Low-Voltage Power Electronics Building Block for Automotive ...

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Low-Voltage Power Electronics Building Block for Automotive Applications L. Solero, V. Serrao, P. Taglioni, F. Crescimbini University ROMA TRE – Dept. of Mechanical and Industrial Engineering Via della Vasca Navale 79 – 00146 Roma, Italy Abstract-This paper investigates the potential use of the power electronics building block concept in low-voltage fed power converters, such as those being envisaged to be soon utilized on board automobiles being equipped with a 42 V electric power system. Concerning such specific applications, various power converter topologies having input voltage in the range from 24 V to 80 V and rating power from few kW up to 30 kW are likely to be used in order to accomplish bi-directional dc-dc or dc-ac power conversion. Thereby, the envisaged development of a low-voltage power electronics building block module including standardized power, thermal, and control interfaces is of great interest for power converter manufacturers as it would be expected to lead to great simplification in converter design and assembly and thereby should allow substantial reduction of mass-production costs.

I. INTRODUCTION The ongoing integration of safety and convenience functions into the passenger cars calls for an increasing demand of electrical power. The electrification of former mechanic, hydraulic and pneumatic functions like power steering, power brakes, or electronic valve control are about to be introduced into the car, yielding better handling and drive-ability as well as an increase in ICE output power. The 14V system with its alternator as a primary energy source is less and less able to meet the increased power demand. This has led the automotive industries to investigate new electric supply options along with hybrid drive train concepts. The hybrid drive train is further more expected to improve fuel efficiency of the vehicles, helping to achieve aggressive fleet mileage targets suggested by the car makers. A review of power converter topologies being used in a number of 42V automotive drives such as starter/alternator and ultracapacitor-based storage system shows that most converter arrangements are based on the use of the wellknown phase-leg structure including two active switches with their respective anti-parallel diodes. Even though such a standard structure is yet being offered in the market in the form of intelligent power modules, in the case of low-voltage high-current applications such power modules do not completely suit the needs of power converter manufactures and often discrete-component converter arrangements are likely to be used. Over the last twenty years, the fundamental approach to electronic power conversion has steadily moved towards “high-frequency synthesis”, resulting in huge improvement in converter performance, size and weight, and hence the cost. Pulse width modulation and other high-frequency techniques

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are already standard in all low- and medium-power applications, and are rapidly penetrating even the highestpower applications. It is important to note, however, that in many high-frequency power conversion technologies, fundamental limits are being reached that will not be overcome without a radical change in the power conversion strategy. The magnitude increase in switching speed, which is possible with new semiconductor device technologies, will require substantial reduction in parasitic capacitances and inductances associated with device and system-level packaging. A major barrier to further advancements in technology and required cost reduction of power electronics products is the striking lack of standardization, as the power electronics industry is often preoccupied with providing partial solutions for specified applications. The idea of a “building block” approach to power electronics design was conceived in the early eighties, influenced by the booming developments in the integrated circuit industry. The approach of constructing different power converters using a smaller number of integrated modules instead of discrete components was initiated by several power semiconductor manufacturers and proposed in some research laboratories. However, the initial attempts were mostly directed towards simplifying packaging of power converters and did not results in a complete shift of the design paradigm, as the concept of integrated circuits did in the signal processing industry. The much more comprehensive concept of the Power Electronics Building Block (PEBB) originated in the nineties. The overall concept is to use intelligent and reconfigurable PEBBs with standardized power, thermal, and control interfaces to develop multitudes of affordable, reliable, and efficient power processing systems. In fact, unlike modern digital technology, which utilizes an array of developed components or cells to build a system, modern power converters still lack a high degree of integration and standardization. As a result, designers are often forced to build entire systems from scratch each time, which is costly in engineering time as well as system reliability. In order to remedy this situation, in the last decade the concept of PEBB has drawn considerable attention and major research centers are being investigating PEBB-based power converter arrangements devoted to a number of applications [1-4]. PEBBs are integrated power modules serving a function that would commonly be found in a wide number of power conversion systems. Depending on the instructions given to the controller, the PEBB would be required to function as, for instance, a dc-dc converter, a voltage-source or current-source

