A brief overview of the requirements for the film capacitors selection

To quickly select a suitable film capacitors, there are usually the following requirements:

1. The rated voltage of the capacitor;

2. Capacitor operating voltage selection;

3. Selection of capacitor capacity and lead span.

Film capacitors are electrodes made of foil, which are made of plastic films such as polyethyl ester, polypropylene, polystyrene or polycarbonate, which overlap from both ends and are coiled into cylinder-shaped capacitors. Depending on the type of plastic film is also known as polyethyl ester capacitor (also known as Mylar capacitor), polypropylene capacitor (also known as PP capacitor), polystyrene capacitor (also known as PS capacitor) and polycarbonate capacitor.

1. Capacitor's rated voltage: refers to the peak of the maximum DC voltage or pulse voltage that can be applied continuously to the capacitor within the rated temperature range. Given the reliability derate requirements, it is generally required that the actual operating voltage should be less than 80% of the rated voltage value.

2. Capacitor operating voltage selection:

Through the pulse voltage and voltage resistance of the capacitor, due to the loss of film capacitors, when used in high frequency and high pulse conditions, if there is a larger current through the capacitor will cause the film capacitors themselves to heat up, in severe cases there will be a risk of hot breakdown and so on (smoke, breakdown), so the use is also limited by the capacitor rated current. Use must ensure that both voltages are within the permissible range. If the actual operating voltage (current) waveform cannot be determined, the self temperature rise of the capacitor can be determined, usually for polyester capacitors, allowing the self-temperature rise to be used at a condition of less than 10C. For polypropylene capacitors, it is allowed to be used at a temperature rise of less than 5 degrees C. (The actual measurement should be tested on the surface of the lead welding site of the capacitor end face)

3. Selection of capacitor capacity and lead span:

1) Capacity selection must meet the E24 series value range: 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1 a total of 24 levels, of which the horizontal line below is the E12 series value, the preferred series value.

2) The capacity range should conform to the capacity range given in the general specifications of various capacitors: the upper and lower limits of the capacity may be slightly different from the specifications provided by different manufacturers, but if the capacity selection value is significantly lower than the lower limit of the category, it should be selected in the ceramic capacitor, and vice versa, if the capacity value is higher than the upper limit of the category, it should be selected in the electrolytic capacitor.

3) Selection of lead molding foot spacing: different models of different specifications of film capacitors, its lead conventional spacing P in the manufacturer's specifications have a definite value, but in practice, according to PCB assembly requirements, manufacturers can be required to mold supply, given the size requirements of the molding after foot distance F.