Publication date: 16 March 2010
The trend in miniaturisation of electronic equipment continues coupled with demand for greater functionality, making devices in turn increasingly vulnerable to electrical interference. As electronic equipment increases in complexity and features, so the processing speed of ICs is becoming faster and number of gates used is increasing significantly. Consequently, power supply circuits supplying power to those devices are gaining in importance. When power supplied to a rapidly switching device is unstable, its operation may also become unstable. This is becoming increasingly evident with semiconductor devices operating under lower voltage. For this reason, stable power supply has become a key issue.
There are a variety of sources of electromagnetic interference that can adversely affect electronics including unstable supply voltage, radiated interference, impulse noise and electrostatic discharge.
An unstable supply voltage can be caused by noise being present on the power line as a result of the power conversion process itself. Digital circuits of electric equipment operating at 30MHz are driven by a DC-DC converter. When an AC adapter is connected to this circuit board, both noise generated directly by the DC-DC converter and noise from digital circuits transmitting through the DC-DC converter are emitted from the power cable. Fig. 1 indicates an example of an experiment demonstrating such noise, emitted from a power cable connected to equipment when an AC adapter is used.
In this instance the noise is suppressed utilising a Murata low profile lead type BNX012 series suppression filter at the connector part of the power cable. We can see that a significant noise suppression effect at 10dB to 20dB can be achieved over a wide range of frequencies between 100MHz to 700MHz. The BNX series, equipped with built-in high capacitance capacitor and high performance feed through capacitor, is characterized by its ability to suppress noise at a wide range of frequencies by using only one unit. This characteristic can also be expected to be used as a measure to suppress external noise (immunity measure) with unpredictable noise frequency.
Switching power supplies and DC-DC converters convert voltage by an internal switching operation. Therefore occasionally noise is emitted from the output voltage caused by the switching operation. Fig. 2 shows an example of switching noise, as well as an illustration of the noise being cleanly suppressed by the BNX002-01 suppression filter.
Radiated impulse noises from the external environment around the circuit can also cause unpredictable noise. At locations with high voltage electric equipment such as factory machine tools, equipment malfunctions can occur from exposure to impulse noise such as switching surges. The strong noise suppression effect of the BNX series makes it applicable for impulse noise measures. Fig. 3 shows its impulse noise suppression effect with the use of an impulse noise generator. This demonstration was carried out with the surface mount type BNX022-01, the 4kV impulse noise is clearly suppressed.
Lower voltage semiconductor devices are particularly vulnerable to damage and the danger of malfunction from external ESD and impulse surge voltages. ESD interference is the transfer of electric charge between bodies of different electrostatic potential in close proximity of through direct contact. The BNX suppression filter series has the ability to protect against exposure to ESD from power supplies. In addition to suffering from noise immunity issues, semiconductors operating at high speed emit higher frequency harmonic noise, and this can render conventional decoupling capacitors ineffective.
Engineers can design for EMC issues by use of discrete components within the circuit itself, although this is often considered to be a specialised discipline as it requires an understanding of the appropriate standards. High performance suppression filters can offer an alternative solution to the conventional approaches of appropriate circuit design and decoupling capacitor selection. An appropriate block filter can offer several advantages over discrete component design. Containing the circuit within one module eliminates tracks between components that could potentially be vulnerable to interference themselves, thus ensuring highly reliable performance. The compact design of a block filter saves space within a unit both on the PCB and potentially in profile height in more compact designs. Placement of a single filter component saves assembly time and thus cost by simplifying the manufacturing process.
Block type EMI suppression filters such as the Murata BNX series have been used for many types of electric equipment due to their high performance. Changes in their configuration are becoming necessary due to recent trends in equipment miniaturization and the accommodation of SMD. In response to such trends, lower profile or SMD type block type EMI suppression filters have also been developed. These block type EMI suppression filters provide a solution to various types of interference, including stabilization of voltage, measures against undesirable radiation, and the suppression of impulse noise and ESD. A suppression block filter solution enables noise suppression on both the positive and negative lines.
Murata's BNX series offers a block type EMI suppression filter in a highly compact package. Its built-in high-performance feed through capacitor and high capacitance multilayer ceramic capacitor allows it to achieve a wide range of noise suppression from low frequency to high frequency. Because an inductor has been built-in on the ground side, noise travelling through the ground can also be significantly suppressed. The high noise suppression performance of the BNX series has placed it as the decisive option for noise suppression for the power supply lines in a variety of equipment.
The BNX012 series offers an ultra low profile at just 8mm thickness, ideal for applications where space is limited. The surface mount type BNX022 and BNX023 can be mounted and reflow-soldered with other surface mount components for easy assembly. This reduces the profile height still further then the leaded versions, to just 3.1mm thick. The surface mount SMD type products have noise suppression performance equivalent to previous products, while offering a rated current of 10A/15A. Typical applications for suppression filters include industrial, IT and entertainment equipment such as noise suppression for large display screens. Additionally high reliability versions BNX024H01 and BNX025H01 are available which are ideal for the requirements of the automotive industry while maintaining a low profile and SMD features.
As electronic equipment continues to reduce in size and gain in features, demand for this type of high-performance EMI suppression filter is expected to rise. Murata intends to be at the forefront of continuing to develop block type EMI suppression filters to accommodate changes in configuration and the environment of equipment to help respond to ever more EMC issues.
