TDK-Lambda News

1. How would you best define EMC and how does it apply to power supplies?
EMC, or Electromagnetic compatibility, is the ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances into that environment. Essentially, EMC can be spit into two – emissions and immunity.

Emissions deals with the electromagnetic energy produced by equipment in operation. When these emissions are transmitted through conducting structures (cables, PCB traces, etc) they are considered as conducted emissions. When these emissions are transmitted through open space as electromagnetic waves, they are considered as radiated emissions. These emissions are often called Radio Frequency Interference (RFI).

Where power supplies are concerned, the main source of RFI is the switching devices – generally FETs, the field-effect transistors. The fundamental switching frequency (usually between 50kHz and 300kHz) and harmonics of this cause the predominant conducted emissions, and the speed of the switching edges (usually between 200ns to <1ns) is the dominant source of radiated emissions.

2. What is the intent of the EMC Directive with regard to conducted emissions?
Directive 2004/108/EC, commonly called “The EMC Directive”, is available to download from the Official Journal of the European Commission [1], came into force on 20th January 2005.

The essential requirements of the EMC Directive with regard to conducted emissions state: ”Equipment shall be so designed and manufactured, having regard to the state of the art, as to ensure that the electromagnetic disturbance generated does not exceed the level above which radio and telecommunications equipment or other equipment cannot operate as intended.”

There is a presumption of conformity that if the requirements of the relevant product standard is met, the essential requirements of the EMC Directive are also met. This is not always the case as the test configuration used to claim compliance may not always be chosen (knowingle or unknowingly) to provide the worst case results – this will be covered later.

3. What standards must be considered for the EMC Directive?
The conducted emissions test standard CISPR16-2-1 specifies the test equipment and the way it is set up, method, frequency range and limits. Specific Product Standards, where they exist, may specify different methods, frequency ranges and limits, or use part or all of the test standard.

In order to CE-mark a power supply to the EMC Directive, the minimum requirements are stipulated in the product standard EN61204-3 – no other standard is required. However, due to the diversity of power supply applications, it is prudent to have considered the requirements of other relevant product standards such as EN55011 and EN55022, as well as the generic standards EN61000-6-3 and EN61000-6-4.

4. How do you select the correct classification of pass?
Most conducted emissions standards specify two classes of pass; this is to allow the manufacturer to cover different intended operating environments. Class A is the least stringent and is intended for commercial and light industrial environments (factories, hospitals, office premises, etc). Class B is more stringent, for domestic and residential environments, or for Class A conditions when the domestic mains supply is shared.

The power supply standard IEC/EN61204-3 provides guidance on which classification is required for a particular environment. It is also prudent to consider the guidance given in the product standard for the end application.

5. What steps must be taken in order to set up the correct test configuration?
It is the responsibility of the manufacturer to demonstrate that reasonable steps have been taken to ensure that the test configurations producing worst case results have been identified.

Generally most applications can be simulated by two different test configurations:
a) The power supply and loads mounted on an earthed metallic sheet
In tests, Lambda has shown that this configuration produces similar results as with a power supply and loads mounted in an earthed-screened metallic enclosure. This configuration simulates end applications where either a partially or fully metallic enclosure is used.
b) The power supply and loads free standing with no additional connection to earth
This configuration simulates end applications where either a non-metallic enclosure or no enclosure is used.

Experience, with certain power supply topologies, has shown that the emissions can increase over time as the power supply reaches its operating temperature. In some cases, this may take up to one hour therefore it may be prudent to allow a warm up time prior to testing in order to simulate worst case results.

6. What are the responsibilities of the Test House and how best to deal with them?
The Test House is responsible for ensuring that the test method is correct and the test equipment is fit for purpose. It is not the responsibility of the Test House to ensure that the product under test is configured to produce worst case results. The Test House will configure the product as advised by the manufacturer.

This may lead to a Test House producing a report that does not show worst case results but shows the product meeting the requirements of the relevant standard/s. This, however, does not necessarily mean the product will meet the essential requirements of the EMC Directive when the power supply is used in its intended application.

