Buck Converter Design Form
Use the form below to size inductance, estimate ripple, compare topology losses, and evaluate optional selected parts.
Example Data Table
| Example Case | Vin | Vout | Iout | Frequency | Ripple Target | Designed L | Estimated Efficiency |
|---|---|---|---|---|---|---|---|
| Industrial 24V to 12V rail | 24 V | 12 V | 5 A | 250 kHz | 30% | 16.0 µH | 96.9% |
| Battery to logic rail | 12 V | 5 V | 3 A | 400 kHz | 25% | 8.9 µH | 95.8% |
| Embedded control supply | 9 V | 3.3 V | 2 A | 500 kHz | 35% | 5.5 µH | 93.6% |
Formula Used
1. Ideal duty cycle
Dideal = Vout / Vin
2. Non-ideal duty cycle
Asynchronous stage: D = (Vout + Vd + Iout × DCR) / (Vin - Iout × RHS + Vd)
Synchronous stage: D = (Vout + Iout × (RLS + DCR)) / (Vin - Iout × RHS + Iout × RLS)
3. Inductance for ripple target
L = VL,on × D / (fsw × ΔIL)
4. Critical conduction boundary
Lcritical = (1 - D) × Rload / (2 × fsw)
5. Output capacitor sizing
Cmin = ΔIL / (8 × fsw × (ΔVtarget - ΔIL × ESR))
6. Switching loss estimate
Psw = 0.5 × Vin × Iout × (tr + tf) × fsw
How to Use This Calculator
Step 1
Enter the source voltage, required output voltage, load current, and switching frequency. These values define the operating point.
Step 2
Choose synchronous or asynchronous operation. Then enter conduction losses, ESR, diode drop, and switching transition times for realistic efficiency estimates.
Step 3
Set the desired inductor ripple percentage and allowed output ripple. The calculator sizes the inductor and minimum output capacitance.
Step 4
Optionally enter selected inductance and capacitance values. This helps compare purchased parts against the design targets and predicted operating mode.
Step 5
Press the calculate button. Review the summary chips, detailed tables, warning notes, and export the result set as CSV or PDF.
Frequently Asked Questions
1. What ripple percentage is common for a buck inductor?
A common design target is 20% to 40% of load current. Lower ripple can reduce output noise, while higher ripple may reduce inductor size.
2. Why does the calculator show both ideal and actual duty cycle?
Ideal duty cycle ignores losses. Actual duty cycle includes switch resistance, diode drop, and inductor DCR, so it better matches practical hardware.
3. What happens when selected inductance is below the critical value?
The converter can move toward discontinuous conduction. Ripple rises, peak current increases, and small-signal behavior changes compared with continuous conduction mode.
4. Why is capacitor ESR important?
ESR creates an immediate ripple component equal to ripple current times ESR. If ESR is large, capacitance alone cannot meet a tight ripple target.
5. Can this tool replace a full simulation?
No. It is excellent for first-pass design and component screening, but parasitics, control loop behavior, layout, and temperature still need deeper verification.
6. Why are switching losses estimated separately?
Conduction and transition losses scale differently. Rise and fall times become increasingly important as voltage, current, and switching frequency increase.
7. Should I use synchronous or asynchronous mode?
Synchronous stages usually improve efficiency at higher current. Asynchronous stages can stay simpler and cheaper, especially when load current is modest.
8. What should I validate after using this calculator?
Check thermal rise, controller limits, current rating, saturation margin, compensation, startup behavior, and PCB layout before releasing the design.