SLYT863 April   2025 LM5066I

 

  1.   1
  2. 1 Introduction
  3. 2 3
  4. 3 Challenges in designing a hot-swap circuit for a 48V AI server
  5. 4 Challenge No. 1: Turnoff delay during an output short-circuit
  6. 5 Challenge No. 2: False gate turn-off during a load transient
  7. 6 Challenge No. 3: Parallel resonance during controlled (slow) turn-on
  8. 7 Proposed circuit enhancements
  9. 8 Improving the turn-off response
  10. 9 Overcoming false turn-off for dynamic loads
  11. 10Damping parasitic oscillations
  12. 11Design guidelines and component selection
  13. 12Cdv/dt discharge circuit
  14. 13Conclusion
  15. 14References
  16. 15Related Websites

12 Cdv/dt discharge circuit

Figure 8 uses a 100V signal diode for DSS. The diode should handle a few tens of milliamperes of forward current. The 8kW hot-swap reference design uses the BAV16W-7-F from Diodes Inc.

You will have to select RSS1, RSS2 and QSS iteratively so that none of the three components become stressed during turn-off. For QSS, you can select any standard PNP transistor with collector-emitter (VCEO) and collector-base (VBEO) voltages of >100VDC and a continuous collector current of >200mA. Select the values for RSS1 and RSS2 and their respective power ratings to limit the current flowing through the QSS transistor to a safe value. You must use a special high-power resistor for RSS2 to manage the transient peak power stress during turn-off. The 8kW hot-swap reference design uses onsemi MMBT5401LT1G for QSS, with RSS1 = 100Ω and RSS2 = 499Ω (the Vishay RCS0805499RFKEA).

Input transient voltage suppression (TVS) diodes are required to protect against transient overvoltages during input hot-plug and output short-circuit events. The TI TVS diode recommendation tool can help you obtain the part number (voltage and power ratings) of the TVS diode and the number of TVS diodes to parallel. The 8kW hot-swap reference design uses three Littelfuse 8.0SMDJ60A TVS diodes. For a deeper analysis into TVS diode selection, see Reference [3].

You will need output Schottky diodes to protect the output pin of the hot-swap controller against a negative transient in the event of an output short-circuit event. The 8kW hot-swap reference design uses three onsemi FSV20100V Schottky diodes.