Since Power Source Equipment like Hubs with Power over Ethernet (PoE) capability are emerging, "Out of the Box" applications for PoE are getting developed such as network/web cameras, security access control, IP Phones, WLAN Access Points and so forth.
Many of these applications are very price sensitive, since they are consumer. To serve the Powered Devices (PD) listed above with a supply voltage of 3.3V or 5V, it is necessary to have a buck converter that scales down the available input voltage coming from the power source equipment (PSE). The supply voltage provided by the PSE is in the range of 30V to 57V. Due to the fact that the source voltage is below 60V, many PD can be operated with a low cost non-isolated buck converter. The featured design, based upon the idea posted by John Betten and Robert Kollman 'Gate-drive method extends supply's input range' (EDN June 26 2003), implements a level shifter and discrete FET driver circuitry. The fundamental difference is that the controller used here is a dual PWM with maximum voltage rating of 50V. To operate this particular low cost dual PWM controller at up to 57V it is necessary to effectively extend its voltage range. A standard zener diode D1 with a breakdown voltage of 12V is used to lower the maximum voltage seen by the TL1451A. Thus the controller will be exposed to a maximum voltage of 45V when the PSE is sourcing at its maximum voltage rating. When the input voltage is applied, current flows through the zener diode D1 and enables the control circuit once the zener voltage plus the UVLO voltage of 3.6V of the TL1451A have been exceeded. Thus the applied voltage must be at least 15.6V to start the converter. The source-switched drivers for the two driver stages consisting of Q4 (Q7), Q1 (Q6) and Q3 (Q8) then begin driving power switches Q2 and Q5. The open collector output of the TL1451A switches to a low state to turn on the main power switch Q2 (Q5). With the base of the bipolar transistors Q4 (Q7) held at 6.8V and the output pin of the TL1451A low, current will flow through Q4's (Q7's) collector-emitter junction.
Transistors Q1 (Q6) and Q3 (Q8) form an NPN/PNP push-pull driver used to switch gate drive current into and out of Q2 (Q5) quickly. This drive circuit is very fast as none of the transistors operate in a saturated mode. This means reasonably high operating frequencies are achievable with the low duty cycles encountered at a high voltage input.
C13 in conjunction with R32 has been implemented to form a soft-start circuit to limit peak currents in the main power switches Q2 (Q5) and limit peak overshoot of output voltage upon turn on. During turn on C13 is in a discharged state and if the reference voltage VREF will be turned on the voltage seen at the Dead Time Control (DTC) pins will almost equal VREF. This will force the controller to limit the DTC to 0%. Due to R32 the capacitor C13 will be charged and the DTC nodes will slowly fall towards ground potential and therefore slowly release the duty cycle limitation which yields to a kind of soft start function. This allows using fairly small main power switches for Q2 (Q5).
An obvious drawback is that during short-circuit or serious overload condition at the output, the main power switches Q2 (Q5) potentially will be damaged and potentially allow the input voltage to appear at the corresponding output pin. It is possible to implement an over-current protection circuit to limit the maximum output current and protect the switching main power switches Q2 (Q5).
To ease the design a Power over Ethernet Powered Device Controller implements the initialization and current limitation. The TPS2375 is a PoE PD controller in 8-Pin package with all features required to develop an IEEE 802.3af compliant PD. The TPS2375 is a second-generation PD controller featuring 100V rating with a 580m switch in the return path and a fixed 450mA current limit function.
The efficiency curves in Figure 2 are reflecting both output currents summed up to a common current for input voltages of 32V, 42V, 48V and 57V. The load current is increased proportionally from both outputs, up to the respective maximum value of each output.
Figure 1: Dual Buck Converter uses Switched Emitter Gate Drive and Zener Diode for extending the controllers input voltage operating range
Figure 2: Efficiency curves are reflecting both output currents summed up to a common current
IC1: Dual Low Cost PWM Controller, TL1451A, Texas Instruments
C1, C7: Aluminum Capacitor CLZ-Series, 47μF, 63V, Vishay
C2, C3: Ceramic Capacitor, 0.1μF, 50V, X7R
C4, C10: Aluminum Capacitor CLZ-Series, 470μF, 6.3V, Vishay
C5, C11: Ceramic Capacitor, 10μF, 6.3V, X7R
C6: Ceramic Capacitor, 0.1μF, 100V, X7R
C8, C9: Ceramic Capacitor, 6800pF
C12, C16: Ceramic Capacitor, 1μF, 16V, X7R
C13: Ceramic Capacitor, 0.1μF, 16V, X7R
C15: Ceramic Capacitor, 330pF
C21, C22: Ceramic Capacitor, 100pF
D1: Diode, Zener, 12V, BZX84C12, Vishay
D2: Diode, Zener, 6.8V, BZX84C6V8, Vishay
D3, D4: Diode, Schottky, MBRS360T3, On-Semi
D5: Diode, TVS, 58V, SMAJ58A, Vishay
Q1, Q4, Q6, Q7: Bipolar, NPN, BC846B, Vishay
Q2, Q5: Mosfet, P-Channel, Si2309DS, Vishay
Q3, Q8: Bipolar, PNP, BC856B, Vishay
R1: 33kΩ, 1/16W, 1%
R2, R12: 1kΩ, 1/16W, 1%
R3: 16.5kΩ, 1/16W, 1%
R4, R11, R18, R19: 100kΩ, 1/16W, 1%
R5, R6, R8, R9, R15: 10kΩ, 1/16W, 1%
R7, R10: 442Ω, 1/16W, 1%
R13: 16.5kΩ, 1/16W, 1%
R14: 10kΩ, 1/16W, 1%
R17: 5.49kΩ, 1/16W, 1%
R27: 4.42kΩ, 1/16W, 1%
R28: 178kΩ, 1/16W, 1%
R29: 24.9kΩ, 1/16W, 1%
R32: 200kΩ, 1/16W, 1%
L2: Inductor, 100μH, 2.2A, 220mΩ, WE-PD XL, 74477020, Wurth
L4: Inductor, 47μH, 2.7A, 100mΩ, WE-PD XL, 744770147, Wurth
* SLVS525 TPS2375 IEEE 802.3af PoE Powered Device Controller
* SLVU108 TPS2375EVM Power-Over-Ethernet Powered Device (PD) Evaluation Module
* SLVS024E TL1451A Dual Pulse-With-Modulation Control Circuit
* EDN June 26th 2003; Gate-drive method extends supply's input range; by John Betten and Robert Kollman
About the author
Dirk Gehrke joined Texas Instruments in 1997, where he has worked as a field applications engineer providing technical support for power supply designs in the U.S., United Kingdom and France. Dirk has authored more than 30 articles, papers and seminar topics. He graduated from the FH Dortmund with a degree in communications engineering.