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Demands on power inductors for today's products include lower profile and better energy efficiency (lower DC resistance). In this article, Murata's Tadashi Tanaka describes recent technology improvements made by Murata in response to these demands.
The operating voltage ranges used by power circuits found in battery operated mobile equipment are becoming increasingly varied. For example, a typical mobile phone might contain an LCD driver, power amplifier module and baseband IC power circuits, all of which require different operating voltage. DC-DC conversion circuits are therefore required to convert from the power voltage supplied to the operating voltage required by each power circuit.
DC-DC converters are required to have properties that can reduce power consumption and extend battery life. For this reason, non-isolated chopper type circuits have been widely adopted. A key component in this type of circuit is the power inductor, and it can contribute significantly to voltage conversion efficiency.
As equipment and devices continue to become faster, smaller and lighter, DC-DC converters need to become smaller, lighter and lower profile too. Higher switching frequencies are also demanded from conventional wirewound inductors. These demands from the market have led to acceleration in the development of new technologies involving materials, design and manufacturing methods.
With wirewound inductors, shielding is used to prevent magnetic flux leakage. Parts in the LQH series (shown in photo) adopt a closed magnetic path structure where the wire portion is shielded all around with a coating of magnetic particles in resin (see Figure 1) to prevent magnetic flux leakage. This series has a simpler structure than other power inductors on the market today, which have a closed magnetic path structure using ferrite sleeve cores. Because of its structure, the LQH series makes maximum use of its wiring space and thus can offer smaller size and lower profile through its compact design.
The allowable current value for a wirewound inductor is pre-determined by the DC bias current characteristic. The DC bias current characteristic is dependent on the inductance-current characteristic and the temperature rise characteristic, which is caused by the heat generation associated with wirewound components.
Both of these characteristics have been improved by optimising the structure and design of the magnetic particle resin. Murata's power inductors are designed with an optimal combination of the two values to provide the highest possible allowable current. Figure 2 gives an indication of the improvements made to the properties of Murata's wire-wound inductors to improve the allowable current.
Figure 3: Voltage conversion efficiency circuit used for evaluation
As an example, Figure 3 shows the voltage conversion efficiency of a DCDC converter circuit equipped with an inductor from Murata's range, the LQH3NPN220N, in this case a 22µH inductor with the dimensions 3.0 x 3.0mm. Murata used a step-up DC-DC converter evaluation circuit with a 1MHz switching frequency for this evaluation. For comparison, Murata also measured another manufacturer's product with a ferrite sleeve core structure with dimensions of 4.0 x 4.0 x 1.2mm. The comparison test confirms that although the product volume of the LQH-series inductor is less than half that of the ferrite sleeve core product, the LQH-series product can provide the same level of voltage conversion efficiency. As a result, this series can contribute to advances in product size and profile reduction of mobile equipment.
Conventional ferrite cores for power inductors are moulded out of ferrite powder, and then fired. However, there are many constraints associated with this method that prevent design flexibility. Murata has developed a method of cutting (rather than moulding) the inductor cores to form the shape which results in faster and more flexible production, and better performance. This new method allows the core to be entirely circular, unlike with previous methods which were closer to rectangular in shape and had a higher DC resistance (See Figure 4). The lower DC resistance exhibited by perfectly circular cores contributes to the overall energy efficiency of the DC-DC converter circuit.
Murata has recently introduced two new power inductors employing these advancements in technology. The LQH6PP and LQH88P series of medium sized wire-wound power inductors with footprints of 6 x 6mm and 8 x 8mm respectively. The lowprofile LQH6PP features height of just 4.5mm, and the LQH88P 4.0mm is even thinner.
These products will be particularly suitable for applications such as DC-DC converters for flatscreen televisions, in which low profile is essential. Despite their compact size, the latest design methods have created a wide range of inductance values, 1.0 to 100µH. Inductors in the LQH6PP have rated current of up to 7.2A (1.0µH model), and the rated current of the LQH88P series is even higher at up to 11.2A.
Advancements in power inductor technology include the introduction of cutting methods for constructing ferrite cores, plus magnetic shielding using magnetic particles in resin. The overall effect on power inductor performance has been significant. Inductors in very small packages are now able to handle large rated currents
