AN4006 STMICROELECTRONICS | Alldatasheet

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Technical content

Datasheet sections

  • 1 High-power PoE converter electrical specific ations
  • 2 High-power converter schematic
  • 3 Bill of material
  • 4 Test results
  • 4.1 Efficiency measurements wi th synchronous rectification
  • 4.2 Converter waveforms
  • 4.2.1 Startup sequence using PowerDsine 9501G injector
  • 4.2.2 Primary-side MOSFET
  • 4.2.3 Secondary-side MOSFET
  • 4.2.4 Output ripple
  • 5 Revision history

February 2012 Doc ID 022454 Rev 1 1/19 AN4006 Application note Designing a high-efficiency (60 W on 4 pairs) PoE converter using the PM8803 and an external current booster Introduction Power over Ethernet (PoE) applications are covered by the IEEE 802.3 working group with specifications released in 2003 (IEEE 802.3af) and in 2009 (IEEE 802.3at). Power at the input of the powered device (PD) increased from 12.95 W (of the .af standard) to 25.5 W (made available by the .at standard). In both cases the power delivery was based on the “2-pair” system, where 4 wires of the Ethernet cable are used (Tx, Rx pairs or spare pairs). Applications requiring more power are constantly emerging and some solutions are already on the market even though there is no standard fully supporting these applications yet. Some of the alternatives are based on a 4-pair delivery system that allows doubling the power delivered along the Ethernet cable with respect to a 2-pair system. This document focuses on a reference design for a high-efficiency, high-power PD (up to 60 W input) power converter based on an active-clamp forward topology with self-driven synchronous rectification using the PM8803 as the main controller. The total power is delivered on the 4 pairs of a single Ethernet cable by a high-power injector. The PM8803 is a highly integrated device embedding an IEEE 802.3at compliant powered device (PD) interfaced with a PWM controller and support for auxiliary sources. To manage the higher input current (up to 1.4 A) of high-power applications, a simple current booster is introduced in parallel to the PM8803 internal hot-swap MOSFET. The proposed converter prototype is built from the PM8803 demonstration board, but several component changes have been introduced in order to manage the higher current on the input/output section of the converter. Schematics of the PoE converter are given in Section 2 while the bill of material is detailed in Section 3. In Section 4 efficiency measurements together with main waveforms of the PoE interface and power converter are shown.

1 High-power PoE converter electrical specifications

Table 1. Specifications for 3.3 V output

2 High-power converter schematic

Figure 1. High-power converter schematic: de tail of the input section including data

Figure 2. High-power converter: detail of the PoE converter based on active-clamp forward

2 K2 K

3 Bill of material

Table 2. Bill of material

4 A 805 TDK

Table 2. Bill of material (continued)

21 Chip resistor 100 k Ω 603 Std

4 Test results

4.1 Efficiency measurements wi th synchronous rectification

Figure 3. Efficiency measurements at 48 V input The difference between dc-dc and overall measurements is about 3-4% from 10 A to 18 A. Figure 4. Efficiency of the different circuits on the converter input stage

4.2 Converter waveforms

4.2.1 Startup sequence using Po werDsine 9501G injector

Figure 7. Startup with 0 A load Figure 8. Startup with 10 A load pairs) is minimum even at high load: see pink and blue traces. For details on the injector please visit www.microsemi.com.

4.2.2 Primary-side MOSFET

Figure 9. Primary-side power MOSFET waveforms at 0 A load Figure 10. Primary-side power MOSFET waveforms at 16 A load

4.2.3 Secondary-side MOSFET

Figure 11. Secondary-side power MOSFET waveforms at 0 A load Figure 12. Secondary-side power MOSFET waveforms at 16 A load

4.2.4 Output ripple

Figure 13. Output ripple measurement at 0 A Figure 14. Output ripple measurement at 16 A

Figure 15. Output ripple measurement at 16 A with infinite persistance

4.2.5 G loop measurement and load transient response

Figure 16. Control loop of the converter at 48 V input and 18 A output Figure 17. Response of the converter to a 8 A - 16 A load transient

5 Revision history

Table 3. Document revision history