Design and analysis of power decoupling based microinverter considering parasitic parameters | Scientific Reports

Scientific Reports volume 14, Article number: 25751 (2024) Cite this article 224 Accesses Metrics details With fossil energy reducing year by year, more and more attention to the new energy sources

Scientific Reports volume 14, Article number: 25751 (2024) Cite this article

224 Accesses

Metrics details

With fossil energy reducing year by year, more and more attention to the new energy sources greatly promotes the development of the distributed power generation system, especially the low-power photovoltaic system. However, the power injected into the single-phase grid is time-varying, an electrolytic capacitor with large bulk should be paralleled with PV to balance the input and output power ripple. But the electrolytic capacitor has a very short lifetime, which could shorten the whole inverter’s lifetime. In this paper, the operational principle of the power decoupling based microinverter considering parasitic parameters is proposed, in which a film capacitor with small capacitance value replaces the electrolytic capacitor to improve the lifetime of the inverter. Further, the operating mode of the transformer and the decoupling capability is analyzed, and obtains the reasons affecting the inverter steady-state performance. Finally, the simulation and experiment validation of the proposed microinverter have been accomplished. Base on the analysis method proposed in this paper, a more accurate device selection can be provided for industrial design.

In recent years, renewable energy generation systems have great benefits dealing with the environmental challenges, which has attracted wider attentions in the past decades1,2,3. Solar photovoltaic (PV) power generation has provided desirable characteristics, such as no noise, no pollution, energy available everywhere, low cost, convenient combination with buildings and so on2,4,5. It has become one of the main forms of renewable energy in the world. The photovoltaic inverter could be divided into three types: centralized inverter, chain type inverter and microinverter, as shown in Fig. 1. Compared with centralized inverter and chain-type inverter, microinverter has more potential feature such as plug-and-play, small size, flexible and safe features. In addition, microinverter avoids the energy loss and battery failure caused by the hot spot effect. Nowadays, microinverters are widely used in smart grids and household applications.

Classification of photovoltaic grid system.

In the PV system, the DC voltage is obtained by the PV panels, which is affected by the light intensity and the output current of the panel. The core technology of PV system is photoelectric conversion, which deliver solar energy of PV modules to the grid. The PV system needs to convert the DC voltage into the AC voltage to meet the power quality standard of the grid4,6,7,8. Since the power of the PV panels is constant when the PV array operates on the MPPT mode, and the gird power is time varying, the system power will instantaneously mismatch. The DC side energy fluctuation of single-stage inverter is a very serious problem as shown in Fig. 2, where Pac represents the AC power and Ppv represents input power of PV side. In order to solve the secondary pulsation problem of DC side, mF level electrolytic capacitors are usually used to stabilize the DC bus voltage. However, electrolytic capacitors have a lifetime of only 1000–7000 hours in the operating environment of 105°C 9. Thus, DC side electrolytic capacitors limit the life and power density of the microinverter 10,11,12,13.

Instantaneous power of input and output.

In order to improve the efficiency and power density of the microinverter, the idea of using power decoupling unit to deal with pulsating power is proposed 14,15,16,17,18,19, as shown in Fig. 3. In paper 14, power decoupling technology is used to reduce the current of decoupling loop, so as to improve the efficiency of the system. However, the voltage and current spikes of the switches are caused for the leakage energy of the transformer cannot be released. What’s more, the system efficiency is low. In paper 15, a new power decoupling unit is added to push-pull circuit and full bridge circuit is adopted in AC port. This decoupled method greatly improves the circuit efficiency, and the maximum efficiency is 95% at 300W. In paper 16, an improved flyback inverter topology with power decoupling is proposed. The traditional flyback micro inverter is improved by changing the control mode.

Schematic diagram of power decoupling method.

In this work, the operational principle of the flyback microinverter considering parasitic parameters is proposed where the flybak-type components, the output capacitance of the main switches is considered. Firstly, the operational principle of the flyback microinverter with decoupling circuit considering parasitic parameters is introduced. In addition, the operating mode of the transformer and the decoupling capability is analyzed. Secondly, the LT-Spice simulations are carried out to analysis influence of the power decoupling circuit. Moreover, a PV microinverter with power decoupling circuit is built to validate the feasibility and effectiveness of the analysis. Finally, the conclusion of the paper is given.

