Ebook Advanced electric drive vehicles: Part 2

Continued part 1, part 2 of ebook Advanced electric drive vehicles presents the following content: low-voltage electrical systems for nonpropulsion loads; 48-V electrification - belt-driven starter generator systems; fundamentals of hybrid electric powertrains; hybrid electric vehicles; fundamentals of chargers; plug-in hybrid electric vehicles;...

9 Low-Voltage Electrical Systems for Nonpropulsion Loads Ruoyu Hou, Pierre Magne, and Berker Bilgin CONTENTS 9.1 9.2 9.3 9.4 Introduction 317 Low-Voltage Electrical Loads 318 Requirements of Auxiliary Power Module 319 Converter Topologies for Auxiliary Power Module 320 9.4.1 Flyback Converter 320 9.4.2 Forward Converter 321 9.4.3 Push-Pull Converter 322 9.4.4 Topologies for the Primary Side 322 9.4.5 Topologies for the Secondary Side 323 9.4.6 Synchronous Rectiication 324 Problems 327 References 328 9.1 INTRODUCTION In conventional vehicles, the traction power is supplied by the internal combustion engine In order to provide power to the vehicle electrical loads, a low-voltage system is utilized, which includes a belt-driven alternator, low-voltage battery, and various electrical loads When the engine is running, it provides torque to the alternator, which then provides electrical energy to the 12 V battery In conventional vehicles, claw–pole synchronous generators are utilized, due to their low-cost structure and reliable operation However, claw–pole alternators usually have low eficiency because of the high leakage lux Depending on the charging current of the low-voltage battery and the load requirements in the vehicle electrical system, the ield current of the claw–pole alternator is controlled by a regulator to keep the system voltage constant In light-duty vehicles, battery voltage is usually 12 V, and when the vehicle is running, system voltage is approximately 13.5 V in summer time and 14.5 V in winter time With the engine stopped, only low-voltage battery supplies power to the electrical loads The battery also behaves like a buffer in the electrical systems and stores energy With the improvements in vehicle technology, safety requirements, and increasing customer demands, many electric and electronic loads have been added to the vehicle electrical system In conventional vehicles, the electrical system has to supply enough power to the entire vehicle network, provided that the quality of the voltage is high enough to ensure the functional safety of electronic loads, especially control units In electriied vehicles, similar low-voltage electric and electronic loads still exist However, traction system voltage is usually much higher than the vehicular electrical system voltage As an example, in 2010 Toyota Prius, battery voltage is 201.6 V (168 NiMH battery cells at 1.2 V) and the voltage supplied to the inverters is varied between 225 and 650 V by a 27 kW boost converter The vehicular system voltage is 12 V In all-electric vehicles, the voltage levels are similar In belt-driven 317 318 Advanced Electric Drive Vehicles starter generator applications, the traction power is usually supplied by lead-acid batteries and the voltage is set around 48 V This is mainly because the precautions required for high-voltage systems not need to be applied in a 48 V system, since high-voltage standards are applied over 60 V DC The traction motors, generator, or starter alternators in electriied powertrain applications are designed for the above-mentioned voltage levels and they cannot be utilized to supply power directly to the vehicular loads For this reason, power converters are required, which convert high voltage from the traction battery to a lower voltage in order to supply power to the vehicular electrical and electronic loads, and also to charge the low-voltage battery This power converter is usually called as auxiliary power module (APM) Depending on the road and weather conditions, many electric loads are on and off when the vehicle is being driven or stopped Therefore, APM can draw power from the high-voltage battery anytime throughout the drive cycle and it might affect the state-of-charge (SOC) of the high-voltage battery In hybrid electric vehicles, if the SOC of the high-voltage battery is low, the engine turns on and charges the battery through the generator This increases vehicle’s emissions and fuel consumption In all-electric vehicles, lower SOC reduces the range of the vehicle Therefore, eficiency of the APM is very important to maintain a higher vehicle performance 9.2 LOW-VOLTAGE ELECTRICAL LOADS % 22 lo ing a tr d s on W ic lo ip ad er s an sy d st wi em n d lo ow ad O s th er lo ad Ac s ce ss or yl oa ds ec El nd co ir A Li gh tin g iti lo on ad s 77 % 10 0% 17 4% 23 3% 31 9% The low-voltage system in a vehicle constitutes many different loads These can be categorized as lighting, air conditioning, wiper and window systems, electronic, and accessory loads As shown in Figure 9.1, air conditioning loads draw most of the power from the electrical