Showing papers on "Pulse-frequency modulation published in 2017"

Dual-Bridge LLC Resonant Converter With Fixed-Frequency PWM Control for Wide Input Applications

Abstract: This paper proposes a dual-bridge (DB) LLC resonant converter for wide input applications. The topology is an integration of a half-bridge (HB) LLC circuit and a full-bridge (FB) LLC circuit. The fixed-frequency pulsewidth-modulated (PWM) control is employed and a range of twice the minimum input voltage can be covered. Compared with the traditional pulse frequency modulation (PFM) controlled HB/FB LLC resonant converter, the voltage gain range is independent of the quality factor, and the magnetizing inductor has little influence on the voltage gain, which can simplify the parameter selection process and benefit the design of magnetic components as well. Over the full load range, zero-voltage switching (ZVS) and zero-current switching (ZCS) can be achieved for primary switches and secondary rectifier diodes, respectively. Detailed analysis on the modulation schedule and operating principle of the proposed converter is presented along with the converter performance. Finally, all theoretical analysis and characteristics are verified by experimental results from a 120-V to 240-V input 24 V/20 A output converter prototype.

PWM and PFM Hybrid Control Method for LLC Resonant Converters in High Switching Frequency Operation

Abstract: High switching frequency is an effective method to improve power density for LLC resonant converters. However, conventional digital controllers, such as general-purpose digital signal processors and microprocessors, have limited frequency resolution, which induces high primary- and secondary-side current variation and leads to poor output voltage regulation. In this paper, a hybrid control method combining pulse frequency modulation (PFM) and pulse width modulation is proposed to overcome the limited frequency resolution issue. The proposed hybrid control method focuses on steady-state operation, and its operating principles are introduced and analyzed. In addition, the proper magnetizing inductance and dead time duration are derived to ensure that the power mosfet s achieve zero voltage switching with the proposed control method. The improved voltage regulation performance is compared with the conventional PFM control and verified through simulation and experimental results using a 240 W prototype converter operating at a switching frequency of 1 MHz.

Modulation Method of a Full-Bridge Three-level LLC Resonant Converter for Battery Charger of Electrical Vehicles

Abstract: This paper proposes a modulation method for a full-bridge three-level LLC resonant converter The target converter is known to enable fixed frequency operation, small resonant inductance ratio, and zero-current switching operation of rectifier diodes under wide output-voltage variation Therefore, it is suitable for an application such as in battery chargers A new parameter named “master duty,” which determines each voltage-level duty, is introduced to this modulation method It simplifies the modulation method; therefore, a modification of the modulation is easy It also makes possible seamless handling of two-level, three-level, and mixed modes The modulation method is represented by the relationship between the master duty and each gate-pulse edge position This representation can clearly depict a complex modulation such as a combination of pulse-width and phase-shift modulations The voltage gain of the converter is analyzed using the first-harmonic approximation technique The result of the analysis is verified by the circuit simulation The voltage gain is also analyzed using lossy components, which is verified by the experimental result A 385-V input 225- to 378-V/66-kW output prototype converter is fabricated for this verification It shows 9814% peak efficiency, and its power density is 1036 W/L

Variable-Angle Phase-Shifted PWM for Multilevel Three-Cell Cascaded H-Bridge Converters

Abstract: Multilevel cascaded H-bridge converters have become a mature technology for applications where high-power medium ac voltages are required. Normal operation of multilevel cascaded H-bridge converters assumes that all power cells have the same dc voltage, and each power cell generates the same voltage averaged over a sampling period using a conventional phase-shifted pulse width modulation (PWM) technique. However, this modulation method does not achieve good results under unbalanced operation per H-bridge in the power converter, which may happen in grid-connected applications such as photovoltaic or battery energy storage systems. In the paper, a simplified mathematical analysis of the phase-shifted PWM technique is presented. In addition, a modification of this conventional modulation method using variable shift angles between the power cells is introduced. This modification leads to the elimination of harmonic distortion of low-order harmonics due to the switching (triangular carrier frequency and its multiples) even under unbalanced operational conditions. The analysis is particularized for a three-cell cascaded H-bridge converter, and experimental results are presented to demonstrate the good performance of the proposed modulation method.

