Challenges in automotive power regulation
Older gas power vehicles had only a minimum of electronics – an electronic component failure may have stopped the radio from working but not prevent anyone from getting home ok. Simple engine control functions where added, to improve engine efficiency and fuel economy. This created the need for quality and reliable components as a single component failure could stop the engine from functioning.
Today, with vehicles that are almost completely controlled by electronics, drivers can face different kinds of reliability risks. Typical, the electronics begin their work when a driver walks up to the car and the passive entry system unlocks it. The interrogator pings the key to allow you to start the car with a press of a button. Numerous power supply, analog and microcontrollers circuits start up and control your engine, transmission and almost everything else in the vehicle.
Of course, in most vehicles now, the GPS points drivers in the right direction. Adaptive cruise control radar keeps cars at safe distances, while the anti-lock braking system keeps drivers in control, and airbags are primed to action in case of an accident. And in many vehicles, the telephone can call for help (and report a position) when drivers are not capable. All electronics, all requiring high levels of reliability in a relatively harsh operating environment.
The standard is vital because the average car today requires an electrical power processing capability of 250W to 1500W, a number that is increasing rapidly due to the high power electrical system required in vehicles for powering the car or truck, and also for entertainment and efficiency. Because a vehicle battery is a relatively unregulated low voltage source, it requires a regulated high voltage DC-DC system, which in most cases means a boost DC-DC converter using a multiphase boost architecture.
For example, in battery electric vehicles, the result is a complete elimination of the mechanical engine and transmission by replacing them with batteries, motors and high voltage electronics. While this improves energy efficiency and reduces greenhouse gases, it does so by the addition of electronic circuitry. Given this expectation for longer service life, and the additional electronic components the only solution is for continuous improvement of electronic component quality and reliability. This is achieved in part by adhering to strict automotive industry standards that are designed to ensure the highest quality and reliability across multiple product lines.
ISO/TS16949 is one of the most important standards impacting electronics development, and is virtually mandatory for leading automakers and their suppliers worldwide. The standard is vital because the average car today requires an electrical power processing capability of 250W to 1500W, a number that is increasing rapidly due to the high power electrical system required in vehicles for powering the car or truck, and also for entertainment and efficiency. Because a vehicle battery is a relatively unregulated low voltage source, it requires a regulated high voltage DC-DC system, which in most cases means a boost DC-DC converter using a multiphase boost architecture.
As an example, consider the simple starting and stopping functions. In the automotive environment, a start/stop system automatically shuts down and restarts the internal combustion engine to reduce the amount of time the engine spends idling, thereby improving the fuel economy. This is most advantageous for vehicles that spend significant amounts of time in traffic jams, requiring frequent start and stop. During the starting period, the battery undergoes what is known as voltage cranking deep, which can be as low as 6 Volts (see Fig 1). In order to protect all the electronics connected to the battery bus, the bus line voltage has to be protected from seeing the battery-cranking transient.
Figure 1. Start/Stop battery voltage cranking profile
Figure 2. Block diagram of a start/stop system
One method to solve this issue is by having a regulated multiphase boost DC-DC converter temporally connected between the battery and voltage bus in order to overcome the dip when a cranking transient happens. Fig. 2 shows the block diagram of a start/stop system. In this configuration, when the battery voltage is below an 11.5V threshold, the battery voltage will be boosted by the multiphase boost DC-DC to provide a stable bus voltage. When the transient event is over and the battery voltage is higher than the 11.5V threshold, the electronic control unit (ECU) will close the bypass relay or switch so that multiphase boost converter is bypassed from the system. Intersil’s multiphase boost controller ISL78220, which meets the important TS16949 standard, is a good candidate for this start/stop application because of it can detect input voltage variations, and its has high light load efficiency due to phase shedding and pulse skipping modes.
In steady state after battery voltage cranking, the multiphase boost converter runs at no load or a very light load condition, and all of the current will come directly from the car battery. To minimize power draw from the battery in this condition, several light load enhancement schemes have been implemented on ISL78220. Automatic phase adding and phase shedding allows for minimal phase count at light load to optimize the efficiency.
Additionally, cycle-by-cycle diode emulation and pulse skipping methods will be activated if the load current is very low. As a result, the system can achieve optimized efficiency over the whole load range. Fig. 3 shows the efficiency comparison with and without the light load efficiency enhancement schemes (phase dropping, diode emulation, and pulse skipping) enabled. We can see the system efficiency is significantly improved in light load range. The start/stop system can gain more than 10% in efficiency by taking advantage of these ISL78220 efficiency enhancement schemes when compared to a typical multiphase boost converter. Moreover, ISL78220 could utilize the lossless DCR current sensing scheme, so no additional losses occur compared to resistor sensing scheme, and continued current information is sensed therefore no sample and hold circuits are required, making the system more accurate and reliable.
