Bootstrap Capacitor
Cboot = Qg / ΔV
Quick answer
This calculator solves Cboot = Qg / ΔV for Bootstrap Capacitance. With Gate Charge 10 nC, Allowed Droop 0.6 V, the result is 16.67 nF. It solves in 3 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
In a high-side gate drive, a bootstrap capacitor stores the charge that turns the high-side MOSFET on. It must be large enough that delivering the gate charge does not droop its voltage below the level needed to keep the MOSFET fully enhanced. This formula gives the minimum bootstrap capacitance for an allowed droop; designers typically use 10–20× this value for margin.
Variables
- Cboot — Bootstrap capacitance (F)
- Qg — Total gate charge of the high-side FET (C)
- ΔV — Allowed bootstrap voltage droop (V)
Practical Notes
Add the high-side quiescent and leakage charge over the on-time for a complete estimate. A factor of 10–20× the minimum keeps the gate drive stiff; too large slows bootstrap refresh at startup.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| Cboot | Bootstrap Capacitance | nF | pF, nF, µF, mF, F |
| Qg | Gate Charge | nC | pC, nC, µC, mC, C |
| dV | Allowed Droop | mV | pV, nV, µV, mV, V, kV |
Step by step
- 1. What you know: Gate Charge 10 nC, Allowed Droop 0.6 V
- 2. The relation: Cboot = Qg / ΔV
- 3. Substitute: Qg = 10 nC, dV = 0.6 V
- 4. Result: Bootstrap Capacitance = 16.67 nF
Nearest standard component values
| Series | Nearest value | Error |
|---|---|---|
| E12 | 18 nF | 8% |
| E24 | 16 nF | 4% |
Worked examples
Minimum range
Given: Gate Charge 100 pC, Allowed Droop 10 mV
Result: Bootstrap Capacitance = 10 nF
Low range
Given: Gate Charge 1 nC, Allowed Droop 100 mV
Result: Bootstrap Capacitance = 10 nF
Typical
Given: Gate Charge 10 nC, Allowed Droop 0.6 V
Result: Bootstrap Capacitance = 16.67 nF
High range
Given: Gate Charge 100 nC, Allowed Droop 0.9 V
Result: Bootstrap Capacitance = 111.1 nF
Maximum range
Given: Gate Charge 1 µC, Allowed Droop 9 V
Result: Bootstrap Capacitance = 111.1 nF
Solving for each variable
Solve for Bootstrap Capacitance
Gate Charge 100 pC, Allowed Droop 10 mV → 10 nF
Solve for Gate Charge
Bootstrap Capacitance 100 pF, Allowed Droop 10 mV → 1 pC
Solve for Allowed Droop
Bootstrap Capacitance 100 pF, Gate Charge 100 pC → 1 V
How Bootstrap Capacitance changes with Gate Charge
| Gate Charge (input) | Bootstrap Capacitance (result) |
|---|---|
| 1 nC | 1.667 nF |
| 2.5 nC | 4.167 nF |
| 5 nC | 8.333 nF |
| 7.5 nC | 12.5 nF |
| 10 nC | 16.67 nF |
| 15 nC | 25 nF |
| 20 nC | 33.33 nF |
| 40 nC | 66.67 nF |
| 70 nC | 116.7 nF |
| 100 nC | 0.1667 µF |
The same result in other units
| Unit | Value |
|---|---|
| pF | 16670 |
| nF | 16.67 |
| µF | 0.01667 |
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<p><a href="https://vbatpower.com/formulas/bootstrap-capacitor/">Bootstrap Capacitor</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Bootstrap Capacitor?
Use Cboot = Qg / ΔV. With Gate Charge 10 nC, Allowed Droop 0.6 V, Bootstrap Capacitance works out to 16.67 nF.
What is the formula for Bootstrap Capacitor?
The formula is Cboot = Qg / ΔV. In a high-side gate drive, a bootstrap capacitor stores the charge that turns the high-side MOSFET on.
How do you find Gate Charge when you know the other values?
Rearrange for Gate Charge: with Bootstrap Capacitance 100 pF, Allowed Droop 10 mV, Gate Charge is 1 pC.
How do you find Allowed Droop when you know the other values?
Rearrange for Allowed Droop: with Bootstrap Capacitance 100 pF, Gate Charge 100 pC, Allowed Droop is 1 V.
What units does Bootstrap Capacitor use?
Bootstrap Capacitance is shown in pF, nF, µF. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
Related Concepts
All Semiconductors formulas →Need more features?
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