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Given PoE’s cost sensitivity, the natural choice is to use the LM5072 to build a flyback converter. The flyback solution in fact offers a multiple isolated output with the right compromise between cost and efficiency and with typical output power between a few watts up to 20-30 watts. Though flyback converters at low power

levels are usually operated in discontinuous conduction mode (DCM), the best efficiency is obtained in the continuous conduction mode (CCM) where, for a given output power, the RMS current in the primary side FET is smaller. As a drawback it presents a right-half-plane zero of close loop transfer function that forces it to roll off the cross-zero-frequency at a relatively low frequency, increasing the dynamic response time of the close loop. Based on the specs shown in Table 1, a basic dimensioning for a flyback converter in CCM is provided. The selection of the correct duty cycle in nominal condition is an important point in the flyback dimensioning since an increase of maximum duty cycle reduces the peak current on the primary side, resulting in lower overall rms current and lower losses.

Vin

Vout1 (main) lout1

Vout2 (aux) lout2

Min Nom 26V 36V 3.3V

Max 48V

0.1A - 3A 10V

1mA - 10mA

Table 1.PoE power converter voltage and current requirements

At the same time the increase of the duty cycle increases the maximum stress voltage between drain to source of the main switching MOSFET, and increases the peak current on the secondary side. The Duty cycle (D) that occurs at nominal input voltage condition can be defined by maximizing the utilisation factor. The utilisation factor (UI) is the total output power divided by the total maximum stress (UI) of the switching MOSFETs. Considering the PoE power levels and input voltage range, it is possible to minimise the FET utilisation factor by choosing a duty cycle for nominal working condition of about 0.4, that translates in to an optimal primary/secondary transformer winding turn ration of 6:1 and a FET with a voltage rating of 80V.

Primary inductance There are several criterions for selecting the primary and secondary inductances, but in practice it’s a compromise between transformer size, peak currents, and RHP zero frequency. All these parameters are functions of duty cycle, load, inductance and input voltage. Figure 3 represents the effects of the primary inductance on primary and secondary currents, and RHP zero for the specified PoE application. The primary inductance and the duty cycle will influence the right half plane zero (RHZ). From Figure 3 it is evident that by increasing the inductance the ripple currents are reduced, therefore input/output ripple voltage and capacitor size could be reduced as well, but increasing the inductance increases the number of primary and secondary windings of the transformer, resulting in

Figure 3: Primary, secondary ripple current, RHP zero versus primary inductance of a typical CCM flyback design

higher transformer losses and size, and it also reduces the RHP zero frequency. The RHZ adds a phase lag of the close loop control characteristic forcing the maximum cross over frequency to be at most 1/4 RHP frequency, resulting in slower transient response and control bandwidth. Common sense suggests not to oversize the inductance otherwise it will compromise the overall close loop performance of the entire system, the size and losses of the flyback-transformer.

A basic overview of the PoE IEEE802.3af has been presented and some of the implementation problems highlighted. An efficient way to solve these issues is to leverage the high integration level of the LM5072 controller enhancing IEEE802.3af compliance design with the minimum number of external components and

providing PWM control to implement a continuous conduction mode flyback converter.

When the LM5072 is used in a CCM flyback supply designed for an output power of 13W with a diode rectifier, the power supply will typically be above 80% efficient at an input voltage of 48V and an output voltage of 3.3V, (the efficiency will increase or decrease as the output voltage respectively increases or decreases) resulting in the best way to squeeze the maximum power out of your existing LAN infrastructure while keeping the system simple and flexible.

National Semiconductor | www.national.com

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Components in Electronics July/August 2011 41

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