Tweet

Posted on 06 November 2019

Metallized Film Capacitors

Free Bodo's Power Magazines!

 

 

 

Considerations for selecting the most suitable types

The expected life cycles of electronic products are continuously increasing, so that today life cycles of up to 10, 15 or 20 years are expected. Of course, each electronic component used in such a product should have at least the same lifetime-expectancy.

By Dieter Burger, Managing Director dbTec electronics GmbH the European sales-office of HJC, Taiwan

In power electronics, metallized film capacitors are being used on the AC-side in EMI-filter or AC-filter applications, e.g. as X2- or Y2- noise suppression capacitors.

On the DC-side, film capacitors are being used e.g. as Pulse-, Snubber-, PFC- and DC-link capacitors.

This article discusses the main stress factors for the life of metallized film capacitors, and the aspects which must be considered to select the most suitable types.

Important considerations for the selection

Depending on the application, the right capacitance-value, voltage withstanding characteristics and current carrying capability must be considered. There are applications with relatively high ripple-currents, e.g. for X2- capacitors in an AC-filter of a PV-inverter, or for DC-link capacitors.

Rationale:

Assuming a constant frequency f and capacitance C, the increase of tan δ (loss factor) in a metallized film capacitor is caused by an increase of the ESR (equivalent series resistance): tan δ = ESR x 2f x C

Not only can an unspecified loss of capacitance have fatal consequences in the application. It is also important to keep the tan δ (ESR) within an acceptable change during the life cycle to limit the power loss and the temperature-rise of the film capacitor.

Temperature increase of the capacitor (ΔT):

ΔT = P/G
ΔT = Thousing – Tambient
- Increase in the housing temperature of the capacitor (°C), maximum 15°C above rated temperature
P = Irms² x ESR
- Power loss of the film capacitor (mW)
G = Thermal conductivity (mW/°C)

The capacitor’s permissible ripple current Irms and its thermal conductivity G must be specified by the capacitor-manufacturer. Following these formulas, the power loss P and the temperature-increase of the capacitor raise by the same factor by which the ESR increases. Therefore, an uncontrolled or unknown increase of the ESR during the life cycle can’t be accepted.

Considering the specified maximum increase of the ESR (tan δ) during the life cycle, the circuit ripple current Irms must be reduced, so that by limiting the power loss, the temperature-increase of the capacitor is limited accordingly.

Regarding the allowed temperature-increase, it must be considered that the internal temperature in the hottest point inside the capacitor (hotspot) is about Tamb + 1.5 to 2 times the ΔT between the housing and the ambient. If the ambient temperature is below 70°C, generally a 15°C temperature increase is acceptable.

The temperature increase of a film capacitor is:

a) proportional to the power loss
b) which is proportional to the tangent delta and to the square of the current.

Consequently if e.g. a doubling of the tangent delta is specified during the lifetime, then the ripple current through the capacitor must be reduced by √2 to maintain the initial power loss. If an increase of the tangent delta is specified by times 5 during the lifetime, the ripple current must be reduced by square root of 5 to maintain the initial power loss.

The above considerations underline that it is essential for a design engineer to know the maximum change of the ESR during the life cycle of his product.

End of Life of a film capacitor

The following end of life criteria are proposed for film capacitors in power electronics:

ΔC/C: ≤ 10%
Δ tan δ/tan δ (at 1kHz and 10kHz): ≤ 200%

The end of life is defined as the point in time from which onwards the capacitor does not fulfill its initial specified values.

Determining factors for the life of a metallized film capacitor

Which facts contribute to an increase of the ESR during the life cycle?

The increase of the ESR expresses the aging of a metallized film capacitor. This is fundamentally determined by the aging of the dielectric, the electrodes and the endspray contacts. The speed of this aging process is accelerated by temperature, voltage and humidity.

Reasons for a capacitance-loss:

  • Self-healing effects
  • Reduction of the electrode metallization by corona-discharges or by humidity-corrosion.

Film metallization with significant humidity-corrosion

Mean values of the capacitance-loss over the time from HJC’s standard X2-capacitors which fulfill the current IEC 60384-14

Reasons for an increase in tan δ (ESR):

  • Aging of the dielectric.
  • Reduction of the metallization by corona- discharges or humidity-corrosion.
  • Humidity corrosion in the endspraycontacts.
  • Break of the endspray-contacts due to excessive stress (heat) from pulsecurrents caused by dU/dt’s.

