Download Bridging the Efficiency Gap in PoE Powered

Bridging the Efficiency Gap in PoE Powered
Device Designs
Application Report
Literature Number: SLUA592
October 1999
Application Report
SLUA592 – October 2008
Bridging the Efficiency Gap in PoE Powered Device Designs
_________________________________________________________________________________________
Donald V. Comiskey
1
Power Management
Introduction
The trend toward higher efficiency is becoming increasingly evident for Power over Ethernet (PoE)
applications. Powered Device (PD) suppliers are looking for solutions that enable them to market their
products as being “Green” or energy efficient. All of the components within the PD’s power path need to
be closely examined to see if any significant improvements in efficiency can be made. This application
report focuses on the full-wave bridge (FWB) rectifier used at the front end of most PD designs. Practical
efficiency gains ranging from 2% to 3.5% are shown to be obtainable by using a FET FWB instead of the
commonly used diode FWB.
2
PD Front-End Circuitry
Figure 1 shows a partial schematic depicting the front-end circuitry commonly used for PoE PD designs.
An input voltage of 48 VDC nominal is applied to the spare line or data line pairs associated with the
RJ-45 input to power the PD. In some nonstandard cases, especially for higher power applications, the
input voltage might be simultaneously applied to both the spare line and data line pairs. In any case, the
PD needs to be insensitive to the polarity of the voltage applied at either of the input pairs, therefore
requiring the two FWB rectifiers shown in the circuit.
The FWB rectifiers at the front end of the PD are typically comprised of standard p-n diodes which can
dissipate an appreciable amount of wasted power and lower the overall efficiency of the PD. The power
dissipated by this type of FWB is equal to the forward voltage drop of two series diodes multiplied by the
input current flowing through these diodes. It’s conceivable that an efficiency increase can be realized by
replacing these diode FWB rectifiers with those constructed using MOSFETs. This application report
explores the benefits of using the FET FWB to increase the overall efficiency of the various PD power
classes. To simplify discussions, the report will assume that the PD is being powered through only one of
the FWB devices, such as the data line pair FWB highlighted in Figure 1.
PoE PD
Data Line Pair
FWB
PoE
Input
1
2
3
4
5
6
7
8
Texas Instruments
TPS23750PWP
RJ-45
1
20
FREQ
19
BL
18
VBIAS
17
MODE
16
AUX
15
SENSP
GATE
14
VDD
COM
13
DET
RSN
12
CLASS
RSP
11
VSS
RTN
PWPD
21
TMR
2
FB
3
COMP
4
SEN
5
9
10
11
14
15
16
6
7
8
9
10
8
7
6
3
2
1
Figure 1. Typical PoE PD front-end circuitry including input FWB rectifiers for polarity insensitivity.
Bridging the Efficiency Gap in PoE Powered Device Designs
1
Application Report
SLUA592 – October 2008
3
Diode FWB vs. FET FWB Operation
Figures 2 and 3 show the operation of the diode FWB for both polarity configurations of the input voltage.
The red highlighted path in Figure 2 indicates that diodes D1 and D4 conduct for the polarity A
configuration, while the blue highlighted path in Figure 3 indicates that diodes D2 and D3 conduct for the
polarity B configuration.
Diode FWB - Input Polarity A
D1
Diode FWB - Input Polarity B
D1
D2
48Vin
Vout
D3
48Vin
Vout
D3
D4
Figure 2. Diode FWB operation for polarity A.
D2
D4
Figure 3. Diode FWB operation for polarity B.
For each of the above configurations, the power dissipation of the diode FWB is equal to the voltage drop
across the two conducting diodes multiplied by the input current:
Pdiss = 2 x VF x Iin
Bridging the Efficiency Gap in PoE Powered Device Designs
2
Application Report
SLUA592 – October 2008
Figures 4 and 5 show the corresponding operation of the FET FWB for both polarity configurations of the
input voltage. Note, in this case, that the upper FETs are P-channel devices and the lower FETs are
N-channel devices. The red highlighted path in Figure 4 indicates that FETs Q1 and Q4 conduct for the
polarity A configuration, while the blue highlighted path in Figure 5 indicates that FETs Q2 and Q3 conduct
for the polarity B configuration. The gate drive components associated with each conducting pair of FETs
are highlighted in green.
FET FWB - Input Polarity A
15V
FET FWB - Input Polarity B
15V
15V
Q1
200K
Q2
Q1
200K
200K
48Vin
Vout
200K
200K
Q3
15V
15V
48Vin
Vout
200K
Q4
15V
200K
Q3
15V
Figure 4. FET FWB operation for polarity A.
Q2
200K
Q4
15V
Figure 5. FET FWB operation for polarity B.
For each configuration, the power dissipation of the FET FWB is equal to the square of the input current
multiplied by the total on-state resistance of the conducting FETs, plus the power dissipated by the active
gate drive components:
Pdiss = ( Iin2 x Rds-on total ) + ( 2 x (P200K res + P15V zener) )
The Rds-on of the FET FWB would normally be selected based on tradeoffs between efficiency gain and
cost for the particular power class of PD involved. The power dissipated by the gate drive components
(the second term of the above equation) is considered to be overhead power dissipation. This overhead
power dissipation, which would be the same for all four PD power classes, should be considered when
selecting the Rds-on for the desired efficiency gain, especially for the lower power classes.
