Santosh Kulkarni

The integration of power inductors is a roadblock in realizing highly miniaturized power supply in package (PSiP) and power supply on chip. Inductors in such power systems are used for energy storage and filtering, but they dominate in size and loss. This paper presents a novel three-dimensional in-silicon through-silicon via (TSV) magnetic-core toroidal inductor for PSiP. The magnetic powder based core is embedded into a TSV air-core inductor using a casting method. The unique air-core inductor design with a hollow core and suspended windings enables a complete core filling with microscale magnetic powders. The proposed casting method is simple, scalable, and generic for a wide range of magnetic powders. TSV magnetic-core inductors are fabricated in a compact size of 2.4 mm × 2.4 mm × 0.28 mm with the core content varying from 63 to 88 wt% of soft ferrite NiZn powders. The TSV magnetic-core toroidal inductors are fabricated and electrically characterized. Small-signal measurements show a threefold higher inductance of 112 nH and a 30% higher quality factor of 14.3 at 12.5 MHz for magnetic-core inductors compared with similar TSV air-core inductors. The small-signal measurement results are verified by the modeled results. The total core loss is characterized by large-signal measurements. A suitable inductor is implemented in a 12-MHz buck converter that operates in a zero-voltage-switching mode. The converter achieves a peak efficiency of 71.6% and an output power of 2.4 W while converting 12 to 5 V _dc .

This paper presents the design, fabrication, and characterization of on silicon integrated thin-film transformers for power applications at switching frequencies between 1 MHz and 10 MHz. Two designs of microtransformers are developed where the footprint of both designs is the same but each design has different winding style. Primary and secondary windings are interleaved in the first design and separated in the second design. Both designs show an inductance of 400 nH and 800 nH, respectively. The microtransformer devices are used and tested in a dc-dc converter that runs at a 4MHz switching frequency. The dc-dc converter with the microdevice achieves a flat efficiency curve with 72% peak efficiency.

This paper details the design, assembly, and detailed characterization of printed circuit board (PCB) embedded thin magnetic film inductors for Power Supply in Package applications. Solenoidal inductors were assembled on copper tracks printed on PCB with wire bonds to complete the copper loop. Furthermore, the solenoid inductors have laminated amorphous soft magnetic thin films embedded between the two conductor layers. The devices have chemically thinned Vitrovac films laminated as core material. Three different designs of solenoid inductors with different numbers of turns are assembled and characterized. The assembled PCB inductor measured a highest quality factor of 8.5 for a three-turn device at 10 MHz.

Integrated magnetic components are key elements of the Power Supply on Chip modules. Due to the application requirements, these magnetic devices work at very high frequency and have low inductances. Conventional small-signal tests do not provide all the required information about the magnetic device. Hence, it is important to develop new set-ups to apply large signals to accurately measure the performance of devices under realistic operating conditions, including non-linear core effects. The proposed experimental set-up is suitable to measure the device impedance under different large-signal test conditions, similar to those in the actual converter, since the excitation current can be configured through every winding: ac current up to 0.5 A at frequencies up to 120 MHz and dc bias current up to 2 A through one or both windings. Voltage and current are measured using commercial instrumentation. Due to the characteristics of the probes and the high frequency of the test, the attenuation and delay due to the probes and the experimental set-up have to be taken into account when processing the voltage and current waveforms to calculate the impedances. The compensation test to calculate this attenuation and delay is described. Finally, the proposed set-up is validated by measuring a two-phase coupled inductors micro-fabricated on silicon.

Coupled inductors offer significant advantages over their uncoupled counterparts; however, with these advantages come a number of additional design caveats. The factors affecting the design and optimization of coupled stripline microinductors for PwrSoC applications are outlined, which include device area, the number of coupled phases, duty cycle, paralleling of coupled inductors, switching frequency, as well as thermal and saturation constraints. Results of this analysis are presented and discussed along with guidelines for the design of coupled stripline microinductors, which show that paralleling coupled inductors is the best route toward higher output current. Analytical models predict a peak inductor efficiency of 86.8% and 86.3% for fabricated 3 and 5-phase coupled stripline microinductors with an Ni ₄₅ Fe ₅₅ core, respectively. Models are verified by measurements on prototype 3 and 5-phase converters with parallel coupled stripline microinductors over a range of operating conditions.

In this paper, models that account for the duty cycle and phase shift angle of the applied phase voltages in coupled stripline microinductors are presented and are shown to have a significant impact on harmonic current amplitudes and therefore microinductor efficiency. The impact of a high coupling factor between two windings surrounded by a single core is also investigated. The models are validated using finite-element analysis and measurements. A three-phase-coupled microinductor has been fabricated with a Ni ₄₅ Fe ₅₅ core and analyzed for a number of operating conditions. A prototype 1.6 W, three-phase converter utilizing this inductor has also been measured and is discussed in detail. The coupled microinductor is predicted to have a peak efficiency of 86.6% at 20 MHz in the prototype circuit.

Realizing an integrated Power Supply in Package solution using a 2.5D or 3D integration technology requires significant advancement in manufacturing processes of magnetic components such as inductors, transformer. Development of these integrated structures involves fabrication of high aspect ratio integrated windings and 3D core topology. These process improvements are required to reduce the losses i.e, copper conduction loss and enhance inductance density through a multilayer closed magnetic core. This work reports development of resist processing techniques to achieve thick copper windings with resist aspect ratio of 5.5. Integrated inductors and transformers require closed magnetic cores such as (Ni ₄₅ Fe ₅₅ ) to maximize the stored flux. At higher operational frequencies, the lower device footprint requires stacked multi-layer magnetic cores with 3D topologies to close the flux path. This work also presents new processing technique using temporary resist to pattern magnetic core deposited in a cavity of over 70 μm depth while minimizing the over-etch of the magnetic core. A 75 μm single spin process is developed with negative tone THB-151N as replacement for existing permanent SU-8 resist process. This process has significant advantage in terms of easy removal, resolution and adhesion to metals for electroplating applications. Core over-etch were reduced by 44% on using THB 151N and excellent adhesion was achieved.

Advances in wide bandgap semiconductors have facilitated an increase in switching frequencies of power supplies. Higher switching frequencies are key to reducing the size of power components. However, miniaturization of magnetic passive components at high frequencies is still a challenge. Soft magnetic high flux density thin films are being widely researched as a replacement for ferrite cores. The existing thin film solutions have an inferior power loss performance compared to the best-in-class ferrites (500-600 kW/m ³ at 500 kHz and 100 mT peak field) for the same frequency. In this paper, we discuss details of a post-processing technique for nanocrystalline thin film to achieve a power loss density of 200 kW/m ³ at 500 kHz with peak flux density of 100 mT.

Accurate short-term power demand forecast is of great importance to utility companies for power system planning and operation. Based on the literature survey, artificial neural networks (ANN) are found to be an alternative to classical statistical methods in terms of the accuracy of the forecasted results. This paper presents the implementation of a Spiking Neural Network (SNN) for short-term load forecasting model to forecast one day ahead and one week ahead hourly demand pattern. The selection of input variables, SNN architecture and training algorithm are discussed in this paper. The suitability and the validation of the proposed model is investigated and compared with a feed forward back-propagation ANN model.