Rachael Born

A Sensorless Implementation of the Parabolic Current Control for Single-Phase Stand-Alone Inverters

Parabolic current control is an attractive current control method with fast transient response and constant switching frequency. Due to the good dynamics of the parabolic current control, it can be employed in voltage source inverters to improve the system performance such as minimizing the distortion of current waveforms or voltage waveforms. To implement the parabolic current control, a current sensor is required, associated with the current conditioning circuit and parabolic carrier generators. Since the parabolic current control is based on the real-time information of the inductor current, any phase delay or propagation delay of the sensor itself and the conditioning circuitry, or limited resolution of parabolic carrier generators, could impact the current control performance. Since the parabolic current control compares analog signals to generate the required control signals, noise from the control board impacts the control precision as well. This paper will explore solutions to these problems. First, the inductor current of the voltage source inverter is analyzed and the parabolic current control strategy is studied, then a sensorless parabolic current control method is proposed. The new sensorless parabolic control method utilizes a current emulator to rebuild the inductor current on a microcontroller. To avoid a dc offset on the ac-side output voltage caused by the current emulator, an additional control loop in the current emulator is added. The effectiveness of the proposed methods is experimentally verified by the use of an H-bridge voltage source inverter.

Solution of input double-line frequency ripple rejection for high-efficiency high-power density string inverter in photovoltaic application

One important type of power conditioning systems in photovoltaic (PV) application is the string inverter which requires small input voltage and current ripple. In addition, high-efficiency and high-power density are also the critical requirements for string inverter system. So the input voltage and current ripple cannot be easily rejected by paralleling large conventional electrolytic capacitors in the inverter input side only, since the huge electrolytic capacitors will limit the efficiency and power density. This paper presents a control strategy to limit the input double-line frequency ripple with a high-efficiency and high-power density front-end buck-type dc/dc converter. The double-line frequency energy is stored at the DC bus capacitor with the control strategy employed and the input current and voltage ripple are highly reduced compared with traditional electrolytic capacitor filter. Experimental results based on a 2-kW 6.6 inch3 hardware prototype with 99.6% peak efficiency are provided to validate the proposed double-line frequency ripple rejection control method.

A Parabolic Voltage Control Strategy for Burst-Mode Converters With Constant Burst Frequency and Eliminated Audible Noise

To improve the conversion efficiency at light load, burst mode control is widely employed by power converters. One drawback of hysteresis burst mode control is that the burst frequency changes with different load, which would introduce audible noise to the system if burst frequency falls into audible frequency range. To solve the frequency variation and audible noise problems of burst-mode-controlled power converters, a parabolic voltage control method is proposed in this paper. A hysteresis burst-mode-controlled quasi-resonant boost converter is employed as an example. First, the operation principles of the boost converter are analyzed and the simplified circuit is obtained. Based on the simplified circuit, the frequency variation problem is studied and burst frequency change over different loads is calculated. Second, by analyzing the constitutive equations of the simplified circuit, inspired the derivation of parabolic current control, a pair of parabolic control carriers are obtained. The proposed parabolic carriers serve as the control band to achieve constant burst frequency, eliminating audible noise, and extending the light load efficiency to the whole load range. A comparison between parabolic voltage control and parabolic current control is conducted. It is found that parabolic voltage control can be seen as the dual of parabolic current control. In addition, the convergence process of the proposed parabolic voltage control is analyzed. The performance of the proposed control strategy is experimentally verified with prototype hardware.

A parabolic voltage control strategy for burst mode converters with constant burst frequency and eliminated audible noise

Burst mode control is widely employed by power converters to improve the light load efficiency. One drawback of burst mode control is that the burst frequency changes with different load, which would introduce audible noise to the system. In order to solve the frequency variation problem of burst mode controlled converters, a parabolic voltage control method is proposed in this paper. By the use of a pair of parabolic carriers as the control bands, the burst frequency is controlled to be constant and audible noise can be eliminated. The derivation of the proposed parabolic voltage control is conducted and operating principles are discussed. The performance of the proposed control method is experimentally verified by the use of a burst mode boost converter.

A High-Efficiency Hybrid Resonant Converter With Wide-Input Regulation for Photovoltaic Applications

A microconverter serves as a front-end dc–dc stage of a microinverter to convert the power from a photovoltaic module to a dc bus. These front-end microconverters require isolation, high-boost ratio, wide-input voltage regulation, and high efficiency. This paper introduces an isolated resonant converter with hybrid modes of operation to achieve wide-input regulation while still maintaining high efficiency. The proposed converter is designed as a series resonant converter with nominal-input voltage and operates under two additional modes: a boost converter with low-input voltage and a buck converter with high-input voltage. Unlike conventional resonant converters, this converter operates at discontinues conduction mode with a fixed frequency, simplifying the design and control. In addition, this converter can achieve zero-voltage switching (ZVS) and/or zero-current switching (ZCS) of the primary-side MOSFETs, ZVS and/or ZCS of the secondary-side MOSFETs, and ZCS of output diodes under all operating conditions. Experimental results using a 300-W prototype achieve a peak efficiency of 98.1% and a California Energy Commission efficiency of 97.6% including all auxiliary and control power at nominal-input voltage.

