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Jan 5 2019

Working Principle and Detection Method of Film Capacitor

Warm hints: This article contains about 4000 words and reading time is about 18 min.

Introduction

Capacitors can be used to provide important ride-through (or hold) energy, or to reduce ripple and noise in power conversion circuits. Choosing the right type of capacitor can have a profound impact on the overall size, cost, and performance of your system.

Article Core

Film Capacitor

Purpose

Introduce what working principle and detection method of the film capacitor are.

Application

Semiconductor industry.

Keywords

Film Capacitor

Catalog

Introduction


Overview of Film Capacitors


Working Principle of Film Capacitor


The Method for Detecting the Quality of A Film Capacitor Is As Follows:


 

Note On the Use of Film Capacitors:

(1) Working Voltage

(2) Working Current

(3) Capacitor Charging and Discharging

(4) Flame Retardancy

(5) Ambient Temperature

The Difference Between Film Capacitors and Electrolytic Capacitors


How to Choose the Right Capacitor



Overview of Film Capacitor

Film capacitors have low equivalent series resistance (ESR) and therefore good ripple current handling capability, high surge voltage rating and self-healing performance, and are important in many important applications such as electric vehicles, renewable energy, and industrial drives. A powerful contender for power conditioning tasks. Film capacitors are particularly suitable for applications that do not need to be held (or traversed), such as between power outages or peaks in line frequency fluctuations, requiring large high frequency ripple currents to be supplied or absorbed with high reliability and low loss.

Film capacitors are also well suited for applications operating at high DC bus voltages to minimize resistive losses. Since aluminum electrolytic capacitors can only provide voltages up to about 550V, applications operating at higher voltages require multiple devices in series, and then it is necessary to prevent voltage imbalance by selecting capacitors with matching values. It is expensive and time consuming; or it adds voltage balancing resistors, which increases the extra energy loss and BOM cost.

On the other hand, aluminum electrolytic capacitors are still a strong choice when pure energy storage density (Joules/cm3) is the main parameter of concern. One example is a commercial off-line power supply that requires cost-effective, high-capacity energy storage to maintain DC output voltage during a power outage without the need for a backup battery. Properly reducing the rating can reduce the life and reliability of aluminum electrolytic capacitors.

However, aluminum electrolytic capacitors can only withstand an overvoltage of about 20%. If there is a higher overvoltage, damage will occur, and the film capacitor can withstand an overvoltage of up to about twice the rated voltage in a short time. As is often the case in practical applications, self-healing capabilities ensure a safer response to accidental overpressure. In addition, film capacitors allow for easier connection and installation, and because they are non-polarized products, there is no reverse connection error. They are typically packaged in an insulated, highly efficient rectangular "box" enclosure for various electrical connections such as screw terminals, lugs, "fastons" or bus bars.

Table 1 compares the characteristics of common film capacitor types. Polyesters can be used at low voltages, and polypropylene exhibits the lowest loss and highest reliability under high stress due to its low loss factor (DF) and high dielectric breakdown per unit thickness. DF is relatively stable and does not vary greatly with temperature and frequency. Segmented high crystalline metallized polypropylene can also be used and can provide energy densities comparable to aluminum electrolytic capacitors.

Table 1: Types of common film capacitors and their characteristics.

Table 1: Types of common film capacitors and their characteristics.


Working Principle of Film Capacitor 

Working Principle of Film Capacitor

The working principle of the film capacitor is the same as that of the general capacitor. It stores the electric charge on the electrode and stores the electric energy. Usually, it is used together with the inductor to form an LC oscillating circuit. The working principle of the capacitor is that the electric charge will move under the electric field, when the conductor With a medium in between, the charge is prevented from moving and the charge is accumulated on the conductor, resulting in cumulative storage of the charge.


The Method for Detecting the Quality of A Film Capacitor Is As Follows:

1) First look at the appearance, if there is a problem with the appearance, the film capacitor is likely to have problems.

2) Test the two legs of the film with a multimeter resistance file to be very high resistance. If there is a capacitance meter, measure the capacitance value to match the mark on the housing.

3) Normal temperature test performance, including capacity, loss, insulation resistance, withstand voltage, ESR, etc. In particular, where is the performance of the capacitor, it is important to test which aspect.

4) Do a simulated life test. There is no problem with the normal temperature test performance, and it depends on whether the life can last.

5) Choose a reputable capacitor manufacturer.

6) If the use requirements are not high, you can buy some general-purpose ones from the market, do your own copy machine test, and pass it, you can use it with confidence.

7) A film capacitor is a capacitor in which a metal foil is used as an electrode, and a plastic film such as polyethylene, polypropylene, polystyrene or polycarbonate is stacked from both ends and wound into a cylindrical shape. According to the type of plastic film, it is called polyethyl capacitor, polypropylene capacitor, polystyrene capacitor and polycarbonate capacitor. Film capacitors have been a capacitor that has gradually increased in usage in recent years, so we must understand the inspection and processing methods of film capacitors.


