# High-Efficiency Isolated Photovoltaic Microinverter Using Wide-Band Gap Switches for Standalone and Grid-Tied Applications

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## Abstract

**:**

## 1. Introduction

## 2. System Design and Considerations

#### 2.1. Active-Clamp Forward-Flyback Resonant Converter

_{in}is assumed to be constant for the following analysis. Eight states are described for time t over a full cycle.

_{0}− t

_{1}): During this time interval, the auxiliary switch S

_{Hi}is off. The main switch S

_{Lo}is turned on with zero-voltage switching (ZVS) at t

_{0}because of the conduction of its body diode. At the secondary side, the diode D

_{1}is on and D

_{2}is off. Load power is supplied alone by the output capacitor C

_{out}. The leakage inductance L

_{r}resonates with capacitor C

_{p}. The resonant current i

_{Lr}can be represented as follows:

_{m}is the magnetizing inductance and i

_{Lm}is its current, i

_{p}is the primary transformer current, and V

_{in}is the input voltage. Resonant angular frequency ω

_{0}is defined as

_{0}is defined as

_{1}− t

_{2}): The transformer primary current reaches zero at t

_{1}, and the diode D

_{1}is off with zero-current switching (ZCS). During this time interval, D

_{2}remains turned off and the load power is still supplied only by output capacitor C

_{out}. The resonant current i

_{Lr}can be represented as follows:

_{2}− t

_{3}): The main switch S

_{Lo}is turned off at t

_{2}, and the equivalent inductor (L

_{m}+ L

_{r}) resonates with S

_{Lo}D-S capacitor C

_{DSLo}. The resonant current i

_{Lr}and resonant voltage v

_{DSLo}can be represented as follows:

_{2}is defined as

_{2}is defined as

_{3}− t

_{4}): The resonant voltage v

_{DSLo}reaches (V

_{in}+ v

_{C}) at t

_{4}, and the body diode of the auxiliary switch S

_{Hi}and output diode D

_{2}are turned on. The resonant current i

_{Lr}, the transformer primary current i

_{p}and diode D

_{2}current i

_{D}

_{2}can be represented as follows:

_{3}is defined as

_{eq}is defined as

_{c}is the clamping capacitor.

_{4}− t

_{5}): During this time interval, the main switch S

_{Lo}remains turned off. The auxiliary switch S

_{Hi}is turned on with ZCS at t

_{4}. The circuit equations are the same with those during State 4.

_{5}− t

_{6}): The transformer primary current reaches zero at t

_{5}. The output diode D

_{2}is turned off with ZCS and the load power is supplied only by the output capacitor C

_{out}. The resonant current i

_{Lr}is as follows:

_{4}is defined as

_{5}is defined as

_{6}− t

_{7}): The auxiliary switch S

_{Hi}is turned off at t

_{6}. During this time interval, the energy stored in S

_{Lo}D-S capacitor C

_{DSLo}is released by the resonance with the equivalent inductor (L

_{m}+ L

_{r}). The resonant current i

_{Lr}and voltage v

_{DSLo}can be represented as follows:

_{6}is defined as

_{6}is defined as

_{7}− t

_{8}): The resonant voltage v

_{DSLo}is reduced to zero at t

_{7}and the body diode of the main switch S

_{Lo}conducts to achieve ZVS. The resonant current i

_{Lr}is as follows:

_{1}is

_{2}is

_{on}

_{,1}is the turn on time of S

_{Lo}and f

_{r}

_{,1}is its associated resonant frequency, and T

_{on}

_{,2}is the turn on time of S

_{Hi}and f

_{r}

_{,2}is its associated resonant frequency.

#### 2.2. System Control

## 3. Results

#### 3.1. Power Switch Comparison

_{g}, the switching speed of the GaN HEMT device is much faster. Figure 11 shows the measured switching waveforms of Infineon CoolMOS 20N60CFD and Transphorm GaN HEMT TPH3006PS with Silicon Labs gate driver Si8261 and a 0-Ω gate resistor. In this work, 20N60CFD and TPH3006PS were tested and compared as high-frequency switches of the inverter stage. Figure 12 shows the gate driver circuit for the GaN HEMT power switches in the experiments. Due to the consideration of a lower V

_{gs}threshold voltage to turn the switch fully on, negative-voltage driving is provided during switch-off time to increase noise immunity. As GaN switches operate with low rise and fall times, parasitic inductances and capacitances in the printed circuit board (PCB) layout must be minimized to avoid excessive ringing in the circuit [20].

