SLVAEE5 June   2020 TPS23754

 

  1.   Alleviating Short Circuit Thermal Concerns for PoE Powered Devices
    1.     Trademarks
    2. 1 Introduction
    3. 2 Detailed Description Using the TPS23754
    4. 3 Thermal Challenge During an Output Short
    5. 4 Alleviating the Thermal Concern at Short Circuit Condition
      1. 4.1 Maintain Power Signature (MPS)
    6. 5 References

4 Alleviating the Thermal Concern at Short Circuit Condition

Decreasing Cvc will increase the frequency of hiccup OCP. Conversely, the frequency of hiccup OCP will decrease by increasing Cvc, but make longer duration of peak fault current. Whatever increasing or decreasing Cvc, the duty cycle of the hiccup OCP is almost the same. The heat issue could not be alleviated. This paper introduces the way how to adjust the duty cycle of peak fault current duration.

In Figure 7, use the 2N3906’s emitter-base diode in parallel with 5.1k to create a current source as current path (b) as 0.6V/5.1k = 117uA.

The rest of current from internal current source is passed through the current path (a).

TPS23754 Figure 7.pngFigure 7. The External Discrete Hiccup Circuit

The new current source is to create the longer recharging time as OFF time of the hiccup OCP, getting a lower average fault current and alleviating the thermal concern on Synchronous FET or Rectifying Diode at secondary. The waveform is shown in Figure 8 and the thermal picture is shown in Figure 9.

TPS23754 slvaee5-short-circuit-2.pngFigure 8. The Hiccup OCP Waveform with External Hiccup Circuit
TPS23754 figure 9.pngFigure 9. IR Thermal Photo with External Hiccup OCP Circuit