Application of Stacked Power Inductor in the Design of High Performance Small Power Supply Circuit

With the multi-function of mobile devices, the working voltage of its power supply circuit has become diversified. Specifically, taking a typical mobile phone as an example, in addition to the original call function, various functions such as camera, radio and television have become common standard functions. The voltage required for the operation of these functions is different. Therefore, the battery voltage must be converted into the voltage required for the normal operation of each circuit through the power conversion circuit. Switching controllers (commonly referred to as DC-DC converters) with high power conversion efficiency are mostly used.

In addition, in the multi-function process of mobile devices, the requirements for small and thin machines are also gradually improved. Therefore, the number of components must be reduced or the components must be smaller. This countermeasure is to increase the switching frequency of DC-DC converter and reduce the necessary rated parameters of power inductance and capacitance, so as to adapt to the miniaturization of components. The switching frequency of the PMIC (power management IC) of the centralized control power supply will be changed from 1MHz to 3MHz, and the mainstream frequency of the DC-DC Converter IC managing the separate power supply is always 3 4MHz. In this case, the power inductance as the main component needs a low sensing value product of 1.0uh to 2.2uh. The improvement of switching frequency makes noise suppression necessary. In order to solve these problems, the development of power inductor is promoted.

Necessary characteristics of power inductance in mobile phone power supply circuit

This paper expounds the requirements for the shape and characteristics of power inductance in the power supply circuit of multifunctional small mobile phone. There are mainly the following three items.

? Small volume and thin thickness

? It can adapt to the characteristics of high power conversion efficiency of power circuit

? It has anti noise ability under the working state of power supply

Next, the performance of the power inductor with the above performance developed by Murata production is explained.

Laminated power inductor developed for small power supply circuit

Power inductor and its main performance for small and thin type

Murata Manufacturing Institute has developed a series of laminated power inductors lqm2hp (2520 size) and lqm2mp (2016 size) corresponding to the miniaturization demand of power circuit, and put them into mass production. These power inductors are ultra-thin designs that meet the trend of 1.0mm thick packaging, such as PMIC and baseband IC at the power supply end. The outline is shown in Figure 1.

Figure 1-1: lqm2hp outline

Figure 1-2: lqm2mp outline

Through the DC current, the induced value has good bias characteristics, which can be used as the specialty of each power inductor. Current inductance bias refers to the property that when the power inductor is energized, the current increases and the inductance decreases. This is caused by the magnetic flux saturation caused by the closed magnetic circuit structure of the laminated power inductor. Therefore, in order to overcome this weakness, new technologies are developed to reduce the magnetic flux distribution in the laminated ferrite, so as to improve the superposition characteristics of the current, which is finally reflected in various products.

Table 1 shows the main characteristics of each product, and Figure 2 shows the current sensing value bias characteristics.

Table 1: main characteristics of lqm2hp (step-up) and lqm2mp (step-down)

Figure 2: current superposition inductance characteristics

High power conversion efficiency characteristics of power supply circuit (DC-DC converter)

The correlation between power conversion efficiency and power inductance performance of DC-DC converter is shown in Figure 3. PFM refers to the mode in which the mobile phone maintains a low current load in the standby mode. At this time, the power conversion efficiency performance is related to the RAC (AC resistance) of the power inductor and the current sensing value paranoia. Fig. 4 shows the RAC characteristics of each power inductor and the power conversion efficiency characteristics of the DC-DC Converter IC with a switching frequency of 4MHz in Fig. 5. Like the RAC characteristic shown in Figure 4, LQM series products can fully inhibit RAC. At the same time, as shown in the current sensing value bias characteristic of Fig. 2, it can also ensure high sensing value when the current is energized, and has good power conversion efficiency characteristics. These performances can achieve high power conversion efficiency of mobile phones in standby mode and contribute to prolonging battery life.

On the other hand, PWM mode refers to the high current load state equivalent to the call state. This mode is related to the RDC characteristic (DC impedance) of the inductor. LQM series with low RDC performance has good power conversion efficiency in this field.

Figure 3: correlation between power conversion efficiency characteristics and inductance characteristics

Figure 4: RAC characteristics

Figure 5: power conversion rate data (used by 4MHz DC-DC Converter IC)

Countermeasures of anti noise performance

The DC-DC converter is used in the power circuit of the mobile phone. The magnetic beam leakage of the power inductor may induce the instability of the power circuit and have a bad impact on the circuit. These problems can be solved by using laminated power inductors with less magnetic beam leakage, which can prevent noise interference and keep the power supply circuit stable. In addition, less magnetic beam leakage can also weaken the electromagnetic coupling between the power supply circuit and the surrounding signal lines and suppress the degradation of signal purity induced by noise. Figure 6 shows the magnetic beam leakage monitoring data of wound power inductor and laminated power inductor. Compared with the wound power inductor which has always been the main power inductor used in DC-DC converter, the laminated power inductor has less magnetic beam leakage. It has been proved that the structure of closed magnetic circuit makes less magnetic beam leakage. These data show that when a small laminated power inductor is used in a small high-speed switching DC-DC converter circuit, it can give full play to its anti noise characteristics.

Figure 6: comparison of magnetic beam leakage between laminated power inductor and wound power inductor

Summary

The demand for small and thin portable devices makes the size of components carried on the device smaller and smaller. The miniaturization and thinness of the DC-DC converter in the power supply circuit can make the switching frequency higher and the surrounding components smaller. During the development of small laminated power inductors, Murata manufacturing office manufactures 1008 and 0806 size products with excellent current inductance bias and less magnetic beam leakage in the energized state. In the future, more miniaturized 0805 and 0603 size laminated power inductors will be further developed to contribute to the design of high-performance small power supply circuits for portable devices.

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