Main functions and applications of thin-film capacitors in AC circuits

I. Film capacitors play the following key roles in AC circuits :

1. Coupling/DC blocking :

Utilizing its characteristic of "allowing alternating current and blocking direct current," it transmits signals from one circuit stage to another while blocking the DC bias voltage between the two stages. It is commonly used in audio and radio frequency circuits.

2. Filtering (directly related to capacitive reactance) :

Power supply filtering : Combined with inductors or resistors, it forms an RC or LC filter network to bypass ripple noise in AC power (such as rectified pulsating DC) to ground, outputting smooth DC power. PP film capacitors are commonly used in such applications due to their low loss and stability.

Noise suppression/bypass : Short-circuit high-frequency interference noise (with a very high frequency f, therefore Xc is very small) to ground to prevent it from interfering with sensitive circuits. Small-capacity film capacitors are commonly used.

3. Power Factor Correction (PFC) :

In inductive loads such as motors and lighting, the current lags behind the voltage, resulting in a low power factor. By connecting a film capacitor in parallel (generating a leading capacitive current), the lagging inductive current can be compensated, aligning the total current with the voltage phase and improving energy efficiency. This is an important application of film capacitors in industrial AC circuits.

4. Resonance and frequency selection :

Connected in series or parallel with an inductor, it utilizes the characteristic of Xc varying with frequency to achieve resonance at a specific frequency fr ( Xc = XL ). It is used to construct oscillators, tuning circuits, filters, etc. For example, in the station selection circuit of a radio.

5. Motor operation and start-up :

Single-phase AC motors require capacitors to generate a phase-shifted current, thereby establishing a rotating magnetic field that enables the motor to start and run. These capacitors need to withstand high AC voltage and current, and film capacitors (especially metallized polypropylene MKP) are widely used due to their high withstand voltage, low loss, and self-healing properties.

II . Why do AC circuits prefer thin-film capacitors?

Compared to electrolytic capacitors, film capacitors have unique advantages in AC circuits:

Non-polar : Can be directly used in pure AC circuits.

Low loss (high Q value) : The dielectric loss tangent is small, resulting in less heat generation and high efficiency, making it especially suitable for high-frequency and high-power applications.

High voltage resistance and reliability : It can withstand high AC voltage peaks and has a long lifespan.

Self-healing : When the metallized film breaks down locally, the metal layer around the breakdown point evaporates instantly, isolating the fault point and allowing the capacitor to return to normal operation, greatly improving reliability.

Capacitance stability : The capacitance value changes little when temperature and frequency change.

Summarize

The working principle of film capacitors in AC circuits can be summarized as follows: 
Utilizing an insulating thin film dielectric, they undergo periodic charging and discharging under the influence of an alternating electric field, thus exhibiting the characteristic of "passing" AC current in the circuit. Their impedance (capacitive reactance) varies with frequency and capacitance value. Based on this principle, film capacitors achieve key functions such as coupling, filtering, compensation, and resonance, becoming indispensable passive components in modern AC and power electronic circuits.