U.S. patent application number 12/552858 was filed with the patent office on 2011-03-03 for ac disconnect of power over ethernet devices.
This patent application is currently assigned to Broadcom Corporation. Invention is credited to James Yu.
Application Number | 20110055598 12/552858 |
Document ID | / |
Family ID | 43626596 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110055598 |
Kind Code |
A1 |
Yu; James |
March 3, 2011 |
AC Disconnect of Power Over Ethernet Devices
Abstract
Embodiments of power sourcing equipment (PSE) utilizing AC
disconnect are provided herein. In one embodiment, a PSE is
provided that includes a DC supply configured to provide a DC
voltage over a data communications medium, a controller configured
to provide an AC disconnect signal over the data communications
medium, and a parallel inductor-capacitor (LC) circuit coupled
between the DC supply and the data communications medium. The
parallel LC circuit is configured to isolate the DC supply from the
AC disconnect signal. In another embodiment, a PSE is provided that
includes a DC supply configured to provide a DC voltage at an
output, an inductor coupled between the output of the DC supply and
a data communications medium, and a capacitor coupled between the
data communications medium and ground. The inductor and capacitor
form a series LC circuit configured to generate an AC disconnect
signal.
Inventors: |
Yu; James; (San Jose,
CA) |
Assignee: |
Broadcom Corporation
Irvine
CA
|
Family ID: |
43626596 |
Appl. No.: |
12/552858 |
Filed: |
September 2, 2009 |
Current U.S.
Class: |
713/300 |
Current CPC
Class: |
G06F 1/26 20130101; H04L
12/10 20130101; G06F 1/266 20130101 |
Class at
Publication: |
713/300 |
International
Class: |
G06F 1/26 20060101
G06F001/26 |
Claims
1. Power sourcing equipment (PSE) for providing DC power to a
powered device (PD) over a data communications medium, the PSE
comprising: a DC supply configured to provide a DC voltage over the
data communications medium; a controller configured to provide an
AC disconnect signal over the data communications medium; and a
parallel inductor-capacitor (LC) circuit coupled between the DC
supply and the data communications medium, the parallel LC circuit
configured to isolate the DC supply from the AC disconnect
signal.
2. The PSE of claim 1, wherein the parallel LC circuit has an
associated resonant frequency that is substantially equal to a
frequency of the AC disconnect signal.
3. The PSE of claim 2, wherein the frequency of the AC disconnect
signal is a fundamental frequency of the AC disconnect signal.
4. The PSE of claim 1, wherein data is transmitted concurrently
with the DC power over the data communications medium.
5. The PSE of claim 4, wherein the data is transmitted in
accordance with the IEEE 802.3 standard.
6. The PSE of claim 1, wherein the controller further comprises: a
charge pump coupled to the DC supply and configured to provided an
output voltage greater than the DC voltage provided by the DC power
supply; and a switch configured to intermittently couple the output
voltage of the charge pump to the data communications medium to
provide the AC disconnect signal.
7. Power sourcing equipment (PSE) for providing DC power to a
powered device (PD) over spare wires of a data communications
medium, the PSE comprising: a DC supply configured to provide a DC
voltage over the spare wires of the data communications medium; a
controller configured to provide an AC disconnect signal over the
spare wires of the data communications medium; and a parallel
inductor-capacitor (LC) circuit coupled between the DC supply and
the spare wires of the data communications medium, the parallel LC
circuit configured to isolate the DC supply from the AC disconnect
signal.
8. The PSE of claim 7, wherein the parallel LC circuit has an
associated resonant frequency that is substantially equal to a
frequency of the AC disconnect signal.
9. The PSE of claim 8, wherein the frequency of the AC disconnect
signal is a fundamental frequency of the AC disconnect signal.
10. The PSE of claim 7, wherein data is transmitted in accordance
with the IEEE 802.3 standard over the data communications
medium.
11. The PSE of claim 7, wherein the controller further comprises: a
charge pump coupled to the DC supply and configured to provided an
output voltage greater than the DC voltage provided by the DC power
supply; and a switch configured to intermittently couple the output
voltage of the charge pump to the spare wires of the data
communications medium to provide the AC disconnect signal.
