The ongoing electrification of cars brings new trends and requirements with every new design cycle. One trend for battery electric vehicles is reducing the size of the low-voltage batteries, which power either 12-V or 48-V systems. Some auto manufacturers are even investigating whether it’s possible to eliminate low-voltage batteries completely. Regardless, you’ll need isolated high- to low-voltage DC/DC converters as a backup or buffer for the low-voltage battery rail. In all of these cases, the high-voltage battery powers the DC/DC converters. Many high-voltage battery systems in cars currently in production or in development use a 400-V or 800-V architecture.

Given the disadvantages of discharging the high-voltage battery more than necessary, high- to low-voltage DC/DC converters need to support operation with the highest possible efficiency. Different activity states in the car require different power levels in the subsystems—for example, 60 W when the driver opens the car, 300 W when the car is in standby but not moving, and 3 kW or more when the car is in drive and fully operational. It is not possible to optimize a single DC/DC converter to cover all three potential output power levels with high-efficiency operation over the whole load range; in the examples given here, you would need two or three independent power converters.

Converter topology selection

In this power tip, I will focus on the 300-W output power range, also known as a micro DC/DC converter. Suitable DC/DC topologies for this output power range include half- and full-bridge converters. Resonant topologies such as half-bridge inductor-inductor-capacitor (LLC) converters offer higher efficiency conversion than their hard-switched counterparts through zero-voltage switching (ZVS) on the primary side and zero-current switching (ZCS) on the secondary side. Another potential topology is the phase-shifted full-bridge (PSFB) topology, which also employs soft switching by leveraging ZVS but is less cost-effective for the 300-W target output power level, since it requires four switches on the primary side.

Figure 1 shows the converter efficiency for various input voltages and load values for the Texas Instruments (TI) Automotive 300 W Micro DC/DC Converter Reference Design Using Half-Bridge LLC. Optimized for 400-V battery inputs and a 48-V output, this design reflects a good compromise between efficiency and cost of the four different topologies.

Figure 1 Efficiency plot of the automotive 300-W micro DC/DC converter reference design. Source: Texas Instruments

In an electric vehicle with a 400-V architecture, the battery voltage can vary from 200 V to 450 V. In general, LLC converters are not known to work well with very wide input voltage ranges because, with peak current-mode control, such a wide input voltage range could lead to the converter prematurely entering light-load efficiency mode (also known as burst mode) under full load conditions, or reaching overload conditions too early under low input-voltage conditions. The reason for both effects is that the feedback voltage is scaled in the controller with the input voltage, making it switching frequency-dependent.

So why should you even consider an LLC for this type of application? The UCC256612-Q1 LLC controller from TI uses input-power proportional control (IPPC), which overcomes these limitations. The feedback voltage only scales with the input power, and stays quasi-constant over the whole input voltage range for a constant load current. Figure 2 shows the differences between IPPC feedback voltage behavior (Figure 2a) and traditional peak current-mode control feedback voltage behavior (Figure 2b).

Figure 2 Feedback over input voltage using (a) IPPC and (b) traditional LLC control. Source: Texas Instruments

Accurate output voltage regulation with isolation

The proper regulation of isolated power supplies in electric vehicles is a tricky topic. Optocouplers, typically used for secondary-side regulation (SSR) in nonautomotive applications, are considered unreliable in automotive applications because of aging effects on the internal glass passivation over their lifetime. An alternative way to provide output feedback to a controller on the primary side is primary-side regulation (PSR) through an auxiliary winding. PSR is not very accurate for high output currents because the voltage drop across the rectifier(s) and droop across traces to the load will be current-dependent but not visible on the auxiliary winding. A second option is to use isolated amplifiers.

For SSR, the reference design uses the TI ISOM8110-Q1 automotive-qualified pin-to-pin replacement for traditional optocoupler devices. Superior aging performance and smaller current transfer ratio (CTR) variations of the ISOM8110-Q1 enable more accurate and reliable designs, which are crucial for automotive systems with expected lifetimes of at least 10 years. In addition, the ISOM8110-Q1 has a slightly different transfer function than traditional optocouplers, enabling higher control loop bandwidths that can ultimately save costs because lower output capacitance values will be able to meet similar load transient requirements.

Figure 3 shows a load transient from 3 A to 6.25 A and back to 3 A for the reference design with a 48-V output. The output voltage deviation with four 82-µF output capacitors is only 400 mV.

Figure 3 Load transient behavior, 400 V_IN, 3 A to 6.25 A, and back to 3 A. Source: Texas Instruments

Apart from dynamic output accuracy, load regulation under static load conditions is important too. Figure 4 shows the load regulation across different input voltages for the reference design.

Figure 4 Load regulation over various input voltage levels, illustrating good load regulation under static load conditions. Source: Texas Instruments

For full functionality, the ISOM8110-Q1 requires a bias current of at least 700 µA on the diode side of the device and 700 µA multiplied by the worst-case CTR on the transistor side, which is 155% with a 5 mA bias current and 180% with a 2 mA bias current. Because some control ICs are optimized for minimum standby power, the feedback pin of such a controller might not be capable of sourcing sufficient current to supply the ISOM8110-Q1 on its own. A simple workaround for such a scenario is to provide the bias current with a pull-up resistor from a regulated voltage rail to the feedback pin. The UCC256612-Q1 generates a 5-V rail with an internal low-dropout regulator, which is externally accessible and can therefore provide the bias current for the opto-emulator IC. The block diagram in Figure 5 demonstrates the implementation of this workaround.

Figure 5 Secondary-side feedback implementation using the ISOM8110-Q1, with external bias from a control IC on the primary side. Source: Texas Instruments

Alternative for micro DC/DC converters

The reference design demonstrates that the half-bridge LLC topology can be a viable alternative for automotive micro DC/DC converters in the 300 W power range, demonstrating good efficiency as well as excellent static and dynamic output voltage regulation.

The ISOM8110-Q1 is a cost-effective, accurate and reliable option to close the loop of isolated power converters in automotive applications. It works well with controllers optimized for low standby power when there is the possibility of an external bias voltage.

Markus Zehendner is a systems engineer and Member Group Technical Staff in TI’s EMEA Power Supply Design Services group. He holds a bachelor’s degree in electrical engineering and a master’s degree in electrical and microsystems engineering from the Technical University of Applied Sciences in Regensburg, Germany. His main focus lies on automotive low-voltage designs for advanced driver assistance systems and infotainment, as well as high-voltage designs for hybrid and electric vehicle applications.

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