The trend of new generation switching power supply EMI filter

Since

    the 1990s, with the miniaturization of electronic equipment and the development of surface assembly technology, electronic components have developed towards miniaturization, chip type, composite, multi-function and high performance. Various surface assembly components have gradually become Mainstream products of electronic components.

    The above negative impacts caused by large-scale integration of semiconductors, especially the rapid development of networking, have directly led to the rapid decline in anti-electromagnetic interference capabilities. According to IBM's regular observation statistics, 88.5% of the causes that seriously threaten the safe and reliable operation of electronic and electrical equipment come from voltage transients and electromagnetic pulses in the power supply. Currently, EMI (electromagnetic interference) filters are still one of the effective means to suppress electromagnetic interference. Therefore, an unavoidable problem currently facing traditional EMI filters is how to adapt to the development needs of miniaturization of electronic equipment.

Development status and trends of chip components

    Currently, DC series chip capacitors are developing rapidly in the world, but power magnetic components adapted to power supply filtering are limited by energy storage capacity. Suppose the energy stored by the inductor L at the transient current i is w, then there is the following formula.

    If the inductor adopts a ring magnetic core, the inductance is:

    where: μ initial magnetic permeability; N turns; A core cross-sectional area; D is the average diameter of the magnetic ring.

    The magnetic field intensity H in the magnetic core is:

    H=0.4Ni / D (3)

    Solve the equation to find the stored energy W:

    Since: μ = B / H (5)

    In the formula: B is the magnetic induction intensity value of the magnetic core

    (4 ) or (6) both indicate that the volume A.D of the magnetic core material and the initial magnetic permeability μ determine the upper limit of the inductor's energy storage. In other words, the energy storage requirements of the power converter circuit determine the minimum size of the power magnetics. Under the same magnetic field strength, the larger the transient current passing through the same magnetic core, the smaller the allowable inductance value is.

    There are two structures of chip inductors: open type and closed type, as shown in Figure 1.

    In the figure, a), b), and c) are ring cores, double E-shaped cores, and pot-shaped cores respectively. Their magnetic circuits are closed, so they are closed cores (which can reduce the generation of EMI). d) It is an I-shaped magnetic core. Since the magnetic circuit is not closed, it is an open magnetic core. It may cause adverse effects due to coupling with other sensitive components, but it can withstand larger currents. This is because the magnetic permeability of the open core is very low, so it can withstand larger currents under the same conditions of storing energy W. For example, the volume length × width × height (approximately) of the Japanese WUMIDA company's open core is 12 × 12 × 6 mm. When the rated current is 10A, its inductance value is 2.1μH; while the double E closed core has the same rated current. When the current is 10A, the inductance value is 2.4μH (slightly larger), but the volume length × width × height (approximately) is 16.4 × 16.4 × 9.6, which is 2.37 times larger than the former (the latter has a maximum rated current of 20A) , the inductance value is 1μH).

    In order to reduce the height (low profile) and increase the current carrying capacity, the current conductors of high-current chip inductor windings have changed from round cross-sections to flat cross-sections (including printed board inductors). The above-mentioned 12×12×6mm, 10A, 2.1μH inductor is made of approximately 1.7×0.2mm flat copper strips.

    In the national "Ninth Five-Year Plan" and the 2010 Long-term Goal Outline, new components are listed as one of the priorities for the development of the electronics industry. Among the six categories of new components, surface mount components are at the top of the list. In 2000, the demand for new chip components in electronic information products was 30 billion pieces (in 1999, the demand for inductors, transformers, and magnetic devices in electronic information products was 16.5 billion pieces). This is the market demand for new chip components due to the development of China's information industry.

Overview of the development of chip EMI filters. International

    chip EMI filters are currently mainly signal filters. There are few EMI filters that can be applied to power supplies. For example, TDK's ACH3218 and ACH4518 series use ferrite beads and chips. Type capacitors form a T-shaped filter circuit. Among them, ACH3218 has a volume of L×W×H=3.2×1.8×2.5mm, a maximum rated voltage of 20VDC, a maximum rated current of 1.5A, and a frequency range of 25dB insertion loss from 11MHz-55MHz to 600MHz-700MHz, a total of 18 varieties.

