Positive Output Luo DC-DC Converters in Power Management Unit for Small Satellites

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Positive Output Luo DC-DC Converters in Power

Management Unit for Small Satellites

(Full text in English)

Andrei BĂESCU1_{, Andrei COCOR}1_{, Adriana FLORESCU}1_{, Dan STOICHESCU}1

1_{University Politehnica of București, Faculty of Electronics, Telecommunications and Information Technology}

Abstract

This paper analyses the possibility of using Luo converters in the structure of a power management unit for small satellites. The efficiency of elementary and self-lift versions of Luo DC-DC converter types is mathematically and by simulations proved. In the final part, a comparison between the parameters of the two converters is made. An overview of the battery management system is offered, with the role of each component being briefly described. A possible design, development phases and the advantages brought by the management unit are also presented.

Keywords: battery management unit, self-lift Luo converters, small satellites

1. Introduction

Small satellites have become a common presence in space during the last years [1]. A significant increase in the number of recent space experiments can also be observed [2]. As a direct consequence, the increased demands and technical challenges for the power system are to be considered [3]. The miniaturized satellites standards like the “nanosat”, “picosat” and, more recently, “femtosat” push the limits even further [4].

Alternative sources of energy, like solar or wind energy, also named clean energy, offer a new perspective for most of the modern technologies, improving the quality of life [5]. Being renewable, they are used in

various domains ranging from

telecommunications [6], to hybrid propulsion systems [7] or to providing energy for small satellites [8]. An efficient system must be capable of saving energy, by managing its own power consumption, just like managing the energy schedule of a building [9].

The engineers face the problem of restricted space, limited amount of the available energy and limited financial resources [10]. The challenge for the small satellite is to maintain cost-effectiveness by maximizing capabilities and performance while meeting difficult design and financial

restrictions [11]. Being relatively cheap and having a simple structure, the positive output Luo converter fits very well in the satellite’s battery management unit and complies all the requirements from financial and complexity point of view [12].

The next chapter presents a diagram of the power management system. The two proposed Luo converter versions are tested and simulated in chapter three. In the last chapter, the research and development phases of the unit are presented along with the advantages and the comparative results given by the two possible Luo converters’ implementations.

2. Overview of the Power Management System

The proposed system incorporates advanced features that can be usually found in more complex entities (i.e. MPPT and battery capacity monitoring), while the complexity and costs are kept under control [13]. It will provide as well a reasonable amount of flexibility for communication with the other components of the power system [14].

The simplified block diagram of the proposed power management system is presented in Figure 1.

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Figure 1. Block diagram of the power management system (PMS) The system consists of three functional

units: the Battery Management Unit (BMU), the Point Of Load (POL) Unit(s) and The System Interface Unit (SI).

The BMU is responsible with the management of the incoming energy from the available sources (photovoltaic panels PVs, fuel cells FCs, radioactive or thermal generators, etc.) as well as with the management of the available storage devices (batteries, super-capacitors) [15]. This unit implements all the necessary algorithms for batteries charging/fitness estimation process, alternative storage elements management (super-capacitors) and power-shedding [16]. The Maximum Power Point Tracking (MPPT) algorithm is also the responsibility of this unit [17].

The Point Of Load Unit (POL) is the main access node of the distributed power architecture. The main role of the POL is to convert the input DC voltage (from the DC bus) to another voltage required by the external load (can be part of the payload). Each POL unit must be able to communicate with the BMU and with the SI via the Energy Management Bus (EMB).

The System Interface Unit (SI) is the main interface between the energy management unit and the entity/payload. His main role is to maintain the communication between the entity/payload and the other components of the energy management unit. This unit is responsible to find alternative energy routes in case of the failure of one of the POLs.

Each energy-processing unit features a high performance DC-DC converter and a digital supervisory system capable to manage the conversion process and communicate with the external systems. In order to maintain the conversion’s

high-efficiency over a wide range of the output current, the DC-DC converters implement the latest state-of-the-art control techniques.

The electronic components of the converter must be carefully dimensioned in order to fulfil the volume and the mass requirements for spacecraft entities [18]. The topology is also of vital importance, as the system must avoid the catastrophic failures that can affect the payload [19].

Energy received from the solar cells is used to power the system during sun-on periods, and to recharge the battery pack for sun-off periods. During solar eclipse, the battery is used as the primary power source. The main power line (DC bus) feeds a number of DC-DC power converters (POLs), which provide the necessary supply voltages for the satellite’s electronics [20]. A battery monitor is also integrated into this system to measure sustainable power consumption and facilitate the power management.

