Automotive systems need to withstand high temperature differences, extreme input transients, and many other types of interference. Almost all electronic products in automobiles are subject to rigorous testing to meet the quality system standards and component qualifications specified by the Automotive Electronics Council (AEC). Most automotive systems use 12V lead-acid batteries, and you may know that the voltage of the battery changes almost in every situation you can think of: ambient temperature, load conditions, age, and so on.
Under normal operating conditions, the voltage can vary from 9V to 16V. Under certain working conditions, it will even be bigger. When starting the internal combustion engine, the 12V lead-acid battery must provide enough energy for the winding of the starter motor to supply a large amount of current in a short time, resulting in a sharp drop in the battery voltage. Very low temperatures can cause the battery voltage to drop to a lower level. This phenomenon is called a cold start. A typical test waveform is shown in Figure 1, where the voltage can be reduced to 3V.
Figure 1: Car battery cold start voltage curve
Suppose we are designing a 12V car audio system that is powered directly from a 12V lead-acid battery. How to keep the sound system constant input voltage? Remember, as mentioned above, the battery voltage can range from 3V to 16V. Is the buck converter useful? No. When the battery voltage is lower than 12V, the input of the audio system will drop, which may cause an undervoltage condition. Let's try the boost converter. Or not. When the battery voltage is greater than 12V, the input of the audio system will rise, which may cause an overvoltage condition and damage the electronic equipment. What we need is a buck-boost converter. As the name implies, this topology can be stepped down or boosted to a stable output voltage regardless of the input voltage. Most importantly, it provides constant energy even if the voltage fluctuates. Figure 2 shows a simple cascade boost for a buck topology.
Figure 2: Cascaded Buck-Boost Converter
There are many variations and simplifications of the buck-boost topology, and there are many advantages over the topology shown in Figure 2. For example, two converters can be simplified and share an inductor, saving board space (see Figure 3A). Want to improve efficiency? Remove the freewheeling diode and add a MOSFET to reduce conduction losses, which is a four-switch buck-boost (Figure 3B). Still worried about the lack of board space? Why not use a four-switch converter and move the MOSFET inside the control IC (Figure 3C), saving valuable PCB space.
Figure 3A: 2-switch buck-boost converter
Figure 3B: 4-switch buck-boost converter
Figure 3C: Buck-Boost Converter with Integrated MOSFET
Want to know more about some variants of the buck-boost converter? Check the new TI Power Management Lab Kit Buck-Boost Board and Lab Guide. This new learning kit focuses on the workings of buck-boost topologies and design-related challenges. The EV kit uses TI's LM5118 controller IC. The kit also provides an experimental guide for practical learning. After mastering the TI-PMLK Buck Boost lab, check out TI's other buck-boost power solutions, such as the four-switch LM5175 and TPS63060 with integrated MOSFETs.
The Applied Power Electronics Conference (APEC) will be held in Tampa, Florida from March 26th to 30th. At the time, you can see a live demonstration of the new TI Power Management Lab Kit buck-boost board and lab board at TI booth #701.
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