12. Power sourcing equipment (PSE) for providing DC power to a
powered device (PD) over a data communications medium, the PSE
comprising: a DC supply configured to provide a DC voltage at an
output; an inductor coupled between the output of the DC supply and
the data communications medium; and a capacitor coupled between the
data communications medium and ground, wherein the inductor and
capacitor form a series inductor-capacitor (LC) circuit configured
to generate an AC disconnect signal.
13. The PSE of claim 12, wherein the series LC circuit is
configured to generate the AC disconnect signal if the PD is
disconnected from the data communications medium.
14. The PSE of claim 12, wherein the series inductor-capacitor (LC)
circuit forms a tank circuit.
15. The PSE of claim 12, wherein the AC disconnect signal has a
frequency substantially equal to the resonant frequency of the
series LC circuit.
16. Power sourcing equipment (PSE) for providing DC power to a
powered device (PD) over spare wires of a data communications
medium, the PSE comprising: a DC supply configured to provide a DC
voltage at an output; an inductor coupled between the output of the
DC supply and the spare wires of the data communications medium;
and a capacitor coupled between the spare wires of the data
communications medium and ground, wherein the inductor and
capacitor form a series inductor-capacitor (LC) circuit configured
to generate an AC disconnect signal.
17. The PSE of claim 16, wherein the series LC circuit is
configured to generate the AC disconnect signal if the PD is
disconnected from the data communications medium.
18. The PSE of claim 16, wherein the series inductor-capacitor (LC)
circuit forms a tank circuit.
19. The PSE of claim 16, wherein the AC disconnect signal has a
frequency substantially equal to the resonant frequency of the
series LC circuit.
Description
FIELD OF THE INVENTION
[0001] This application relates generally to Power over Ethernet
(PoE) devices and, more specifically, to apparatuses for AC
disconnect of PoE devices.
BACKGROUND
[0002] The IEEE 802.3af and 802.3at specifications, also known as
Power over Ethernet (PoE), provides a framework for delivering DC
power concurrently with data over standard Ethernet cabling. A PoE
system includes three basic components: power sourcing equipment
(PSE) for providing power, a powered device (PD) for receiving and
consuming the power, and cabling for transferring the power from
the PSE to the PD. The PSE, as defined by the IEEE 802.3af/t
specifications, performs much of the basic power provisioning
process, including detection, classification, operation, and
disconnection.
[0003] Detection is first performed by the PSE to determine if a
valid PD is connected to its power providing output. Detection is
carried out by inducing a small voltage at the output of the PSE to
detect a specific 25 K.OMEGA. signature resistor. This signature
indicates that a valid PD is connected and that the provision of
power to the PD can begin.
[0004] After a valid PD is detected, an optional classification
stage can be performed to estimate the amount of power required by
the PD. To perform classification, the PSE again induces a voltage
around 15.5-20.5 Vdc for a period of time within 10 to 75 ms. The
current consumed by the PD during this time period indicates to the
PSE its power classification.
[0005] Following detection and optional classification, the output
power of the PSE can be increased, during the operation stage, to
its full voltage capacity, which is typically around 48 Vdc. The
output voltage of the PSE is gradually increased to its full
voltage capacity to prevent high frequency noise from disrupting
data being transferred concurrently with the power.
[0006] The final stage of the power provisioning process involves
removal of power following the disconnection of the PD connected to
the PSE. The IEEE 802.3af/t specifications define two specific
techniques for power disconnection; namely, DC disconnect and AC
disconnect. Both methods provide the same desired result--the
detection of a disconnected PD and the removal of power within 300
to 400 ms thereafter. The removal of power when a PD is
disconnected is important because the PD may be replaced by a
non-PoE-ready device, which may result in damage.
[0007] DC disconnect is performed at the PSE by measuring the
current consumed by the PD. If the PD is disconnected at any point,
the consumption of current by the PD would cease, indicating
disconnection of the PD. AC disconnect, on the other hand, entails
the addition of a low AC signal on top of the 48 Vdc operating
voltage. The returned AC signal amplitude is monitored at the PSE.
