AN2783 datasheet(22/47 Pages) STMICROELECTRONICS

22 / 47 page

Power transformer and operating input voltage

AN2783

22/47

Power transformer and operating input voltage

The PM8800 demonstration kit can contain two different types of power transformers:

Table 4.

Characteristics of the power transformer for the PM8800 evaluation kit

The standard EP13 transformer is designed to operate at full IEEE802.3af power when the

input voltage is in the range 36-72 V. When working from auxiliary voltages lower than 36 V

the output power must be reduced in order to not saturate the transformer.

The custom transformer has been designed to work optimally over the full input range

between 12 V to 60 V in order to exploit the AUXII connection option made available by the

PM8800.

FA2706/7-BL can be also used to draw more power than 10 W when restricting the input

operational voltage range. For example about 20 W can be drawn with a minimum input

voltage of about 30 V, the minimum operating voltage of the PoE standard range.

Figure 10.

Output power vs. VAUXII

An auxiliary winding has been added in the FA2706/7-BL to directly drive a synchronous

power MOSFET as a secondary rectifier, in order to reduce the power losses associated

with a standard diode rectifier at high output currents.

When working at very low input voltage, as in the case of AUXII, the diode D6 can be added,

(actually not mounted on the reference board) to directly supply the VCC pin of PM8800.

Warning:

In this condition please do not increase the AUXII voltage

above 15 V as permanent damage can occur to the device.

Type

Lprimary

Ipeak sat

Rsec

PoE13P-33L / 50L

127

µH

6 / 4

1 A

24 m

Ω/ 39 mΩ

FA2706-BL / 07-BL

µH

6.8 / 4.85

3.5 A

8 m

Ω/ 18.5 mΩ

Pout vs VAUXII w ith FA270x-BL

VAUXII ( V )

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