Meeting the Challenges of AC-DC Converters for Industrial Applications

The ongoing automation of industrial facilities includes paradigms such as Industry 4.0 and lights-out warehouses. Whether the application provides power to an advanced robotic arm for welding or the sensors used in remote condition monitoring, AC-DC converters are a critical design element. The ongoing demand for AC-DC converters has also led to engineering constraints and specifications that these power supply controls must meet. 

 

AC-DC Converters and Industrial Applications

AC-DC converters are necessary for many aspects of industrial factory automation, including robotics, AC servos, and manufacturing equipment. In this design context, they are used in high-voltage applications such as commercial air condition systems, general inverters, and auxiliary power supplies for industrial lamps. Furthermore, AC-DC converters are an integral part of motors, heaters, sensors, PLCs, and controller systems. And while these industrial uses are wide and varied, there are certain challenges related to AC-DC converters common to all designs.

 

Figure 1. AC-DC converters are a necessity in every industrial facility and warehouse. Image provided courtesy of Pixabay. 

 

Primary Challenges for Today’s AC-DC Converters

The equipment that depends on AC-DC converters for power continues to raise the bar when it comes to certain characteristics and aspects of design. Many of the challenges faced by modern AC-DC converters are related to efficiency, heat generation, downsizing, protection functions, and obsolescence.

Efficiency

Higher levels of efficiency, defined as output power/input power, are increasing in demand. Efficiency is related to the power consumed when a device is in standby mode, leading to the challenge of reducing the power draw even when the device is not actively being used. Efficiency is also closely related to size. Typical efficiencies for AC-DC converters are on the order of 80% but can reach up to 95% with the right selection of parts and systems.

Heat Generation

The difference between the input power and output power represents a loss that takes the form of heat generation. Controlling and minimizing heat generation is critical for many reasons, including the damage it can cause to surrounding electronic components. 

 

Figure 2. Heat sinks are a common approach to controlling the effects of heat generation. Image provided courtesy of Pixabay.

 

In the context of industrial AC-DC converters, heat generation is most often addressed through the use of a heatsink to aid in heat dissipation. What’s more, a cooling fan may also be required to keep temperatures at an acceptable level. The necessity of heatsinks and cooling fans further decreases efficiency and increases the size of the system. 

Downsizing

Downsizing is another major challenge for AC-DC converters and involves not just the number of parts but also their size. The need for a small footprint and low part count does lead to more flexibility in installations and reduced manufacturing and shipping costs, but it can be extremely difficult to achieve. And the goal of downsizing is further complicated by the need for heatsinks and cooling fans. 

Protection Functions

AC-DC converters require effective safety functions that support both safety and reliability. There are three basic types of protection functions needed in AC-DC converters:

• Input (over-voltage, input voltage drop with non-functional)
• Output (overload, short circuit, reverse voltage, and over-voltage)
• Temperature (excessive self-heating and increased ambient temperature)

However, including the right protection functions can complicate downsizing and miniaturization unless a control IC with integrated protection functions is used.

Obsolescence

Another problematic issue faced by engineers and designers is obsolescence. Is the selected AC-DC converter guaranteed for the life of the product? Being forced to do a redesign is serious enough, but doing so because of the obsolescence of an AC-DC converter is even worse. The aftermath of such a redesign is often more extensive than it may initially appear. Not only is another revision of the PCB necessary, but there is a high probability that EMC testing will need to again be performed for the product. This, in turn, means associated paperwork must be resubmitted (and even a complete redo of the safety analysis).

 

Solutions

There are three specific approaches to the design and implementation of AC-DC converters that help meet the challenges just discussed. Each of these measures--power supply control ICs, switching systems, and high voltage SiC MOSFETs--combine to provide an AC-DC converter solution that meets the needs of today’s industry. 

Power Supply Control ICs

Power supply control ICs can integrate multiple features onto a single IC chip. First, note that such an approach is far easier to implement in a design than a discrete configuration and supports efforts at downsizing, both in terms of part count and footprint. Power supply control ICs also offer enhanced reliability, multiple built-in protection functions, and better efficiency. In addition, most manufacturers will provide support during the IC design process, contributing to a shortened design time and time to market.

Switching Systems

Another approach to more effective industrial AC-DC converters is the use of switching systems as opposed to the more traditional transformer circuit. The implementation of switching systems contributes to downsizing despite requiring additional parts. Although the circuitry is more complex, it can be simplified using a control IC. 

