High Frequency PCB Layout

Document Sample
High Frequency PCB Layout Powered By Docstoc
					                High Frequency PCB Layout

                                                 24 March, 2008


                                                   Kyle Swartz
                                                   Chris Koliba
                                                  Namruth Nalla
                                                  Matt Cloutier
                                                   Sean Hatch




Michigan State University Team 05—ESD Gun Design. 3/24/08.        1
                        Objective
 When dealing with high frequency circuits, it becomes important to
 analyze the circuit from a electromagnetic standpoint—basic circuit
 begins to break down.

 Electromagnetic effects and interactions are especially important
 when a high frequency circuit is put on a PCB. Component proximity
 and orientation, and trace characteristics will effect the characteristics
 of the circuit.

 This presentation will give an overview of electromagnetics topics
 relevant to laying out PCBs, and apply these topics to a few specific
 examples.




Michigan State University Team 05—ESD Gun Design. 3/24/08.                2
                        Presentation Overview
 Electromagnetics Review:
             - Lumped Vs. Distributed Circuits
             - Transmission Lines
             - Radiation/Reception
 PCB Layout Issues:
             -Component Orientation
             -Circuit Geometry
             -Trace Length
             -Trace Geometry
 Selected Examples:
             -Transmission Lines
             -Traces Acting as Antennas
             -MOSFET System Arrangement
             -Clock Signal Traces
             -Trace Corners

Michigan State University Team 05—ESD Gun Design. 2/11/08.   2
                        Lumped Vs. Distributed Circuits
 When is my circuit “high frequency”?:
             -Depends on tr, the rise time of the signal
             -Also depends on td, the travel time between two points
                         -need to know l, length, and v, propagation velocity of trace
             -Electromagnetic effects must be considered when tr/td < 2.5
             -Example:
                     CMOS technology can be used to create signals with rise
                     times of 0.5 ns.

                         Common PCB traces have a propagation velocity of about
                         0.47*c

                          Circuit becomes high frequency when distance falls
                         below 2.82 cm

                          The traces on the PCB must be taken as transmission
                         lines, not nodes.

Michigan State University Team 05—ESD Gun Design. 3/24/08.                               4
                        Transmission Lines
 Important Aspects:
             -Characteristic Impedance, Z0:
                         - depends on width of trace
                         - depends on material (permittivity)
                         - depends on thickness of board
                         - depends on thickness of trace

             -Reflections:
                      - Voltage waves can bounce off of load!
                      - ΓL= R L – Z0 / R L + Z0
                      - Can lead to:
                               - Wasted power
                                     - Damaged components
                                     - Attenuated signals




Michigan State University Team 05—ESD Gun Design. 2/11/08.      2
                        Transmission Lines
 Other Aspects:
             -Propagation time
             -Realize that node voltages can no longer be assumed
             -Distort Signals

 Solutions:
             - Impedance matching can fix these problems
                         - Computer programs can do this for you
                         - Smith Charts




Michigan State University Team 05—ESD Gun Design. 2/11/08.          2
                        Radiation
 Electromagnetic Radiation :
             - High frequency signals in traces cause EM radiation
             - Traces act as antennas (antennas receive and transmit)
             - Time varying fields can induce currents and voltages on nearby
             conductors and system components, especially inductors
             - See Maxwell’s equations for more specific details on how these
             problems are caused


 Crosstalk/EMI:
             - The induction of voltages and currents on nearby traces from
             radiated EM fields is referred to as crosstalk or EMI (electromagnetic
             interference
             - These currents and voltages can falsely trigger switches and logic in
             a circuit
             - PCB traces in loops (or similar shapes) make especially good
             antennas


Michigan State University Team 05—ESD Gun Design. 3/24/08.                         7
                        Radiation (cont’d)
 Some sources of EMI:
             - Switching; MOSFETS, for example
             - Oscillators
             - Motors
             - Reflection interfaces
             - Outside sources too, but shielding can fix this

 How to Prevent EMI on a PCB:
             - Shielding
             - Short traces
             - No parallel inductors
             - Package types act as shields (PGA and BGA)
             - EMI Filtering:
                      - Near output to prevent noise
                         - Near EMI source to prevent Crosstalk




Michigan State University Team 05—ESD Gun Design. 3/24/08.        8
                        Ex: MOSFET System Arrangement
 Schematic:




 Description:
       - Gate of MOSFET driven with high frequency signal
             - Driver should be able to handle high frequency signal




Michigan State University Team 05—ESD Gun Design. 3/24/08.             9
                        Ex: MOSFET System Arrangement (cont’d)
 High Frequency Considerations:
             -Distance between driver output and gate  Time Delay
             -Distance between driver output and gate  Inductance
                     -When combined with gate capacitanceRinging!




