Briefly describe the selection requirements of film capacitors

A novice quickly chooses a suitable film capacitor, usually the following requirements:

1. The rated voltage of the capacitor;

2. Selection of the working voltage of the capacitor;

3. Selection of capacitor capacity and lead span.

Film capacitors are capacitors in which metal foil is used as an electrode, and plastic films such as polyethylene, polypropylene, polystyrene or polycarbonate are overlapped from both ends and then wound into a cylindrical structure. According to the type of plastic film, they are called polyethylene capacitors (also known as Mylar capacitors), polypropylene capacitors (also known as PP capacitors), polystyrene capacitors (also known as PS capacitors) and polycarbonate capacitors.

1. Rated voltage of capacitor:

Refers to the peak value of the highest DC voltage or pulse voltage that can be continuously applied to the capacitor within the rated temperature range. Taking into account the reliability and derating requirements, it is usually required that the actual working voltage should be less than 80% of the rated voltage value.

2. Selection of the working voltage of the capacitor:

The pulse voltage and withstand voltage of the capacitor, due to the loss of the film capacitor, when used under high frequency and high pulse conditions, if there is a large current through the capacitor, the film capacitor will heat itself, and there will be thermal breakdown in severe cases. Etc. (smoke, breakdown), so the use is also limited by the rated current of the capacitor. When using, you must ensure that both voltages are within the allowable range. If the actual working voltage (current) waveform cannot be determined, the self-temperature rise of the capacitor can be used to determine it. Generally, for polyester capacitors, the self-temperature rise is allowed to be used under the condition of less than 10C. For polypropylene capacitors, they are allowed to be used when their own temperature rise is less than 5°C. (The actual measurement should be tested on the surface of the lead welding part of the capacitor end surface)

3. Selection of capacitor capacity and lead span:

1) The capacity selection must be within the range of E24 series values: 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, There are 24 levels in 6.2, 6.8, 7.5, 8.2, and 9.1, among which the horizontal line drawn below is the E12 series value, which is the preferred series value.

2) The capacity value range should conform to the capacity range given in the general specifications of various capacitors:

The specifications provided by different manufacturers may have slightly different upper and lower limits of capacity, but if the selected value of capacity is significantly lower than the lower limit of the category, it should be selected from ceramic capacitors, otherwise if the capacity value is already high The upper limit of this category should be selected among electrolytic capacitors.

3) Selection of lead forming pitch:

For film capacitors of different types and specifications, the conventional lead spacing P has a certain value in the manufacturer’s specifications, but in actual use, according to the PCB assembly requirements, the manufacturer can be required to form and supply, and the given pitch F after the formation is Size requirements.