Hoai Nam Le

Circuit-Parameter-Independent Nonlinearity Compensation for Boost Converter Operated in Discontinuous Current Mode

This paper proposes a current control method for discontinuous current mode (DCM) in order to achieve the same control performance as continuous current mode (CCM) in a boost converter. By utilizing the duty ratio at the previous calculation period to compensate for a DCM nonlinearity, the controller, which is designed for CCM, can also be used in DCM. In the frequency analysis, the cutoff frequency of the proposed DCM current control agrees exactly to the design value, which is 2 kHz, whereas the cutoff frequency of the conventional DCM current control results in high error of 47.5%. In the current step response experiment with a 360-W prototype and the switching frequency of 20 kHz, the experimental DCM current response almost agrees with the conventional CCM current response, which are 380-μs rise time for both CCM and DCM, 9% and 8% overshoot for CCM and DCM, respectively. Furthermore, the computation time of the proposed DCM current control is shorter 35% than the conventional DCM current control.

Minimization of interconnected inductor for single-phase inverter with high-performance disturbance observer

When a inductance of a single-phase grid-connected inverter is minimized, the effect of the disturbance to the current control such as the dead-time error voltage increases. This paper proposes a disturbance compensation method by using a high-performance disturbance observer. In particular, in the control system, the current controller is implemented by the digital signal processor, whereas the high-performance disturbance observer is implemented by the field-programmable gate array in order to suppress the disturbance at high switching frequency. This results in the improvement of the current control performance. With the experimental prototype, in case of the minimized inductance (%Z = 0.5%), the total harmonic distortion (THD) of the output current is improved from 11.1% to 2.9% at rated load by applying the proposed disturbance compensation method. Furthermore, with the proposed disturbance compensation method, it is confirmed that THD of the output current is improved by over 68% at all load range.

Clarification of relationship between current ripple and power density in bidirectional DC-DC converter

This paper clarifies the relationship between the current ripple and the power density in bidirectional DC-DC converters. In the conventional power density design method, in order to obtain the pareto-front curve of the power density and the efficiency, the current ripple is designed as constant value, whereas the switching frequency is varied. As a result, the possibilities of higher power density or higher efficiency at different current ripple are not considered. Therefore, in this paper, the current ripple is also varied in order to evaluate all the designable power density. Specifically, a design flow chart is introduced to show step-by-step how to express all the losses and the volume of the converter as functions of the current ripple. Several 1-kW prototypes are constructed in order to confirm the validity of the design flow chart. By varying the current ripple, the highest power density of 10.1 kW/dm3 with the efficiency of 98.55% is achieved at the current ripple of 60%. Furthermore, the maximum error between the calculated and experimental power density and efficiency are 19.5% and 0.22 pt. respectively.

Efficiency improvement at light load in bidirectional DC-DC converter by utilizing discontinuous current mode

This paper proposes a feedback current control for bidirectional DC-DC converter which is operated in Discontinuous Current Mode (DCM) at light load and Continuous Current Mode (CCM) at heavy load in order to improve light load efficiency. In the proposed method, the nonlinearity compensation for DCM operation is constructed by using the duty ratio at previous calculation period. Moreover, the introduction of DCM current feedback control into bidirectional power conversion is accomplished by detecting the operation mode at the output of the control system. This make the control becomes parameter-independent. The validity of the proposed control is confirmed by a 1-kW prototype. In the ramp response, the slope of the DCM current almost agrees to the design value with the error of 0.8%. Moreover, the smooth transition among 4 current modes: CCM-powering, DCM-powering, DCM-generation, CCM-generation, is also confirmed. On the other hand, in order to further improve the efficiency at light load, the synchronous switching for DCM is proposed. As a result, at load of 0.1 p.u. the efficiency of the DCM synchronous switching is improved by 1.5% from 97.2% to 98.7% compared with the CCM synchronous switching. Besides, it is confirmed that, the efficiency of the CCM/DCM synchronous switching is higher by 0.2% than that of the CCM/DCM asynchronous switching at all range of load. Furthermore, the efficiency at rated load is 98.8%, whereas the maximum efficiency is 99.0% at load of 0.45-0.65 p.u.

