By Maury Wood, VP Strategic Marketing

Bidirectional, power-dense DC–DC converters are the ideal solution for the new and challenging use cases presented by machine electrification across numerous industries. This paper demonstrates how high-efficiency, fixed-ratio DC–DC converter modules are capable of supporting transient regenerative loads without the cost and complexity of liquid cooling.

Electrification, the societal movement away from fossil fuel powered machines, is sweeping across all industrial, vehicle and aerospace/defense equipment categories. The economic and cultural forces driving this movement are well known and generally undisputed. Electrification has both environmental benefits (lower associated carbon emissions, for example) and key performance benefits such as high-torque motors enabling high levels of acceleration in electric vehicles ^{[1], [2]}.

High DC voltages ranging from 270V up to 1,000V are commonly used in electric equipment and vehicles as a means to reduce power losses in the bussing or cabling between the power source and the power load (linear and rotational motors, actuators, sensors, processors, point-of-load low voltage regulators, etc.). High voltages also enable the delivery of high levels of transduced mechanical force, both linear displacement and rotational displacement.

DC–DC converters play a vital role in transforming high voltages to lower voltages, with or without isolation, regulation and reverse operation, in electric vehicles, data centers, communications systems and industrial equipment of all types. These power converters can be implemented using discrete components or in modular package form. DC–DC converters power modules are the focus of this article.

Over the past 10 years or so, the dominant DC subsystem power delivery network (PDN) voltage of 12V has begun to transition to 48V (54V in data centers), driven by the significant increase in load power requirements, and the need to maintain safety extra-low voltage (SELV) levels, heralding the emergence of HV-to-48V_DC converters. Coincident with this evolutionary change of subsystem PDN voltage has been the adoption of 48V-centric dc–dc converter power modules, which have numerous ease-of-use, power density, power scaling and weight benefits, and which support regeneration (the return of energy to the primary power source).

The use of high DC voltage is ramping in equipment, vehicles and infrastructure

Electrolytic batteries, using a fast-evolving variety of chemistries, are often used as both high-voltage and low-voltage dc power sources and are obviously ideal for mobile (non-tethered) and handheld applications. The rechargeable nature of many battery types, from lead acid cells to the latest sodium-ion and graphene types, as well as modern supercapacitors, support regenerative energy use cases that in aggregate are destined to save enormous amounts of energy globally.

In electric vehicles, it is common to see nominal battery pack voltages of 400V_DC and 800V_DC. In the future, 800V battery packs are likely to dominate, due to escalating energy density trends. Mild hybrid vehicles generally use 48V_DC batteries, with some manufacturers electing to use 12V_DC multi-cell packs. Electric vehicles include not only automobiles but also industrial and agricultural vehicles (including construction vehicles such as excavators and tractors) as well as all types of recreational vehicle platforms (personal watercraft, 4x4 off-road vehicles, snow machines, motorcycles, etc.). With few exceptions, notably driving range and the time required to refuel or recharge the vehicle, the electric versions of these vehicle types tend to have superior end-user performance (such as acceleration, torque and ride quality) than their internal combustion engine counterparts.

Why is 48V_DC power distribution replacing 12V_DC power distribution?

Higher distribution voltage delivers the same power with lower current. Because distribution power losses (typically using copper or aluminum busbars or cables) are a function of the square of the current (P = I²R), in high power applications these substantial conduction losses due to busbar and cable resistance can be reduced by using higher distribution voltages. Busbars and cables are sized according to current carrying capacity (referred to as ampacity), and a 4x increase in voltage and a 4x decrease in current has a substantial impact on sizing, weight and cost. For example, to conduct 200A, a copper busbar needs a cross-sectional area of about 0.0625 square inches, whereas to conduct 800A, a copper busbar needs a cross-sectional square of about 0.3125 square inches, a difference of a factor of five ^[3]. The size and weight of busbars and cabling associated with 48V_DC are thinner, lighter and thus cheaper than those associated with 12V_DC power delivery networks.

Exploring the capabilities of high voltage to 48V conversion with a fixed-ratio converter module

The technical capabilities of advanced 48V power modules unlock new levels of efficiency and performance. For example, the Vicor BCM6135 series is a family of fixed-ratio, isolated (4242V), bus converter power modules with integrated magnetics and by design are inherently bidirectional, supporting regenerative battery applications.

