A Triple-Output ±5% Regulated High Efficiency POE (Power over Ethernet) Supply
General Description
The maximum input power available to a POE power supply depends on the PD power classification. Maximum PD power is 12.95 watts for a class 0 PD power classification. The maximum PD power for a class 1 PD power classification is 3.85 watts. This application note describes a triple-output, 3.12 watts, high-efficiency POE power supply with ±5% regulation on all the outputs.
Description of Power Supply Operation
Figure 1 is the circuit diagram of the triple-output POE power supply using the MAX5941A IEEE802.3af-compliant POE interface plus a current-mode PWM controller(U2). The power supply has a flyback topology with synchronous rectifiers on the 3.3V and 2.5V output, and a buck converter with synchronous rectification for the 1.2V output of the 3.3V. The circuit diagram does not include the diode bridge portion of the input section.
IEEE802.3af-Compliant POE Interface Section of the Circuit
One half of the MAX5941A IC is dedicated to the POE interface section. The DC output from the diode bridge appears across capacitor C10. Zener diode D4 prevents overvoltages at the MAX5941A. The MAX5941A's internal hot-swap MOSFET turns on at 39V and slowly charges capacitor C6 to 40V. The inrush current is limited by the controlled turn-on of the MOSFET. The PGOOD signal goes high when capacitor C6 is almost fully charged to the input voltage. R10 is the PD detection resistor and R25 is the classification resistor.
PWM Section
The second half of the MAX5941A is the PWM section. The PWM starts operation when power is applied to the PWM portion of the controller. Soft-start capacitor C14 slowly increases the peak primary current in the power transformer.
Figure 1. Schematic of power supply
| Designator | QTY | Description |
| C1, C5, C7 | 3 | Ceramic capacitor 100µF, 6.3V, X5R, 10% (1210) |
| C10 | 1 | Ceramic capacitor 0.1µF, 100V, X7R, 20% (1206) |
| C11 | 1 | Ceramic capacitor 0.47µF, 16V, X7R, 20%, (0805) |
| C12, C15, C17, C19, C20 | 5 | Ceramic capacitor 0.1µF, 16V, X7R, 20% (0603) |
| C13 | 1 | Ceramic capacitor 4700pF, 100V, X7R, 20% (0603) |
| C14 | 1 | Ceramic capacitor 1000pF, 50V, X7R, 20% (0603) |
| C16 | 1 | Ceramic capacitor 0.33µF, 25V, X7R, 20% (0603) |
| C18 | 1 | Ceramic capacitor 47pF, 50V, COG, 20% (0603) |
| C2 | Electrolytic capacitor 4.7µF, 35V | |
| C21 | 1 | Ceramic capacitor 47µF, 6.3V, X5R, 10% (121) |
| C24, C25, C4 | 3 | Ceramic capacitor 0.1µF, 16V, X7R, 20% (0603) |
| C26 | 1 | Ceramic capacitor 220pF, 50V, X7$, 20% (0603) |
| C6 | Electrolytic capacitor 22µ, 63V | |
| Panasonic EEVFK1220XP | ||
| C8 | 1 | Panasonic EEVFK1220XP |
| C9 | 1 | Ceramic capacitor 2.2nF, 250VAC, X7R, 20% (2220) |
| D1, D2, D3, D6 | 4 | Diode 1N4148W (SOD323) |
| D4 | 1 | Zener Diode SMBJ54 (SMB) |
| D5-D8 | 2 | Schottky Diode BAT54 (SOT23) |
| L1 | 1 | Inductor 4.7µH, Coilcraft DO1608C-472 |
| L2, L3 | 2 | Inductor 1µH, Coilcraft DO1608C-102 |
| Q1 | 1 | NPN small signal transistor MMBT3904 (SOT23) |
| Q2 | 1 | n channel MOSFET 5A, 150V (DPAK) |
| Fairchild Semiconductor FQD5N15 | ||
| Q3 | 1 | PNP small signal transistor MMBT3907 (SOT23) |
| Q6 | 1 | n Channel, logic-level, power trench MOSFET (SOT23) |
| R1 | 1 | Resistor 4.75k, 1% (0603) |
| R10 | 1 | Resistor 25.5k, 1% (1206) |
| R11 | Open | |
| R12 | 1 | Resistor 221Ω, 1% (0603) |
| R14 | 1 | Resistor 2.10K, 1% (0603) |
| R16 | 1 | Resistor 33Ω, 1% (0603) |
| R17, R21 | 2 | Resistor 1k, 1% (0603) |
| R18 | 1 | Resistor 22Ω, 1% (0603) |
| R19 | 1 | Resistor 1.5Ω, 1% (0805) |
| R2 | 1 | Resistor 47Ω, 1% (0603) |
| R22 | 1 | Resistor 7.87k, 1% (0603) |
| R22 | 1 | Resistor 1M, 1% (0603) |
| R23 | 1 | Resistor 1M, 1% (0603) |
| R24 | 1 | Resistor 2.32k, 1% (0603) |
| R25 | 1 | Resistor 255Ω, 1% (0805) |
| R26 | 1 | Resistor 10k, 1% (0603) |
| R3 | 1 | Resistor 100Ω, 1% (0603) |
| R4 | 1 | Resistor 11.30k, 1% (0603) |
| R6, R7 | 2 | Resistor 10kΩ, 1% (0603) |
| R8 | 1 | Resistor 49.9k, 1% (0603) |
| R9 | 1 | Resistor 22.60k, 1% (0603) |
| T1 | 1 | Custom transformer Coilcraft C1154-B |
| T2 | 1 | Gate-drive transformer Pulse Engineering PA0184 |
| U1 | 1 | Dual n Channel 2.5V (G-S) MOSFET (SO8) |
| Vishay Si9926BDY | ||
| U2 | 1 | IEEE802.3aF-compliant POE/PWM Controller (SO16) |
| Maxim MAX5941A | ||
| U3 | 1 | 8-Pin SO Error Amplifier Optocoupler (SO8) |
| Fairchild Semiconductor FOD2712 | ||
| U4 | 1 | 3A 1MHZ Buck Regulator with Internal Switches (QSOP16) |
| Maxim MAX8505 |
Features
- 275kHz switching frequency
- IEEE802.3af-compliant power source
- Utilizes an IEEE802.3af-compliant POE interface plus PWM controller in a single 16-pin SO package (MAX5941A)
- Flyback topology for the power supply
- Synchronous rectification on all outputs
- Isolated outputs with an isolation voltage of 1500VAC
- ±5% regulation on all outputs
- Output +3.3V at 0.2A, +2.5V at 0.6A, and +1.2V at 0.8A
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