Converter Topology Choices:

Flyback Vs. Forward

Flyback or forward converter topologies are normally used in a

powered device DC-DC converter, depending on the PD’s voltage and

current requirements.

Flyback Topology

(See Figure 1)

B

Beenneeffiitt:: Lowest cost for <6A output currents.

W

Whheenn ttoo U

Ussee:: Typically for output voltages greater than 2.5V.

Recommended for multi-output, voltages (>12V) and for applications

requiring lowest cost.

Advantages:

• Output isolated from input.

• No output inductor required.

Disadvantages:

Higher output ripple current results in higher output capacitor cost

and medium efficiency due to higher peak and rms currents.

Figure 1. Transformer Flyback Topology

PoE - the Cost-Effective Means of Power Supply

In addition to data communications between the two devices, Power-

over-Ethernet (PoE) enables an Ethernet Switch/Hub to power a

remote device over an existing LAN cable. Without PoE, any remote

device needs to be powered independently. Typically, device power is

derived from an AC-DC adapter or at board level inside the device.

This adds cost and requires locating the device near a source of AC

power.

Device installation is further complicated by different safety codes,

AC voltages and wall plugs around the world. PoE avoids these

problems by supplying low-voltage DC power directly to the

Powered

Device (PD) from the Power Sourcing Equipment (PSE) that is located

in the Ethernet Switcher or mid-span Hub and eliminates the

complexity of AC power sources.

To ensure that all equipment is protected and devices operate

properly when connected, the IEEE established a universal standard

(IEEE802.3af). This specifies the power requirements of the PSE and

the PD, and the communication protocol between the two -

regardless of manufacturer or configuration.

Traditional Power-over-Ethernet

The 12.95W power limit was adequate for IP telephony when

IEEE802.3af was initially approved in 2003. Network cameras, were

initially the most prevalent application enabled by the Power-over-

Ethernet standard.

However, the popularization of WLAN applications triggered

developers’ and marketers’ inventiveness for creating more power-

hungry derivative devices, such as Video IP Phones, Multi-channel

WLAN Access Points (like the ones in the IEEE802.11n upcoming

MIMO standard) and Pan-Tilt-Zoom IP Cameras.

All these devices typically require between 13W and 30W - more

than the normally available 13W.

Enhancing Power-over-Ethernet

In December, 2004, the IEEE802 established a PoEPlus group to

study the market needs and possible technical solutions to address

higher power applications while maintaining backward compatibility

with the original standard. In September, 2005, the group set and

approved, as binding, objectives for the IEEE802.3at Power-over-

Ethernet Enhancements. One of the main objectives was providing

maximum power to PDs as allowed within practical limits of at least

30W.

Power-over-Ethernet (PoE) Application Notes

Input

Output

Vo

+

_

C

Vin

vL

Vx

+

_

+

_