DC/DC Converter Integrated Architecture for 48V ... - IEEE Xplore

DC/DC Converter Integrated Architecture for 48V Supplies in Micro/mild Hybrid Vehicle Electrical Engine Control Module Sergio Saponara*, Pierre Tisserand#, Pierre Chassard#, Dieu My-Ton# *

Dipartimento Ingegneria della Informazione, Università di Pisa, via G. Caruso 16, 56122, Pisa, Italy # Valeo Engine and Electrical Systems, 2 rue André Boulle, 94046 Créteil, France

Abstract— The paper shows the design in low-cost HV-CMOS technology of an integrated DC/DC converter for a 48 V electrical belt system generator-alternator used in micro/mild hybrid vehicles. Aiming at interfacing the 48 V power domain with a lower voltage domain, but without using cumbersome inductors and transformers, difficult to integrate in CMOS technology, the DC/DC converter exploits a multi stage architecture. Each stage is a switched capacitor unit. Multiple voltage outputs are supported with a configurable regulation factor, so that the same converter acts in step down or step up mode sustaining an input voltage variation from 6 V (in case of cranking) up to 60 V. Keywords — 48 V power systems, DC/DC converters, hybrid vehicles

I. INTRODUCTION The trend in automotive industry is to decrease the CO2 emitted in the air. One way to follow this objective is to increase the presence of electrical machines on-board vehicles producing standard electrical energy reused to produce torque for hauling. Toyota has recently announced that by 2050 will cut by 90% all petrol/diesel vehicles. The “dieselgate” in Europe and US [1] is accelerating the evolution towards electric/hybrid mobility. Usually, synchronous machines are used on-board vehicles with power ranging from kW to hundreds of kW. To control such machines, electronic devices are more and more placed on the engine with a need of strong integration to keep low the size and cost of the final product while increasing reliability. In automotive operating environment, also other constraints have to be considered [2-4]: vibrations, electromagnetic interference/compatibility (EMI/ EMC), high voltages and currents, high temperatures, highdensity of integration. The global industry hybrid roadmap shows development from micro-hybrid to electrical vehicle. One trigger in mid-hybrid segment (up to 15 kW) is the voltage that shall be below 60 V for security reasons. To face higher power levels hybrid vehicles will rely on a 48 V DC bus to be interfaced with 12 V DC domain for existing-power loads [5-9]. The 48 V network is fully insulated (floating) from the 12 V one by using power DC/DC converters. To reduce the cost of migrating from ICE (Internal Combustion Engine)-based vehicle generation [4] to new hybrid or pure electric generations most of automotive components will be reused. One difficulty of the 48 V electronics architecture is the insulation with lower voltages. The state of art of DC/DC converters is dominated by

switching converters, of step-down (buck) type [7], using large off-chip inductors to filter the undesired switching frequency, thus reducing distortions and ripple at the DC output. Large transformers are also used to provide galvanic isolation between input and output domains. The lower the output ripple and harmonic distortion specification, the higher the inductor values to be used. Large inductors lead to systems with a large size, weight and cost since they cannot be integrated in semiconductor devices but require ferromagnetic cores. In particular, for 48 V network, the low-power electronic is supplied with 5 V – 3.3 V – 1.65 V low voltages that need converters. One issue, due to safety requirements, is to avoid 48 V propagating in case of failure, weeding out the use of DC/DC converters without transformers. Moreover, in case of cranking at the key “ON”, the battery voltage may drop to few Volts [10]. Hence, a step-up regulator is also required, thus increasing weight, cost and size of the power system. On the other hand, inductorless linear regulators have a poor efficiency Ș= Vout/Vin when trying to regulate a low output voltage (12 V or even lower to few Volts) from a 48 V input voltage. To address the above issues, this paper proposes an innovative architecture with an inductorless DC/DC converter where power efficiency is kept high by using a switchedcapacitor circuit. Developed by an industry-academia collaboration within the Athenis3D [6] EU funded project, the work has been patented in [9]. Hereafter, Section II presents the design of a new electrical belt system generator-alternator for 48 V micro/mild hybrid applications including the DC/DC converter. This unit has to be interfaced to the in-vehicle networking system and to the 12 V domain, see Fig. 1.

