SLUAAV9 March   2024 LM76003 , UCC27282 , UCC27288

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Design and Potential Risk in Certain Application Scenario
  6. 3Analysis of Potential Problem
    1. 3.1 High Duty Cycle Causes High Current Stress in Bootstrap Diode
      1. 3.1.1 Mode 1
      2. 3.1.2 Mode 2
      3. 3.1.3 Mode 3
      4. 3.1.4 Mode 4
    2. 3.2 Influence by the Extra Voltage Source
  7. 4Design Recommendation
  8. 5Summary
  9. 6References

3.1.2 Mode 2

Mode 2 is a dead time period after Q­1 turns off and Q2 still remains off. This mode is similar to mode 1. The only difference is that the inductor current can flow through body diode of Q1. And in most cases, the voltage drop in body diode is higher than the VRDSON. This means that voltage difference between bootstrap capacitor can higher than the difference in mode 1 and may overcharge the bootstrap capactitor. Using equation to express bootstrap voltage value is:

Equation 2. VCBOOT2=V1-Vd1+Vd2

VCBOOT2: voltage in bootstrap capacitor in mode 2

Vd1: forward voltage drop of bootstrap diode

Vd2: voltage drop of MOSFET body diode

Because Vd2 is larger than VRDSON, so VCBOOT2 can be larger than the VCBOOT1. In facts, the voltage in bootstrap capacitor can be the maximum value in this mode if we do not count the potential oscillation in HS to ground voltage. Figure 3-4 shows equivalent circuits in mode 2.

GUID-20240313-SS0I-BP6L-4Z7X-3KXPMKGG1GGN-low.svgFigure 3-4 Equivalent Circuit in Mode 2