Abstract:
In one embodiment, the invention is an apparatus (e.g., an Ethernet switch) having an isolated side and a line side. A line-side connector is connectable to a cable. An isolated-side physical-layer module (1) is electrically coupled to the connector via a signal-isolation transformer and (2) processes signals transmitted over the cable. A line-side power conditioning module (1) is electrically coupled to an isolated-side power switcher via a power-isolation transformer that converts an AC power signal received from the power switcher into a transformed AC power signal and (2) converts the transformed AC power signal into a cable power signal to be supplied via the connector to the cable in order to power a cable-powered device connected to the cable. An isolated-side control module performs a detection function in which the control module determines whether or not a cable-powered device is connected to the cable.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of the filing date of U.S. provisional application No. 60/706,512, filed on Jul. 21, 2005, the teachings of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to electronics, and, in particular, to Power Sourcing Equipment (PSE) for communication systems conforming to the IEEE 802.3 Ethernet and IEEE 802.3af Power over Ethernet (PoE) standards. 
   2. Description of the Related Art 
     FIG. 1  shows a block diagram of a portion of a conventional Ethernet switch  100  for a communication system that conforms to both the IEEE 802.3 Ethernet standard and the IEEE 802.3af PoE standard, the teachings of both of which are incorporated herein by reference. As shown in  FIG. 1 , for each port in the switch, Ethernet switch  100  comprises Ethernet Physical-Layer (PHY) module  102 , RJ-45 Ethernet connector  104 , 48-volt switcher  106 , line-side PSE control and power conditioning module  108 , isolated-side PSE control module  110 , four-pair signal-isolation transformer  112 , power-isolation transformer  114 , and optical isolator  116 . 
   According to the IEEE 802.3 Ethernet standard, Ethernet PHY module  102  communicates with a Media Access Controller (MAC) and switching function and processes incoming and outgoing differential data signals that are transmitted over an Ethernet cable (not shown) that is connected to switch  100  at Ethernet connector  104 . According to the IEEE 802.3af PoE standard, switch  100  is a PSE-capable switch that can provide a 48-volt DC (cable power) signal on the Ethernet cable to power a so-called powered device (PD), such as an Internet Protocol (IP) telephone, such that the PD device does not require any additional power source to operate. The 48-volt DC signal is provided by switcher  106  and modules  108  and  110 . 
   Transformers  112  and  114  and optical isolator  116  provide high-voltage electrical isolation between (1) circuitry, such as Ethernet PHY module  102 , switcher  106 , and PSE control module  110 , located on the so-called isolated side of switch  100  (i.e., the primary side of the transformers) and (2) circuitry, such as Ethernet connector  104  and PSE control and power conditioning module  108 , located on the so-called line side of switch  100  (i.e., the transformers&#39; secondary side), to protect the isolated-side circuitry from high voltages that might appear on the line side, such as those that can occur when lightening strikes near an Ethernet cable connected to connector  104 . 
   Switcher  106  provides an AC power signal that is converted by power-isolation transformer  114  into a transformed AC power signal that can be converted by line-side PSE control and power conditioning module  108  into the 48-volt (differential) DC signal that is applied to the center taps of the secondary-side coils of two of the four transformer pairs in signal-isolation transformer  112 . 
   In the embodiment shown in  FIG. 1 , signal-isolation transformer  112  has four pairs of transformer coils, where the 48-volt DC signal is applied to the center taps of two of the four secondary-side coils. The IEEE 802.3 Ethernet standard also covers lower-rate (e.g., 10 Mbit and 100 Mbit) Ethernet systems that have only two pairs of coils, where two of the four wire pairs in the Ethernet cable are spares. According to the IEEE 802.3af PoE standard, the 48-volt DC signal can be applied directed to the two spare wire pairs to power a PD device. Although this specification describes Ethernet switches with ports having four pairs of transformer coils, the teachings of this specification apply equally well to Ethernet switches with ports having only two pairs of transformer coils and two spare wire pairs. 
   According to one conventional implementation, line-side PSE control and power conditioning module  108  provides two PSE control functions (i.e., detection and classification) and three PSE power conditioning functions (i.e., rectification, filtering, and impedance control). 
