Download Homework 6

ECE 477
Digital Systems Senior Design Project
Rev 8/09
Homework 6: Printed Circuit Board Layout Design Narrative
Team Code Name: ______________Defender____________ Group No. ___6___
Team Member Completing This Homework: _________Stephen Wolf_______________
E-mail Address of Team Member: ___sjwolf____ @ purdue.edu
Evaluation:
SCORE
DESCRIPTION
Excellent – among the best papers submitted for this assignment. Very few
corrections needed for version submitted in Final Report.
Very good – all requirements aptly met. Minor additions/corrections needed for
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version submitted in Final Report.
Good – all requirements considered and addressed. Several noteworthy
8
additions/corrections needed for version submitted in Final Report.
Average – all requirements basically met, but some revisions in content should
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be made for the version submitted in the Final Report.
Marginal – all requirements met at a nominal level. Significant revisions in
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content should be made for the version submitted in the Final Report.
Below the passing threshold – major revisions required to meet report
*
requirements at a nominal level. Revise and resubmit.
* Resubmissions are due within one week of the date of return, and will be awarded a score of
“6” provided all report requirements have been met at a nominal level.
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Comments:
ECE 477
Digital Systems Senior Design Project
Rev 8/09
1.0 Introduction
Defender is a coilgun based turret defense system. There are two PCBs that comprise the
surface mount components of Defender: a high voltage power board and a separate board for all
communication and control. The high voltage board comprises of a rectifier circuit for charging a
capacitor bank to 400V, a safety discharge circuit for this bank, and the ability to discharge this
bank through the coilgun. Also present on this board are another transformer and two other
rectifiers for the low voltage systems on the control board. The main control board talks to the
power board through an optically isolated I2C bus. On the main board there are two power
supplies: one at 3.3V for the microcontroller and all other components and one at 12V for the
atom board and motors. The microcontroller interfaces through an I2C bus to a variety of
components, such as the motor controller, a keypad encoder, and the I2C port expander on the
power board. A USB connection to the atom board is also present.
2.0 PCB Layout Design Considerations - Overall
The current size of our boards is 170mm x 85mm and 100mm x 75mm which comes out
to 34 in2 of area, slightly larger than half of the maximum 60. The size of these boards leaves
more than enough room for component placement and signal routing as shown in Appendix B.
The only limitations with large parts are the power supply components, such as large filter
capacitors and inductors. On the communication and control board most signal traces will be 12
mils in width following the recommendations from the lecture. The exception to this is the motor
to connector traces which will be handling higher currents and voltages (.4A at 12V) as well as
some of the power supply traces as discussed in section 4.0. The trace width on the power board
is not as critical as the trace spacing to avoid discharge of voltage along the board. According to
table 6-1 in IPC2221[1] the spacing for a coated board such as ours needs to be at least 1.5mm (60
mils) for the 500V peak AC voltage. Due to the high pulsed currents of up to 400A through the
coilgun no part of that circuit will be on the board and all parts will be chassis mounted.
Signal routing is simplified greatly by the presence of the I2C bus. All peripherals except
for the external phototransistors are connected to this bus either directly or through the use of an
I2C 16 pin port expander. These port expanders are positioned closely to the part they are
controlling for short traces and simple routing. The phototransistors will have direct routing to
the microcontroller pins through an external header and some ESD protection circuitry.
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Digital Systems Senior Design Project
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There are a few manufacturing concerns for soldering a few parts that need to be addressed.
Several parts, such as the motor controller and power supplies, have thermal pads that will be
soldered using vias underneath the pad. The keypad encoder is an extremely small QFN package
that could be difficult to solder and could very easily be done incorrectly with no easy way to
remove the part. To solve this there will actually be two spots for the keypad to encoder to be
soldered to on the chance that pads are destroyed. Thermal management of some parts is an issue
as well. Parts which will be generating large amounts of heat will have large (>.5 in2) copper
pours to act as heatsinks. Where possible these heatsinks will be linked directly to the ground
plane, as in the case of the motor controller[2]. These heatsinks will be placed as far away from
other components as the design allows to avoid thermal effects on other components.
EMI on the power board is a large concern with the voltage and switching levels present.
The components on the power board such as the ADC’s will have input filters associated with
their input resistors to reduce high frequency noise. The I/O port expander on this board is for
digital switching of MOSFETs only and will not be easily affected by EMI. The filter capacitors
for the rectifier networks are placed on the communication boards separate from the power
boards to also reduce this EMI. The 45° trace rule will be observed to reduce reflections and
radiated noise as much as possible.
