Laili Wang

An Interleaved LLC Resonant Converter Operating at Constant Switching Frequency

The interleaving and phase shedding techniques on LLC resonant converters will increase the load capacity, reduce the output current ripple and improve the efficiency. However, conventional frequency-controlled LLCs will lose regulation in individual phases if all phases are synchronized for interleaving, causing current imbalance. Existing load-sharing solutions for multiphase LLCs cannot achieve voltage regulation, phase shedding, and expandable load capacity all at the same time. In this paper, a switch-controlled capacitor (SCC) modulated LLC converter (SCC-LLC) is presented for multiphase paralleling. It features constant switching frequency, which results in a simple structure for paralleling and the ability for phase shedding to improve light-load efficiency. In the proposed structure, a SCC unit is used in each individual LLC stage to regulate the output gain, thereby inherently solves the conflict between the interleaving and the load sharing.

Improving Light and Intermediate Load Efficiencies of Buck Converters With Planar Nonlinear Inductors and Variable On Time Control

It has been addressed that light and intermediate load efficiencies of high-frequency buck converters in portable electronic devices are very important for extending lifetime of batteries. This paper proposes to utilize wide range, gradually changeable nonlinear inductors with variable on time control to improve light and intermediate load efficiencies. Integrated multipermeability magnetic cores are advanced to realize the nonlinear inductors. And based on the nonlinear inductors, variable on-time control scheme is proposed to reduce the frequency-dependent loss. The theoretical loss reduction is demonstrated by comparing loss of a converter employing conventional constant on-time control scheme and employing the proposed control scheme. A two-permeability nonlinear inductor prototype is fabricated. Its inductance could vary from 0.13 μH at full load to 0.66 μH at no load. The nonlinear inductor is employed in a 12 V input, 1.6 V output buck converter to evaluate its performance. With the purpose of comparison, other two commercial chip inductors are also tested in the converter. The results show the nonlinear inductor with variable on time control could effectively improve efficiencies of light and intermediate load conditions as theoretical analysis does.

Design of Ultrathin LTCC Coupled Inductors for Compact DC/DC Converters

It is found out that using reverse coupled inductors could effectively improve power density and dynamic performance of multiphase interleaved point of load converters. Thus, it is of great significance to conduct a research about design and fabrication of ultrathin coupled inductors for system 3-D integration. The aim of this paper is to explore the structures, modeling, and fabrication of ultrathin coupled inductors based on low-temperature cofired ceramic (LTCC) technology. Four structures classified by shapes and relative positions of windings are introduced and compared. Simple but effective analytic models are set up to calculate self-inductance, leakage inductance, and coupling coefficient for quick design, and a design guideline is summarized according to the models. We made a 1.3-mm-thick LTCC coupled inductor and two competing coil coupled inductors, and compared their characteristics and performance. They are tested in a 12-V input and 1.2-V/40-A output two-phase interleaved buck converter at three different switching frequencies. Core loss and winding loss of the inductors are quantified by simulation. Compared with conventional coil coupled inductors, the LTCC coupled inductor has higher power density and light load efficiency. Besides, it could also help to improve the transient performance of converters for its higher coupling coefficient.

An interleaved LLC resonant converter operating at constant switching frequency

The interleaving technique is necessary for LLC resonant converters to achieve high power level. The advantages include expanded power capacity, lower output ripple current, and higher light-load efficiency by using phase shedding. However, conventional frequency-controlled LLC converters will lose regulation in individual phases if all the phases are operating at the same switching frequency, causing load sharing problem. Existing load sharing solutions for interleaved LLC converters all have limitations. In this paper, a switch-controlled capacitor (SCC) modulated LLC converter (SCC-LLC) is presented to solve the load-sharing problem. With constant switching frequency, interleaving and phase shedding can be achieved. A 600-W, two-phase interleaved constant frequency SCC-LLC prototype is built to verify the feasibility and demonstrate the advantages.

Bipolar Ripple Cancellation Method to Achieve Single-Stage Electrolytic-Capacitor-Less High-Power LED Driver

Conventional topologies for high-power LED drivers with high power factors (PFs) require large capacitances to limit the low frequency (100 or 120 Hz) LED current ripples. Electrolytic capacitors are commonly used because they are the only capacitors with sufficient energy density to accommodate high-power applications. However, the short life span of electrolytic capacitors significantly reduces the life span of the entire LED lighting fixture, which is undesirable. This paper proposes a bipolar (ac) ripple cancellation method with two different full-bridge power structures to cancel the low-frequency ac ripple in the LED current and minimize the output capacitance requirement, enabling the use of long-life film capacitors. Compared with the existing technologies, the proposed circuit achieves zero double-line-frequency current ripple through LED lamps and achieves a high PF and high efficiency. A 100-W (150 V/0.7 A) LED driver prototype was built which demonstrates that the proposed method can achieve the same double-line-frequency LED current ripple with only 44-μF film capacitors, compared with the 4700-μF electrolytic capacitors required in the conventional single-stage LED drivers. Meanwhile, the proposed prototype has achieved a peak power efficiency of 92.5%, benefiting from active clamp technology.

