Power over Ethernet
265
Ryan Huff
Active bridge rectifiers reduce heat dissipation within PoE security
cameras
positive DC power from either 24V AC, +12V DC or −12V DC. The resulting DC power and the PoE inputs are diode- ORed with the winning supply fed to a wide input voltage isolated switching power supply, which in turn powers the camera electronics.
This power architecture presents a few challenges. When the camera is powered from the auxiliary input, three diodes (circled in Figure 265.1) fall into the power path. In addition to the inefficiency of this design and possible heat problems from the power dissipated by the diodes, the three diodes lead to a significant voltage drop at the input to the switching power supply. With a 10W to 15W camera, these challenges are easily surmountable, but the latest security cameras have doubled this power consumption. Features like pan/tilt/zoom (PTZ) and camera lens heaters for outdoor operation have made this power architecture unsuitable for this new wave of cameras.
To illustrate the architecture’s deficiencies, consider a 26W camera. For a 12V DC auxiliary input (assumed to actually be 9V DC due to use of unregulated wall warts/AC adapters) and three 0.5V drop Schottky diodes, the input voltage of
Introduction
Power over Ethernet (PoE) has been embraced by the video surveillance industry as a solution to an age-old problem: com- plicated cabling. For instance, a basic, traditional fixed-view security camera requires two cables: one for power (10W to 15W from a 24V AC or 12V DC), and a separate, coax cable for the video signal. With PoE, a single Ethernet cable carries both video data and power. Everything is simplified. Right?
Not quite. To meet compatibility with existing systems, camera manufacturers must produce PoE-enabled cameras that are also compatible with legacy power sources—they must accept PoE 37V to 57V DC from an RJ-45 jack or 24V AC, +12V DC, or −12V DC from an auxiliary power connector.
The old way loses power
Figure 265.1 shows the power architecture used by many PoE camera manufacturers to solve this problem. A full-bridge diode rectifier after the auxiliary (old-school) input produces
Figure 265.1 • Auxiliary Input and PoE Power Architecture Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00265-9
the switching power supply is 7.5V (9V − 3 ã 0.5V). The input current for this camera is approximately 3.5A (26W/7.5V).
The resultant power dissipation of the three Schottky diodes in the power path is 5.2W (3.5A ã 3 ã 0.5V). This power dis- sipation leads to higher temperature within the camera, which is difficult, time consuming and expensive to mitigate.
Improve performance with ideal diodes
Figure 265.2 shows a way to counter this shortcoming. Here, the two diodes of the full-bridge rectifier are replaced by ideal diodes, circled (black) in Figure 265.2. Ideal diodes are sim- ply MOSFETs controlled to behave like regular diodes. The advantage of an ideal diode is that one can use MOSFETs with low channel resistance (RDS(ON)), thus reducing the forward voltage drop (IDSã RDS(ON)) to much less than a Schottky diode drop. The LT4320 ideal diode bridge controller enables the control of four MOSFETs in a full-bridge configuration.
The third diode drop due to the diode-OR in Figure 265.1 is eliminated by the LT4275 LTPoE++/PoE+/PoE PD control- ler. Its topology allows the use of a few small-signal diodes, circled together in Figure 265.2, for auxiliary input sensing.
These diodes are not in the power path as in the traditional architecture, so they do not contribute any additional voltage drop or heat issues.
Results
The power architecture shown in Figure 265.2 significantly reduces overall power losses when compared to that of Figure 265.1. To quantify, the LT4320 combined with low channel resistance MOSFETs results in a 20mV drop across each ideal diode bridge MOSFET. This produces an input at the isolated supply of 8.96V (9V − 2 ã 20mV). The higher input voltage drops the required input current to only 2.9A (26W/8.96V) versus the original 3.5A.
The resulting power dissipation of the improved architec- ture is now a scant 116mW (2.9A ã 2 ã 20mV), versus 5.2W for the original architecture—a 45× reduction! Additionally, the lower input current further reduces power dissipation in the isolated power supply’s power components (i.e., input filter inductor, power transformer and switching MOSFETs) due to the reduction of their I2R power losses. A simple calculation puts this reduction at 31% (100% − 2.9 A2/3.5A2).
Conclusion
Adding the LT4320 and LT4275 to the auxiliary and PoE inputs of a PoE-enabled security camera recovers more than 5W (5.2W − 116mW) of power dissipation over traditional full- bridge/diode-OR designs. This reduction of power eases the thermal design time and complexity of PoE security cameras.
