What is the bead
Bead has very high resistance and magnetic conductivity. It is equivalent to series connection of resistance and inductance, but both resistance and inductance values vary with frequency. The bead has preferable high frequency filter characteristics over ordinary inductance and offers resistance when in high frequency, therefore, it maintains higher impedance over a wide range of frequency domain and thus improves the filter effect.
Inductance can be used for the filter of power supply, and the circuit symbol for bead is just inductance, but anyhow, from the type it can be seen that bead is used and in the circuit function. The bead and inductance are all the same in principle but differ only in frequency characteristics.
Bead is composed of oxygen magnet while the inductance is composed of magnetic core and coil. The bead translates alternate current signal into heat energy while inductance stores the alternate current and gives it off slowly.
Bead has large inhibition effect only to high-frequency signal which normally has specification of 100 Ohm / 100 m MHZ and its resistance is much lower than inductance when in low frequency.
Ferrite bead is an anti-jamming unit that has been quickly applied and developed, it is cheap, easy to use and has remarkable effect in the filter for high frequency noise.
In circuit, we just need the wire to pass through it (what I use is just like ordinary resistance type with wire passing through and glued, form of pasted ones are also used but seldom seen on sale). When current passes through the wire, the ferrite has little or no impedance to low-frequency current but it has larger damping effect on higher frequency current. High frequency current emits thereof in form of heat and its equivalent circuit is in series connection with one inductor and one resistor and the values of the two units are in proportion to length of the bead. Bead has a large varieties and the manufacturer shall provide description of technical specifications, especially the curve showing the relationship between bead impedance and frequency.
Some beads have various holes, passing of wire will increase impedance of the unit (the square of times that wire passes the bead), but when in high frequency, the increased ability to noise suppression can not be obtained as expected, therefore use of series connection of more beads will bring better results.
Ferrite is a magnetic material, and will cause saturation if the current passing is too large, leading to a sharp decline of magnetic inductivity. For strong current, beads that custom-designed in structure shall be used and heat radiation measures shall also be introduced.
Ferrite beads can be used not only for the filter of high frequency noise in power supply circuit (for DC and AC output), but also for other circuits and can be manufactured into very small size. Bead can play an important role especially in case of digital circuit as pulse signal contains very high frequency of upper harmonic which is the major source of high frequency radiation.
Principle of Bead
The principal raw material of Bead is ferrite, which is a lattice structural ferrimagnetic material. Ferrite material is an iron magnesium alloy or an iron nickel alloy with its manufacturing process and mechanical property same as ceramics, and the color is grey black. The magnetic core commonly used in EMI filter is made of ferrite material and many manufacturers are providing ferrite material exclusively used for EMI suppression. This material features a very large high frequency loss with very high magnetic inductivity that can produce minimal capacitance between inductive coil windings in case of high frequency and high-resistance. To ferrite used for suppression of EMI, the most important performance parameters are magnetic conductivity μ and saturation magnetic flux density Bs. Magnetic conductivity μ can be expressed as plural，the real part forms inductance while the imaginary part represents loss which will increase with increase of frequency. Therefore, its equivalent circuit is a series circuit consisting of inductance L and resistance R, both L and R are functions of frequency. When wire passes through this kind of ferrite core, the inductance impedance then formed will increase with rise of frequency in shape, but the mechanism is quite another under different frequencies in reality.
In low-frequency range, impedance is composed of inductive reactance and when in low frequency R is small and magnetic conductivity of the magnetic core is higher, therefore the inductance amount is larger, while L plays a major part, EMI is suppressed as it is reflected and the loss of magnetic core then is relatively small, the entire device features a low loss, high Q inductance which is easier to cause resonance. On this account, when in low-frequency range, enhancement of interference may occasionally occur after use of ferrite bead.
In high frequency section, impedance is composed of resistance component and rises with frequency. The lowering of magnetic conductivity of the magnetic core results in reduction of inductive inductance amount, and inductive reactance component is reduced, but the loss of magnetic core then increases, so does resistance component, causing increase of total impedance. When high-frequency signal passes through the ferrite, EMI is absorbed and converted into a form of heat energy and gets dissipated.
Ferrite suppression elements are widely used in printed boards, power lines and data wires. For instance, adding of ferrite suppression elements at entry end of power line of printed board will smooth the high-frequency interference. Ferrite bead or bead is exclusively used to suppress high-frequency interference and peak interference in signal line and power line, it also has the ability of absorbing ESD impulse interference.
The size of numerical value of two elements is proportional to the length of bead which has an obvious impact on the suppression effect, the longer is the length of the bead, the better is the suppression effect.
Selection and Use of Bead:
Please take special care that the unit for bead is Ohm, not Hunter, because the unit for bead is rated as per the impedance it produces at a certain frequency, and the unit for impedance is also Ohm. A characteristic curve diagram of frequency and impedance will normally be provided on the datasheet of bead and normalized as 100MHz, for example, 1000R@100MHz, it means the impedance of bead at 100MHz frequency is equivalent to 600 Ohm.