   During detection, line-side PSE module  108  sequentially applies two different, low-power signals to the transformer coils (either low-current or low-voltage depending on the implementation, such as a first low-power signal of approximately 3 volts followed by a second low-power signal of approximately 8 volts, instead of the full 48-volt DC signal) to enable line-side PSE module  108  to detect whether a valid PD device (which is required by the PoE standard to have a 25-Kohm impedance across the lines that provide the detection signal) is currently connected to Ethernet connector  104  via an Ethernet cable. 
   If such a PD device is detected, then line-side PSE module  108  (optionally) performs classification, during which line-side PSE module  108  increases the level of the applied signal to approximately 18 volts and measures the PD signature current draw to determine the power classification of the detected PD device. 
   If a PD device is detected and (optionally) classified, then line-side PSE module  108  performs power conditioning, during which line-side PSE module  108  generates and applies the appropriate 48-volt DC signal via connector  104  to the Ethernet cable to power the PD device. This power conditioning function involves rectification and filtering of the transformed AC signal that is applied to line-side PSE module  108  from the secondary side of transformer  114  to generate the 48-volt DC signal. The power conditioning function also involves impedance control for the 48-volt DC signal. If no PD device is detected, then line-side PSE module  108  does not generate and apply a 48-volt DC signal to the Ethernet cable. 
   Depending on the particular implementation, switch  100  may be configured to support multiple Ethernet ports, like the port associated with Ethernet connector  104 . According to one such conventional implementation, line-side PSE control and power conditioning module  108  is implemented as a relatively large integrated circuit (IC) that is capable of simultaneously supporting four different Ethernet ports. Examples of such line-side PSE control and power conditioning modules are:
         The 12-channel PoE Manager, Product No. PD64012, sold by PowerDsine Ltd. of Israel;   The Quad Integrated Power Sourcing Equipment Power Manager, Product No. TPS2384, sold by Texas Instruments Incorporated of Dallas, Tex.; and   The Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection module, Product No. LTC4258, sold by Linear Technology Corporation of Milpitas, Calif.       

   These conventional PSE control and power conditioning modules provide a certain (relatively limited) level of operating functionality. In certain conventional Ethernet switches, such as switch  100  of  FIG. 1 , additional (e.g., switch-vendor value-added) functions are provided by isolated-side PSE control module  110 , which is typically implemented using a microcontroller. One exemplary additional function that may be provided by isolated-side PSE control module  110  is power balancing between multiple (e.g., as many as 48 or more) PD devices connected to a single Ethernet switch, where power balancing is based on the results of the PD power classification performed on each PD device. To implement this additional operating functionality, isolated-side PSE control module  110  receives (explicit) information about the status of the operations at line-side PSE module  108  via optical isolator  116  and transmits control signals to control the operations at line-side PSE module  108  via optical isolator  116 . 
   Such a conventional configuration for switch  100  has a number of disadvantages. First of all, there is a considerable dollar cost to providing all of this circuitry associated with these functions. Furthermore, when two or more PD devices are connected to different Ethernet connectors supported by a single line-side PSE control and power conditioning module, there is no electrical isolation to protect the rest of the PD devices from a lightening strike near any one of the PD devices. 
   SUMMARY OF THE INVENTION 
   In one embodiment, the present invention is an apparatus having an isolated side and a line side. The apparatus comprises a physical-layer module, a power switcher, a power conditioning module, and a control module. The physical-layer module is (1) located on the isolated side of the apparatus, (2) adapted to be electrically coupled via a signal-isolation transformer to a cable connector located on the line side of the apparatus, and (3) adapted to process signals transmitted over the cable. The power switcher is located on the isolated side of the apparatus. The power conditioning module is (1) located on the line side of the apparatus, (2) electrically coupled to the power switcher via a power-isolation transformer adapted to convert an AC power signal received from the power switcher into a transformed AC power signal, and (3) adapted to convert the transformed AC power signal into a cable power signal to be supplied via the connector to the cable in order to power a cable-powered device connected to the cable. The control module is located on the isolated side of the apparatus and adapted to perform a detection function in which the control module determines whether or not a cable-powered device is connected to the cable. 
