SLYT862 March   2025 LM5066I , TPS25984B

 

  1.   1
  2. 1 Introduction
  3. 2 Understanding EOS
  4. 3 Enterprise server system example
  5. 4 TVS diode selection
  6. 5 Design steps
  7. 6 Output Schottky diode selection
  8. 7 Placement and PCB layout considerations
  9. 8 Conclusion
  10. 9 References
  11. 10Related Websites

3 Enterprise server system example

eFuses are widely used in rack server modules at the front end for input protection and to enable hot-swapping functionality. Figure 2 shows the typical power distribution architecture of a rack server, where the input comes from a 12V backplane and is then distributed from the eFuse to all downstream loads. The power path, which involves the backplane, PCB traces and interfacing connectors, introduces parasitic inductance (L) that then creates unintended transient voltages during fault events.

A typical block diagram of the power distribution of a 12V rack server. Figure 2 A typical block diagram of the power distribution of a 12V rack server.

Let’s quantitatively analyze the impact of L on the eFuse, as illustrated in Figure 3. In the case of an output short circuit, the eFuse interrupts a large amount of current instantaneously from approximately 200A (overcurrent) to 0A (shutoff for protection) within 1µs, resulting in a large current transient (di/dt), as shown in Equation 1:

Equation 1. d i / d t   =   ( 0 A   –   200 A ) / 1 µ s   =   – 2   ×   10 8 A / s

This current will be trapped as energy in the parasitic inductance and produce a surge, expressed by Equation 2:

Equation 2. V L   =   L   ×   d i / d t   =   100 n H     – 2   ×   10 8 A / s   =   – 20 V

That –20V surge will be in series with the 12V input power supply and will effectively create a positive voltage spike of 32V, exceeding the 20V VIN absolute maximum rating of the TPS25984B eFuse from Texas Instruments (TI). Similarly, the output inductance creates a negative voltage spike on the output.

To prevent this, a transient voltage suppressor (TVS) diode will clamp the voltages on the positive side, while a low-forward-voltage freewheeling Schottky diode will clamp the voltages on the negative side. Careful selection of these components is necessary to ensure reliable system protection.

Inductive kickback voltages caused by an abrupt interruption of fault current in a power switch. Figure 3 Inductive kickback voltages caused by an abrupt interruption of fault current in a power switch.