system These include radiator fan, blowers, and seat heaters In conventional vehicles, cabin heating is usually maintained from the waste heat of the engine In hybrid electric vehicles, the engine waste heat can still be utilized; however, in all-electric vehicles, the entire cabin heating should be provided by an electrical heating system Lighting loads consume around 24% of the total power in a vehicular electrical system They are composed of many different loads, including headlights, fog lamps, park lamps, lashers, turn signals, and so on Among these, back lights, headlamps, and fog lamps draw most of the power In a typical vehicle, wiper and window system-related loads draw around 10.30% of the total power Electronic loads include the control units and displays Power outlets, CD player, and Bluetooth are FIGURE 9.1 Typical low-voltage loads in a vehicle electric system Low-Voltage Electrical Systems for Nonpropulsion Loads 319 some of the accessory loads Electric power steering and motor engaging park brake are some other loads for the low-voltage electric power system Most of the loads in the low-voltage electrical system are resistive loads The resistance that is seen from the supply side changes with the current drawn by the load The fans, pumps, wipers, and power windows all have electric motors, which are usually controlled by their corresponding control system The power drawn by the fans is dependent on the fan speed Ambient temperature and the cabin temperature set by the driver determines the coolant low rate, and, hence, the electrical power drawn by the coolant pump In some circumstances, many of these loads can operate together However, the status of the vehicle and the driving conditions usually determine the activation of the loads In a typical vehicle, low-voltage electrical system should be sized at around kW This is the maximum power that the APM should supply In vehicles where additional luxury loads are requested, such as power sunroof, active suspension system, or entertainment systems, the power level of the APM could be higher 9.3 REQUIREMENTS OF AUXILIARY POWER MODULE APM draws power from the high-voltage battery and powers the loads in the low-voltage system In an electriied powertrain, the size of the high-voltage battery determines the range and the emissions of the vehicle The more current the APM draws, the higher the drop in the SOC of the highvoltage battery This might have a signiicant effect on the vehicle performance Therefore, the most important requirement for APM is its eficiency With a higher eficiency, APM draws less power from the high-voltage battery, and the battery charge can be utilized more to power the drivetrain In practice, the eficiency of APM is expected to be higher than 95% in the medium and heavy load conditions The reliability of APM is also very important since it powers all microprocessors in the vehicle and, thus, keeps the vehicle awake As the APM creates an electrical conversion between the high-voltage/power system and the low-voltage/power system of the vehicle, a galvanic isolation must be used for safety reasons This ensures that a failure within the high-voltage system will not affect the low-voltage system and shut down the vehicle The opposite is also true; galvanic isolation would protect the high-voltage system from a failure happening on the low-voltage system, which is directly accessible to the driver and passengers within the vehicle The other important requirement for APM is the quality of the output voltage Especially electronic loads, such as the control units, radio, and the CD player, are very sensitive to the ripple content of the voltage supplied by the APM For this reason, the output voltage ripple of APM should be quite low, which might require designing output ilters As such, a ilter is generally bulky in comparison to the converter; it brings challenges in deining the switching frequency, which strongly affects the iltering requirements, but also losses, as well as the output capacitance and inductance of the converter The SOC of the high-voltage battery varies depending on the traction power requested from the high-voltage battery The terminal voltage and, hence, the input of the APM changes in this case Therefore, APM is required to operate in a certain input voltage range and provide the output voltage speciications for the entire input voltage range Finally, APM should be designed to operate in various temperature conditions In automotive system, the operating temperature usually varies between −40°C and 85°C, so that the vehicle can operate in different climatic regions around the world For a power converter with high eficiency requirements, the ambient temperature is very important when deining the size of the cooling system As an example, the resistance of the transformer and inductor windings and the conduction losses of the power semiconductor switches are dependent on temperature Therefore, the designer should design the thermal management system for the