48伏特起動發電機
2017年，整個汽車行業都實現了顯著增長–從汽車OEM到小型器件製造商。汽車電氣化成為備受矚目的熱門話題，特別是48伏特架構。事實上，在任何搜尋引擎中搜索“48伏特”這個術語，您都會搜索到大量結果，這表明面向汽車系統的這個工程解決方案已在市場上占據了一席之地。
在前一篇“48伏特輕度混合動力系統的出現”中，我簡要介紹了新型48伏特汽車架構及其與現有12伏特系統的差別。在這篇我們的話題將觸及確定新電壓水平背後的原因，並深入探討它的主要應用之一——48伏特起動發電機。
為什麼是48伏特？
顯而易見的首要問題是：“為什麼偏偏是48伏特？”。這是一個非常重要的問題，要記住，在上世紀90年代，人們提出了42伏特電功率標準來取代12伏特標準。雖然這個標準沒有得到發展，但其意圖是解決我們當今面臨的一些相同問題，例如更大功率的電驅動配件及更輕的線束。選擇48伏特作為標準有兩個主要原因 –安全性和效率。
在提高電壓時，涉及的一個主要問題是它可能危及人身安全。雖然有人仍在爭論48伏特是否足夠安全，但這個電壓水平提供了我們需要的更高功率而沒有進入“高電壓”範圍。圖1顯示了48伏特電池在不同電壓水平下的工作情況。正如ZVEI的文檔“電動出行的電壓等級”所述，60伏特（直流）是安全上限最大值，高於它的電池電壓都被視為過於危險。在“正常工作”範圍內，電池能夠達到最佳性能，但汽車內部的電子器件應該能夠耐受最壞情況的高電壓條件。
Figure 1: Safety voltage margins
As I’ve previously stated, the current 12-volt system is unable to cope with the growing demand for electrical power within conventional vehicles. However the 42-volt proposal was a complete replacement of the 12-volt electrical architecture while 48-volt complements it. The 48-volt battery simply adds an additional power source for new applications which also contribute to a smoother driving experience. Furthermore, the size and cost of wiring and components is significantly reduced due to the higher voltage of the 48-volt battery.
A closer look at the 48-volt starter generator options
Having a similar appearance as the car alternator (Figure 2a) but slightly bigger in size, the 48-volt starter generator’s initial topology position is at the engine’s belt. The belt-driven starter generator (BSG), also known as P0 architecture (Figure 2b) is a cost-effective solution which can provide up to a 15% reduction in CO2. Looking at some boost recuperation systems (such as Bosch’s), the maximum power ratings are around 10 kW for mechanical output in boost mode and 12 kW for electrical output during recuperation – both at 48-volts. While these numbers are rated for short periods of time, the BSG’s continuous power can reach up to 5 kW with maximum efficiency of 85%.
Figure 2a and 2b: Car alternator and P0 starter generator topology
However with tightening emission regulations, automotive Tier 1 suppliers have developed different starter generator topologies to further reduce the CO2 footprint of 48-volt mild hybrid vehicles. In ascending order, these configurations offer better emission reduction but become increasingly complex and costly.
Figure 3: 48-volt mild hybrid starter generator topologies
Crankshaft mounted starter generator (P1)
As the name suggests this solution has the starter generator mounted directly on the crankshaft (which converts the linear motion of pistons into rotary motion). This provides higher torque than the P0 architecture due to the absence of a belt drive, and with no belt losses there is greater efficiency. The maximum power required is 10 kW but the efficiency goes up to 94%. However one significant limitation of this solution is that torque requirements can be demanding, due to no torque/speed ratio between the crankshaft and the starter generator. An example of this topology is the 2010 Mercedes-Benz S400 BlueHybrid.
Shaft mounted machine (P2/P3)
Both P0 and P1 architectures are mounted on the engine, but there are other mounting options such as having the 48-volt electrical machine on the gearbox’s input/output shaft (P2/P3 respectively). By providing a mechanical disconnect, this translates to improved energy flow efficiency and allows for the provision of hybrid functions (e.g. e-drive).
The P2 architecture is ether integrated into the transmission on the input shaft or attached on the side, and results in increased energy recuperation and electrical drive capabilities. Mounting the solution on the output shaft (P3), provides the highest level of the above-mentioned benefits. The obvious disadvantage of the shaft mounted electrical machine is the cost of integration.
Rear axle mounted electrical machine (P4)
The ultimate architecture at this time involves mounting at the rear axle drive (P4). This provides the vehicle with 4-wheel drive capabilities, with the combustion engine at the front and the electrical machine at the back. Maximum power requirement of the P2-P4 architectures can reach up to 21 kW with an efficiency of 95%. Moving the starter generator closer to the rear axle also provides more hybrid functionality to the vehicle. The new 48-volt machine is able to reduce the CO2 emissions by up to 21% in urban driving environment depending on its architecture.
What is more, this high-power application requires a significant portion of electronics to drive it. Naturally power MOSFETs play a key role in these electronic modules, but they need to be capable of withstanding worst case scenarios such as excessive currents and thermal leakages. Hence, the topic of my next blog will be to delve into the semiconductor content inside 48-volt applications such as the starter generator. Stay tuned!
作者:安世半導體 技術營銷工程師 Ivan Petrov
Ivan近期獲得曼徹斯特大學電氣和電子工程專業工程學學士（榮譽）學位，其於2016年9月加入安世半導體擔任技術營銷工程師一職，並在短短的一年後晉 升為研究生營銷工程師。他負責功率半導體業務的市場分析。在過去的一年裡，他發表了幾篇深受好評的傳統動力系統報告，並開始致力於汽車電氣化這一迅速發展 的領域。Ivan對電動車(xEV)系統的了解來自於其靜態研究、參與的眾多xEV會議和全球客戶拜訪。在工作之餘，他是一名活躍的網球選手，他的比賽風 格和外表曾被人拿來與羅傑·費德勒和格里戈爾·季米特洛夫比較。
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