Bridging the Efficiency Gap in PoE Powered Device Designs
3
Application Report
SLUA592 – October 2008
Efficiency Gain of FET FWB over Diode FWB
The graph in Figure 6 shows the efficiency gains that can be obtained when using the FET FWB over the
standard diode FWB. The series of plots indicate how the efficiency gain for each of the four PD power
classes would depend on the selection of the FET FWB Rds-on. The plots take into consideration the
overhead power dissipation discussed in Section 3 above and the different values of VF that would
normally be encountered when using a standard diode FWB at the various current levels associated with
the different PD classes. Operation at 48 VDC, full load, and +25°C has been assumed for each PD class.
Efficiency Gain vs. FET FWB Rds-on
(as com pared to standard diode FWB)
3.75%
Class 1 (3.84W)
3.50%
Class 2 (6.49W)
Class 3 (12.95W)
3.25%
Class 4 (25.5W)
3.00%
2.75%
Efficiency
Gain
2.50%
2.25%
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
2.00%
4.5
4
FET FWB Total Rds-on (ohms)
Vin = 48V
Full Power
+25°C
Figure 6. Efficiency Gain vs. FET FWB Rds-on for the four PD power classes.
Bridging the Efficiency Gap in PoE Powered Device Designs
4
Application Report
SLUA592 – October 2008
Cost-Benefit Analysis of using FET FWB
The decision to use a FET FWB over the standard diode FWB would normally be based on a cost-benefit
analysis of the particular PD application. The graph in Figure 6 uses a minimum efficiency gain of 2% as a
practical criteria for indicating when the benefits provided by a FET FWB may be worthwhile. The cost of
the FET FWB would typically be inversely proportional to Rds-on.
Use of a schottky diode FWB instead of a standard p-n diode FWB might also be considered when
performing a cost-benefit analysis of the PD application. The schottky diode FWB would typically provide
an efficiency gain on the order of 1% and its cost would normally be between that of the standard diode
FWB and the FET FWB.
Figure 7 shows an example of efficiency gain vs. FWB relative cost for the standard diode, schottky diode,
and FET FWB options. The example is based on comparably sized and rated devices offered in an IC
package. The FET FWB option is assumed to have an Rds-on of 1.2 ohms and uses the external gate drive
components discussed in Section 3 above. All of the component costs are based on 1000 pc. pricing
through distribution. This relative cost information on the three FWB options can be compared to the
overall PD cost to provide an overall cost-benefit analysis.
Efficiency Gain vs. FWB Relative Cost
3.5%
Class 1 (3.84W)
3.0%
1.2 ohm
FET
FWB IC
Class 2 (6.49W)
Class 3 (12.95W)
2.5%
Class 4 (25.5W)
2.0%
Schottky
Diode
FWB IC
Standard
Diode
FWB IC
1.5%
Efficiency
Gain
1.0%
0.5%
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.0%
0.5
5
FWB Cost Multiplier
Vin = 48V
Full Power
+25°C
Figure 7. Efficiency gain vs. FWB relative cost example for providing overall cost-benefit analysis of PD.
Bridging the Efficiency Gap in PoE Powered Device Designs
5
Application Report
SLUA592 – October 2008
6
Considerations
The information provided in this report has been based on operation at +25°C. It’s important to note that at
elevated temperatures the efficiency of the standard diode FWB will increase due to decreasing VF, while
the efficiency of the FET FWB will decrease due to increasing Rds-on. It’s not uncommon for the Rds-on to
double over a 100°C increase in junction temperature. This needs to be considered when selecting the
appropriate Rds-on for the desired efficiency gain when using the FET FWB.
7
Summary
As the demand for “Green” and energy efficient PoE devices increases, PD suppliers are looking for cost
effective ways to increase the efficiency of their products. This application report has explored the use of
the FET FWB over the standard diode FWB commonly used at the front-end of a PD. The report shows
that practical efficiency gains ranging from 2% to 3.5% can be obtained when using the FET FWB and
selecting its Rds-on. An example of the relative cost of the various FWB options has also been presented as
an aid in determining the overall cost effectiveness of using the FET FWB in PD designs.
8
References
1) TPS23754: High Power/High Efficiency PoE Interface and DC/DC Controller, Datasheet SLVS885,
Texas Instruments, October 2008
2) TPS23750: Integrated 100-V IEEE 802.3af PD and DC/DC Controller, Datasheet SLVS590A,
Texas Instruments, August 2005
3) IEEE P802.3at Draft 3.1, July 31, 2008
4) Rectifier Bridge Has No 2Vf Drop, Design Ideas, Theta Engineering Inc.
Bridging the Efficiency Gap in PoE Powered Device Designs
6
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,
and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should
obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are
sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard
warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where
mandated by government requirements, testing of all parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right,
or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a
warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual
property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied
by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive
business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional
restrictions.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all
express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not
responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably
be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing
such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be
provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in
such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products
Applications
Audio
www.ti.com/audio
Communications and Telecom www.ti.com/communications
Amplifiers
amplifier.ti.com
Computers and Peripherals
www.ti.com/computers
Data Converters
dataconverter.ti.com
Consumer Electronics
www.ti.com/consumer-apps
DLP® Products
www.dlp.com
Energy and Lighting
www.ti.com/energy
DSP
dsp.ti.com
Industrial
www.ti.com/industrial
Clocks and Timers
www.ti.com/clocks
Medical
www.ti.com/medical
Interface
interface.ti.com
Security
www.ti.com/security
Logic
logic.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Power Mgmt
power.ti.com
Transportation and Automotive www.ti.com/automotive
Microcontrollers
microcontroller.ti.com
Video and Imaging
RFID
www.ti-rfid.com
OMAP Mobile Processors
www.ti.com/omap
Wireless Connctivity
www.ti.com/wirelessconnectivity
TI E2E Community Home Page
www.ti.com/video
e2e.ti.com
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2011, Texas Instruments Incorporated