Constant-frequency control of quasi-resonant converters

An additional independent control needed to eliminate the undesirable variable switching frequency of quasi-resonant (QR) converters is obtained by replacing the output rectifier by an active switch. The concept is applicable to all classes of converters. Compared to QR converters with conventional switch realization, constant-frequency quasi-resonant (CF-QR) converters exhibit the same type of switching transitions and similar switch voltage and current stresses. Advantages of CF-QR converters are not restricted to the constant-frequency control. In all classes, operation at zero load is possible, so that the available load range is unlimited. The range of attainable, conversion ratios is significantly extended in the classes of zero-voltage quasi-square-wave (CF-ZV-QSW) and zero-voltage multiresonant (CF-ZV-MR) topologies. A practical design example of a 25 W CF-ZV-MR buck converter is constructed and evaluated. The converter operates at 2 MHz from zero load to full load, with a full-load efficiency of 83%. Simple duty ratio control is used to maintain the output voltage constant for all loads. The circuit is inherently immune to the short-circuit condition at the output. Disadvantages of CF-QR converters are the increased gate-drive losses and increased complexity of the power stage and the control circuitry     

Mathematical modelling of quasi-resonant converters

A method to determine the small-signal control-to-output transfer function of quasi-resonant converters operating with continuous current is presented. The design of the feedback loop and compensation network becomes easier by this approach. Investigation is carried out using the boost converter topology     

Multi-loop control for quasi-resonant converters

A multiloop control scheme for quasi-resonant converters (QRCs) is described. Like current-mode control for pulse width modulation (PWM) converters, this control offers excellent transient response and replaces the voltage-controlled oscillator (VCO) with a simple comparator. In this method, referred to as current-sense frequency modulation (CSFM), a signal proportional to the output-inductor current is compared with an error voltage signal to modulate the switching frequency. The control can be applied to either zero-voltage-switched (ZVS) or zero-current-switched (ZCS) QRCs. Computer simulation is method applied to a ZCS buck QRC. A circuit implementation is presented that allows multiloop control to be used on circuits switching up to 10 MHz. This circuit requires few components and produces clean control waveforms. Experimental results are presented for zero-current flyback and zero-voltage buck QRCs, operating at up to 7 MHz. Good small-signal characteristics have been obtained     

Linear multiloop control of quasi-resonant converters

The aim of this paper is to study the stability of linear multiloop control of quasiresonant converters (QRC). It is shown that a P-current feedback, plus an I-voltage feedback suffices to stabilize a QRC. A systematic procedure of control synthesis is presented and stability conditions for the control parameters are derived from nonlinear systems theory. Performance of the proposed controller is verified through an experimental application in a boost type QRC.     

Lateral power MOSFET for megahertz-frequency, high-density DC/DC converters

DC/DC converters to power future CPU cores mandate low-voltage power metal-oxide semiconductor field-effect transistors (MOSFETs) with ultra low on-resistance and gate charge. Conventional vertical trench MOSFETs cannot meet the challenge. In this paper, we introduce an alternative device solution, the large-area lateral power MOSFET with a unique metal interconnect scheme and a chip-scale package. We have designed and fabricated a family of lateral power MOSFETs including a sub-10 V class power MOSFET with a record-low R/sub DS(ON)/ of 1m/spl Omega/ at a gate voltage of 6V, approximately 50% of the lowest R/sub DS(ON)/ previously reported. The new device has a total gate charge Q/sub g/ of 22nC at 4.5V and a performance figures of merit of less than 30m/spl Omega/-nC, a 3/spl times/ improvement over the state of the art trench MOSFETs. This new MOSFET was used in a 100-W dc/dc converter as the synchronous rectifiers to achieve a 3.5-MHz pulse-width modulation switching frequency, 97%-99% efficiency, and a power density of 970W/in/sup 3/. The new lateral MOSEFT technology offers a viable solution for the next-generation, multimegahertz, high-density dc/dc converters for future CPU cores and many other high-performance power management applications.     

Zero-voltage-switching multiresonant technique-a novel approach toimprove performance of high-frequency quasi-resonant converters

A novel multiresonant switching concept is proposed to overcome the limitations of high-frequency quasi-resonant converters. A novel family of zero-voltage-switching (ZVS) multiresonant converters is generated. The unique arrangement of the multiresonant network results in absorption of all major parasitic components in the resonant circuit, including transistor output capacitance, diode junction capacitance, and transfer leakage inductance. This allows the new converters to provide favorable switching conditions for all semiconductor devices. Experimental results show that ZVS multiresonant converters are superior to ZVS quasi-resonant converters due to their reduced transistor voltage stress and improved load range and stability     

