David Gilham

Gallium Nitride based 3D integrated non-isolated point of load module

The introduction of Gallium Nitride (GaN) based power devices offers the potential to achieve higher efficiency and higher switching frequencies than possible with Silicon MOSFETs. This paper will discuss the GaN device characteristics, packaging impact on performance, gate driving methods, and the integration possibilities using GaN technology. The final demonstration being an integrated 3D point of load (POL) converter operating at a switching frequency of 2MHz for a 12V to 1.2V buck converter with a full load current of 20A. This 3D converter employs a low profile low temperature co-fired ceramic (LTCC) inductor and can achieve a full load efficiency of 83% and a power density of 750W/in3 which doubles the power density of current integrated POL converters on the market today.

Optimization of a high density gallium nitride based non-isolated point of load module

The demand for future power supplies to achieve higher output currents, smaller size, and higher efficiency cannot be achieved with conventional technologies. There are limitations in the packaging parasitics, thermal management, and layout parasitics that must be addressed to push for higher frequencies and improved power density. To address these limitations, the use of integrated 3D point of load (POL) converters utilizing GaN transistors, low profile magnetic substrates, and ceramic substrates with high thermal conductivity will be considered. This paper will discuss the effect of parasitics on the performance of high frequency GaN POLs, methods to improve the circuit layout of a highly integrated 3D integrated POL module, and the thermal design of a high density module using advanced substrates. The final demonstration is a 900W/in3 12V 2MHz Alumina DBC GaN converter which offers unmatched power density compared to state of the art industry products and research.

A high frequency core loss measurement method for arbitrary excitations

Recently, point of load (POL) converter are pushed to higher switching frequency for higher power density. As the frequency increases, magnetic core loss becomes a significant part of the total loss of POL converters. Accurate measurement of this part of loss is important for the converter design. And the core loss under non-sinusoidal excitation is particularly interesting for pulse-width modulation (PWM) converters. However, precise measurement is difficult with classical four-wire methods, because of high frequency and non-sinusoidal flux waveform (like triangular flux). In this paper, a new method is proposed for high frequency core loss measurement with arbitrary excitation. The principle is to cancel the reactive power in the core under test with a lossless or low-loss core and reduce the sensitivity to phase discrepancy. By doing this, phase discrepancy induced error can be significantly reduced, and accurate measurement can be achieved for higher frequency than conventional method.

New core loss measurement method for high frequency magnetic materials

Magnetic core loss is an important concern for power converters. As the switching frequency increases and converter size reduced, the core loss will have significant impact to the converter efficiency and temperature. Accurate evaluation is important for magnetic design and converter loss estimation. The classic two-winding method is limited to low frequencies (usually below 1 MHz) because it is sensitive to phase discrepancy. In this paper, a new method is proposed for high-frequency core loss measurement that utilizes capacitive cancellation, which is suitable for HF and VHF core loss measurement. The new method greatly reduces the sensitivity to phase discrepancy, which is the dominating error source in the conventional two-winding method. An experimental demonstration is performed at 10 MHz, and the possible errors are analyzed in detail. With the proposed method, the high-frequency core loss can be accurately measured.

High-Density Low-Profile Coupled Inductor Design for Integrated Point-of-Load Converters

Low-profile integrated point-of-load (POL) converter is todays industry trend for portable electronic applications. Magnetics is the major challenge and bottleneck for achieving a low-profile high-power-density integrated POL. So, how to design a low-profile magnetic becomes one of the key technologies for integrated POL. Inverse coupling is one of the possible methods used to reduce inductor size due to the dc flux cancelling effect. Several integrated low-profile coupled inductor structures with different flux patterns (vertical flux and lateral flux) are proposed and studied in this paper based on low-temperature co-fired ceramics (LTCC) technology. Two LTCC coupled inductor prototypes are designed and fabricated to verify the theoretical analysis. A 1.5-MHz, 5-1.2 V, 40-A 3-D integrated buck converter with LTCC coupled inductor substrate is also fabricated. The peak efficiency of this integrated converter is as high as 89%. The power density of this integrated converter is as high as 680 W/in3, which is almost six times higher than todays industry products with the same current level.