7. What are the key elements to understanding reports, results and margin?
Care must be taken when reviewing EMC reports for conducted emissions that all the necessary information is included:

• Setup information, preferably with a photograph, in enough detail to replicate the test. Details of test parameters: IF (input frequency) bandwidth, measurement times, input lines measured
• Load conditions
• Line voltage and frequency

• Dated references to test standards used.

It’s a fine line between marginal pass and marginal fail – the test standards require that with 80% confidence, 80% of product will pass the limit. This is often called the 80/80 margin. As with all statistical distributions, this has the consequence that some power supplies can have reduced margin or in certain circumstances even fail the limit and yet still be compliant with the requirements.

However, most engineers will always insist that 100% of product shipped meets the published specification. When designing power supplies, Lambda stipulates a 3dB minimum margin to meet the conducted emissions limits; this ensures the statistical chance of individual power supplies failing due to manufacturing tolerances is minimal.

8. Due diligence – what is it, when should it be applied and by whom?

When power supply products are not CE-marked to the EMC Directive, it is the responsibility of the end equipment manufacturer to exercise due diligence. If the PSU product is CE-marked to the EMC Directive, it is the responsibility of the organisation that places the product on the market in Europe to exercise due diligence.

For products manufactured inside the European Community (EC) it is the responsibility of the manufacturer to be able to demonstrate due diligence in their testing of the product. For products manufactured outside of the European Community the responsibility lies with the reseller to be able to demonstrate they have exercised due diligence in confirming the test results and reports provided by the manufacturer are correct. And, be warned, it is not due diligence to just accept results and reports from outside of the European Community at face value.

The path to exercising good due diligence is not that easy to follow, however some frequent-used examples are:

• Checking the test report covers relevant standards and contains clear and appropriate information.
• Repeating conducted emissions testing [2] to confirm results detailed in the test report.
• Ensuring that test methods used are appropriate for worst case.
• Visiting the manufacturer and their test facilities/partner.

9. Declarations of conformity – what are they?
As stated previously in Question 8, it is the responsibility of either the manufacturer, if located in the European Community, or the reseller, if the manufacturer is outside the European Community (EC), to raise and sign a Declaration of Conformity (DofC).

This is a legally binding document and as a minimum should contain:

• The name and address of the manufacturer
• The name and address of the organisation selling the product in the European Community (EC), if the manufacturer is outside the European Community (EC)
• An unambiguous product description
• A reference to the current EMC directive, correct when the D of C was raised.
• A statement of compliance to the EMC directive
• A reference to the product standard used for compliance testing. In the case of Power Supplies this is EN61204-3
• The identity and signature of the person empowered to bind the manufacturer or his authorised representative (European Community (EC) based)

Additionally, it is common to find references to the generic EMC standards – EN61000-6-X, other product standards such as EN55011 and EN55022 and individual test standards such as EN61000-3-X, and EN61000-4-X. Whilst listing these is not mandatory, care should be taken to ensure that the provisions of all listed standards have been complied with. This information should be available in the test report.

10. Is there additional support and knowledge available?
Even after comprehensive testing there is still a risk that applications in the real world can present EMC problems. The additional testing carried out by Lambda based on experience and knowledge of real-life applications can include such tests as: off load, low load, grounded output and effects of temperature. This is in addition to the requirement Lambda seeks for additional margin to minimise this risk.

Lambda’s knowledge and flexibility can provide information to assist customers in meeting a variety of EMC requirements for their end equipment. In addition, its own EMC test facility enables a rapid response to individual customer issues. This coupled to close ties with both UK and German based EMC test houses provides a high level of customer support across a broad range of EMC application issues. As well as conducted emissions, this support and knowledge also covers the other aspects of EMC testing.


References
1. http://eur-lex.europa.eu/LexUriServ/site/en/oj/2004/l_390/l_39020041231en00240037.pdf
2. Full due diligence would cover other aspects of EMC testing, but for the purpose of this article, only conducted emissions has been considered.