While the PD is connected, the low impedance of the PD lowers the
returned AC signal. During disconnection, however, the AC signal
level will increase, indicating disconnection of the PD.
[0008] In conventional PSEs implementing AC disconnect, a diode is
used to isolate the DC source, providing the 48 Vdc operating
voltage, from the AC disconnect signal. Depending on the type of
diode utilized, the diode can have a forward voltage drop of
0.3-0.7 Vdc at 600 mA, or a total power consumption around 0.2-0.4
W, for example. Not only does the isolation diode increase overall
power consumption, but further increases the overall temperature at
the media dependent interface (MDI) of the PSE. This excess power
consumption and temperature becomes even more apparent in
multi-port hubs or switches that are PoE-ready. For example, in a
24-port hub that is PoE-ready, 24 separate diodes (one for each
port) can be required to isolate the DC supply from the AC
disconnect signal(s).
[0009] Therefore, what is needed is an apparatus for isolating a DC
supply of a PSE from an AC disconnect signal, while limiting any
additional power consumption and heat produced as a result
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
[0010] The accompanying drawings, which are incorporated herein and
form a part of the specification, illustrate the present invention
and, together with the description, further serve to explain the
principles of the invention and to enable a person skilled in the
pertinent art to make and use the invention.
[0011] FIG. 1 illustrates an exemplary PoE system in which
embodiments of the present invention can be implemented.
[0012] FIG. 2 illustrates portions of an exemplary PSE, utilizing a
conventional isolation technique.
[0013] FIG. 3 illustrates portions of an exemplary PSE, utilizing a
parallel inductor-capacitor (LC) circuit, according to embodiments
of the present invention.
[0014] FIG. 4 illustrates portions of an exemplary PSE, utilizing a
series inductor-capacitor (LC) circuit, according to embodiments of
the present invention.
[0015] The present invention will be described with reference to
the accompanying drawings. The drawing in which an element first
appears is typically indicated by the leftmost digit(s) in the
corresponding reference number.
DETAILED DESCRIPTION
[0016] In the following description, numerous specific details are
set forth in order to provide a thorough understanding of the
invention. However, it will be apparent to those skilled in the art
that the invention, including structures, systems, and methods, may
be practiced without these specific details. The description and
representation herein are the common means used by those
experienced or skilled in the art to most effectively convey the
substance of their work to others skilled in the art. In other
instances, well-known methods, procedures, components, and
circuitry have not been described in detail to avoid unnecessarily
obscuring aspects of the invention.
[0017] References in the specification to "one embodiment," "an
embodiment," "an example embodiment," etc., indicate that the
embodiment described may include a particular feature, structure,
or characteristic, but every embodiment may not necessarily include
the particular feature, structure, or characteristic. Moreover,
such phrases are not necessarily referring to the same embodiment.
Further, when a particular feature, structure, or characteristic is
described in connection with an embodiment, it is submitted that it
is within the knowledge of one skilled in the art to effect such
feature, structure, or characteristic in connection with other
embodiments whether or not explicitly described.
1. Operating Environment
[0018] FIG. 1 provides a diagram of an exemplary PoE system 100 in
which embodiments of the present invention can be implemented. PoE
system 100 includes a switch/hub 102 and a powered end station 104.
Powered end station 104 can be any one of several different network
nodes; for example, powered end station 104 can be an internet
protocol (IP) phone or a wireless access point, to name a few.
Switch/hub 102 provides power to powered end station 104 over
conductor pairs 106 and 108, which are further configured to carry
differential data between switch/hub 102 and powered end station
104.
[0019] As illustrated in FIG. 1, switch/hub 102 includes a
transceiver 110 that achieves full duplex transmit and receive
capability using a differential transmit port (TX) 112 and a
differential receive port (RX) 114. A first transformer 116 couples
high speed data between transmit port 112 and conductor pair 106.
Likewise, a second transformer 118 couples high speed data between
receive port 114 and conductor pair 108. The respective
transformers 116 and 118 pass the high speed data to and from
transceiver 110, but isolate any low frequency or DC voltage from
the transceiver ports, which may be sensitive to large magnitude
voltages.