   In another embodiment, the present invention is a method for powering a cable-powered device using an apparatus having a line side and an isolated side, wherein the cable-powered device is connected to the line-side of the apparatus via a cable. According to the method, signals transmitted from the cable-powered device over the cable are received at the line side of the apparatus. The signals from the cable-powered device are (1) transmitted from the line side of the apparatus to the isolated side of the apparatus via a signal-isolation transformer located between the line side of the apparatus and the isolated side of the apparatus and (2) processed on the isolated side of the apparatus. An AC power signal on the isolated side of the apparatus is converted into a transformed AC power signal on the line side of the apparatus via a power-isolation transformer located between the line side of the apparatus and the isolated side of the apparatus. The transformed AC power signal is converted, on the line side of the apparatus, into a cable power signal, which is supplied to the cable in order to power the cable-powered device. A determination is made, on the isolated side of the apparatus, that the cable-powered device is connected to the cable based on signals received at the isolated side of the apparatus from the line side of the apparatus via the power-isolation transformer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. 
       FIG. 1  shows a block diagram of a portion of a conventional Ethernet switch for a communication system that conforms to both the IEEE 802.3 Ethernet standard and the IEEE 802.3af PoE standard; 
       FIG. 2  shows a block diagram of a portion of an Ethernet switch for a communication system that conforms to the IEEE 802.3 Ethernet and 802.3af PoE standards, according to one embodiment of the present invention; 
       FIG. 3  shows a schematic block diagram of the switcher, the line-side PSE control and power conditioning module, the power-isolation transformer, and the optical isolator of the conventional Ethernet switch of  FIG. 1 ; 
       FIG. 4  shows the schematic block diagram of  FIG. 3  annotated to indicate the migration of functions associated with particular elements in the line-side circuitry of the PSE control and power conditioning module of  FIG. 1  to a combined switcher/PSE control module corresponding to a combined implementation of the switcher and the isolated-side PSE control module of  FIG. 2 ; 
       FIG. 5  shows a schematic block diagram of the switcher, the line-side PSE power conditioning module, the isolated-side PSE control module, and the power-isolation transformer of the Ethernet switch of  FIG. 2 , according to one possible embodiment of the present invention; 
       FIG. 6  shows a schematic block diagram of the switcher, the line-side PSE power conditioning module, the isolated-side PSE control module, and the power-isolation transformer of the Ethernet switch of  FIG. 2 , according to one possible low-voltage, mixed-signal, CMOS-technology embodiment of the present invention; 
       FIG. 7  shows a schematic block diagram of only those elements of  FIG. 6  that are involved in the PSE detection mode of operation; 
       FIG. 8  shows a schematic block diagram of only those elements of  FIG. 6  that are involved in the PSE classification mode of operation; and 
       FIG. 9  shows a schematic block diagram of only those elements of  FIG. 6  that are involved in the PSE power-on mode of operation. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows a block diagram of a portion of an Ethernet switch  200  for a communication system that conforms to the IEEE 802.3 Ethernet and the IEEE 802.3af Power over Ethernet (PoE) standards, according to one embodiment of the present invention. As shown in  FIG. 2 , Ethernet switch  200  comprises Ethernet PHY module  202 , RJ-45 Ethernet connector  204 , 48-volt switcher  206 , line-side PSE power conditioning module  208 , isolated-side PSE control module  210 , four-pair signal-isolation transformer  212 , and power-isolation transformer  214 , where switcher  206  and power conditioning module  208  function together as an isolated switching power supply. Ethernet PHY module  202 , connector  204 , and transformers  212  and  214  are similar to the corresponding elements in conventional switch  100  of  FIG. 1 . Note that, unlike switch  100  of  FIG. 1 , switch  200  does not have any optical isolator. 
   According to this embodiment of the present invention, the PSE detection and (optional) classification functions that were performed by line-side PSE control and power conditioning module  108  of  FIG. 1  and all of the PSE control functions that were performed by isolated-side PSE control module  110  of  FIG. 1  are now performed by isolated-side PSE control module  210  of  FIG. 2 . The only PSE functions that remain on the line side of switch  200  of  FIG. 2  are the power conditioning functions implemented by PSE power conditioning module  208  of  FIG. 2 . These power conditioning functions include, but are not limited to, rectification, filtering, and impedance control. Note that, in this embodiment, isolated-side PSE control module  210  does not receive any explicit information from the line side of switch  200  about the status of the operations at line-side PSE power conditioning module  208 . 
