Solutions for power conversion efficiency and power inductor performance of DC-DC converters-EEWORLD

Collect

With the continuous development of high-tech, various devices have entered the high-end stage. As a reserve force, power circuits have also made significant progress. 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 universal standard functions. The voltages required for the operation of these functions are different. For this reason, the battery voltage must be converted into the voltage required for the normal operation of each circuit through a power conversion circuit. Most of them use a switching controller (usually called a DC-DC converter) with high power conversion efficiency.

In addition, as mobile devices become more multifunctional, the requirements for smaller and thinner machines are gradually increasing. To this end, it is necessary to reduce the number of components used or make the components smaller. This countermeasure is to increase the switching frequency of the DC-DC converter and reduce the rated parameter values of the necessary power inductors and capacitors to adapt to the miniaturization of components. The switching frequency of the PMIC (Power Management IC) that centrally controls the power supply will be changed from the 1MHz that has been used to 3MHz, and the mainstream frequency of the DC-DC converter IC that manages the separate power supply has always been 3~4MHz. In this case, the power inductor as the main component requires a low-inductance product of 1.0uH to 2.2uH. The increase in the number of switching frequencies makes noise suppression a necessity. In order to solve these issues, the development of power inductors has been promoted.

Necessary characteristics of power inductors in mobile phone power circuits

Here we will explain the requirements for the shape and characteristics of the power inductor in the power circuit of a multifunctional small mobile phone. There are mainly three items.

• Small size and thin thickness

• Possessing characteristics that can adapt to high power conversion efficiency of power supply circuits

• Noise immunity in power supply operation

Next, we will explain the performance of the power inductor developed by Murata Manufacturing that has the above performance.

Multilayer power inductor developed for small power supply circuits

Power inductors that meet the needs of small and thin products and their main performances

Murata Manufacturing Co., Ltd. has developed the LQM2HP (2520 size) and LQM2MP (2016 size) series of multilayer power inductors to meet the needs of miniaturization of power circuits and has put them into mass production. These power inductors are ultra-thin designs that meet the trend of 1.0mm thick packages for baseband ICs that combine PMICs and power supply ends. The appearance is shown in Figure 1.

Figure 1-1: LQM2HP appearance

Figure 1-2: LQM2MP appearance

The good inductance bias characteristic after passing the DC current can be used as the feature of each power inductor. The current-inductance bias characteristic 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 multilayer power inductor. Therefore, in order to overcome this weakness, new technology is developed to reduce the magnetic flux distribution in the multilayer ferrite, thereby improving the superposition characteristics of the current, which is finally reflected in each product.

Table 1 shows the main characteristics of each product, and Table 2 shows the current-inductance bias characteristics.

Product Name Inductance
(μH) Inductance Measurement Frequency
(MHz) Rated Current
(mA) DC Resistance
(Ω)
LQM2HPNR47MG0 0.47±20% 1 1800 0.040 ±25%
LQM2HPN1R0MG0 1.0±20% 1600 0.055 ±25%
LQM2HPN1R5MG0 1.5±20% 1500 0.07 ±25%
LQM2HPN2R2MG0 2.2±20% 1300 0.08 ±25%
LQM2HPN3R3MG0 3.3±20% 1200 0.10 ±25%
LQM2HPN4R7MG0 4.7±20% 1100 0.11 ±25%

Product Name Inductance
(μH) Inductance Measurement Frequency
(MHz) Rated Current
(mA) DC Resistance
(Ω)
LQM2MPNR47MG0 0.47±30% 1 1600 0.060 ±25%
LQM2MPN1R0MG0 1.0±30% 1400 0.085 ±25%
LQM2MPN1R5MG0 1.5±30% 1200 0.11 ±25%
LQM2MPN2R2MG0 2.2±30% 1200 0.11 ±25%

Figure 2: Current superposition inductance characteristics
High power conversion efficiency characteristics of power supply circuit (DC-DC converter)

The relationship between the power conversion efficiency of the DC-DC converter and the performance of the power inductor is shown in Figure 3. PFM refers to the mode in which the mobile phone maintains a low current load in the standby state. At this time, the power conversion efficiency performance is related to the Rac (AC resistance) of the power inductor and the current-inductance bias characteristics. Figure 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 Figure 5. As shown in the Rac characteristics in Figure 4, the LQM series products can fully suppress Rac. At the same time, as shown in the current-inductance bias characteristics in Figure 2, it can also ensure high inductance when the current is energized, and has good power conversion efficiency characteristics. These performances can achieve high power conversion efficiency of mobile phones in the standby state, contributing to extending battery life.

On the other hand, the PWM mode refers to a high current load state equivalent to a call state. This mode is related to the Rdc characteristics (DC resistance) of the inductor. The LQM series with low Rdc performance has good power conversion efficiency in this area.

Figure 3: Correlation between power conversion efficiency characteristics and inductor characteristics

Figure 4: Rac characteristics

Figure 5: Power conversion rate data (using a 4MHz DC-DC converter IC)

Noise immunity measures

DC-DC converters are used in the power circuits of mobile phones. The magnetic flux leakage of the power inductors used may induce instability in the power circuit and have a negative impact on the circuit. These problems can be solved by using multilayer power inductors with less magnetic flux leakage, which can prevent noise interference and keep the power circuit stable. In addition, less magnetic flux leakage can also weaken the electromagnetic coupling between the power circuit and the surrounding signal lines, etc., and suppress the degradation of signal purity induced by noise. Figure 6 shows the magnetic flux leakage monitoring data of winding power inductors and multilayer power inductors. Compared with winding power inductors, which have always been the main power inductors used in DC-DC converters, multilayer power inductors have less magnetic flux leakage, and it has been confirmed that the closed magnetic circuit structure has less magnetic flux leakage. These data show that when small multilayer power inductors are used in small high-speed switching DC-DC converter circuits, their anti-noise characteristics can be brought into play.

Figure 6: Comparison of magnetic flux leakage between multilayer power inductors and wire-wound power inductors

summary

The demand for smaller and thinner portable devices has led to the size of the components mounted on them becoming smaller and smaller. The smaller and thinner DC-DC converters in the power circuit can make the switching frequency higher and the surrounding components more compact. In the process of developing small multilayer power inductor products, Murata Manufacturing has manufactured 1008 and 0806 size products that have excellent current-inductance bias characteristics and less magnetic flux leakage when powered. In the future, we will further develop more compact 0805 and 0603 size multilayer power inductors to contribute to the design of high-performance small power circuits for portable devices. ■

Reference address：Solutions for power conversion efficiency and power inductor performance of DC-DC converters

Previous article：Discussion on Optimization of Buffer Circuit of High Frequency Power Module
Next article：The influence of cooling method on performance and service life of communication switch power supply

Popular Resources
Popular amplifiers