Does DC generate EMI? Why do we need EMI filters for equipment running on DC power supply? If you are also having similar queries in your mind, then read on. This study elaborates on the sources and reasons for DC EMI. It also touches upon the prevailing EMI emission compliance standards across the world.
We also bring to you the popular MF 610 series product range from EMI Solutions – EMIS DC EMI Filters.
There is a slight misconception about what we call as DC power. Though DC technically refers to non-varying or static current (0Hz), the term is also used to refer to low frequency fields, typically under 10Hz. To be more accurate, it is referred to as quasi-DC.
These low frequency currents lead to creation of DC magnetic fields by both natural and man-made causes. Lightning, earth’s magnetic flux are natural sources of DC fields. Welding equipment, MRI and other medical diagnostic scanners, DC battery power supply are common man made sources. These quasi-DC magnetic fields are a source of EMI.
EMI sources and receptors
When two electrical / electronic devices are close to each other, the electrical noise of one can interfere with the operation of another — known as Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI). Devices in Industrial automation, medical equipment, power supplies are not only susceptible to EMI, they are also constant emitters of EMI. With the drastic increase in the use of these machinery in our everyday lives, EMI concerns are rising in the world.
There are two major challenges in EMI. One is due to the interference between electronic circuits on the PCB itself. This is near-field coupling within the components and the circuits. The other challenge is radiated emission which goes mainly about the cabling where the cable itself acts as an antenna for transmitting/receiving.
Out of these options, use of EMI filters is the most simple, cost effective and reliable solution. An EMI Filter is used to suppress the electrical noise that stems from electric currents in the device’s wiring, reducing EMI for a clearer signal. EMI noise currents have very low amplitude. Due to the fact that most power supplies operate using DC voltages, DC EMI Filters allow DC and low-frequency currents to pass through while blocking harmful high-frequency currents. This arrangement allows for full functionality of the device while diverting unnecessary noise.
Differential mode electromagnetic noise is only on the power lines and is not present on the earth ground lead and can be measured with respect to the power lines. This noise is conducted on the supply and ground lines in opposite directions. The reason for the noise is because cross-cancelling does not happen effectively. This type of noise is suppressed by installing a filter on the on the signal /power supply line. Designing filters with differential mode chokes is an economic way to combine low losses, low voltage drop, minimal signal impact with efficient EMC/EMI filtering.
Common mode electromagnetic noise can only be measured from earth ground to one of the power lines. This noise is conducted simultaneously on all lines (supply and ground) in the same direction. But it is also coupling back to ground. Designing filters with common mode chokes is an economic way to combine low losses, low voltage drop, minimal signal impact with efficient EMC/EMI filtering.
CM-DM equivalent circuits
CISPR 22 is a widely used European standard for electromagnetic compatibility for Information Technology Equipment (ITE). The conducted and radiated EMI emission limits from CISPR are used to benchmark equipment across the world. There is also another related standard – EN 55022 which is the CENELEC standard. EN 55022 is a modified derivative for CISPR 22. FCC (Federal Communications Commission) Part 15 is prevalent in the USA.
For conducted EMI, CISPR 22 covers frequencies from 150KHz to 30MHz. CISPR 25 automotive specifications cover higher frequencies of upto 108MHz.
For radiated EMI, CISPR 22 covers frequencies from 30MHz to 1GHz. CISPR 25 automotive specifications cover higher frequencies of upto 2.5GHz.
CISPR 22 Standard EMI emission frequencies and permitted limits
These specifications are very stringent. So it is very difficult to pass these standards without adequate steps taken at the design stage itself. Inserting an EMI filter in the DC line input after the inverter is a simple, affordable and reliable solution to EMI management.
The use of DC EMI filters can ensure the CISPR standards compliance of electrical/electronic equipment. Choosing the attenuation required from a filter is done by first measuring system performance without a filter fitted. Subsequently, the extent of filtering needed from an external filter to meet specifications, is calculated. EMI filters themselves must meet stringent safety standards to minimize the device’s risk of electric shock and fire hazards.
Many countries in the world have their own safety standards for EMI filters. So they should be selected based on what is approved by the safety standards of the countries to which the device will be exported.
For testing, LISN is a line impedance stabilization network which is used in the test setup for conducted and radiated noise measurements.
Although AC/DC Power Supplies feature their own internal EMI Filters, sometimes, the systems power generate more electrical noise than the internal filter can handle on its own. Separate power filters are useful in these situations to bring the noise down to acceptable levels. DC EMI filters help to combat interference in applications such as:
The impact of Radiated EMI in switching power supply is quite significant. The circuit layout, components placement within the power supply and impact of external components – all of them contribute to the EMI.
Before we improve an EMI, we need to understand where the noise is coming from. High-frequency conducted and radiated emissions from power converters occur based on the transient voltage (dv/dt) and transient current (di/dt) generated during hard switching. The best way is to look in the schematic first, and identify the high transient currents. Such EMI is an increasingly complex issue in the design and qualification phases, especially given the increased switching speed of power MOSFETs.
To address the frequency interference on the Input side of Frequency Inverter, a DC EMI filter should be employed. For upper frequencies, an AC EMI filter is recommended on the output side of Frequency Inverter. To filter the low frequencies, a sine wave filter can be included. The sine wave filter typically allows decoupling of the inverter from the grid along with its prime directive of improving signal purity for utility requirements.
Schematic diagram of PV system incorporating line filtering
EMI filters fitted in power supplies are connected to the primary side of an electronic device. Therefore, the highest adherence to safety standards is required to protect against accidental electric shock, smoking or fire.
EMI Solutions Pvt Ltd is an ISO 9001-2015 and 14001-2015 certified company that exclusively caters to EMI related requirements. We deliver both standard manufactured and custom designed EMI filters and components. In addition, our state-of-the art EMI test facility and technical consultancy services give our customers the convenience of TOTAL SOLUTIONS TO EMI/EMC under one roof.
With satisfied customers in over 15 countries, EMIS has a steadily expanding global footprint. We pride in our commitment to deliver EMI solutions that meet our customer’s exact requirements. We have been consistently chosen as a preferred EMI solutions provider to several industries in the Military, Medical electronics, Home appliances, Renewable Energy, Telecom sectors.
Several products are available to specifically address the renewable energy market from EMIS. EMIS had designed the MF 610 series of DC EMI Filters for Photo Voltaic Inverters and DC power supply. These filters are available over a very wide power range.
EMIS filters product range includes a wide variety of filters that are cost effective, compact size, and come with multiple mounting options. They support high contact reliability with shock proof terminals. Our filters also provide good attenuation from incoming interference.
All the MF610S filters are specified upto 1,200 V. For low-current applications, the MF610S is rated for 25 A. Power loss is only 8 W while providing attenuation of at least 50 dB for frequencies from 50 kHz to 10 MHz as tested to CISPR specifications at 50 Ω. For higher power applications, MF610S is rated for 2500 A. Attenuation performance is inversely related to the power specification.
These filters come with additional differential mode rejection in low frequency. They are widely used with Switched Mode Power Supplies to reduce noise from power supply, back into the mains. These filters are compact and also provide good attenuation of incoming interference.
Fig: Typical Circuit Diagram
The DC EMI filters from EMIS are widely used by customers across the world.
Our EMI filters are fully compliant with international Industry Standards such as CISPR and also come with CE Marking. Custom built filters are available on customer request.