SNVAA94 November   2023 LM5113-Q1 , LMG1205 , LMG1210

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1 Introduction
  5. 2 Bootstrap Overcharge
  6. 3 Modeling Bootstrap Overcharge
  7. 4 Changing Bootstrap Components
  8. 5 Zener Diode Method
  9. 6 Schottky Diode Method
  10. 7 Overvoltage Clamp Method
  11. 8 Active Switch Method
  12. 9 Synchronous GaN Bootstrap Method
  13. 10Other Methods of Preventing Bootstrap Overcharge
    1. 10.1 Reducing Dead Time
    2. 10.2 Opting for a Bias Supply
    3. 10.3 Adjusting for Gate Voltage
  14. 11Summary
  15. 12References

5 Zener Diode Method

A common, simple option to bootstrap overcharging is to place a Zener diode with approximately 5 V breakdown (Vz) parallel to Cboot. Any excess charge is dissipated in the Zener diode once Cboot is charged to Vz rather than charging Cboot.

The Zener diode method prevents the Cboot voltage from exceeding Vz in all conditions compared to the bootstrap option discussed previously. Additionally, 5-V to 6-V Zener diodes have very small temperature coefficients. Thus, the Zener diode method is reliable under different load and temperature conditions.

Another benefit to using this method is that the Zener diode allows for some overcharging. A small amount of overcharging is beneficial, as overcharging cancels the voltage drop in the bootstrap diode. Canceling the voltage drop increases the bootstrap voltage and reduces conduction losses in the high-side FET.

There are drawbacks to using the Zener diode method. First, Zener diodes do not transition instantly to and from breakdown. A Zener diode data sheet often includes a Reverse Voltage versus Current plot. This curve has a knee, which results in extreme variations of the Zener voltage depending on the current it must sink. This drawback presents trade-offs: a low Vz Zener diode has more leakage current at nominal voltage, but a higher Vz diode clamps at a higher voltage. A Zener diode with a low enough Vz to prevent overcharge always adds leakage current at nominal voltages.

Second, the Zener diode power dissipation is excessive if there is low bootstrap resistance to limit Iboot. The power dissipated by the Zener diode is the product of Vz and Iz. Iz approaches Iboot when the bootstrap voltage exceeds the breakdown voltage of the Zener diode. The instantaneous power dissipation exceeds 10 W if Iboot is more than 2 A. High power dissipation hurts efficiency and damages the Zener diode if the ratings are exceeded.