Planar Transformer Research Articles

Forced Air-Cooling Thermal Design Methodology for High-Density, High-Frequency, and High-Power Planar Transformers in 1U Applications

Due to increased loss per unit volume in power transformers when operating at higher frequency, the increase in cooling system size can outweigh high-frequency transformer (HFT) size reduction benefits. Conventional air-cooling approach simply adopts oversized cooling fans with high airflow to keep the transformer below a temperature limit, resulting in a large footprint and high power consumption. For many applications using air cooling, more challenges arise when multiple transformers are placed in a dimension-constrained space in which free airflow from the cooling system is easily disrupted. This work seeks to address this tradeoff by presenting a high-density, low-profile, forced air-cooling system for HFTs. The combination of an epoxy resin cast transformer winding and converging air duct is proposed to minimize the heat transfer barrier from both the internal and external structures of the high-frequency power transformer. The air-cooling system design process is demonstrated with a 15-kW load test featuring two 500-kHz planar Litz-wire transformers. The proposed transformer cooling system achieves a power density of 635 W/in3, a peak height of 43 mm, and a cooling power consumption of 6.8 W. Compared to using a 120 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times120$ </tex-math></inline-formula> mm cooling fan, peak winding temperature is reduced to 8%, the cooling system volume is reduced to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times $ </tex-math></inline-formula> , and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times $ </tex-math></inline-formula> reduction of cooling power consumption is achieved.

Compact MV-Insulated MHz Transformer-Coupled Gate Driver With Staged Turn-Off Scheme for Series-Connected Power Devices in DC Circuit Breaker Applications

The short-circuit protection equipment of dc circuit breaker (DCCB) is important for the dc grid. Also, the series-connected power devices are usually employed in the solid-state circuit breaker (SSCB) or hybrid circuit breaker (HCB) to meet the clamping voltage requirement. A simple passive gate driver and power supply solution is a critical component to drive the circuit breakers more cost-effective and reliable. This article proposed a novel compact MV-insulated transformer-coupled gate driver method, which combines the auxiliary power and gate signal together. The proposed high-frequency-modulated multilevel transformer voltage enables both the simultaneous and the staged turn-off schemes. Besides, the cascade high- and low-voltage transformer structures simplify the insulation design and demonstrate better scalability. The common-mode current could be suppressed as well using this structure. The design example of a compact 2-MHz high-voltage planar transformer with >13-kV partial-discharge-free insulation capability is illustrated. Finally, the simulation and experimental results are also given to demonstrate the feasibility of the proposed gate driver method.

Design of a 7.5 kW Dual Active Bridge Converter in 650 V GaN Technology for Charging Applications

High-voltage GaN switches offer low conduction and commutation losses compared with their Si counterparts, enabling the development of high-efficiency switching-mode DC–DC converters with increased switching frequency, faster dynamics, and more compact dimensions. Nonetheless, the potential of GaN switches can be fully exploited only by means of accurate simulations, optimal switch driving, suitable converter topology, accurate component selection, PCB layout optimization, and fast digital converter control. This paper describes the detailed design, simulation, and implementation of an air-cooled, 7.5 kW, dual active bridge converter exploiting commercial 650 V GaN switches, a compact planar transformer, and low ESL/ESR metal film capacitors. The isolated bidirectional converter operates at a 200 kHz switching frequency, with an output voltage range of 200–500 V at nominal 400 V input voltage, and a maximum output current of 28 A, with a wide full-power ZVS region. The overall efficiency at full power is 98.2%. This converter was developed in particular for battery charging applications, when bidirectional power flow is required.

