SLUAAT9 January   2024 UCC27201A

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Start-Up Operation
  5. 2Design Considerations
  6. 3Proper Sizing of Bootstrap and VDD Capacitors
  7. 4High Power Continuous Operation
  8. 5Design Considerations
  9. 6Summary
  10. 7References

2 Design Considerations

There are considerations in the design to help reduce the stress on the boot diode and ways to mitigate possible damage to the boot diode in the bidirectional DC-DC applications.

Firstly, proper sizing of the bootstrap capacitor is important to properly driving the power MOSFETs and also resulting in the fastest start-up of the HB-HS bias. We do not recommend oversizing the bootstrap capacitor in these applications.

There are additional design considerations that can address the gate driver and bootstrap start-up as follows:

  1. Incorporate a low frequency LO pulse on the phase that is not actively delivering power. This makes sure that the HB-HS bias is fully precharged prior to the phase being enabled.
  2. Make sure that when a phase is enabled, that the 1st initial LO pulse is a long pulse adequate to charge the bootstrap capacitor where the boot diode forward current has gone to zero or near zero. Random enabling of the phase during the normal HI and LI pulse widths can result in short LI initial pulse widths leading to high forward current flowing in the boot diode and then higher reverse recovery current when HS goes high.
  3. Add an external Schottky diode in parallel with the driver internal boot diode. Use a small package diode to allow close placement to the IC VDD and HB pins with low VF and low RDYN.