signal lines within the 150 mil range around the differential line, especially if the edge is steep.

The above is what problems USB should pay attention to in the design of electromagnetic compatibility EMC, Xiaobian believes that some knowledge points may be seen or used in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.