Electrolytic Capacitor Placement And Loop Area Control

An electrolytic capacitor is the largest component in most power circuits and the one whose position has the widest consequences. It sets the area of the loop that the switching current flows around, it is the component that dries out first when it runs hot, and it is usually the tallest part on the board and therefore the one that interferes with everything placed near it. Moving it a few millimetres changes the electromagnetic behaviour, the thermal life and the assembly of the board at the same time.

This article explains why the placement of an electrolytic capacitor is a layout decision rather than a mechanical one, how the loop area and the thermal path are arranged, and how the result is verified.

Why Placement Matters

The capacitor supplies the current that the switching stage draws in pulses, and it supplies it through the loop formed by the capacitor, the switch and the ground return. The inductance of that loop, multiplied by the rate of change of the current, is the voltage spike that the switch and the rest of the circuit have to tolerate. A capacitor placed a few millimetres further away enlarges the loop and raises the spike, which is seen as noise, as ringing and eventually as a component failure.

The same component is also a thermal device. Its life is set by the evaporation of the electrolyte, and that process roughly doubles in rate for every ten degrees of temperature rise. The position of the capacitor relative to the hot parts of the circuit, to the copper that spreads heat and to the airflow in the enclosure therefore decides how long the product will last, and that is a layout decision made once and lived with for the life of the product.

Loop Area And Ripple Current

The loop that matters is the one that carries the pulsating current, and it includes the capacitor, the switching device, the ground connection between them and any sense resistor in the path. Keeping the loop small means placing the capacitor so that its terminals face the switch and returning the ground directly beneath the current path rather than around the outside of the circuit.

The ripple current also has a rating. A capacitor has a maximum ripple current at a stated frequency and temperature, and exceeding it raises the internal temperature and shortens life. Placing the capacitor where it shares the current with a parallel part, or where the impedance of the connection forces more of the current through one device, changes the ripple that each one sees. Parallel capacitors placed asymmetrically will not share equally, and the one nearest the switch will run hotter.

Heat, Life And Airflow

The internal heating of the capacitor is the product of its equivalent series resistance and the square of the ripple current, and it adds to the heat that arrives from the board and from the surrounding air. A capacitor placed next to a hot inductor or a hot diode receives both, and the copper that connects it conducts heat in from the rest of the circuit as well.

The remedies are geometric. The capacitor is placed upstream of the hot components in the direction of airflow, so that it sees air that has not already been warmed. Where the board is enclosed, a thermal via field under the capacitor and a copper area on the other side act as a heat sink, and a small amount of clearance from the hot parts allows convection to work. Measuring the case temperature of the capacitor in the finished assembly is the only way to know whether the arrangement succeeded.

Bulk And Local Decoupling

An electrolytic capacitor is a bulk device and it is not the right part for high frequency decoupling, because its equivalent series inductance makes it ineffective above a few hundred kilohertz. The ceramic capacitors placed at the pins of the switching device do that job, and the electrolytic one supplies the energy that the ceramics cannot store. The two work as a pair, and the layout has to keep the ceramic close to the device and the electrolytic close to the ceramic, so that the current path between them is short.

The arrangement of the whole power stage, including the inductor, the sense resistor and the feedback path, is described under DC-DC converter layout and routing, and the noise that the same loop radiates is discussed under radiated EMI in switching regulator layout. Where the switching noise couples into the rest of the design, the general measures are set out under EMI suppression design principles.

Mechanical And Assembly Considerations

An electrolytic capacitor is usually the tallest component on the board and sometimes the heaviest. On a board that is assembled on both sides, it has to be placed so that it does not interfere with the reflow of the opposite side or with the support pins of the oven. On a board that is wave soldered it has to be placed so that the wave does not shadow it, and on a board that is conformally coated it has to be placed so that the coating can reach every part of its body and its seal.

The mechanical fixing also matters for a large can. A capacitor that is only held by its leads will vibrate, and the vibration fatigues the leads and the solder joints. An adhesive bead, a clip or a mounting ring is used where the can is large or where the product is exposed to vibration, and the space for that fixing has to be reserved in the layout rather than found later.

Verification

Verification is thermal and electrical. The case temperature of the capacitor is measured with the product running at full load and at the highest ambient it will see, and the result is compared with the temperature used in the life calculation. The ripple current is measured with a current probe around the capacitor lead or with a shunt in series, and the figure is compared with the rating.

The third check is the waveform. A capacitor that is working correctly shows a small, smooth voltage ripple, while one that is too far from the switch or has too little capacitance shows a large ripple with high frequency content on top of it. The shape of the waveform, viewed at the capacitor terminals, is the most direct evidence that the placement has done its job.

FAQ

How close should the electrolytic capacitor be to the switch? As close as the layout allows, with the positive terminal facing the current source and the ground return beneath it. The distance is limited by the loop area target rather than by a fixed number of millimetres.

Can two smaller capacitors replace one large one? They can, and two placed symmetrically often share the current better than one, but each has its own equivalent series resistance and the total ripple rating is not simply the sum.

Does the capacitor have to be on the same side as the switch? It is better if it is, because a via adds inductance to the loop. Placing it on the opposite side adds two vias and increases the spike that the switch sees.