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A Fault-Tolerant Strategy for Three-Level Flying-Capacitor DC/DC Converter in Spacecraft Power System^{ †}

^{1}

^{2}

^{3}

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^{†}

## Abstract

**:**

## 1. Introduction

## 2. Control Scheme of Fault-Tolerant Strategy for Three-Level Flying Capacitor DC/DC Converter

_{2}or S

_{3}occurs, the voltage reference of the flying capacitor changes from half of the output voltage to zero. The normal inner power devices are opened immediately, and the driver signals of the normal inner switches are set as low. If a short-circuit fault of outer switch S

_{1}or S

_{4}occurs, the reference of the flying capacitor voltage changes from half of the output voltage to the output voltage, and the driver signals of the normal outer switches are set as high. When the voltage of the flying capacitor reaches the reference value, the converter changes its mode to the two-level mode. If the inner switch is at fault, the driver of the other inner switch is set as high in the two-level mode. The driver signals of the two outer switches operate in the complementary state. If the outer switch is at fault, the driver of the other outer switch is set as high. The driver signals of the two inner switches operate in the complementary state.

## 3. Analysis of Operation Modes for the Converter under Short-Circuit Fault Conditions

_{L}< 0.5 V

_{H}) and the situation in which the ratio is larger than 0.5 (V

_{L}> 0.5 V

_{H}).

_{3}when the ratio of the input and output voltage is smaller than 0.5 (V

_{L}< 0.5 V

_{H}). When the short-circuit fault of switch S

_{3}is detected by the detection circuit, the converter is regulated from the three-level mode to the flying capacitor voltage control mode. The reference of the flying capacitor voltage decreases from half of the output voltage V

_{H}to zero. The voltage of the flying capacitor is changed by the flying capacitor voltage loop from half of the output voltage to zero during this stage. After the voltage of the flying capacitor is detected to have reached zero, the converter finishes the flying capacitor voltage control mode and begins to operate in two-level mode.

_{1}, g

_{2}, g

_{3}, and g

_{4}are gate drivers of switches S

_{1}, S

_{2}, S

_{3}, and S

_{4}, respectively. When V

_{L}< 0.5 V

_{H}, the duty cycle of inner switches g

_{3}and g

_{4}is larger than 0.5. The three-level mode is the pre-fault mode. The stages of the three-level mode are shown in Figure 5. There are four stages in the three-level mode. In the three-level mode, the driver signal of g

_{1}and g

_{4}are complementary, and the driver signal of g

_{2}and g

_{3}are complementary.

_{3}is short-circuited and S

_{2}turns on, then the flying capacitor is short-circuited. There is a large current spike occurring in S

_{2}, so the fault is detected. After the fault is detected, the converter operates in Stage VI and VIII; in these two stages, g

_{4}is in the PWM state, and the values of g

_{1}, g

_{2}, and g

_{3}are 0 when the short-circuit fault occurs in S

_{3}. Similarly, g

_{1}is in the PWM state, and the values of g

_{2}, g

_{3}, and g

_{4}are 0 when the short-circuit fault occurs in switch S

_{2}. The two-level mode is the post-fault mode. In the two-level mode, the driver signal of g

_{1}and g

_{4}are complementary, and g

_{2}is set as 1 when the short-circuit fault occurs in switch S

_{3}. Similarly, g

_{3}is set as 1 when S

_{2}is where the fault occurs.

_{4}when V

_{L}is lower than 0.5 V

_{H}. Before the fault occurs, the converter operates in three-level mode. When the short-circuit fault of the switch S

_{4}is detected, the converter begins to operate in flying capacitor voltage control mode. The flying capacitor voltage is regulated gradually from half of the output voltage to output voltage. When the flying capacitor voltage reaches the output voltage, the converter turns to operate in two-level mode.

_{3}occurs. The stages of the flying capacitor voltage control mode after a short-circuit fault are shown in Figure 9. There are three stages after the short-circuit fault occurs in the outer switches. In Stage V of the flying capacitor voltage mode, g

_{4}is short-circuited and g

_{1}turns on; then the output capacitor is connected directly with the flying capacitor. Due to the existence of the voltage difference between the output voltage and the flying capacitor voltage, there is a current spike occurring in S

_{1}, and the fault can be detected. After Stage V, the converter operates in Stages VI and VII. In these two stages, g

_{3}is in the PWM state, and g

_{1}, g

_{2}, and g

_{4}are 0 when the short-circuit fault occurs in S

_{4}. Similarly, g

_{2}is in the PWM state, and g

_{1}, g

_{3}, and g

_{4}are 0 when the short-circuit fault occurs in S

_{1}. In the two-level mode, the driver signal of g

_{2}and g

_{3}are complementary, and g

_{1}is set as 1 when the fault occurs in S

_{4}. Similarly, g

_{4}is set as 1 when the fault occurs in S

_{1}.