The topology of flyback microinverter with power decoupling circuit is shown in Fig. 4. The flyback inverter consists of main switch Sm, transformer T1, secondary switches Sac1 and Sac2 and diode Dac1 and Dac2. The main switch is controlled by SPWM, and the secondary side switches work in power frequency. The power decoupling circuit consists of a switch SX and a decoupling capacitor Cx. At the same time, in order to cut off the reverse current flowing through Sm, the diode Dm is connected in series with Sm. The grid connected filter circuit is composed of Lf and Cf. Cdc represents the dc link capacitor. Vdc and vac represent the PV voltage and the grid voltage respectively. Idc and i1 are the current before and after dc link capacitor Cdc. ip is the current of transformer. i2 and iac are the currents before and after the filter. Lm and L2 are the primary and secondary inductance of the transformer. In addition to necessary components, the output capacitors of the main switches and the leakage inductance of the transformer are considered.

Topology of flyback microinverter with power decoupling circuit.

The effect of output capacitances of Sm, Sx, Sac1 and Sac2 and the primary side leakage inductance of the transformer are considered in this analysis. Five operating modes are included in each switching cycle. The working process of this microinverter will be analyzed and the equivalent circuits are shown in Fig. 5. The working waveforms of the circuit and the driving signal waveforms of each switch are shown in Fig. 6.

Equivalent circuits of the five operation modes. (a) Mode I (b) Mode II. (c) Mode III (d) Mode IV. (e) Mode V.

Operation waveforms of the microinverter with decoupling circuit.

At this stage, Sm is on and the other switches are off as shown in Fig. 5, therefore, the positive input voltage is applied to the primary winding of the transformer and it is in the forward magnetizing mode. The primary current ip rises linearly. until it reaches the set value i1p. Then, Sm turns off, and mode I ends. In this period, all energy is stored in the primary winding of the transformer. The equivalent circuit of mode I is pictured in Fig. 5 (a). The input current is.

Where D1 is the duty cycle of mode I. The circuit works in discontinuous conduction mode (DCM) and peak current i1p and ixp can be calculated.

Where Lm is the magnetizing inductance in the primary side of the transformer, and ω is the angular frequency of the grid.

The voltage of Sx is.

where vx is the voltage of the decoupling capacitor and Vdc is the input voltage.

In this section, the current i1 is.

Where.

The effective value of the current flowing through Sm is.

Where Tg is the period of the grid and m = Tg/Ts.

In this mode, Sm turns off, Sx turns on, and, Sac1, and Sac2 keeps off. when Sm turns off, there will be voltage spike on Sm due to the hard switching. At the same time, the current ip immediately transferrers to the decoupling loop, therefore, the primary current of the transformer does not have spike. The decoupling capacitor is charged through the body diode of Sx. Therefore, Sx achieves zero voltage switching (ZVS) turn-on. The following can also be obtained according to working principle.

Where Ts is the time of one switching cycle.

In this section, the current flowing through Sx could be expressed as.

The decoupling capacitor is discharged after the ip reduced to zero. Then, the transformer is reversely magnetized. The equivalent circuit of mode II is pictured in Fig. 5 (b). Sx turns off when ip increases to the given value ixp, and this mode ends. In this process, the voltage of Sm is always clamped to Vdc+vx.

Sx turns off, and the junction capacitor of Sx is charged. There will be a large spike on the current ip and the drain-source voltage of Sx due to the leakage inductance junction capacitance of the switch. The equivalent circuit of mode III is shown in Fig. 5 (c). In this mode, the following relationship exists.

Where N is ratio of the transformer. Then, the following equation is derived.

Where D3 is the duty cycle of mode III. Substituting (2) and (3) into (11) gives a result as follow.

According to the polarity of the grid voltage, the switches Sac1 and Sac2 work in turn for half cycle. At this stage, the diode Dac1 or Dac2 is turned on, and the corresponding switch Sac1 or Sac2 is on. The primary side energy of the transformer is transmitted to the secondary side. This process ends when the current i2 linearly decreases to zero. The equivalent circuit of mode IV is shown in Fig. 5(d).

In this section, the duty cycle D4 of mode IV is constant. Then the time of this period is.

Then, the secondary side current of the transformer is.

The effective value of the current flowing through Sac1 will be.

The circuit operates in current interrupt mode (DCM) and uses sequential magnetization modulation. The instantaneous pulsating power is processed by the decoupling circuit, so that smaller capacitor can be used instead of the original electrolytic capacitor, which will improve the life of the inverter.