   The migration of the PSE detection and classification functions from the line side of switch  100  to the isolated side of switch  200  is enabled by the fact that all of the information needed to implement those functions appears on both sides (i.e., on both the primary- and secondary-side coils) of power-isolation transformer  214 . Note that Ethernet PHY module  202 , switcher  206 , and PSE control module  210  are preferably, but do not have to be, implemented in a combined manner on a single integrated circuit. 
   This function migration from line side to isolated side also enables practical implementation of a separate line-side PSE power conditioning module, similar to module  208  of  FIG. 2 , for each Ethernet connector, similar to connector  204 , supported by switch  200 . As such, a switch of the present invention can be configured with multiple Ethernet ports, while providing electrical isolation between all of the Ethernet ports. In particular, the high voltage associated with a lightening strike near an Ethernet cable connected to one of the Ethernet ports will not reach any of the other Ethernet ports (or their associated cables and PD devices). In one possible implementation, a single module, like isolated-side PSE control module  210  of  FIG. 2 , can be designed to support multiple Ethernet ports, with each port having its own switcher (like switcher  206 ), transformer (like transformer  214 ), PSE power conditioning module (like module  208 ) and Ethernet connector (like connector  204 ), while still allowing very high levels of integration. 
     FIG. 3  shows a schematic block diagram of switcher  106 , line-side PSE control and power conditioning module  108 , power-isolation transformer  114 , and optical isolator  116  of conventional switch  100  of  FIG. 1 . In  FIG. 3 , the elements in the upper half of the diagram that are to the left of transformer  114  correspond to switcher  106 . Diode  302  and capacitor  304  symbolically represent the line-side circuitry that provides the power conditioning functions of line-side PSE module  108 , while the elements in the lower half of the diagram represent the line-side circuitry that provides the detection and classification functions of line-side PSE module  108 . 
   As represented in the upper half of  FIG. 3 , switcher  106  is a stand-alone, isolated, 48-volt, flyback switching supply that provides power to the PSE circuitry. Pulse Width Modulation (PWM) control logic  306  is provided information regarding output voltage (via transformer winding  308 ) and drive current (via current-sensing resistor  310  and current reference amplifier  312 ) and uses this information to adjust the pulse width of the drive signal applied to the gate of power FET  314 , such that the desired voltage, with an appropriate current limit, is applied to the output. 
   As represented in the lower half of  FIG. 3 , optionally under the control of isolated-side PSE control module  110 , PSE control logic  316  of line-side PSE module  108  performs the PoE detection and classification functions by sequentially providing two different fixed currents  318  and  320  onto the output, while the output voltage is measured by PSE control logic  316 . This is followed by the application of a fixed voltage  322  during which the current is measured by resistor  324  and amplifier  326 . During a normal power-up sequence, this is followed by PSE control logic  316  turning on power FET  328  and continuing to monitor current draw and voltage for health/fault/disconnect status. 
     FIG. 4  shows the schematic block diagram of  FIG. 3  annotated to indicate the migration of functions associated with particular elements in the line-side circuitry of PSE control and power conditioning module  108  of  FIG. 1  to a combined switcher/PSE control module corresponding to a combined implementation of switcher  206  and PSE control module  210  of  FIG. 2 , located on the isolated side of switch  200 . As represented in  FIG. 4 , all of the line-side functions on the lower half of the diagram either migrate to the isolated side or are eliminated (i.e., in the case of the optical isolator). 
   In particular, as represented in  FIG. 4 :
         The functions of power FET  328  are migrated to power FET  314 ;   The functions of current sources  318  and  320 , resistor  324 , and amplifier  326  are migrated to resistor  310 , amplifier  312 , and PWM control logic  306 ;   The functions of PSE control logic  316  are migrated to PWM control logic  306 ;   The functions of voltage source  322  are migrated to voltage reference  330 ; and   The detection and classification power provided to the Ethernet connector is migrated to the output of the line-side power conditioning function.       

