DC Link Capacitor Calculator

The DC bus is the energy store that smooths the rectifier ripple on the input side and absorbs switching ripple on the inverter side. It keeps the bus voltage from collapsing during regeneration (motor braking) and sustains motor flux during supply dips. Without it the inverter draws lumpy current from the rectifier and experiences poor torque control — the capacitor is what makes PWM inversion work.

Two formulas: the ripple formula C = P × Δt / (V × ΔV), where Δt ≈ 1/(2·fsw) for SVPWM, and the hold-up formula C = 2 × P × t_hold / (V² − V_min²). Compute both and use the larger. Most industrial VFDs use 8–12% ripple; EV inverters use 3–5% ripple. Higher switching frequency reduces Cmin directly.

Every switching cycle charges and discharges the bus capacitor by ΔQ = C × ΔV. With SVPWM modulation the ripple current at the switching frequency can be 20–30% of the output phase current RMS. This is why DC link caps are almost always aluminum electrolytics or polymer capacitors — they handle high RMS ripple current better than ceramic of the same energy content.

When the inverter powers up, the DC link is empty so it looks like a dead short to the rectifier, drawing 100+ A inrush that can blow the input fuse or damage diodes. A precharge resistor in series with the DC bus limits this to a safe charging current (~10 A) while the capacitor charges, then a contactor or relay bypasses it. τ = R × C ≈ 1–2 s is standard — the resistor only conducts during power-up, so power rating is modest (V²/R for a few seconds).

Film capacitors handle higher RMS ripple and lower ESR but cost ~5× per microfarad. They are used in EV and solar inverters where reliability trumps cost. Aluminum electrolytics deliver far more capacitance per dollar but have limited ripple current and finite life (~5000–10 000 h). Most grid-tie inverters parallel strings of 100–470 µF 400 V aluminum electrolytics for the DC bus.