One of the most exciting aspects of the new USB type-c standard is its power transmission component. Powered by USB, the device can successfully get more power, enabling features that were previously impossible. Portable devices such as mobile phones, tablets and laptops will be able to charge faster. High-power devices such as displays will be able to obtain power and data over the same cable.
The number of devices and hosts is still relatively small, but it is increasing. With the popularity of USB type-c devices, consumers also want to use them at home and on the go, especially in cars.
Automotive systems have a unique set of requirements and design barriers that go beyond USB power requirements. Table 1 shows typical voltages in automotive systems.
Table 1 Typical voltages in automotive systems
The automotive system will use protection and/or regulation of the input to limit the voltage of the load. This voltage is typically limited to two times higher than the truck voltage or to the battery voltage, but below 40V. Input protection allows input voltages between 3 and 40V.
The USB Type-A device operates only at 5V, so the buck converter can create a charging or communication port. However, the USB Type-A system does not work under start-up conditions. In the past, this was not a big problem because the driver of the vehicle only started the vehicle once and it took only a short time to start. However, with the adoption of the stop-start idle speed, this interruption has gradually become a serious problem. Imagine that you are sitting in the car listening to the music played on your phone, and the music is interrupted each time the car starts and stops. The USB supply allows voltages between 5V and 20V, and providing the proper voltage for the load is a real problem.
A simple buck converter is no longer able to perform a power conversion. A simple boost converter is not enough for a specific input and output voltage range. Automotive system designers need a converter that can step down or boost depending on operating conditions.
Some topologies conform to these standards, including single-ended primary inductor converters (SEPIC), flyback or non-inverting buck-boost. The non-inverting buck-boost category also includes the option of a two-switch or four-switch. Figure 1 shows a simplified schematic of each topology.
Figure 1. Simplified schematic of a non-inverting buck-boost topology
Each of these topologies contains trade-offs, as listed in Table 2.
Table 2: Tradeoffs when choosing a non-inverting buck-boost topology
The performance of SEPIC and flyback converters is highly similar; however, SEPIC's clamped input voltage and off-the-shelf inductance make it more attractive for automotive applications. The dual-switch buck-boost is power limited and has a range similar to SEPIC, but for dual-switch buck-boost controllers, there are few options available. Therefore, SEPIC can only be used for low power applications (5-50W), while four-switch buck-boost is used for high power applications (30-100W).
Many applications in the automotive environment can benefit from USB power, including:
· Up to 100W charging port in vehicles (5, 9, 15 or 20V).
· Infotainment port for charging and receiving data from portable devices.
• Infotainment output ports—for example, ports that connect to the rear monitor of the vehicle and provide power and data through the same cable.
· Diagnostic port for power and data from the car.
The USB type-c has not been applied to the vehicle, but will soon be available. There are still many power barriers that need to be addressed to ensure a trouble-free user experience. Changes in input and output voltages make non-inverting buck-boost power supplies an ideal choice. For low cost and low power solutions, SEPIC may be the right choice. For power and more efficient designs, the four-switch buck-boost method provides a very attractive solution. The number of applications powered by USB in a car environment will only increase and will not decrease, which means that the opportunities for power supply design will be more diverse.
For more information on power tips, check out the Power Tips blog series on TI Power Home.
other information1. Singh, Atul Automotive Backlight LED Power Supply Dump and Start Protection, TI Application Report (SNVA681), March 2015.
2. Download the LM5175 and LM5118 data sheets.
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