     FIG. 5  shows a schematic block diagram of switcher  206 , line-side PSE power conditioning module  208 , isolated-side PSE control module  210 , and power-isolation transformer  214  of switch  200  of  FIG. 2 , according to one possible embodiment of the present invention. In  FIG. 5 , the elements to the left of transformer  214  correspond to switcher  206  and PSE control module  210 , while diode  502  and capacitor  504  symbolically represent the line-side circuitry that provides the power conditioning functions of PSE power conditioning module  208 . In this particular implementation, the PSE control module  210  is shown being implemented as a set of logic separate from enhanced PWM control logic  506  of switcher  206 . In a combined implementation, a single logic device can be used to implement all functions to the left of power-isolation transformer  214  in  FIG. 5 . 
   Alternatively, the functions of PSE control module  210  may be implemented in software and/or hardware in two or more different processing modules, including one or more processing modules that support multiple ports. For example, in one possible implementation, a single logic device implements all of the functions to the left of power-isolation transformer  214  that are associated with a single port, while another (shared) logic device, such as a microcontroller, implements additional functions that are associated with multiple ports, such as power balancing. 
   In any case, PSE control module  210  monitors the current and voltage from transformer  214  and current sense amplifier  512  and determines the appropriate PSE state for commanding enhanced PWM control logic  506 , which is enhanced (relative to PWM control logic  306  of  FIG. 3 ) to support the detection and classification functions that migrated to the isolated side. 
     FIG. 6  shows a schematic block diagram of switcher  206 , line-side PSE power conditioning module  208 , isolated-side PSE control module  210 , and power-isolation transformer  214  of switch  200  of  FIG. 2 , according to one possible low-voltage, mixed-signal, CMOS-technology embodiment of the present invention. The PSE circuitry of  FIG. 6  supports all three modes of operation described previously: detection, classification, and power conditioning (i.e., power on). 
   In this embodiment, PSE control module  210  is implemented in digital logic as logic modules  602 - 610 . Similarly, enhanced PWM control logic  506  of  FIG. 5  is implemented in digital logic as PWM control logic module  612  and Pulse Frequency Modulation (PFM) control logic module  614 , where PWM control logic module  612  controls pulse width modulation during the classification and power-on modes, and PFM control logic module  614  controls a PFM loop comprising detection (MOSFET) transistor  616  and current-limiting resistor  618  used during the detection mode. 
   When power is first applied to the PSE circuitry of  FIG. 6 , analog bias circuitry is stabilized, clocks (e.g., from clock generator  620 ) are started, and all circuitry is reset or initialized. Following initialization, operation of the PSE circuitry is under the control of master sequencer  602 , which will determine the mode of operation for the PSE circuitry. Per the IEEE 802.3af PoE Standard, the basic sequence of operation is: (1) detection, (2) classification, and (3) power conditioning. Provisions may be made for various fault conditions and/or user interventions to override this basic sequencing. 
   Not explicitly shown in  FIG. 6  (or in subsequent  FIGS. 7-9 ) are paths whereby digital logic functions are clocked at appropriate times taking into account proper settling of the sensed levels. Also not explicitly shown are (a) control signals, whereby sequencers (e.g., master sequencer  602 , detection sequencer  606 , and classification sequencer  608 ) can adjust (i) loop filter parameters (e.g., of PFM loop filter  622  and PWM loop filter  624 ), (ii) gains (e.g., of amplifiers  626 ,  628 , and  630 ), or (iii) other parametric settings and (b) the control and test interface paths (e.g., from interface  632 ) that allow a user or surrogate processor to adjust voltages and current thresholds and to fine-tune timing, as appropriate. 
   Operation of the PSE circuitry of  FIG. 6  for the three different modes of operation (i.e., detection, classification, and power conditioning) is described below in the context of  FIGS. 7-9 , respectively. 
     FIG. 7  shows a schematic block diagram of only those elements of  FIG. 6  that are involved in the PSE detection mode of operation. Following initialization, master sequencer  602  will set variable load  648  to mimic the expected detection load and then direct detection sequencer  606  to begin the detection process, which continues under the control of detection sequencer  606 . 