A Carrier-Based PWM Scheme for Neutral Point Voltage Balancing in Three-Level Inverter Extending to Full Power Factor Range

Abstract: This paper proposes a unique carrier-based pulse width modulation strategy for a three-level neutral-point-clamped inverter that works satisfactorily and with uniform convergence time over the full power factor (PF) range of the load variation. It has excellent control over capacitor voltages and mitigates low-frequency oscillations in the neutral point for the full PF range. First, an investigation is made to see how the load currents get modulated to generate compensating neutral current causing charging and discharging of dc-link capacitors. It is observed that when an additional modulating voltage signal that is in phase with the corresponding phase current is added in each phase, a neutral current is produced in the right direction which can be used to compensate for any prior unbalance in capacitor voltages. It is demonstrated that the performance of this scheme is insensitive to the load PF variation (full range from zero to unity). It performs satisfactorily throughout the entire linear modulation range and eliminates any undesirable low-frequency harmonic component in the compensating neutral current. The effectiveness of the proposed scheme is verified through simulation and experimental results by varying load PF as well as modulation index with both passive and motor loads.

An evaluation of frequency methods for creating a system to control semiconductor converters

Abstract: I explore the possibility of increasing the response speed of the semiconductor converters that operate in the pulse mode by increasing the carrier frequency or rejecting pulse-width modulation (PWM) in favor of frequency PWM. The relation between the PWM carrier frequency and the limit response speed of the current loop achievable in closed control systems using classical frequency methods for creation of linear systems has been established. A model of mathematical simulation of a semiconductor device is proposed. The model is approximated by ideal elements without considering the voltage drop on the semiconductor keys and the dead-time interval. Using the simulation model, the frequency characteristics of of the frequency converters are compared with one converter operating in the linear mode and the other in the pulse mode. The limiting value of the frequency starting from which the frequency characteristics of the linear and pulse systems diverge has been determined for different kinds of the transfer functions of the “unchangeable” part. It has been established that this frequency approaches the Nyquist frequency most closely at resonance-peak amplitude A m > 1.

A Unified Space Vector Pulse Width Modulation for Dual Two-level Inverter System

Abstract: This letter proposes a unified space vector pulse width modulation (SVPWM) for a dual two-level inverter system with two isolated dc voltage sources. The gate pluses for the dual inverter are generated by the concept of one unified SVPWM in accordance with the voltage-second integral principle and the ratio of the two dc-link voltages can be an arbitrary positive value. This proposed technique also simplifies the region identification in sectors and reduces total switching frequency of the dual inverter. The beneficial performance of this proposed technique is verified experimentally.

A Design of a 92.4% Efficiency Triple Mode Control DC–DC Buck Converter With Low Power Retention Mode and Adaptive Zero Current Detector for IoT/Wearable Applications

Abstract: This paper presents a retention/ pulse frequency modulation (PFM)/ pulse width modulation (PWM) mode dc–dc buck converter with adaptive zero current detector (AZCD) and spread spectrum clock generation (SSCG) for IoT/Wearable systems. The proposed dc–dc buck converter is capable of handling loads from 10 μA to 20 mA with high efficiency by applying triple mode (retention mode, PFM mode, and PWM mode), gate split technique, and AZCD. Retention mode is proposed to extend wide load range at ultralight load. Gate split technique adjusts the conduction loss and switching loss. AZCD reduces the operation duty of the high-speed comparator and avoid reverse current below light load. In IoT applications, each node communicates with other nodes through a Bluetooth low energy transceiver, which consumes very low current and is highly sensitive to supply noise. Therefore, the proposed dc–dc buck converter adopts the SSCG technique to reduce electromagnetic interference by up to 18 dB. This chip is implemented using 0.13 μm CMOS technology with an active area of $\text{820}\times \text{800}\;\mu \text{m}^{2}$ . The maximum power efficiency of the proposed dc–dc buck converter is 92.4% at a switching frequency of 2.5 MHz when the load current range is 10 to 20 mA. The input voltage range and the regulated output voltage are 2.2–3.3 and 1.7 V, respectively. In addition, the proposed dc–dc buck converter achieves over 74.2% efficiency in retention mode when the load current range is from 10 to 500 μA.

