ST AN4006 Application note

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.

February 2012

Doc ID 022454 Rev 1

1/19

www.st.com

Contents	AN4006

Contents

1	High-power PoE converter electrical specifications . . . . . . . . . . . . . .			. 4
2	High-power converter schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			. 5
3	Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			7
4	Test results		. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
	4.1	Efficiency measurements with synchronous rectification . . . . . . . . . . . . .		10
	4.2	Converter waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		12
		4.2.1	Startup sequence using PowerDsine 9501G injector . . . . . . . . . . . . . .	12
		4.2.2	Primary-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
		4.2.3	Secondary-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
		4.2.4	Output ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
		4.2.5	Gloop measurement and load transient response . . . . . . . . . . . . . . . . .	17
5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			18

2/19	Doc ID 022454 Rev 1

AN4006	List of figures

List of figures

Figure 1. High-power converter schematic: detail of the input section including data transformers, bridges, protection and optional CM choke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 2. High-power converter: detail of the PoE converter based on active-clamp forward topology with self-driven synchronous rectification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. Efficiency measurements at 48 V input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 4. Efficiency of the different circuits on the converter input stage. . . . . . . . . . . . . . . . . . . . . . 10 Figure 5. Booster current characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 6. Booster power dissipation vs. input current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 7. Startup with 0 A load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. Startup with 10 A load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 9. Primary-side power MOSFET waveforms at 0 A load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 10. Primary-side power MOSFET waveforms at 16 A load . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 11. Secondary-side power MOSFET waveforms at 0 A load . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 12. Secondary-side power MOSFET waveforms at 16 A load . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 13. Output ripple measurement at 0 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 14. Output ripple measurement at 16 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 15. Output ripple measurement at 16 A with infinite persistance . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 16. Control loop of the converter at 48 V input and 18 A output. . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 17. Response of the converter to a 8 A - 16 A load transient . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Doc ID 022454 Rev 1

3/19

High-power PoE converter electrical specifications							AN4006
1	High-power PoE converter electrical specifications
	Table 1.	Specifications for 3.3 V output
		Parameter	Description	Min	Typ	Max		Unit
	Input voltage range		applied at J3 connector	0		57		V
	Operative input voltage			42		57		V
		UVLO	Vin rising edge			36		V
			Vin falling edge	30				V
	Auxiliary input voltage range			42	48	54		V
	Output voltage (Vout)		Vin= 42 V to 57 V, Iout 0 to Imax	3.25	3.35	3.45		V
	Output current (Iout)		Vin= 42 V to 57 V	0		18		A
	Peak-to-peak output ripple		48Vin, Iout=Imax		50	70		mVpp
	Efficiency DC-DC only		Vin=48 V, Iout=Imax		91			%
	Overall efficiency		Vin=48 V, Iout=Imax		88			%
	Switching frequency				220			kHz

4/19	Doc ID 022454 Rev 1

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forward clamp-active on	AN4006