   Detection sequencer  606  directs voltage-control logic  604  to select and apply the first detection voltage point V 1  (e.g., nominally about 3 volts) as reference voltage  642  to be used by voltage-sense ADC (analog-to-digital converter)  634  of voltage-sense block  636 . 
   Detection sequencer  606  then enables PFM control logic  614 , which sends short-duration pulses (e.g., typically about 100 to 500 ns long) to detection transistor  616 , which in turn sends precision low-current pulses to the isolated side of power transformer  214 , such that the resulting output voltage on the line side of power transformer  214  ramps up slowly. This transformer output voltage is sensed through third winding  638  in power transformer  214  by voltage-sense block  636 , in which ADC  634  compares sensed voltage  640  with reference voltage  642 . 
   The resulting digitized signal  644  from ADC  634  is appropriately filtered by PFM loop filter  622  in order to maintain loop stability, and the resulting filtered signal  646  is used by PFM control logic  614  to determine an appropriate pulse sequence that stabilizes the transformer output voltage at set point V 1 . 
   Detection sequencer  606  monitors the duration taken to arrive at set point V 1 . If the duration is too long, then detection sequencer  606  will time out, resetting the sequence. Such a time-out is indicative of an improper detection load on the output, such as the excessive capacitance required to be detected by the IEEE 802.3af PoE Standard. If the initial voltage set point V 1  is reached successfully (e.g., within a specified duration), then detection sequencer  606  will record the frequency (f 1 ) that PFM control logic  614  employed to stabilize at that level. 
   Detection sequencer  606  will then direct voltage-control logic  604  to select the second detection voltage point V 2  (e.g., nominally about 8 volts), and an analogous ramp-up sequence will be implemented until either set point V 2  is reached or a time-out occurs. If the second voltage set point V 2  is reached successfully, then detection sequencer  606  will record the frequency (f 2 ) that PFM control logic  614  employed to stabilize at that level. 
   If the detection process gets this far without timing out, then detection sequencer  606  estimates the detection load from the difference between frequencies f 1  and f 2 . This is possible because the frequency, pulse width, peak current, and peak voltage at both set points V 1  and V 2  are known and can be used to calculate the average current and the average voltage. After compensating for parasitic losses, the value of the detection resistor in the PoE powered device can be calculated from the slope of the average voltage-current curve between set points V 1  and V 2 . 
   If the detection mode was not successful (e.g., detection sequencer timed out while trying to achieve either set point V 1  or set point V 2 ), then master sequencer  602  will wait a specified period (e.g., one half second) before re-initiating the detection sequence, as required by the IEEE 802.3af PoE Standard. 
     FIG. 8  shows a schematic block diagram of only those elements of  FIG. 6  that are involved in the PSE classification mode of operation. If the detection process was successful, then master sequencer  602  will initiate a classification process under the control of classification sequencer  608 . 
   Classification sequencer  608  directs voltage-control logic  604  to select the classification voltage (e.g., nominally about 18 volts). During classification, switcher  206  operates as a conventional current-mode, pulse-width modulated, switching power supply. Reference voltage  642  from voltage-control logic  604  is compared by ADC  634  to sensed voltage  640 , which depends on the output voltage of power transformer  214  as reflected to third winding  638 . 
   To ensure that the voltage from third winding  638  is closely representative of the actual transformer output voltage (in order to maintain high accuracy), an adjustable matching load  648  is utilized and set by master sequencer  602  to mimic the actual load expected in the classification mode. (Master sequencer  602  analogously controls variable load  648  during the detection and power-on modes.) 
   ADC  634  digitizes the error between the desired output voltage and the actual output voltage. This digitized error  644  is conditioned through PWM loop filter  624  to maintain loop stability. The resulting filtered error signal  650  is processed through digital current-limit function  652  and applied to current-sense block  654 , which senses the transformer current, to enable PWM control logic  612  to determine the peak current at which the pulse width modulator should turn off the power (MOSFET) transistor  656 . 
   In the PSE classification mode, the programmable current-limit function  652  is set and monitored by fault monitor  610  to limit the average output current, e.g., to under 100 ma, as required by the IEEE 802.3af PoE Standard. 