266
Dilian Reyes
High power PoE PD interface with integrated flyback controller
PD interface controller
The PD interface controller provides the same 25k signature detection resistance defined in the standard PoE. An extended optional class can be read by a customized PSE that looks for such a class. Once a PSE detects and classifies the PD, it fully powers on the device. The LTC4268-1 provides a low inrush current limit, allowing load capacitance to ramp up to the line voltage in a controlled manner without interference from the PSE current limit. After the load capacitance is charged up, the LTC4268-1 switches to the high input current limit and provides a power good signal to its switching regulator indi- cating that it can start its operation. During this time, the LTC4268-1 remains in its high current limit state allowing for up to 35W delivered to the load.
Introduction
To this day, Power over Ethernet (PoE) continues to gain pop- ularity in today’s networking world. The 12.95W delivered to the Powered Device (PD) input supplied by the Power Sourcing Equipment (PSE) is a universal supply. Each PD pro- vides its own DC/DC conversion from a nominal 48V supply, thus eliminating the need for a correct voltage wall adapter.
However, higher power devices cannot take advantage of standard PoE because of its power limitations, and must rely on a large wall adapter as their primary supply. The new LTC4268-1 breaks this power barrier by allowing for power of up to 35W for such power-hungry 2-pair PoE applications.
The LTC4268-1 provides a complete solution by integrating a high power PD interface control with an isolated flyback controller.
Figure 266.1 • High Efficiency, Triple Output, High Power PD Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00266-0
Synchronous flyback controller
Once power is switched over to the synchronous flyback con- troller, the LTC4268-1 regulates the output voltages by sens- ing the average of all the output voltages via a transformer winding during the flyback time. This allows for tight out- put regulation without the use of an opto-isolator, providing improved dynamic response and reliability. Synchronous rec- tification increases the conversion efficiency and cross-regula- tion effectiveness above a conventional flyback topology. No external driver ICs or delay circuits are needed to achieve synchronous rectification; a single resistor is all that is needed to program the synchronous rectifier’s timing.
High efficiency, triple output, high power PD
Figure 266.1 shows a design using the LTC4268-1 in a high power, triple output PD. A high power PSE connects through an Ethernet cable to the RJ45 connector. PSE detection and power is passed through the data pairs’ high power Ether- net transformer or directly to the spare pairs in this 2-pair 10/100BaseT PoE system. The PSE power is then controlled by the LTC4268-1 PD interface and forwarded on to its switching regulator. An auxiliary supply option can also be connected to bypass and disable the PD interface which gives the auxiliary priority in power supply over PoE. Power con- version is then from the auxiliary supply down to the output voltages.
The small supply of the LTC4268-1 utilizes an isolated fly- back topology with synchronous rectification that requires no opto-isolator, lowering the parts count. This circuit gives effi- ciencies at full load of 83% when powered from a PSE and over 85% power sourced from an auxiliary supply.
PSE and auxiliary supplies
Standard PSEs are capable of providing as low as 15.4W at the port output. This would not be sufficient power for a high power PD operating at full load. Here, a customized PSE capable of delivering higher power must be used, or a PSE controller designed for high power such as an LTC4263-1 sin- gle port PSE controller. In cases where a high power PSE is not available, an auxiliary supply can be used.
2-pair vs 4-pair PD
2-pair power is used today in IEEE 802.3af systems. One pair of conductors is used to deliver the current and a second pair is used for the return while two conductor pairs are not pow- ered. This architecture offers the simplest implementation method but suffers from higher cable loss than an equivalent 4-pair system.
4-pair power delivers current to the PD via two conductor pairs in parallel allowing for an even higher level of power.
This lowers the cable resistance but raises the issue of current balance between each conductor pair. Differences in resistance of the transformer, cable and connectors along with differ- ences in diode bridge forward voltage in the PD can cause an imbalance in the currents flowing through each pair. Using two independent LTC4268-1s (Figure 266.2) allows for interfac- ing and power from two independent PSEs, and independent DC/DC converters resolve the current imbalance.
Conclusion
The LTC4268-1 is a highly integrated solution for the next generation of PD products. It offers PoE PD functionality with control for efficient high power delivery to the output load.
Figure 266.2 • 4-Pair, High Power PD Diagram
267
Mark Gurries
Simple battery circuit extends Power over Ethernet (PoE) peak current
adapters or other external power sources. The PoE specifi- cation defines a hardware detection protocol where Power Sourcing Equipment (PSE) is able to identify PoE Powered Devices (PDs), thus allowing full backwards compatibil- ity with non-PoE-aware (legacy) Ethernet devices. The PoE specification also sets an upper limit on the power that can
Introduction
Power over Ethernet (PoE) is a new development that allows for the delivery of power to Ethernet-based devices via standard Ethernet CAT5 cable, precluding the need for wall
Figure 267.1 • Simple Battery Charger/PowerPath Controller (LTC4055) Augments PoE Regulator’s (LTC4267) Peak Output Power to Overcome PoE Power Constraints Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00267-2
be drawn by a PD. The problem is: what happens when a PD must draw more power than allowed by the PoE stand- ard? Examples may be the spin up of a disk drive or a period of sustained transmission of data from an RF transmitter. If the average power load of these applications is less than the available PoE power, one solution is to store power in the PD when power consumption is low and then tap the reserve to augment PoE power when needed. For many applications, a rechargeable battery fits the bill.
