USB interface electromagnetic compatibility (EMC) solution

usb The interface has fast transmission speed , Support hot plug and connect multiple devices , At present, it has been used in all kinds of computers 、 Widely used in consumer products .

　　 One 、 usb The interface faces EMC problems

　　 because usb The interface runs at a high speed , Easy to pass through usb The external high frequency radiation of the connecting cable exceeds the standard , At the same time, due to the hot plug , Susceptible to transient voltage shock and electrostatic interference . So when we design the product interface , We need to focus on the design of interface filtering , Protective design ,PCB Design 、 EMC design is considered in many aspects of structural cable .

 

　　 In this paper, EMC solutions are mainly combined with usb2.0 Interface circuit features , Starting from the interface circuit of the product schematic diagram , Provide specific... That meets the actual design requirements of the product emc design scheme , So that the product can meet the electromagnetic compatibility standards and specifications , Get good emc The quality of , Improve product reliability .

　　 Two 、 usb Interface standard requirements

　　 with usb Typical consumer products of interface , Need to meet the relevant EMC requirements , And usb Relevant EMC project requirements are as follows , Other applications such as military products 、 Automotive electronics 、 Railway Electronic requirements are different , Please refer to relevant EMC standards for details .

　　

　　 3、 ... and 、 Schematic diagram emc Design ：

　　

　　 Four 、 Key points of schematic design ：

　　4.1 Filter design points ：

　　L1 For common mode filter inductors , It is used to filter out common mode interference on differential signals ;

　　L2 For filter beads , It is used to filter out the interference on the power supply ;

　　C3、C4 Filter capacitor for power supply , Filter out the interference on the power supply ;

　　C1、C2 Design for reservation , Pay attention to the capacitance as small as possible , Such as the actual impact on signal transmission , No welding .

　　4.2 Key points of protection design ：

　　D1、D2、D3 form usb Interface protection circuit , It can discharge static electricity quickly , Avoid electrostatic interference on internal circuits .

　　C5、C6 It is the jumper capacitance between interface ground and digital ground , The typical value is 1000pF, The withstand voltage shall meet the requirements of 2KV above .

　　4.3 Special requirements ：

　　4.3 R1、R2 Current limiting resistance , The coupling between the differential lines will affect the external impedance of the signal line , This resistor can be used to achieve the best terminal matching , Adjust it according to the actual situation .

　　4.4 Device selection requirements ：

　　L1 For common mode inductance , The selection range of common mode inductance impedance is 60Ω/100MHz~120Ω/100MHz, Typical value selection 90Ω/100MHz

　　L2 Choose magnetic beads , The impedance range of the bead is 100Ω/100MHz~1000Ω/100MHz, Typical value selection 600Ω/100MHz ; Flux of magnetic beads shall meet the requirements of circuit current when selecting , Beads for power supply are recommended

　　C3、C4 The two capacitors should be different in value 100 times , Typical values for 1000pF、0.1uF; Filtering high frequency interference on power supply for small capacitance , Large capacitance is used to filter out the ripple interference on the power line ;

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

　　C5、C6 It is the jumper capacitance between interface ground and digital ground , The typical value is 1000pF, The withstand voltage shall meet the requirements of 2KV above .

　　4.5 List of recommendations for selection of relevant EMC devices

　　

　　 5、 ... and 、PCB Design description

　　5.1 Key points of layout design

　　 The layout of components should follow the signal flow direction ;

　　 The protective device should be placed as close to the interface as possible , Make sure that the lead inductance is minimum , In order to ensure that the protective device can normally carry out protective action .

　　 The chip should be placed in the signal layer closest to the formation , And try to get as close as possible to usb Socket , Shorten the distance of differential line .

　　5.2 Key points of wiring design

　　 No other signal wire can be used under the common mode inductance .

　　 If usb The interface chip needs to be connected in series with the terminal resistance or D When the wire is connected to the pull resistance ． Be sure to place these resistors as close to the chip as possible .

　　 take usb The differential signal line is distributed in the signal layer closest to the formation .

　　 keep usb Formation integrity at the lower end of the differential line , If we divide the strata at the bottom of the difference line , It will cause the discontinuity of differential line impedance , And it will increase the influence of external noise on the differential line .

　　 stay usb In the process of differential wiring , Avoid placing vias... On differential lines （via）, Through hole will cause impedance mismatch of differential line .

　　 Ensure the consistency of the line spacing of the differential line in the process of routing , If the distance between the differential lines changes during the routing , It will cause the discontinuity of differential line impedance .

　　 In drawing the difference line , Use 45° A bend or arc to replace 90° Corner , And try to be around the difference line 150 mil Do not use other signal lines in the range , Especially the digital signal line with steep edge should pay more attention to its routing usb Differential line .