Overview
A switch-mode power supply turns the mains into one or more regulated DC rails, and in almost every stage a capacitor sets the performance: the input bulk holds the rectified bus, the DC link smooths the intermediate voltage, and the output capacitor sets the ripple that the load sees. Choosing the wrong capacitor raises the ripple, wastes board space and shortens the life of the whole product. CapXon builds the aluminum electrolytic capacitors, and BeiLuo supplies them with genuine traceability and FAE support.
Input Bulk Capacitance
The input bulk capacitor sits across the rectified mains and performs two jobs: it smooths the ripple from the rectifier and it holds the bus up for a short time after the mains dips. The capacitance is chosen from the acceptable bus ripple and the hold-up time, and the voltage rating must exceed the peak bus with margin for the surge and the regulation tolerance. For a high-voltage primary a slim radial such as the CapXon KY series keeps a small diameter, while a snap-in capacitor such as the HU series is the choice when the bus is lower and the capacitance is higher.
Inrush and Hold-Up
A large bulk capacitor draws an inrush current at power-up, so an inrush limiter is normally fitted ahead of the capacitor to keep the diode and the switch within their ratings. The same capacitor provides the hold-up that carries the control through a brief mains drop, so the capacitance trade is between a larger, more expensive capacitor and a longer ride-through. Confirm the inrush and hold-up with the real load, because both depend on the converter.
The DC Link and Intermediate Bus
Inside the supply the intermediate DC link carries the switching ripple, so it needs a capacitor with a high enough ripple current rating and a low enough ESR for the switching frequency. A snap-in capacitor such as the CapXon HU series offers a high capacitance in a compact can with no clamp, which keeps the assembly simple, and its standardized terminal spacing drops into a common footprint. Where the link voltage is high, the UJ series adds a 5000-hour life for continuous duty.
Output Filtering and Ripple
The output capacitor sets the ripple the load sees, and the ESR often dominates at the switching frequency, so a low-ESR part and a short output loop matter as much as the capacitance value. Place the output capacitor close to the load, keep the loop small and use a part rated for the ripple at the case temperature. For a low-voltage rail a high-CV snap-in such as the CapXon HU103M010N200A packs the capacitance into a small can.
Ripple Rating and Frequency
The ripple current rating depends on the frequency, and a capacitor rated at 120 Hz delivers a higher permissible ripple at the switching frequency, so use the frequency multiplier in the datasheet rather than the headline number. Verify the actual ripple on the bench, because a low-ESR part carries more ripple than the calculation suggests.
Lifetime and Thermal Design
The life of an aluminum electrolytic capacitor depends strongly on its core temperature, so the thermal design is part of the capacitor choice, not an afterthought. Keep the ripple within the rating, space the capacitors for airflow and keep them away from the hottest components, and choose a long-life series such as the CapXon LE range where the equipment runs continuously.
Ripple Heating
The ripple current heats the core above the ambient, and the temperature rise depends on the ESR and the can size, so a bigger can runs cooler for the same ripple. Measure the case temperature at worst-case load and ambient, then compare it with the datasheet derating to confirm the margin. A capacitor that runs cooler lasts far longer.
Choosing the Case and Terminal
Match the capacitor to the mechanical design: a snap-in part mounts directly on the board without a clamp and suits a larger can, while a radial leaded part keeps a small footprint for a dense board. The lead or terminal spacing must fit the footprint, and the case height must clear the enclosure. Choosing the case early avoids a late mechanical change.
Getting Help
Send your bus voltage, ripple, hold-up and life requirements to our FAE team and we will propose a capacitor shortlist with the capacitance, voltage, case and lifetime, then quote from stock. BeiLuo holds mainstream CapXon snap-in and radial models in regional warehouses and ships every order with an import declaration, a certificate of origin and a RoHS compliance file, so a supply design can move from prototype to production without a supply gap. Engineer-level support covers the whole selection, from the first calculation through validation and volume supply.