A simple large-signal nonlinear modeling approach for fastsimulation of zero-current-switch quasi-resonant converters

A nonlinear modeling approach for zero-current-switch (ZCS) quasi-resonant converters (QRC) is proposed which can be derived easily using simple analytical techniques. The converter model obtained is readily absorbed by MATLAB for analysis and design of both the open- and closed-loop configurations in fast speed. Simulations have shown its accuracy, even for large-signal transient responses. Applications of this modeling approach to the three basic topologies of buck, boost, and buck-boost converters are given as illustrative examples. The condition for zero-current switching is identified from the model. The feasibility of applying this proposed modeling approach to the extended period QRC topologies is to be discussed. Simulation results for the three basic topologies are given to show the merits of the proposed modeling approach     

Computer-aided modeling of quasi-resonant converters in thepresence of parasitic losses by using the MISSCO concept

The DC and small-signal models of quasi-resonant converters, operating in both half-wave and full-wave modes, are developed in a suitable form for computer simulation. The starting step is the extraction of a minimum separable switching configuration (MISSCO) containing all power switches but a minimum number of other components (resonant ones). By using the step-response analysis and average technique, and by perturbing and separating the DC and AC components in the resulting equations, the equivalent models of MISSCO are derived. They are introduced in the converter structure to replace the circuit initially extracted. Models of different quasi-resonant converters can be obtained by this general approach. The analysis takes into account the conduction losses of the switching devices and reactive elements, which improves considerably the model accuracy. Model-based computer simulation agrees with the experimental results     

A novel method oriented to evaluate the real characteristics ofpractical buck zero-voltage switching quasi-resonant converters

The output characteristics of buck zero-voltage switching quasi-resonant converters (ZVS-QRCs), presented in previously published works, are calculated considering equal input and output energies. Nevertheless, measurements that were taken with breadboarded buck ZVS-QRCs demonstrate an important deviation between the results of the theoretical analysis and the experimental data. This deviation becomes greater when the load current increases. This paper deals with a novel simulation method to approximate the real behavioral characteristics of Buck ZVS-QRCs. Analytical expressions are derived to calculate the power losses in each power converter component as well as the DC-voltage conversion ratio. Computer simulation results are confronted to experimental ones proving the effectiveness of the proposed method     

The design of sigma-delta modulation analog-to-digital converters

The author examines the practical design criteria for implementing oversampled analog/digital converters based on second-order sigma-delta (ΣΔ) modulation. Behavioral models that include representation of various circuit impairments are established for each of the functional building blocks comprising a second-order Σ2gD modulator. Extensive simulations based on these models are then used to establish the major design criteria for each of the building blocks. As an example, these criteria are applied to the design of a modulator that has been integrated in a 3-μm CMOS technology. An experimental prototype operates from a single 5-V supply, dissipates 12 mW, occupies an area of 0.77 mm², and has achieved a measured dynamic range of 89 dB     

Analysis and design of clamped-mode resonant converters withvariable load

In this paper, a design procedure for clamped-mode resonant converters working with variable load is proposed and analyzed. The operation of these converters with general parallel or series loads is reviewed first. The operation of the transistors as zero-voltage switches is identified and characterized as mode A. Mode A is preferred to simplify the implementation of the power switches and to increment the converter efficiency. As a case study, the design of an RF power amplifier is presented. The amplifier is connected to a variable load with reactive characteristics, through a low-pass filter of three elements. The validity of the proposed design approach is verified with an experimental setup     

Comparison of the square-wave and quasi-resonant topologies

The waveforms of a square-wave DC-DC converter and a quasiresonant DC-DC converter are examined in detail and a comparison is made between the switching losses and conduction losses for each topology. Using data from commercially available semiconductor devices, conservative estimates are then given for the switching frequency at which the resonant approach becomes advantageous. The effect of an isolation transformer on this comparison is also addressed     

High-frequency quasi-resonant converter technologies

Resonant switch topologies operating under the principle of zero-current switching (ZCS) and zero-voltage switching (ZVS) are introduced to minimize switching losses, stresses, and noises. Using the resonant switch concept, a host of new quasi-resonant converters (QRCs) are derived from conventional PWM converters. They are capable of operating in the megahertz range, with a significant improvement in performance and power density. Performances of ZCS and ZVS QRCs are compared. Power stages, gate drives, and feedback controls are discussed     

Buck quasi-resonant converter operating at constant frequency:analysis, design, and experimentation

A buck pulsewidth modulated zero-current switching quasi-resonant converter (buck PWM ZCS QRC) operating at constant frequency is discussed. Operating principle and design-oriented analysis are presented with normalized design curves, design procedure, design example, simulations, and experimental results. The new topology, which can be considered as a particular one, is compared with the well-established buck frequency-modulated zero-current switching quasi-resonant converter (buck FM ZCS QRC) proposed by Fred C. Lee (1988)     