Impact of Switching Harmonics on Capacitor Cells Balancing in Phase-Shifted PWM-Based Cascaded H-Bridge STATCOM

Abstract: The purpose of this paper is to investigate the impact of switching harmonics on the instantaneous power distribution in the cells of a cascaded H-bridge-based STATCOM when using phase-shifted pulse width modulation. The case of high- and low-switching frequency for the converter cells is investigated and the interaction between voltage and current harmonics is analyzed. It is shown that in both cases, this interaction results in an uneven power distribution among the cells in the same phase leg, leading to drifting of the dc-capacitor voltages and thereby the need for proper stabilization control loops. It is also shown that the selected frequency modulation ratio affects the active power distribution among the cells. In particular, the selection of a noninteger frequency modulation ratio helps in providing a more uniform power distribution among cells of the same phase leg, thus contributing to the capacitors balancing. A methodology for optimal selection of the frequency modulation ratio is given. Theoretical conclusions are validated through simulation and experimental results.

A Carrier-Based Phase-Shift Space Vector Modulation Strategy for a Nine-Switch Inverter

Abstract: By using a carrier-based pulse width modulation, this paper presents a developed phase-shift space vector modulation strategy for a nine-switch inverter to overcome one of major drawbacks, i.e., output voltage and current distortion. This is due to the crossover of modulating waves per phase based on the different phase-shift angle between dual terminals. The proposed strategy done by mathematical algorithms based on the critical operating point principle accomplishes in the wide range of phase-shift conditions that contribute in both common- and difference-frequency modes with its expediency of uncomplicated computation and handy implementation. Even more, the identical and nonidentical modulation index techniques are wholly brought up. The feasibility of the proposed strategy is confirmed by results of the simulation directly compared with the experiment, showing in good accordance with the theoretic analysis.

A Modulation Strategy to Operate Multilevel Multiphase Diode-Clamped and Active-Clamped DC–AC Converters at Low Frequency Modulation Indices With DC-Link Capacitor Voltage Balance

Abstract: This paper proposes a modulation strategy for multilevel multiphase diode-clamped dc–ac converters (also applicable to other functionally equivalent topologies) able to keep the dc-link capacitor voltages balanced under passive front ends, low frequency modulation indices (i.e., low number of switching transitions per fundamental cycle), any value of the amplitude modulation index, and any ac-side displacement power factor. A suitable phase voltage pattern with minimum number of switching transitions is presented for the n -level three-phase case. Subsequently, it is extended to higher number of switching transitions per fundamental cycle and to higher number of phases. Simulation results with three, four, and five levels; three and five phases; and several frequency-modulation-index values are presented to validate the proposed modulation strategy. Experimental results obtained with a four-level three-phase dc–ac converter prototype are also provided. The proposed modulation strategy enables the use of this type of converters in applications where the ac fundamental frequency may be close to the switching frequency, such as in high-power systems and variable-speed motor drives.

A Sensitivity-Improved PFM LLC Resonant Full-Bridge DC–DC Converter With LC Antiresonant Circuitry

Abstract: An LLC resonant circuit-based full-bridge dc–dc converter with an LC antiresonant tank for improving the performance of pulse frequency modulation (PFM) is proposed in this paper. The proposed resonant dc–dc converter, named as LLC-LC converter, can extend a voltage regulation area below the unity gain with a smaller frequency variation of PFM by the effect of the antiresonant tank. This advantageous property contributes for protecting overcurrent in the case of the short-circuit load condition as well as the start-up interval in the designed band of switching frequency. The circuit topology and operating principle of the proposed converter is described, after which the design procedure of the operating frequency and circuit parameters is presented. The performances on the soft switching and the steady-state PFM characteristics of the LLC-LC converter are evaluated under the open-loop control in experiment of a 2.5-kW prototype, and its actual efficiency is compared with an LLC converter prototype. For revealing the effectiveness of the LLC-LC resonant circuitry, voltages and currents of the series and antiresonant tanks are analyzed, respectively, with state-plane trajectories based on calculation and experiment, whereby the power and energy of each resonant tank are demonstrated. Finally, the feasibility of the proposed converter is evaluated from the practical point of view.