Note On the Use of Film Capacitor:

(1) Working Voltage

The choice of film capacitor depends on the highest voltage applied and is affected by factors such as applied voltage waveform, current waveform, frequency, ambient temperature (capacitor surface temperature), capacitance, and the like. Before use, check that the voltage waveform, current waveform and frequency across the capacitor are within the rated value.


(2) Working Current

The pulsed (or alternating current) current through the capacitor is equal to the product of the capacitance C and the rate of voltage rise, i.e., I = C & TImes; dt / dt.

Due to the loss of the capacitor, when used under high frequency or high pulse conditions, the pulse (or alternating current) current through the capacitor causes the capacitor to heat itself and rise in temperature, which may cause thermal breakdown. Therefore, the safe use conditions of the capacitor are not only limited by the rated voltage, but also limited by the rated current.

When the actual operating current waveform is different from the given waveform, the polyester film capacitor is generally used when the internal temperature rise is 10 ° C or less; the polypropylene film capacitor has an internal temperature rise of 5 ° C or less. In the case of use, the surface temperature of the capacitor must not exceed the rated upper limit temperature.

The internal temperature rise formula of the metallized film capacitor is as follows:

△T=I2rms*DF*ω/(β*S)

△T: internal temperature rise of the capacitor

Irms: effective current value through the capacitor

DF: loss tangent

Ω: capacitive reactance (1/2πfc)

β: thin film heat transfer coefficient

S: capacitor surface area


(3) Capacitor Charging and Discharging

Since the capacitor charging and discharging current depends on the product of the capacitance and the voltage rising rate, even a low voltage charging and discharging may generate a large instantaneous charging and discharging current, which may cause damage to the performance of the capacitor. When charging and discharging, connect a current limiting resistor of 20 Ω/V to 1000 Ω/V or higher to limit the charge and discharge current to a specified range. If there is a phenomenon of capacitor short-circuit charge and discharge, please include it in the scope of defective products and do not use it.


(4) Flame Retardancy

Although a fire-retardant flame-retardant material, a combustion-supporting epoxy or an outer casing, is used in the outer casing of the film capacitor, the external high temperature or flame can deform the capacitor core to cause package cracking, causing the capacitor core to melt or burn.


(5) Ambient Temperature

The capacitor is rated for use at a temperature of 85 °C. When the actual operating temperature of the capacitor exceeds the rated operating temperature (within the maximum operating temperature range), the rated voltage of the capacitor will decrease as the temperature increases. Standard formula for capacitor voltage reduction:

VC=VR*(165-TA)/80

VC: Capacitors can withstand voltage at high temperatures

VR: capacitor rated voltage

TA: capacitor surface temperature rise


The Differences Between Film Capacitor and Electrolytic Capacitor

(1). For power conversion applications, understanding the relative advantages of electrolytic and film capacitors can help designers make the right choice for the best overall size, weight and BOM cost. Can be summarized as follows:

Electrolytic capacitor:

Has a higher energy storage density (Joules / cm 3);

Low cost for "straight through" bulk capacitors for DC bus voltages;

Maintain ripple current rating at higher temperatures;

Film capacitors:

Lower ESR for excellent ripple handling;

Higher surge voltage rating;

Self-healing improves system reliability and service life;


(2). In terms of overvoltage withstand capability, the film capacitor has a stronger overvoltage impact resistance than the electrolytic capacitor; in terms of temperature resistance, the film capacitor has a temperature range of -40 ° C to -70 ° C, while the electrolytic capacitor is It is easy to cool at low temperature and has low safety factor. In terms of cost, the film capacitor is easy to be connected in series and parallel, and the cost is low. The electrolytic capacitor has the possibility of explosion and increases the cost. In terms of safety, the film capacitor is non-polar and is affected by the environment. The effect is small, and the electrolytic capacitor is polar and is affected by the environment during use. See the table below for details:

Aspects

Electrolytic capacitor

Film capacitor

life

thousands of hours

generally 10w hours or more

error

20%

5%-10%

volume (same pressure and capacity)

large

small

Price (same pressure and capacity)

cheaper

more expensive

performance characteristics (overvoltage withstand / temperature resistance)

weak

strong

capacity range

high

Under 10 uF

polarity

yes

no

explosion-proof design

yes

no

storage conditions

1-2 years

less affected by the environment, unrestricted


How to Choose the Right Capacitor

Analysis of some common power conversion circuits can show how choosing different capacitor technologies can profoundly impact the size, weight, and cost of the system, depending on whether the capacitors need to be used to store energy or handle ripple noise.

For example, for a bulk capacitor used as a 1 kW off-line converter, the difference in characteristics between the two types of capacitors can be clearly illustrated by comparing the electrolytic capacitor and the film capacitor. As shown in Figure 1, the converter has a power factor corrected front end and has a nominal DC bus voltage (Vn) of 400V.

Figure 1: Capacitor used as energy storage for power outages.

Figure 1: Capacitor used as energy storage for power outages.

Assuming an efficiency of 90% and a voltage drop (Vd) of 300V, below this value, the output regulation function will be lost. In the event of a power outage, the bulk capacitor C1 provides energy to maintain a constant output power when the bus voltage drops from 400V to 300V. We can calculate the C1 value required to make a 20ms crossing before the voltage drops below 300V:

How to Choose the Right Capacitor

The 680μF 450V aluminum electrolytic capacitor of the TDK-EPCOS B43508 series is 35mm x 55mm in diameter and meets the total volume of 53cm3 (about 3 cubic inches).