[0020] Transformer 116 includes primary and secondary windings,
where the secondary winding (coupled to conductor pair 106)
includes a center tap 120. Transformer 118 includes primary and
secondary windings, where the secondary winding (coupled to
conductor pair 108) includes a center tap 122. Power Sourcing
Equipment (PSE) 124 generates an output voltage that is applied
across center taps 120 and 122 of transformers 116 and 118 on the
conductor pair sides of the transformers. Center tap 120 is coupled
to a first output of PSE 124, and center tap 122 is coupled to a
second output of PSE 124. As such, transformers 116 and 118 isolate
the DC voltage provided by PSE 124 from sensitive data ports 112
and 114 of transceiver 110. An example DC output voltage provided
by PSE 124 is substantially 48 volts, but other voltages could be
used depending on the voltage/power requirements of powered end
station 104.
[0021] PSE 124 includes a PSE controller (not shown) that controls
the provisioning of DC power to powered end station 104. More
specifically, the PSE controller of PSE 124 performs the basic
power provisioning process defined by the IEEE 802.3af/t
specifications, including detection, classification (optionally
performed), operation, and disconnection. In an embodiment, the PSE
controller of PSE 124 utilizes an AC disconnect technique to
discontinue the supply of DC power over conductor pairs 106 and 108
when powered end station 104 is disconnected.
[0022] Still referring to FIG. 1, the contents and functionality of
powered end station 104 will now be discussed. Powered end station
104 includes a transceiver 126 having full duplex transmit and
receive capability that is achieved using differential receive port
(RX) 128 and differential transmit port (TX) 130. A transformer 132
couples high speed data between conductor pair 106 and receive port
128. Likewise, a transformer 134 couples high speed data between
transmit port 130 and conductor pair 108. Transformers 132 and 134
pass the high speed data to and from transceiver 126, but isolate
any low frequency or DC voltage from sensitive data ports 128 and
130.
[0023] Transformer 132 includes primary and secondary windings,
where the secondary winding (coupled to conductor pair 106)
includes a center tap 136. Likewise, transformer 134 includes
primary and secondary windings, where the secondary winding
(coupled to conductor pair 108) includes a center tap 138. Center
taps 136 and 138 supply the power carried over conductor pairs 106
and 108 to a powered device (PD) 140.
[0024] PD 140 can include a PD controller (not shown) to monitor
the voltage and current provided to it. The PD controller can
further provide the necessary impedance signatures on the return
conductor 108 during detection, so that the PSE controller,
implemented within PSE 124, can recognize PD 140 as a valid
PoE-ready device.
[0025] During operation, a direct current (I.sub.DC) flows from PSE
124 through center tap 120 and divides into a first current
(I.sub.1) and a second current (I.sub.2) that are carried over
conductor pair 106. The first current (I.sub.1) and the second
current (I.sub.2) recombine at center tap 136 to reform the direct
current (I.sub.DC) used to power PD 140. On return, the direct
current (I.sub.DC) flows from PD 140 through center tap 138, and
divides for transport over conductor pair 108. The return DC
current recombines at center tap 122 and returns to PSE 124.
[0026] As discussed above, data transmission between switch/hub 102
and powered end station 104 can occur concurrently with the
provisioning of DC power by PSE 124. Data is carried differentially
over conductor pairs 106 and 108 between switch/hub 102 and powered
end station 106. Because data is carried differentially over
conductor pairs 106 and 108, the data is ideally unaffected by the
DC power transfer, which appears as common mode.
[0027] It should be noted that other alternative configurations for
PoE system 100 can be used without departing from the scope and
spirit of the present invention. For example, in an alternative
configuration of PoE system 100, DC power, supplied by PSE 124, is
transmitted over the spare wire pairs of the Ethernet cabling as
specified by the IEEE 802.3af/t Ethernet specifications.