given speciications, which ensures that the required eficiency can be maintained in various ambient conditions 320 Advanced Electric Drive Vehicles 9.4 CONVERTER TOPOLOGIES FOR AUXILIARY POWER MODULE In a typical electriied powertrain architecture shown in Figure 9.2, the APM is required to deliver power from high-voltage (HV) DC bus to 12 V loads The converter must incorporate galvanic isolation to protect the low-voltage (LV) electronic system from the potentially hazardous high voltage [1,2] This requirement restricts the available topologies to those containing a transformer [3] In the following, possible candidates for the APM are introduced and discussed 9.4.1 FlybaCk ConVerter As shown in Figure 9.3, lyback converter has a single switch and it employs its transformer’s magnetizing inductance for energy storage However, the magnetic lux in the lyback transformer has a DC component Because of this, the size of the transformer core increases as the power requirements increase [4] Especially, in high-input-voltage applications where high conversion ratio is required, the voltage stress on the lyback converter switch can be a limiting factor in the design The switch voltage stress in a lyback converter can be represented as N  Vin + Vo ×    N2  where Vin is the input voltage, Vo is the output voltage, N1 and N2 represent the number of turns of the primary and the secondary windings, respectively The output voltage of the lyback converter can be expressed as  D   N2  Vo = Vin   − D   N1  where D is the duty cycle For 300 V input voltage, if the converter operates at 50% duty cycle, the transformer turns ratio (N1/N2) would be 25:1 to achieve 12 V at the output In this case, the switch voltage stress ends up at 600 V Considering the voltage overshoots due to the stray inductance in Bidirectional DC/DC converter Motor control Energy conversion High-voltage battery AC/DC converter Charger APM DC/DC converter High-voltage system 12-V accessories Low-voltage battery Low-voltage system FIGURE 9.2 Typical electriied powertrain system with low-voltage network Alternator Grid/charging station DC/AC inverter Electrical motor/ generator 321 Low-Voltage Electrical Systems for Nonpropulsion Loads N1 N2 D L Co Vin + – + – Vo S FIGURE 9.3 Flyback converter the circuit, the switch should be rated higher than this value This increases the cost and reduces the power density Indeed, switches enabling high switching frequency (several kHz) are required to keep the transformer size reasonable Hence, MOSFET is usually the preferred choice However, for a 600 V voltage stress, most of the current MOSFETs available in the market might not be capable of handling that high voltage, and the ones rated for these values are usually more expensive than insulated gate bipolar transistor (IGBT) for the same power rating IGBTs can handle higher voltages, but they usually might not be capable of operating at high switching frequencies In either case, there is a restriction to achieve high power density and reasonable cost of the converter at the same time 9.4.2 ForWard ConVerter Compared to the lyback converter, forward converter does not need to store the energy in the transformer The energy is transferred from the source to the load while the switch is closed As shown in Figure 9.4, a third winding is applied to provide a path for the magnetization current when the switch is open in order to reduce the magnetizing current to zero before the start of each switching period This provides a smaller transformer size for the forward converter [4] However, the transformer in a forward converter still employs DC lux similar to the lyback converter The semiconductor switch in the forward converter is still exposed to high-voltage stress, which can be represented as N   Vin ×  +  N3   where N3 is the number of turns of the third winding Since the magnetizing current must be zero before the start of the next switching period, the following condition must be followed in forward converter: N   D 1 +  < N1   N3 D3 D1 Lx D2 Co N1 N2 Vin + – Lm S FIGURE 9.4 Forward converter + – Vo 322 Advanced Electric Drive Vehicles Np:Ns D1 L + – Vo Co + Vin – D2 S1 S2 FIGURE 9.5 Push–pull converter Thus, N3 must be smaller than N1 For the same operating conditions with the lyback converter (Vin = 300 V, D = 50%), the switch voltage stress in the forward converter will be greater than 600 V 9.4.3 puSh-pull ConVerter Figure 9.5 shows the typical circuit diagram of the push-pull converter In steady state, the input and output voltage relationship can be represented as N  Vo = 2Vin  s  D  NP  where D is the duty cycle for each switch Compared to lyback and forward converters, the number of semiconductor switches is higher in push-pull converter The voltage stress on the switches is also twice the input voltage However, unlike lyback and forward converters, the transformer of the push-pull converter has AC lux Therefore, the transformer does not need to store energy, yielding a relatively smaller transformer core, which