Multilayer ferrite coils for decoupling circuits
High current capability in a small package
Variations in the power source voltage caused by switching at the DC/DC converters can spread to other circuits through the power source line as noise. Multilayer ferrite coils from the TDK MLZ-H series offer the high current capability required in the decoupling circuits of electronic devices such as digital cameras, video cameras and notebook PCs.
The miniaturization of semiconductors improves the working speed of circuits and has led to a corresponding decrease in drive voltage. For example, the drive voltage of circuits which use ICs has been progressively lowered from 5 V to 1 V or below. However, as the drive voltage decreases, greater stability in the power supply is required, as even the slightest variance in voltage can cause the IC to malfunction. With the increase in speed of electronic devices in recent years, it has become common to fit small DC/DC converters in close proximity to each IC or at the point of load (POL). Figure 1 shows the use of a coil in a decoupling circuit.
|Figure 1: Use of coils for decoupling in power circuits of DC/DC converters|
LC filters improve circuit efficiency
The coil for decoupling is series-connected to the power circuit and has two major functions (Figure 2). One is the suppression of high-frequency noise caused by the DC/DC converters. The higher the frequency of the alternating current, the more resistance it encounters. An LC filter (low-pass filter) is often used to improve efficiency. Unlike the coil, in the case of the capacitor the higher the frequency the more easily the current passes. High-frequency noise reflected by the coil, therefore, travels through the capacitor and is bypassed to the earth side.
The second function of the coil is to stabilize the power supply voltage. When there are fluctuations in the current, the coil emits an electromotive force in a direction that stops the fluctuation (the coil’s self-induction function), and stores the electric energy as magnetic energy. This enables the coil to prevent fluctuations in response to an instantaneous rise or drop in the power supply voltage and maintain the magnitude of the current, thus stabilizing the power supply voltage.
Significantly increased rated current
Because currents in power circuits are much higher than the currents in signal circuits, wound coils have until now been the most commonly used coil for decoupling. Compared with multilayer coils, wound coils have been distinguished by their superior current capability. For this reason, it has been possible to use multilayer coils in relatively few devices. The TDK MLZ series of multilayer ferrite coils changes that. The MLZ series utilizes a ferrite material that features a higher rated current, making it compatible with many circuits that were previously only compatible with the wound type. Employing its decades of experience with ferrites and using advanced materials technology, TDK has engineered the finely textured polycrystalline structure of ferrite materials to improve the rated current.
While the latest ferrite material has the same basic composition as previous materials, its saturation flux density was vastly improved. Based on these advances, the TDK MLZ-W series of multilayer ferrite coils was introduced in 2009, which featured a rated current of up to 2.5 times higher than previous products. The latest addition to the MLZ family is the MLZ-H series with even higher current capability (Figure 3).
|Figure 3: Comparison of wound coils and multilayer coils|
Thanks to its enhanced DC superimposition characteristics and optimized layer structure, the new MLZ2012-H series features a rated current of 700 mA at an inductance value of 1.0 µH, an increase of up to 2.5 times over the MLZ-W series (Figure 4) and equivalent to that of wound coils. This was also made possible through the use of low-resistance electrodes. Moreover, high-density packaging is possible thanks to the MLZ-H series’ magnetic shield structure. Finally, the new series is RoHS-compatible and can be processed with lead-free soldering.
|Figure 4: Inductance DC superimposition characteristics|
Coil for decoupling in a wider range of applications
With the TDK MLZ-H series, a high-spec multilayer ferrite coil is now available that can be used in many applications previously reserved for larger wound coils – thanks to the unique ferrite material and optimal layered structure. With DC superimposition characteristics that are now equivalent to those of wound coils, the miniature, low-resistance coil is ideal for decoupling in small electronic devices such as digital cameras, video cameras and notebook PCs.
|Typ. DC |