Texas Instruments TPS23755 User Manual

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User's Guide
PoE Powered Devices Debug Guidelines
This document provides the debug process for TI Power over Ethernet (PoE) Powered Device (PD) designs.
Most problems occur in the DC/DC design. PoE uses the IEEE802.3 standard that includes IC vendors
and end-equipment designs, resulting in strict requirements for PoE operation. However, there are no such
requirements on DC/DC design. Unfortunately, the complex nature of DC/DC design leaves less margin of error
than one might expect, yet many degrees of freedom in design choices. This combination allows errors to slip in
undetected that can result in designs that do not work or cause damage.
1 Introduction.............................................................................................................................................................................2
2 Preparation and Measurement
2.1 Important PoE Signals.......................................................................................................................................................
2.2 Lab Equipment...................................................................................................................................................................
2.3 Measurement Techniques..................................................................................................................................................
2.4 Board Preparation..............................................................................................................................................................
3 Narrowing Down the Problem
3.1 Schematic Areas................................................................................................................................................................
3.2 Narrowing Down the Area On
4 Common Issues....................................................................................................................................................................
5 Conclusion............................................................................................................................................................................
Figure 2-1. PoE Start-up..............................................................................................................................................................3
Figure 2-2. Switching Waveform..................................................................................................................................................4
Figure 3-1. PoE Input Section......................................................................................................................................................5
Figure 3-2. PoE IC Settings.........................................................................................................................................................
Figure 3-3. Input Filter.................................................................................................................................................................
Figure 3-4. DC/DC Pin Settings...................................................................................................................................................7
Figure 3-5. VCC Input Circuit for Active Clamp Forward.............................................................................................................
Figure 3-6. VCC Input Circuit for Primary-Side Regulation Flyback............................................................................................
Figure 3-7. VCC Input Circuit for Optocoupler Flyback...............................................................................................................
Figure 3-8. Primary Side Feedback PSR Flyback.......................................................................................................................
Figure 3-9. Primary Side Feedback Optocoupler Flyback...........................................................................................................
Figure 3-10. Primary Side MOSFETs and Accompanying Components
Figure 3-11. Primary Side MOSFET RCD Snubber...................................................................................................................
Figure 3-12. Secondary Side Diode Rectified Flyback..............................................................................................................
Figure 3-13. Secondary Side Self Driven Synchronous Flyback
Figure 3-14. Secondary Side Self Driven Synchronous Flyback
Figure 3-15. Secondary Side Driven Synchronous Flyback......................................................................................................
Figure 3-16. Secondary Side Active Clamp
Figure 3-17. Output Capacitance...............................................................................................................................................14
Figure 3-18. Secondary Side Optocoupler Feedback Network.................................................................................................
Figure 3-19. PoE Handshake....................................................................................................................................................
Figure 3-20. PoE DC/DC Start-up.............................................................................................................................................
Figure 3-21. Current Sense Switching
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SLVAF74 - JULY 2021
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ABSTRACT

Table of Contents

Techniques..........................................................................................................................2
Area.......................................................................................................................................5
Board................................................................................................................................15
List of Figures
Forward.................................................................................................................14
Waveform......................................................................................................................17
Copyright © 2021 Texas Instruments Incorporated
(Flyback)....................................................................10
A............................................................................................12
B............................................................................................13
PoE Powered Devices Debug Guidelines
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