How is the capacitance tolerance of film capacitors controlled

How are film capacitors with low capacitance tolerance produced?

Generally, after film capacitors are manufactured, their actual capacitance varies—some may be slightly higher or lower than the nominal value. The finished capacitors undergo a sorting process where a machine automatically measures the capacitance deviation. To obtain capacitors with tighter tolerances, the sorting machine can automatically select only those that meet the specific requirements; however, this increases production costs, naturally resulting in a higher price for the capacitors.

Is a smaller capacitance tolerance always better for film capacitors?

Capacitance tolerance is a crucial parameter when selecting film capacitors. It refers to the deviation between a capacitor's actual capacitance and its nominal value, typically expressed as a percentage (e.g., 1% or 5%). Generally, a smaller tolerance is preferable, as it enhances circuit precision and stability.

However, a smaller tolerance is not always ideal. In certain applications, an excessively low tolerance can lead to issues. For instance, when a circuit utilizes multiple film capacitors, having extremely tight tolerances across all of them means their actual capacitance values may be nearly identical; this can potentially trigger problems such as circuit resonance.

Therefore, the selection of film capacitors requires comprehensive consideration of the specific application scenario. Generally, for applications demanding high precision and stability—such as precision measuring instruments or audio amplifiers—capacitance tolerance should be as low as possible. Conversely, in applications where multiple film capacitors are used to form a circuit, the tolerance should not be excessively low; otherwise, issues such as circuit resonance could arise.

There are four common types of errors in film capacitors

1. During the winding process, periodically measure the capacitance and height of the winding element to control capacitance variation. If testing occurs shortly after packaging, the capacitance may shift after a period of storage—often resulting in an increase (positive shift)—a phenomenon particularly noticeable in polyester film capacitors.

2. Monitor the capacitance distribution of finished products and provide immediate feedback on any deviations so that the capacitance adjustment value can be modified.

3. Perform spot checks on the capacitance of elements after flattening and shaping; if deviations are detected, promptly adjust the target winding capacitance value.

4. To accurately determine the target winding capacitance (also known as the correction value), the rate of capacitance change occurring during hot/cold pressing, heat treatment, and packaging must be factored into the element's capacitance correction value. These adjustment values vary depending on the model, specifications, and even the specific winding machine used.

Solutions for addressing capacitance deviation in film capacitors

Control deviation values during testing based on observed patterns; for instance, if polyester film capacitors tend to exhibit positive deviation, the tolerance criteria for negative deviation can be relaxed.

Address manufacturing process factors that cause capacitance deviation by subjecting encapsulated products to an additional heat treatment.

Allow the capacitors to sit undisturbed for one week after encapsulation before testing. For wound film capacitors, capacitance (C) is determined by the formula C = 0.177 εS/d, where ε is the dielectric constant of the material, S is the effective electrode area, and d is the dielectric thickness. Capacitance is directly proportional to ε and S, and inversely proportional to d.

Although specifications are precisely defined, these three parameters can fluctuate during the manufacturing process, resulting in capacitance deviation. The focus of the production process is to minimize these deviations and improve the rate at which target capacitance values are achieved.

Winding process: Inherent variations exist in the film layer's width and thickness. Issues include tension fluctuations as the roll diameter changes and tension errors across winding machines. The pressure roller force is insufficient. Misalignment or edge deviation occurs during the winding process. High air humidity causes the core to swell (increase in effective diameter).

Hot-pressing process: Uneven pressure distribution due to core thickness variations leads to inconsistent core tightness and a wide spread in capacity. Uneven hot-press platens.

Temperature deviation. Errors in heat treatment time or temperature, soaking, vacuum level, timing, and curing temperature.