How to Efficiently Generate High-Voltage Power Rails in Automotive Applications

Although 12V lead-acid batteries are still the mainstream of automotive power supply, there are some new applications that require higher voltages, such as mains audio power amplifiers and window defrost devices. To meet the requirements of these high-voltage applications, a new generation of AEC-Q100 certified synchronous boost controllers has appeared on the market. This controller is designed to increase the 12V battery voltage, can withstand peak voltages up to 60V, and has the high reliability required by new car models.

This article introduces a pair of easy-to-use 2-phase 55V synchronous boost controllers that can generate 24V, 36V, or 48V rails in an automotive environment with only 12V supply. We will examine some of the key features they integrate, including comprehensive protection features that help achieve an optimized solution, thereby reducing costs and improving efficiency, safety, and reliability. We will also discuss an integrated PMBus interface that provides advanced control, telemetry, and diagnostics and simplifies the task of achieving ISO 26262 compliance.

Boosting 12V battery voltage

A constant challenge for system designers is how to achieve higher power efficiency while minimizing board space. The ISL78227 and ISL78229 55V synchronous boost controllers address this problem by integrating advanced FET drivers that adaptively adjust switching times to prevent cross conduction while simplifying power stage design. The 2-phase configuration used in these two controllers reduces ripple current, allowing for smaller input and output capacitors, which helps reduce board footprint. Two controllers can be used in parallel to increase the number of phases to four, supporting higher power output levels.

The ISL78227 and ISL78229 feature a PMBus interface, support a wide operating frequency range of 50kHz - 1.1MHz, and can be configured to optimize operating frequency using smaller external components to help improve efficiency or minimize board space. They include many features designed to maximize efficiency, which is important because the peak output current from a 12V battery at a 400W load can exceed 30A.

Synchronous FETs for output rectification

Since most buck converters have relatively low output voltages, FETs are often used in buck converters instead of diodes to perform the output rectification function. In this configuration, a large portion of the power loss in generating the output voltage comes from the voltage drop across the rectifying elements. Replacing the output rectifier diode with a synchronous FET that can be turned on and off at the appropriate times can greatly improve efficiency. This is because the FET losses are usually only a small fraction of the losses in the rectifying diode. In a buck converter, the reference voltage for the synchronous FET is ground, so the drive circuit is relatively simple.

Synchronous FETs bring several benefits to the boost configuration. In boost converter applications, the output voltage is typically several times the input voltage, so the power losses in the output rectifier components are a small percentage of the total output power. The boost converter benefits from the improved efficiency of synchronous FETs, and the bidirectional current flow provided by synchronous FETs supports continuous mode operation (even under light load conditions) - an important advantage for applications that require low electromagnetic interference (EMI). Bidirectional current flow is also an important capability for achieving effective envelope tracking functions, which we will discuss below. In addition, the use of synchronous FETs does not preclude operation in discontinuous mode. The boost controller can detect negative current flow and can choose to disable the synchronous FET to emulate the function of a synchronous rectifier diode.

Improving Light-Load Efficiency with Diode Emulation

Audio signals often change dramatically in very short periods of time. One moment the amplifier may need a burst of high power and the next moment a burst of very low power. There may even be silence between audio sessions. When this happens, the power usage of the amplifier drops significantly, and because of this, the power required by the boost regulator also drops to a lower value. In fact, under light load conditions, the boost inductor current can drop to zero. When this happens, the output voltage of the inductor (the boost voltage) is higher than the input voltage (the battery voltage). If the synchronous FET is kept on during this condition, current will start to flow in the reverse direction through the inductor and draw charge from the output capacitor.

Figure 1. Efficiency vs. load comparison, 2-phase boost configuration, three operating modes, f _SW = 200kHz,

V _IN =12V, V _OUT =36V , T _A =+25°C

These 55V boost controllers include optional circuitry to avoid this reverse conduction loss by causing the synchronous FET to mimic the current blocking behavior of a real diode. This smart diode operation, called diode emulation mode (DEM), works by turning off the synchronous FET when the circuit senses that the inductor current is beginning to flow in the wrong direction. If the controller enters diode emulation mode and the load is still decreasing, the controller will enter pulse skipping mode to reduce the number of switching cycles, thereby improving its efficiency when very light loads occur on the output.

While DEM can improve efficiency at light loads, it also introduces some EMI challenges due to the changing switching characteristics. To avoid EMI issues, it is often desirable to maintain continuous conduction mode (CCM) operation. Of course, this sacrifices the efficiency gains provided by diode emulation, as shown in Figure 1. However, in applications such as audio amplifiers, an alternative method to achieve light load efficiency gains is to have the amplifier power supply use envelope tracking to track the input requirements.

