In this article, we will discuss some design techniques to achieve higher power density without affecting performance.
Infotainment Power Architecture
Many infotainment power designs follow a similar architecture. Automobile battery is used as the input of power supply, and usually works in a wide input voltage range due to cold start and sudden load drop conditions. The battery supplies power to the wide input voltage step-down converter, which outputs the intermediate bus voltage. The common intermediate voltage is 5V or 3.3V. The power rail supplies power to downstream devices such as LDO and low input voltage step-down converter, which can generate the required power for various loads. Examples of these loads include network protocol interfaces, connection modules, and sensors. Input filters are usually added to the front end of off battery buck converters to mitigate EMI challenges at specific frequencies.
Power tree example for infotainment applicationsAs shown in Figure 1As shown in. The load switch is used to mediate between wide input and low input step-down. This helps to reduce quiescent current consumption, thereby maximizing battery life. In addition, linear voltage regulator (LDO) is used for 3.3v/10ma power rail. For such a low current rail, it is meaningful to use LDO instead of Buck Converter in order to save design cost and space.
Figure 1The power tree shows the power supply mode of infotainment system in automobile design. Source: Texas Instruments
Some of the technologies used by power designers to increase the power density of such solutions are taking advantage of higher switching frequencies (considering and reducing the main power loss sources in the design) and compact layout technology.
Switching frequency and passive component dimensions
One way to increase the power density is to increase the switching frequency of the overall solution. In the buck converter, each passive component in the circuit stores and releases energy in each switching cycle. At faster switching speeds, the amount of energy buffered per cycle will be reduced. Higher switching frequencies can produce smaller passive components, such as capacitors and inductors. Due to the small input voltage ripple, the input capacitance can be reduced. Because the loop bandwidth is faster, the output capacitance can also be reduced.
The inductance is inversely proportional to the switching frequency, as shown in the following formula:
L = (V OUT – VI N ) * D) / F sw * Δ I L = V L * D / F sw * Δ I L
Where l = inductance, d = duty cycle, f SW = switching frequency, I L = inductor current ripple, V L = voltage at both ends of inductor (can also be written as V out – V in). In the solution of infotainment power tree in Figure 1, the switching frequency of all converters is 2.1 MHz.
Increased power loss
Unfortunately, increasing the switching frequency comes at the cost of increasing power loss. The power loss of each regulator and its related components will determine how much power density we can actually increase.Figure 2Shows the main loss types of various external components in the power supply circuit.
Figure 2Common loss types in power circuit components. Source: Texas Instruments
In addition to optimizing the above external components, pay attention to the thermal performance of the package when deciding which IC to use. The better the heat dissipation effect of a package, the greater the power loss you can bear without seeing an extreme rise in temperature. A special consideration for automotive systems is to select components and passive components that meet automotive standards. These devices meet automotive reliability and robustness requirements and may include functions for improving EMI, such as spread spectrum modulation.
Basic layout skills
Even the best designed power solution will not work well if placed in a less ideal layout. After maximizing power density at the schematic level, we still need to mitigate possible problems due to improper placement and routing of parts. One of them is EMI.
In the synchronous buck converter, the conducted radiation is caused by the change of voltage with time (DV / DT) and the change of current with time (di / DT) caused by switching action. These waveforms contain higher harmonics and can be easily coupled to other devices on the circuit board. As we increase the switching speed, dealing with EMI becomes more complex because there will be more sudden changes in voltage or current levels.
Figure 3Shows the layout of the infotainment power tree in Figure 1. The color box around the PCB assembly corresponds to the color of the block diagram in Figure 1. The compact solution size of the layout is 1.20 inches x 1.06 inches, and no components are placed at the bottom of the PCB.
Figure 3The layout size of infotainment power solution is 1.20 inches x 1.06 inches source: Texas Instruments
When arranging components, keep input connectors away from any potential noise sources. This helps to avoid bypassing front-end filtering through parasitic elements. stayIn Figure 4, the input connectors are marked in red. The EMI filter is marked in pink and the input voltage of the wide input converter is marked in yellow. Grounding shielding around the filter also helps to reduce EMI and isolate the filter from other noise components.
Figure 4This is what the EMI front-end filter layout looks like in the power supply design. Source: Texas Instruments
The designer should also pay attention to minimizing the inductance in the high frequency switching loop of the step-down converter. The path includes an input capacitor, a high side FET, a low side FET, and a ground loop to the input capacitor. In this particular infotainment system, one of the four load point (POL) converters (U4) is used asIn Figure 5AExamples of. The input capacitor (C19) and high frequency input capacitor (C22) are placed as close to the IC as possible to minimize loop inductance. These capacitors are marked in red and the minimized critical path is marked in yellow. The high side and low side FETs are integrated into the IC.
Figure 5The capacitor is placed close to the IC (top 5a) and the converter moves to the right (bottom 5b). Source: Texas Instruments
As shown in Figure 5BAs shown, this converter and other similar converters move to the right of the overall solution to maximize the efficiency of EMI filters and increase layout compactness.
Meeting EMI requirements is one of the most challenging parts of power supply design. Therefore, although it is a good practice to configure filters in the design, it is likely that the filter components will need to be adjusted during board testing to meet specific EMI standards.
Figure 6Shows the physical boards built and tested for this power solution.
Figure 6This PCB solution is built and tested for infotainment power supply system. Source: Texas Instruments
stayIn Figure 7, we show the thermal image of the circuit board to prove that we can obtain good thermal effect even if the layout is compact. After operating the circuit board without air flow for 10 minutes, the hottest temperature is 69.3 ° C. See pmp22648 reference design for more details.
Figure 7In the thermal image at the top of this board, V in = 13.5 V and all power rails are at maximum load. Source: Texas Instruments
As we can see in this article, the focus of today’s automotive infotainment system is to install the solution in a small area while still achieving high performance. Focusing on key design considerations such as switching frequency and power loss will enable you to optimize individual components for compact size. Followed by the use of good layout technology to reduce the main source of EMI, which will be the key to achieve high power density and high-performance solutions.