One member of this family, a 2.5kW steady-state-rated module, has a ratiometric conversion “K factor” (the equivalent of transformer turns ratio) of 1/16 and is rated to convert nominal 800V to nominal 50V.

Due to its circuit topology and zero-voltage and zero-current switching, its peak efficiency is 97.3%, resulting in 67.5W of power loss as heat (2.7% x 2.5kW) to thermally manage at peak power delivery (3.1kW) for a given case temperature of T_CASE of 70°C. The volumetric power density is high at 159kW/L (module dimensions are 61.3mm by 35.4mm by 7.3mm); module weight is 58g, yielding a continuous massimetric power density of 43.1W/g.

The BCM6135 (see Figure 1) supports instantaneous bidirectional start-up and steady-state operation. Furthermore, the BCM6135 acts as a capacitance multiplier, scaling the bulk capacitance across the high-voltage (HI) bus to the low-voltage (LO) bus by the square of the K factor (16² = 256). This attribute saves the cost, weight and space of the bypass or bulk capacitors that otherwise would be required on the low-voltage bus.

Additionally, the high switching frequency of the BCM® enables extremely fast load step transient (di/dt) performance of 8 MA/s, allowing the module to replace auxiliary batteries and supercapacitors otherwise needed to support the transient load steps in demanding applications, including those in high-performance computing and electric vehicles.

The BCM’s wide input voltage range (from 520V to 920V) supports a wide variety of DC voltage distribution standards. Wide input voltage range is one of the attributes of the proprietary Sine Amplitude Converter (SAC™)  topology used in the BCM. The importance of wide input voltage range is well illustrated by the recommendation of the German Association of the Automotive Industry, or VDA, VDA 320: Electric and Electronic Components in Motor Vehicles – 48V On-Board Power Supply–Requirements and Tests (version 01/20/2025), also known as LV 148, developed by the automotive OEMs Audi, BMW, Daimler, Porsche and Volkswagen as a common OEM standard for 48V_DC voltage range components^[4]. This guidance recommends that the battery possess an unlimited voltage operating range between 36V and 52V, limited operating modes between 24V and 54V (see Figure 2).

The thin (7.3mm) BCM6135 module family is overmolded and electroplated for thermal agility, shielded and interconnected through surface mount terminals or through-hole pins, and the three-dimensionally interconnected ChiP™ package offers low thermal impedance and high thermal adeptness, including a coplanar thermal interface to heat sinks and cold plates ^[5].

Regenerative active suspension without active cooling

The BCM6135 conversion efficiency at high ambient temperature (70°C) and sourcing 50A output current at 48V is typically 97.3%. This high-voltage to 48V power module is often used in continuous load applications, but it is also well suited to transient pulse load applications and depending on the pulse duty cycle of the load, potentially can be used with passive cooling (no forced air or liquid cooling). Regenerative electric vehicle active suspension (which can be combined with active anti-roll control) is an excellent example of a bidirectional use case that is characteristically transient. The linear motors that actuate the active suspension are activated only when bumps and potholes are encountered. This type of system application is best modeled and described using peak power conversion metrics.

In years past, 12V_DC has been shown to be inadequate (within reasonable size, weight and cost constraints) to power active suspension motors. Note that an electric vehicle’s 800V_DC main battery could be used to power an active suspension subsystem, but running 800V_DC to the vehicle’s periphery reduces safety, particularly to first responders to crash accidents.

The guaranteed peak power rating of this BCM6135 model is 3.1kW for 20ms, with 25% duty cycle, at the low end of its operating voltage range (i.e., low line operation; the full continuous operating range is 17V to 57.5V). As can be expected, the peak power output derates for longer durations of the transient demand. Developing an application- level peak power specification for an active suspension is complex, as worst-case road surface profile, cooling method, size, weight and cost constraint goals can vary enormously. However, to minimize size, weight and cost, it is typically strongly preferred to use passive thermal management for the active suspension DC-DC converter subsystem (i.e., conductive/convective heatsinks but no fan forced air or circulating liquid cold-plates).

The design challenge to meet these constraints amounts to validating that the power converter module can meet the peak transient load demands without incurring thermally activated module shut down. The BCM6135 is electroplated on both sides, and the heat sink(s) should ideally contact both sides of the package. The module, which has a package thermal capacitance of 44.5 J/K, includes an internal temperature sensor, which in combination with the two sided thermal model ^[6], enables estimation of the maximum internal MOSFET “junction” temperature as shown in Figures 3 and 4.

