Mingkai Mu

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.

Design and evaluation of GaN-based dual-phase interleaved MHz critical mode PFC converter

This paper presents the design consideration and performance evaluation of gallium nitride (GaN) high electron mobility transistor (HEMT) based dual-phase interleaved MHz critical conduction mode (CRM) power factor correction (PFC) converter. A 1.2kW 1-3MHz interleaved boost PFC converter prototype is built with 97.9% peak efficiency and 120W/in3 power density. The significant impact of MHz frequency is demonstrated as dramatically size reduction of boost inductor and electro-magnetic interference (EMI) filter. Several inductor designs are discussed. The corner frequency of EMI filter is pushed to several hundreds of kHz. Finally, the limitation of conventional boost PFC converter is discussed as high conduction loss on diode rectifier bridge and high switching loss caused by valley switching, which is negligible in other low frequency PFC converter but significant in MHz PFC converter. The totem-pole bridgeless PFC converter is introduced to further improve the efficiency with no rectifier bridge and zero-voltage switching (ZVS) extension strategy.

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.

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.

Three-level driving method for GaN power transistor in synchronous buck converter

The emerging Gallium-Nitride (GaN) based power transistors offers the potential to achieve higher efficiency and higher switching frequencies than possible with Silicon MOSFETs. This paper will discuss the GaN device characteristics, and based on this, the driving method will be discussed. Then a three-level driving method is proposed to overcome the high reverse conduction loss issue of the GaN power transistor. Finally, a 12V to 1.2V Synchronous Buck converter with a full load current of 20A is built to verify the proposed method. The experimental results show that the proposed method is necessary and effective for efficiency improvement in high switching applications of GaN power transistor.

The optimal design of GaN-based Dual Active Bridge for bi-directional Plug-IN Hybrid Electric Vehicle (PHEV) charger

A high frequency, high efficiency bi-directional battery charger for Plug-in Hybrid Electric Vehicle (PHEV) is built with high voltage normally-off GaN-on-Si HFETs. This paper characterized the multi-chip-model both statically and dynamically. The optimal design of the isolated 500 kHz Dual Active Bridge DC/DC stage is detailed, taking account the wide battery voltage range and sinusoidal charging, to eliminate large DC link capacitor. Experimentally result shows a 500 kHz DAB converter with discrete inductor and transformer can achieved 97.2% efficiency at 1kW and 96.4% efficiency at 2.4 kW. By integrating the inductor into the transformer, 98.2% efficiency is achieved at 1 kW.

High frequency integrated Point of Load (POL) module with PCB embedded inductor substrate

In this paper, a novel alloy flake composite material is used to demonstrate the PCB integrated magnetic component with low temperature fabrication process. The most important benefit of the alloy flake composite core is easy to be patterned into any desired shape for integration. Compared with the traditional flake composite, the permeability and core loss of the new flake composite are improved prominently, by doing some lateral alignment of the flake and increasing the volume ratio of the alloy. The layerwise magnetic core is sandwiched into multilayer PCB using conventional PCB laminating technique. It has been proved that the manufacturing process, such as laminating, cutting and drilling, has very little impact on the magnetic properties of the flake core. Based on a simple 4-layer PCB substrate with embedded core, the megahertz 3D integrated Point of Load (POL) modules are built, which achieve more than 700 W/in3 power density. The PCB modules survive after hundreds of thermal variation cycles, validating the reliability and compatibility of the alloy flake composite material with PCB integration. In addition, the application of standard PCB process reduces the cost for manufacturing such integrated modules due to the easy automation and low temperature process.

Influence of High-Frequency Near-Field Coupling Between Magnetic Components on EMI Filter Design

This work presents a detailed analysis of magnetic component coupling and its influence on EMI filter design. In contrast with other literature, the novelty of this paper reveals that magnetic coupling should be divided into two categories: low-frequency coupling and high-frequency coupling. It is proven that coupling is frequency related and high-frequency near-field stray flux distribution can be differ dramatically from low frequency condition. The change of the near-field stray flux distribution is caused by displacement current from stray capacitors. By using the Biot-Savart equation, high-frequency near-field distribution can be well predicted and matched with experimental results. In addition, single stage DM LC filters are utilized to demonstrate the influence of high-frequency coupling.

Bi-directional PHEV battery charger based on normally-off GaN-on-Si multi-chip module

A high frequency, high efficiency bi-directional battery charger for Plug-in Hybrid Electric Vehicle (PHEV) is built with high voltage normally-off GaN-on-Si HFETs. The battery charger topology consists of a 500 kHz Full Bridge (FB) AD/DC stage and a 500 kHz Dual Active Bridge (DAB) DC/DC stage. The system functionality is verified and measured efficiency is 97% for the AC/DC stage and 97.2% for the DC/DC stage, which leads to a total efficiency 94.2%. By doing sinusoidal charging, the DC link capacitance can be reduced to much less than one third but efficiency will drop to 92.0%.

Characterization of Low Temperature Sintered Ferrite Laminates for High Frequency Point-of-Load (POL) Converters

Low profile magnetic components and associated integration techniques are desired for design and fabrication of highly integrated point-of-load (POL) converters working at high frequency. The multilayer low-fire ferrite inductors can be fabricated as the magnetic substrate in an integrated POL converter with active components on top. This paper reports the characterization of magnetic property, microstructure, and chemical composition of commercially available low-fire Ni-Cu-Zn ferrites (ESL 40010, 40011, and 40012) and their mixed laminates. Permeability and core loss density were measured on toroidal cores sintered at 885 ° C for 3.5 h. The influence of superimposed dc bias on the magnetic property of ferrite laminates was also evaluated. The microstructure and chemical composition of low temperature sintered ferrite laminates were analyzed. The mixed laminate with alternating layers of ESL 40010 and ESL 40012 in 1:1 ratio presents the highest permeability and the lowest core loss density among all examined samples when dc bias is above 1000 A/m. Finally, a 12 V to 1.2 V, 15 A, high frequency (1.5-5 MHz) integrated POL converter with laminated ferrite inductor was fabricated and demonstrated to work at high efficiency with a power density as high as 1000 W/in3.