Power Tips #75: USB Power Delivery for automotive systems
One of the most exciting aspects of the new USB Type-C standard is the power delivery portion. With USB Power Delivery, devices can negotiate for more power, thus enabling features that were previously not possible. Portable devices like phones, tablets and laptops will be able to charge faster. Higher-power devices like monitors will be able to receive both power and data over the same cable.
The number of devices and hosts is still relatively low, but momentum is building. As the popularity of USB Type-C devices increases, consumers will want to use them at home as well as on the go, particularly in automobiles.
Automotive systems have a unique set of requirements and design obstacles beyond the requirements for USB Power Delivery. Table 1 shows the typical voltages in automotive systems.
Table 1 Typical voltages in automotive systems
Automotive systems will use protection and/or regulation on the input to limit the voltage that loads see. This voltage is usually limited to voltages above truck voltages or double the battery voltages, but below 40V. Input protection leaves you with an input-voltage range of 3-40V.
USB Type-A devices operate at 5V only, so a step-down converter can create a charging or communication port. A USB Type-A system does not work during cranking conditions, however. In the past, that was not a huge deal because vehicle operators only cranked vehicles once, and cranking takes only a short time period. But with the adoption of stop-start idling, this interruption is becoming a larger problem. Imagine that you are in your car listening to music streaming from your phone. Every time your car stops and starts, the music would cut out. With USB Power Delivery allowing voltages that range from 5V up to 20V, there is a real problem delivering the proper voltage to the load.
No longer will a simple step-down buck converter be able to perform the power conversion. Given the input and output voltage ranges, a simple boost converter will not be sufficient either. What automotive system designers need is a converter that can step down or step up the voltage depending on the operating conditions.
A few topologies meet these criteria, including the single-ended primary-inductor converter (SEPIC), flyback or noninverting buck-boost. The noninverting buck-boost category also includes options for two switches or four switches. Figure 1 shows a simplified schematic for each topology.
Figure 1 Simplified schematics for noninverting buck-boost topologies
Each of these topologies comes with trade-offs, as listed in Table 2.
Table 2 Trade-offs for choosing a noninverting buck-boost topology
The SEPIC and flyback converters offer very similar performance; however, the clamped input voltage and off-the-shelf inductors of the SEPIC make it a little bit more attractive for automotive applications. The two-switch buck-boost is limited in power to a similar range as the SEPIC, but there are very few choices available for two-switch buck-boost controllers. So that leaves the SEPIC for lower-power applications (5-50W) and the four-switch buck-boost for higher-power applications (30-100W).
Many applications within an automotive environment could benefit from USB Power Delivery. These applications include:
- Charge ports in vehicles (5, 9, 15, or 20V) up to 100W.
- Infotainment ports that charge and accept data from a portable device.
- Infotainment output ports – for example, a port that connects to a monitor in the rear of the vehicle and provides both power and data over the same cable.
- Diagnostic ports that can provide power and data from the automobile.
USB Type-C has not made its way into vehicles just yet, but it will soon. There are many power-supply obstacles to resolve along the way to ensure a trouble-free user experience. The varying input and output voltages make noninverting buck-boost supplies an ideal choice. For low-cost and low-power solutions, a SEPIC might be appropriate. For higher-power, higher-efficiency designs, a four-switch buck-boost approach can provide a very attractive solution. The number of applications for USB Power Delivery in an automotive environment is only going to increase, which means many more opportunities for power-supply designs.
For more Power Tips, check out TI’s Power Tips blog series on Power House.
- Singh, Atul. Load Dump and Cranking Protection for Automotive Backlight LED Power Supply, TI Application Report (SNVA681), March 2015.
- Download the LM5175 and LM5118 data sheets.