Estimation of internal module temperature profile

The thermal capacitance is used to calculate the thermal time constant of the module during a transient thermal event. This time constant is the product of thermal capacitance and thermal resistance. The value given for thermal capacitance in product datasheets is a calculated value that assumes the product is at a uniform temperature internally (throughout the module) at all times during the transient thermal event. This is a linearized simplification, but it allows the product designer a method to quickly estimate the temperature-versus-time behavior of the product early in the product design cycle. The simplification of uniform internal temperature also implies that the thermal time constant better reflects actual product performance when utilized for double-sided cooling of the 48V power module using heat sinks.

For example, an equivalent circuit to model the thermal resistance of the BCM6135 is shown in Figure 5 ^[7]. Electrical resistors are analogous to thermal resistances in units of degrees Celsius per watt [°C/W]. A current source is analogous to a heat source in units of watt [W]. A voltage source is analogous to a temperature source in this circuit model with units of degrees Celsius [°C].

The equivalent circuit assumes package top and bottom cooling with an equivalent thermal resistance of 0.7°C/W and case temperature of 35°C, thermal capacitance of the module of 44.5J/K, and that the module dissipates 130W during 30 seconds-on, 30 seconds-off continuously repeated pulses.

Simulation results for this circuit are shown in Figure 6; the operating conditions are 520 VHI, 32.5 VLO, 80A low-side peak output current (2.6kW peak output power). During the first power pulse, the maximum internal temperature increases to about 90°C. The next pulse shows an increase in maximum internal temperature to about 115°C. Repeated pulses would show maximum internal temperature remains around 115°C.

Application testing of the module should always be conducted to validate initial modeled estimates of transient performance and to properly design a passive convective heatsink.

Lab test results

The BCM6135 is inherently bidirectional with instantaneous switching of the directional mode of operation. The conversion efficiency of the module is the same regardless of the direction of current flow.

In the regenerative active suspension application, the 800V battery is sourcing current when the vehicle is traveling over smooth road surfaces and the suspension actuation motor is the 48V load. When a pothole is encountered by the vehicle, the motors in the suspension momentarily become generators (compression), and the voltage on the low side of the BCM increases above the voltage of the 800V battery divided by the conversion K factor (K = 1/16 in this application). This difference in potential causes the bus converter to swap the direction of current flow, without internal loop controller intervention. The 800V battery then momentarily becomes the load (rebound) and recovers energy by charging through its battery management system circuit.

Once the displacement from the pothole has subsided, the bus converter will once again step down the 800V battery and supply current to the suspension linear motors. All of this occurs without intervention by the vehicle’s on-board processors. The frequency of these suspension actuations ranges from about 1Hz to 10Hz. Interestingly, the road surface profile is essentially an analog of the bus converter load step dynamics.

It is the potential difference between the bus converter high and low sides that defines the current amplitude and direction.

Imagine that the load on the low side is a passive load (such as a resistor) and on the high side there is a battery with a potential of 800V. The BCM will act as a K = 1/16 voltage transformer and create a potential on the low side equal to 50V. Current will flow through that resistor and is determined by voltage applied across the resistor.

If an energy source is added to the low side with potential of 51V and replaces the resistor, the potential difference between the output of BCM (50V) and that energy source (51V) will be negative (–1V), and the current will start flowing in the opposite direction. The level of this current will be defined by the total path resistance inside the BCM and the battery.

This can be visualized with the BCM connected to 800V source on the high side and a bidirectional power supply on the low side. By varying the voltage on the bidirectional power supply ±100mV, current will flow alternatively in both directions, and the peak current will be 100mV divided by the BCM output resistance. For a bus converter output impedance of 25mΩ, this yields a peak current of approximately 4A flowing bidirectionally under these assumptions (Figure 7).

In lab tests (Figure 8), the BCM6135 has demonstrated peak power of 4kW (80A at 50V) for 60ms, an indicator that the module design is thermally robust across dynamic loads.

