How to design a low-cost power supply for automotive electronic systems

Automotive electronics design must provide a more competitive cost advantage in the face of multiple design requirements such as performance, reliability, and time to market. This article will address these design requirements to discuss how to design a low-cost power supply for automotive electronics systems, as well as system design (and redesign), reliability, and chip cost, and explore solutions to problems.

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The new car uses more and more sophisticated electronic components. This trend has led chip makers to face many new challenges when developing new chips: automotive electronic systems have extremely harsh working conditions, such as longer periods of service, wide operating temperature range, and electromagnetic interference. And electromagnetic compatibility, even mechanical properties and other issues need to be resolved. Under the pressure of the overall cost of the chip product, the technical need to meet the strict requirements like military products. The factory car must be safe and reliable, and can maintain the best performance state for a long time to meet the user's requirements. In addition, the design cycle of the car must be shortened to ensure that the new car can be launched as soon as possible.

Design Flow

National Semiconductor has long introduced a very versatile WEBENCH online design tool to assist system design engineers in designing power supplies. Based on this tool, the system design engineer can input the design parameters of the highest and lowest input voltage, output voltage and current, temperature range, etc. to WEBENCH even if the design of the power supply is not deep. The design software will select some suitable for them according to these parameters. The power management chip required for its design.


Figure 1: Circuit diagram of the airbag system.

System designers can then pick the ones that best fit their needs, including the latest Simple Switchers series of switching regulator chips with power switches and controllers. After the chip is selected, WEBENCH's design software provides engineers with a system solution with a circuit diagram so that engineers can use the circuit diagram for simulation testing to test the electronic characteristics of the system and its ability to dissipate heat.

The WEBENCH online design tool is very easy to use, and the system design engineer can quickly complete the power supply design by using this design tool. The database of this design site stores hundreds of chips and all design tools support a variety of different architectures. After the design is completed, the engineer can refer the entire design to a colleague or supplier. They can even order the prototype board online. The supplier guarantees that the product can be delivered to the customer within a few days. The required discrete components will also be associated with the template. And send it out, engineers only need to solder discrete components on the board, which ensures that the product will be launched faster.

There is no need to pay very expensive simulation tests and licensing fees to use this design tool. From the idea of ​​determining the technical parameters to the circuit layout, the entire design process is supported. Engineers can pick the right chip for simulation testing in terms of electronics and thermal performance.

System designers can save design and debug time by using this design tool, and save the company more development costs. In addition, many companies are good at designing digital systems, but they don't know much about the design of power supplies. In the past, they may need to hire outside consultants to complete this work, but now they only need to use WEBENCH design tools, so they do not need to seek foreign aid, so that the expenses can be saved.

New design challenges

Quiescent current

The development of the automotive industry is on the rise, and new challenges are constantly emerging. For example, the quiescent current standards required for automotive electronic systems are becoming more stringent. More and more automakers are asking chip suppliers to provide ECU chips with quiescent currents below 100uA. This is because the quiescent current is not low enough for the car to After several weeks of continuous parking, the battery in the car will not be restarted due to the lack of use for many days. One way to solve this problem is to shorten the power supply line between the battery and the ECU chip. However, starting this switching regulator still consumes a certain amount of power because the switching regulator is fabricated using metal oxide semiconductor (MOS) technology, and the switch generates a small resistance when it starts. Since a large amount of current is to be output, many switches are required to increase the power consumption. The reason why this scheme is rarely used is also here.

2. Load dump

The second problem to be faced is load-dump (load-dump, load dump will occur immediately at the start of the car engine, from battery power to generator power supply, 40V to 60V with a duration of 100mS to 500mS) Conversion), the solution is completely different. In the past, National Semiconductor customers required the chip to have overvoltage protection. However, more and more automotive electronic systems, such as the steering wheel power control system, have already started working when the engine is ignited, so the chip must be able to operate normally during a load dump. For this reason, National Semiconductor is currently re-adding overvoltage protection for several of the low dropout regulator series. The LM9070, LM9071 and LM9072 chips are several low-dropout regulators with this capability. For versions with a wider input voltage range, two letters of HV are added after these models for identification.

3. 42 volt power bus


Figure 2: Power distribution structure of the airbag system - system circuit diagram.