All the switches turn off in this mode. Due to the leakage inductance energy of the transformer, the primary current ip and the voltage Vds-Sx will resonate greatly, as shown in Fig. 5(e).

The primary current of the transformer rises to i1p in the first stage of each switching cycle. The current changes from positive i1p to negative sinusoidal ixp, as shown in Fig. 7. The hysteresis curve of its transformer is shown in Fig. 8. Due to the increase of the decoupling circuit, the working process of the inverter changes. The primary side of the transformer is magnetized forward and then magnetized reverse.

Primary current waveforms of the transformer.

B/H curve of transformer.

The PV panels provide constant power in microinverter systems.

Where Pin is the input power of the PV panels. And the voltage and the current of the grid is as follow. Assume the power factor is one.

Then the power translate to the grid can be calculated.

The average value of Pac equal to the power provided by the PV panels. Thus, the frequency of the pulsating power is twice of the grid, as shown in Fig. 9. The pulsating power is.

Input/output power and power ripple waveforms.

This part of the pulsating power will transfer to the decoupling capacitor. The voltage across the decoupling capacitor can be approximated as the sum of a dc component Vx and a fluctuation voltage \(\Delta\)V. The energy stored in the capacitor during the 1/8 cycle is

Where Tgrid is the period of the grid voltage. This part of energy can also be expressed as.

Because of \({E_d}(t)={E_c}\), the value of the capacitor can be expressed as:

It can be seen from the above equation that when ω and input power are constant, the decoupling capacitor value is only related to Vx and ΔV, and the size of Cx can be reduced by increasing Vx and ΔV. Thus, a thin film capacitor can be used instead of an electrolytic capacitor. In this application, Cx is selected as 40uF/250V.Therefore, the capacitance value can be effectively reduced. The size comparison between electrolytic capacitors E36D101LPN262UAA5U (2.6mF/100V) and film capacitors 406PHC250K (40uF/250V) is shown as Table 1. Therefore, the power density of the converter could be reduced.

This circuit works in DCM mode, so the switch Sm achieves Zero Current Switching (ZCS) turn-on, and its loss mainly includes on-state loss and turn-off loss. The on-state loss depends on the effective value of the current flowing through Sm and the on-state resistance of the switch, which can be expressed as follows:

Where RSm.on is the on-state resistance of Sm.

During turn off, the voltage of Sm is:

The turn off loss of Sm can be expressed as.

Where tf.Sm is the turn off time of Sm.

According to the theoretical analysis, the voltage of Dm is zero at the first and second stage. Therefore, at the end of the first stage, the current of Dm decreases to zero, and the voltage of Dm is always zero. Therefore, there is no switching loss of Dm. The loss of Dm is mainly on state loss, which can be expressed as:

Where VF.1 is the forward conduction voltage drop of Dm.

When the switch is off, the voltage of Sx is the sum of the voltage on the decoupling capacitor and the primary side voltage of the Transformer as follow.

The turn off loss of Sx can be derived as.

Where tf.Sx is the turn off time of Sx.

Sac1 and Sac2 achieved ZVS turn on. In addition, after the secondary energy is transferred, the secondary side current naturally drops to zero, and there is no reverse recovery current. So Sac1, Sac2, Dac1 and Dac2 are ZCS off. Since the secondary current is very small, the switching losses of Dac1 and Dac2 can be ignored. Therefore, the losses of the secondary switches and diodes are mainly on state losses as follow.

Where RSac1.on is the on-state resistance of Sac1 and VF.2 is the forward conduction voltage drop of Dac1.

The simulation of the flyback microinverter with decoupling circuit is carried out in PISM to verify the feasibility of analyze. In the PISM simulation model, all of the parasitic components in the flyback microinverter with power decoupling circuit are considered. The power stage parameters of the system are shown in Table 2.

In order to study the influence of parasitic parameters, the influence of parasitic parameters is added in the simulation. The parasitic components considered in the simulation is in Table 3. What’s more, other parameters are the same as ideal simulation.