System design of a 3D integrated non-isolated Point Of Load converter

Thermal issues in the design of high-power and small-size non-isolated Point of Load (POL) converters have been significant and are becoming more so. If a heat sink is dispensed with, then a large area of multi-layer PCB material with significant amounts of copper is required for necessary cooling. A high-rate cooling fan is also necessary. So what at first seems like a small and elegant converter becomes bulky and challenging to position in the overall system when its thermal management is taken into account. This work finds a solution to these problems and investigates the use of Low- Temperature Co-fired Ceramic (LTCC) for integrating the inductor with the active stage. A buck POL is used to demonstrate these methodologies. The end result is a POL converter that can achieve full 20 A output current with no need for an external heat sink and no need for a cooling fan while providing a large step-down of 5 V to 1.2 V. High light- load efficiency of 92.2% is achieved and a high power density of 156 W/in3 including all the necessary thermal management, in natural convection condition, has been demonstrated.

New Core Loss Measurement Method for High-Frequency Magnetic Materials

Magnetic core loss is an emerging concern for integrated POL converters. As switching frequency increases, core loss is comparable to or even higher than winding loss. Accurate measurement of core loss is important for magnetic design and converter loss estimation. And exploring new high frequency magnetic materials need a reliable method to evaluate their losses. However, conventional method is limited to low frequency due to sensitivity to phase discrepancy. In this paper, a new method is proposed for high frequency (1MHz∼50MHz) core loss measurement. The new method reduces the phase induced error from over 100% to <5%. So with the proposed methods, the core loss can be accurately measured.

High-Density Integration of High-Frequency High-Current Point-of-Load (POL) Modules With Planar Inductors

Planar inductors made by mixed laminates of low-temperature sintered Ni-Cu-Zn ferrite tapes and metal-flake composite materials are used for high-density integration of point-of-load (POL) modules. Incremental permeability and core loss density were characterized on toroidal samples under high dc bias to demonstrate that both materials are suitable for application in high-frequency high-current POL converters. In order to realize a high power density POL module, a multilayer ferrite inductor laminated with alternating layers of ESL 40010 and ESL 40012 in a 1:1 ratio has been fabricated and integrated with the active layer. Meanwhile, standard printed circuit board (PCB) processing has been adopted for the POL integration with a PCB-embedded inductor using NEC-TOKINs metal-flake composite materials. These developed 3-D integration approaches can be used to reduce the footprint and increase the power density for POL converters. It has been demonstrated that the power efficiency of both POL modules with integrated planar inductors can achieve above 87% at an operating frequency of 2 MHz and an output current of 15 A. Additionally, no obvious efficiency degradation was observed on the integrated POL modules after a certain number of thermal cycling from -40 °C to +150 °C.

Low profile LTCC inductor substrate for multi-MHz integrated POL converter

The emerging Gallium Nitride (GaN) based power device enables high frequency point-of-load (POL) converter with high current capability. This paper presents the low profile LTCC inductor substrate design and evaluation for multi-MHz 3D integrated POL module with GaN device. In the design process, the core thickness and loss of the inductor are main considerations. The impact of different frequency on the LTCC inductor substrate design is discussed. The LTCC inductors and some commercial discrete inductors are experimentally evaluated on a 12V to 1.2V 15A POL converter built with EPCs GaN devices. The comparison demonstrates that the LTCC inductor can dramatically improve the converter light load efficiency due to its nonlinear inductance, while maintain almost the same full load efficiency as the discrete inductor. Because of the low profile design, the power density of the POL module with LTCC inductor is much higher than that with discrete inductor. The 3D integrated POL module achieves 84.5% efficiency and 700W/in3 power density at 2MHz, 81.8% efficiency and 800W/in3 power density at 3MHz.

Modeling of planar inductors with non-uniform flux distribution and non-linear permeability for high-density integration

High-density integrated point-of-load (POL) converter is todays industry trend for portable electronic applications. Low-profile planar inductors can be used as substrates for 3D integrated POL converters to achieve very high power density. However, most planar inductors have non-uniform flux distribution. Some planar inductors, such as low-temperature co-fired ceramics (LTCC) inductors, even have non-linear permeability. This means that the conventional inductance calculation method, which is based on uniform flux and constant permeability, cannot be used directly for these planar inductors. This paper proposes some simple but sufficiently accurate models (one numerical, the other analytical) to calculate inductance for planar inductors with non-uniform flux distribution and non-linear permeability. Based on these models, several low-profile LTCC inductor substrates are designed for 3D integrated POLs. The measured inductance closely matches the predicted result, which means the proposed models have very good accuracy.