Designing a power supply that meets the demands of Power over Ethernet (PoE) and Voice over Internet Protocol (VoIP) applications can be challenging. The low component-count power supply depicted in the figure meets these specifications without the need for complicated circuitry.
The circuit shows a typical converter for a powered device (PD) as defined by the PoE standard. The schematic includes the necessary circuitry for "discovery" and "classification," along with a DP423G high-voltage power-conversion IC implementing the dc-dc converter.
When an input voltage is first applied in the range of 2.5 to 10 V dc, the PD must present the correct "discovery" signature impedance. R1 supplies this impedance. As the input voltage reaches the 15- to 20-V dc range, R1 also programs a "classification" current draw between 0.5 and 4 mA, identifying this as a Class "0" PD.
At voltages above approximately 30 V dc (the total of the zener diode VR1 voltage, R2 voltage, and Q1 gate-source threshold voltage), the MOSFET pass-switch (Q1) turns on and connects the input voltage to the power supply. Zener diode VR2 limits the Q1 gate-source voltage to safe levels when the input voltage is high (greater than 43 V dc).
Resistor R3 prevents spurious turn-on, and along with VR1 and R2, it should be chosen to prevent Q1 turn-on below 28 V dc. Resistor R2 limits the power dissipation in zener diodes VR1 and VR2—the higher the resistance, the lower the dissipation.
The DPA423G (U1) switches at 400 kHz. Resistor R5 programs in the undervoltage and overvoltage thresholds. Resistor R6 controls the U1 current limit, and with R4, it provides a relatively flat output-power limit versus input voltage. When the drain of the device turns on, energy is stored in transformer T1.
At drain turn-off, the transformer voltage inverts (a clamp is provided by zener VR3 in parallel with capacitor C3) and is rectified by D1. Parallel output capacitors C7 and C8 filter the output and share the output ripple current. A post filter (L2, C9) attenuates switching ripple, with a high-frequency decoupling capacitor (C10) near the output pins.
The overall power supply achieves an efficiency of 78%, an impressive feat for such a simple flyback supply delivering 3.3 V at 2 A. For most applications, the cost and simplicity may well be the most important features. If situations dictate, efficiency could be increased by approximately 5% to 10% by changing the topology to a synchronous-rectified forward converter and moving up to the DPA424G, a higher-power device with lower conduction losses.
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