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
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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)
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August 2008
Rev 2
1/32
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
3.1.2
3.1.3
Power supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Transient protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2
STHS4257 powered device interface controller . . . . . . . . . . . . . . . . . . . . 11
3.2.1
3.2.2
3.2.3
3.2.4
3.2.5
Sequence of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
POE Signature resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Startup sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Maintain power signature (MPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3
Power converter operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.1
3.3.2
3.3.3
3.3.4
3.3.5
3.3.6
Input filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Critical conduction operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Miscellaneous component functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4
Transformer details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1
4.2
Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
STHS4257 powered device interface controller . . . . . . . . . . . . . . . . . . . . 17
4.2.1
4.2.2
4.2.3
Signature current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Classification current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Undervoltage lockout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3
Powered device supply solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.3.1
Current draw at low input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2/32
AN2487
4.3.2
4.3.3
Contents
Power Good signal and capacitor charging current . . . . . . . . . . . . . . . . 19
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.4
4.5
4.6
4.7
Startup loading on the Power Sourcing Equipment . . . . . . . . . . . . . . . . . 19
4.4.1
Maintain power signature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
STHS4257A1 voltage drop under load . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Input current limit vs input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Characteristics of entire system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.7.1
4.7.2
4.7.3
4.7.4
4.7.5
4.7.6
Output vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Cross regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Startup and shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Transient response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Overload characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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
3/32
List of figures
AN2487
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Block diagram of demonstration board PSAL06-17 rev 2. . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Front of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Rear of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Bottom of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Test and ethernet source connection circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Power converter circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Signature resistance slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
42 V input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
57 V input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Full-Load startup input current with cable resistance - 44 V source, 37 V at PD . . . . . . . . 20
Full-Load startup input current with cable resistance - 57 V source, input . . . . . . . . . . . . . 20
No load current - DC maintain power signature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Cross regulation, 42 V In, 12 V at half load, 3.3 V load varied . . . . . . . . . . . . . . . . . . . . . . 22
Cross regulation, 42 V In, 3.3 V at half load, 12 V load varied . . . . . . . . . . . . . . . . . . . . . . 22
Cross regulation, 50 V In, 12 V at half load, 3.3 V load varied . . . . . . . . . . . . . . . . . . . . . . 22
Cross regulation, 50 V In, 3.3 V at half load, 12 V load varied . . . . . . . . . . . . . . . . . . . . . . 22
Cross regulation, 57 V In, 12 V at half load, 3.3 V load varied . . . . . . . . . . . . . . . . . . . . . . 22
Cross regulation, 57 V In, 3.3 V at half load, 12 V load varied . . . . . . . . . . . . . . . . . . . . . . 22
42 V Full load startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
57 V Full load startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
57 V Shutdown full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.3 V Half-full transient, 12 V at half load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
12 V Half-full transient, 3.3 V at half load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Switching frequency Ripple, two cycles shown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Expanded switching transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Spectrum of noise between output and input ground planes . . . . . . . . . . . . . . . . . . . . . . . 25
Component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Top copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Bottom copper (mirrored) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Bottom of board (mirrored) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4/32
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 signal-
carrying 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.
1.1
Limitations 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
5/32
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