8 EV Challenges of Converting High Voltage to SELV

The shift to 800V electric vehicle (EV) battery architectures is inspired by the need to increase charging rates, reduce resistive losses (I²R), and improve drivetrain efficiency. In parallel, zonal electrical architectures are emerging, using 48V local buses to reduce harness weight.

Converting 800V high-voltage (HV) battery power reliably and efficiently to 48V safety extra-low voltage (SELV) power introduces significant constraints in terms of packaging, thermal management, system safety, and transient demands. Rather than relying on distributed DC-DC converters paired with 48V batteries or supercapacitors, the key to delivering better performance is a fundamental change to a more centralized, efficient conversion architecture that incorporates high-density power modules.

What’s driving high-voltage adoption?

The current trend in EV architecture is a move toward higher voltages in both primary and auxiliary systems. In both cases, this is driven by a need to increase vehicle efficiency and reduce weight by concentrating power electronics density.

In the primary system, for example, a shift from a 400V battery to an 800V battery reduces current for the same power delivery (P = IV). As a result, OEMs are able to design solutions with thinner cables, resulting in lighter systems and higher overall efficiency. Auxiliary systems are transitioning from centralized 12V distribution to zonal designs that deliver 48V from multiple nodes, each with onboard DC-DC stages for 12V regulation. Similar to the 800V transition, the 48V zonal architecture allows OEMs to reduce copper usage and simplify wiring.

While both cases yield major benefits for the EV system, they introduce a number of pressing technical challenges for automotive and power engineers converting high voltage to SELV. Here are eight of the most common:

8 challenges in HV-to-SELV conversion and the power module solution

1. Voltage regulation and efficiency

Historically, alternators used in vehicles with internal combustion engines have provided a consistent output voltage to power the system electronics. In EVs, batteries provide power to the system, but their voltage output is inconsistent due to factors such as droop and charging/discharging states.

The VDA320 specification overseen by the German Association of the Automotive Industry recommends that 48V systems operate between 36V and 52V. One approach to provide these voltages is to use a regulated DC-DC converter. Most standard HV-to-48V converters achieve 95-97% peak efficiency under ideal conditions, but these conditions are rarely met, and the partial loads that most converters manage in real-word operation degrade that efficiency metric.

Sine Amplitude Converter (SAC) power modules can be used in place of regulated DC-DC converters. SAC modules work by converting the input voltage at fixed ratios, such as 2:1, 4:1, 6:1, 16:1, and 32:1. In the instance of a 16:1 ratio, a VDA320-defined range of 36V to 52V can be achieved from an 800V battery output ranging from 576V to 832V, which is well within the battery’s expected variance.

SAC modules are more efficient than regulated DC-DC converters, achieving 98-99% peak efficiency at 25°C ambient with a curve optimized around 50% output load (Figure 1). Half load is a sweet spot for real-world average usage in EV power distribution (non-coincident loads) and is therefore the correct operating condition for system optimization.

2. Safety and isolation at higher voltages

As voltages increase, safety becomes a more significant design consideration. As anything above 60V_DC (SELV threshold) is considered potentially lethal, 800V systems introduce a substantial risk to the user and technicians. Because of this, isolation is particularly important for HV-to-SELV designs.

Discrete designs based on switching topologies are limited in their ability to achieve high isolation ratings because of parasitic capacitance between components, inadequate creepage and clearance distances, and the difficulty of synchronizing high-speed switching while maintaining dielectric integrity across isolation barriers.

Power modules based on SAC topologies can achieve extremely high voltage isolation ratings thanks to zero-voltage and zero-current switching. These soft-switching techniques reduce EMI and minimize voltage stress across the isolation barrier, which enables the use of compact magnetic structures without compromising insulation performance. As a result, power modules can integrate high-isolation transformers and maintain efficiency even in dense, high-voltage environments where discrete solutions typically fail.

3. Creepage and clearance constraints at high voltage

Unfortunately, system safety and area are at odds with one another as voltage scales up. As system voltage increases, so too does the required minimum spacing between conductors, both across air (clearance) and insulating surfaces (creepage). These increased spacing requirements constrain layout and increase enclosure size in higher voltage EV systems.

As such, emerging 800V systems now require larger a physical separation for creepage/clearance in order to protect against arcing. In discrete designs, larger physical separations for safety consume more area and limit the power density of automotive systems. To compound matters, plastic aging and surface contamination can increase the risk of component breakdown over the system’s lifetime, which is addressed by adding space and material.

Instead of approaching the design with discrete components, power modules guarantee safety without compromising power density. Power modules can densely integrate components into smaller areas, protected by features like overmolding to prevent arcing and provide ingress protection against conductive dust and moisture that can reduce dielectric strength.

4. Packaging and power density

OEMs are constantly seeking greater power density, since lower system weight and area increase efficiency and allow for the placement of more battery cells. Traditional discrete converters with output regulation and filtering can occupy more than 2L of volume and weigh over 2kg at 4kW. In automotive or e-Mobility applications, where space and weight matter, this is not the best option.

