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

5 Design steps

When designing an input protection for the rack server, select the supporting component values for the TPS25984B eFuse from its data sheet and then follow the following design steps for TVS selection. First, select a unidirectional TVS with a reverse standoff voltage equal to or greater than VDC_max. We chose the Littlefuse SMDJ12A diode [4] as a starting point. Next, determine the IP, which is nothing but the circuit breaker current. Then calculate clamping voltage. Because Rd is a function of tP, use Equation 6 to find tP:

Equation 6. t p = L I p V c ( m a x ) - V I N = 100 n H   ×   200 A 20 V - 12 V = 2.5 μ s

For pulse widths below 20μs, you can approximate the dynamic resistance to that at an 8/20μs test pulse. From the SMDJ12A data sheet, our calculations were:

Equation 7. V B R ( m a x = 14.7 V
V C m a x a t I P P 8 / 20 μ s = 25.71 V
I P P ( 8 / 20 μ s ) = 754 A

Therefore:

Equation 8. R d ( 8 / 20 μ s = V C ( m a x ) - V B R ( m a x ) I P P = 25.71 - 14.7 754 = 14.6 m Ω

Now, using the Rd of 14.6mΩ, calculate the clamping voltage:

Equation 9. V C = V B R ( m a x ) + I P     R d = 14.7 V + 200 A   ×   14.6 m Ω = 17.6 V

Because the clamping voltage is less than maximum tolerable voltage, VC(max) (the 20V absolute maximum rating of TPS25984B eFuse), you can proceed further with the SMDJ12A; otherwise, you will have to consider a TVS diode with a lower Rd, or parallel TVS diodes.

Calculate the peak power using:

Equation 10.   P P P = V C     I P = 17.6 V ×   200 A = 3.52 k W

Because the SMDJ12A supports a peak power of 60kW for 2.5µs (see Figure 6), you can proceed further.

Now, derate the power rating with temperature by using Figure 6. The maximum power support at 75° is:

Equation 11. P P P   ×   d e r a t i n g _ f a c t o r = 0.8   ×   60 k W = 48 k W

Because 48kW > 3.52kW and VC < 20V, the SMDJ12A is a good choice for this application.

Peak pulse power rating (left) and peak pulse power derating curve (right). Figure 6 Peak pulse power rating (left) and peak pulse power derating curve (right).

Figure 7 shows the clamping performance of the SMDJ12A on the TPS25984B system.

Transient protection with the SMDJ12A diode at the input of the TPS25984B eFuse. Figure 7 Transient protection with the SMDJ12A diode at the input of the TPS25984B eFuse.