Energy Suggestions #144: Designing an environment friendly, cost-effective micro DC/DC converter with excessive output accuracy for automotive functions

August 27, 2025

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Energy Suggestions #144: Designing an environment friendly, cost-effective micro DC/DC converter with excessive output accuracy for automotive functions

The continued electrification of automobiles brings new traits and necessities with each new design cycle. One development for battery electrical autos is lowering the scale of the low-voltage batteries, which energy both 12-V or 48-V programs. Some auto producers are even investigating whether or not it’s attainable to remove low-voltage batteries fully. Regardless, you’ll want remoted high- to low-voltage DC/DC converters as a backup or buffer for the low-voltage battery rail. In all of those instances, the high-voltage battery powers the DC/DC converters. Many high-voltage battery programs in automobiles presently in manufacturing or in growth use a 400-V or 800-V structure.

Given the disadvantages of discharging the high-voltage battery greater than vital, high- to low-voltage DC/DC converters must help operation with the best attainable effectivity. Completely different exercise states within the automotive require completely different energy ranges within the subsystems—for instance, 60 W when the motive force opens the automotive, 300 W when the automotive is in standby however not transferring, and three kW or extra when the automotive is in drive and totally operational. It’s not attainable to optimize a single DC/DC converter to cowl all three potential output energy ranges with high-efficiency operation over the entire load vary; within the examples given right here, you would want two or three unbiased energy converters.

Converter topology choice

On this energy tip, I’ll deal with the 300-W output energy vary, often known as a micro DC/DC converter. Appropriate DC/DC topologies for this output energy vary embrace half- and full-bridge converters. Resonant topologies reminiscent of half-bridge inductor-inductor-capacitor (LLC) converters supply greater effectivity conversion than their hard-switched counterparts by zero-voltage switching (ZVS) on the first aspect and zero-current switching (ZCS) on the secondary aspect. One other potential topology is the phase-shifted full-bridge (PSFB) topology, which additionally employs gentle switching by leveraging ZVS however is much less cost-effective for the 300-W goal output energy stage, because it requires 4 switches on the first aspect.

Determine 1 reveals the converter effectivity for varied enter voltages and cargo values for the Texas Devices (TI) Automotive 300 W Micro DC/DC Converter Reference Design Utilizing Half-Bridge LLC. Optimized for 400-V battery inputs and a 48-V output, this design displays a great compromise between effectivity and value of the 4 completely different topologies.

Determine 1 Effectivity plot of the automotive 300-W micro DC/DC converter reference design. Supply: Texas Devices

In an electrical car with a 400-V structure, the battery voltage can range from 200 V to 450 V. Generally, LLC converters aren’t identified to work effectively with very extensive enter voltage ranges as a result of, with peak current-mode management, such a large enter voltage vary might result in the converter prematurely getting into light-load effectivity mode (often known as burst mode) below full load situations, or reaching overload situations too early below low input-voltage situations. The rationale for each results is that the suggestions voltage is scaled within the controller with the enter voltage, making it switching frequency-dependent.

So why do you have to even think about an LLC for this kind of software? The UCC256612-Q1 LLC controller from TI makes use of input-power proportional management (IPPC), which overcomes these limitations. The suggestions voltage solely scales with the enter energy, and stays quasi-constant over the entire enter voltage vary for a relentless load present. Determine 2 reveals the variations between IPPC suggestions voltage conduct (Determine 2a) and conventional peak current-mode management suggestions voltage conduct (Determine 2b).

Determine 2 Suggestions over enter voltage utilizing (a) IPPC and (b) conventional LLC management. Supply: Texas Devices

Correct output voltage regulation with isolation

The right regulation of remoted energy provides in electrical autos is a difficult matter. Optocouplers, sometimes used for secondary-side regulation (SSR) in nonautomotive functions, are thought-about unreliable in automotive functions due to ageing results on the inner glass passivation over their lifetime. An alternate approach to supply output suggestions to a controller on the first aspect is primary-side regulation (PSR) by an auxiliary winding. PSR will not be very correct for top output currents as a result of the voltage drop throughout the rectifier(s) and droop throughout traces to the load will likely be current-dependent however not seen on the auxiliary winding. A second choice is to make use of remoted amplifiers.

For SSR, the reference design makes use of the TI ISOM8110-Q1 automotive-qualified pin-to-pin alternative for conventional optocoupler units. Superior ageing efficiency and smaller present switch ratio (CTR) variations of the ISOM8110-Q1 allow extra correct and dependable designs, that are essential for automotive programs with anticipated lifetimes of a minimum of 10 years. As well as, the ISOM8110-Q1 has a barely completely different switch operate than conventional optocouplers, enabling greater management loop bandwidths that may in the end save prices as a result of decrease output capacitance values will be capable of meet related load transient necessities.

Determine 3 reveals a load transient from 3 A to six.25 A and again to three A for the reference design with a 48-V output. The output voltage deviation with 4 82-µF output capacitors is barely 400 mV.

Determine 3 Load transient conduct, 400 V_IN, 3 A to six.25 A, and again to three A. Supply: Texas Devices

Aside from dynamic output accuracy, load regulation below static load situations is essential too. Determine 4 reveals the load regulation throughout completely different enter voltages for the reference design.

Determine 4 Load regulation over varied enter voltage ranges, illustrating good load regulation below static load situations. Supply: Texas Devices

For full performance, the ISOM8110-Q1 requires a bias present of a minimum of 700 µA on the diode aspect of the machine and 700 µA multiplied by the worst-case CTR on the transistor aspect, which is 155% with a 5 mA bias present and 180% with a 2 mA bias present. As a result of some management ICs are optimized for minimal standby energy, the suggestions pin of such a controller may not be able to sourcing adequate present to provide the ISOM8110-Q1 by itself. A easy workaround for such a situation is to supply the bias present with a pull-up resistor from a regulated voltage rail to the suggestions pin. The UCC256612-Q1 generates a 5-V rail with an inside low-dropout regulator, which is externally accessible and may due to this fact present the bias present for the opto-emulator IC. The block diagram in Determine 5 demonstrates the implementation of this workaround.

Determine 5 Secondary-side suggestions implementation utilizing the ISOM8110-Q1, with exterior bias from a management IC on the first aspect. Supply: Texas Devices

Different for micro DC/DC converters

The reference design demonstrates that the half-bridge LLC topology is usually a viable different for automotive micro DC/DC converters within the 300 W energy vary, demonstrating good effectivity in addition to glorious static and dynamic output voltage regulation.

The ISOM8110-Q1 is a cheap, correct and dependable choice to shut the loop of remoted energy converters in automotive functions. It really works effectively with controllers optimized for low standby energy when there’s the potential for an exterior bias voltage.

Markus Zehendner is a programs engineer and Member Group Technical Employees in TI’s EMEA Energy Provide Design Companies group. He holds a bachelor’s diploma in electrical engineering and a grasp’s diploma in electrical and microsystems engineering from the Technical College of Utilized Sciences in Regensburg, Germany. His foremost focus lies on automotive low-voltage designs for superior driver help programs and infotainment, in addition to high-voltage designs for hybrid and electrical car functions.