3.0 PCB Layout Design Considerations - Microcontroller
The microcontroller, a PIC24FJ64GB004, is being used primarily as a communication router
and translator in our design. It will take signals in from a USB line and send them out to the
correct I2C bus. The I2C bus requires external pull-up resistors and the USB bus also require
some special layout measures; these are described in the datasheet[3]. None of the more
complicated functions of the microcontroller are being utilized, such as an external oscillator,
and we do not need to take any particularly detailed precautions on the PCB layout around the
microcontroller due to this fact. All bypass capacitors will be located very close to the multiple
Vdd/Vss pin pairs of the micro according to the datasheet[3]. A special header for In-Circuit
Debugging (ICD) will be routed to allow for easier programming and debugging. Any micro pin
that is routed to an outside header such as those for ICD or those associated with external input
will have a small (<470Ω) resistor to reduce the chances of ESD damage. Care will have to be
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ECE 477
Digital Systems Senior Design Project
Rev 8/09
taken with the circuit associated with MCLR input. This input needs to have a filter and switch
associated with it as well as clear from any noise sources as to avoid spurious resets.
4.0 PCB Layout Design Considerations - Power Supply
There are three power supplies in Defender: a high voltage AC charging system for the
capacitor bank, a 3.3V rail for logic, and a 12V rail for the motors and the Atom board. The
communication and control board will utilize a ground plane for all digital and control logic. The
power board will not have a ground plane since most of the signals that are grounded are high
voltage AC signals and separating the grounds from the digital logic will reduce noise. The 3.3V
regulator is the simplest to route traces.
The 3.3V rail is a LDO regulator from National Semiconductor, the LP3872EMP-3.3[4].
This chip has no special considerations other than closely placing the input filter capacitor from
the rectifier and the output tank capacitor close to the chip. The 3.3V rail will be powering
mostly digital components and will be connected to the ground plane to reduce noise. The few
analog components, such as voltage references and ADCs, will have a separate power rail from
the digital components. This chip does require a large copper area beneath the ground tab for
heatsinking.
The 12V supply is a switching power supply also from National Semiconductor, the
LM2677S-12[5]. This switching power supply has a more complicated layout pattern than the
LDO regulator with more requirements on which components need to be closest. These
guidelines are detailed out in National Semiconductor AN-1229[6]. These guidelines can be best
summarized with having traces that carry switching currents be as small as possible to reduce
inductance and therefore voltage spikes. The placement of the inductor, catch diode, and large
input capacitor are the most critical parts and will be placed first. The 12V rail powers two main
components: the Atom board and the motor controller. These devices will each have their own
power rail going to a star point at the 12V input. The same ground plane as the 3.3V rail will be
utilized under the assumption that the ground plane is not a source of noise, although separate
ground rails can be utilized if this is a concern.
The high voltage power supply on the power board uses a chassis mount transformer to step
up the AC voltage to 360V rms and then rectify onboard to produce a charging voltage. The
critical parameter here is trace clearance as discussed in section 2.0 to avoid discharging across
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Digital Systems Senior Design Project
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the board. The ground rail for this supply will be isolated from the other grounds of the system.
The traces for anything carrying these high voltages will need to be as wide and short as possible
to reduce resistance and inductance. They must also be near the edges of the board to carry the
power rail to the capacitor bank.
5.0 Summary
The overall PCB structure of Defender was presented in its current two board form. Each
board has its own complications to be considered in laying out the PCB. Issues such as trace
width and clearance, component placement, thermal management, EMI reduction, bypass
capacitor placement, ground plane and power routing, and high voltage isolation are looked at.
The microcontroller routing was examined and it was found that no special considerations must
be taken other than those standard in the data sheet. The layouts required for the various power
supplies were also examined, taking into account more variables such as trace clearance, ground
plane routing, and star point construction of the power rails. The
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ECE 477
Digital Systems Senior Design Project
Rev 8/09
List of References
[1] The Institue for Interconnecting and Packaging Electronic Circuits. Generic Standard on
Printed Board Design [Online]. Available: http://www.the-bao.de/divers/ipc2221.pdf.
[Accessed: Feb 24, 2011]
[2] Texas Instruments. Dual Stepper Motor Controller/Driver [Online]. Available:
http://focus.ti.com/docs/prod/folders/print/drv8821.html. [Accessed: Feb 4, 2011
[3] Microchip. PIC24FJ64GB004 [Online]. Available:
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en536118. [Accessed:
Feb 4, 2011].
[4] National Semiconductor. LP3872 [Online]. Available:
http://www.national.com/mpf/LP/LP3872.html#Overview. [Accessed: Feb 20, 2011].
[5] National Semiconductor. LM2677S-12 [Online]. Available:
http://www.national.com/pf/LM/LM2677.html#Overview. [Accessed: Feb 20,2011].
[6] Sanjaya Maniktala. Simple Switcher PCB Layout Guidelines [Online]. Available:
http://www.national.com/an/AN/AN-1229.pdf. [Accessed: Feb 24,2011].
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Digital Systems Senior Design Project
Appendix A
Current Board Schematic
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Digital Systems Senior Design Project
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Digital Systems Senior Design Project
Appendix B
Current Board Layout
Communication and Control Board
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