A Single-Stage PFM-APWM Hybrid Modulated Soft-Switched Converter With Low Bus Voltage for High-Power LED Lighting Applications

Abstract: In order to design a high-efficiency cost-effective high-power LED driver, a single-stage integrated boost- LLC -type soft-switched ac–dc converter is usually employed. The minimized number of semiconductors and the reduced power loss are its main advantages. However, the high bus voltage is its well-known disadvantage, which is caused by the employment of a conventional pulse frequency modulation (PFM) control strategy with the duty cycle of 0.5. The range of the input ac voltage is severely limited. In order to overcome this problem, a hybrid PFM-asymmetric pulse width modulation strategy is proposed in this paper. The proposed strategy can significantly reduce the bus voltage while maintaining the soft-switching operation for both primary MOSFETs and secondary diodes. Furthermore, the electrolytic capacitors are removed by increasing the bus voltage ripple. Hence, high efficiency, long lifetime, low cost, and universal operation are achieved on the proposed LED driver, which is well matched to the virtues of LED lighting source. The detailed operation principles and design consideration for the proposed modulation approach are analyzed and discussed. The feature of the proposed solution was demonstrated using a universal-input 100-W hardware prototype. The experimental results verified the full-range zero-voltage-switching (ZVS) operation for primary switches and the efficiency achieved 91.6% under the 130-V input voltage.

All-Digital High Resolution D/A Conversion by Dyadic Digital Pulse Modulation

Abstract: In this paper, the limitations of digital-to-analog (D/A) conversion by Digital Pulse Width Modulation (DPWM) are addressed and the novel Dyadic Digital Pulse Modulation (DDPM) technique for all-digital, low cost, high resolution, Nyquist-rate D/A conversion is proposed Thanks to the spectral characteristics of the new modulation, in particular, the requirements of the filter needed to extract the baseband component of DPWM signals can be significantly released so that to be suitable to inexpensive integration on silicon in analog interfaces for nanoscale integrated systems After the new DDPM technique and its properties are introduced on a theoretical basis, the implementation of a D/A converter (DAC) based on the proposed modulation is addressed and its performance in terms of noise and linearity is discussed A 16-bit DDPM-DAC prototype is finally synthesized on a field-programmable gate array (FPGA) and experimentally characterized

Laser frequency stabilization by combining modulation transfer and frequency modulation spectroscopy.

Abstract: We present a hybrid laser frequency stabilization method combining modulation transfer spectroscopy (MTS) and frequency modulation spectroscopy (FMS) for the cesium D2 transition. In a typical pump-probe setup, the error signal is a combination of the DC-coupled MTS error signal and the AC-coupled FMS error signal. This combines the long-term stability of the former with the high signal-to-noise ratio of the latter. In addition, we enhance the long-term frequency stability with laser intensity stabilization. By measuring the frequency difference between two independent hybrid spectroscopies, we investigate the short-and long-term stability. We find a long-term stability of 7.8 kHz characterized by a standard deviation of the beating frequency drift over the course of 10 h and a short-term stability of 1.9 kHz characterized by an Allan deviation of that at 2 s of integration time.

Current Modulation of Nanoconstriction Spin-Hall Nano-Oscillators

Abstract: A single nanoconstriction spin-Hall nano-oscillator (NC-SHNO) in out-of-plane fields is presented as a nonlinear amplitude and frequency modulator operated by radio-frequency (RF) current modulation. The current modulation was carried out in different NC-SHNO nonlinearity regimes corresponding to negative, zero, and positive values of $df/dI$ in order to investigate the device response to an 80 MHz modulating current. Our study shows that current modulation of SHNOs can be quantitatively predicted by a nonlinear frequency and amplitude modulation (NFAM) model using the values of $df/dI$ and $d^2f/dI^2$ extracted from the free-running frequency $f$ versus current $I$ profile. The NFAM model reproduces the asymmetric sideband amplitude as well as the red and blue shift of the frequency in excellent agreement with the experimental results. The ability to predict the modulation process is a necessary benchmark in designing SHNO modulators for future integrated microwave circuits.