Parameter Shift Prediction of Planar Transformer Based on Bi-LSTM Algorithm

Parameter Shift Prediction of Planar Transformer Based on Bi-LSTM Algorithm

Butterfly Interleaving Winding Arrangements for Multiphase Coupled Inductors

Multiphase coupled inductors in pulse-width modulated (PWM) converters benefit from the reduced steady-state current ripple and fast transient performance. This article proposes a magnetic core geometry together with a butterfly interleaving winding arrangement to integrate the four-phase negative coupled inductors. It adopts the interleaving winding strategy usually used in planar transformers to reduce the proximity effect. Basically, each winding has two overlapped areas with other two windings. The two currents in the overlapped area are in opposite directions. The proximity effect is, therefore, reduced. Compared with the conventional nonoverlapped winding arrangement, finite element analysis simulation indicates more than 60% ac resistance reduction and almost 30% footprint minimization. Both design considerations and design guidelines for the multiphase buck converter with coupled inductors are presented. The magnetic core and winding structure are demonstrated in a 36–75 V (nominal at 48 V) to 30 V four-phase negative coupled buck converter as a front stage for datacenter applications. It is implemented on a six-layer printed circuit board (PCB) without any parallel winding connections, and can operate up to 1165 W. The experiment shows the prototype achieves 99.3% peak efficiency with 912 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> power density.

Inserted-Shunt Integrated Planar Transformer With Low Secondary Leakage Inductance for LLC Resonant Converters

The leakage inductance of an integrated transformer is usually utilized as the series inductor of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> topology. However, leakage inductance exists on both primary and secondary sides of an integrated transformer and secondary leakage inductance leads the control and design of the converter to difficulty. In this article, a novel topology forinserted-shunt integrated transformers is proposed that has low secondary leakage inductance. The inserted shunt of the proposed topology is not segmental and can be located conveniently within the transformer. In addition, the inserted shunt does not require low permeability core material, simplifying its manufacture. The design and modeling of the proposed transformer topology are presented and verified by finite-element analysis and experimental implementation. The proposed topology is also compared with a recently published inserted-segmental-shunt integrated transformer. It is shown that the proposed transformer provides higher efficiency and lower ac resistance. Finally, an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant converter is implemented to examine the performance of the proposed integrated transformer in practice.

Design of a 350 kW DC/DC Converter in 1200-V SiC Module Technology for Automotive Component Testing

In this paper, the design and implementation of a DC/DC converter for automotive component testing with state-of-the art performance is described. The converter is the core of a battery emulator for the characterization and development of automotive batteries, electronic chargers, traction inverters, DC-DC converters, E-motors and E-axles. Cutting edge performance, flexibility and compactness are obtained by exploiting 1200-V SiC modules, high switching frequency, planar transformer technology, suitable topology solutions and fast digital control strategies. The implemented system is a liquid-cooled, bidirectional converter with galvanic isolation capable of 350 kW continuous output power, output voltage range 48–1000 V, continuous output current up to 800 A (1600 A peak), voltage/current ramp-up time below 10/2 ms and 0.1% current/voltage accuracy. The entire instrument is implemented in a standard full-height 19-inch rack cabinet.

Low-Loss Segmented Shielding Technique for PCB-Winding Planar Transformers

Printed-circuit-board (PCB) winding planar transformers have been widely adopted in isolated high-efficiency and high-power-density converters. Inserting the shielding layer is an effective method to attenuate the common-mode noise induced by the large interwinding capacitance of PCB winding. This letter identifies that a part of the magnetic flux will pass through the shielding layer due to the planarized structure of the planar transformer. Furthermore, the generation mechanism and the distribution of eddy current and related loss in the shielding layer are revealed. In order to reduce the eddy current loss in the shielding layer, a segmented shielding technique is proposed. The shielding layer is divided into multiple segments to reduce the coupled flux and the eddy current loss in each segment of the shielding layer. The experimental results on a 300 kHz 1000 W/28 V output resonant converter validate that, compared with the conventional shielding method, the proposed shielding technique can improve the efficiency by 0.15% at full load.

