Chao Fei

State-trajectory control of LLC converter implemented by microcontroller

This paper investigates how to integrate state-trajectory control functions of LLC resonant converter within a commercial low-cost microcontroller, including Simplified Optimal Trajectory Control (SOTC) for load transient, burst mode and soft start up. Simplified calculation process is proposed for SOTC to minimize digital delay. Guideline to select a proper microcontroller is presented to limit calculation delay and ADC delay within 1 switching cycle to ensure good transient performance. Optimized transient processes are proposed to eliminate large oscillation between burst mode and normal operation. Soft start-up is implemented with sensing the output voltage only. The whole control system structure is proposed to integrate all these control functions and for the first time all these control functions are integrated in one low-cost MCU. The results are demonstrated on a 130kHz 300W 380V/12V halfbridge LLC resonant converter with MCU TMS320F2808.

Soft start-up for high frequency LLC resonant converter with optimal trajectory control

This paper investigates the soft start-up for high frequency LLC resonant converter with optimal trajectory control. Two methods are proposed to realize soft start-up for high frequency LLC converter by commercial low-cost microcontrollers (MCU). Both methods can achieve soft start-up with minimum stress and optimal energy delivery. One method is mixed-signal implementation by sensing resonant tank to minimize the digital delay. Another method is digital implementation with look-up table. Experimental results are demonstrated on 500kHz 1kW 400V/12V LLC converter.

Digital Implementation of Soft Start-Up and Short-Circuit Protection for High-Frequency LLC Converters With Optimal Trajectory Control (OTC)

Achieving soft start-up and short-circuit protection have always been challenging for resonant converters due to severe stresses in the resonant tank. Optimal trajectory control (OTC) has been proven to be the most effective control method to optimize energy delivery with given stresses. This paper proposes a method to implement soft start-up and short-circuit protection for LLC converters by using low-cost microcontrollers (MCUs) with minimum stresses and optimal energy delivery. Our current understanding of the relationship between the switching frequency and the output voltage is based on the state-plane analysis, and the requirement for the controllers is significantly reduced when using the lookup table. Further improvement enables the application of the proposed control method to high-frequency LLC converters without increasing the cost for the controllers. This paper proposes a method to protect the LLC converter from abrupt short-circuit with low-cost MCUs, which improves transient response to short-circuit significantly, and investigates limitations when operating the high-frequency LLC converter under short-circuit conditions. The proposed methods minimize the CPU resource requirement and can be further integrated with other state-trajectory control functions within one MCU. Experimental results are demonstrated on a 500-kHz 1-kW 400-V/12-V LLC converter with 60-MHz MCU TMS320F28027.

Multi-step Simplified Optimal Trajectory Control (SOTC) for fast transient response of high frequency LLC converters

This paper proposes Multi-step Simplified Optimal Trajectory Control (SOTC) for the high frequency LLC converters. SOTC can improve the transient performance of the LLC converters by immediately changing the pulse width of the primary driving signal during the load transient. However, the implementation of SOTC requires high-performance digital controllers. The proposed Multi-step SOTC calculates the trajectory based on only the output voltage and the load current, and settles the resonant tank within multi steps. With the proposed Multi-step SOTC, low-cost controllers can be used to control the high frequency LLC converters with the state-trajectory control concept. The number of steps in multi-step SOTC is selected based on the speed of the controller and the switching frequency of the LLC converters. Experimental results are demonstrated on a 500kHz 1kW 400V/12V LLC converter.

Light load efficiency improvement for high frequency LLC converters with Simplified Optimal Trajectory Control (SOTC)

The light load efficiency has drawn more and more attention for the LLC resonant converters. And the high frequency LLC converters are emerging in recent years. The conventional burst mode control cannot fix to the efficiency-optimal state during the burst-on time. Optimal Trajectory Control (OTC) for burst mode can solve this problem by using fixed 3-pulse pattern; however, the burst operation range of OTC is limited when it is applied to the high frequency LLC converters due to the fixed 3-pulse pattern and digital delay. To extend the burst operation range for the high frequency LLC converters, Simplified Optimal Trajectory Control (SOTC) for burst mode with adaptive burst on-power and adaptive multi-step are proposed in this paper. The burst mode operation range can be extended by increasing either the burst on-power or the burst on-time. The experimental results are demonstrated on a 500kHz 1kW 400V/12V LLC converter with 92.5% efficiency at 10% load and 91.8% efficiency at 2.4% load.

High-efficiency high-power-density 48/1V sigma converter voltage regulator module

A high efficiency and high power density sigma converter for 48/1V voltage regulator module (VRM) is proposed in this paper. The Sigma converter is a quasi-parallel converter that uses a high efficiency unregulated converter to deliver most power to the load with small power flowing through buck converter responsible for regulating the output voltage. The unregulated isolated converter is LLC converter designed with matrix transformer structure integrating 4 transformers in one core structure with integrating the Synchronous Rectifiers (SRs) with the winding to minimize the termination losses of the transformer so a high efficiency can be achieved. The buck converter is designed with discrete GaN devices and PCB winding inductor to regulate the output voltage. The designed Sigma converter is 48/1V-80A achieving a power density of 420W/in3 and maximum efficiency of 93.4%.