   Classification sequencer  608  controls all aspects of voltage application, timing, and time-out processing for the PSE classification mode defined in the IEEE 802.3af PoE Standard. If the classification process completes successfully, then the current observed (i.e., the output of current-limit function  652 ) is reported to classification sequencer  608 , which determines the power class of the PoE powered device per the IEEE 802.3af PoE Standard. 
     FIG. 9  shows a schematic block diagram of only those elements of  FIG. 6  that are involved in the PSE power-on mode of operation. Following the classification process, master sequencer  602  will initiate the power-on mode. 
   Master sequencer  602  will set variable load  648  to mimic the expected load and then direct voltage-control logic  604  to select the power-on voltage level (e.g., nominally about 48 volts). During power-on, switcher  206  operates as a conventional current-mode, pulse-width modulated, switching power supply. Reference voltage  642  from voltage-control logic  604  is compared by ADC  634  to sensed voltage  640 , which depends on the output voltage of power transformer  214  as reflected to third winding  638 . 
   ADC  634  digitizes the error between the desired output and the actual output voltage. This digitized error  644  is conditioned through PWM loop filter  624  to maintain loop stability. The resulting filtered error signal  650  is processed through current-limit function  652  and applied to current-sense block  654 , which senses the transformer current, to enable PWM control logic  612  to determine the peak current at which the pulse width modulator should turn off the power (MOSFET) transistor  656 . 
   In the PSE power-on mode, the programmable current-limit function  652  is set and monitored by fault monitor  610  to limit the average output current, e.g., to nominally about 425 ma during initial power application (in-rush conditions) and then to a current limit dependent upon the results of the classification process, not to exceed nominally about 375 ma, as required by the IEEE 802.3af PoE Standard. 
   Master sequencer  602  controls all aspects of voltage application, timing, and time-out processing for the PSE power-on mode defined in the IEEE 802.3af PoE Standard. Various current limits, such as Icut, Ilimit, and Iinrush, are programmed and monitored by fault monitor  610  using information received from current-limit function  652 . Appropriate levels of ramp-up rates, ramp-down rates, noise, and ripple currents are ensured by a combination of loop characteristics and hardware components utilized in the output circuits (e.g., power conditioning module  208 ). 
   Off-load block  658  in power conditioning module  208  senses when switcher  206  is shutting down in order to apply a small additional load to the output to ensure that the transformer output decays within the duration allotted by the IEEE 802.3af PoE Standard. 
   Depending on the particular implementation, the isolated-side PSE control module of the present invention can provide other PSE control functions that are normally implemented on the line side of certain conventional Ethernet switches. In general, these functions may be those required by the IEEE 802.3af standard, and, in particular, by the requirements of  FIG. 33-6  (entitled “PSE State Diagram”) and  FIG. 33-7  (entitled “PSE Monitor Overload, Monitor Short, and Monitor MPS State Diagram”) of the IEEE 802.3af standard. 
   Although the present invention has been described in the context of communication systems conforming to the IEEE 802.3 Ethernet and IEEE 802.3af PoE standards, the invention is not necessarily limited to communication systems that conform to either or both of those two standards. Moreover, as those standards may evolve over time, it is expected that implementations of the present invention can also evolve in a corresponding manner. 
   Although the present invention is described in the context of switches in which a 48-volt (differential) DC signal is applied to the secondary-side coils of two signal-isolation transformers, the invention is not necessarily so limited. For example, the present invention may be implemented in the context of (1) DC power signals having voltage levels other than 48 volts, (2) non-differential (i.e., single-sided) DC power signals, and (3) even differential or single-sided AC power signals. Moreover, the power signals may be provided to the cables via other means, such as direct connection to the connector. Furthermore, the present invention may be implemented in contexts other than switches, such as routers or other suitable apparatus. 
   The present invention may be implemented as circuit-based processes, including possible implementation as a single integrated circuit (such as an ASIC or an FPGA), a multi-chip module, a single card, or a multi-card circuit pack. As would be apparent to one skilled in the art, various functions of circuit elements may also be implemented as processing steps in a software program. Such software may be employed in, for example, a digital signal processor, micro-controller, or general-purpose computer. 
   It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims. 
   Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.”