Parasitic Component Inclusive Optimum Phase-Frequency Contour Enabled Synchronous Rectification of Asymmetric CLLC Resonant Converter

With an objective to reduce the switching losses in a bidirectional resonant CLLC DC/DC converter for electric vehicle (EV) charging applications, this paper presents an elaborate frequency dependent general harmonic approximation (GHA) based secondary side turnoff current minimization technique by investigating the optimum operating point to achieve synchronous rectification (SR). Formulation of an accurate all-inclusive gain model is presented, specifically focusing on effect of parasitic components on the resultant gain-frequency trend, backed with thorough experimental validation. Further, meticulous modeling of the GHA based state equations is presented to obtain a contour of feasible operational frequencies and corresponding phase shifts to ascertain accurate SR operation with a multi-dimensional optimization technique to ensure reduced switching losses. In addition to that, to precisely characterize the resonant tank equivalent circuit, stressing on its effect on SR phase calculation, a detailed 3D finite element analysis (FEA) based R-L-C modelling of the employed high frequency planar transformer (HFPT) is explained. To validate and benchmark the performance of the proposed gain model while ensuring accurate SR operation, a 1kW all-GaN based CLLC experimental prototype is developed for a resonant frequency of 500kHz, with a power density of 106 W/inch <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Experimental waveforms at corner conditions are presented for a wide-gain bidirectional operation, portraying a peak converter efficiency of 98.49%.

Finite Element Study of Core Materials of a Planar Transformer and Comparison of its Electromagnetic Properties with a High Frequency Transformer

Finite Element Study of Core Materials of a Planar Transformer and Comparison of its Electromagnetic Properties with a High Frequency Transformer

AC Resistance and Leakage Inductance Estimation for Planar Transformers With Parallel-Connected Windings

Regarding the ever-increasing switching frequency of power converters, there is a lack of tools to perform a quick and effective optimization of the high-frequency transformers embedded in isolated power converters. Indeed, at high operating frequency, the geometry of the core and windings deeply impacts the overall transformer performance. Specifically, assessing accurately the current distribution in parallel-connected windings is a challenging and key issue. Winding parallelization, frequently implemented in low-voltage and high-current applications so as to decrease Joule losses, may be an ineffective solution because of skin and proximity effects. They create an unbalanced current distribution in the parallel-connected windings, which greatly impacts the ac resistance and leakage inductance of the transformer. This article proposes an innovative frequency analytical model permitting to simply and accurately compute the current distribution in the transformer windings, the ac resistance, and the leakage inductance. The proposed approach is comprehensively presented, validated using both measurements and two-dimensional finite element analysis and analyzed giving guidelines to engineers designing high-frequency transformers.

High-Frequency Planar Transformer Based on Interleaved Serpentine Winding Method With Low Parasitic Capacitance for High-Current Input LLC Resonant Converter

High-Frequency Planar Transformer Based on Interleaved Serpentine Winding Method With Low Parasitic Capacitance for High-Current Input LLC Resonant Converter

Differential Power Processing Converter With Active Clamp Structure and Integrated Planar Transformer for Power Generation Optimization of Multiple Photovoltaic Submodules

In recent years, in order to improve the fuel efficiency of electric vehicles, a lot of research has been conducted to adopt photovoltaic (PV) systems to electric vehicles. In addition, various Differential Power Processing (DPP) converters that can mitigate power imbalance among submodules in PV systems are being studied and developed. However, most conventional DPP converters suffer from large volume, low-price competitiveness, and low efficiency. Therefore, in this paper proposes a novel DPP converter. The proposed DPP converter can significantly reduce the volume of converter by integrating multiple transformers into single transformer. Moreover, because of the active clamp structure, it is possible to eliminate secondary side circuits. Furthermore, more than 95% efficiency under 10-60% load and maximum 96.654% efficiency about 20% load were achieved. Thus, the proposed DPP converter can achieve high power density, high price competitiveness, as well as high efficiency. To support the validity of the proposed converter, a prototype of the proposed DPP converter was built, and experiments were proceeded with 8V input and 60W/30V output specifications.

