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

4 Challenge No. 1: Turnoff delay during an output short-circuit

With the increase in load current, more MOSFETs need to be parallel to limit the maximum steady-state MOSFET junction temperature to a safe value (100°C to 125°C). For example, to support a steady-state load current of 150A at an ambient temperature of 70°C, eight Texas Instruments (TI) CSD19536KTT MOSFETs need to be in parallel to limit the steady-state MOSFET junction temperature to 100°C. Paralleled MOSFETs help thermally, but increase the effective capacitance on the GATE pin of the hot-swap controller and impact the turnoff response.

During an output short-circuit, the MOSFETs need to turn off fast enough to prevent further buildup of fault current and avoid damage to the MOSFETs, input power supply, or printed circuit board (PCB). The gate pull-down strength of the TI LM5066I hot-swap controller is limited to 160mA, which is not enough to turn off all eight MOSFETs completely during a short-circuit event, as shown in Figure 5.

Short-circuit response of the LM5066I controller with eight MOSFETs. Figure 5 Short-circuit response of the LM5066I controller with eight MOSFETs.