ST AN2487 Application note

AN2487

Application note

STEVAL-TSP001V1 power over Ethernet powered device demonstration board

Introduction

This application note describes the STEVAL-TSP001V1 demonstration board for the evaluation of a power over Ethernet (PoE) system used to transmit electrical power, along with data, to remote devices over a standard twisted-pair cable in an Ethernet network.

A complete power solution for Ethernet-connected powered devices is presented. The power supply fully complies with IEEE 802.3-2005 PoE specifications and delivers the rated output from any compliant source.

Example outputs of 3.3 V and 12 V are given in this document, but other requirements can easily be met by implementing small changes.

Key features

■Low profile, small size: 5"w x 1.5"d x 0.65"h (0.5" except RJ-45 Ethernet connectors)

■Complies with all IEEE 802.3-2005 (IEEE 802.3af) Power over Ethernet specifications

■Respects source limitations

■Does not pollute power source

■1500 V isolated outputs eliminate ground loops

■Highest possible economical output power (total power at output approximately 10 W)

■Useful output voltages

–12 V @0.65 A output (7.8 W, loose tolerance)

–3.3 V @0.65 A output (2.145 W, tight tolerance)

■Increased cost-effectiveness due to SMT construction (through-hole connectors for ruggedness)

August 2008

Rev 2

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www.st.com

Contents	AN2487

Contents

1	Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
	1.1	Limitations on available power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
2	Powered device demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	2.1	Design choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
3	Circuit descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
	3.1	Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10

3.1.1 Power supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1.2 Transient protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1.3 Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 STHS4257 powered device interface controller . . . . . . . . . . . . . . . . . . . . 11

3.2.1 Sequence of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.2 POE Signature resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.3 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.4 Startup sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.5 Maintain power signature (MPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.3 Power converter operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3.1 Input filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.3.2 Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.3.3 Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.3.4 Critical conduction operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.3.5 Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.3.6 Miscellaneous component functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.4 Transformer details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4	Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
	4.1	Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
	4.2	STHS4257 powered device interface controller . . . . . . . . . . . . . . . . . . . .	17

4.2.1 Signature current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.2.2 Classification current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.2.3 Undervoltage lockout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.3 Powered device supply solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3.1

Current draw at low input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19

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AN2487	Contents

4.3.2 Power Good signal and capacitor charging current . . . . . . . . . . . . . . . . 19 4.3.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.4 Startup loading on the Power Sourcing Equipment . . . . . . . . . . . . . . . . . 19

4.4.1 Maintain power signature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.5 STHS4257A1 voltage drop under load . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.6 Input current limit vs input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.7 Characteristics of entire system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.7.1 Output vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.7.2 Cross regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.7.3 Startup and shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.7.4 Transient response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.7.5 Overload characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.7.6 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.8 Ripple and noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.8.1 Radiated EMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Appendix A Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Appendix B Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Appendix C Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

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List of figures	AN2487

List of figures

Figure 1. Block diagram of demonstration board PSAL06-17 rev 2. . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 2. Front of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 3. Rear of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 4. Bottom of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 5. Test and ethernet source connection circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. Power converter circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 7. Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 8. Signature resistance slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 9. 42 V input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 10. 57 V input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 11. Full-Load startup input current with cable resistance - 44 V source, 37 V at PD . . . . . . . . 20 Figure 12. Full-Load startup input current with cable resistance - 57 V source, input . . . . . . . . . . . . . 20 Figure 13. No load current - DC maintain power signature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 14. Cross regulation, 42 V In, 12 V at half load, 3.3 V load varied . . . . . . . . . . . . . . . . . . . . . . 22 Figure 15. Cross regulation, 42 V In, 3.3 V at half load, 12 V load varied . . . . . . . . . . . . . . . . . . . . . . 22 Figure 16. Cross regulation, 50 V In, 12 V at half load, 3.3 V load varied . . . . . . . . . . . . . . . . . . . . . . 22 Figure 17. Cross regulation, 50 V In, 3.3 V at half load, 12 V load varied . . . . . . . . . . . . . . . . . . . . . . 22 Figure 18. Cross regulation, 57 V In, 12 V at half load, 3.3 V load varied . . . . . . . . . . . . . . . . . . . . . . 22 Figure 19. Cross regulation, 57 V In, 3.3 V at half load, 12 V load varied . . . . . . . . . . . . . . . . . . . . . . 22 Figure 20. 42 V Full load startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 21. 57 V Full load startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 22. 57 V Shutdown full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 23. 3.3 V Half-full transient, 12 V at half load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 24. 12 V Half-full transient, 3.3 V at half load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 25. Switching frequency Ripple, two cycles shown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 26. Expanded switching transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 27. Spectrum of noise between output and input ground planes . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 28. Component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 29. Top copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 30. Bottom copper (mirrored) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 31. Bottom of board (mirrored) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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AN2487	Overview

1 Overview

The IEEE 802.3af specification is intended to greatly simplify the wiring to remote equipment by providing power over the same medium that carries the signal to/from the equipment. Applications that particularly benefit from this concept are security cameras, wireless access points, and internet telephones. Power wiring, with its associated conduit installation and licensed electrician fees, is eliminated. The wiring voltage is kept to a safe level, less than 60 volts DC. This voltage is not even applied to a connector until the downstream equipment is determined to be connected and asking for power. Lower voltage signaling is used to determine the need for power.

