Automotive USB charger reference must support EMI standards and include differential EMI filters Universal Serial Bus (USB) is an industry standard that defines cables, connectors, and communication protocols for easy connection, communication, and power exchange between computers and devices. Initially, USB was used in vehicles by consumers who wanted to easily charge their electronic devices, but recent USB applications are used to handle data transfer around the car. There are three types of USB port connectors: A, B, and C. The port connector type depends on whether the system port is downstream or whether the USB cable carries data. Type A in Figure 1 is used only between hubs.
Figure 1: USB Type A connector
TI has developed a 21W dual automotive USB charger solution (Figure 2) that is typical of today’s modern vehicles. The solution includes all the front-end protection required to address issues such as double batteries, reversed battery polarity, and load dump conditions (compliant with International Organization for Standardization [ISO] 7630 Pulse 1, 2a, 3a/b), as well as support for deep cold crank conditions. The 21W dual car USB design supports 2 x 2.1A for each USB port, capable of charging two tablets or smartphones.
Figure 2: 21W Dual Port USB Charging Solution
The CISPR 25 Class 5 Rated Automotive USB Charger Reference Design with Full Front-End Protection supports stringent automotive electromagnetic interference (EMI) standards such as CISPR 25 Class 5 and includes differential EMI filters to support conducted EMI suppression. This support is especially important because any system-level noise introduced in the design can affect other electronic automotive systems. For example, poor filtering can result in a buzzing noise in the audio system.
Figure 3: Switch node LMS3655-Q1, you can see that no ringing occurs
TI designed the LMS3655-Q1 to support very low EMI switching noise. This was achieved by controlling the regulator’s switch slew rate, optimizing the PCB layout and component placement, using jittering techniques such as spread spectrum, and reducing the parasitic capacitance and inductance of the package. Limits wideband noise generation by controlling fundamental EMI generation and limiting switch node ringing.
The design makes it easier to address EMI at the system level by controlling noise generation. Since the problem does not occur in the first place, there is no need to address it later in the PCB layout or by adding additional components.
Figure 4: EMI configured on PCB without shielding
To verify the results, we tested the design in an EMI chamber under documented conditions specified in the CISPR 25 standard (see Figure 4). This design addresses one of the fundamental problems seen in other designs, which is noise leakage from the switch node to the edge connector. Since the design limits the EMI at the switch node, there is no need to add metal shielding for suppression. By eliminating such components, the design is not only more cost-effective, but also smaller, thinner, and can operate at lower temperatures.
Operating the design at lower temperatures can limit additional noise associated with high temperatures, along with the power savings. Heat limits the operating life of any system, and automotive systems must last longer than the average consumer electronics device.
Figure 5: Efficiency curve of LMS3655-Q1
The LMS3655-Q1 operates below the AM band to maximize efficiency. In addition, it is designed at the integrated circuit (IC) level by combining internal field-effect transistor (FET) selection, dead-time reduction, and other techniques to minimize parasitic resistance to operate at the highest levels of integration, thermal performance, and EMC.
USB designs are getting smaller and more integrated into a limited space. This requires smaller PCB copper for heat dissipation of individual components, and the plastic material used to contain the USB design itself generates additional heat. The LMS3655-Q1 provides high efficiency, minimizing self-heating within the design, and enables operation at higher temperatures where tight thermal constraints exist. The high level of integration minimizes size, noise, and solution cost. The product helps overcome these challenges and develop competitive solutions.
기사입력2018.05.16 15:10
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