Figure 3. Efficiency comparison between enable and disable the light load efficiency enhancement schemes
Another automotive multiphase boost converter application lies in the infotainment system, where high power car audio amplifiers often need a main supply rail in the 25-50V range with the ability to supply peak power levels approaching 800W. The solution becomes much simpler with a multiphase boost controller. Splitting the power stage into multiple paralleled phases reduces stress on the power components, speeds up the response to load and improves system efficiency. Fig. 4 shows a typical system configuration for the automotive audio amplifier system.
Figure 4. Power supply diagram of a Class-D full bridge power amplifier. For higher resolution, click here.
There are several special requirements for automotive audio amplifier applications. First of all, when the battery voltage is reducing due to slow discharge, the audio amplifier’s output power could be set to reduce accordingly, keeping enough energy in the battery to start the car. The ISL78220 contains a dedicated VREF2 input pin that could be connected to any analogue signal and the internal reference will follow VREF2 pin voltage when it is below 2V. ISL78220 can also provide the tri-level PWM signal for the external driver to turn off both the lower and upper MOSFETs at the same time in synchronous boost structure, and therefore prevent the current flow from output to the input – the so called “energy pumping” issue in audio environment.
In the hybrid electrical vehicle (HEV) and electrical vehicle (EV) systems, a 200-400V high voltage battery stack is used as energy storage and a 12V traditional battery is required for the legacy systems. The charging of the high battery pack is done trough an isolated AC-DC and the charging of the low battery pack is done through an isolated DC-DC. Given the large fluctuation of the high voltage battery pack, a pre-regulator is normally inserted between the low voltage battery and the input of the isolated DC-DC converter, such that the transformer designs can be optimized. Fig. 5 illustrates the block diagram of HEV/EV system.
Figure 5. Block diagram of HEV/EV system
Fuel cell powered electric vehicles require an energy storage device to start up the fuel cells and to store the energy captured during regenerative braking. The fuel cell is the main power source but its power density is low, so a storage unit such as battery must be integrated with this system to supply the peak power demand during transient conditions. Low voltage batteries are preferred as the storage device to maintain compatibility with the majority of today’s automobile loads, while the voltage of the fuel cell is on the order of 60V (for the commercially available 10kW module). A DC-DC converter is therefore needed to interface the low-voltage batteries with the fuel cell–powered higher-voltage DC bus system. Fig. 6 illustrates the block diagram of fuel cell interface system.
Figure 6. Block diagram of the fuel cell interface system
Intersil’s ISL78220 is a good candidate for both HEV/EV and fuel cell applications. For battery-powered systems, the most important thing is to monitor and control the battery charging current. The total current flowing out of ISL78220’s IOUT pin is proportional to the summed sensing current of all the phases (i.e. the input current of the multiphase boost converter). Simply connecting the IOUT pin to the VREF2 pin via a simple voltage divider filter, the total current can be accurately monitored and controlled. On the other hand, the ISL78220’s dedicated PWM invert pin makes the system very flexible. By connecting this pin to VCC or to GND, the ISL78220 can be configured as either a multiphase boost controller or multiphase buck controller, respectively. Synchronous boost or buck structures can be used such that the current can flow in both directions.
In summary, the increasing power density and output power requirements makes the automotive power regulation design more challenging in the automotive market. The multiphase DC-DC converter structure can ease the automotive design in many aspects. The reduced ripple current of multiphase operation allows it to reduce EMI and boost efficiency over the load range as compared to single-phase alternatives. The ISL78220 from Intersil is the industry’s first multiphase boost controller that is specifically designed for automotive start/stop systems, car audio systems, and HEV/EV/fuel cell systems. By achieving TS16949 certification, Intersil demonstrated its commitment to quality for the design, manufacturing and distribution of power management and analog products in these systems, to move the vehicle toward its highest reliability and efficiency.
About the authors:
Ken Lenk is senior marketing manager for Automotiave Products at Intersil Corp. A BSEE graduate of Penn St. University, he was formerly employed by Freescale Semiconductor, Tyco Electronics and Maxim Integrated Products.
Niall Lyne is the Automotive Product Line Director at Intersil. He is responsible for the company’s product and technology development of ICs for the Automotive industry. He has more than two decades of hands-on engineering and management experience in the semiconductor industry.
Greg Miller is Intersil’s Vice President of Applications Engineering and Advanced Power Architectures. He received his BSEE from Rochester Institute of Technology in 1987 and his MSEE from Syracuse University in 1994. He has 20 years experience in the Power Electronics field with a focus on low voltage, high current dc-dc converters.
Jifeng Qin is a senior application engineer and has been with Intersil since June 2008 He received his BSEE degree from Zhejiang University, China and MSEE degree from North Carolina State University, USA.
Zaki Moussaoui holds a bachelor degree in Mathematics, Masters Degree in control system, and PhD in Power Electronics. He has sixteen years experience in the Power Electronics field and is currently a Sr. Applications Engineering Manager.