How to design low-cost power supply for automotive

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How to design low-cost power supply for automotive electronic system

more and more sophisticated electronic parts are used in new vehicles. This development trend makes chip manufacturers face many new challenges when developing new chips: the working environment of automotive electronic systems is extremely harsh, such as the long service life of cars and a wide range of working temperatures; There are also electromagnetic interference, electromagnetic compatibility, and even mechanical properties that need to be solved. Under the condition that the overall cost of chip products is under great pressure, the technology also needs to meet strict requirements, such as the cube made of military materials with a side length of 1 cm. The car leaving the factory must be safe and reliable, and can maintain the best performance state for a long time, so as to meet the requirements of users. In addition, the design cycle of cars must be shortened to ensure that new cars can be launched into the market as soon as possible

design process

the price and factors affecting the price of the national semi conducting hydraulic testing machine in the United States. The company has long introduced a set of widely used design tools on WEBENCH to assist system design engineers in designing power supplies. Based on this tool, system design engineers can input design parameters such as maximum and minimum input voltage, output voltage and current, and temperature range into WEBENCH even if they do not have a deep understanding of power supply design. The design software will select some power management chips that meet their design needs according to these parameters

As long as the system is used in strict accordance with the instructions,

system design engineers can then select the one that best meets their needs from these chips, including the newly launched simple switches series switching regulator chip with power switch and controller. After the chip is selected, WEBENCH's design software will provide engineers with a system solution with circuit diagrams, so that engineers can use the circuit diagrams to conduct simulation tests to test the electronic characteristics of the system and its heat dissipation capacity

the design tool on WEBENCH is very easy to use, and the system design engineer can quickly complete the design of the power supply by using this set of design tools. The database of this design station stores the information of hundreds of chips, and all design tools can support a variety of different architectures. After the design is completed, the engineer can refer the whole design to colleagues or suppliers. They can even order prototype circuit boards on the. The supplier guarantees that the relevant products can be delivered to the customers within a few days, and the required discrete components will be sent together with the template. The engineer only needs to weld the discrete components on the circuit board, which can ensure that the products can be launched to the market faster

there is no need to pay very expensive simulation testing and licensing fees to use this set of design tools. From the determination of technical parameters to the idea of circuit layout, the whole design process can be supported. Engineers can select appropriate chips for simulation tests of electronic and thermal performance

as long as the system design engineer adopts this set of design tools, it can save design and debugging time, and also save more development costs for the company. In addition, although many companies are good at designing digital systems, they do not have a deep understanding of power supply design. In the past, they may need to hire an external consulting company to complete this work, but now as long as they use the WEBENCH design tool, they don't need to seek foreign assistance, so that they can save money in this area

new design challenges

(1) quiescent current

with the rapid development of the automotive industry, new challenges continue to emerge. For example, the quiescent current standard required by automotive electronic systems is becoming more and more stringent. More and more automotive manufacturers require chip suppliers to provide them with ECU chips with a quiescent current lower than 100ua. This is because if the quiescent current is not low enough, the battery in the car will not restart after the car has been parked for several weeks. One way to solve this problem is to shorten the power supply line between the battery and ECU chip. However, starting this switching regulator will still consume a certain amount of power, because the switching regulator is made of metal oxide semiconductor (MOS) technology, and the switch will produce a small resistance when starting. Since a large amount of current needs to be output, many switches need to be installed to increase the power consumption accordingly. This is also the reason why this scheme is rarely used

(2) sudden load drop

the second problem to face is the sudden load drop (load dump, when the car engine starts, there will be a sudden load drop, which will switch from battery power supply to generator power supply, and there will be a voltage conversion of 40V to 60V lasting from 100ms to 500ms). The solution method is completely different. In the past, customers of national semiconductor required that the chip must be equipped with overvoltage protection function. However, at present, more and more automotive electronic systems, such as the driving wheel power control system, have been working since the engine was ignited, so the chip must be able to operate normally when the load drops suddenly. For this reason, national semiconductor is currently adding overvoltage protection function to several models of low-voltage drop regulator series. Lm9070, lm9071 and lm9072 chips are several low-voltage drop regulators with this function. For versions with wider input voltage range, two letters HV will be added after these models as identification

(3) 42 volt power supply bus

it is expected that in the next few years, new vehicles will adopt 42 volt power supply bus. The advantage of using 42 volt power bus is that it can reduce the power consumption of the car and the weight of the cable. However, if the power supply voltage is increased to 42 volts, the conversion efficiency of the power supply will decline, and the result is that the gain is not worth the loss

a. efficiency considerations

if we use a 3.3V battery to provide 100mA power supply for the electronic system, the actual power consumption of the system is only 330MW. If the low-voltage drop 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 included. For the system with 42 volt battery, the system power consumption is equal to 42v*100ma=4.2w. In other words, if the low-voltage drop regulator uses a 42 volt battery to provide regulated power supply for the electronic system, its efficiency is more than three times lower than using a traditional 12 volt battery. The efficiency of the low-voltage drop regulator is directly proportional to the output voltage/input voltage (vout/vin), and its efficiency (= (vout*iout)/(vin* (iout+iq)). However, the conversion efficiency of Buck DC/DC converter will be higher. The efficiency of lm2675 chip can reach 90% if it works with 12 V input voltage, and 82% if it works with 40 V input voltage. The conversion efficiency of LM5007 chip can be as high as 93%

