Automotive body electronics and gateway modules must operate without interruption, regardless of variations in the car battery. A car’s battery voltage can drop down to below 3V during cold crank and may surge as high as 40V during load dump, thus necessitating a DC/DC power supply to both step up and step down and maintain a regulated operating voltage from 5V to 12V.
In addition, DC/DC power supplies should have a small solution size in order to save space, operate with low quiescent current for minimal drainage of the car battery, and be capable of 2MHz switching to avoid electromagnetic interference (EMI) in the AM band. In this blog post, I will compare typical conventional DC/DC power-supply solutions and examine the advantages offered by integrated wide input voltage (VIN) buck-boost converters.
In order to produce the regulated voltage of 5V or 12V, the DC/DC power supply should be able to boost up the battery voltage from as low as 3V during cranking and buck down the battery voltage when it is higher than the required output voltage.
Figures 1 through 4 show simplified power architectures of the conventional solutions. For presentation simplicity, these figures use diodes as the rectifiers, but you can replace them with power metal-oxide semiconductor field-effect transistors (MOSFETs) for higher power-conversion efficiency.
Figures 1 and 2 offer straightforward and quick solutions by cascading two power stages, using one buck and one boost. You can save development time when both the buck and boost stages are existing designs. A main drawback is the low efficiency caused by the two conversion stages. For instance, even if each stage can achieve 90% efficiency, the overall efficiency is the product of the two, yielding 81%. Another drawback is the relatively higher bill-of-materials (BOM) cost and larger solution size – owing to the use of two power inductors, two controllers and more peripheral components, which may duplicate some functions of the two controllers.
Figure 3 shows a single-ended primary-inductor converter (SEPIC) converter, achieving the power conversion in a single stage. However, it still requires two inductors. Although you can use coupled inductors to replace two separate ones, the former undoubtedly costs more than the latter. The AC coupling capacitor that the SEPIC requires also adds to the BOM cost.
Figure 4 shows a buck-boost converter. It is a single-stage converter and only needs one power inductor. The nonsynchronous rectifier reduces overall efficiency.
Figure 1: Conventional architecture No. 1: pre-boost followed by buck
Figure 2: Conventional architecture No. 2: buck followed by post-boost
Figure 3: Conventional architecture No. 3: SEPIC
Figure 4: Conventional architecture No. 4: buck-boost
Using the synchronous rectifiers as shown in Figure 5 improves the efficiency, but four external MOSFETs raises the BOM cost. There are also challenges associated with difficulty in the printed circuit board (PCB) layout and the routing of power components for optimal performance.
Figure 5: Synchronous buck-boost
Highly integrated wide VIN buck-boost converters
A buck-boost converter with all four power MOSFETs integrated with the controller overcomes the drawbacks of conventional solutions. Figure 6 is a simplified block diagram of an integrated buck-boost converter. Typical products include TI’s TPIC74100-Q1, TPS55060-Q1 and the recently released TPS5516x-Q1 family. These products support a maximum load of up to 1A in a miniature solution size.
Figure 6: Integrated single-stage single-inductor solution: buck-boost converter
An application example
Figure 7 shows a typical solution with the TPS55165-Q1 for 5V at 1A applications. The full solution requires less than a dozen external components.
Figure 8 shows the miniature solution size. For reference, the integrated circuit (IC) size is 6.5mm by 4.5mm. It’s possible to further reduce the solution size for double side mounting.
Figure 9 shows the circuit’s performance under cold cranking. You can see that the output voltage is solidly regulated at 5V even during transient conditions.
Figure 7: A complete solution with the TPS55165-Q1 for a 5V/1A application
Figure 9: Battery voltage cranking response (conditions: IOUT = 0.5A, VIN transient: 12V to 4V)
Four-switch buck-boost converters are optimal for automotive applications by using a single conversion stage, a single power inductor and a minimal number of external components. With 2.2MHz frequency switching, 15µA low IQ operation, optional spread-spectrum technique and a miniature solution size, the highly integrated wide VIN TPS5516x-Q1 family is a good fit for body control and gateway modules.
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