아크 용접기 DC-DC 컨버터용 10kW급 평면 변압기의 설계 및 분석

In this paper, a design method for a 10kW planar transformer is presented. The problems and design considerations when applying a planar transformer to a 10kW arc welder are described. We present a design technique considering power density, heat generation, and efficiency, which are important elements of a planar transformer. In addition, in order to verify the validity of the design, general winding transformers, winding type planar transformers, and printed circuit board type planar transformers are designed, respectively, and finite element method (FEM) simulations and experiments are performed for efficiency and heat generation. As a result of the experiment based on the proposed planar transformer, it is confirmed that the rated system efficiency is improved by 2.1%, the power density by 204%, and the transformer temperature by 28.3℃ compared to the normal winding transformer designed under the same conditions.

On-Chip GaN Planar Transformer Design for Highly Integrated RF Systems

The work presented in this paper concerns the design of an on-chip GaN transformer. The integrated transformer is composed of two planar stacked coils with spiral octagonal geometry. A comparison is done on different analytical methods to calculate the inductance of the transformer spiral planar coils. Three transformers with different outer diameters are compared to illustrate the influence of the coil geometry. The estimated inductance and dc series resistance are evaluated. Using COMSOL Multiphysics 5.3 software, the thermal effect is illustrated in the integrated transformer operating at high frequencies. The different parasitic effects created by the planar stacked layers are validated by the equivalent electrical circuit, and the different electrical parameters are calculated.

Design of a High-to-Low Voltage, Low-Power, Isolated DC/DC Converter for EV Applications

The design of a low-cost 800–12 V, 4 W isolated dc/dc converter for automotive applications is investigated. Such a converter is required to supply tiny systems directly from the high-voltage battery of an electric vehicle. In particular, a selected topology based on a multilevel flying-capacitor stage is analyzed. The converter operating mode selection is studied and simulation results reveal the interest of operating at the limit of the zero voltage switching (ZVS) mode. The design of a planar transformer adapted to the application is described. Design considerations of an IC brick that includes the main blocks required to control the power stage are discussed. The performances of a 4-W prototype with two transformer designs validate the choice of the operating mode. The peak efficiency is 87.4% which confirms the interest of the selected approach compared to existing solutions.

Investigation of ZVS Criteria and Optimization of Switching Loss in a Triple Active Bridge Converter Using Penta-Phase-Shift Modulation

Triple active bridge (TAB) as an isolated multiport dc–dc converter is a promising solution for integrated energy management systems to maintain an efficient power flow between the ports. This article presents the design, modeling, and switching loss optimized phase-duty-controlled PWM modulation scheme of a TAB converter composed of three full-bridge modules and a high-frequency planar transformer. The work aims at the derivation and analysis of the zero-voltage switching (ZVS) conditions for a TAB, where the turn-on switching loss is mitigated along with attaining an improved EMI performance, comprising of reduced noise peaks. Moreover, an optimized five-variable control-based TAB PWM modulation technique is proposed in this article in order to achieve minimized switching loss and the overall system loss for a wide voltage gain and load range. The instantaneous switching currents, responsible for the switching losses at the devices, are derived from the transformer winding current expressions, formulated employing the generalized harmonic approximation (GHA) technique. The various loss minimization techniques and the theoretically obtained criteria for ZVS are experimentally verified using a laboratory-developed prototype of an 800-W TAB converter. With the implementation of the proposed optimal phase-duty control, the experimental results show a nonunity gain light-load efficiency increment up to 10% compared to the conventional phase-shift modulation alone.