Figure 1. Efficiency of lm2675 step-down DC/DC converter chip

b. consideration of power consumption. The efficiency of low-voltage step-down regulator chip is very low, which is easy to cause a waste of energy, and the dissipated energy will accumulate together, causing the ambient temperature to rise, so that the junction temperature of the chip is also c) there is no vibration around, which will rise. We can use the following formula to calculate the approximate temperature rise:

P wasted power = (TJ TA)/θ (j,a)=(Vin﹣Vout) × IOUT

C. chip design that can meet future needs

prudent system design engineers should look to the future and strive to meet the needs of future generations. They should choose voltage regulators that can support 42 volt batteries from now on, so as not to redesign the power system from scratch in a few years. Lm2936hv low voltage drop regulator chip and lm295x switching regulator chip are designed for 42 volt power supply bus. These two series of regulator chips and related devices are introduced in detail on this page

airbag power supply system

even small cars currently have six airbags, and the safety standards adopted are extremely high. Responsible for inflating the airbag is a special chip called squib driver, which must be able to start immediately in the emergency of a crash. As shown in Figure 2, the airbag system is composed of several parts

Figure 2 circuit diagram of airbag system

the tube burst driver is located in the compartment, and the battery is located under the bonnet. During a crash, the line connection between the burst tube driver and the battery may be disconnected due to the impact. Therefore, the safety capacitor will generally be set next to the burst tube driver, so that there is enough power storage near the driver to provide power to inflate the airbag

the inflation process of the airbag system is briefly introduced below. The airbag system is equipped with boost converters. Generally speaking, these boost converters adopt SEPIC topology (SEPIC topology requires two inductors in the circuit) or flyback topology (a transformer is required in the flyback topology). Before inflation, the boost converter of the airbag system will first increase the battery voltage (Vbat) (this battery voltage can be as high as 40 Volts when the load drops suddenly) until it reaches the level required by the safety voltage (vsafe), which can ensure that the safety capacitor stores a large amount of electric energy (see Figure 3). After that, the higher safety voltage is reduced to a few volts, which is called remote voltage (vremote). This inflation preparation procedure must be strictly followed to ensure that there is less pressure drop in the subsequent low-pressure drop regulators. The less the voltage drop, the lower the power consumption and the higher the efficiency

Figure 3 power distribution structure of airbag system - system circuit diagram

due to the low value of vremote, the input voltage range of subsequent low-voltage drop regulators can meet the relevant requirements even if they are not automotive products. Lp2985 series chips belong to this kind of regulator. Its input voltage is as high as 16 volts, output current is 150mA, and junction temperature can be as high as 125 ℃, which is suitable for most applications

lm9076 regulator chip is suitable for higher temperature working environment, and its junction temperature can be as high as 150 ℃. This voltage regulator can output 150mA current and 5V voltage with 8V input voltage. Within the rated operating temperature and load range, this voltage regulator chip can maintain extremely high output voltage accuracy (up to 2%). Because this structure adopts safety capacitor, it is more safe and reliable when working. From the perspective of cost, the use of remote low-voltage drop regulators does not need to add any radiators, so it helps to save costs. Since vsafe (Vbat (maximum value) and primary converter can be used as boost converter, SEPIC or flyback converter with complex layout is not required. Buck converters are suitable for Buck Regulators, which are also low cost. Since the low-voltage drop regulator does not need to use high-voltage power supply, there are many different CMOS or low-voltage bipolar chips to choose from

there are many different voltage outputs to choose from

if we use a switching regulator as the input stage, and then use multiple low-voltage drop regulators to provide output, we can reduce the overall cost of the power supply system. The high-power power supply system must even adopt such a design. Figure 4 is an example

Figure 4 can provide multiple output configurations

when determining the optimal local voltage (vlocal), the following parameters must be considered in detail:

vlocal must be low enough to ensure that the low-voltage drop regulator will not dissipate too much power, IOUT (maximum) = (TJ TA)/( θ (j,a) × (Vin﹣Vout))。

the low voltage drop regulator will stabilize the voltage only when the overall voltage exceeds the specified voltage drop, so vlocal must be higher than vx+vdox

this paper concludes that if the power supply is properly designed, it will help to reduce the overall cost of system development. If the application environment is extremely harsh, such as battery voltage, output current and

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