A New High-Switching-Frequency Modulation Technique to Improve the DC-Link Voltage Utilization in Multilevel Converters

Abstract: This paper presents a new high-frequency modulation method for multilevel converters The proposed method provides a broad linear operating range and can be digitally implemented with minimal computational effort This modulation method creates a phase voltage composed of a rectangular component superimposed on the top of a quasi-square-shaped reference function The reference functions are defined such that the utilization of the dc-link voltage is maximized in any modulation index, while the dv/dt of the switches is always the minimum possible value In order to implement the proposed method, an update time that is much shorter than the fundamental period is defined in the algorithm for updating the rectangular components of the reference voltage The high-frequency rectangular component can then be imposed on the reference function to generate the final switching function by switching between two voltage levels during the time between two update instances Several experimental results are provided to evaluate the performance of the proposed method and to compare its operation to conventional methods

Novel efficiency-Optimal Frequency Modulation for high power density DC/AC converter systems

Abstract: State-of-the-art high power density AC/DC and DC/AC converter systems typically employ Triangular Current Mode (TCM) modulation or conventional PWM. TCM is characterized by a wide variation of switching frequency over the mains period and ensures soft-switching in all operating points, but results in increased conduction and high-frequency losses due to the necessary large current ripple. In contrast, PWM with constant switching frequency features a lower RMS current and thus reduced conduction losses, but cannot achieve soft-switching over the entire mains period and suffers from turn-on losses. In this paper it is investigated if an optimal combination of these two operating modes, i. e. an optimal adjustment of switching frequency and/or current ripple amplitude throughout the mains period, can lead to an increase in conversion efficiency and how such an Optimal Frequency Modulation (OFM) control scheme can be implemented in practice. The presented analysis is based on an ultra compact 2kW, 400V DC/AC converter system designed to overcome the GOOGLE Little Box Challenge.

Design and implementation of a series resonant solid state transformer

Abstract: The focus of this study is on the design of a full-bridge unidirectional resonant LLC Solid State Transformer. The proposed topology uses a high-frequency transformer to optimize the size and weight of the converter. This converter has the capability of operating at fluctuating load conditions while it keeps the voltage regulated in different operation point. The converter is designed to maintain soft switching by using a resonant circuit in this design to minimize the switching loss of the high frequency converter. ZVS in the leading leg for turn on mode and ZCS commutation in the lagging leg for all of the modes are achieved in the H-bridge through the suggested circuitry which is analyzed mathematically in detail in this study. A combination of Pulse frequency modulation (PFM) and Phase Shifting Modulation (PSM) are utilized to control this resonant converter. The experimental setup for the suggested configuration was implemented and the results of the simulation and calculations have been verified with test results. The hardware set up was tested with two different power levels and the output results confirm that the control method works properly to feed the load and keeps the converter working in the expected frequency range and maintaining the soft switching to decrease switching loss. The results shows conversion efficiency of 97.18% is achieved.

Error-feedback control of multistability

Abstract: We propose the use of error-feedback control for converting a multistable system into a monostable one. The method is tested with a multistable erbium-doped fiber laser which exhibits the coexistence of four periodic orbits. A desired orbit is selected by an external harmonic modulation which frequency is equal to the frequency of one of the coexisting attractors. Additionally, the error signal, proportional to the difference between the external modulation and the laser output intensity, is applied to the pump current of the diode pumped laser. For certain combinations of the amplitude of the external modulation and the error signal, the laser displays monostability with a single periodic orbit at frequency determined by the external modulation. The efficiency of the control is demonstrated by both numerical simulations and experimentally.

