Claudio Adragna

Design-oriented steady-state analysis of LLC resonant converters based on FHA

The aim of this paper is to present a comprehensive design methodology for an LLC resonant converter, based on a detailed quantitative analysis of the steady-state operation of the circuit. This analysis follows the first harmonic approximation (FHA) approach, which tremendously simplifies the system model, leading to a linear circuit, which can be dealt with through the classical complex AC-circuit analysis. Two of the major benefits of the LLC resonant topology are the ability of the power MOSFETs and secondary rectifiers to be soft-switched and the capability of operating down to zero load. The design-oriented steady-state analysis presented in this paper addresses these two constraints quantitatively, allowing the designer to derive the circuit parameters which not only fulfil input voltage and output power specification data but also soft-switching and no-load operation constraints

Implementation of open-loop control for interleaved DCM/CCM boundary boost PFC converters

This paper is focused on the implementation of the open-loop control method for interleaved DCM/CCM boundary boost PFC converters where the slave converter is synchronized to the turn-on instant of the master converter and both converters operate with current-mode control. It is shown that this method is the only open-loop control method that provides a stable operation. Implementations of the master-slave synchronization circuit in both analog and digital technology are described. Experimental results obtained on a 400-W, universal input, 400-V output prototype circuit with two interleaved DCM/CCM boundary boost PFC converters controlled by an integrated control circuit currently being developed are provided.

Analysis of multi-phase LLC resonant converters

In this paper a topology for multi-phase interleaved LLC resonant converter is presented. The proposed solution, based on three LLC modules with transformer primary windings star connection allows to drastically reduce the output current ripple and consequently to minimize the output filter capacitor size. Differently from other multi-phase solutions, that are greatly susceptible to resonant component mismatch and consequently can be affected by a considerable current imbalance among modules, the proposed topology exhibits an inherent balancing capability. Small-signal analysis is presented and the possibility to turn-off one or two modules (phase shedding) at reduced output current levels is discussed, highlighting the trade-off between converter efficiency and output capacitor current ripple reduction. Measurements on a prototype will be included in the paper as validation of assertions and proposals.

Design guideline for magnetic integration in LLC resonant converters

The aim of this paper is to present a comprehensive design methodology for the magnetic integration of the series and shunt inductances of the resonant tank in an LLC resonant converter within the transformer. The design procedure applies to symmetrical core-bobbin structures with two separate slots for the primary and secondary windings. A specific leakage inductance Asigma. (per square turn) that depends on geometrical parameters only is derived. This is used, together with the cross section and winding area of the core, to define two other core parameters that allow the designer to identify the minimum ferrite core size suitable for the application.

Analysis and Performance Evaluation of Interleaved DCM/CCM Boundary Boost PFC Converters Around Zero-Crossing of Line Voltage

In this paper, causes of improper interleaving of two DCM/CCM boundary boost PFC converters around zero crossing of the line voltage are analyzed. The converters have a master-slave relationship. The slave is synchronized to the turn-on instant of the master by using an open-loop interleaving method. It is shown that phase shifting the gate drive signals of the two stages by 180° does not provide 180° phase-shift between the individual inductor currents, which is the real purpose of interleaving. Fortunately, the improper interleaving around zero crossing of the line voltage does not deteriorate the power factor (PF) and total harmonic distortion (THD). It is shown that although the interleaving method can be improved to achieve close to 180 phase shift between the master and slave currents even around zero crossing of the line voltage, the improved interleaving has only a minor effect on the PF and THD.

Ripple steering AC-DC converters to minimize input filter

In this paper, a boost topology with a coupled inductor to achieve zero-ripple input current for power factor correction (PFC) application is proposed. By using a coupled inductor as the PFC transformer, this topology not only can comply with any standards or regulations, but it also provides clean power in a universal ac line (90 Vac to 264 Vac). The zero ripple current PFC topology is designed to work in critical conduction mode (DCM boundary) to eliminate diode reverse recovery loss and provide automatic control. This is done so that the complexity of the controller is reduced; the control strategy of PFC stage is similar to that of a conventional boost converter. By using a coupled inductor boost topology, one can remove electromagnetic interference (EMI) filter without sacrificing the input current harmonics. Consequently, smaller passive components can be used, and a lighter weight and lower cost can be achieved. Experimental results are given to confirm the theoretically predicted behavior.

New Fixed-Off-Time PWM modulator provides constant frequency operation in boost PFC pre-regulators

The objective of this paper is to describe a new fixed-off-time (FOT) PWM modulator and its application in boost PFC pre-regulators. The uniqueness of this new modulator, suitable for integration in a control IC, is that it provides the OFF-time modulated with the instantaneous line voltage in such a way that the resulting switching frequency is constant and not dependent on the rms input voltage or on the output load as long as the boost stage operates in continuous conduction mode (CCM).

Enhanced constant-on-time control for DCM/CCM boundary boost PFC pre-regulators: Implementation and performance evaluation

This paper shows an innovative improvement of the constant on time (COT) control of transition mode boost PFC. The proposed control method employs a minimum current comparator to improve the harmonic distortion and power factor of the converter while maintaining the same complexity of the constant on time control. An overview and the enhancements of this method are presented in comparison to the standard COT. The algorithms, the equations, the analog and digital implementations are detailed for both improved methods. The comparison of the standard, enhanced COT (eCOT) and ramp enhanced COT (ReCOT), in terms of THD, PF and computational load, are shown with physical measures based on real implementations. The measures confirm the analysis and the good performance reached.

High-power-factor quasi-resonant flyback converters draw sinusoidal input current

This paper presents a novel control methodology that enables high-power-factor quasi-resonant flyback converters with peak current mode control to ideally draw a sinusoidal current from the input source, thus performing like boost converters operated in the same way. It is a well-known fact that the traditional quasi-resonant control methodology, used in a number of control chips commercially available, when applied to high-power-factor flyback converters provides an inherently distorted input current. Although the resulting distortion level is such that compliancy with IEC61000-3-2 regulation on the limits for harmonic current emissions can be easily achieved, recently this inherent distortion is becoming a problem in Solid-state Lighting (SSL), where this topology is extremely popular. In fact with the traditional method it is difficult to meet the Total Harmonic Distortion (THD) target (<; 10% or even lower) that is becoming a market requirement in some geographical areas [1]-[3]. The novel control method aims to overcome this issue.

Digital implementation and performance evaluation of a time-shift-controlled LLC resonant half-bridge converter

Time-shift control (TSC) is a novel control technique for resonant converters that outperforms the traditional direct frequency control (DFC) method implemented in all of the currently available control ICs. In particular, TSC makes the dynamic behavior of LLC resonant converters closely resembling that of a first-order system. Thus, closing the control loop to meet stability and even demanding dynamic performance specifications becomes a much easier task. This paper, after briefly reviewing the concepts related to TSC, describes its implementation with a digital controller specific for power conversion applications. An LLC resonant half-bridge converter is operated with both TSC and DFC and the dynamic performance achieved with both control methods is compared. This benchmark confirms the superior performance of TSC in terms of load transient response and input ripple rejection. Furthermore, the experiments show that TSC is able to prevent hard-switching of the half-bridge switches at start-up.