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PM8805 датащи(PDF) 36 Page - STMicroelectronics

номер детали PM8805
подробное описание детали  IEEE 802.3bt PoE-PD interface with embedded dual active bridge
PDF  45 Pages
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производитель  STMICROELECTRONICS [STMicroelectronics]
домашняя страница  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM8805 датащи(HTML) 36 Page - STMicroelectronics

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Thermal aspects
PM8805
36/45
DS12813 Rev 1
5
Thermal aspects
5.1
Power dissipation
In this paragraph some worst case calculations are performed in order to show the thermal
behavior of the PM8805 in 2-pair and 4-pair PoE operation modes.
Assumptions:
– Worst case current on the 4 MOSFETs at the 4 corner of package: 1A
– MOSFET Ron at 25C 90 mΩ, 1.6x at 125C = 150mΩ
– Worst case current on the hot swap MOSFET: 1A per each 2-pairs
– PoE input voltage= 54 V
– Controller quiescent current consumption= 2 mA
Figure 11 shows four possible 2-pair configurations depending on the polarity and the
Alternative by the PSE.
The max. total power dissipated on each of the 2-pair cases is:
Equation 4
Figure 11. 2-pair power dissipation configurations
The wide central pad has a total thermal load of 0.11+0.15=0.26 W, but it is connected to a
wide copper plane of the PCB (VOUT signal) and not considered critical.
The hot swap is estimated to dissipate about 0.15 W, and can be considered the most
critical point of the device together with the one powered corner pad, also having a thermal
Ptot_2P = 2x ((1A)
2 x 0.15Ohm) + ((1A)2 x 0.15Ohm) + (54V x 2mA)
= 0.3W + 0.15W + 0.11W = 0.56W



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