can be designed in a smaller volume This results in better potential power density than lyback and forward converters Flyback, forward, and push-pull converters all provide galvanic isolation using a transformer It is also possible to design the converter by selecting different topologies for the primary and the secondary sides of the transformer Depending on the operational requirements of the APM, various topologies can be used on both sides, which will also affect the design of the transformer 9.4.4 topologieS For the primary Side In general, full-bridge and half-bridge topologies can be utilized for the primary side Figure 9.6 shows the circuit diagram of these two topologies For high-power applications, full-bridge converter is usually applied as it is relatively simple and robust, and it offers good power density and eficiency The switch voltage stress is equal to the input voltage, which leads to a lexible switch (a) (b) S1 Vin + – S3 S1 Vin + – C S2 C1 S4 C2 S2 FIGURE 9.6 Primary-side topology candidates: (a) full bridge and (b) half bridge 323 Low-Voltage Electrical Systems for Nonpropulsion Loads Ds1 Ds2 α T = 1/fsw D Ds3 Ds4 Dtransformer FIGURE 9.7 D–α Phase-shift full-bridge control scheme selection for the APM In addition, zero voltage switching (ZVS) technique can be implemented on the full bridge by employing phase-shift control in order to reduce the switching loss [5], as shown in Figure 9.7, where D is the duty cycle for each switch and α is the phase-shift angle between S1 and S4 In the 2004 model of Toyota Prius, an isolated APM topology has been used with a full-bridge converter on the primary side [6] Compared to the full-bridge converter, half-bridge converter only needs two switches instead of four However, these two switches are required to carry two times as much current as compared to the full-bridge converter Meanwhile, the voltage stress for these two switches still equals the input voltage Thus, the switch requirements for the half-bridge topology are higher than the full-bridge topology, which restricts its feasibility in high-current applications In addition, half bridge requires two input capacitors instead of one for the full bridge 9.4.5 topologieS For the SeCondary Side Owing to the low output voltage and high current requirements, conduction losses dominate on the secondary side For a kW application, an output voltage of 12 V results in an output current around 250 A This yields large conduction loss and strongly affects the eficiency of the secondary side converter [8] Hence, it is critical to select the most suitable topology to maximize the converter eficiency for high-current operations This point is especially important because power requested in modern vehicles is continuously increasing This results in higher current rating on the secondary side As a result, topologies proposing better capabilities in handling higher current are appropriate for the secondary side in APM converters Figure 9.8 shows the center-tapped rectiier and current doubler rectiier topologies, which can be used as the secondary-side topology in a unidirectional APM The main waveforms for these topologies are shown in Figure 9.9 (a) (b) L D1 D2 + – L1 D1 L2 D2 + – FIGURE 9.8 Secondary-side topology candidates: (a) center-tapped rectiier and (b) current doubler rectiier 324 Advanced Electric Drive Vehicles (a) Vsec IL (b) Vsec T = 1/fsw IL1 ΔI T = 1/fsw ΔIL Io Io IL2 Io ISR1 Io IL1 + IL2 Io ΔI Io ISR1 ISR2 Io Io ISR2 ton toff ton toff ton toff ton toff FIGURE 9.9 Main waveforms of (a) center tapped and (b) current doubler rectiier (From P Alou et al In Proceedings of Applied Power Electronics Conference and Exposition, Dallas, TX, Mar 2006.) From the inductor aspect, since current doubler has two switches and two inductors, each inductor operates at the same switching frequency as the semiconductor device Center-tapped rectiier obtains two switches with one inductor; therefore, the inductor current ripple oscillates at twice the switching frequency of the switches From the transformer aspect, the current doubler might be more attractive than the center-tapped rectiier One of the drawbacks of the center-tapped rectiier is that its transformer winding has double winding The secondary side in the current doubler rectiier has a single winding This decreases the utilization factor of the transformer in the center-tapped rectiier Owing to the single secondary winding, it is possible to parallel more coils in the current doubler rectiier for the same window area, enabling lower resistance for high-current operation 9.4.6 SynChronouS reCtiFiCation High-current requirement on the secondary side usually results in high conduction losses The conduction loss of diode rectiiers contributes signiicantly to the overall power loss due to the high voltage drop A typical PN-junction power diode voltage drop is 1.2 V and even Schottky barrier diode (SBD) still has 0.6 V voltage drop [9] For a 12 V output APM application, this becomes a signiicant