Weiyi Feng

Optimal Trajectory Control of Burst Mode for LLC Resonant Converter

The LLC resonant converter has drawn much attention for high efficiency. However, its light-load efficiency still cannot meet the ever increasing requirements. This paper proposes a novel burst-mode control for the LLC resonant converter to improve the light-load efficiency. Based on the state-plane trajectory analysis, during the burst-on time, a three-pulse switching pattern is implemented. By optimizing the first pulse width, the state variables are settled to the steady state of the highest efficiency load in the minimal time, and then, the next two pulse operation follows this steady-state trajectory to achieve high burst efficiency. Moreover, when the load changes, the burst-on time with the optimal three-pulse switching pattern is maintained, but the burst-off time is modulated. As a result, the output-voltage low-frequency ripple minimally keeps constant, and no additional filter is needed. Experimental results validate the proposed techniques.

Optimal Trajectory Control of LLC Resonant Converters for LED PWM Dimming

In this paper, a novel PWM dimming solution with the optimal trajectory control for a multichannel constant current (MC3) LLC resonant LED driver is proposed. When PWM dimming is on, the LLC resonant converter operates under the full-load condition. The LED intensity is controlled by the ratio between the on-time and off-time of the PWM dimming signal. To eliminate the dynamic oscillations when the MC3 LLC starts to work from the idle status, the switching pattern is optimized based on the graphic state-trajectory analysis. Thus, the full-load steady state is tracked within the minimum time. Moreover, under low dimming conditions, the LED intensity can be controlled more precisely. Finally, the optimal PWM dimming approach is verified on a 200 W, 4-channel MC3 LLC LED driver prototype.

Simplified Optimal Trajectory Control (SOTC) for LLC Resonant Converters

In this paper, a simplified optimal trajectory control (SOTC) for the LLC resonant converter is proposed. During the steady state, a linear compensator such as proportional-integral or proportional-integral-derivative (PID) is used, controlling the switching frequency fs to eliminate the steady-state error. However, during load transients, the SOTC method takes over, immediately changing the pulse widths of the gate-driving signals. Using the state-plane analysis, the pulse widths are estimated, letting the state variables track the optimal trajectory locus in the minimum period of time. The proposed solution is implemented in a digital controller, and experimental results show that while the digital logic requirement is very small, the performance improvement is significant.

Optimal Trajectory Control of LLC Resonant Converters for Soft Start-Up

In this paper, the soft start-up process for the LLC resonant converter is investigated and optimized based on a graphical state-trajectory analysis. By setting a current limitation band, several optimal switching patterns are proposed to settle the initial condition. After that, by sensing the output voltage, the optimal switching frequency is determined within the current limiting band. This virtually guarantees that there will be no current and voltage stress in the resonant tank during the soft start-up process. Meanwhile, the output voltage is built up quickly and smoothly.

A Universal Adaptive Driving Scheme for Synchronous Rectification in LLC Resonant Converters

In this paper, a universal adaptive driving scheme for synchronous rectification (SR) is proposed. The drain to source voltage of the synchronous rectifier is sensed so that the paralleled body diode conduction is detected. Using the proposed SR driving scheme, the SR turn-OFF time is tuned to eliminate the body diode conduction. The SR gate driving signal can be tuned within all operating frequency regions. Moreover, a simple digital implementation is introduced. Compared with analog ones, it enables more intelligent and precise SR control, improving converter efficiency. For rapid prototyping purposes, the digital SR tuning system is realized in Cyclone III field-programmable gate array (FPGA).

Digital implementation of driving scheme for synchronous rectification in LLC resonant converter

A digital implementation of synchronous rectification (SR) driving scheme for LLC resonant converter is proposed in this paper. Compared with analog ones, digital implementation enables more intelligent and precise SR control, improving converter efficiency. In the proposed SR driving scheme, the drain to source voltage of synchronous rectifier on the secondary side is sensed, so that the paralleled body diode conduction is detected. The SR turn-off time is digitally tuned based on the body diode conduction information. For rapid prototyping purposes, the digital tuning system is realized in Cyclone III FPGA.

LLC converters with automatic resonant frequency tracking based on synchronous rectifier (SR) gate driving signals

In this paper, a novel switching frequency control scheme to track the resonant frequency point (f0) is proposed. First, the synchronous rectifier (SR) gate driving signals are well tuned with proposed universal adaptive SR driving scheme by eliminating the body diode conduction. Then, the novel pulse width locked loop with digital implementation is presented to minimize the pulse width difference between main switch and well tuned SR gate driving signals, making LLC always operate at f0 point.

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.

LLC resonant converter burst mode control with constant burst time and optimal switching pattern

In this paper, a novel burst mode control solution for LLC resonant converters is proposed for the highest efficiency and small output voltage ripple. The burst time is set to be constant, the off time is modulated by load conditions. During the burst time, three pulse switching pattern is implemented to keep output voltage low frequency ripple minimal. The switch first turned on is controlled to be last turned off to reduce the initial hard switching turn on loss when burst starts. The pulse width of first switch is optimized to settle LLC to the steady-state under the highest efficiency load condition in one pulse. Zero current detection (ZCD) of the resonant current is implemented to achieve first switch partial ZVS, further increasing light load efficiency.

A hybrid strategy with Simplified Optimal Trajectory Control for LLC resonant converters

In this paper, a Simplified Optimal Trajectory Control (SOTC) for the LLC resonant converters is proposed. During the steady state, linear compensator like PI or PID is used, controlling the switching frequency (fs) to eliminate the steady error. However, during load transients, the Simplified Optimal Trajectory Control (SOTC) method takes over, controlling the pulse widths of the driving signals. Using the state-plane analysis, the pulse widths are estimated, letting the state variables track the optimum trajectory locus in the minimum period of time. Experimental results show that digital logic requirement is very small, but the performance improvement is significant.