Introduction to Mylar Capacitors

Mylar capacitors are a type of film capacitor that uses Mylar (biaxially-oriented polyethylene terephthalate) as the dielectric material. They were introduced in the 1950s and have since become widely used in various electronic applications due to their unique characteristics and performance advantages over other types of capacitors.

Key Features of Mylar Capacitors

1. High dielectric strength
2. Low dissipation factor
3. Excellent stability over a wide temperature range
4. High insulation resistance
5. Self-healing properties

These features make Mylar capacitors suitable for applications that require high reliability, long service life, and stable performance under varying environmental conditions.

Construction and Materials

Dielectric Material: Mylar Film

The core component of a Mylar capacitor is the Mylar film, which serves as the dielectric material. Mylar is a trademark name for biaxially-oriented polyethylene terephthalate (BOPET), a polyester film developed by DuPont. The film is stretched in both the machine and transverse directions during the manufacturing process, resulting in a material with excellent mechanical, electrical, and thermal properties.

Electrodes: Metallized or Foil

Mylar capacitors can be constructed using either metallized or foil electrodes:

1. Metallized Mylar Capacitors: In this construction, a thin layer of metal (usually aluminum or zinc) is deposited directly onto the Mylar film through a vacuum deposition process. This results in a self-healing capacitor, as the metal layer can vaporize around a fault site, effectively isolating the defect and preventing a short circuit.

2. Foil Mylar Capacitors: Foil Mylar capacitors use separate metal foil electrodes (usually aluminum) that are wound together with the Mylar film. This construction offers higher current-carrying capacity and lower equivalent series resistance (ESR) compared to metallized Mylar capacitors.

Packaging and Termination

Mylar capacitors are available in various packaging options, including:

1. Radial leads
2. Axial leads
3. Surface mount (SMD)

The choice of packaging depends on the specific application requirements, such as available space, mounting preferences, and environmental conditions.

Electrical Characteristics

Capacitance Range

Mylar capacitors are available in a wide range of capacitance values, typically from 0.001 µF to 100 µF. The achievable capacitance depends on factors such as the dielectric thickness, electrode area, and the number of layers in the capacitor.

Voltage Ratings

Mylar capacitors offer a broad range of voltage ratings, from a few volts to several kilovolts. The maximum voltage rating is determined by the dielectric strength of the Mylar film and the capacitor’s construction.

Typical voltage ratings for Mylar capacitors include:

Dissipation Factor

The dissipation factor (DF) or loss tangent (tan δ) is a measure of a capacitor’s energy losses. Mylar capacitors have a low dissipation factor, typically ranging from 0.1% to 0.5% at 1 kHz, depending on the specific type and construction.

A low dissipation factor is essential for applications that require high efficiency and low heat generation, such as in power supplies and filtering circuits.

Insulation Resistance

Mylar capacitors exhibit high insulation resistance, typically exceeding 100,000 MΩ. This high insulation resistance helps to minimize leakage current and ensures stable performance over time.

Temperature Coefficient

The temperature coefficient of a capacitor describes how its capacitance value changes with temperature. Mylar capacitors have a relatively low temperature coefficient, typically ranging from -2% to +2% over a temperature range of -55°C to +125°C.

This low temperature coefficient makes Mylar capacitors suitable for applications that require stable capacitance values across a wide temperature range, such as in automotive and industrial electronics.

Mylar Capacitor Applications

Coupling and Decoupling

Mylar capacitors are commonly used for coupling and decoupling applications in electronic circuits. Coupling capacitors are used to transfer AC signals between stages while blocking DC, while decoupling capacitors help to reduce noise and stabilize power supply voltages.

Audio and Signal Processing

The low dissipation factor and high insulation resistance of Mylar capacitors make them well-suited for use in audio and signal processing applications. They are often used in audio filters, tone controls, and equalizers to achieve high-quality sound reproduction.

Power Factor Correction

Mylar capacitors, particularly those with higher voltage ratings, are used in power factor correction (PFC) circuits to improve the efficiency of power systems. By compensating for reactive power, PFC capacitors help to reduce the load on power generation and distribution equipment, leading to energy savings and improved system stability.

Motor Run Capacitors

Motor run capacitors are used to improve the starting and running performance of single-phase AC motors. Mylar capacitors with high voltage ratings and capacitance values are commonly used in this application due to their stability and reliability.

