Calculator Inputs
Example Data Table
The sample below demonstrates a typical high-voltage bus design case. Values are illustrative and should be checked against your topology, mission profile, and capacitor datasheet.
| Case | Power (W) | Vdc (V) | Vmin (V) | Ripple (Vpp) | Hold-Up (ms) | Rectifier | Suggested C (µF) |
|---|---|---|---|---|---|---|---|
| Motor Drive Example | 2200 | 700 | 560 | 20 | 15 | Three-Phase 6-Pulse | 754.00 |
| UPS Front End | 1500 | 400 | 320 | 15 | 20 | Single-Phase Full-Wave | 1596.00 |
| Inverter DC Bus | 5000 | 800 | 650 | 18 | 10 | Custom 900 Hz | 535.00 |
Formula Used
1) DC Bus Load Current
The capacitor sees current based on DC input power, not only output power. Efficiency is included so the bus current better reflects real demand.
I_load = P_out / (η × V_dc)
2) Capacitance from Ripple Limit
This estimates the capacitance needed so bus discharge between charging pulses stays inside the allowed peak-to-peak ripple band.
C_ripple = I_load / (f_ripple × ΔV_pp)
3) Capacitance from Hold-Up Requirement
This uses the available energy difference between the nominal and minimum acceptable bus voltages during constant-power discharge.
C_hold-up = (2 × P_dc × t_hold-up) / (V_dc² − V_min²)
4) Recommended Capacitance
The calculator chooses the stricter requirement, then applies safety and aging multipliers so your final recommendation is more practical.
C_recommended = max(C_ripple, C_hold-up) × Safety Factor × Aging Factor
5) Energy, Hold-Up, and ESR Estimates
These outputs help evaluate stored energy, ripple heating, and how much extra ripple comes from ESR under AC current stress.
E = 0.5 × C × V²
t_hold-up = C × (V_dc² − V_min²) / (2 × P_dc)
P_ESR = I_ripple² × ESR
V_ESR = I_ripple × ESR
How to Use This Calculator
- Enter the converter output power and nominal DC bus voltage.
- Set the minimum bus voltage your system can tolerate during discharge.
- Enter the allowed capacitor ripple, required hold-up time, and efficiency.
- Select the ripple source model or enter a custom ripple frequency.
- Provide switching frequency, ESR, ripple current factor, safety factor, and aging factor.
- Press Calculate DC Link Capacitor to show results above the form.
- Review recommended capacitance, hold-up result, ripple estimate, ESR loss, and the voltage decay graph.
- Download the result set as CSV or PDF for reporting or design review.
FAQs
1) What does this calculator estimate?
It estimates DC link capacitance from ripple and hold-up requirements, then adds design and aging margins. It also reports bus current, stored energy, ESR loss, ripple current demand, and voltage decay over time.
2) Why are two capacitance values calculated?
One value controls bus ripple between charging pulses. The other protects hold-up performance during input dips or interruptions. The higher value dominates the design because the capacitor bank must satisfy both conditions.
3) Why is minimum bus voltage important?
Hold-up energy depends on the voltage window between nominal bus voltage and the lowest acceptable operating voltage. A lower minimum voltage increases usable energy and can reduce the required capacitance.
4) What is ripple current factor?
It is an engineering estimate that relates bus load current to capacitor AC ripple current. Different topologies and modulation methods produce different stress levels, so confirm the final value against measurements or detailed simulation.
5) Does ESR change the required capacitance?
ESR does not directly set the ideal capacitance equations here, but it adds ripple voltage and self-heating. High ESR can make a theoretically correct capacitance behave poorly in real hardware.
6) When should I use a custom ripple frequency?
Use a custom frequency when the bus ripple source is not well represented by simple line-rectifier assumptions. That is common in special front ends, active rectifiers, and custom charging pulse patterns.
7) Should I still check datasheets after using this?
Yes. Always verify voltage rating, surge rating, ripple current capability, temperature derating, lifetime curve, mounting limits, and parallel bank behavior before finalizing the capacitor part number.
8) Can I use multiple capacitors in parallel?
Yes. Designers often parallel capacitors to reach the needed capacitance, reduce ESR, and increase ripple current capability. Keep lead layout short and symmetrical so current sharing stays controlled.