A design of single-switch two-stage DC-DC converters with PWM and PFM for off-grid solar power system

Abstract: In this paper, a single-switch two-stage DC-DC conversion circuit is proposed for an off-grid solar power application. A photovoltaic(PV) panel powers the load and a storage unit(battery) via the proposed circuit. The battery is designed to balance the supply and the demand of power under different irradiation situations. Based on conventional cascaded DC-DC converters, the proposed design is developed with the single switch technique reducing size, cost and power loss. The control scheme in this design is pulse width modulation(PWM) with pulse frequency modulation(PFM). The PWM module is similar to conventional design except its ramp signal with a variable frequency is provided by a resettable integrator. As a result, the PWM and PFM modules regulate the two stages of the proposed circuit separately with the same switching control signal. In this paper, the modes of operation of the circuit are discussed as well as the control schemes. The design process is described along with the circuit analysis and comparisons with conventional design are made as well. A prototype has been built to verify the proposed circuit with simulation and experimental results.

Modulation Index Adjustment for Recovery of Pure Wavelength Modulation Spectroscopy Second Harmonic Signal Waveforms

Abstract: A new technique of modulation index adjustment for pure wavelength modulation spectroscopy second harmonic signal waveforms recovery is presented. As the modulation index is a key parameter in determining the exact form of the signals generated by the technique of wavelength modulation spectroscopy, the method of modulation index adjustment is applied to recover the second harmonic signal with wavelength modulation spectroscopy. By comparing the measured profile with the theoretical profile by calculation, the relationship between the modulation index and average quantities of the scanning wavelength can be obtained. Furthermore, when the relationship is applied in the experimental setup by point-by-point modulation index modification for gas detection, the results show good agreement with the theoretical profile and signal waveform distortion (such as the amplitude modulation effect caused by diode laser) can be suppressed. Besides, the method of modulation index adjustment can be used in many other aspects which involve profile improvement. In practical applications, when the amplitude modulation effect can be neglected and the stability of the detection system is limited by the sampling rate of analog-to-digital, modulation index adjustment can be used to improve detection into softer inflection points and solve the insufficient sampling problem. As a result, measurement stability is improved by 40%.

New Modulation Strategy Combining Phase Shift and Frequency Variation for Dual-Active-Bridge Converter

Abstract: This paper proposes a new modulation strategy for a dual-active-bridge converter. The target of the new strategy is to suppress the circulating current in the converter by minimizing the load angle. The soft-switching operational area is expanded by employing frequency variation together with phase shift modulation. Unlike published modulation methods based on frequency variation, the strategy proposed in this paper can operate under the light or heavy power transmission regardless of the frequency limitation as well as the voltage ratio. As confirmed by experiments, the new strategy can boost the overall efficiency by up to 7% compared to the conventional single-phase-shift method.

A 71% efficient energy harvesting and power management unit for sub-μW power biomedical applications

Abstract: This paper presents an Energy Harvesting and Power Management Unit (EH-PMU) to power battery-less sub-μW systems-on-chip (SoCs) and wireless sensors for emerging Internet-of-Things (IoT) applications. The EH-PMU can harvest energy from either photovoltaic or thermoelectric sources and provides regulated outputs of 0.5V, 1V, and 1.8V. To reduce the power conversion overhead in < 1μW-power systems and thus to extend the system lifetime, the EH-PMU employs a hybrid architecture consisting of nW-quiescent power switched-capacitor DC-DC converters and low-dropout (LDO) regulators. The platform uses a 1.3nW gate-leakage based voltage reference generator, operational from 0.5V, along with Pulse Frequency Modulation (PFM) control to further lower the quiescent power of the switching regulators. The EH-PMU achieves a peak end-to-end efficiency of 71.1% while powering a 1 μW load in a 0.13μm chip.

A PFM-Based Digital Pixel With an Off-Pixel Residue Measurement for Small Pitch FPAs

Abstract: Digital pixels based on pulse frequency modulation employ counting techniques to achieve a very high charge-handling capability compared to their analog counterparts. Moreover, extended counting methods that make use of leftover charge (residue) on the integration capacitor help to improve the noise performance of these pixels. However, focal plane arrays with small pixel pitch are constrained in terms of pixel area, which makes it difficult to benefit from in-pixel extended counting circuitry. Thus, in this brief the authors propose a novel approach to measure the residue outside the pixel using an analog-to-digital converter (ADC). A first prototype of the proposed pixel, in the form of a testbed, has been developed. It is aimed at medium-wave infrared imaging arrays that have a small pixel pitch. The prototype is composed of a pixel front end, a 12-bit successive approximation register ADC, a counter, and a comparator. The front end is a modified version of the conventional design and has been designed and fabricated in 90-nm CMOS, whereas off-the-shelf discrete components have been used to implement the ADC, comparator, and counter. A measured signal-to-noise ratio at low illumination levels is 55 dB.