The design of serpentine-mode converters for high-power microwaveapplications

In this paper, we report the design methodology and numerical results for two mode converters that are suitable for high-power microwave applications. Both converters are designed to operate at 11.424 GHz and utilize periodic serpentine structures to convert between modes with different azimuthal-mode indexes. The first converts about 98.5% of an incident linearly polarized TE₁₂ mode to the TE ₀₁ mode when oriented as an H-plane bend, but has just 1% mode conversion to all modes when oriented as an E-plane bend. The second device converts a linearly polarized TE₁₁ mode to a TM ₀₁ mode with over 99.99% effectiveness. The performance of both devices with respect to parametric variations is detailed. Experimental measurements of the radiation patterns from the TE₁₂ -to-TE₀₁-mode converter are consistent with the theoretical predictions     

Secondary LC filter analysis and design techniques forcurrent-mode-controlled converters

Small-signal characteristics of current-mode-controlled PWM converters with a second-stage LC filter are analyzed. It is shown that a secondary filter can be designed to provide good attenuation of the switching ripple while maintaining adequate stability margins with capacitive loading. The resonant frequencies and damping coefficients of the second filter are derived, and design guidelines are given. It is shown that the current-loop gain of the buck converter is not affected by the addition of the second-stage filters when a small filter inductance is used. Three design examples are presented to demonstrate the use of analysis results. Two filter examples are designed for a buck converter. One of the second filters shows the problems that arise with a poor design. A third example is the design of a second-stage filter for a buck-boost converter. In each of the design examples, the small-signal analysis was performed using EASY5 software and the circuits were simulated using the state-space simulation program COSMIR     

Small-signal analysis and control design of asymmetrical half-bridge DC-DC converters

This paper presents the small-signal modeling, dynamic analysis, and control design of the asymmetrical half-bridge dc-dc converter that employs a clamp capacitor and a magnetizing inductor to accommodate pulsewidth-modulated operation with asymmetrical duty ratios. The circuit averaging technique is applied to extract the small-signal dynamics of the power stage, and a graphical loop-gain method is used to design the feedback compensation and analyze the closed-loop performance of the converter. The distinctive power-stage dynamics of the converter are addressed and design guidelines for voltage feedback compensation are established. The results of the control design and closed-loop analysis are substantiated by experiments using an experimental converter.     

A zero-current-switched off-line quasi-resonant converter withreduced frequency range: analysis, design, and experimental results

A half-bridge zero-current-switched (ZCS) offline quasi-resonant converter (QCR) operating in full-wave mode is implemented to reduce the modulation frequency range due to load variations. The design and characteristics of the converter are described and compared with their half-wave counterpart in frequency range, component stress, and efficiency. An experimental converter which delivers output power from 10 to 100 W with a modulation frequency from 700 kHz to 1.4 MHz is presented. The converter showed superior transient response compared to the corresponding half-wave converter. The efficiency of the full-wave converter at full-load was several percent lower than that of the half-wave converter and decreases at a much faster rate as the output power decreases     

DC-to-AC inversion using quasi-resonant techniques

It is demonstrated that two basic approaches can be taken to achieve DC-to-AC inversion. The first, the differential load excitation (DLE) scheme, uses two converters, driven out of phase, to drive a load differentially. Unfortunately, this scheme requires a large number of components when it is implemented using quasi-resonant techniques. A more component-effective approach uses switched source excitation (SSE) schemes. One SSE approach uses two converters, each handling one polarity of the signal. By appropriate switching, an amplified signal is reconstructed at the output. The second SSE approach entails only one converter but with a slightly more involved switching requirement. It allows a significant overall reduction in the number of components needed and reduced performance requirements on most of the switches, since most switch at the lower inversion-frequency rate. A number of quasi-resonant inverter topologies from two broad categories are presented. For applications that require inverters to supply reactive power, three schemes which can be applied to various inverters, thus affording them reactive load handling capability, are discussed     

Evaluation of synchronous-rectification efficiency improvementlimits in forward converters

Design trade-offs for different implementations of the forward power converter with synchronous rectifiers are presented, and effects of the synchronous rectifier driving method on the conversion efficiency are evaluated. Specifically, the merits and limitations of the RCD-clamp and active-clamp reset approaches for a power converter with self-driven synchronous rectifiers are discussed. Estimates of the upper limits of the efficiency improvements of the discussed synchronous rectification approaches relative to the Schottky diode implementation are derived. Finally, experimental comparisons of efficiencies for an offline, 3.3 V/20 A forward power converter power stage are presented and compared with the theoretically estimated ones