By Why Capacitor Selection Deserves Care
The capacitor is often the component that decides the life of a power converter. An electrolytic DC link dries out, an under-rated snubber fails to clamp a spike, and a motor capacitor of the wrong value overheats the winding. Choosing the right part early is far cheaper than a field failure, which is why a structured selection process pays for itself. This guide walks through a repeatable method for selecting a Faratronic film capacitor, using DC-Link, snubber and AC motor capacitors as the candidate set.
Step 1: Decide the Function
Start with the job the capacitor has to do. A DC-Link capacitor holds the bus voltage steady and supplies the switch current, so its design is driven by ripple current and energy storage. A snubber capacitor clamps the voltage spike across a switch, so its design is driven by loop energy and equivalent series inductance. A motor run capacitor sets the phase shift in a single-phase motor, so its design is driven by capacitance accuracy and end-of-life safety. The function determines which parameters matter and which datasheet figures to read first.
Voltage Class
The voltage class follows from the circuit, not the nominal voltage. A DC-Link capacitor must survive the bus voltage plus the switching overshoot, so a designer leaves at least fifteen percent margin between the worst-case peak and the rating. A snubber must survive the spike it exists to clamp, so its rating is set by the loop inductance and the peak current. Never choose the class from the average voltage alone.
Step 2: Set the Capacitance
For a DC link, the capacitance is set by the allowable bus ripple and confirmed by the ripple-current rating. For a snubber, the capacitance is set by the energy to absorb and the acceptable clamp level. For a motor run capacitor, the capacitance comes from the motor maker specification, because the wrong value changes torque and winding current. In every case, the capacitance and the ripple or pulse current are checked together, not separately.
Ripple and Pulse Current
The current a capacitor can carry is limited by its ESR and its thermal path. A film capacitor has a much lower ESR than an electrolytic, so it carries far more ripple current for the same case size, which is why it lasts longer in a high-frequency converter. Check the current at the hot case temperature, not at 25 degrees Celsius.
Step 3: Choose the Case and the Connection
The case follows the space and the environment. A PCB plastic case suits a compact board, an aluminum case with screw terminals suits a high-current power stack, and an explosion-proof aluminum cylinder suits a compressor. The connection matters as much as the case: the shortest loop from the capacitor to the switch gives the cleanest bus voltage and the most effective snubber. The BeiLuo FAE team helps finalise the choice and validate it on the bench.