Fig. 1: 48 V starter alternator interfaced to in-vehicle network

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As shown in Fig. 2, the aim of the project is realizing the whole power mechatronic system as a low-size and low-weight embedded system using CMOS technology. Section III presents the DC/DC converter architecture and its performances when integrated in a HV CMOS technology. Section IV draws some conclusions. II. 48 V POWER SYSTEM FOR MICRO/MILD HYBRID VEHICLES Fig. 3 shows the circuit schematic of the proposed electrical belt system generator-alternator for 48 V micro/mild hybrid cars. The system is composed by a synchronous machine and its dedicated electronics. The electrical machine is made with a double three-phase stator and a wounded rotor. Fig. 2: Power system implementation as embedded mechatronic unit

. Fig. 3: Electrical belt system generator-alternator 48 V circuit

Fig. 4: Multistage architecture of the 48V DC/DC converter

For the electronics, there are two powers parts: 1- The stator connected power MOSFETs, 6 transistors for each stator, which can act as 3-phase inverter or rectifier. They are used for synchronous rectification in case the power system is working in generator mode, and for PWM-based DC/AC power conversion inverter) in case the power system is working in motor mode. 2- The rotor current control, which in Fig. 3 is managed by an H-bridge using 2 power MOSFETs and 2 diodes, with PWM waves applied on the gates of the transistors. A full H-bridge is necessary for rotor current control for fast demagnetization in case of load dump, and also to guaranty safety against overvoltage in case of component failures. In Fig. 3, the control electronic is split in two domains: the 12 V domain, which is used for usual communications with the engine control unit, and the 48 V domain dedicated to the rotor and stator control. The latter generates all signals needed to drive the 12 power MOSFETs of the 2 rectifier/inverter stages for the two 3-phase stators. It also generates the command signals for the 2 power MOSFETs used in the H-bridge to control the rotor current. This part also includes the analog and digital circuits to sense all sensors used in stator and rotor control: stator thermal sensors, phase current sensors, phase voltage sensors, rotor position sensors, and power module thermal sensors. From an algorithmic point of view, the electrical machine whose scheme is proposed in Fig. 3 can be controlled with standard voltage loop control, as in [11]. However, with new Lithium generation battery, a closed-loop flux vector control [12] is more and more used. The control algorithm in the scheme in Fig. 3 is implemented through an FPGA. Analog, digital and power devices have to be integrated in the same platform, realizing an embedded mechatronic system. To be noted that in Fig. 3, for prototyping, all grounds are connected together. The advantages of a double stator configuration of the electrical machine, as in Fig. 3, are discussed in literature [13]. III. DC/DC CONVERTER ARCHITECTURE A key block of the new power system is the DC/DC converter producing lower voltages needed by analog and digital circuits. A new DC/DC converter architecture is proposed using switching technique to keep high the power efficiency, but avoiding inductors and transformers. The DC/DC converter uses a multi-stage architecture shown in Fig. 3, where the isolator block works according to the circuit in Fig.

5 and the DCDC_1 and the DCDC_2 blocks work according to the circuit schematic in Fig. 6. The principle of operation of the insulator block in Fig. 5 is to insert one or several serial capacitors voltage to guaranty the insulation. Fig. 5 describes one application. The behavior is based on two clock cycles: when the Red switches are ON, then C1 is loaded and C3 supplies loads. Instead, when Black switches are ON, the charge is transferred from C1 to C2. In Fig. 5 if at least one switch (or more) fails, there is no possibility to have overvoltage at the output. C1 P8 6V..60V

C2 P7

P6

P5 P4

P0 G0

LDO

G3

5V out

G6

+ C3 -

G1

G2

G4

G5

P1

P3

Ve

Control

Vs

P2 Start / Stop

Fig. 5: Insulated DC/DC converter principle

Fig. 6: Basic circuit of the switching capacitor stage

For the demonstrator, the DC/DC converter regulates the 48 V power DC domain to lower voltage outputs required by analog parts, sensors and digital parts (memory, processors) at 12 V, 5 V, and 1.65 V. As a specification, the input voltage can rise from the nominal 48 V up to 60 V or can drop in case of cranking down to 6 V. The circuit has been first designed and simulated in all PVT (process-voltage-temperature) corner cases at layout level in Cadence CAD environment. Then, the DC/DC converter has been realized as an integrated circuit in a 0.35 μm HVMOS technology with transistors operating up to 70 V at a main switching frequency of 90 kHz. The frequency value has been derived after EMI/EMC analysis in the automotive environment. To keep the power efficiency