The cable voltage was selected so that existing 48 V telecom equipment could be used to supply the power without requiring DC-DC conversion. Careful consideration was given to existing communications wiring. Category 5 and 6 wiring and connectors can be used unmodified to carry power, if the current is limited to values that cannot cause overheating.

Category 5 and 6 cables contain extra wire pairs that are unused in the present Ethernet structure. These can be used to carry power (PSE(a) Pinout Alternative B), or the signalcarrying twisted pairs themselves can carry power if center-tapped transformers are used to inject and recover DC current (PSE Pinout Alternative A).

For further safety, the powered device (PD) must present a “maintain power” signature. If the load is disconnected, the source cuts off power to the cable.

Isolation is required between the power source and the PD ground, if it exists. In some applications this is not an issue, such as in plastic-cased standalone Ethernet devices without external connections.

If external connections are required, the powered device's accessible metal must have a local ground to avoid hazards caused by lightning-induced ground jumps.

1.1Limitations on available power

Voltage must be kept below 60 V to meet IEC60950 and other specifications' safety limits. The standard telecom battery voltage range is 44 V to 57 V.

Steady-state current must be kept under 350 mA to prevent excessive temperature rise in the wire and connectors. (Higher current, 400 mA for 50ms max, can be used for startup.)

The 802.3af specification permits only one power scheme per channel, either unused pair or simplex leads. Both schemes cannot be used together to increase available power. Consequently, at low input voltage, a minimum power of only 15.4 W is available for any load (350 mA at 44 V).

The specification also requires operation with 20 ohms of cable and connector resistance, further limiting the minimum power available at the powered device input to 12.94 W.

The specification also requires that power of either polarity be accepted. A bridge rectifier with its forward drop loss further reduces input power. At 350 mA, a 1N4004 diode drops 0.7 volts. Loss in a bridge built of parts similar to these is 0.49 W, leaving only 12.45 W at the PD power supply input. At about 82% efficiency, the power converter can deliver no more than approximately 10 W.

a. PSE: power sourcing equipment

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Powered device demonstration board	AN2487

2 Powered device demonstration board

2.1Design choices

The following points are considered as requirements for powered device supplies:

Input

Three inputs are provided; one for each of the two PoE powering schemes, and one additional input for lab test and demo purposes.

Output

There are two outputs which cover most applications.

1.Low voltage DC, tightly regulated and relatively ripple-free, for logic circuitry.

2.Relatively high voltage DC for relays, actuators, motors, lighting, etc. High precision is typically not required, nor is low ripple.

Selected voltages and currents:

●3.3 V at 0.65 A, ripple less than 50 mV P-P

●12 V at 0.65 A, ripple in the range of 250 mV P-P

The 12 V output is cross-regulated from the 3.3 V output. It will maintain ±10 to ±15% regulation.

1500 V isolation of the output is included because it is usually required and is easy to eliminate for additional small cost savings. Where appropriate, the demo board uses 2 mm clearance between the Ethernet input and circuitry that may need to be grounded locally.

Power over Ethernet interface

The ST STHS4257 power over Ethernet interface controller was chosen for the signature, classification, source connect, and converter start functions. This chip combines these functions in a small SO-8 package.

DC-DC Converter

The converter accepts, at a low cost, a wide range of input voltage and load currents.

Topology, operating mode, and control scheme

The converter uses the flyback topology. It is the simplest, lowest cost solution at the 10 W power level. It also supports multiple cross-regulated outputs at minimum possible cost.

Transition mode is used to maximize transformer utilization and minimize electro-magnetic interference (EMI). The converter starts each transformer charge cycle (transistor on) immediately after the output diodes shut off. There are no stability problems over 50% duty because there is no energy left in the transformer at the end of each cycle.

The converter uses peak current-mode control, in which the error between output and setpoint controls the peak current in the transformer primary.

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AN2487	Powered device demonstration board

Enable

It was decided to use the Power Good signal from the STHS4257 to start the converter. This ensures that the converter's input capacitor is fully charged well before the converter is allowed to start. If the converter was started from its own input voltage, there would be a coordination problem to be solved - the converter could start before the input capacitors were charged if conditions were incorrect. The cost is only a resistor and a PNP transistor, but the benefits include peace of mind.

Control chip

ST's L6565 quasi-resonant SMPS controller allows easy implementation of the transition mode peak current control scheme. It was designed for line voltage, but it can operate at much lower voltages with minor changes in the startup circuitry. The benefits include lower EMI and much lower turn-on losses than any other scheme.