High-density power modules enable the tight integration of components in the X, Y, and Z axes through the use of multi-layer PCBs, a method that would be unachievable with discrete solutions. For example, integrating filtering within the converter module saves space and improves power density by eliminating the need for bulky output filters.

And, by using power modules, designers can place the conversion solutions directly within the battery housing. In that way, OEMs also benefit from leveraging existing thermal and mechanical protection infrastructure. Additional weight and space savings are realized by eliminating the need for separate enclosures or added cooling system loops (Figure 2).

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Figure 2: SAC-based solutions are more compact and lighter weight, consuming a smaller footprint.  They enable a near 2x improvement in volumetric and gravimetric power density compared to similar solutions

5. Thermal bottlenecks

Many designers believe that power modules incur thermal management challenges due to their high-power densities and the close physical proximity of the internal components. However, power modules can integrate many power MOSFETs, controllers, and other components within the same package without driving up thermals. For example, a multi-stage, high-frequency module has been shown to achieve thermal resistances as low as 1.4°C/W (both PIN and NON-PIN sides), which is on par with a single discrete power MOSFET (Figure 3).

Power modules also simplify cooling. In discrete solutions, which consist of many components spaced far apart to meet creepage and clearance requirements, cooling is a challenge due to the varying component heights and locations. In contrast, power modules integrate all components in a single package, unlocking direct-to-baseplate cooling that eliminates the need for heat spreading or external thermal vias.

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Figure 3: Despite being a fully integrated solution, power modules offer thermal performance that is on par with that of a single power MOSFET

6. Transient response

The performance and real-time responsiveness of EV systems can often be a matter of life or death. For example, subsystems such as steering and braking require immediate power delivery under millisecond-scale dynamic load steps, lest they fail and endanger the vehicle occupants.

In some ways, this is a fundamental limitation of battery-powered systems, as conventional EV batteries have a transient response around 250A/second. When the voltage is regulated through conventional switching-based discrete converter solutions, the transient response is limited by the switching frequency of the converter, which is normally at 100kHz or below.

Because high-density power modules don’t rely on conventional switching topologies, their transient response is not limited by converter switching frequencies. As a result, SAC-based power modules can achieve transient responses (di/dt) of greater than 8,000,000A/s (Figure 4). Because the device’s response is intrinsic to the topology and passive behavior, a high-density modular implementation also eliminates control-loop delays for even faster responses.

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Figure 4:  BCM modules offer fully linear behavior between VIN and VOUT, delivering clean current without any risk of overshoot or ringing

7. 48V battery and supercaps

Most standard EV architectures require either a 48V lithium-ion pack or a supercapacitor array to buffer transient load steps and maintain stability. This is not ideal, as the batteries, capacitors and related circuitry impose huge cost, weight and space penalties.

High-density power modules solve this by removing DCM/PRM stages, replacing them with a single high-speed, high-efficiency module (Figure 5). With a transient response 32,000x faster than that of a battery alone, SAC-based modules meet the load demands of auxiliary systems.

Because of their bidirectional operation and low impedance, power modules also allow energy from capacitive or regenerative loads to flow directly back into the HV bus without external logic or relays. Integration is easy and requires no overhead, as the modules’ zero-delay polarity reversal eliminates the need for MCU-managed direction control. Behavior is guaranteed to be both passive and symmetrical.

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Figure 5: The performance of BCM modules enables complete replacement of the 48V battery

8. Peak power demands

Conventional discrete DC-DC converters are power-limited and require safety features such as current limits, which hinder their ability to operate beyond their nominal operating points. The typical peak power of these DC-DC converters is equal to continuous power, meaning that if the system occasionally demands 100A, the converter must be sized to handle 100A at all times – even if the average current is much lower. Overprovisioning converters for transient events in this way results in higher cost, size and thermal overhead.

High-density power modules are thermally limited – not power limited – meaning they provide peak power beyond their continuous power ratings. If a modular DC-DC converter is rated for 80A continuous, it can still sustain 100A peak for < 20ms or at 25% duty cycle.

With the ability to operate at these transient levels, OEMs can right-size power delivery around average, not peak, current and save significant vehicle cost and weight. This is valuable for handling inductive loads like motors and actuators, which often experience start-up surges, especially in zonal systems where loads are intermittent and non-coincident.

Eliminating legacy power constraints

The transition to 800V platforms and 48V zonal architectures has exposed the limitations of traditional converter design, including poor transient response, excessive bulk and dependence on local energy storage.

For power designers, the Vicor BCM6135 SAC-based power module enables a rethinking of HV-to-SELV conversion at the system level. Using high-density power modules, designers can eliminate the 48V battery, reduce weight and cost, and allow real-time bidirectional energy flow with unmatched density and transient speed.

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