#### 3.2. Experimental Verification

_{r,}

_{1}and f

_{r,}

_{2}are 66 kHz and 95 kHz, respectively. Figure 15 show the measured resonant current and soft-switching waveforms, which are consistent with the theoretical waveforms shown in Figure 5b. Figure 16 shows the voltage and current waveforms of the power switches, which show that the converter achieves ZVS and ZCS. The efficiency performance of the DC-DC stage, which reaches an efficiency of 97%, is shown in Table 3.

_{o}of 240 V

_{rms}. In the grid-tied mode, experimental results showed that the maximum power point was tracked with an accuracy of approximately 99%. The power efficiency of the microinverter system reaches 95% and maintains high efficiency over the full power range.

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 2.**Block diagram of the proposed microinverter system in standalone and on grid mode. MPPT, maximum power point tracking.

**Figure 5.**(

**a**) Circuit schematic and (

**b**) theoretical waveforms of the active-clamp forward-flyback resonant converter.

**Figure 9.**Control block diagram for the DC-DC stage and inverter in standalone mode. ADC, analog to digital converter; OVP, over voltage protection; UVP, under voltage protection; 2P2Z, 2 pole 2 zero; EN, interleaved enable signal; PWM, pulse-width modulation; NCO, numerically controlled oscillator; PR, proportional-resonant.

**Figure 14.**Waveforms of the gating signals for the DC-DC stage in (

**a**) single and (

**b**) interleaved modes.

**Figure 16.**Waveforms of the switches showing (

**a**) zero-voltage switching (ZVS) and (

**b**) zero-current switching (ZCS) achieved in the circuit.

Device | On-Resistance (Ω) | Gate-Charge (nC) | FOM |
---|---|---|---|

GaN HEMT TPH3006PS | 0.15 | 6.2 | 0.93 |

CoolMOS 20N60CFD | 0.19 | 95 | 18.05 |

Parameter | Symbol | Specification |
---|---|---|

Input voltage | V_{in} | 18–40 V |

Output voltage | V_{o} | 240 V_{rms} |

Output current | I_{o} | 2.1 A_{rms} (rated) |

Efficiency | ŋ | ≥0.95 (at 375 W) |

Voltage THD | THD_{v} | ≤5% (at nominal output) |

Mode | P_{o} (W) | Efficiency (%) |
---|---|---|

single | 100 | 97.10 |

interleaved | 250 | 97.25 |

interleaved | 375 | 97.10 |

interleaved | 500 | 96.50 |

V_{bus} (V) | V_{o} (V_{rms}) | P_{o} (W) | Efficiency (%) |
---|---|---|---|

380 | 240 | 100 | 96.65 |

380 | 240 | 250 | 97.80 |

380 | 240 | 375 | 98.00 |

380 | 240 | 500 | 98.15 |

V_{pv} (V) | V_{o} (V_{rms}) | P_{o} (W) | Efficiency (%) |
---|---|---|---|

34 | 240 | 100 | 94.15 |

34 | 240 | 250 | 95.25 |

34 | 240 | 375 | 95.17 |

34 | 240 | 500 | 94.70 |

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**MDPI and ACS Style**

Liu, Y.-C.; Chen, M.-C.; Yang, C.-Y.; Kim, K.A.; Chiu, H.-J.
High-Efficiency Isolated Photovoltaic Microinverter Using Wide-Band Gap Switches for Standalone and Grid-Tied Applications. *Energies* **2018**, *11*, 569.
https://doi.org/10.3390/en11030569

**AMA Style**

Liu Y-C, Chen M-C, Yang C-Y, Kim KA, Chiu H-J.
High-Efficiency Isolated Photovoltaic Microinverter Using Wide-Band Gap Switches for Standalone and Grid-Tied Applications. *Energies*. 2018; 11(3):569.
https://doi.org/10.3390/en11030569

**Chicago/Turabian Style**

Liu, Yu-Chen, Ming-Cheng Chen, Chun-Yu Yang, Katherine A. Kim, and Huang-Jen Chiu.
2018. "High-Efficiency Isolated Photovoltaic Microinverter Using Wide-Band Gap Switches for Standalone and Grid-Tied Applications" *Energies* 11, no. 3: 569.
https://doi.org/10.3390/en11030569