2. Conventional Apparatus for Isolation
[0028] FIG. 2 illustrates portions of an exemplary PSE 124a,
utilizing a conventional isolation technique. PSE 124a includes a
PSE controller 200 and a DC supply 202. In an embodiment, DC supply
202 is configured to provide 48 Vdc across nodes 120 and 122, which
are coupled to PD 140 as illustrated in FIG. 1. PSE controller 200
is configured to control PSE 124a to perform the basic power
provisioning process defined by the IEEE 802.3af/t specifications,
including detection, classification (optionally performed),
operation, and disconnection.
[0029] Detection is first performed by PSE controller 200 to
determine if a valid PD is coupled to output nodes 120 and 122.
Detection is specifically carried out by producing a small voltage
across nodes 120 and 122 to detect a specific signature resistor,
such as 25 K.OMEGA.. This signature indicates that a valid PD, such
as PD 140 illustrated in FIG. 1, is coupled to PSE 124a and that
the provision of power to the PD can begin.
[0030] After a valid PD is detected, an optional classification
stage can be performed to estimate the amount of power required by
the PD. To perform classification, PSE controller 200 again
produces a voltage (e.g., around 15.5-20.5 Vdc) for a predetermined
period of time (e.g., 10 to 75 ms). The current consumed by the PD
during this predetermined period of time indicates to PSE
controller 200 the power classification of the PD.
[0031] Following detection and optional classification, the output
power of PSE 124a can be increased, during the operation stage, to
its full voltage capacity, which is typically around 48 Vdc. The
output voltage of the PSE is gradually increased to its full
voltage capacity to prevent high frequency noise from disrupting
data being transferred concurrently with the power.
[0032] It should be noted that portions of the entire structure for
performing detection, classification, and operation (i.e., the
first three stages of the basic power provisioning process defined
by the IEEE 802.3af/t specifications) have been omitted from the
illustration in FIG. 2 for the sake of clarity.
[0033] The final stage of the power provisioning process involves
removal of power following the disconnection of the PD inductively
coupled to PSE 124a at nodes 120 and 122. The IEEE 802.3af/t
specifications define two specific techniques for power
disconnection; namely, DC disconnect and AC disconnect. Both
methods provide the same desired result--the detection of a
disconnected PD and the removal of power within 300 to 400 ms
thereafter. The removal of power when a PD is disconnected is
important because the PD may be replaced by a non-PoE-ready device,
which may result in damage of the device.
[0034] PSE 124a is configured to perform AC disconnect, which
entails the addition of a low AC signal on top of the 48 Vdc
operating voltage provided by DC supply 202. To generate the low AC
signal, PSE controller 200 includes a charge pump 204 that is
coupled at an input to the 48 Vdc signal produced by DC supply 202.
Charge pump 204 is configured to generate a voltage signal that is
higher than the 48 Vdc signal received at its input. In embodiment,
the voltage signal produced by charge pump 204 is substantially 3 V
higher than the 48 Vdc signal; that is, charge pump 204 produces a
51 Vdc signal that is applied across charge pump capacitor
C.sub.CP. Capacitor C.sub.CP can be utilized at the output of
charge pump 204 to smooth variations in the 51 Vdc signal
produced.
[0035] Switch S1, further included in PSE controller 200 and
coupled to the 51 Vdc signal, can be switched on and off to produce
the AC disconnect signal, which transitions back and forth from 48
V to 51 V at the frequency in which switch S1 is switched on and
off. In an embodiment, switch S1 is controlled by a control signal
206 provided by an oscillator (not shown). In a further embodiment,
switch S1 is switched on and off at a frequency of 27 Hz. The
resulting AC disconnect signal (of frequency 27 Hz) is coupled to
node 120 through resistor R.sub.1 and diode D.sub.1, where diode
D.sub.1 provides reverse isolation.
[0036] After generation and provisioning of the AC disconnect
signal, the returned AC signal amplitude is monitored at PSE 124a.
While the PD is connected, the low AC impedance of the PD lowers
the returned AC signal. During disconnection, however, the AC
impedance across terminals 120 and 122 increases significantly and,
as a result, the AC signal level will increase, indicating
disconnection of the PD. Switch S.sub.2 is controlled by control
signal 208 to discontinue the provision of DC power when
disconnection is detected. Specifically, to discontinue the
provision of DC power, switch S.sub.2 is opened. It should be noted
that the structure used to monitor the returned AC signal has been
omitted from FIG. 2 for the sake of clarity.