Multipermeability Inductors for Increasing the Inductance and Improving the Efficiency of High-Frequency DC/DC Converters

Distributed air-gap inductors such as iron powder chip inductors and low-temperature cofired ceramic (LTCC) inductors have the advantage of low-fringing effect loss. However, the flux density nonuniformly distributes in the magnetic cores, which results in the magnetic material closer to the conductor becoming saturated while the magnetic material further away from the conductor is still not fully utilized. This paper proposes a multipermeability distributed air-gap inductor structure to increase inductance without the necessity of increasing the inductor volume. The best discrete permeability value is investigated. Based on the best discrete permeability value, inductance as well as the inductance density trends is calculated by varying the number of permeability layers under the condition that thickness for each layer is constant. Also, the inductance variations versus the number of permeability layers are also obtained under the condition that the inductor thickness is constant. A three-permeability inductor and a single-permeability inductor are fabricated to evaluate the proposed method. The measured results show that the three-permeability inductor has a much higher inductance than the single-permeability inductor for the entire load range. Both inductors are tested in a 5-V input, 3-V output dc/dc converter to compare their performances. The results show that the three-permeability inductor can further improve the efficiency of high-frequency dc/dc converters.

A horizontal-winding multi-permeability distributed air-gap inductor

Distributed air-gap inductors have the advantage of reducing winding loss in high switching frequency DC/DC converters. However, they also have the disadvantage of uneven distribution of flux density, which inevitably leads to the incomplete utilization of magnetic material. This paper proposes a horizontal-winding multi-permeability distributed air-gap inductor structure to increase inductance without the increase of inductor volume. To evaluate the proposed method, a two-permeability inductor together with a single permeability inductor is fabricated. The measured results show the two-permeability inductor has higher inductance than the single-permeability one, especially at light load. Both inductors are tested in a 12V input, 1.6V output DC/DC converter to show their performance. The results show the two-permeability inductor could further improve light load efficiency of high frequency DC/DC converters.

Electrolytic-capacitor-less high-power LED driver

Conventional topologies for high-power LED drivers with high power factors require large capacitances to mitigate the output current ripples. Electrolytic capacitors are commonly used because they are the only capacitors with sufficient energy density to accommodate high power applications. However, the short life span of electrolytic capacitors significantly reduces the life span of the entire LED lighting fixture, which is undesirable. This paper proposes a single-stage high-power LED driver using ripple compensation concept to minimize the output capacitance requirement, enabling the use of long-life film capacitors. Compared to existing technologies, the proposed circuit achieves zero ripple current through LED lamps and achieves a high power factor and high efficiency. A 100W (150V/0.7A) LED driver prototype was built which demonstrates that the proposed method can achieve the same LED current with only 44μF film capacitors, compared to the 4700μF electrolytic capacitors required in conventional single-stage LED drivers. Meanwhile, the proposed prototype has achieved a peak power efficiency of 92%, benefiting from active clamp technology.

Bang-Bang charge control for LLC resonant converters

This paper proposes a Bang-Bang Charge Control (BBCC) method for LLC resonant converters. It utilizes the series resonant capacitor to directly control the cycle-by-cycle input electric quantity, thus can provide very fast dynamic performance in all input and output conditions. The proposed method can be extended to other topologies that include a series capacitor.

A Horizontal-Winding Multipermeability LTCC Inductor for a Low-profile Hybrid DC/DC Converter

Distributed air-gap inductors have the advantage of reducing winding loss in high switching frequency dc/dc converters. However, they also have the disadvantage of uneven distribution of flux density, which inevitably leads to uncompleted utilization of magnetic material. This paper proposes a horizontal-winding multipermeability low-temperature cofired ceramic (LTCC) inductor to increase inductance and improve efficiency without the necessity of increasing inductor volume. For the purpose of simplicity, a two-permeability LTCC inductor is taken as an example for analysis and comparison. Design of such an inductor is demonstrated with the aid of 2-D finite elementary analysis simulation. A two-permeability LTCC inductor together with a single-permeability LTCC inductor is fabricated for measuring and testing. The measured results show the two-permeability inductor has higher inductance than the single-permeability inductor. Both inductors are tested in a 5-V input, 3.3-V output dc/dc converter to test their performances. The testing results show the two-permeability LTCC inductor could further improve the efficiency of high-frequency dc/dc converters compared with the single-permeability LTCC inductor.