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inverter, a synchronous rectifier, or a motor controller. The PEBB is envisioned to be scalable in the level of power processing and on-board intelligence. The goal of the PEBB development is to create a power-processing component that moves most of the design away from specific circuit topology consideration and moves up to a systems level power electronic switch and associated inductors, capacitors, and other ancillary components selection. In few words, the PEBB concept removes the low-level design problems and treats converters as a functional assembly. As PEBB modules can be connected together to form several power system topologies, greatly reduced design efforts as well as increased system simplicity and reliability are achieved. The PEBB concept can be also the best choice to minimize both the layout and packaging parasitics, because all the power semiconductor devices, control circuits, and the busbar would be integrated together as a large power device. In addition, maintenance cost is reduced since individual modules are easily replaced and the number of stock spare parts is reduced. The designer is thus concerned with information and control data flowing in and out of PEBBs and between PEBBs in the larger power systems design. This means that less specialty training in the field of power electronics is required, thus opening the way for higher quality and efficiency, and increased use of power processing technology. This paper investigates the potential use of the PEBB concept in low-voltage fed power converters, such as those being envisaged to be soon utilized on board automobiles being equipped with a 42 V electric power system including both engine directly-coupled starter/alternator and ultracapacitor-based storage system. Concerning such specific applications, various power converter topologies having input voltage in the range from 24 V to 80 V and rating power from few kW up to 30 kW are likely to be used in order to accomplish bi-directional dc-dc or dc-ac power conversion. Thereby, the envisaged development of a lowvoltage PEBB (i.e., LV-PEBB) module including standardized power, thermal, and control interfaces is of great interest for power converter manufacturers as it would be expected to lead to great simplification in converter design and assembly and thereby should allow substantial reduction of mass-production costs [5, 6]. II. THE LOW-VOLTAGE PEBB A review of power converter topologies being used in a number of 42V automotive drives such as starter/alternator and ultracapacitor-based storage system shows that most converter arrangements are based on the use of the wellknown phase-leg structure including two active switches with their respective anti-parallel diodes. Even though such a standard structure is yet being offered in the market in the form of intelligent power modules, in the case of low-voltage high-current applications such power modules do not completely suit the needs of power converter manufactures and often discrete-component converter arrangements are likely to be used.

Concerning such particular converter applications, this paper investigates a LV-PEBB topology suitable to be used as either dc-dc converter or ac inverter or synchronous rectifier. As an example the same LV-PEBB topology could be used to assembly each one of the power electronic converters required for the hybrid power-train shown in Fig. 1. UltraCap Storage

Battery Storage

Buck/Boost DC-to-DC

42V Power Net

CRPW M Inverter

ICE

Gear

AFPM Clutch

Clutch

Fig. 1. Example of hybrid power-train.

Fig. 2. Proposed LV-PEBB configuration.

A. LV-PEBB Power Section The power section of the LV-PEBB is accomplished by means of a 6-pack module based on Mosfet technology, temperature, current and voltage sensors, input and output filters. The 6-pack module configuration has been chosen in order to form a 5-poles PEBB where the parallel connection of two or more switches can be easily achieved for high current applications. The proposed LV-PEBB configuration is shown in Fig. 2; where, for dc-ac mode of operation, the input filter is accomplished by means of a dc-link capacitor CPN and the output filter is achieved with a small inductor LL for each phase. The small inductors are included in the LVPEBB with the purpose to make possible a safe parallel connection of two or more switches of the same LV-PEBB or even of two or more LV-PEBBs. The small inductors are as well responsible of limiting the variation of the phase current in case of short-circuits which are external to the LV-PEBB; besides, the proposed power electronics block includes software active protections for both over-current, overvoltage and over-temperature. The suitable combination of

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the protections’ operating time and phase inductor value makes possible a very small current de-rating for the power switches with respect to traditional power converter design. A certain amount of dc-link capacitance is directly placed in the LV-PEBB in order to meet the requirement for the voltage ripple (e.g. ΔVpp=1%VPN) and to reduce harmonics and EMI emissions at the power interface. The mathematical expression for the dc-link voltage ripple in three-phase converter topologies is as follow: 3 ⋅ (1 − M ) ⋅ M ⋅ I Lpk ΔVpp = (1) 2 ⋅ fs ⋅ CPN where ΔVpp is the peak-to-peak dc-link voltage ripple, ILpk is the peak value of the ac-phase current, fs is the switching frequency, CPN is the dc-link capacitance for LV-PEBB and M is the modulation index (defined as M = Vllpk VPN , Vllpk is the peak value of the line-to-line voltage and VPN is the dclink voltage value). To select properly the PEBB capacitor, also the rms value of the ripple current in the LV-PEBB dc-link capacitor must be considered during the design: I Lpk

⎡1 ⎛4 3 ⎞⎤ ⋅ M ⎢ + cos 2 ϕ ⋅ ⎜ − ⋅ M ⎟ ⎥ (2) 2 ⎝π 2 ⎠⎦ ⎣π where ICPNrms is the ripple current in the capacitor tank and cosϕ is the displacement power factor. Eq. (1) and (2) reach their maximum value for M=0.5 and cosϕ=1. As EMI requirement calls for very low inductance capacitor and short connections, the dc-link cap is directly connected to the switching module and then – for safety reason - it must be assured that the total energy stored in the capacitor is not large enough to destroy the power module case when a short-circuit inside the LV-PEBB occurs. The ac-phase small inductor is sized on the basis of the following equation in order to limit the current variation occurring at external short-circuits for the operating time of the active protection: 2 ⋅ VPN (3) ΔI L = ⋅ Tpr 3 ⋅ LL ICPNrms =

where ΔIL is the maximum tolerable current variation, LL is the inductance value of the small inductor and Tpr is the protection operating time. The proposed LV-PEBB is devoted to applications rated 10kW-42Vdc, however the opportunity of parallel connections between two or more LV-PEBBs gives the chance to use this configuration for higher power applications. The selected 6pack power module is the VWM350-0075P which is based on Mosfet technology, the inductors are rated 1.5μH-250A and the dc link capacitor is 3.1mF-115A. The chosen value for the inductors allows a parallel connection of LV-PEBB modules with a very small current de-rating (