comparable to that of conventional DC/DC switching converters using inductors, in our design the jump between input and output voltages is realized in multiple cascade stages, see Fig. 4. Indeed, in switched capacitor power converters poor efficiency values are obtained with large ratios between input and output voltages. Each stage in Fig. 4 implements the basic switching circuitry with flying capacitor of Fig. 6 where the possible regulation factor K is 2, 1, 1/2 for DC/DC_1 stage in Fig. 4 or 1, 1/2, 1/3 for the DCDC_2 stage. The isolator in Fig. 4, whose circuit is detailed in Fig. 5, has conversion factor 1. The first stage DC/DC_1 regulates the 48 V nominal input voltage in a Vout1 of 24 V, (equal to Vouti) which is regulated down to 12 V (Vout in Fig. 4) by DC DC_2 with a power efficiency of 85 %. DC/DC_2 can be also configured to regulate the Vout1 of 24 V down to a Vout of 8 V. In case of cranking, if the input voltage, Vbat in Fig. 4, drops down to 6 V, the DC/DC_1 and DC/DC_2 can be configured so that the Vout is still 12 V. In this case, the same converter acts as a step up. Once regulated from 48 V to 12 V the supply domain, 2 parallel low drop-out (LDO) regulators allow for lower output voltages: 5 V and 1.65 V. Thus, the DC/DC converter has 3 multiple outputs: a main Vout at 12 V plus 5 V and 1.65 V. Operating with an input voltage much lower than 48 V the LDO regulators have now acceptable efficiency. For the LDOs, integrated architectures we already proposed in literature [7] have been adopted According to Eq. (1) the regulation capability of each switching DC/DC converter stage depends on the switching frequency fSW and on the size of the capacitor Cf plus a terms n depending on the conversion ratio. (1) ܸ‫ ݐݑ݋‬ൌ ‫ ܭ‬ή ܸ݅݊ െ ‫ݐݑ݋ܫ‬ൗሺ‫ ܨܥ‬ή ݊ ή ݂ ሻ ௌௐ

By cascading the multi-stage DC/DC switching core with the LDOs, as in Fig. 4, the regulation performances of the whole conversion system are further improved. The power supply rejection ratio (PSRR) of the whole converter in Fig. 4 is at least 40 dB at low frequencies and -90 dB at the switching frequency. The voltage ripple is about 10 mV for the 1.65 V output and 25 mV for the 5 V output. As result, without using inductors but just capacitors (with values from 100 nF to 10 μF) the design allows to regulate high input voltages (one order of magnitude of variation from 6 V to 60 V). Target load current values are from few tens of mA to 300 mA. The efficiency as a function of the input battery voltage Vbat to be regulated is reported in Fig. 7 considering the proposed multi-stage DC/DC switched cap architecture in Fig. 4. Fig. 7 considers as reference also a single-stage DC/DC switched cap realization where just one block, working according to the scheme in Fig. 6, is sized to regulate to 12 V an input voltage from 6 V to 60 V. The data in Fig. 7 refers to the case of a current load of 100 mA, and working temperature of 27 0C. With the proposed multistage DC/DC switched cap converter at the nominal battery voltage of 48 V the efficiency is about 85%, whereas the efficiency is above 60% when the battery voltage drops to few volts or increases up to 60 V. The galvanic isolation is ensured by the intermediate stage in Fig. 4, detailed in Fig. 5, with series capacitive isolator without using heavy and cumbersome ferromagnetic cores. The

switching phases are generated by an on-chip control unit (not shown in Fig. 4), starting from a ring oscillator operating at 32 MHz, and can be modulated according to different switching strategies: classic PWM, skip-mode and variable switch frequency modulation. The skip-mode and the variable switch frequency modulations allow for a spreading of the generated electromagnetic noise and hence they improve the EMI performance of the circuit vs. classic PWM. The skip mode is simpler to implement since it does not require voltage controlled oscillators but it can create substrate injection problems when skipping for a long time period in case of low current values.

Fig. 7: Power efficiency of the DC/DC converter vs. Vbat

The area including pads of the whole converter of Fig. 4 (including also the skip mode control) in 0.35 μm HV-CMOS is 36 mm2. The first layout version is conceived with SMD capacitors mounted on the PCB board on the side of the IC. Future work aims at stacking capacitors on top of the converter IC, further minimizing board area. The architecture in Fig. 4 can be scaled to whatever output voltage by integrating single-chip only the multi-stage switching core and using COTS devices for the LDOs. A saving x5 in PCB size is expected vs. previous power mechatronic generation [6]. IV. CONCLUSION The paper has presented the design, in a 0.35 μm HV-CMOS technology, of 48 V DC/DC converter for electrical belt system generator-alternator control unit in micro/mild hybrid vehicle. Without using inductors, the DC/DC converter generates multiple voltage outputs with a configurable regulation factor (acting in step down or step up modes) sustaining an input voltage variation of one order of magnitude, from 6 V to 60 V. The PSRR is at least 40 dB at low frequencies, and 90 dB at switching frequency (90 kHz). The switched capacitor DC/DC stages are controlled according to a skip-mode technique, where control signals are internally generated starting from a 32 MHz reference clock generated by an integrated ring oscillator. ACKNOWLEDGMENT Discussions with E. Wachmann, AMS ag, and L. Ferrari, A. Sisto, C. Mattaliano, G. Ciarpi, University of Pisa in the framework of the ATHENIS 3D EU project are gratefully acknowledged.

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