A Novel Isolated Resonant Gate Driver With Adjustable Duty Ratio for SiC MOSFET

Resonant gate driver is a promising technique to save gate driver loss at high switching frequencies to further promote the integration level of gate driver with power modules. State-of-the-art isolated resonant gate drivers (RGDs) have to keep duty ratio fixed at 0.5 to avoid transformer saturation, which significantly limits the practical applications. A new isolated RGD (IRGD) with adjustable duty ratio to drive silicon carbide (SiC) MOSFET is proposed in this article by employing switching period expansion technique with a dual-secondary transformer. Transformer secondary windings are involved in the turn-on and turn-off processes alternatively and are switched at the falling edge of each input driving cycle. The effective switching frequency at gate driver transformer is reduced to 1/2 to further save power losses at high switching frequencies. This article further discusses the optimal damping approach to suppress <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$v_{\mathrm {gs}}$ </tex-math></inline-formula> overshoot and oscillation due to an adjacent second driving stage brought by parasitic gate resistor for IRGDs. The proposed IRGD is built with printed circuit board (PCB) integrated planar transformers, designed at leakage inductance of 303 nH with primary-side silicon (Si) devices and secondary bidirectional gallium nitride (GaN) devices. The duty ratio is proven to be adjusted from 0.15 to 0.9. The power consumption of the prototype is 0.883 W at 500 kHz and 1.473 W at 1 MHz driving a 900-V SiC device with 87-nC gate charge, which is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6\times $ </tex-math></inline-formula> less power consumed than a conventional gate driver (CGD). The adopted SiC power device achieves dv/dt on of 18 V/ns and dv/dt off of 35 V/ns with the proposed IRGD.

Analysis of a Capacitive Sensing Circuit and Sensitive Structure Based on a Low-Temperature-Drift Planar Transformer.

In space gravitational-wave-detection missions, inertial sensors are used as the core loads, and their acceleration noise needs to reach 3×10−15 ms−2/Hz at a frequency of 0.1 mHz, which corresponds to the capacitive sensing system; the capacitive sensing noise on the sensitive axis needs to reach 1 aF/Hz. Unlike traditional circuit noise evaluation, the noise in the mHz frequency band is dominated by the thermal noise and the 1/f noise of the device, which is a challenging technical goal. In this paper, a low-frequency, high-precision resonant capacitor bridge method based on a planar transformer is used. Compared with the traditional winding transformer, the developed planar transformer has the advantages of low temperature drift and low 1/f noise. For closed-loop measurements of capacitive sensing circuits and sensitive structures, the minimum capacitive resolution in the time domain is about 3 aF, which is far lower than the scientific measurement resolution requirement of 5.8 fF for gravitational wave detection. The capacitive sensing noise is converted to 1.095 aF/Hz in the frequency band of 10 mHz−1 Hz. Although there is a gap between the closed-loop measurement results and the final index, the measurement environment is an experimental condition without temperature control on the ground; additionally, in China, the measurement integrity and actual measurement results of the capacitive sensing function have reached a domestic leading level. This is the realization of China’s future space gravitational wave exploration.

Neural-Network-Based Automated Synthesis of Transformer Matching Circuits for RF Amplifier Design

Rich experience and intuition play important roles in designing planar transformers (TFs) for contemporary radio frequency integrated circuits (RFICs). In general, RFIC designers have been heavily relying on multiple iterations of full electromagnetic (EM) simulations, which consumes much time and effort. Here, we propose an automated matching circuit synthesizer (AMCS) using neural networks (NNs). The proposed AMCS directly synthesizes a matching circuit combined with a TF throughout the entire design process, ranging from the desired performance to layout. In the AMCS, which is a “spec-to-layout” synthesizer, one NN returns physical parameters of matching circuits, and another NN estimates the electrical performance in two-port <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> -parameters from the desired impedances. Before the NNs are trained, input feature design is conducted to avoid the one-to-many problem, which cannot be well characterized with an inverse NN. This significantly reduces the time and effort for iterative circuit and EM simulations. The AMCS generated the matching circuit layouts for simple single-stage amplifiers operating at different frequencies up to 70 GHz or in different bandwidths of up to 32.5%. The estimated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> -parameters of the amplifiers show good agreement with the EM simulation results.