Fault-tolerance wide voltage conversion gain DC/DC converter for more electric aircraft

Binxin ZHU, Jiaxin LIU, Yu LIU, Kaihong WANG

college of Electrical Engineering and New Energy (Hubei Provincial Research Center on Microgrid Engineering Technology),China Three Gorges University, Yichang 443002, China

KEYWORDS Cuk converter;Fault tolerance;High reliability;More Electric Aircraft(MEA);Wide voltage conversion gain

Abstract In this paper,a fault-tolerance wide voltage conversion gain DC/DC converter for More Electric Aircraft (MEA) is proposed.The proposed converter consists of a basic Cuk converter module and n expandable units.By adjusting the operation state of the expandable units, the voltage conversion gain of the proposed converter could be regulated,which makes it available for wide voltage conversion applications.Especially, since mutual redundancy can be realized between the basic Cuk converter module and the expandable units, the converter can continuously work when an unpredictable fault occurs to the fault-tolerant parts of the proposed converter,which reflects the fault tolerance of the converter and significantly improves the reliability of the system.Moreover,the advantages of small input current ripple, automatic current sharing and low voltage stress are also integrated in this converter.The working principle and features of the proposed converter are mainly introduced, and an experimental prototype with 800 W output power has been manufactured to verify the practicability and availability of the proposed converter.

1.Introduction

To reduce greenhouse gas emissions and fuel consumption,much attention has been focused on More Electric Aircraft(MEA).1–3At present,the bus voltage of MEA is divided into 270 V and 28 V, and its power supply system is shown in Fig.1.Normally,270 V DC bus is preferred to reduce the size,weight,and cost of wire and machine.4Since the rated voltage of some DC energy sources, such as fuel cells, batteries and supercapacitors, is at 24–48 V, the output voltage will be accompanied by the variation with working conditions.Therefore,converters with wide voltage gain are required to connect DC energy sources and DC bus.Meanwhile, fault tolerance is also required to ensure the reliability of the power supply system.5–6.

Fig.1 Power supply system in MEA.

Fault tolerance means that when an unintended sudden failure occurs to a component or components, the system can continuously work.Generally, fault tolerance of the converter can be achieved by hardware redundancy, which can be divided into four categories: switch-level, leg-level,module-level, and system-level.7–8Switch-level fault-tolerant method can be simply summarized as adding additional auxiliary switches in the converter to make the topology have the fault-tolerant ability.9–12As mentioned in Ref.,12an additional switch connects the input terminal and output terminal of the buck converter.The converter can form a buck-boost converter through the additional switch to continue to operate when original switch of the converter is failed.However, this topology cannot be extended,which severely limits its applications.For leg-level fault-tolerant design,a redundant branch is usually added to replace the faulted branch to ensure the normal operation of the converter.13–17For example, the interleaved connection can not only reduce the ripple of the input current,but also realize the redundancy of branches.Although the converter has high reliability, the parasitic parameters in the circuit may greatly affect its input current, which results in the imbalance of input current between phases.Consequently, sophisticated control is needed to balance the current of each phase.In addition,system-level redundancy places the entire converter as a redundant backup, which greatly improves system reliability, but incurs extremely high costs.8In order to balance the hardware redundancy and cost of the converter,leg-level design or module-level design is considered as a promising choice.In the module-level,redundant modules are connected in series or in parallel in the converter.18–24The common way to achieve redundancy is adopting the Input-Parallel Output-Series (IPOS) structure.This method can improve the voltage conversion ratio of the converter, and meanwhile the interleaved conduction mode of the switches can also decrease the input current ripple of the converter.20.

However, the input and output voltage of the boost IPOS converter are not in common ground, and the anti-jamming capability is poor.While the duty cycle of the switches is less than 0.5, the input current of each phase is imbalanced.21–24The imbalance of input current problem is solved by fixing the duty cycle of some switches equal to 0.5 in Ref.22.Limited duty cycle and imbalanced input current are solved by adding an auxiliary diode to the converter in Ref.,23but the input and output voltage are still not in common ground.In Ref.,24the above-mentioned problems can be solved by adding capacitors and diodes,but the voltage of capacitors at the output terminal is imbalanced.Therefore, it is indispensable to search for a more valuable topology with redundancy ability and apply it to aerospace systems.

In this paper,a fault-tolerance wide voltage conversion gain DC/DC converter for MEA is proposed,which is composed of a basic Cuk converter module and n expandable units.A structure similar to the IPOS is used in the proposed converter,which can effectively solve the above problems.The input and output voltage of the converter are in the common ground, and the automatic input-current sharing can be realized whenever the duty cycle of the switches is greater than 0.5 or less than 0.5.Each module can be redundant to other modules, which makes the converter with reliability.Moreover,by controlling the working state of each expandable unit,the proposed converter can achieve a wide voltage conversion gain.