Forced PWM operation mode

Many power system applications require that the switching frequency of the converter remain constant to minimize the possibility of glitches. Due to this requirement, the ISL78227 and ISL78229 can also operate in PWM mode (without pulse skipping). However, in forced PWM mode, there are situations where reverse current can flow, such as when starting up into a pre-biased output state or when the output voltage rises to a higher than expected voltage. In a typical system, there is no way to limit the reverse current, which can damage the synchronous FET. The ISL78227 and ISL78229 address this issue by providing a reverse current limit function. Limiting negative current reduces output voltage transients and improves system reliability. As a result, design engineers can configure the boost controller in forced PWM mode without having to worry about reverse current getting out of control.

Improve light load efficiency through phase shedding

The ISL78227/29 synchronous boost controller supports 2-phase boost operation, and we can connect the two devices together to achieve 4-phase operation (see Figure 2). Under heavy load conditions, the main system losses are due to conduction losses and switching losses, but under light load conditions, switching losses begin to dominate. To improve efficiency, both controllers can be configured to monitor the system current level. If the load drops below a certain threshold, the controller will drop a phase, which can reduce switching losses under light load conditions. The phase blanking process is completed within 15 switching cycles to prevent load transients. If the load subsequently increases above the threshold, a phase is immediately added to manage the increased load.

Figure 2. Connecting two devices supports 4-phase operation, meeting the requirements of higher power applications.

Reference voltage control and audio envelope tracking

The boost controller output voltage can be regulated using the 1.6V on-chip reference voltage, or it can be regulated to an external tracking voltage used to drive the control loop. Unique to the ISL78227 and ISL78229 controllers is that the external signal used to drive the tracking function can be configured as an analog voltage or a PWM signal. These TRACK functions dynamically support changes in the output boost voltage. These controllers include negative current limiting and protection features, which are useful when envelope tracking flows from a higher voltage to a lower voltage.

The output boost voltage’s job is to track the control signal, but when going from a higher voltage to a lower voltage, the output capacitor must be discharged to allow the voltage to drop. If the load itself does not draw enough current, the synchronous FET can help discharge the output capacitor without worrying about FET damage due to excessive current. This is because both controllers include negative current limiting and protection circuitry for these conditions.

The ability to support envelope tracking without worrying about excessive reverse current is very useful for audio applications where the supply voltage changes rapidly over a wide range. In audio applications, the TRACK signal can be used to control the boosted output voltage so that it tracks the signal amplitude changes of the audio amplifier. This keeps the supply voltage stable and prevents short-term pulse wave interference when the load changes, thereby preventing popping and popping from the audio power amplifier.

Remember that the power delivered to a speaker is a function of the amplifier's peak output voltage, as shown in the following equation:

P _avg = V _rms • I _rms = V ² _rms / R = V ² _peak / 2R

In automotive audio power amplifier applications, it is common to boost the voltage of a 12V battery. These boost controllers can boost the battery voltage to 48V or any desired voltage to support the power level of the audio power amplifier. Audio amplifier power in the 100-800W range is very common. Some multi-channel systems of premium audio systems may include a 30-40W amplifier and a higher power amplifier for driving a subwoofer.

In analog audio amplifiers, efficiency can be improved if the supply voltage is only large enough to support the audio signal. Efficiency improvements in digital audio amplifiers depend on the digital amplifier architecture.

PMBus Control

The ISL78229 boost controller shown in Figure 3 includes a PMBus interface that helps designers achieve ISO 26262 compliance and Automotive Safety Integrity Level (ASIL) compliance for their systems. The PMBus interface is useful in systems that require real-time telemetry, error reporting to a microcontroller, and system control. It provides a way to remotely enable or disable the boost controller and monitor and report variables such as input voltage, input current, and output voltage. In addition, the boost controller includes a pin to support the measurement of an external negative temperature coefficient (NTC) resistor for temperature monitoring. It then digitizes the signal and can also report the reading over the PMBus. Overtemperature fault limits for external temperature monitoring can also be set.

Figure 3. Typical application of ISL78229 with PMBus control

The boost controller also features fault reporting capabilities such as input overvoltage, output overvoltage or undervoltage, overcurrent, and overtemperature faults. Each function can be monitored via PMBus. Adding a PMBus interface helps avoid the need for dedicated telemetry circuitry .

in conclusion

The ISL78227/29 multiphase 55V synchronous boost controllers offer many features to meet many different power system requirements. Individually these features may not be significant, but when combined, the whole is more than the sum of its parts. Voltage quality modules for start-stop systems, mains audio amplifiers, and window defrosters are just a few of the high voltage applications that require a robust boost controller solution.

About the Author

Jerome Johnston is an Application Engineer in the Central Applications team at Intersil. He has over 30 years of experience in analog system design and applications and is the holder of 13 US patents. Jerome holds a Bachelor of Science in Electrical Engineering from the University of Nebraska.