portion of the voltage drop (10%) and penalizes the eficiency MOSFET presents lower conduction loss than diode As a result, the concept of synchronous rectiication (SR) came to reduce the conduction loss and maximize the conversion eficiency on the secondary side [3] In SR, rectifying diodes are replaced by synchronous MOSFETs Corresponding topology for the current doubler circuit is shown in Figure 9.10 The synchronous MOSFETs operate in the third quadrant The body diode of the MOSFET conducts prior to the turn on of the switch In other words, conduction loss of the body diode is 325 Low-Voltage Electrical Systems for Nonpropulsion Loads S1 + L1 Co – L2 S2 FIGURE 9.10 Synchronous rectifying current doubler generated just before the synchronous MOSFET turns on However, it can be turned on in ZVS, which results in negligible switching loss at turn-on At turn-off, the MOSFET stops conducting prior to the body diode, which means that the synchronous rectiier still has the reverse recovery losses from its body diode [10] If the voltage stress across the semiconductor is relatively high, MOSFETs with high voltage rating need to be used High-voltage MOSFETs have larger on-state resistance, Rds, which might reduce the system eficiency In this case, a Schottky diode-based coniguration might provide a comparable eficiency in the secondary side with a lower cost as compared to SR MOSFET-based coniguration Typically, there are two different techniques to control the SR: external-driven SR (EDSR) and self-driven SR (SDSR) [11] As shown in Figure 9.11a, in the EDSR technique, the control signals are generated by an external controller, which guarantees the appropriate timing By doing so, the switches can be turned on during the whole rectiication period, and the eficiency can be maximized [12] However, circuitry to generate the gate pulses and drivers to charge the gate capacitance of the MOSFETs are required [11] Unlike the EDSR, the control signals as well as the energy to drive the SDSR switches are obtained from the secondary side of the transformer and no driver is needed [11], as shown in Figure 9.11b As a result, a simple, low-cost rectiication control can be implemented However, there are mainly two drawbacks for SDSR The irst one is the voltage with which the MOSFETs are driven is variable, and it depends on the input voltage Second, not too many topologies are suitable for SDSR The most suitable topologies for using SDSR are the ones that drive the transformer asymmetrically with no dead time: lyback and half bridge with complementary control, and so on [9] The concept of a half-bridge converter with SDSR control and its main waveforms are shown in Figures 9.12 and 9.13a, respectively [9] For topologies with symmetrically driven transformers, as the full-bridge and push–pull converters, the synchronous rectiiers are not activated during the dead time of the transformer The main waveforms are shown in Figure 9.13b It is clear that during the dead time of the transformer, the body diode of the MOSFET, which usually creates very large forward voltage drop in the circuit, has to conduct This fact causes a noticeable decrease in eficiency (a) (b) Synchronous rectification switches Controller and driver Control signals FIGURE 9.11 (a) EDSR and (b) SDSR Synchronous rectification switches Self-driven control signals 326 Advanced Electric Drive Vehicles S1 L C1 + Va – DC + – S2 C2 FIGURE 9.12 Half-bridge converter with SDSR (From A Fernandez et al IEEE Transactions on Industry Applications, vol 41, no 5, pp 1307–1315, Sep 2005.) (a) (b) Va Va T = 1/fsw VGS1 VGS1 VGS2 VGS2 T = 1/fsw MOSFET on MOSFET off (body diode on) MOSFET on FIGURE 9.13 Transformer voltage and SDSR gate drive signal waveform of (a) asymmetrical-driven waveform and (b) symmetrical-driven waveform Therefore, it is important to extend the conduction period of the SDSR MOSFETs over the period when the voltage across the transformer is null The basic idea to improve the system eficiency with SDSR under symmetric transformer waveform is shown in Figure 9.14 One possible implementation method to generate these extended gate driver signals is to apply an additional winding and an additional voltage source VA to force the synchronous rectiiers to be on Vtransformer T = 1/fsw Extended conduction Ideal VGS1 Ideal VGS2 MOSFET on FIGURE 9.14 Ideal SDSR gate drive signal voltage for symmetrical transformer voltage waveform ... VP2 – + V – P1 + Vin – S2 P1 S1 + VS2 – + VS1 – L D2 Sw2 FIGURE 9.16 P2 Sw1 Push-pull converter iL Co + – Vo 328 Advanced Electric Drive Vehicles S1 Vin + – S5 S3 + Vp – C Np:Ns L1 + Vo – L2... HEV, PHEV, and EV can provide the most 120 108 Voltage (V) 96 kW 84 10 kW 72 16 kW 60 20 kW 48 36 24 12 FIGURE 10.3 125 25 0 375 500 625 750 875 1000 1 125 125 0 1375 1500 Current (A) Effect of increasing... 1 +  < N1   N3 D3 D1 Lx D2 Co N1 N2 Vin + – Lm S FIGURE 9.4 Forward converter + – Vo 322 Advanced Electric Drive Vehicles Np:Ns D1 L + – Vo Co + Vin – D2 S1 S2 FIGURE 9.5 Push–pull converter