The BMU provides all the necessary information regarding the available incoming energy and the battery pack status in real-time [21]. The main decision regarding the power-shedding comes from the BMU and is send to the POL units through EMB.

The battery-charging unit is part of the BMU and can handle any available chemistry (Ni-Cd, Ni-MH, Li-ion, etc.). As the performance of the battery pack decreases over time, a monitoring system capable to estimate the fitness/capacity of the batteries is considered highly valuable [22]. Prognostics and health management technologies are used to assess the reliability and performance of an autonomous system and, under its actual lifecycle conditions, to determine the advent of failure and mitigate

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system risk. The system provides an interface with the external components of the entity.

3. Battery Management Unit block analysis

According to recent technology used by the European Space Agency (ESA), a typical value for the voltage that feeds a small satellite (also called line voltage) is 50 V. The latest spacecraft battery models are available in 8 V and 32 V versions as standard, but also models with custom voltages in this range are created on demand [23].

In this case, an essential aspect of the previous proposed schema is the DC-DC converter used in the BMU. Choosing the converter type for such a tricky application as charging the battery of an isolated system, often for an undetermined period, depends a lot on the simplicity, cost and efficiency of the selected unit.

A. Mathematical Analysis and Simulation of Positive Output Luo converter

Positive output Luo converter fits this description and its electronic schema can be seen in Figure 2.

Figure 2. Electronic diagram of the positive output Luo converter designed in OrCAD Capture The capacitor C main role is to store and

transfer energy from the power source input to the output through L1. If the capacity of C

is enough large, the variation of voltage across C, at steady state, will be ignored.

During the conduction period of the static contactor, source current is:

2 1 L L

I i i

i = +

The inductance of coil L1 receives energy

from the source. The currents iL1 and iL2 are

increasing, while the coil inductance L2

absorbs energy from both the source and capacitor C.

During the lock period of the static contactor, the source current is equal to zero. Capacitor C is loaded by current iL1,

which flows through the diode D.

The energy that was stored in L1 is now

transferred to the capacitor C. In this period,

iL2 remains continuous while it crosses the

circuit through R, C0 and D. Both currents iL1

and iL2 are decreasing. Because the ripples of iL1 and iL2 are small,

1

1 L

L I

i ≈ _and

the static contactor is locked and the charging of C increases:

(

1 D

)

TIL1

Q+ = − (3.1)

This charging decreases during the conduction period of the static contactor:

2 L DTI Q− = _(3.2) Because Q+ = Q—: (3.3)

Because C0 has the role of a low pass

filter,

(3.4)

during the start-up, iL is equal to zero during the lock period.

The average current through the source is: 1 1 2 1 ) 1 1 ( ) ( _L _L _L _L L I I I D D D i i D Di I = = + = + − = (3.5) 2 2 L L

I

i

≈

1 2 1 L L I D D I = − 0 2 I I_L = 2 1 L L L

i

=

+

(4)

The output current is:

(3.6)

The output voltage is:

(3.7)

Conversion ratio of the converter in continuous conduction mode is:

(3.8)

The current IL1 increases and is sustained

by VI during start-up time. In the lock period,

it decreases and is inversely polarized by – VC.

(3.9) (3.10)

The current’s ripple through L1 is:

(3.11) Because 1 2 1 ) 1 1 ( ) ( _L _L _L L I I D D D i i D Di I = = + = + − ,

the relative ripple of current iL1 is:

(3.12) 2 2

L

DTV

i

L

=

I

∆

_(3.13) Because , it results: (3.14)

During the lock state, the next relations are valid: 2 1 L L D i i i = + (3.15) (3.16) (3.17) I c TI C D C Q v = = − ∆ + 1 _(3.18) (3.19) As O O

V

C

Q

=

,

∆

Q

=

C

_O

∆

V

_O, 2 1 2

L

DTV

i

_L

=

∆

it results: 2 1 2

9

2

1

L

DTV

T

i

Q

=

∆

L

=

∆

_(3.20)

so the voltage ripple of V0 is:

(3.21)

and the relative ripple results:

(3.22)

In discontinuous conduction mode, the current iD becomes zero during blocking. Discontinuous conduction mode condition is ζ ≥ 1, (D2 / N2) and (R / 2R) ≥ 1, so:

2

N

z

D

fL

R

D

N

≤

=

_(3.23)

The output voltage in discontinuous mode is: (3.24) while

D

fL

R

−

≥

1

2

I O I D D I =1− 1 1 DV D V_O − =

D

V

N

I O

−

=

1

C TV D DTV₁ =(1− ) O I C V V D D V = − = 1 1 1

L

DTV

i

I L

=

∆

1 1 1 1 1

)