In a second lab test (Figure 9), the load was pulsed from 16A to 80A with a 10% duty cycle (900ms at 16A and 100ms at 80A). The operating condition is 520V_HI and 32.5V_LO; this is the low end of the supported BCM6135 voltage range. The average power delivery was 720W (22A at 32.5V). Over the course of 30 minutes (1800s), the internal sensor “read temperature” (a proxy for junction temperature) indicated a steady-state temperature of ~100°C, considerably below the maximum allowable junction temperature of 125°C. The test setup was passive cooling with a single-sided heat sink. This is another positive indicator for the targeted passively cooled application.

On the other hand, in a third lab test (see Figure 10), with the same thermal management setup, the average power delivery was increased to 1.1kW (22A at 50V). In this test, the operating condition is 800V_HI and 50V_LO; this is the high end of the supported BCM6135 voltage range. The load was pulsed from 17.5A to 70A with a 10% duty cycle (900ms at 17.5A and 100ms at 70A). In 7.5 minutes, the sensed internal temperature was 100°C and was still rising (not in steady state). But 7.5 minutes (450 seconds) is a much longer duration than 20 seconds, so this is a positive indicator that the BCM6135 may meet some active suspension design requirements.

Ultimately, the BCM6135 was lab characterized to support an average power of 1.3kW for 30 seconds with a passively-cooled heat sink across the sealed-enclosure operating temperature range.

Active suspension design objectives include road surface profile assumptions (amplitude and duration of bumps and holes that can be mitigated), and these assumptions bear directly on the required peak power capability of the DC-DC converter. The electromagnetic characteristics of the linear motor also impact the DC-DC converter requirements. That said, the BCM6135 is an indispensable bus converter module for contemporary active suspension, active anti-roll control DC-DC converter subsystems.

Conclusion

The economic and quality-of-life benefits of electrification is driving the adoption of HV to 48V DC-DC conversion across equipment types throughout the global economy. Integrated high-voltage to 48V power modules are becoming more common in EVs and HEVs as battery voltages increase and 48V low voltage buses become more widespread.

Next-generation bidirectional fixed-ratio bus converter modules are capable of meeting electrically and thermally demanding requirements in transient regenerative use case applications such as active electric vehicle suspensions. The passively cooled findings presented are significant in light of the accelerating trend towards more costly liquid-cooled power delivery systems.

Acknowledgments

The author wishes to thank his colleagues at Vicor for their assistance in preparing this article, especially Haris Muhedinovic, Lap Nguyen, and Alexander Parady.

References

[1] The Slow Road to Electrification of Everything. Accessed: Feb. 4, 2025. [Online]. Available: https://www.forbes.com/sites/hessiejones/2023/09/26/the-slow-road-tothe- electrification-of-everything

[2] With Greater Voltage Comes Responsibility. Accessed: Feb. 5, 2025. [Online]. Available: https://www.fleetowner.com/equipment/media-gallery/ 21134251/with-greater-voltage-comes-greater-responsibility

[3] Ampacities and Mechanical Properties of Rectangular Copper Busbars: Table 1. No. 110. Accessed: Feb. 4, 2025. [Online]. Available: https://copper. org/applications/electrical/busbar/bus_table1.php

[4] VDA 320 (01/2015). Accessed: Feb. 6, 2025. [Online]. Available: https:// webshop.vda.de/VDA/en/vda-320-012015

[5] Automotive Power Modules. Accessed: Feb. 6, 2025. [Online]. Available: https://www.vicorpower.com/all-products/automotive

[6] Thermal Models of Vicor Power Components. Accessed: Feb. 10, 2025. [Online]. Available: https://www.vicorpower.com/documents/whitepapers/ wp-Thermal-Models-Vicor-Power-Components.pdf

[7] Thermal Management for VIA™ and ChiP™ Modules. Accessed: Feb. 10, 2025. [Online]. Available: https://www.vicorpower.com/documents/application_ notes/an_Thermal_Management_VIA_ChiP_Modules.pdf.

Maury Wood is Vice President of Strategic Marketing at Vicor Corporation. Prior to joining Vicor, Maury held senior roles at optical fiber test and semiconductor companies, including EXFO, AFL, Broadcom, NXP, Analog Devices, and Cypress. He holds a BSEE from the University of Michigan and did graduate studies at Northeastern University, Babson College, and MIT. He enjoys climbing, back-country skiing, mountain biking, and playing jazz bass.

Maury Wood, VP Strategic Marketing