Of course, one cannot just throw a battery and a battery charger into the mix. The power path must be able to change seamlessly, on the fly, from PoE-powers-device-and-charges- battery, to PoE-and-battery-power-device, to battery-powers- device. Figure 267.1 shows a complete and compact solution.
The PoE circuit
By default, power over the Ethernet is not available. The standard calls for a protocol to be implemented that allows the Ethernet hub to identify the device needing power. The LTC4267 simplifies the design of PDs by providing whole- sale implementation of the protocol and power management functions.
PoE power comes in the form of −48V at 350mA. If the PoE current is allowed to exceed 400mA, the standard calls for the PSE to break the circuit. This is a problem for devices that occasionally need a little more juice than PoE will offer.
Another problem is that −48V does not easily convert to commonly used positive voltage supply rails. Designers are forced to provide DC isolation along with the inverted down conversion to a more usable voltage. To meet these require- ments, the LTC4267 used in Figure 267.1’s circuit imple- ments an input current limited DC input isolated flyback converter, providing a user-settable regulated low voltage.
The LTC4267 circuit in Figure 267.1 supplies 5V at 1.8A.
5V is a popular supply voltage to run logic, interface with other devices such as USB, and of primary concern in this application, to charge a single Li-Ion cell to its target termina- tion voltage of 4.2V.
PowerPath and charger circuit
In Figure 267.1, the LTC4055 provides triple PowerPath con- trol and Li-Ion battery charging. One path is created by con- necting an external Schottky diode to the LTC4055’s OUT pin and the built-in wall adapter detection circuits. In this case, the “wall adapter” power comes from the LTC4267
power, not used in this application. The third path is the bat- tery discharge path. When the 5V PoE power goes away or drops out of regulation, the LTC4055 automatically switches the battery power over to the OUT pin using its internal ideal diode circuit. There is no delay in the switchover, so power is never lost.
When 5V PoE power is restored, the battery is dis- connected from the load and charging is permitted. The LTC4055 charge current is adjustable and in Figure 267.1, the circuit is limited to 900mA which is drawn from the OUT pin. That leaves 900mA to run the system while charging.
Powered devices connected to the OUT pin must be com- patible with the Li-Ion voltage range. The ACPR pin of the LTC4055 can be used to indicate which power source is pro- viding power, allowing the PD to configure itself accordingly.
High transient load or continuous current load operation
When the power limit of the 5V PoE supply is reached, the voltage drops and the battery charger shuts down to relieve the PSE of the charge current load. If the voltage continues to collapse, the battery automatically is placed into paral- lel operation with the 5V PoE power supply, thus increasing the available peak load current. The LTC4055 ACPR signal is active high during the overload. Battery charging automati- cally resumes once the overload goes away and the 5V PoE voltage has risen enough to show recovery.
Optimization options
If sustained currents approaching 1.8A are expected from the 5V PoE and there are thermal management issues related to the diode’s heat dissipation, the diode D9 can be replaced with the LTC4411 ideal diode for more efficient operation.
Recommended DC/DC converters to generate logic supplies in this application include the LTC3443 buck-boost and/or the LTC3407-2 dual buck regulators.
Conclusion
The highly integrated LTC4267 and LTC4055 simplify the design of compact, simple and complete battery-based power systems that run from Ethernet power. More importantly, seamless PowerPath control enables circuits that can use a battery to augment Ethernet power when an application
268
Dilian Reyes
Fully autonomous IEEE 802.3af Power over Ethernet midspan PSE requires no microcontroller
quad PSE controller designed for both endpoint and midspan PSEs that integrates PD signature detection, power level clas- sification, AC and DC disconnect detection and current limit without the need for a microcontroller.
A PSE’s duties
The responsibilities of the PSE are to correctly detect if a compliant PD has been connected to a port, optionally clas- sify the PD and properly apply power to the PD while
Introduction
The IEEE802.3af Power over Ethernet (PoE) standard defines how power will be delivered over CAT5 lines. Despite the differences between legacy devices and those that adhere to the new standard, there is no need to completely replace existing systems. The nominal 48V required by powered devices (PDs) can be delivered by midspan power sourc- ing equipment (PSE), which is connected to the front end in series with legacy routers and switches. The LTC4259A is a
Figure 268.1 • Autonomous 4-Port Power over Ethernet Midspan PSE Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00268-4
protecting the port from fault conditions. Once a PD is pow- ered on, the PSE monitors a PD’s presence and switches off power when the device is removed. A PSE must also provide overcurrent protection to prevent damage to the PSE and PD.