[0037] PSE 124A further implements a conventional technique to
isolate the generated AC disconnect signal from the DC supply 202.
Specifically, PSE 124A utilizes a diode D.sub.2 to isolate DC
supply 202, which provides the 48 Vdc operating voltage across
nodes 120 and 122, from the AC disconnect signal. Depending on the
type of diode utilized, the diode can have a forward voltage drop
of 0.3-0.7 Vdc at 600 mA, or a total power consumption around
0.2-0.4 W, for example. Not only does diode D.sub.2 increase
overall power consumption, but further increases the overall
temperature at the media dependent interface (MDI) of PSE 124a.
This excess power consumption and temperature becomes even more
apparent and prohibitive in multi-port hubs or switches that are
PoE-ready. For example, in a 24-port hub that is PoE-ready, 24
separate diodes (one for each port) can be required to isolate the
DC supply from the AC disconnect signal.
[0038] Therefore, what is needed is an apparatus for isolating DC
supply 202 from the AC disconnect signal, while limiting additional
power consumption and heat produced as a result thereof.
[0039] It should be noted that protection capacitor C.sub.P and
protection diode D.sub.P, further illustrated in FIG. 2, can be
used to neutralize surge events on and across nodes 120 and 122. In
an embodiment, diode DP is a transient voltage suppression (TVS)
diode used to limit the differential voltage across nodes 120 and
122.
3. Parallel Inductor-Capacitor (LC) Circuit
[0040] FIG. 3 illustrates portions of an exemplary PSE 124b,
according to embodiments of the present invention. The
implementation of PSE 124b eliminates the need for diode D.sub.2
illustrated in FIG. 2 and used to isolate DC supply 202 from the
generated AC disconnect signal. Diode D.sub.2 has been replaced by
a parallel combination of an inductor L.sub.1 and a capacitor
C.sub.1 in the implementation of PSE 124b illustrated in FIG.
3.
[0041] The parallel inductor-capacitor (LC) circuit 300, formed
from inductor L.sub.1 and capacitor C.sub.1, has a resonant
frequency given by:
f = 1 2 .pi. L 1 C 1 ##EQU00001##
At resonance, the effective impedance of parallel LC circuit 300 is
extremely large; in fact, the theoretical impedance is infinite.
Thus, the value of L.sub.1 (specified in Henries) and the value of
C.sub.1 (specified in Farads) can be selected such that their
parallel combination has a resonant frequency equal to the
fundamental frequency of the AC disconnect signal generated by PSE
controller 200. For example, assuming that the AC disconnect signal
generated by PSE controller 200 has a fundamental frequency of 4
kHz, a value of 680 .mu.H for inductor L.sub.1 and 2.2 .mu.F for
capacitor C.sub.1 establishes a resonant frequency of approximately
4 kHz for the parallel LC combination. Therefore, this specific
implementation of the LC combination, formed by inductor L.sub.1
and capacitor C.sub.1, will present an extremely large impedance to
the 4 kHz AC disconnect signal and effectively isolate DC supply
202 from the 4 kHz AC disconnect signal.
[0042] Moreover, the power consumed by inductor L.sub.1 and
capacitor C.sub.1 is considerably less than diode D.sub.2,
illustrated in FIG. 2. Inductor L1 and capacitor C1 will generally
only dissipate power as a result of their parasitic resistances,
which are typically fairly low valued, especially compared to the
effective resistance of a forward biased diode.
[0043] It should be noted that parallel LC circuit 300 can include
additional active and passive components and is no way limited to
the structure illustrated in FIG. 3. For example, parallel LC
circuit 300 can further include a resistor or additional inductive
and capacitive components.
4. Series Inductor-Capacitor (LC) Circuit
[0044] FIG. 4 illustrates portions of an exemplary PSE 124c,
according to embodiments of the present invention. The
implementation of PSE 124c eliminates the need for charge pump 204,
capacitor C.sub.CP, switch S.sub.1, resistor R.sub.1, and diode
D.sub.1, all of which were used, at least in part, to generate the
AC disconnect signal as illustrated in FIG. 2. In addition, diode
D2, used in FIG. 2 to isolate DC supply 202 from the generated AC
disconnect signal, has been eliminated and replaced by an inductor
L.sub.1.