High efficiency two-stage 48V VRM with PCB winding matrix transformer

High efficiency power supply solutions for data centers are gaining more attention, in order to minimize the fast growing power demands of such loads, the 48V Voltage Regulator Module (VRM) for powering CPU is a promising solution replacing the legacy 12V VRM by which the bus distribution loss, cost and size can be dramatically minimized. In this paper, a two-stage 48V/12V/1.8V–250W VRM is proposed, the first stage is a high efficiency, high power density isolated — unregulated DC/DC converter (DCX) based on LLC resonant converter stepping the input voltage from 48V to 12V. The Matrix transformer concept was utilized for designing the high frequency transformer of the first stage, an enhanced termination loop for the synchronous rectifiers and a non-uniform winding structure is proposed resulting in significant increase in both power density and efficiency of the first stage converter. The second stage is a 4-phases buck converter stepping the voltage from 12V to 1.8V to the CPU. Since the CPU runs in the sleep mode most of the time a light load efficiency improvement method by changing the bus voltage from 12V to 6 V during light load operation is proposed showing more than 8% light load efficiency enhancement than fixed bus voltage. Experimental results demonstrate the high efficiency of the proposed solution reaching peak of 91% with a significant light load efficiency improvement.

Digital implementation of adaptive synchronous rectifier (SR) driving scheme for LLC resonant converters

In this paper, an adaptive synchronous rectifier (SR) driving scheme for the LLC resonant converters using the ripple counter concept is proposed, along with two methods of implementation. With the proposed scheme, the SR drain to source voltage is sensed to detect the body diode conduction, based on which the SR on-time can be well tuned to eliminate the body diode conduction. One proposed implementation tunes the SR on-time every switching cycle based on the ripple detection; another proposed implementation tunes the SR on-time every nth switching cycle (n = 1, 2, 3 ...) based on the ripple counter, which is suitable for the high frequency LLC converters. The proposed SR driving scheme has the simple implementation, requires only low-cost digital controllers and occupies very few controller resources. More importantly, since the digital controllers have already been widely adopted in the control of the LLC converters, the proposed adaptive SR driving method can be embedded into these digital controllers with little extra cost. Furthermore, how to integrate the proposed SR driving method with closed-loop control is explained in details. Experimental results are demonstrated on a 130kHz LLC converter with 100MHz microcontroller (MCU) and a 500kHz LLC converter with a 60MHz MCU and a ripple counter.

Shielding Technique for Planar Matrix Transformers to Suppress Common-Mode EMI Noise and Improve Efficiency

Isolated high output current dc/dc converters are critical for power supplies in information technology industries. LLC converters with a matrix transformer are suitable for these applications due to their high efficiency and high power density. By pushing switching frequency up to MHz with GaN devices, planar matrix transformers with printed circuit board (PCB) windings are advantageous over conventional transformer designs, but the large interwinding capacitance of the PCB windings provides a critical coupling path for common-mode (CM) noise. In order to analyze the CM noise, the interwinding capacitance of the PCB windings is analyzed, and a related CM noise model is provided in this paper. To suppress the CM noise, shielding layers can automatically be inserted into the PCB windings between the primary and secondary windings in the fabrication process. This paper proposes a novel shielding technique, in which half of the shielding serve as the primary winding. The proposed shielding technique not only suppresses CM noise but improves the efficiency as well. The proposed shielding is verified by experiments on 1-MHz 400 V/12 V 800-W LLC converters with a peak efficiency of 97.7% and a power density of 900 W/in3. The proposed shielding reduces CM noise by 30 dB and improves full-load efficiency by 0.2%.

High-Efficiency High-Power-Density LLC Converter With an Integrated Planar Matrix Transformer for High-Output Current Applications

Isolated high-output current DC/DC converters are critical for future data center power architecture. LLC converters with matrix transformer are suitable for these applications due to its high efficiency and high power density. Different matrix transformer structures are investigated in this paper. To improve the current design practice, a high-frequency transformer loss model is developed and a detailed design methodology is proposed. Moreover, a novel matrix transformer structure is proposed to integrate four elemental transformers into one magnetic core with simple four-layer print circuit board windings implementation and further reduced core loss. By pushing switching frequency up to megahertz with GaN devices, the proposed matrix transformer can achieve high efficiency, high power density, and automatic manufacturing for magnetic components. A 1-MHz 380 V/12 V 800-W LLC converter with GaN devices is demonstrated. The prototype achieves a peak efficiency of 97.6% and a power density of 900 W/in3.