_{L}is higher than 0.5 V

_{H}is analyzed in detail. The stage analysis is shown in Figure 10 for when S

_{3}is short-circuited in this situation. This situation is different from the situation when V

_{L}is lower than 0.5 V

_{H}. The duty cycle of g

_{3}and g

_{4}is less than 0.5.

_{3}is detected, the converter is regulated by the flying capacitor voltage control loop. Then S

_{4}is opened. Because V

_{L}> 0.5 V

_{H}and the initial value of v

_{fly}is half of v

_{H}, inequality (1) can be derived at the beginning of the fault. The voltage difference between the inductor is always positive. It should be noticed that if inequality (2) holds, the current of the inductor will continue to increase during Stage VI. When v

_{fly}is lower than (v

_{H}− v

_{L}), the converter operates in Stages VII and VIII. When the flying capacitor voltage reaches 0, the converter turns into the two-level mode, and the S

_{2}is closed.

_{3}is short-circuited is shown in Figure 11.

_{fly}is the flying capacitor voltage, V

_{H}is the output voltage, V

_{L}is the input voltage, L is the inductance of the input inductor, C is the capacitance of the flying capacitor, i

_{L}is the current value of the inductor, and I

_{L}is the initial value of the inductor current. If the output voltage is 100 V, the input voltage is 80 V, the inductor is 1 mH, and the initial current of the inductor is 10 A, then it can be calculated that the maximum inductor current is 17.2 A. The maximum current under different levels of inductance when S

_{3}is short-circuited is shown in Figure 12.

_{4}is short-circuited is shown in Figure 13. When the short-circuit fault of S

_{4}is detected, S

_{3}is turned off. Because v

_{L}is larger than 0.5 V

_{H}and the initial value of V

_{fly}is half of V

_{H}, inequality (4) can be derived at the beginning of the fault. The voltage difference between the inductor is always positive. It should be noticed that if inequality (5) holds, the current of the inductor will continue to increase during Stage VI. When v

_{fly}is higher than 0.5 V

_{H}, the converter operates in Stages VII and VIII.

_{H}, S

_{1}is turned off. The proper inductance should also be designed carefully. The flying capacitor voltage and inductor current can be calculated by (4). The current spike analysis is given in Figure 14. If we take the same parameters as the situation when S3 is faulty, the maximum inductor current is 17.2 A. The maximum current under different levels of inductance when S

_{4}is short-circuited is shown in Figure 15.

_{o}is 100 V, the input voltage U

_{IN}is 30~80 V, the switching frequency f

_{s}is 10,000 Hz, the duty cycle D is 0.2~0.7, the input current I

_{IN}is 33 A, the output current Io is 10 A, the ripple ratio of the input current δ

_{I}is 0.02, the ripple ratio of the flying capacitor voltage δ

_{f}is 0.1, and the ripple ratio of the output capacitor voltage δ

_{o}is 0.01. Then the inductor is 0.9 mH, the flying capacitor is 200 μF, and the output capacitor is 1 mF. With the consideration of the current spike analysis after a short-circuit fault in the simulation, the inductance of the inductor is chosen as 1 mH, and the capacitances of the flying capacitor and the output capacitor are chosen as 200 μF and 1 mF, respectively.

## 4. Simulation and Experiment Verifications of the Fault-Tolerant Strategy

_{L}<0.5 V

_{H}) and 80 V (V

_{L}> 0.5 V

_{H}). The inductance of the inductor is 1 mH. The capacitance of the flying capacitor is 200 uF, and the capacitance of the output capacitor is 1 mF. The parameters of the simulation are shown in Table 1.

_{4}under two load conditions are shown in Figure 16. It can be seen that the output voltage of the converter is uninterruptible when a short-circuit fault of S

_{4}occurs. The mode transfer after the fault is seamless.

_{3}under different load conditions are shown in Figure 17a,b. The two load conditions are 100 W for a light load and 1000 W for a heavy load. It can be seen that the output voltage of the converter is uninterruptible after the S

_{3}fault occurs. The spike of the current and voltage is in the normal range during the mode transition after the fault.

_{3}under two different load conditions including a light load and a heavy load are shown in Figure 18a,b, and the waveforms when a short-circuit fault occurs in S

_{4}under two different load conditions are shown in Figure 19a,b. It can also be seen that the output voltage is uninterruptible when two types of faults occur. The spike and surge of the current and voltage are also in the normal range.