             Gate Voltage: Fairchild BSS123, L(trace) = 200n, Z0 = 10 ohms, RL = 1k

                                   -Voltage spikes can damage transistor.
                                   -Transistor can bounce
                                   -Current travels into driver

Michigan State University Team 05—ESD Gun Design. 3/24/08.                            10
                        Ex: MOSFET System Arrangement (cont’d)
 High Frequency Considerations (cont’d):
             -Characteristic Impedance  Reflections
                     -Different voltage at gate (could be more, or less)
                         -Could damage driver

 Solutions:
       -Ringing
                         -Shorten distance between driver and gate to reduce inductance
                         -Narrow trace to increase characteristic impedance (dampen).
                         -Comes at the cost of increased power dissipation.




            Gate Voltage: Fairchild BSS123, L = 20n, Z0 = 50 ohms, RL = 1k.
Michigan State University Team 05—ESD Gun Design. 3/24/08.                                11
                        Ex: MOSFET System Arrangement (cont’d)
 Solutions (cont’d):
       -Impedance mismatches
                         -Adjust trace width to match selected impedance
                         -Compromise with ringing.


 Other Notes:
             -Same problems exist between logic signal generator and driver IC.
             -The solutions can be applied to many similar problems, but as
             proximity increases:
                     -Capacitance increases
                         -Crosstalk increases.




Michigan State University Team 05—ESD Gun Design. 3/24/08.                        12
                        Ex. Clock Signal Traces
 Description:
             -Two clock signal traces travelling parallel to each other, the two clock
             signals are 180 degrees out of phase:




 High Frequency Considerations:
             - The radiated fields from the two lines will nearly cancel each other.
                         -decreases EMI throughout board.




Michigan State University Team 05—ESD Gun Design. 3/24/08.                             13
                        Ex. Clock Signal Traces (cont’d)
 Images:




         Left: Two current sources of equal magnitude placed near each other, 180 degrees
         out of phase. Right: Two current sources placed relatively further away from each
         other, 90 degrees out of phase.




Michigan State University Team 05—ESD Gun Design. 3/24/08.                                   14
                        Ex. Trace Corners
 Description:




 High Frequency Considerations:
             - Remember that the impedance of the trace depends on its width
             - The width of a trace at its corner is greater than it is elsewhere
             - This causes reflections at the corner of the trace
             - EMI is also prevalent near these reflection centers


 Solution:
             - Round the corners (constant width, less reflection)
             - CAD programs can help you

Michigan State University Team 05—ESD Gun Design. 3/24/08.                          15
                        Summary
 Some High Frequency PCB Effects, & Solutions:
             -Increased trace length greater inductance waveform deformation
                         -Solution: shorter traces
             -Increased trace density capacitance waveform deformation
                         -Solution: reduced board density
             -Increased board density crosstalk false triggering, noise
                         -Solution: reduced board density, orthogonal positioning, phase
                         cancellation (clock signals), filtering
             -Characteristic Impedance reflections circuit damage or
             attenuation
                         -Solution: impedance matching



 Remember: At high frequencies, board layout is governed more by
 electromagnetic interactions, and less by electronic circuit theory. MOSFET
 arrangement, trace corners, and clock signal positioning are just a few
 applications of this idea.
Michigan State University Team 05—ESD Gun Design. 3/24/08.                                 16
                        References
 Texas Instruments. “PCB Layout Guidelines for Power Controllers.”
          http://focus.ti.com/lit/ml/slua366/slua366.pdf

 Baker, Bonnie. Microchip Technology Inc. “Circuit Layout Techniques And Tips.”
          http://www.analogzone.com/acqt0729.pdf

 Olney, Barry. “EMC Design for High Speed PCBs.”
          http://www.icd.com.au/articles/emc.html

 Pawson, James. “Leakage Inductance.”
         http://thedatastream.4hv.org/gdt_leakage.htm

 Inan; Inan. Engineering Electromagnetics. 1998.

 Paul Falstad’s Home Page. http://www.falstad.com/index.html




Michigan State University Team 05—ESD Gun Design. 3/24/08.                        17