Power converters offer a high capability to efficiently manage electrical energy flows. Until a few years ago, their primary use was in supplying motors in industrial applications and in electric traction systems. Nowadays, in addition to those fields they are employed in a very wide range of low, medium, and high power applications including residential applications, renewable energy systems, distributed generation, and automotive. Since digital control represents a key element of modern power converters, this paper presents a review on digital devices [microcontrollers, Field Programmable Gate Arrays (FPGA)], hardware and software design techniques as well as implementation issues useful for designing modern high-performance power converters.

Digital Control of Power Converters—A Survey

In this article, based on the LLC resonant converter fundamental harmonic analysis model, the modulation strategies for LLC resonant converter can be categorized into the following three groups: 1) input voltage fundamental harmonic modulation; 2) resonant tank elements modulation; and 3) secondary equivalent impedance modulation. Operational principles and control diagrams for different modulation strategies are given. Comprehensive comparisons between these modulation strategies in terms of topology complexity, control complexity, and voltage gain range are presented with respect to the same system specifications. The advantages and disadvantages for each modulation strategy are summarized to provide guidance for engineers when analyzing and designing an LLC resonant converter. The hybrid modulation strategies are categorized into different groups based on the specific applications. Brief introductions of these hybrid modulation strategies are presented. Future trends regarding the modulation strategies of LLC resonant converters are presented.

Overview of Modulation Strategies for LLC Resonant Converter

This paper introduces a novel concept for implementing constant current (CC) and constant voltage (CV) charges of batteries for electric vehicles using a dual full-bridge LLC (FBLLC) resonant converter, which shares its primary switches. In the proposed concept, the CC and CV charges are implemented using the inherent characteristics of an LLC converter as a current source and a voltage source. One FBLLC resonant converter operates with a fixed-resonant network whereas the other operates with a variable resonant network for the CC and CV charge operations. The proposed converter can achieve zero voltage switching (ZVS) and nearly zero current switching (ZCS) for all of the primary switches in the CC charge operation, and ZVS for all of the primary switches in the CV charge operation, which lead to a high efficiency. In addition, fixed switching frequency operation is possible in both the CC and CV charges thanks to the variable resonant network. A 3.3-kW prototype converter is implemented to verify the feasibility and validity of the proposed converter and a maximum efficiency of 98% was achieved.

A Novel Dual Full-Bridge LLC Resonant Converter for CC and CV Charges of Batteries for Electric Vehicles

Recently, the full-bridge (FB) LLC resonant converter is getting more attention due to its zero-voltage switching of the primary switches, zero-current switching of rectifier diodes, and high power capability. However, the conversion efficiency is degraded under light-load conditions due to the core loss in magnetic components and the switching loss in semiconductor devices. In this paper, a new control method is proposed for the FB LLC resonant converter. In the proposed method, the frequency control is basically used to regulate the output voltage over entire load conditions. Moreover, the proposed method utilizes phase-shifted gate signals between switch legs, based on predetermined optimal duty ratio under light-load conditions. At this point, the optimal duty ratio is determined to get minimum power loss under each light-load condition through the loss analysis of all components. Therefore, the proposed method makes high efficiency under light-load conditions. To confirm the validity of this paper, the prototype of the network power supply with 320–385 V dc input and 48 V/720 W dc output is tested.

Analysis on Load-Adaptive Phase-Shift Control for High Efficiency Full-Bridge LLC Resonant Converter Under Light-Load Conditions

This manuscript focuses on a novel actively controlled hybrid magnetic battery/ultracapacitor based energy storage system (ESS) for vehicular propulsion systems. A stand-alone battery system might not be sufficient to satisfy peak power demand and transient load variations in hybrid and plug-in hybrid electric vehicles (HEV, PHEV). Active battery/ultracapacitor hybrid ESS provides a better solution in terms of efficient power management and control flexibility. Moreover, the voltage of the battery pack can be selected to be different than that of the ultracapacitor, which will result in flexibility of design as well as cost and size reduction of the battery pack. In addition, the ultracapacitor bank can supply or recapture a large burst of power and it can be used with high C-rates. Hence, the battery is not subjected to supply peak and sharp power variations, and the stress on the battery will be reduced and the battery lifetime would be increased. Utilizing ultracapacitor results in effective capturing of the braking energy, especially in sudden braking conditions.

A Novel Integrated Magnetic Structure Based DC/DC Converter for Hybrid Battery/Ultracapacitor Energy Storage Systems

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 performance is readily predicted by the proposed model. It is found that in an N-phase coupled inductor converter, the voltage waveforms driving the leakage inductors are no longer the phase node voltages but are the modified voltages with a frequency N times the original switching frequency. In addition, their magnitudes also vary with the coupling coefficient among the coupled windings. Through coupling, a converter is capable of responding faster to load transient depending on the coupling coefficient and control mechanism, and that dependency is analytically revealed in the paper. Finally, a two-phase buck regulator is experimentally tested to verify the proposed model.

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.

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.

LLC performance enhancements with frequency and phase shift modulation control

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.

Digital synchronous rectification controller for llc resonant 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 [1][2].

Brent McDonald

Papers

Modeling and Analysis of Coupled Inductors in Power Converters

Modeling and Analysis of Coupled Inductors in Power Converters

LLC performance enhancements with frequency and phase shift modulation control

Digital synchronous rectification controller for llc resonant converters

A novel adaptive synchronous rectification method for digitally controlled LLC converters