Datasheet

+
-
49.9kW
10kW4.99kW
6.04kW
10kW
0.01mF
301kW
1 Fm
V
B
To
FRS
ARTN
TL331IDBV
TPS23757
www.ti.com SLVS948D –JULY 2009–REVISED NOVEMBER 2013
Special Switching MOSFET Considerations
Special care must be used in selecting the converter switching MOSFET. The TPS23757 minimum switching
MOSFET V
GATE
is ~5.5 V, which is due to the V
C
lower threshold. This will occur during an output overload, or
towards the end of a (failed) bootstrap startup. The MOSFET must be able to carry the anticipated peak fault
current at this gate voltage.
Thermal Considerations and OTSD
Sources of nearby local PCB heating should be considered during the thermal design. Typical calculations
assume that the TPS23757 is the only heat source contributing to the PCB temperature rise. It is possible for a
normally operating TPS23757 device to experience an OTSD event if it is excessively heated by a nearby
device.
Frequency Dithering for Conducted Emissions Control
The international standard CISPR 22 (and adopted versions) is often used as a requirement for conducted
emissions. Ethernet cables are covered as a telecommunication port under section 5.2 for conducted emissions.
Meeting EMI requirements is often a challenge, with the lower limits of Class B being especially hard. Circuit
board layout, filtering, and snubbing various nodes in the power circuit are the first layer of control techniques. A
more detailed discussion of EMI control is presented in Practical Guidelines to Designing an EMI Compliant PoE
Powered Device With Isolated Flyback, TI literature number SLUA469. Additionally, IEEE802.3at sections 33.3
and 33.4 have requirements for noise injected onto the Ethernet cable based on compatibility with data
transmission.
Occasionally, a technique referred to as frequency dithering is utilized to provide additional EMI measurement
reduction. The switching frequency is modulated to spread the narrowband individual harmonics across a wider
bandwidth, thus lowering peak measurements. The circuit of Figure 32 modulates the switching frequency by
feeding a small ac signal into the FRS pin. These values may be adapted to suit individual needs.
Figure 32. Frequency Dithering
ESD
The TPS23757 has been tested to EN61000-4-2 using a power supply based on Figure 1. The levels used were
8 kV contact discharge and 15 kV air discharge. Surges were applied between the PoE input and the dc output,
between the adapter input and the dc output, between the adapter and the PoE inputs, and to the dc output with
respect to earth. Tests were done both powered and unpowered. No TPS23757 failures were observed and
operation was continuous. See Figure 24 for additional protection for some test configurations.
ESD requirements for a unit that incorporates the TPS23757 have a much broader scope and operational
implications than are used in TI’s testing. Unit-level requirements should not be confused with reference design
testing that only validates the ruggedness of the TPS23757.
Layout
Printed circuit board layout recommendations are provided in the evaluation module (EVM) documentation
available for these devices.
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