How to select film capacitors

In voltage regulators, the following three types of capacitors are commonly used as voltage input and output bypass capacitors: multilayer ceramic capacitors, solid tantalum electrolytic capacitors, and aluminum electrolytic capacitors. Designers must consider these factors when choosing bypass capacitors, as well as when film capacitors are used in filters, integrators, sequential circuits, and other applications where the actual capacitance value is very important. Improper selection may result in circuit instability, excessive noise and power consumption, shortened product life cycle, and unpredictable circuit behavior.

Selection requirements for film capacitors

1. The rated voltage of the 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. Capacitor working voltage selection:

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 on the surface of the lead welding part of the capacitor end face)

3. Selection of capacitor capacity and lead span:

1) The 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, There are 24 levels in 6.2, 6.8, 7.5, 8.2, 9.1, among which the horizontal lines drawn below are the E12 series values, which are the preferred series values.

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 the capacity, but if the selected value of the 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 pitch 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 pitch F after the formation is given Size requirements.

Common problems of film capacitor selection

A. Improper selection of rated voltage

The rated voltage is improperly selected, and the most current part of the state is the resonant circuit part (C5). R&D personnel should make preliminary calculations after comprehensive consideration of the equipment rated power, input voltage, circuit topology, inverter control mode, load material, load magnetic ratio, circuit Q value and other parameters. After the prototype meets the requirements, you need to use an oscilloscope to add a high-voltage voltage probe, and actually measure the peak-to-peak voltage, peak voltage, rms voltage, resonant frequency and other parameters across the resonant capacitor when the device is at maximum power. Determine whether the selected resonant capacitor model and parameters are correct.

B. Improper selection of rated current

The rated current is improperly selected, and the most common places are C3 (DC support) and C5 (resonance). If the actual current value required is higher than the allowable current value of the capacitor, it will cause serious heating of the capacitor, long-term high temperature T. operation, resulting in a greatly reduced capacitor life, serious explosion or even fire. In the development of equipment, you can measure the actual peak current and the root mean square value of the current through a dedicated current probe or other methods, and then adjust the parameters of the capacitor. Finally, in the full power aging test of the equipment, the temperature rise of a F capacitor is measured, and the selection of the capacitor is determined according to the allowable parameters of the temperature rise of the capacitor. (Comprehensive evaluation by current measurement and temperature rise)

C. Improper way of pressing the line

Improper wiring method mainly occurs in the parallel use of multiple capacitors. Due to the wiring method and the inconsistent wiring distance and other factors, each parallel capacitor is inconsistent in the circuit. In the end, there are multiple capacitors connected in parallel, and the temperature rise of each is different. The temperature rise of capacitors in individual locations is too high, and the state is now burning. Therefore, it is necessary to make reasonable wiring and connection for the parallel use of capacitors, and try to achieve current sharing to improve the service life of capacitors.