How to make a PoE PD system more efficient by using a flyback converter

PoE sends power over the Ethernet cable by injecting 50 V DC into the center taps of the Ethernet transformers. Power is sent via two or four twisted pairs of cables. One of the key aspects of the IEEE standard is that it allows power to be injected from the Ethernet switch to both positive and negative polarity. Therefore, the PD must have a rectifier bridge at the input to adjust the DC polarity for the system.

Most PoE PD systems require less voltage than the 50V PoE input, and a DC-DC converter is required to reduce the PoE voltage to something usable by the system. PoE input and DC-DC converter are combined to determine the overall performance of the PoE PD system.

Figure 1: PoE PD interface

Flyback converters are one of the most common DC / DC converters in PoE systems due to their efficiency, low cost and isolation. Flyback is commonly used in systems that require 30 watts of power. A flyback converter uses a transformer to transmit power and reduce (or increase) the input voltage. Most Flyback Converter designs use custom transformers to meet efficiency and EMI requirements. The transformer may be an insulating DC/DC converter, and has a very high impedance between the primary side and the secondary side. Separation can reduce ground loops and secure PoE PD devices with user-enabled ports such as USB.

The flyback is based on a transformer that operates with two combined chokes. When the FET is on, current flows through the inductor on the primary side. Since the secondary diode is reverse biased, the secondary inductor does not carry current. This condition is caused by the polarity of the transformer. This is because when the polarities are reversed, the secondary voltage is the opposite of the primary voltage. The controller opens the switch when the current through the primary inductor reaches its peak. The transformer attempts to maintain the magnetic flux by inducing current to the secondary inductor and biasing the diode forward. The current flowing from the inductor then charges the output capacitor and powers the load. The output voltage is mainly determined by the turn’s ratio between the primary and secondary sides of the transformer. When the switch is closed, the output capacitor repeats this cycle and is loaded when the switch is closed.

The efficiency of the overall system is increased by using active clamp and secondary side FET and is discussed below.

Active clamp

Active clamp flyback improves efficiency by recirculating instead of wasting energy. When the main switch is off, the clamp switch sends power from the clamp capacitor to the secondary side just before the main switch turns on. The clamp also ensures that the main switch is on when the VDS is close to zero, reducing switching losses and reducing EMI generation. Active terminals allow low rated voltage synchronous FETs to be activated at the output, reducing ON resistance which reduces costs and further increase efficiency.

Figure 2: Active clamp

Secondary side FET driver

In this configuration, the secondary-side diode is replaced by a FET. The FET controller turns on when the switch is open and off when the switch is closed. This eliminates the voltage drop across the diode and is replaced by the voltage drop across the FET (resistance * current flowing through the FET), thus enhancing the efficiency. In an isolated flyback converter, a pulse transformer is required to send the gate drive signal through the dividing wall to the secondary-side FET. Depending upon the output voltage and power level of the system, the efficiency can be improved by up to 5%.

Figure 3: Secondary side FET

PoE PD Evaluation Board

The STEVAL-POE002V1 reference design is a two-stage converter for a powered device (PD), and is able to deliver up to 40 W (5 V/8 A) DC from appropriate 4-pair PoE power sourcing equipment (PSE), or an external auxiliary supply. The first stage is the Ethernet interface is based on the PM8805 controller with integrated full active bridge rectification. The PoE controller is compliant with the third generation IEEE 802.3bt specification (as well as the preceding 802.3af/at standards), with improved features and increased power capacity that allow new application possibilities for PoE networks, including point of sales and retail logistics devices.