Emerging Solutions to Hybrid & Electric Vehicle DC: DC Converter Design and Test

By Mike Hawes, R&D Planning Manager Automotive & Energy Solutions, Keysight Technologies

Introduction

The latest news continues to support extensive investments and development in vehicle electrification as hybrid and electric vehicle sales grow. Although electric vehicles (EV) still account for less than 1% of the passenger cars sold in 2016, EVs grew 60% from 2015 to 2016 (download the full App Note to view more). Some of the major detractors of purchasing an EV are being addressed by the likes of Tesla, Chevrolet and others. Range anxiety is less of a concern with ranges over 200 miles on a single charge (Chevy Bolt – 238 miles, Tesla Model 3 – 220 miles). This range allows commuters and ‘short day’ trippers to complete their round trip without worrying about charging station locations and charge time. Price is coming down, as Tesla recently shipped their first Model 3, with a base price of $35,000. The Model 3 is Elon Musk’s first mass market focused EV, with plans to increase their total EV production by 10 times. The Chinese government has goals in their latest 5-year plan to install 4.8 million charging stations by 2020 (download the full App Note to view more). With their ever-increasing air pollution and over 100 cities with a population more than 1 million, they have little choice but to move to zero carbon vehicles.

However, many manufacturers are only making EV ‘compliance’ cars to bring their fleet into compliance with CO2 emission regulations. The industry is still not making profitable EVs. Experience has shown that new powertrain technology typically takes more than one design cycle to turn a profit. The cost pressure on EV power train components (traction motors/ converters, power converters and batteries) continues to drive new fundamental technologies. For example, to extend the range of EVs, Li-Ion cells are being developed with higher capacities, reaching 60 Ah and more. This technology will help extend the range of EVs, but at a cost of less reliability than Lead-acid, requiring additional validation testing and continual manufacturing process monitoring. The cost pressures for EV manufacturers will continue to be very strong, as the industry tries to win a more significant part of the traditional Internal Combustion Engine (ICE) vehicle market.

Hybrid electric vehicles (HEV), on the other hand, are being made profitably and have been for some time. According to the Nikkei Newsletter, both Honda and Toyota have been making a profit on every HEV they’ve sold since 2009 (download the full App Note to view more), with profit margins equivalent to traditional ICE powered vehicles. HEVs are selling at much higher volumes than EVs and are expected to dominate the market for the foreseeable future (see Figure 1). European car OEMs are making a significant commitment to put mild hybrid technology in many of their vehicles. In fact, Volvo recently announced all new cars will have electric motors by 2019. Mild Hybrid (MH) technology claims ~50% less investment than a full hybrid technology, while still providing CO2 reductions up to 15-20%. The MH approach to reducing CO2 emissions balances the need to meet regulations while minimizing investment costs to enable MHs to stay competitively priced with ICE powered vehicles.

Figure 1

Figure 1. Global HEV/EV shipment estimates.

HEVs and EVs have multiple architectural variations. Figure 2 shows a simplified block diagram of a couple of these architectures. For the strong (or parallel) hybrid and the pure EV (no engine), a high voltage (HV) bus supplied by the large battery, drives the electric powertrain. Power levels of the inverter and motor/generator range from ~ 60 kW up to and over 180 kW. Along with the large Li-Ion battery, a significant investment is required to develop these architectures. Most of the components are bidirectional, allowing for power to go from the battery to the inverter, which turns the motor and moves the vehicle (traction drive). When decelerating, the momentum of the vehicle turns the generator, which drives power back through the inverter and charges the battery (regenerative braking).

In the Mild Hybrid (MH), the motor/generator, inverter and battery are also bidirectional. They are not large enough to drive the vehicle by themselves (as in the HEV or EV), but instead are used to supplement the engine power during acceleration and recharge the battery during deceleration. The voltage level for MHs is typically 48 V, keeping the bus structure under the 60V safety rating for HV, but also providing 4 times the potential power of the 12 V…


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