Prove for the sensitivity of metallized film capacitors for humidity:

Due to market pressure for miniaturization and cost-savings, film capacitors are being produced with increasingly thinner dielectric films and metallization layers. The result of this tendency is greater sensitivity of the capacitors to environmental conditions.

However, most specifications of metallized film capacitors don’t take into account the sensitivity of film capacitors for the simultaneous stress by voltage and humidity. The same is true for the current IEC standard for AC- and DC- metallized film capacitors.

Current IEC standard regarding the humidity resistance:

X- and Y- capacitors:
IEC 60384-14 4.12 (Damp heat)
→ 40°C, 90-95% humidity, 21 or 56 days (→ without voltage!)

DC capacitors:
IEC 60068-2-78 (Test Ca, Damp heat)
→ 40°C, ~ 95% air humidity, 21 or 56 days (→ without voltage!)

As both tests are without voltage, it just can be considered as storage tests.

Current IEC standard regarding the endurance performance:

X- and Y- capacitors:
IEC 60384-14 4.14 (Endurance):
→ 1.25 Urated (X-capacitors) or 1.7 Urated (Y-capacitors) at upper category temperature for 1000 hours (through a 47ohm resistor), once every hours increase to 1.5 Urated for 0.1seconds. (→ without humidity!)

DC capacitors:
IEC 61071 Capacitors for power electronics – 5.15 (Endurance Test):
“The purpose of the endurance test is to demonstrate the performance of the capacitor under the conditions which will actually occur in service.”
→ Same as for X- and Y- film capacitors, to verify the endurance performance the standard IEC 61071 just refers to simultaneous stress by temperature and voltage, but without humidity!

However, it can be easily verified that humidity in combination with voltage leads to an accelerated capacitance-loss and increase of tan δ (ESR).

The test-result show that conventional X2-capacitors can pass the test 4.2 (damp heat test) from the latest issue of the standard IEC 60384- 14 with excellent results (green curve). However, the same capacitors may exceed their specified capacitance-values already after about 250 hours (blue curve), if in addition to the temperature of 40°C and humidity of 95% there will be also applied the mains-voltage.

The chemical process of humidity-corrosion in the metallization occurs faster when humidity and voltage are present at the same time. This is true for both, AC- and DC- film capacitors.

The curves in Figure 4 and 5 demonstrate the need to verify the capacitor’s robustness when temperature, voltage and humidity are applied simultaneously. Humidity can enter a film capacitor either during its production-process or later in the application due to an untight packaging (box and potting).

Capacitance loss during a test with 85°C, 85% RH and 700VDC

Mean values of the loss factor increase at 10 kHz of the same types in the same test

Accelerated lifetime-test:

Whether a sufficiently robust film capacitor has been selected can be verified by using an accelerated life cycle test with the following parameters:

  • 85°C, 85% humidity, nominal voltage UN, 1000 hours.

Whereby the following criteria apply:
ΔC/C ≤ 10%
Δ tan δ/tan δ≤ 200% at 1 kHz, and at 10 kHz or 100 kHz.

This test is called either THB-test (Temperature, Humidity, Bias) or “85/85-test with voltage”.

Next to the verification of the humidity-robust construction of the film capacitor, it is essential to verify if its failure-mode shows a Fail-safe characteristics.

Conclusion:

Metallized film capacitors stand out from other capacitor types due to their numerous technical advantages. However, a metallized film capacitor is not a static component, like almost all electronic components it is subject to aging processes.

For a designer of a power electronics board it is essential to know the aging-characteristics of the selected film capacitors. Therefore he needs to know what is the maximum capacitance-loss and increase of the ESR during the life cycle of his product. It is a weakness from most current datasheets of film capacitors that the electrical data are given for new capacitors, but the aging-effects are just specified for the influence of temperature and voltage. It can be easily verified that film capacitors may show an accelerated aging under the simultaneous influence of temperature, voltage and humidity. As long as the current IEC standards for AC- and DC film capacitors are not updated accordingly, the robustness of the selected film capacitor against a climatic aging should be verified within the homologation process. A suitable test (THB-test or 85/85-test with voltage) has been proposed in this article. The manufacturer of the film capacitor should be able to specify what will be the maximum capacitance-loss and increase of the tangent delta in this accelerated lifetime-test.

Further information can be found under: www.dbtec-electronics.com

 

VN:F [1.9.17_1161]
Rating: 5.5/6 (2 votes cast)
Metallized Film Capacitors, 5.5 out of 6 based on 2 ratings

This post was written by:

- who has written 791 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.