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 3.**The diagram of the key quantity under the situation in which short-circuit fault occurs in switch S

_{3}.

**Figure 7.**The diagram of the key quantity under the situation in which short-circuit fault occurs in switch S

_{4}.

**Figure 10.**Short-circuit fault analysis (V

_{L}> 0.5 V

_{H}). (

**a**) The stages when S

_{3}is short-circuited; (

**b**) stage analysis.

**Figure 11.**The analysis of the stages under S

_{3}fault. (

**a**) Equivalent circuit; (

**b**) the simulation curve (V

_{L}80 V, V

_{H}100 V, initial value of I

_{L}-10 A).

**Figure 13.**Short-circuit fault analysis (V

_{L}> 0.5 V

_{H}). (

**a**) The stages when S

_{4}is short-circuited; (

**b**) stage analysis.

**Figure 14.**The analysis of the stages under S

_{4}fault. (

**a**) Equivalent circuit; (

**b**) the simulation curve (V

_{L}80 V, V

_{H}100 V, initial value of I

_{L}10 A).

**Figure 16.**The waveforms of S

_{4}short-circuit fault (V

_{L}< 0.5 V

_{H}). (

**a**) Light load 100 W; (

**b**) heavy load 1000 W.

**Figure 17.**The waveforms of S

_{3}short-circuit fault (V

_{L}< 0.5 V

_{H}). (

**a**) Light load 100 W; (

**b**) heavy load 1000 W.

**Figure 18.**The waveforms of S

_{3}short-circuit fault (V

_{L}> 0.5 V

_{H}). (

**a**) Light load 100 W; (

**b**) heavy load 1000 W.

**Figure 19.**The waveforms of S

_{4}short-circuit fault (V

_{L}> 0.5 V

_{H}). (

**a**) Light load 100 W; (

**b**) heavy load 1000 W.

**Figure 20.**The experimental waveforms of S

_{3}short-circuit fault. (

**a**) Overall stages; (

**b**) the waveform of three-level mode before S

_{3}fault; (

**c**) the waveform of S

_{3}short-circuit fault during fault; (

**d**) the waveform of two-level mode after S

_{3}fault.

**Figure 21.**The experimental waveforms of S

_{4}short-circuit fault. (

**a**) Overall stages; (

**b**) the waveform of three-level mode before S

_{4}fault; (

**c**) the waveform of S

_{4}short-circuit fault during fault; (

**d**) the waveform of two-level mode after S

_{4}fault.

Item | Case I (Figure 16) Value | Case II (Figure 17) Value | Case III (Figure 18) Value | Case IV (Figure 19) Value |
---|---|---|---|---|

Input voltage | 30 V | 30 V | 80 V | 80 V |

Output DC bus voltage | 100 V | 100 V | 100 V | 100 V |

Input inductor | 1 mH | 1 mH | 1 mH | 1 mH |

Flying capacitor | 200 μF | 200 μF | 200 μF | 200 μF |

Output bus capacitor | 1 mF | 1 mF | 1 mF | 1 mF |

Load power | 100 W/1000 W | 100 W/1000 W | 100 W/1000 W | 100 W/1000 W |

Fault location | Inner switch S_{3} | Outer switch S_{4} | Inner switch S_{3} | Outer switch S_{4} |

Item | Value |
---|---|

Input voltage | 10 V |

Output DC bus voltage | 32 V |

Input inductor | 100 μH |

Flying capacitor | 20 μF |

Output bus capacitor | 470 μF |

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## Share and Cite

**MDPI and ACS Style**

Li, H.; Gu, Y.; Zhang, X.; Liu, Z.; Zhang, L.; Zeng, Y.
A Fault-Tolerant Strategy for Three-Level Flying-Capacitor DC/DC Converter in Spacecraft Power System. *Energies* **2023**, *16*, 556.
https://doi.org/10.3390/en16010556

**AMA Style**

Li H, Gu Y, Zhang X, Liu Z, Zhang L, Zeng Y.
A Fault-Tolerant Strategy for Three-Level Flying-Capacitor DC/DC Converter in Spacecraft Power System. *Energies*. 2023; 16(1):556.
https://doi.org/10.3390/en16010556

**Chicago/Turabian Style**

Li, Haijin, Yu Gu, Xiaofeng Zhang, Zhigang Liu, Longlong Zhang, and Yi Zeng.
2023. "A Fault-Tolerant Strategy for Three-Level Flying-Capacitor DC/DC Converter in Spacecraft Power System" *Energies* 16, no. 1: 556.
https://doi.org/10.3390/en16010556