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49.9kW

10kW4.99kW

6.04kW

10kW

0.01mF

301kW

1 Fm

FRS

ARTN

TL331IDBV

FBU

FBL

TLV431

FromRegulated

OutputVoltage

TPS23754

TPS23754-1

TPS23756

SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013

www.ti.com

Figure 36. Error Amplifier Softstart

Special Switching MOSFET Considerations

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

MOSFET V

GATE

is about 5.5 V, which is due to the V

lower threshold. This will occur during an output overload,

or toward 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 TPS23754 device 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.

Frequency Dithering for Conducted Emissions Control

The international standard CISPR 22 (and adopted versions) are often used as a requirement for conducted

emissions. Ethernet cables are covered as a telecommunication port in 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 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