Operating frequency

Because the L6565 operates in transition mode, it operates at variable frequency. The frequency is lowest at high output current and minimum input voltage. The converter is designed to operate at about 30 kHz under full load at 36 V input. The transformer is designed to go into saturation at a slightly lower frequency, and run at a higher frequency with a higher input voltage and lighter loads.

If a fixed operating frequency is required, this is NOT the recommended method. In some applications, the power supply must be synchronized with other circuitry so that its noise spikes occur between signal sampling times. In this case, another controller should be used.

Noise generated by the transition mode scheme is quite low, however. It is more than sufficient for the applications best suited for PoE technology.

Physical/mechanical

It was decided early on to keep the converter as small as possible consistent with the power requirements. SMT was a requirement, as was low profile

Early experiments with small surface-mount power transformers in the 10 W range required very high operating frequencies, in the 70 to 100 kHz range. The converters worked very well, but it was very difficult to keep the 2 mm clearance required for the 1500 V isolation requirement. Safety considerations made it necessary to include two unused pins between the cable-connected side and the load side of the transformer. The smaller SMT bobbins available would not support the wire required for 10 W of output. The smallest 12-pin SMT bobbin available was finally chosen, and the transformer core that fit into it became the core of choice.

The transformer height became the limiting height for components. Parts selection then proceeded.

The final height was set at ½”, including the 1/16” thickness of the PC board, but not including the RJ-45 connectors or their leads, or the terminal strips. These serve demonstration functions only.

The supply dimensions (with the above exclusions) give a volume of 4" x 1.5" x 0.5", or 3 in3. At 10 W output, power density is a respectable 3.3 W per in3, including the PoE functions.

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8/32															Powered
8		16			1		1							diagram Block .1 Figure	board demonstration device
		15			2
7							2
		14			3
6							3
		13			4
5							4
		12			5
4							5
		11			6
3							6
		10			7
2							7
		9			8
1							8
ETHERNET					ETHERNET		ISOLATED
TO POWER					ISOLATION		ETHERNET							of
							TOOTHER		\
SOURCING					TRANSFORMER					1	FEEDBACK
EQUIPMENT	UNUSED						EQUIPMENT		4					demonstration
		SIMPLEX TAP									TL431
	PAIR								3	2
	POWER	POWER
		4	BR1		1			Power Pos
			AC	V+									TB2
											RECTIFIER	1	+12v
		3	AC	V-	2									board
						TRANSIENT	POE-PD		POWER		STPS2L40U	2	+3.3v
											STPS3150U
						CLAMP	I/FChip		CONVERTER
			BR2									3		PSAL06
													OUTGND
		4			1	SMAJ58A	STHS4257	Power Good	L6565DControl
			AC	V+
									STD5N20T4 FET
													ISOLATED
									SMAJ188
		3	AC	V-	2		Signature						POWER
								Power Neg					OUTPUT	17-
TB1		4	BR3		1			Classification						rev
1			AC	V+				IFNEEDED						2
	TEST
2	POWER	3	AC	V-	2
AUXILIARY
POWERINPUT															AN2487

AN2487	Powered device demonstration board

Figure 2. Front of demonstration board

Figure 3. Rear of demonstration board

Figure 4. Bottom of demonstration board

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Circuit descriptions	AN2487

3 Circuit descriptions

Figure 5. Test and Ethernet source connection circuitry

			T2
	J1	16		1		J2
		15		2
OVERHANG BOARD EDGE0.062						OVERHANG BOARD0.062EDGE
	1	14		3	1
	2	13		4	2
	3	12		5	3
	4	11		6	4
	5	10		7	5
	6	9		8	6
	7				7
	8		POE Signal XFMR		8
	RJ-45
						RJ-45
TO POWER SOURCING EQUIPMENT					UNPOWERED, ISOLATED, SIMPLEX LOOPBACK
			3	D1
				DF02M
			4		1						48V+
											48V+
									TO POWER CONVERTER, SHEET 2 OF 2
			2			K					/PG
											/PG
							D3			1
							SMAJ58A				R2
						A
			3	D2							100K
											1/8W
				DF02M						2
								8	U1
								V+	/PG	6
			4		1			2
								CLASS		5	48V-SW
								-RTN			48V-SW
								V
						C1	1	4	STHS4257A
						0.1uF		R1
			2			100V		DNI
								1/8W
							2	1%
			3	D4
				DF02M
	J3
	1		4		1
	2
	Phoenix 2 pin
			2

3.1Inputs

3.1.1Power supplies

There are three methods of powering the demonstration board:

1.Unused pairs (J1)

The unused twisted pairs in the Ethernet cable can carry power to RJ-45 pins 4 and 5, and pins 7 and 8.

2.Simplex leads (J1)

The transformer windings are center tapped, and power is carried over the same pairs as the signal leads. The DC power current balances out in the transformer - saturation of the core does not occur, and the signals can pass. Power is drawn off the centertaps (simplex leads) and signals are coupled to another winding.

3.Auxiliary DC input (TB1)

For convenience, this input method is provided for test and evaluation. Each input is sent through its own bridge rectifier so that it is polarity insensitive.

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