Wide-Load-Range Efficiency Improvement for High-Frequency SiC-Based Boost Converter With Hybrid Discontinuous Current Mode

This paper proposes a hybrid current mode between triangular current mode (TCM) and discontinuous current mode (DCM) in order to improve a wide-load-range efficiency for a high-frequency SiC-based boost converter. At rated load, TCM, where the zero-voltage switching (ZVS) is achieved, is used in order to increase the switching frequency and minimize the boost converter. At light load, the hybrid discontinuous current mode (HDCM), where the TCM current is flown during the zero-current interval of DCM, is applied in order to achieve both ZVS and a reduction of a current ripple. This maintains a high efficiency over a wide load range. A 1-kW prototype is realized to compare HDCM over continuous current mode (CCM), DCM, and TCM. Compared to TCM, the root-mean-square current is reduced up to 56.6% with HDCM. Consequently, the efficiency of HDCM at light load of 0.1 p.u. is improved by 3.1%. Compared to DCM, the average-current ripple in HDCM is reduced by 82.3%, whereas the efficiency of HDCM at light load of 0.1 p.u. is improved by 1.5%. Finally, when the current ripple of CCM is designed to be half of TCM, the efficiency of HDCM at rated load is improved by 0.9% compared to CCM.

Requirements for circuit components of single-phase inverter applied with power decoupling capability toward high power density

This paper discusses how to achieve high power density with high efficiency for a single-phase inverter with an active power decoupling circuit. In conventional PV inverters, bulky electrolytic capacitors are connected to DC-link in order to absorb power pulsation with twice the grid frequency. On the other hand, in the active power decoupling circuit, the small capacitor can be used. However, the additional inductors and switching devices are necessary. Thus, the power density of the active power decoupling circuit is reduced. In this paper, the Pareto optimization of power density and efficiency is used to clarify the maximum power density points of the power decoupling circuits. As a result, the maximum power density of the conventional boost type active buffer, which connects a boost chopper to DC-link, is 90% of that of electrolytic capacitor topology. In addition, this paper proposes a DC-DC converter with the power decoupling capability in order to achieve higher power density than that of the passive topology. The proposed circuit, which requires no additional inductor for the power decoupling circuit, uses discontinuous current mode (DCM) for the power decoupling capability. As a result, the maximum power density is obtained to 1.1 times higher than that of passive topology. However, the total loss of switching devices is 1.5 times higher. Thus, in order to surpass the efficiency of the passive topology by the active power decoupling, the switching device is required to reduce the total loss by 35% compared to the present products.

Zero-voltage switching for bidirectional buck/boost converter using hybrid discontinuous current mode

This paper proposes a hybrid current mode between Triangular current mode (TCM) and Discontinuous current mode (DCM) in order to achieve Zero-voltage switching (ZVS) for a bidirectional buck/boost DC-DC converter. In the proposed Hybrid Discontinuous Current Mode entitled HDCM, the TCM operation is applied during the zero-current interval of DCM. Therefore, both ZVS and the variable current ripple, which result in the high efficiency at wide load range, are achieved. The achievement of ZVS with HDCM is confirmed by a 600-W prototype. Compared to TCM, the Root-Mean-Square current is reduced by 47.2% at most with HDCM, which further contributes to the loss reduction. Moreover, under the condition of the same boost inductor, the efficiency of HDCM at load of 0.2 p.u. is improved by 1.5% compared to TCM.

Current THD reduction for grid-connected inverter operating in discontinuous current mode

This paper proposes a Discontinuous current mode (DCM) feedback current control for the grid-connected inverter in order to achieve the low THD of the grid current at wide load range. In the DCM operation, the nonlinearity between the duty ratio and the current occurs in the current command response, whereas another nonlinearity between the disturbance voltage and the current occurs in the disturbance response. Therefore, the nonlinearity in the current command response is compensated by utilizing the duty ratio at the previous calculation period, whereas the nonlinearity in the disturbance response is utilized in order to reduce the effect of the disturbance and improve the grid current THD at light load. As a result, the grid current THD of the proposed DCM feedback current control is reduced by 89.0% at light load of 0.1 p.u. compared to the conventional Continuous current mode (CCM) feedback current control. Furthermore, the smooth current regulation is confirmed even when the load step change occurs from 1.0 p.u. to 0.1 p.u.