Brent McDonald

Modeling and Analysis of Coupled Inductors in Power Converters

This paper describes a new approach to the analysis of switched mode power converters utilizing coupled inductors and presents a novel canonical circuit model for N-winding coupled inductors. Waveform and ripple of the winding current in a coupled inductor converter can be easily determined using the developed model similar to those obtained in an uncoupled inductor converter. Influence of coupling coefficient on converter steady state and transient performances is readily predicted by the proposed model and a comparison of coupled and uncoupled inductor converters is provided. It is found that coupling among windings effectively alters the phase node voltage waveforms driving the coupled inductors. Through coupling, a converter is capable of responding faster to load transient depending on the coupling coefficient and control mechanism, and this dependency is analytically revealed in the paper. Some design constraints regarding coupling coefficient are also discussed for two-winding and multi-winding coupled inductors in power converter applications. Finally, a two-phase buck regulator is experimentally tested to verify the proposed model.

LLC performance enhancements with frequency and phase shift modulation control

This paper presents an algorithm that uses a combination of frequency modulation (FM) and phase shift modulation (PSM) to enhance both the controllability and efficiency of an LLC dc-dc converter. It will be shown that, the proper “marriage” of FM and PSM will result in the ability to support a wider range of operating conditions without comprising efficiency or control monotonicity. These operating points include but are not limited to: monotonic start up, light load regulation, high input voltage, constant-current and constant-power.

Digital synchronous rectification controller for llc resonant converters

This paper introduces a robust, hardware efficient, mixed-signal control loop that governs the switching actions of the secondary-side synchronous rectifier (SR) switches of LLC resonant converters. The new SR control method minimizes switching and conduction losses of the SR switches through online optimization of their on-off timing using an auto-tuning process that takes into account the effect of parasitic elements, mainly leakage inductances. In this controller, the information from body diode conduction detection circuits across the SR switches and switching frequency available from digital controller are utilized by a digitally implemented auto-tuning algorithm to determine optimal switching times to achieve zero current switching. In comparison with the existing SR controllers the introduced solution has more precise driving scheme. Moreover, the controller requires simple hardware implementation. Experimental results obtained with a 350 W, 400 V to 12 V, isolated LLC experimental resonant converter, verify the operation of the introduced SR driving scheme and show efficiency improvement over the whole operating range resulting in up to 9% reduction of converter power losses during light load operation.

Auto-tuned minimum-deviation digital controller for LLC resonant converters

This paper presents a practical auto-tuned digital controller for LLC resonant dc/dc converters that results in virtually minimal possible output voltage deviation during transients. During transients the controller applies a two-step frequency change algorithm such that the minimum deviation and seamless transition to the new steady state is achieved. The fast voltage regulation is obtained through the output voltage measurements only and fairly simple calculations of the frequency-changing sequence eliminating fast current sensors and complex calculations usually existing in fast controllers for LLC converters. Based on the initial voltage deviation the first frequency step of the controller is determined and adjusted through an auto-tuning process. Smooth transition to the new steady state is achieved through the estimation of the new switching frequency from the ripple amplitude. Experimental results obtained with a 350 W, 400 V to 12 V, isolated LLC converter confirm recovery with practically smallest possible deviation and bump-less transitions to the new steady state.

A novel adaptive synchronous rectification method for digitally controlled LLC converters

In this paper, a novel adaptive synchronous rectification method for digitally controlled LLC converters is proposed. By sensing the synchronous rectifier (SR) body diode forward drop, both the SR turn-on and turn-off edges are optimized for efficiency. Negative current prevention is utilized to improve the system robustness and is enhanced by simple digital control capabilities. Compared with a conventional analog SR control approach, this method achieves higher system efficiency and flexibility. This control method has been implemented in a Texas Instruments digital power controller UCD3138A and a companion gate driver UCD7138.

Practical transformer modeling and characterization for a high performance LLC converter

This paper will present a method to model and characterize a typical LLC dc-dc converter transformer with a center tapped secondary. The emphasis of the model will be on the ease of characterization using practical lab equipment and its subsequent suitability for simulation in a number of practical venues (e.g. Saber, Simplis, Pspice [9][10][11]). Practical design implications for the LLC converter will also be explored.

Parameterization for power solutions

PMBus™ has done an excellent job of standardizing the way digital controlled power supplies communicate. However, this standardization has not included many of the traditional design requirements present in power supplies. A standardization proposal for the parameterization of power supply design requirements into a PMBus™ format is made. Benefits to the power supply design engineer and the host system are discussed. Examples are presented in the context of both isolated and non-isolated applications.

Hardware efficient auto-tuned linear-gain based minimum deviation digital controller for indirect energy transfer converters

This paper introduces a robust, hardware-efficient auto-tuned digital controller applicable to various hard switching dc-dc converters, including indirect energy transfesr topologies. Unlike existing fast transient controllers for indirect energy transfer converters, the controller achieves fast transient response and practically minimum deviation of the output voltage without depending on information about converter parameters, i.e. inductor and output capacitor values. This is achieved by utilizing an auto-tuned non-linear controller that, based on the load-step information during a transient, finds the switching sequence for the converter to ramp up/down the inductor current to its new steady state average value in a single on/off switching action. Experimental results obtained from a 1.5 V to 3.3 V, 1A, 500 kHz boost prototype verify response with practically minimum output voltage deviation and demonstrate a more than 50% reduction of both output voltage deviation and recovery time compared to a voltage mode, fast PID-based controller.

A hardware-efficient self-tuned output capacitor current and time constant estimator for indirect energy transfer converters based on dynamic voltage slope adjustment

This paper introduces a novel hardware-efficient auto-tuned output capacitor current and time-constant estimator for indirect energy transfer converters, which is based on well-known principle of the utilization of an auxiliary RC circuit and time constant matching. To provide accurate current measurement and early fault detection of the system while avoiding complex calculations/hardware usually existing in other auto-tuned methods, the reconstruction of the time constant is performed through a simple detection of the polarity of the slope of the estimator voltage during the main switch off time. The effectiveness of the estimator has been experimentally verified with a 20 W boost-based prototype, demonstrating about 97% of accuracy in the instantaneous current and time-constant measurements.

Resolution requirements to avoid limit cycling in LLC resonant converter

This paper introduces a general method for finding minimum resolution of digital pulse frequency modulators (DPFM) of LLC converters to avoid limit cycling oscillation (LCO) in the output voltage under worst operating conditions. This work is built on the prior art [1], [2] where detailed analysis of limit cycling has been investigated for different hard switching converters and for constant load current controlled soft switching converters. The analysis introduced here takes into account that both the switching frequency and the gain characteristic of the converter change with operating conditions and that the resolution of the DPFM can be selected from a constrained set of the combinations of those two values. Experimental results obtained from a 350 W, 400 V to 12 V LLC converter prove the effectiveness of the introduced analysis by varying DPFM resolution and comparison of LCO with the theoretically predicted results for different loads.