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

3.1.1 Mode 1

In this mode, Q1 is on and Q2 is off. The equivalent circuit of this mode is shown in Figure 3-3.

Because when Q1 turns on, FET can be modeled as a resistor, the value is equal to RDSON. V1 can charge the bootstrap capacitor, so the voltage in bootstrap capacitor can increase gradually. One thing we can take into consideration is in this mode, inductor current can continue flowing from the ground to HS point. A voltage drop is created by the inductor current, furthermore, the HS voltage refer to the ground can be negative. This phenomenon can let bootstrap capacitor charge to a higher value. If we assume the voltage drop in bootstrap diode is Vd1 and Vd2 in body diode of FET, we can obtain the equation of voltage value in bootstrap capacitor, which is:

Equation 1. V CBOOT 1 = V 1 - V d 1 + V RDSON

VCBOOT1: voltage in bootstrap capacitor in mode 1

Vd1: forward voltage drop of bootstrap diode

VRDSON: voltage drop in FET

GUID-20240313-SS0I-T450-QQ35-Q07HV7TBH8K5-low.svg Figure 3-3 Equivalent Circuit in Mode 1

But in practice, a certain amount of time is needed to charge the bootstrap capacitor, if the time is limited, voltage in bootstrap diode can not reach the theoretic maximum value VCBOOT1. Also, a bootstrap capacitor can not be charged if the capacitor has been charged to a value higher than VCBOOT1 in mode 2 or 4.