The structure of this paper is organized as follows.First,the operation principle and performance of the converter are described.Then,fault redundancy and adjustable voltage conversion gain of the proposed converter are analyzed.In addition, the characteristics and performance of the proposed converter are compared with those of other similar converters.Finally, experimental results and correlations analysis are described.

2.Operation principle and performance

2.1.Operation principle

As shown in Fig.2, the topology of the proposed converter consists of a basic Cuk converter module and multiple expandable units.Each expandable unit includes two capacitors, two inductors, a switch, and a diode.The basic Cuk converter module with one expandable unit is taken as an example to simplify analysis process,which is shown in Fig.3.The following hypotheses are made in the analysis: (A) All components are ideal devices,taking no account of the parasitic parameters of components in the circuit; (B) Owing to the fact that the capacitance of all capacitors is large enough, the voltage ripples are much smaller than their DC values; (C) When converter is in steady state, it works in continuous conduction mode (CCM); (D) The driving signals of S1and S11are interleaved with 180° phase shift.

The operation waveforms of the proposed converter in one switching period are shown in Fig.4.When duty cycles of the switches range from 0.5 to 1,there are three working modes of the converter, corresponding to Mode 1 to Mode 3, and their relevant waveforms are shown in Fig.4(a).When duty cycle is less than 0.5,it is also divided into three working modes:Mode 2 to Mode 4, and the relevant waveforms are shown in Fig.4(b).The relevant equivalent circuits of the converter in one switching period are shown in Fig.5.The details can be described as follows.

Fig.2 Proposed converter with n expandable units.

Fig.3 Proposed converter with one expandable unit.

Fig.4 Key waveforms for one switching period (Duty cycle D1 = D11 = D).

Fig.5 Equivalent circuits of working states (1: switch is turned on, 0: switch is turned off).

Mode 1 [t0–t1] & [t2–t3] (corresponding to Fig.5(a)): Both switches S1and S11are conducting.During this mode,voltage source uinsupplies power to inductors L1,L11,while inductors L2,L12,capacitors C2,C12and load R are charged reversely by capacitors C1, C11.The currents of inductors L11, L12reach their peak values at t1.

Mode 2[t1–t2]&[t5–t6](corresponding to Fig.5(b)):Switch S11and diode D1turn on, switch S1and diode D11turn off.During this mode, the current of inductor L1continuously increases linearly.Capacitor C11is charged by voltage source uinand inductor L11.Inductor L12and capacitor C12supply power to load R.The current of inductor L11decreases linearly.

Mode 3[t3–t4]&[t7–t8](corresponding to Fig.5(c)):Switch S1and diode D11turn off, switch S11and diode D1turn on.During this mode, the current of inductor L11continuously increases linearly.Voltage source uinand inductor L1supply power to capacitor C1.Load R is charged by inductor L2and capacitor C2.The current of inductor L1decreases linearly.

Mode 4 [t6–t7] & [t8–t9] (corresponding to Fig.5(d)): Both switches S1and S11are in the off-state.During this mode,capacitors C1and C11are charged by voltage source uinand inductors L1, L11, while inductors L2, L12and capacitors C2,C12supply power to load R.

2.2.Conversion ratio

Suppose that duty cycles of S1and S11are different, and the volt-second balance formulas for inductors L1, L2, L11and L12are obtained as follows:

2.3.Voltage and current stress of components

The voltage stresses of the switches and diodes can be calculated as

When duty cycle D1=D11=...=Dn1=D(n=1,2,...),the voltage stresses of the semiconductor devices can be expressed as

According to Eq.(7), the voltage stress of semiconductor device of the proposed converter is lower than that of the conventional Cuk converter when the input voltage and output voltage are the same.

By neglecting the effect of inductor current ripple on the results, the current stress analysis is simplified.The amperesecond balance formulas for capacitors C1, C2, C11and C12are deduced:

Compared with Eq.(9) and Eq.(10), it can be concluded that when all switches have the same duty cycle, the average current flowing through each phase is equal.Consequently,automatic input-current sharing of the proposed converter can be realized.

3.Fault redundancy and adjustable voltage conversion gain

The basic Cuk converter module and expandable units of the converter can be redundant for each other,which can improve the reliability of the converter.The unit including inductor L1,switch S1and capacitor C1in the basic Cuk converter module,and the units containing inductors Ln1,switches Sn1and capacitors Cn1in expandable cells are fault-tolerant parts of the proposed converter.When any fault-tolerant part of the above unit is failed, the converter can work continuously.Take the basic Cuk converter module with one expandable unit as an example as shown in Fig.6.Since switches are the most vulnerable component in a converter,switch failures of the converter are discussed here.The basic Cuk converter module can work continuously when the switch S11of the expandable unit is not working, and vice versa.And the voltage conversion gain of the converter will change accordingly.The detailed analysis is given as follows.