[0045] Inductor L.sub.1 and capacitor C.sub.P form a series
inductor-capacitor (LC) circuit that produces an oscillation (i.e.,
an AC disconnect signal) when a PD, coupled to nodes 120 and 122,
is disconnected. The occurrence of this oscillation can be
monitored for by PSE controller 200 to detect the disconnection of
the PD and to discontinue the provisioning of DC power.
[0046] Specifically, during operation, DC supply 202 is configured
to provide 48 Vdc (for example) to the PD inductively coupled to
nodes 120 and 122. Current flows through inductor L.sub.1 during
operation and out node 120. When the PD is disconnected, inductor
L.sub.1 resists changing the current flowing through it and
continues to supply current, which now charges capacitor C.sub.P.
Eventually, the energy stored in the magnetic field of inductor
L.sub.1 is exhausted and the current supplied by inductor L.sub.1
ceases. However, the charge now stored on capacitor C.sub.P will
begin to flow back through inductor L1, re-establishing its
magnetic field. When all the charge stored on capacitor C.sub.P has
been dissipated, energy will again be extracted from the magnetic
field of the inductor to continue the flow of current.
[0047] This flow of charge, back and forth between capacitor
C.sub.1 and inductor L.sub.1, following disconnection, produces an
oscillation that can be detected by PSE controller 200 to signal
disconnection. Once detected switch S.sub.2 can be opened by
control signal 208 to stop the provisioning of DC power. The series
LC circuit is often referred to as a tank circuit, which has
similar properties to water sloshing back and forth in a tank.
[0048] It should be noted that the frequency of oscillation,
produced by the series LC circuit, is specified by its resonant
frequency, given by:
f = 1 2 .pi. L 1 C P ##EQU00002##
For example, assuming a value of 4.7 .mu.H for inductor L.sub.1 and
a value of 0.01 .mu.F for capacitor C.sub.P, the frequency of
oscillation produced by the series LC circuit will be approximately
700 kHz. This 700 kHz oscillation provides an effective AC
disconnect signal that can be monitored for and detected by PSE
controller 200 to stop the provisioning of DC power.
[0049] It should be further noted that an additional capacitor,
other than protection capacitor C.sub.P can be utilized to form the
series LC circuit illustrated in FIG. 4. In addition, diode D.sub.P
can be configured to prevent possible over voltages from occurring
across nodes 120 and 122 as a result of the oscillations produced
by the series LC circuit illustrated in FIG. 4.
5. Conclusion
[0050] It is to be appreciated that the Detailed Description
section, and not the Abstract section, is intended to be used to
interpret the claims. The Abstract section may set forth one or
more but not all exemplary embodiments of the present invention as
contemplated by the inventor(s), and thus, are not intended to
limit the present invention and the appended claims in any way.
[0051] The present invention has been described above with the aid
of functional building blocks illustrating the implementation of
specified functions and relationships thereof. The boundaries of
these functional building blocks have been arbitrarily defined
herein for the convenience of the description. Alternate boundaries
can be defined so long as the specified functions and relationships
thereof are appropriately performed.
[0052] The foregoing description of the specific embodiments will
so fully reveal the general nature of the invention that others
can, by applying knowledge within the skill of the art, readily
modify and/or adapt for various applications such specific
embodiments, without undue experimentation, without departing from
the general concept of the present invention. Therefore, such
adaptations and modifications are intended to be within the meaning
and range of equivalents of the disclosed embodiments, based on the
teaching and guidance presented herein. It is to be understood that
the phraseology or terminology herein is for the purpose of
description and not of limitation, such that the terminology or
phraseology of the present specification is to be interpreted by
the skilled artisan in light of the teachings and guidance.
[0053] The breadth and scope of the present invention should not be
limited by any of the above-described exemplary embodiments, but
should be defined only in accordance with the following claims and
their equivalents.
* * * * *