Special Switching MOSFET Considerations

Thermal Considerations and OTSD

Frequency Dithering for Conducted Emissions Control

+

-

6.04kW

10kW

301kW

1 F

m

V

B

To

FRS

ARTN

TL331IDBV

ESD

Layout

TPS23754

TPS23754-1

TPS23756

Special care must be used in selecting the converter switching MOSFET. The TPS23756 minimum switching

towards the end of a (failed) bootstrap startup. The MOSFET must be able to carry the anticipated peak fault

current at this gate voltage.

Sources of nearby local PCB heating should be considered during the thermal design. Typical calculations

assume that the TPS23754 is the only heat source contributing to the PCB temperature rise. It is possible for a

normally operating TPS23754 device to experience an OTSD event if it is excessively heated by a nearby

device.

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, IEEE802.3-2005 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 37 modulates the switching frequency by

feeding a small ac signal into the FRS pin. These values may be adapted to suit individual needs.

Figure 37. Frequency Dithering

The TPS23754 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 TPS23754 failures were observed and

operation was continuous. See Figure 29 for additional protection for some test configurations.

ESD requirements for a unit that incorporates the TPS23754 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 TPS23754.

Printed circuit board layout recommendations are provided in the evaluation module (EVM) documentation

available for these devices.

Copyright © 2008–2009, Texas Instruments Incorporated

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Product Folder Link(s): TPS23754 TPS23754-1 TPS23756