Fig.6 Equivalent circuit for single switch operation.

Suppose that switch S1is open circuit and switch S11is working normally.The relevant waveforms and equivalent circuits are shown in Fig.7 and Fig.8, respectively.

Interval 1 [t0–t1] (corresponding to Fig.8(a)): Switch S11and diode D1are conducting,voltage source uinsupplies power to inductor L11,inductor L12and load R are charged by capacitors C11and C12,and capacitor C2supplies power to inductor L2.

Interval 2 [t1–t2] (corresponding to Fig.8(b)): The current flowing through capacitor C12is reversed at t1.

Interval 3 [t2–t3] (corresponding to Fig.8(c)): Switch S11is turned off, diode D11is turned on, and voltage source uinand inductor L11supply power to capacitor C11.Inductor L2and capacitors C2and C12supply power to load R.

Fig.7 Key waveforms for case that only switch S11 is in operation.

Fig.8 Equivalent circuit for case that S1 is open circuit and switch S11 is working.

Interval 4 [t3–t4] (corresponding to Fig.8(d)): The current flowing through capacitor C12is reversed at t3.

During one switch period, the voltage of capacitor C1equals the voltage source uinand remains constant,so the current of capacitor C1is 0 and capacitor C1corresponds to an open circuit.Therefore, the current flowing through inductance L1is 0.The voltage of inductor L1is also 0, which is equivalent to a wire.

Therefore, the volt-second balance formulas for inductors L2, L11and L12are obtained:

When the converter is extended to a basic Cuk Converter module with n expandable units, it can continue to operate normally regardless of the failure of the basic Cuk converter module or the expandable unit.When total of m modules and units fail (m ≤n) and the duty cycles of all switches are the same,the voltage gain conversion ratio formula of the converter can be expressed as

4.Performance comparison

Compared with other similar converters, advantages of the proposed converter are shown in Table 1.20,25,26

The interleaved parallel structure can effectively reduce the input current ripple of Cuk converter and realize the mutual redundancy between phases.However, conventional interleaved parallel structure cannot improve the voltage conversion gain of the converter, and the input current of each phase cannot achieve automatic current sharing which means complicated control is needed to realize the current sharing of each input phase.The converter proposed in this paper can solve the above problems perfectly without changing the number of devices.

The topologies in Ref.,20Ref.,25and Ref.26adopt structure IPOS structure, which realizes automatic input-current sharing.However, the input voltage and output voltage of these topologies are not in common ground, which results in poor anti-interference ability of the converter.In addition, none of these topologies can be extended, so that the voltage conversion gain of these converters and their applications are limited.The converter proposed in Ref.20 combines a boost converter with an elementary self-lift Cuk converter which has strong fault-tolerance and can continue to work normally even after any switch is failed.Ref.25 improves the voltage gain of the converter by adding a switch and a diode to the IPOS buckboost converter.However, the switches S1and S3of the converter are floating ground, which makes the driving circuit design complicated.When switch S2is failed,the input current is imbalanced and the reliability of the converter is reduced.The IPOS quasi-Z-source DC-DC converter is proposed in Ref.26, which evidently improves the voltage conversion gain of the converter, but this structure requires more components and the duty cycle of the converter must be less than 0.5,otherwise the converter cannot work properly.

In practice, the voltage conversion gain of the converters proposed in Ref.20, Ref.25 and Ref.26 and conventional interleaved Cuk converter can only be regulated by changing the duty cycle; however, the proposed converter can adjust the output voltage conversion ratio by controlling both the duty cycle and the working state of the expandable units.Thus,a wide voltage conversion gain can be achieved in the proposed converter.

5.Simulation and experimental results

5.1.Simulation results

To verify the practicability of the proposed converter in highpower states,the converter with three expandable units is simulated.The relevant simulation parameters are shown in Table 2.

Voltage waveforms of switches, diodes and capacitors are shown in Figs.9(a)-(d).The average voltages satisfy the calculation results of Eq.(4) and Eq.(7).

Current waveforms of inductors are shown in Figs.9(e)and(f).The average currents satisfy the calculation results of Eq.(11).

Table 1 Comparison with other interleaved parallel converters.

Table 2 Specification of simulation circuit.

Fig.9 Simulation waveforms.

The input and output voltage and current waveforms are represented in Fig.9(g).When input voltage uinis 48 V,output voltage uois 270 V,the input current is 2083 A and the output current is 520 A.