Coherent population trapping atomic clock by phase modulation for wide locking range

Abstract: A method of detecting Coherent Population Trapping resonance by phase modulation (PM) was studied to expand the frequency locking range (LR). We calculated error signals by density matrix analysis using an eigenvector algorithm and verified the calculated result using an 87Rb vapor cell and a Rb-D1 vertical cavity surface-emitting laser. By comparing the error signal for PM with that for conventional frequency modulation (FM), it was found that the LR with PM is one order of magnitude wider than that with FM without degrading the frequency stability.

Analysis of the photon–photon resonance influence on the direct modulation bandwidth of dual-longitudinal-mode distributed feedback lasers

Abstract: The paper explores the possibilities to extend the direct modulation bandwidth in dual-longitudinal-mode distributed feedback lasers by exploiting the photon–photon resonance induced by the interaction of the two modes in the laser cavity. The effects on the direct amplitude modulation and on the direct modulation of the difference frequency between the two modes are analyzed using simulation and experimental results. When the photon–photon resonance, which occurs at the difference frequency between the two modes, is properly placed at a higher frequency than the carrier-photon resonance, the small-signal amplitude modulation (AM) bandwidth of the laser can be significantly increased. However, both simulations and experiments point out that a high small-signal AM bandwidth does not lead to a high large-signal AM bandwidth if the small-signal modulation response has significant variations across the modulation bandwidth. The paper shows that a high large-signal AM bandwidth is obtained when the two modes are significantly unbalanced, whereas a high-bandwidth difference frequency modulation can be best detected when the two modes are balanced and the DC bias is properly chosen.

A bridgeless electrolytic capacitor-free LED driver based on series-resonant converter with constant frequency control

Abstract: High-brightness light-emitting diode (LED) is next generation of green lighting with high luminous efficiency and long life. The bulky electrolytic capacitors are required to balance the pulsating power difference, thus restricting the driver lifetime. However, the existing LED drivers using pulse frequency modulation (PFM) is bound to arouse heavy electromagnetic interference(EMI). In this paper, a series resonant converter (SRC) is proposed to eliminate the low frequency ripple. Meanwhile, the switching controlled capacitor (SCC) is used to replace the resonant capacitor to achieve full range operation with constant switching frequency. Half-bridge switches are shared by integrating the bridgeless boost power factor correction (PFC) circuit and the power control (PC), so as to realize the single-stage drive. Furthermore, the SCC is shared by the outputs of upper and lower half-bridge, which reduces the volume. All the superior performance of proposed LED driver is verified by simulation with 2-channel LEDs, rated output power of 100W.

The design of cascaded DC-DC converters with single-switch PWM and PFM for standalone PV power applications

Abstract: This paper proposes a design of cascaded DC-DC converters for transferring energy from solar panels to loads in standalone cases. The storage units are connected to the proposed system via the converters to solve the instability of the renewable power source. The voltage levels of the power supply, storage and the load are different to each other in the design. Single-switch simplification is utilised to reduce the number of switches in the topology. The control circuit of the system is developed based on the conventional cascaded converters. Resettable integrators are introduced in the control circuit to perform Pulse Frequency Modulation (PFM) to regulate one of the converters. The ramp signal from the integrator is used to achieve Pulse Width Modulation (PWM) simultaneously. The advantages of the proposed design include the fewer components, the smaller size and more control freedom than the conventional converters for PV power system. In this paper, the design process of the proposed converters is introduced with the operation principles in different situations. Moreover, the design of the control circuit is illustrated as well as the operation of PWM and PFM. Last, the proposed design is verified by the simulation and experimental results.