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A typical calculation of a bootstrap capacitor’s value

A typical calculation of a bootstrap capacitor’s value

Infineon_Team
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The bootstrap capacitor on the high side of a gate driver provides the energy required to turn on a high-side power switch. The bootstrap capacitor discharges as the gate driver tries to turn on the high-side power switch.

The value of, the total electric charge required to turn on a power switch, is crucial. Infineon_Team_1-1713179649570.png can be expressed as:

Infineon_Team_2-1713179728086.png

which,

Infineon_Team_3-1713179745238.png is the gate charge,

Infineon_Team_4-1713179745240.png is the quiescent current volume responsible for supplying gate driver turn ON a power switch,

Infineon_Team_5-1713179745240.png is the volume of leakage current from the bootstrap-capacitor to the driver IC, subsequently passes to the IC’s ground,

Infineon_Team_6-1713179745240.png is the maximum duty of the frequency,

Infineon_Team_7-1713179745241.png is the operation frequency.

            Therefore, the value of the bootstrap capacitor can be derived as:

Infineon_Team_8-1713179827312.png

which the

Infineon_Team_9-1713179838826.png is the voltage of ripple on bootstrap-capacitor. The value of "D" can be set to "1" in order to take the margin into consideration.

In most cases, the magnitudes of 

Infineon_Team_10-1713179838826.pngand

Infineon_Team_11-1713179838827.png range from a few Pico-amperes to a few nano-amperes. As a result, (2) can be approximated as,

Infineon_Team_13-1713180002643.png

Furthermore, the volume of

 

Infineon_Team_14-1713180011560.png  is generally less than or equal to 0.1V to ensure proper functioning of the high-side signal. Therefore, (3) can be expressed as,
Infineon_Team_15-1713180046127.png

Additionally, consider the following recommendations:

  1. Use a multilayer ceramic capacitor (MLCC) as a bootstrap capacitor.
  2. Place the bootstrap capacitor close to the driver IC with a short pathway.
  3. When determining the value of the bootstrap capacitor, consider the DC bias characteristic of the MLCC.
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