Bench Talk

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Industrial facilities can be noisy environments, both acoustically and electrically, filled with the hum of solar inverters, the low roar of large high-voltage electric motors, and constant thrum of HVAC systems. Along with this assortment of equipment, all of which is prone to emitting electromagnetic interference (EMI), engineers need reliable low-voltage DC power supplies to operate essential components like controllers, sensors, and communications links. Traditionally, this requires a step-down transformer to convert AC input to a lower voltage, followed by a rectifier and switching or linear regulator to obtain 5VDC to 48VDC.

Unfortunately, this approach can be bulky and expensive. Magnetic parts—such as transformers—are large, while linear regulators are generally inefficient. Although switching regulators provide greater efficiency, both can require heat sinks and may introduce additional EMI, requiring additional filtering in the form of magnetic components like chokes.

A modern alternative is emerging. Silicon carbide (SiC)-based integrated converters are reshaping auxiliary power supply design. This blog explores how these compact, efficient devices can deliver up to 48W without heat sinks, simplifying design and ultimately reducing costs and space in industrial systems.

High-Voltage FETs

Another option for deriving auxiliary DC power is to use a high-voltage field-effect transistor (FET) that can operate directly from the AC line power (Figure 1).

Figure 1: A line-connected auxiliary power supply configuration. (Source: ROHM, redrawn by Mouser)

For designers looking to implement this solution, ROHM Semiconductor offers a variety of devices that prioritize efficiency, reliability, low standby power, and safety. These include silicon metal-oxide-semiconductor (Si-MOS) FETs rated at 730V and above. As shown in Figure 2, the internal architecture of the 730V/800V models separates the power MOSFET, startup circuit, and control circuit to provide high noise immunity.

Figure 2: Diagram of isolated functional blocks for enhanced noise immunity. (Source: ROHM)

A recent addition to ROHM’s product lineup is the BM2SC12xFP2 series, which integrates an SiC MOSFET with quasi-resonant (QR) controller to boost efficiency and reduce EMI. One version, housed in a TO263 surface-mount-device (SMD) package, features a built-in 1,700V SiC MOSFET with a drain-source on-resistance (RDS(on)) of 1.12W. It also includes an internal gate driver and control circuitry, eliminating the need for external components. This approach not only cuts down on bill of materials (BOM) costs and printed circuit board (PCB) space but also minimizes loop inductance typically introduced by external gate drivers, thereby lowering switching losses.

Figure 3 illustrates four possible implementations of an auxiliary power supply utilizing Si-MOS and SiC devices. The blue horizontal bars represent the achievable power output without a heat sink, while the pink bars indicate the range where a heat sink is required. At the top, a design using twelve discrete components can operate only to approximately 8W without a heat sink, and scales to 25W when one is added.

Figure 3: Power output with heat (pink) and without (blue) heat sinks across four circuit types. (Source: ROHM)

The design at the bottom of Figure 3 highlights the efficiency of a single BM2SC121FP2 in a TO263 package, delivering an impressive 48W of power without a heat sink. The integrated BM2SC12xFP2 series is an ideal choice for compact designs where minimizing heat sink size, cost and maintenance is critical.

Looking at the second row of Figure 3, a 1,500V Si-MOS device with additional components can achieve 12W output without a heat sink and up to 100W with one. The third row highlights a BM2SC12xFP2 device in a leaded TO220 package capable of reaching 100W with a heat sink and 36W without one. While impressive, this still falls short of the 48W that the SMD version can offer without a heat sink.

QR Switching

A standout feature of the BM2SC12xFP2 series is the use of quasi-resonant (QR) switching instead of traditional pulse-width modulation (PWM). QR switching enables soft-switching, allowing transitions to occur at zero-current or low-voltage points in the switching cycle to minimize voltage spikes, ringing, and interference, while simplify filtering design. The BM2SC12xFP2 series achieves QR switching through built-in gate control, eliminating oscillations and ensures stable operation. This integration also reduces the need for external components, enabling simpler, more compact designs. QR switching also improves thermal performance, contributing to the ability of the SMD version to deliver up to 48W without a heat sink.

Evaluation Kits

To help jump-start auxiliary power supply development, ROHM offers a variety of evaluation boards, including options for the BM2P06xMF-Z series of PWM converters, which support  5V, 12V, or 24VDC output from a range of input voltages. For example, the BM2P060MF-EVK-001 evaluation board (Figure 4) features a BM2P061MF-Z converter with PWM controller and 650V switching MOSFET in a SOP-20A package. This board can provide up to 1.67A at 24V from a 90V to 264VAC input voltage.

Figure 4: The ROHM BM2P060MF-EVK-001 evaluation board provides an example reference circuit of an isolated flyback converter controlled by PWM. (Source: ROHM)

To further assist designers, ROHM offers an AC/DC design tool that simulates circuit behavior and displays waveforms to verify operation before prototyping. Automated calculation sheets are also available to generate key design parameters for QR and buck topologies.

Conclusion

Auxiliary supplies are used to power controllers, sensors, data loggers, and communications devices across a range of applications, from 400V three-phase industrial equipment, renewable energy systems, and robotics to industrial HVAC and uninterruptible power supplies, as well as pumps, elevators, conveyors, and cranes.

To meet these needs, ROHM Semiconductor offers a comprehensive portfolio of AC/DC conversion solutions, including PWM and QR controllers, power FETs, and fully integrated solutions that combine FETs, controllers, and gate drivers to simplify designs. Key products include the BM2SC12xFP2 series, which integrates a SiC FET and QR controller. A surface-mount variant in this series can operate directly from a high-voltage AC supply and deliver up to 48W of DC power without a heat sink, helping engineers reduce both system costs and board space in auxiliary power supply designs.

Author

Rick Nelson is a technical journalist who has served as executive editor of Test & Measurement World, chief editor of EDN, and executive editor of EE-Evaluation Engineering. He has also contributed to publications including Vision Systems Design and Electronic Design, and he has participated in many live panel discussions and webcasts. Rick has also held systems-engineering and product-development positions at General Electric and Litton Industries. He received his B.S.E.E. degree from The Pennsylvania State University.