It is expected that in the next few years, the new car will use a 42 volt power bus. The benefit of using a 42 volt power bus is to reduce the car's power consumption and cable weight. However, if the supply voltage is increased to 42 volts, the conversion efficiency of the power supply will decrease, and the result will not be worth the candle.

a. Efficiency considerations

If we use a 3.3 volt battery to supply the required 100mA power to the electronic system, the actual power consumption of the system is only 330mW. If the low-dropout regulator chip is powered by a 12-volt battery, the overall power consumption of the system will not be less than 12V*100mA=1.2W even if the quiescent current is not counted. For a system with a 42 volt battery, the system power consumption is equal to 42V*100mA=4.2W. In other words, the low-dropout regulator uses a 42-volt battery to provide regulated power to the electronic system, which is three times less efficient than a conventional 12-volt battery. The efficiency of the low-dropout regulator is proportional to the output voltage/input voltage (Vout/Vin), and its efficiency (=(Vout*Iout)/(Vin*(Iout+Iq)). But the step-down DC/DC converter The conversion efficiency will be higher. The LM2675 can operate up to 90% with an input voltage of 12 volts and 82% with an input voltage of 40 volts. The conversion efficiency of the LM5007 chip can be as high as 93%.

b. Power consumption considerations

The low-dropout regulator chip is very inefficient, which is more likely to cause waste of energy, and the energy dissipated will accumulate, causing the ambient temperature to rise, so that the junction temperature of the chip will also rise. We can use the following formula to calculate the approximate increase in temperature:

P (wasted power) = (Tj-Ta) / ((j, a) = (Vin-Vout) * Iout

c. Chip design to meet future needs

A prudent system design engineer should look to the future and strive to meet the needs of future generations. They should pick a regulator that can support a 42-volt battery from now on, so that it won't need to redesign the power system from scratch in a few years. The LM2936HV low-dropout regulator chip and the LM295X switching regulator chip are designed for the 42-volt supply bus. These two series of regulator chips and related devices are available at The .html page has a detailed introduction.

Airbag power supply system

Even small cars currently have six airbags and the safety standards required are extremely high. Responsible for inflating the airbag is a special chip called a squib driver that must be able to start immediately at the emergency of the crash. According to Figure 1, the airbag system consists of several parts.

The squib driver is located inside the compartment and the battery is located under the hood. When the vehicle crashes, the line connection between the squib driver and the battery may be disconnected due to the impact, so the safety capacitor is usually placed next to the squib driver so that there is enough power stored nearby to provide power to inflate the airbag. .

The following briefly describes the inflation process of the airbag system. The airbag system is equipped with boost converters. In general, these boost converters use a sepic topology (these features require two inductors in the circuit) or a flyback topology (a transformer is required in the flyback topology). Before inflating, the booster converter of the airbag system first raises the battery voltage (Vbat) (this battery voltage can be as high as 40 volts during load dump) until it reaches the level required by the safe voltage (Vsafe), which ensures The safety capacitor stores a large amount of electrical energy (see Figure 2). This higher safety voltage is then turned down to a few volts. This lower voltage is called the remote voltage (Vremote). This charge preparation procedure must be followed exactly to ensure that there is less pressure drop in the lower-level regulators of the subsequent stages. The lower the voltage drop, the lower the power consumption and the higher the efficiency.


Figure 3: Power distribution structure of the air bag system - system circuit diagram

Due to the low value of Vremote, the low-voltage drop regulators of the subsequent stages can meet the relevant requirements even if they are not automotive-grade products. The LP2985 series is a voltage regulator of this type with an input voltage of up to 16 volts, an output current of 150 mA, and a junction temperature of up to 125 °C, making it suitable for most applications.

The LM9076 regulator chip is suitable for use in higher temperature environments with junction temperatures up to 150 °C. This regulator can output 150mA and 5V with an input voltage of 8V. This regulator chip maintains extremely high output voltage accuracy (up to 2%) over the rated operating temperature and load range. Because this structure uses a safety capacitor, it is safer and more reliable when working. From a cost perspective, the use of a remote low-dropout regulator eliminates the need for additional heat sinks, which helps save costs. Thanks to Vsafe (Vbat (highest value), and the primary converter can be used as a boost converter, there is no need to use a more complex set of sepic or flyback converters. The buck converter is suitable for buck regulators. The latter is also less expensive. Since low-dropout regulators do not require high-voltage power supplies, there are many different CMOS or low-voltage bipolar chips to choose from.

Multiple different voltage outputs are available

If we use a switching regulator as the input stage and then provide output with multiple low-dropout regulators, we can reduce the overall cost of the power system. High-power power systems must even adopt such a design. Figure 4 is one of the examples.


Figure 4: Configuration that provides multiple outputs

When determining the most ideal local voltage (Vlocal), the following parameters must be considered in detail:

Vlocal must be low enough to ensure that the low-dropout regulator does not dissipate too much power. Iout (highest value) = (Tj-Ta) / (((j, a) * (Vin-Vout))

The low-dropout regulator stabilizes the voltage only if the overall voltage exceeds the specified voltage drop, so Vlocal must be higher than Vx+VDox.

Summary of this article

Proper design of the power supply will help reduce the overall cost of system development. If the application environment is extremely harsh, for example, when the battery voltage, output current, and temperature are extremely high, we must consider a multi-conversion stage structure. There are many regulator chips available on the market for automotive power supply systems. The use of free online design tools such as WEBENCH also helps reduce design costs and time-to-market. Automakers are hoping to shorten the design cycle of new cars, so regulator chips will be more popular with automakers. Even in the same car, different subsystems have different limitations in practical applications, so engineers who design these electronic systems must have a good understanding of this aspect.

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