What is often missed in an automotive power supply when choosing an integrated circuit is not the specification on the data sheet but how the device will operate in the final design. This is because the datasheet covers the integrated circuit performance and can miss how the device will work in a closed loop or how it will interact with other components in the system.
This is even more important when choosing a front-end power supply for an automotive design. The power supply that connects directly to the battery needs to offer high efficiency, a small solution size and be very quiet. But why is this so?
The power supply that connects to the battery is more than a simple regulator, it is also a power-conditioning circuit connecting the noisy and harsh environments of automotive electromechanical systems – such as the transmission, engine and braking – to highly sophisticated electronic control units (ECUs) using leading-edge processors and sensors that are susceptible to harsh conditions.
The front-end power supply also has to manage fault and user conditions, like double battery or load dump. In this article, I will describe what you should consider when designing automotive power supplies and what additional functionality to add to your design to make the challenges more manageable.
Saving fuel
A modern automobile includes many fuel-saving features; one of these is start-stop operation. This start-stop feature activates when the automobile rolls to a stop and the engine switches off to save fuel while in a traffic queue or waiting at a stoplight (see Figure 1). Another condition, called cold crank, places similar electrical loads on the system. The cranking of the engine applies a large load to the system, causing a drop in battery voltage.
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Figure 1: Start-stop feature available in many of today’s automobiles.
Under start stop or cold crank the voltage can drop quite dramatically – and to a level that causes the buck converter to go out of regulation. In some instances you may need to use a boost converter in the circuit to assist the system. If you design your power supply to support sudden voltage drops, it is possible to save the expense of the boost, as well as the cost, space and power consumption associated with the additional components.
A combination of close to 100 percent duty cycle, very short minimum off-time and a low RDSON high-side field-effect transistor (FET) resistance enables the power supply (see Figure 2) to support a deep dropout of lower than 0.6 Volt under full load and full operating temperature range, whilst maintaining regulation for downstream power supplies without adding additional design complexity.
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Figure 2: Drop-out for a device supporting cold-crank conditions.
Stability is key
Maintaining regulation and stopping the power supply from becoming unstable is critical in automotive power supplies around the drop-out and high VIN conditions, as this relates to system-related conditions like cold crank or even load dump from faulty alternators. Supporting low drop-out is one condition, but it is also important when hitting other power supply conditions like minimum switch on-time (TMIN-ON), that the device does not oscillate as this translates to high-frequency noise and can cause disturbance in the system.
Controlling the stability of the power supply is critical and a smooth operation into and out of a wanted regulation range are required as this controls the noise that is generated. Figure 3 shows two identical power components: one that oscillates when hitting TMIN-ON; the other when hitting TMIN-OFF.
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Figure 3: Two identical devices, one operating in TMIN-ON and the other in TMIN-OFF conditions.
Offering low electromagnetic interference (EMI) power supplies is becoming more important due to many factors, including the increasingly more complex wire harnesses used in automobiles and the number of ECU nodes being added to the system. Each harness wire and each ECU has the ability to create noise that can be transmitted around the automobile and affect other applications. Given the increasing number of safety applications, the ability to offer low EMI is highly desirable. Several techniques can be used in the design of the power supply to minimize EMI.
One technique to lower the EMI is to design the package layout for optimum placement and positioning of the passive components used in regulation and filtering. As you can see in Figure 4, using a pinout that offers symmetrical placement of the high-frequency input capacitors – which are grounded on either side of the switch node – creates reduced inductance between ground (GND) and the switch and thus cancels noise.
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Figure 4: Placement of high-frequency CIN capacitors.
Additionally, by constructing the package in a certain manner, it is possible to remove the parasitic loop inductance and capacitance inside the package and reduce the switch-node ringing, which is a major contributor to the generation of noise (see Figure 5). The idea is that if you can reduce noise at the source, then you don’t need to tackle problems with additional components.
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Figure 5: Switch node of an automotive power supply without any ringing.
Spread spectrum
Another technique for reducing noise in the system is spread-spectrum technology, which modulates the central switching frequency (in this case 2.1 MHz) and suppresses the harmonics and sub-harmonics. Spread-spectrum is very effective for reducing the overall noise peaks, but will not influence the noise floor due to the spreading of the noise, as the name suggests.
However, because spread-spectrum helps with high-frequency harmonics, it can help meet the often-tough original equipment manufacturer (OEM) standards for EMI, make printed circuit board (PCB) designs simpler, and reduce filtering component size and cost.
Figure 6 shows the 30 MHz to 108 MHz frequency band, with the spread-spectrum turned off (left) and turned on (right). One of the major challenges to overcome is thermals due to space constraints and the ambient environment. A lot of automotive designs today are often incorporated into existing form factors and sizes used previously by mechanical functionality or just an empty space in the automotive design.
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Figure 6 shows the 30 MHz to 108 MHz frequency band, with the spread-spectrum turned off (left) and turned on (right).
A good example is the rearview mirror. In the past, rearview mirrors were simple hand-operated devices, but increasingly we are seeing the integration of electronics to accommodate new features like cameras. Given the radiated heat from the sun as well as the material encasing the mirrors (often plastics), the electronics can reach temperatures as high as 105°C on the PCB.
The power supply has to be small enough to manage the integration of the electronics, but it also has to be highly efficient so that it does not cause self-heating. The power supply needs to support high efficiency to minimize losses, and also to operate 2.1 MHz to make the additional components needed to make the power supply as small as possible to manage the thermals and size (see Figure 7).
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Figure 7: Thermal image of a device operating at 2.1 MHz.
Reliability
Maintaining quality is critical for automotive designs, even more so when designing for safety-based systems. Solderability is often a piece of the reliability puzzle. To enhance the quality of the soldering joint, you can use several techniques; one of these is wettable flanks (see Figure 8). Wettable flanks is a process of cutting a cavity in the edge of the package to enable more visible wetting of the pin side for optical inspection. This is helpful as it eliminates the need to x-ray the package to ensure adequate soldering, while ensuring joint integrity.
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Figure 8: Wettable flanks used on automotive devices.
As highlighted in this article, there is much more to making an automotive DC/DC regulator than just converting the voltage and maintaining regulation under different load conditions. Several system- and package-related issues can be solved by close consideration of what problems the engineer is trying to solve. By understanding these issues, the end result can be a faster design, smaller solution and lower cost of ownership.
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