Minimizing switching losses and optimizing components for a compact size are crucial in power designs for automotive infotainment systems.
In today’s society, it’s no surprise that people want more entertainment and information within the comfort of their own vehicles. The term “infotainment” encompasses these features. When we look at the elements that power these systems, there is a constant drive to fit more features into smaller spaces. The challenge here is that even while we aim to increase power density, we still need to maintain a high level of performance.
However, the compact layout that comes with higher power density increases the potential for challenges such as electromagnetic interference (EMI) if parts placement and routing are done poorly. In this article, we will discuss some design techniques to achieve increased power density without compromising on performance.
Infotainment power architecture
Many infotainment power supply designs follow a similar architecture. The car battery serves as the input to the power supply and often operates over a wide input voltage range due to cold crank and load dump conditions. The battery supplies a wide-input voltage buck converter which outputs an intermediate bus voltage. Common intermediate voltages are a 5-V or a 3.3-V rail. This rail powers downstream devices such as LDOs and low-input voltage buck converters which generate the needed power for various loads. Examples of these loads include network protocol interfaces, connectivity modules, and sensors. An input filter is typically added to the front-end of the off-battery buck converter to mitigate EMI challenges at specific frequencies.
An example power tree for infotainment applications is shown in Figure 1. A load switch is used at the intermediary between the wide-input and the low-input bucks. This can help to reduce quiescent current consumption in order to maximize battery life. Furthermore, a linear regulator (LDO) is used for the 3.3-V/10-mA rail. For a low current rail like this one, it makes sense to use an LDO rather than a buck converter in order to save on cost and space in the design.
Figure 1 The power tree shows how infotainment systems are powered in automotive designs. Source: Texas Instruments
Some techniques that power supply designers employ to increase the power density of such solutions are utilizing higher switching frequencies—accounting for and reducing major sources of power loss within the design—and techniques for compact layout.
Switching frequency and passive component size
One way to increase power density is to increase the switching frequency of the overall solution. In a buck converter, each passive element in the circuit stores and releases energy during each switching cycle. The amount of energy buffered each cycle will be reduced at faster switching speeds. A higher switching frequency can result in smaller passive components such as capacitors and inductors. The input capacitance can be reduced due to a smaller input voltage ripple. The output capacitance can also be reduced due to faster loop bandwidth.
The inductance is inversely proportional to the switching frequency as shown in the following equation:
L= (VOUT – VIN) * D)/Fsw * ΔIL = VL * D/Fsw * ΔIL
Where L = inductance, D = duty cycle, Fsw = switching frequency, IL = inductor current ripple, and VL = voltage across inductor (which can also be written as VOUT – VIN). In the solution for the infotainment power tree from Figure 1, all converters are switching at 2.1 MHz.
Power loss increase
Unfortunately, increasing the switching frequency comes at the cost of increased power loss. Power losses from each regulator and its associated components will dictate just how much we can actually increase power density. Figure 2 shows the major types of loss for various external components in the power circuitry.
Figure 2 Types of losses that commonly occur in power circuit components. Source: Texas Instruments
In addition to optimizing the external components above, pay attention to the thermal performance of the package when deciding which ICs to use. The better a certain package can dissipate heat, the more power loss you can afford without seeing extreme rises in temperature. A special consideration for automotive systems is to select automotive-qualified devices and passive components. These devices are qualified to meet the automotive reliability and robustness requirements and can include features for EMI improvement—such as spread-spectrum frequency modulation.
Basic layout tips
Even the best designed power solutions will not work well if they are placed within a less-than-optimal layout. After maximizing on power density at the schematic level, we still need to mitigate problems that can arise with poor parts placement and routing. One of these is EMI.
In synchronous buck converters, conducted emissions result from changes in voltage over time (dv/dt) and changes in current over time (di/dt) that come from switching action. These waveforms contain higher order harmonics that can easily become coupled into other devices on the board. As we increase switching speed, EMI becomes more complex to deal with because there are more sudden changes in voltage or current levels.
Figure 3 shows the layout for the infotainment power tree from Figure 1. The colored boxes around the PCB components correspond with the block diagram colors in Figure 1. The compact solution size of the layout is 1.20 in. x 1.06 in. with no components placed on the bottom side of the PCB.
Figure 3 The layout size of infotainment power solution is 1.20 in. x 1.06 in. Source: Texas Instruments
When laying out the components, keep the input connectors away from any potential noise sources. This helps to avoid bypassing the front-end filtering via parasitic elements. In Figure 4, the input connectors are outlined in red. The EMI filter is outlined in pink, and the wide-input converter input voltage is outlined in yellow. The ground shielding around the filter also helps with EMI reduction and isolating the filter from other noisy components.
Figure 4 This is how EMI front-end filtering placement looks like in power designs. Source: Texas Instruments
Designers should also take care to minimize inductances in the high-frequency switching loop of a buck converter. This path comprises input capacitor, high-side FET, low-side FET, and ground return to the input capacitor. In this particular infotainment system, one of the four point-of-load (PoL) converters (U4) is used as an example in Figure 5a. The input capacitor (C19) and high-frequency input capacitor (C22) are placed as close to the ICs as possible to minimize loop inductance. These capacitors are outlined in red, and the critical path to minimize is drawn in yellow. The high-side and low-side FETs are integrated into the IC.
Figure 5 Capacitors are placed close to the IC (5a on top) while converters are moved toward the right side (5b on bottom). Source: Texas Instruments
As shown in Figure 5b, this converter and the other ones like it are moved toward the right side of the overall solution to maximize the effectiveness of the EMI filter and to increase the layout compactness.
Complying with EMI requirements is one of the most challenging parts of a power design. So, while provisioning for a filter in the design is good practice, it’s very likely that the filter components will need to be tuned during board testing to meet a specific EMI standard.
Figure 6 shows the physical board that was built and tested for this power solution.
Figure 6 This PCB solution was built and tested for the infotainment power system. Source: Texas Instruments
In Figure 7, we show a thermal image of the board to demonstrate that we were able to achieve good thermal results even with the compact layout. After running the board for 10 minutes with no airflow, the hottest temperature was 69.3 °C. Check out the PMP22648 reference design for more detailed information.
Figure 7 In this thermal image of board’s top side, VIN = 13.5 V and all rails are at maximum load. Source: Texas Instruments
As we have seen in this article, the focus for today’s automotive infotainment systems is on fitting the solution into a small area while still achieving high performance. Paying attention to crucial design considerations such as switching frequency and power loss will allow you to optimize the individual components for a compact size. Following this up with good layout technique to mitigate major sources of EMI will be the key to a power-dense, high-performing solution.
This article was originally published on EE Times.
Sahana Krishnan is studying power electronics as a graduate student at the University of California, Berkeley. She was previously an application engineer at Texas Instruments.
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