In order to realize the ZVS turn on of the decoupling switch Sx, dead time is added between the driving signals of Sm and Sx considering the parasitic parameters. The dead time is set to 200ns, and the main simulation waveforms are shown in Fig. 10. The drive signal waveforms of the main switches are shown in Fig. 10(a), and the waveforms of the decoupling circuit is shown in Fig. 10 (b). It can be noted that the pulsation frequency of the decoupling capacitor is 100 Hz. Through charging and discharging of the decoupling capacitor, the power decoupling is achieved. Due to the parasitic parameters, distortion of iac occurs at the zero crossing. In mode I, the transformer is positively magnetized, and the output capacitor of Sac1or Sac2 is charged through the diode Dac1or Dac2. Therefore, the spike current occurs as shown in Fig. 10(c). At the same time, the current value of iac at zero crossing point is very tiny. Therefore, current waveform of iac is distorted.

Simulation results with parasitic parameters. (a) Driving signals of Sm, Sx and Sac1. (b) Waveforms of the decoupling circuit. (c) The impact of the parasitic parameters. (d) Waveforms of the current i1, i1p and ix.

The waveforms of the input current i1, primary current of the transformer i1p and the current of the decoupling circuit ix are shown in Fig. 10 (d). The current spike will occur when the decoupling switch Sx turne off, due to the parasitic capacitor of the switch. Additionally, when all switches are turned off, the current resonates, which is consistent with the theoretical analysis.

A 250 W prototype was built to verify the performance of the analyze of the flyback microinverter with decoupling circuit. The nominal input dc voltage is designed at 40 V and the output ac voltage is 220 V for the PV connection. The transformer turns ratio k is 1:4:4. The designed parameters are the same as the simulation, which is listed in the Table 1.

The voltage of the decoupling capacitor vx, the output voltage vac, input current iin, and the output current iac is shown in Fig. 11(a). It can be seen that the voltage of decoupling capacitor vx fluctuates at twice the power frequency. The power decoupling of the input and output energy is achieved by using the decoupling circuit and the efficiency of the prototype is 73%. The drive signals of Sm and Sac1 and secondary current of the transformer iac1 during the zero-crossing point is shown in Fig. 11 (b). It can be clearly seen that the current spike in the mode I is much larger than the actual output current, which cause zero-point distortion of iac.

Experimental waveforms of the prototype. (a) Waveforms of vx, vac, iac and iin. (b) Waveforms of the Vgs_Sm, Vgs_Sac1 and iac1. (c) The Wavefomrs of the Vds_Sm and ip.

The waveforms of Vds_Sm and the primary current of the transformer ip is shown in Fig. 11 (c). It can be seen that due to the existence of parasitic parameters, the circuit has a turn-off spike and resonance. The period of ip reduceing from positive to negative is the decoupling loop operation time. Thus, the decoupling loop current ix is same as ip in this period. Further, the waveforms of the traditional flyback type microinverter is shown in Fig. 12, it could be seen that the DC bus and the DC current still have secondary pulsation, in which three 2200uF/63V parallel capacitors are used as input capacitors. Through power decoupling technology, the secondary pulsation on the DC side is transferred to the power decoupling circuit, which can effectively reduce the pulsation on the DC side, as shown in Fig. 11(a).

The operational principle of the flyback microinverter with decoupling circuit considering parasitic parameters is proposed in this paper. Besides the flybak-type components, the output capacitance of the main switches is considered. It explains the voltage spike and oscillation reasons during the switches are all off. The influence on the transformer and the design of the decoupling capacitor are proposed. A 250 W prototype was built to verify the performance of the proposed microinverter. The correctness of the theoretical analysis is verified by the simulation and experimental results, which is useful in the application of the microinverter. The analysis method proposed in this paper can effectively help designers select power switches, passive components of the system reasonably, which will be crucial for the rational design of engineering applications.

Waveforms of vin, vac and iin for the traditional flyback type microinverter.

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This work was supported by National Natural Science Foundation of China (52237008, 52107176).

School of Automation, Beijing Information Science and Technology University, Beijing, China

Yajing Zhang

School of Instrument Science and Opto-electronics Engineering, Beijing Information Science and Technology University, Beijing, China

Binqiang Si

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Y.J. Zhang, conceptualization, methodology, writing-review and editing; Y.J. Zhang, formal analysis, investigation, and validation; B.Q. Si resources and supervision;

Correspondence to Binqiang Si.

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Zhang, Y., Si, B. Design and analysis of power decoupling based microinverter considering parasitic parameters. Sci Rep 14, 25751 (2024). https://doi.org/10.1038/s41598-024-77291-z

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Received: 10 July 2024

Accepted: 21 October 2024

Published: 28 October 2024

DOI: https://doi.org/10.1038/s41598-024-77291-z

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