Principal EMC/ESD Compliance Testing Considerations

Author : John Stone, Relec

07 September 2023

Compliance testing is an essential element of product development and subsequent market introduction. In an electronic and electrical system context, the key standards that will need adherence to are EN62368-1, EN61010 or EN60335 with regard to safety, plus the relevant electromagnetic compatibility (EMC) ones.

The EMC immunity of a system or constituent system component will gauge its capacity to handle exposure to electromagnetic interference (EMI) and still continue to function correctly. EMI can emanate from numerous different sources - including electrical, magnetic and RF, as well as those caused by fluctuations on the incoming supply. Immunity is typically measured through a set of standardised tests, which evaluate the EMI resilience of the system or system component at various frequencies and power levels. These tests are encompassed within the EN61000 standard series. They involve subjecting the device under test (DUT) to EMI at defined levels, while simultaneously monitoring its performance to check that predefined performance criteria are satisfied. Tests results are classified in relation to loss of function or degradation in operational performance.

Defining performance criteria

The key performance criteria that tests can be set against are:

1.Normal performance within limits specified by the manufacturer. This can vary, but here the limits specified within the Melcher family of power supply products will be used. Melcher specifies normal performance as the output voltage of a supply not deviating by more than 3% from the value prior to the test.

2.Temporary loss of function or degradation in performance, which ceases after the disturbance stops. Again, there will be a particular level of acceptable deviation being stipulated.

3.Loss of function or degradation in performance where correction will not happen once the disturbance stops, but will call for operator intervention or system reset. For a Melcher power supply, operator intervention can mean turning the unit off and on, by removal and reapplication of the input supply or via the inhibit input.

4.Loss of function or degradation of performance which is not recoverable at all - because of damage caused to hardware or software, or the loss of data. Here, the DUT has failed and will not subsequently restart.

EN61000 series overview

Parts 1 and 2 of the EN61000 standard series deal with the general methodology and environment for testing, while part 3 looks at harmonic currents. This article will concentrate on part 4, which outlines specific tests designed to ascertain if equipment is sufficiently protected against different EMC disturbances.

It is important to highlight here, that compliance of a system component (such as a power supply, display, etc.) with a particular standard does not equate to the final system also being guaranteed as compliant. Whilst many of the tests detailed will be directed the power inputs of a system, it is still feasible that energy can enter a system by other means and cause a failure.

Figure 1: Contact discharge test curve

EN61000-4-2 testing

Static electricity discharges may be present in environments with low relative humidity levels, typically being generated from low conductivity artificial fibres (such as carpets, vinyl garments, etc.). EN61000-4-2 specifies the requirements and tests for the immunity of electrical and electronic equipment which are subject to electrostatic discharge (ESD) events that could be generated in this way.

There are 4 test levels, which can be evaluated using 2 different test methods (contact and air discharge). Contact discharge is the most common method, where an ESD gun is brought into contact with the equipment under test (EUT) and a high voltage is applied directly. Under certain circumstances this might not be possible (for instance, some equipment might be sensitive to touch or pressure) - in which case an air discharge can be made.

Test levels

The discharge levels associated with a particular type of equipment will be detailed in their own specific product standards. Typically, the following will apply:

Levels 1 and 2 -Systems which are installed in a controlled environmental setting and with additional anti-static precautions. The test voltages are 2kV for contact/air discharge at level 1 and 4kV for contact/air discharge at level 2.

Level 3 - Systems which are only occasionally handled. The test voltages are 6kV for contact discharge and 8kV for air discharge.

Level 4 - Systems which are continuously being handled. The test voltages are 8kV for contact discharge and 15kV for air discharge.

10 discharges will be conducted per polarity on each selected point, with a discharge rate of 1/s being employed.

Table 1: Contact discharge current characteristics

ESD pulse characteristics

The waveform shown in Figure 1 describes the output of a typical ESD generator (or ESD gun) that is used to test compliance in accordance with EN61000-4-2. The waveform is defined by its first peak current (Ip), rise time from 10-90% of Ip and the current at 30ns and 60ns are compared with the ideal generator characteristics.

Designing for ESD immunity

There are a number of simple procedures that can be followed to ensure a system complies with EN61000-4-2. These include:

•Providing short electrical paths for ESD currents to system ground. These can be through the use of transient voltage suppressors or Class Y capacitors. Special care should be taken to ensure that the tracks to these devices are as short as possible.

•Inputs and outputs that can be exposed to ESD impulses should be individually protected.

•The selection of system components (power supplies, displays and suchlike) that have proven compliance to EN61000-4-2.

•The use of metal shielding.

•Having adequate track separation on PCBs.

Relec’s team of experienced account managers and application engineers are able to draw upon huge amounts of relevant EMC/ESD knowledge. They are thus well positioned to support customers throughout the design process - so that potential obstacles can be avoided which could otherwise hold up project completion and result in time-to-market delays or unwanted cost penalties.