Traditional PSE solutions use a microcontroller to per- form the detection measurements and calculations and con- trol additional circuitry that switches power to a PD. The LTC4259A in Figure 268.1, by contrast, requires no micro- controller and runs autonomously carrying out signature detection. It automatically interprets the loading conditions and powers on a valid PD.
The midspan PSE also must not interfere with an end- point’s operation. An endpoint PSE applies power on either the signal pairs or the spare pairs of the CAT5 cable, while the midspan PSE must apply power to only the spare pairs.
To avoid conflict if the two were to be connected at the same time, the circuit in Figure 268.2 implements an LTC1726 watchdog timer to periodically disable the LTC4259A’s detec- tion scheme for two seconds. Midspan devices are required to have a backoff capability after a failed attempt of detection to allow for a potentially present endpoint PSE to detect and power on a port.
After the backoff interval is complete, LTC4259A detec- tion is re-enabled for at least one full detection cycle. If a midspan or endpoint PSE is able to detect a valid signature 25kΩ (RSIG) and power up the PD, a compliant PD would no longer display the RSIG to prevent any further good sig- nature detects and power ups from a second PSE. Hardware implementation of the backoff timer eliminates the need for a microcontroller software timing routine.
Disconnect detection
When a PD is unplugged from a powered port, the IEEE standards specify that a PSE must implement at least one
The LTC4259A auto mode uses the AC disconnect method by default. The LT1498 in Figure 268.3 is a dual rail- to-rail op amp used to output a sine wave to drive OSCIN of the LTC4259A. The LTC4259A applies the AC signal to the lines and detects its presence when a PD has been removed and the port power is to be switched off.
Supplying 3.3V from − 48V
A 3.3V supply powers the digital portion of the LTC4259A.
The LTC3803 circuit in Figure 268.4 converts −48V to 3.3V eliminating the need for a second power supply. This boost regulator circuit achieves a tight 2% regulation and outputs 400mA, enough for up to 12 LTC4259As and port indicator LEDs in a 48-port application.
LTC4259A options
Figure 268.2 • Midspan PSE Backoff Timer
Figure 268.3 • Sine Wave Circuit for AC Disconnect
Figure 268.4 • −48V to 3.3V Boost Converter
269
Jesus Rosales
Power over Ethernet isolated power supply delivers 11.5W at
90% efficiency
Ethernet) applications, the choices are few. When trying to maximize efficiency every milliwatt counts. MOSFET gate driving losses become significant so the fewer switches to turn on and off the better. A push-pull converter could be used, but the additional complexity is not justified at this power level. A single transistor forward converter is another option, but requires an additional output inductor and rectifier. A fly- back converter is the simplest choice. Flyback converters are thought to be less efficient than forward and push-pull con- verters, but that changes when the output is synchronously rectified (a MOSFET is used instead of a diode to rectify the output).
The LT1725 switching regulator controller greatly simpli- fies the design of PoE supplies. The LT1725 is specifically designed for the isolated flyback topology and includes fea- tures that make it a good match for PoE supplies, including programmable input undervoltage lockout, hysteretic start-up and a patented feedback circuit that eliminates the need for an optocoupler while providing excellent output regulation.1
Note 1: U.S. Patent No. 05438499, 05305192, 0584163.
When powering IP telephones, wireless access points, PDA charging stations and other PDs (Powered Devices) from an Ethernet cable, designers have at most 12.95W of available power per the IEEE 802.3af standard. Increased demands for power mandate a very efficient power converter especially for class 3 devices (consuming between 6.49W and 12.95W).
The more power lost in the converter, the less power available for the PD.
The voltage available from the PSE (Power Sourcing Equipment) ranges from 44V to 57V, but PDs need to oper- ate with as much as 20Ω of series wire resistance. A PD can never draw more than 350mA or 12.95W continuously. With the maximum input current of 350mARMS, the input voltage can droop as much as 7V bringing the lower side of the input range to 37V. To avoid interfering with the classification signa- ture impedance measurement, a PD must not draw significant current below 30V.
There are many topologies to choose from when design- ing an isolated DC/DC converter, but for PoE (Power over
Figure 269.1 • 36V–72V Input to 3.3V at 3.5A Output, Isolated Synchronous Flyback Converter
http://dx.doi.org/10.1016/B978-0-12-800001-4.00269-6 Analog Circuit Design: Design Note Collection.