When switch S31is failed or stops working at 0.25 s, the simulation results are shown in Fig.10.The current flowing through L31drops rapidly to 0 A,and the average output voltage uoremains 270 V which effectively verifies the reliability of the proposed converter when working in high-power states.

5.2.Experimental results

To validate the aforementioned theoretical analysis,an 800 W prototype with two expandable units is built.The specific models and parameters of the components used in the prototype are collected in Table 3,and the expandable cell and main circuit of the prototype are displayed in Fig.11.

The voltage stresses of the switches and diodes are shown in Figs.12(a) and(b), respectively.Initial phase difference of the three PWM waves is 120°,and duty cycle of all switches is kept at 0.65.The voltage stress of the switches and diodes is about 138 V, which is consistent with the theoretical analysis.

The voltage waveforms of capacitors C1, C11and C21are shown in Fig.12(c).The voltage values are about 138 V,228 V and 318 V respectively.Fig.12(d)represents the voltage waveforms of capacitors C2, C12and C22, all about 90 V,which correspond to the calculation results of Eq.(4).

Fig.12(e) shows the current waveforms of three input inductors L1,L11and L21,and the value of the current is about 5.56 A.The current waveforms of three output inductors L2,L12and L22are shown in Fig.12(f) and the current value is about 2.96 A.

The input and output voltage and current waveforms are represented in Fig.13.When input voltage uinis 48 V, output voltage uois 270 V, the input current is about 18A, and the output current is about 3 A.

When the working state of the expandable unit changes,the output voltage of the converter connected to the DC bus is required to be stable.Take the sudden change of the working state of switch S21as an example.As can be seen from Fig.14,when the three switches are working normally,the output voltage is about 270 V, the input current is about 18 A, and the current flowing through L21is about 5.6 A.When switch S21stopped working, the current flowing through L21was rapidly reduced to 0 A.After 25 ms,the input current and output voltage were stabilized at 18 A and 270 V.The experimental result shows that a wide voltage conversion gain can be achieved in the proposed converter by controlling the switches of each expandable unit, and the basic Cuk converter and expandable units can be redundant for each other, which proves that the fault tolerance of the converter can make the system highly reliable.

Fig.10 Waveform when switch S31 operating state changes in simulation.

Table 3 Specification of experimental prototype.

Fig.11 Photograph of prototype.

Fig.12 Voltage and current waveforms of prototype.

Fig.15 shows the efficiency curves of the proposed converter with different expandable units.The peak efficiency of the converter reaches 94.21%, when output power is 500 W.When output power of the converter with two expandable units is 800 W, the efficiency is 92.5%.The specific losses include diode loss PD, switch loss PS, inductor loss PL,capacitor loss PCand other losses Pothercaused by circuit parasitic parameters.Fig.16 reveals the loss percentage distributions of the devices of this converter.

Fig.13 Input and output waveforms.

Fig.14 Switch S21 operating state changes.

Fig.17 shows the comparison between the theoretical and actual voltage conversion gain of the proposed converter.Due to the presence of parasitic components, the voltage conversion gain of the actual circuit will not reach its theoretical value.With the increase of duty cycle, the deviation between theoretical value and actual value will increase.When the voltage gain is fixed to 5.25 and the number of expandable units is 1 and 2, the duty cycle is adjustable, ranging from 0.7377 to 0.6521.When the number of expandable units is 1 and 2 and duty cycle ranges from 0.7377 to 0.6521, the voltage gain is in the range of 8.44 to 3.81.Therefore,by controlling the duty cycle and the working state of the expandable units, the proposed converter can achieve wide-range voltage conversion gain.

Fig.15 Power efficiency curves of proposed converter when expandable units are 1 and 2.

Fig.16 Calculated loss distribution.

Fig.17 Comparison of theoretical and actual voltage conversion gain.

6.Conclusions

A fault-tolerance wide voltage conversion gain DC/DC converter for MEA is proposed in this paper.Theoretical analysis and experimental results show that the proposed converter has the following advantages:

(1) The voltage conversion gain of the proposed converter can be adjusted by controlling the working state of each expandable unit, which makes the proposed converter achieve a wide voltage conversion gain.

(2) The basic Cuk converter module and expandable units of the converter can be redundant for each other.When any unpredictable fault occurs to the fault-tolerant parts, the proposed converter can work continuously,which reflects the fault tolerance of the converter and effectively improves the reliability of the system.

(3) Compared with the conventional Cuk Converter, the voltage stress of the semiconductor devices of the proposed converter is lower, and automatic input-current sharing can also be realized.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by the National Natural Science Foundation of China(No.51707103)and the Hubei Provincial Key Laboratory on Operation and Control of Cascaded Hydropower Station, China (No.2022KJX08).