1

(

NfL

R

D

I

L

DTV

I

i

I I L L

=

−

∆

=

ξ

1 2 1 L L I D D I = − 2 2 2 2

NfL

DR

I

i

L L

=

∆

=

ξ

L

TV

D

i

D L L O

)

1

(

2 1

−

=

∆

+

∆

=

∆

D

I

O L L D

=

+

=

−

1

2 1 fL N R D I i D D 2 2 = ∆ =

ξ

fCR

D

CV

TI

D

v

O I C C

=

(

1

−

)

=

1

∆

=

ρ

2 1 2 9C L V DT C Q v O O O = ∆ = ∆ 2 2 2 2 1 9 9 N f C L D V V L C DT V v O O I O O O ₌ ₌ ∆ =

ε

I O V fL R D D V 2 ) 1 ( − =

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Figure 3. Simulation of the positive output Luo converter in PSPICE

For simulating the converter, the following values were chosen:

VI = 20 V; VO = 30 V; T = 20 us; f = 50 kHz; D = 0.6; L1 = 10 m; C = 20 u; L2 = 10 m; CO = 20 u; R = 20; the ideal diode with snubber D. For these values, using the above formulas of the converter, the following results were obtained, according to Figure3:

VO = 30 V; N= 1.5; VC = 30 V; ∆iL1 = 24 mA; ξ1= 10.6; ∆iL1 = 24 mA; ξ2 = 16; ∆iD = 48 mA;

ξ = 6.4; ∆vC=0.9 V; ρ= 0,03; ∆vO= 3 mV; ε= 0.1 m.

B. Mathematical Analysis and Simulation of Self-Lift Positive Output Luo converter

The self-lift positive output Luo converter (see Figure 4) is an improved version of the elementary Luo converter, using the voltage-lift technique.

Figure 4. Electronic diagram of the positive output self-lift Luo converter designed in OrCAD Capture The functioning of the circuit is defined

by the two diodes that are conducting alternatively; the current flows through D1

during the conduction period and diode D is locked, while during the lock period the current flows through D and D1 is locked.

From here on, in the same way as for the previous converter, the functioning parameters are defined:

(3.25) (3.26) (3.27) (3.28) (3.29)

T

D

I

T

D

I

Q

₋

=

_C₋_OFF

(

1

−

)

=

_L

(

1

−

)

_(3.30) During commutation, − + =Q Q _(3.31) O CO C V V V = = I O V D V − = 1 1 D V V N I O − = = 1 1 O I I D I − = 1 1 DT I DT I Q₊= _C₋_ON = _O

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O L I D D I − = 1 (3.32)

During the lock period, O LO L D I D D I I I − = + = 1 (3.33)

fL

R

N

2 1

2

1

=

ξ

_(3.34) O

fL

R

N

2 2

2

1

=

ξ

_(3.35) (3.36)

)

/(

_O O eq

LL

L

=

+

The relative ripples of the voltages are:

fCR

D

1

2

=

ρ

_(3.37) (3.38) (3.39)

The variation report of the current iD is 1, when the circuit functions on the limit:

(3.40)

The limit between the continuous mode (CCM) and discontinuous mode (DCM) is:

(3.41)

where zN is R/(fLeq)

When N>MB, the circuit functions in DCM mode. In this case, the current through diode

iD decreases to zero at t = t1 = [D + (1−D) m]T,

where DT < t1 < T and 0 < m < 1 and “m” is

the fill factor, defined as:

)

/

(

1

2 eq

fL

R

D

M

m

=

ζ

(3.42) (3.43) I O

V

m

D

V

]

)

1

(

1

[

−

+

=

(3.44) I eq O

V

fL

R

D

V

)

2

)

1

(

1

(

+

2

−

=

(3.45)

The transformation report of the converter in discontinuous mode is:

(3.46)

The output voltage will linearly increase as long as the load resistance R is increased.

Figure 5. Simulation of the positive output self-lift Luo converter in Pspice For simulating the converter, the

following numerical values were chosen:

VI = 20 V; VO = 30 V; T = 20 us; f = 50 kHz; D

= 0.6; L = 10 m; C = 20 u; C1= 20 u; Lo = 1m; Co = 20 u; R = 40; D and D1 the ideal diodes with snubber. eq

fL

R

N

2

1

=

ξ

R fC N 1 1 1 2 =

σ

O OL C f N D 2 1 8 =

ε

1

2

1

2

=

eq

fL

R

N

ξ

eq fL R D D MDCM 2 ) 1 ( 1 2 ₋ + = ) ( ) 1 ( ₁ 1 D mT VC VC DTV = − − eq fL R D D MDCM 2 ) 1 ( 1 2 ₋ + =

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For these values, using the formulas of the converter, the following results were obtain, according to fig.5:

VO = 50 V; N = 2.5; VC = 50 V; ξ1 = 6.4; ξ2 = 96; ξ = 64; ρ = 0.0075; σ1 = 0.01875; ε = 1 4. Conclusions and results

For both converters, a 20 V input voltage was used, for the same period and at the same frequency. In the case of the elementary positive output Luo converter, the output voltage (30 V) is 150 % greater than the input voltage. When using the self-lift version of the converter, the output voltage is increased up to 250 % of the input voltage value (Figure 6).

