The EMC test is known as Electromagnetic Compatibility, a certification for electronic devices to maintain their limitation of electromagnetic waves. As discussed in the previous article, there are two types of EMC tests – emission (EMI) and immunity (EMS). EMI (Electromagnetic Interference) tests measure the magnetic waves emitted by the device, and EMS (Electromagnetic Susceptibility) tests are performed to test the device's emissions handling immunity.
This article is about some of the most common EMS or immunity tests performed in EMC testing laboratories on electronic devices. Tests performed in conjunction with EMI testing may vary depending on the equipment under test (EUT) application. The most common immunity test is given below.
EMC immunity test is commonly performed on continuous or transient phenomena. EMS is also known as EMC immunity. The EMC immunity test measures the device's ability to survive in the presence of interference generated by other devices.
Types of EMC Immunity Tests
As shown in the block diagram, there are two types of EMC immunity tests: continuous and transient. Both have their testing methods. Below is the diagram showing the most common immunity tests.
Fig. 1 EMC immunity test block diagram
Continuous Immunity Tests
In continuous immunity testing, the device is placed in an environment where different types of continuous waves are applied to the device for several minutes or several hours. This test simulates the RF interference signal present in the real world. Some common ongoing immunity tests are explained below.
Fig.2 Continuous immunity test
Transient Immunity Test
In a transient immunity test, a small burst of energy is applied to the product – the EUT or equipment under test – for a short period of time. Like continuous immunity, transient immunity is applied to a product's signal/data ports, cabinet ports, and power ports.
Fig.3 Transient immunity test
Types of Continuous Immunity Test
- Immunity test performed
An RF signal generator and amplifier generate an electromagnetic field. This electromagnetic field is injected into the cables going to the device, such as signal cable, data cable or power cable, by an injection device. The most common injection device is CDN, BCI probes, EM Clamp and direct voltage injection equipment. Because this test is continuous, many standards call it “Radio Frequency”. As seen in the diagram, the signal is transmitted to the EUT.
Fig.4 Immunity test performed
Fig.5 Injection devices
- Irradiated immunity test
Radiated immunity testing is performed to evaluate the device's ability to function normally in the presence of electromagnetic radiation generated by other sources through the air. Electromagnetic radiation may vary with different devices, such as cell phones, Wi-Fi routers, microwaves, etc.
The closed device and its housing and cables are exposed to electromagnetic radiation generated through an RF amplifier. An RF signal generator is used to generate electromagnetic waves, which are the input to the RF amplifier. The output of the RF amplifier is generated by a variable frequency electromagnetic field and transmitted through an antenna. As shown in the diagram, the antenna transmits electromagnetic waves to EUT.
Fig.6 Irradiated immunity test
- Continuous Magnetic Field Immunity Test
A magnetic field is generated in a loop of wire (antenna). The magnetic field in the wire varies according to the variation in voltage in the AC electrical network. The device is exposed in the loop of wire where a magnetic field is produced. The ESE is exposed in this field to evaluate the product's performance for a specific time. As shown below, the ESE is placed between magnetic fields, which oscillate according to the power supply.
Fig.7 Magnetic immunity test
Types of Transient Immunity Tests
- Electrostatic Discharge (ESD)
When two electrically charged objects come into contact, they discharge electricity, such as when the human body comes into contact with an electrical device. This discharge produces a short pulse burst, which can damage electrical components such as ICs, LCDs, memory, etc.
Fig.8 ESD simulator
An ESD simulator performs this discharge. The ESD pulse is applied to the device casing or part of the device where humans can touch.
There are two types of tips available: air discharge tip and contact discharge tip. The air discharge simulator discharges through the air with the arc, while the contact discharge simulator discharges through contact with the device. A very common test level is 8kv for air discharge and 4kv for contact discharge.
Fig. 9 ESD immunity test
- Electrical Fast Transient (EFT)
The fast transient is caused by the switching of inductive loads such as motor, switches and relays. A burst generator is used to simulate the same situation as switching with an inductive load. The series of short pulses with high amplitude and repetition frequency with short rise time is applied to the EUT. This test applies to AC and DC ports and signal cables over 3 m in length.
As shown in the image, the EFT generator generates the signal, which is fed to the EUT with the help of a capacitive coupling clamp.
Fig.10 EFT immunity test
- Outbreak
Surges can be caused by many factors, such as high-power switching events, magnetic/inductive coupling, and even lightning. A small power surge can cause arcing, broken wiring, motor damage, and many other problems. Surge testing is mainly applicable on EUT AC ports or sometimes DC ports. Some standards apply to signal ports, signal cables longer than 30 m, or if the cable runs outside a building.
Fig.11 Surge immunity test
- Voltage Drop Test
In the real world, the main AC power supply may fluctuate or a failure may occur. This failure can occur due to a power cut or sudden and significant load changes. Voltage drop is the voltage drop of the main supply or the power supply if it is cut off suddenly. A continuous change in voltage causes a continuous change in voltage drops.
Voltage drop testing ensures that the device does not lose functionality under these voltage drop conditions. The image shows that an autotransformer is connected to the immunity test system, which controls voltage drops and switches the power supply.
Fig. 12 Voltage drop test
- Transient Magnetic Field Immunity Test
This test is similar to the continuous magnetic field immunity test. The ESE is placed in a loop of wire, where a magnetic field is generated. In the continuous magnetic field immunity test, the EUT device is exposed to a continuous fluctuating magnetic field to the main power supply (at 50/60 Hz). In a transient magnetic field immunity test, the EUT is exposed to the magnetic field generated by the transient generator. The amplitude of this pulse is high, but the rise time is short.
Fig.13 Transient magnetic field test
Pass/fail criteria
EMC testing laboratories decide the criteria according to the performance of the device. These criteria depend on country standards. Below is data taken from European standards.
Criterion A
If the product performed perfectly in all tests, it would meet Criterion A.
Criterion B
If the device lost its functionality in the test and after the test regains its normal functionality, then the product falls into Criterion B.
Criterion C
If the device loses functionality and needs to be turned on again, it falls into Criterion C.
Criterion D
If the device completely loses functionality and cannot be recovered, the hardware or software will be damaged. Then the device meets Criterion D.