The idea of ​​a new generation of EMI wave device:

    Among the noise generated by switching power supplies, the differential mode noise component accounts for the main component in the low frequency band, and the common mode noise component accounts for the main component in the high frequency band. Therefore, it is necessary to use common mode and differential mode chokes. But how to reduce their size is the biggest headache for designers. Recently, Murata Corporation of Japan has developed the PLY series of hybrid chokes. This device compactly combines common mode and differential mode chokes in a very clever way. As one, the PLY10 series has common mode magnetic circuits and differential mode magnetic circuits respectively. Each magnetic circuit has its own ferrite core. Its maximum installation size is L×W×H=18×16×17.5mm. The rated current range is 0.5~2.0A, the 100VDC voltage-resistant products can withstand 40~150W power, and the 200VDC voltage-resistant products can withstand 75~300W power. Such a wide range of use makes the PLY10 series devices compatible with most switching power supply equipment. compatible.

    The example of the PLY series gives everyone an inspiration. Effective suppression of common and differential mode noise of switching power supplies must rely on specialized inductive devices - common and differential mode chokes, which are composed of simple chip (capacitor, inductor) components. EMI filters are difficult to suppress the common and differential mode noise generated by switching power supplies.

    For this reason, a new generation of EMI filter that effectively suppresses switching power supply noise should be a related circuit composed of effective common and differential mode chokes and a number of capacitors. In order to reduce the size, these inductors and capacitors should be chip components as much as possible. .

    Another method that can be used for reference is to develop dedicated matching EMI filters for the series of power modules produced by professional power module manufacturers through CEO3 of MIL-STD-461 or other specifications. For example, Advanced Analog Company of the United States provides AFC461 series EMI filters for AHE, ATO, ATW and AHF series DC/DC power modules. Its rated voltage is 28~40VDC, rated current is 4A, 100kHz-50MHz insertion loss is 40dB, and the operating temperature is -55~ 155°C, minimum size L×W×H=53.85×28.45×9.65mm. Similarly, the American Interpoint Company provides FMH-461EMI filters for MHE/MLP, MHL, MTO and MTW series DC/DC power modules. Its insertion loss index emphasizes a minimum attenuation of 50dB at 500kHz and a minimum attenuation of 35dB at 200kHz.

    VICOR also configures its new generation power module with a MICRO packaged input filter module FILTMOD and a MICRO packaged input protection module LAM2. The former can effectively eliminate conducted interference noise, and the latter can effectively limit surge current and peak interference, such as Shown in Figures 2 and 3.

    It is recommended that the circuit of the switching power supply EMI filter adopts the standard mode circuit shown in Figure 4, which is conducive to development into a standard module and final integration. In actual application, if the suppression of the first-level filter module is not enough, a second-level filter module can be connected in series.

    Among domestic products, the experimental sample CX series circuit developed by Beijing Zhongbei Innovation Technology Development Co., Ltd. adopts the standard model shown in Figure 4. The structure uses a choke coil, the volume is L×W×H=15×15×9.5mm, and the rated voltage is 50VDC. Rated current 0.5~2A.

Outlook

    In order to adapt to the needs of the continuous miniaturization of electronic equipment, the emergence and development of a new generation of EMI filters based on chip capacitors and inductors is a historical necessity. Like the development history of standard module power supplies, the new generation of EMI filters should also develop in the direction of standard mode circuits. Due to the rapid development of DC series chip capacitors, the development of a new generation of DC series EMI filters may become a reality. Unfortunately, in the development of AC chip capacitors, currently there are only Murata's CHM2143, GHM2145 and GHM2243 series, and their AC withstand voltage performance is worse than that of traditional polyester film capacitors. Therefore, the new generation of AC series EMI filters still needs the in-depth development and development of AC chip capacitors.