Figure 6. Output voltage comparison for the proposed Luo converters

The steps of developing an efficient battery management unit can be resumed at defining the needed characteristics and finding the best solution on the market for the DC-DC converter and for the control and management unit:

1. For the DC-DC converter:

− Investigate the available topologies for this system;

− Chose the suitable topology;

− Investigate the large and small signal response of the proposed topology;

− Propose a control strategy and a control system for the proposed topology;

− Investigate the opportunity of implementing more complex control strategies in order to improve the performance of the proposed topology. 2. For the control and management unit:

− Investigate the available MPPT

algorithms advantages and

disadvantages.

− Analyse the required computational power in the context of limited available energy.

− Investigate the opportunity of implementing more advanced MPPT

algorithms (i.e. Fuzzy Logic or Neural Networks).

− Investigate the available batteries charging algorithms for the specific chemistry.

− Investigate the available methods for batteries fitness estimation.

− Following these steps, the main development phases can be defined:

− Design the proposed DC-DC converter.

− Implement the DC-DC converter using the available commercial components

− Design the proposed digital control and management system using commercial available components.

− Design the battery protection unit.

− Develop the prototype (PCB, etc.).

− Develop the firmware for the digital unit.

− Validate and verify the prototype. This paper offers solutions for the first two main development phases, proposing two suitable converter types: the Luo elementary positive output converter and the modern Luo self-lift positive output converter. Although both types of converters are adequate for the battery management unit, checking all the structure and costs requirements, the self-lift version of the converter brings a considerable advantage by obtaining greater output figures for the same input parameters.

The design of the proposed digital control management system and the other block components will be presented in future works.

5. Acknowledgment

The work has been funded by the Sectorial Operational Program Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/134398, also through POSDRU/174/1.3/S/149155, as well as by a grant of the Romanian National Authority for Scientific Research, Program for research – Space Technology and Advanced Research – STAR, project number 80/29.22.2013, code CDI_ID 230.

6. References

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[6] COCOR A., FLORESCU A., POPESCU A., STOICHESCU D., “Power supply blocks with Cúk and Self-Lift Cúk converters for telecommunication sites”, International Symposium on Fundamentals of Electrical Engineering, Nov. 2014, Cat. No.:CFP1493Y-ART, ISBN: 978-1-4799-6821-3

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7. Biography

Andrei BĂESCU was born in Constanţa (România), on May 18, 1988.

He graduated the Politehnica University, Faculty of Electronics, Telecommunications and Information Techno-logy, in București (România), in 2011.

He is a PhD student in applied electronics and information technology at the Politehnica

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University, in București (România), since 2013. His research interests concern: modern energy conversion systems, power electronics, clean energy systems.

Correspondence address: Universitatea Politehnica din București, Splaiul Independenţei, nr.313, 060042 București, România, e-mail: [email protected]

Andrei COCOR was born in

Constanţa (România), on October 28, 1988.

He graduated the Politehnica University, Faculty of Electronics, Telecommunications and Information Technology in București (România), in 2011.

He is a PhD student in applied electronics and information technology at the Politehnica University, in București (România), since 2013. His research interests concern: modern energy conversion systems, power electronics, clean energy systems.

Adriana FLORESCU was born in București (România), on December 3, 1964.

She graduated the Politehnica University, Faculty of Electronics,

Telecommunications and Information Technology in București (România), in 1988.

She received the PhD degree in electronic engineering from the Politehnica University of București (România), in 2001.

She is Professor at the Politehnica University, in București (România).

Her research interests concern: power electronics, artificial intelligence and computer assisted analysis.

Dan STOICHESCU was born in

București (România), on November 7, 1942.

He graduated the Politehnica University, Faculty of Electronics, Telecommunications and Information Technology in București (România), in 1965.

He received the PhD degree in electronic engineering from the Politehnica University of București (România), in 1981.

He is Professor at the Politehnica University, in București (România).

His research interests concern: automated systems, power electronics and theory of information transmission.