EMI/RFI Suppression

Mylar capacitors are used in electromagnetic interference (EMI) and radio-frequency interference (RFI) suppression circuits to reduce unwanted noise and interference in electronic systems. Their low equivalent series resistance (ESR) and high voltage ratings make them effective in filtering out high-frequency noise.

Comparing Mylar Capacitors with Other Types

Mylar vs. Ceramic Capacitors

Ceramic capacitors are another popular choice for many electronic applications. Compared to Mylar capacitors, ceramic capacitors offer:

• Higher capacitance density
• Lower ESR
• Wider temperature range

However, Mylar capacitors have several advantages over ceramic capacitors, including:

• Higher voltage ratings
• Better stability over temperature
• Lower piezoelectric effects

Mylar vs. Electrolytic Capacitors

Electrolytic capacitors, particularly aluminum electrolytic capacitors, are commonly used in power supply and filtering applications. Compared to Mylar capacitors, electrolytic capacitors offer:

• Higher capacitance values
• Higher energy density
• Lower cost per microfarad

However, Mylar capacitors have several advantages over electrolytic capacitors, including:

• Longer service life
• Lower ESR and dissipation factor
• Better stability over temperature
• Non-polarized construction

Mylar vs. Polypropylene Capacitors

Polypropylene capacitors are another type of film capacitor that shares many characteristics with Mylar capacitors. Compared to Mylar capacitors, polypropylene capacitors offer:

• Lower dissipation factor
• Higher insulation resistance
• Better high-frequency performance

However, Mylar capacitors have some advantages over polypropylene capacitors, such as:

• Higher dielectric strength
• Better mechanical stability
• Lower cost

Frequently Asked Questions (FAQ)

1. What is the self-healing property of Mylar capacitors?

The self-healing property of Mylar capacitors refers to their ability to recover from minor dielectric breakdowns. In metallized Mylar capacitors, when a fault occurs, the thin metal layer around the fault site vaporizes, effectively isolating the defect and preventing a short circuit. This self-healing mechanism helps to extend the service life of the capacitor and improve its reliability.

2. Can Mylar capacitors be used in high-temperature applications?

Yes, Mylar capacitors can be used in high-temperature applications, typically up to 125°C. They offer excellent stability over a wide temperature range, with a low temperature coefficient and minimal changes in capacitance and dissipation factor. However, for extreme high-temperature environments, other types of capacitors, such as ceramic or mica capacitors, may be more suitable.

3. Are Mylar capacitors polarized?

No, Mylar capacitors are non-polarized, meaning they can be connected in either direction in a circuit without affecting their performance. This is in contrast to some other types of capacitors, such as aluminum electrolytic capacitors, which are polarized and must be connected with the correct polarity to avoid damage.

4. How do I choose the right Mylar capacitor for my application?

When selecting a Mylar capacitor for your application, consider the following factors:

• Required capacitance value
• Voltage rating
• Temperature range
• Dissipation factor and ESR requirements
• Physical size and packaging constraints

Consult the manufacturer’s datasheets and application notes to ensure that the chosen Mylar capacitor meets your specific requirements and is suitable for your intended use.

5. What are the main failure modes of Mylar capacitors?

The main failure modes of Mylar capacitors include:

1. Dielectric breakdown: This occurs when the applied voltage exceeds the dielectric strength of the Mylar film, leading to a short circuit.

2. Overheating: Excessive current or high ambient temperatures can cause the capacitor to overheat, leading to premature aging and failure.

3. Moisture ingress: If the capacitor’s packaging is damaged or inadequate, moisture can penetrate the capacitor, causing corrosion and deterioration of the dielectric material.

To minimize the risk of failure, ensure that the Mylar capacitor is operated within its specified ratings, properly installed, and protected from environmental factors such as moisture and excessive heat.

Conclusion

Mylar capacitors offer a unique combination of high performance, stability, and reliability, making them a popular choice for a wide range of electronic applications. Their high dielectric strength, low dissipation factor, and excellent temperature stability enable them to deliver consistent performance across various operating conditions.

By understanding the construction, electrical characteristics, and application-specific requirements of Mylar capacitors, engineers and designers can make informed decisions when selecting the most appropriate capacitor for their projects. As technology continues to advance, Mylar capacitors are likely to remain a key component in the electronics industry, offering reliable and efficient solutions for power conditioning, signal processing, and noise suppression.