In contrast, the use of film capacitor solutions can lead to unrealistic bulk: up to 15 TDK-EPCOS B32678 film capacitors may be required in parallel, resulting in a total volume of 1500 cm3 (91 cubic inches).

If a capacitor is only needed to control the ripple voltage on a DC line such as an electric vehicle power system, it will be significantly different when selecting a capacitor. The bus voltage may be 400V as before, but it is powered by a battery, so there is no traversal requirement. When the downstream converter extracts 80Arms pulse current at a switching frequency of 20kHz, it is very realistic to limit the ripple to within 4Vrms. The required capacitance is:

How to Choose the Right Capacitor

The 180μF 450V electrolytic capacitor of the TDK-EPCOS B43508 series has a ripple current rating of approximately 3.5 Arms at 60 °C, which includes frequency correction. Processing the current of 80A requires 23 capacitors in parallel, which will result in an unnecessary 4140 μF large capacitance and a total volume of approximately 1200 cm3 (73 cubic inches). This is consistent with the 20 mA / μF rule of thumb for electrolytic capacitor ripple current ratings.

Using the TDK-EPCOS B32678 series of film capacitors, the 132Arms ripple current rating can be achieved with only four devices in parallel, with a volume of only 402cm3 (24.5 cubic inches). In addition, if the ambient temperature can be kept below 70 ° C, a smaller capacitor can be selected.

There are other reasons why film capacitors are the best choice. The parallel connection of multiple electrolytic capacitors results in an excessive capacitance, which may cause problems such as controlling the energy in the surge current. In addition, DC-connected transient overvoltages are common in light-duty traction applications such as electric vehicles, and film capacitors are more robust.

Similar analysis is also suitable for applications such as UPS systems, power conditioning for wind or solar generators, general grid-tied inverters and welders.


Price

The relative cost of a thin film or electrolytic capacitor can be analyzed from the perspective of mass storage or handling of corrugation capabilities. As shown in the results summarized in Table 2, Data 1 published in 2013 compares the typical cost of a DC bus powered by a rectified 440 VAC power supply:


Per joule (J)

Ripple current ampere (A)

Film capacitor

20-50 cents

1 dollars

Electrolytic capacitor

5-10 cents

3 dollars

Table 2: Cost comparison of films and electrolytic capacitors.

Considering the above analysis, film capacitors are an excellent choice for filtering applications such as decoupling, switching buffering and EMI suppression or inverter output.

Decoupling capacitors placed on the inverter or converter DC bus provide a low inductance path for high frequency current cycling. The rule of thumb is to use approximately 1μF of capacitance per 100A switch. It is worth noting that the connection to the capacitor should be as short as possible to avoid transient voltages. The magnitude of the change of 1000 A/μs is possible at high currents and high frequencies. Considering that the PCB traces may have an inductance of about 1 nH / mm, a transient voltage of 1 V can be generated per millimeter according to the following equation:

How to Choose the Right Capacitor

In the switching buffer circuit, the capacitor is connected in series with the resistor/diode and connected by a power switch (usually an IGBT or MOSFET) to control dV/dt, as shown in Figure 2.

Figure 2: Switching buffer for IGBT or MOSFET.

Figure 2: Switching buffer for IGBT or MOSFET.

The snubber capacitor reduces ringing, controls EMI, and prevents spurious on/off. The size of the snubber capacitor is usually chosen to be approximately twice the sum of the switch output capacitor and the mounted capacitor. The resistor value is chosen to prevent all ringing as standard.


EMI Suppression

As shown in Figure 3, the film capacitors are also ideally used as X-type and Y-type capacitors with their self-restoring and transient overvoltage capabilities, reducing differential mode and common mode noise, respectively. Safety grade X1 (4kV) or X2 (2.5kV) capacitors are connected by a power cord, usually of the polypropylene type, typically with a capacitance of a few μF, which is required to comply with applicable EMC standards.

A Y-type capacitor with a low connection inductance is at the position where the input line is connected to ground. Here, the nominal transient voltages of the Y1 or Y2 capacitors are 8kV and 5kV, respectively, as shown in the input-to-ground connection. Considerations for leakage current limit the amount of capacitance that can be applied. Although the low connection inductance of the film capacitor helps maintain high self-resonance, the external grounding system should be kept short.

Figure 3: X- and Y-type capacitors for EMI suppression, inverter output filtering.

Figure 3: X- and Y-type capacitors for EMI suppression, inverter output filtering.

Non-polarized film capacitors and series inductors can typically be integrated into a single module, which forms a low-pass filter that attenuates high-frequency harmonics in the AC output of the driver and inverter (Figure 4). These are increasingly used to meet system EMC requirements and reduce the stress associated with dV/dt on cables and motors, especially when the load is remote from the drive unit.

Figure 4: Film capacitors in motor driven EMC filtering.

Figure 4: Film capacitors in motor driven EMC filtering.


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