Interleaved auxiliary resonant snubber for high-power, high-density applications

Power density has become increasingly important for applications where weight and space are limited. New wide-bandgap (WBG) devices, combined with softswitching, now allow inverters to shrink in size by pushing to higher switching frequencies while maintaining efficiency. This paper proposes a novel interleaved auxiliary resonant snubber for high-frequency soft-switching to reduce volume while maintaining efficiency. The design of an auxiliary resonant snubber is discussed; this allows the main GaN MOSFETs to achieve zero voltage switching (ZVS). The auxiliary switches and SiC diodes achieve zero current switching (ZCS). While soft-switching minimizes switching loss, conduction loss is simultaneously reduced for high-power applications by interleaving two high frequency legs.

A sensorless parabolic current control method for single phase standalone inverters

Parabolic current control is an attractive current control method with fast transient response and constant switching frequency. Due to the good dynamics of parabolic current control, it can be employed in voltage source inverters to improve the system performance such as minimizing the distortion of current waveforms or voltage waveforms. To implement parabolic current control, a current sensor is required, associated with the current conditioning circuit and parabolic carrier generators. Since parabolic current control is based on the real-time information of the inductor current, any phase delay or propagation delay of the sensor itself and the conditioning circuitry, or limited resolution of parabolic carrier generators, could impact the current control performance. Since the parabolic current control compares analog signals to generate the required control signals, noise from the control board impacts the control precision as well. This paper will explore solutions to these problems. First, the inductor current of voltage source inverter is analyzed and the parabolic current control strategy is studied, then a sensorless parabolic current control method is proposed. The new sensorless parabolic control method utilizes a current emulator to rebuild the inductor current on a micro-controller. To avoid a dc offset on the ac-side output voltage caused by the current emulator, an additional control loop in the current emulator is added. The effectiveness of the proposed methods are experimentally verified by the use of an H-bridge voltage source inverter.

A parabolic current control based digital current control strategy for high switching frequency voltage source inverters

Since parabolic current control has fast transient response and high control precision with constant switching frequency, it is an attractive current control strategy for voltage source inverters. The employment of parabolic current control delivers great performance on current waveform control or voltage waveform control. The implementation of parabolic current control usually requires high speed digital-to-analog converters (DACs) to generate a pair of parabolic carriers as the control band. With increases in switching frequency, this requirement becomes more critical because the speed of DAC limits the update rates of the parabolic carriers. This impacts the smoothness and punctuality of the parabolic control band. To solve this issue, a parabolic current control based digital current control strategy is proposed in this paper. The proposed method uses the sensed current of the output inductor to calculate the next duty-cycle. To improve the transient response speed, a duty-cycle adjustment is presented. The performance of the proposed digital control method is verified in simulation.

Output capacitance effect on the voltage gain in the high step-up series resonant converter

For the series resonant converter (SRC), the voltage gain defined as voltage conversion ratio is one when the switching frequency is equal to resonant frequency. However, it can be greater than one at light load condition especially in the high step-up SRC, which is caused by the high voltage side devices output capacitance. Also the current through the resonant tank is no longer the sinusoidal waveform. The steady-state analysis of this phenomenon based on the state trajectories is presented in this paper. The relationship between voltage gain and device output capacitance is given. The experimental results of 300-W hardware prototype with 30-V normal input and 380-V output validated the derived voltage gain under the effect of devices output capacitance.

Single-Step Current Control for Voltage Source Inverters With Fast Transient Response and High Convergence Speed

Current control loop response in voltage source inverters impacts the quality of output current and output voltage waveforms. Parabolic current control provides a fast transient response with approximate-constant switching frequency, solving the frequency variation problem of hysteresis current control. This makes it a good candidate for the current control loop of voltage source inverters to achieve a good system performance. Yet, parabolic current control is often implemented with digital-to-analog converters, analog comparators, and field-programmable gate array circuits where increasing switching frequency pushes update speed and bandwidth requirements. Concurrently, even if the transient response of parabolic current control is fast, it can still take up several switching cycles converging to steady state. In order to solve both problems, a new current control strategy, motivated by the convergence analysis of parabolic current control but with a convergence process that takes just one switching operation, is proposed: single-step current control. Since single-step current control samples two data points in a switching cycle instead of using continuous parabolic carriers, it can be easily implemented in a digital microcontroller then high switching frequency can be achieved. The small signal model, dead-time compensation, and stability analysis are also studied in this paper. The convergence speed of the algorithm is verified with experimental hardware prototype and the transient performance of the designed voltage source inverter meets expectation.