 

Figure 3. Simplified comparison of a transformer circuit and a switching circuit. Image courtesy of ROHM Semiconductor. 

 

Compared to traditional transformer systems, switching systems offer higher efficiency and less heat dissipation along with smaller size and weight. They do, however, require high-voltage tolerant components, including high-power MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors).

SiC MOSFETs

One approach to effectively configure switching systems is to use high-power SiC (Silicon Carbide) MOSFETs. SiC MOSFETs are available in a single compact surface-mount package and offer several benefits for industrial AC-DC converters. They are more efficient than Si (Silicon) MOSFETs and contribute to downsizing by simplifying circuitry requiring fewer components. This translates to a shortened design cycle and reduced costs. And their superior reliability and efficiency minimize the risk of failure and contribute to overall system efficiency.

 

Figure 4. A comparison of the efficiency of Si MOSFETs vs SiC MOSFETs when used in an AC-DC converter. Image provided courtesy of ROHM Semiconductor. 

 

Usage Example

Consider an AC-DC converter needed for a servo drive for an industrial robot. The specifications are the following:

• 400 VAC 48 W 
• Normal operating current: 600 µA
• Burst operating current: 500 µA
• Maximum operating frequency: 120 kHz
• Operating temperature: 20 °C to 95 °C
• Available in a TO263-7L package
• Protections that include under voltage lockout and over voltage protection

Also included are the following design constraints:

• Low EMI emission
• Excellent efficiency and reduced losses
• Low current consumption during standby
• Reduced electric power when a light load is present 
• Supports miniaturization
• Reduced part count and BOM
• Obsolescence guarantee of at least 2 years
• Low cost

An example of AC-DC converters suitable for industrial servo drive are ROHM’s BM2SC12xFP2-LBZ series shown in Figure 5. Specifically, consider the BM2SC121FP2-LBZ.

 

Figure 5. One of the models in ROHM’s new industrial AC-DC converter product line. Image used courtesy of ROHM Semiconductor. 

 

Shown here are the basic specifications, which all fall within those of the industrial AC servo drive under consideration:

• Optimized for 400 VAC 48 W applications
• Normal operating current: 800 µA
• Burst operating current: 500 µA
• Maximum operating frequency: 120 kHz
• Operating temperature: -40 °C to +105 °C
• TO263-7L package
• Includes multiple protection circuits, such as under voltage lockout, over voltage protection, and high accuracy thermal shutdown function

A quasi-resonant (QR) switching control system enables soft switching for low EMI. Also included is a 1700 V SiC MOSFET (1.12 Ω) in a single compact surface-mount package (TO263-7L) . The SMP for this particular line of converters is particularly well-adapted for industrial AC servo drives and general-purpose inverters. 

 

ROHM’s BM2SC12xFP2-LBZ also falls within the following constraints:

• Low EMI emission: quasi-resonant operation results in low EMI emission
• Excellent efficiency and reduced losses: exhibits 28% lower losses with 5% higher efficiency using SiC MOSFET (see Fig. 4) compared to conventional Si MOSFETs
• Low current consumption during standby: 19 µA
• Reduced electric power when a light load is present: burst operation reduces an electric power at light load
• Supports miniaturization: measures just 10.18 mm x 15.5 mm x 4.43 mm; in addition, high withstand voltage and superior voltage noise resistance characteristics of the internal SiC MOSFET enable smaller components used for noise suppression
• Reduced part count and BOM: replaces up to 12 components (AC/DC converter IC, 800V SiC MOSFET x 2, Zener diode x 3, resistor x 6) and heat sink
• Obsolescence: guaranteed supply for at least five years (and sometimes ten years)
• Low cost: supports automated board mounting and fewer external parts, both of which serve to reduce assembly costs, while built-in protection reduces the time needed to select components and evaluate their reliability for drive and clamp circuits

Figure 6. Comparison of conventional AC-DC converter solution to a BM2SC12xFP2-LBZ solution. Image courtesy of ROHM Semiconductor.

 

Note that this AC-DC converter achieves breakthrough levels of miniaturization, contributing significant downsizing. Along with multiple protection functions, a high-accuracy thermal protection circuit is built in. The monolithic design reduces the probability of component failure, simplifying the circuit considerably, as illustrated in Fig. 6.

 

Conclusion

Every year, the demand for AC-DC converters grows, and with that demand, there are evolving requirements and challenges that must be met. Industrial applications that require power supply controls, in particular, must meet design requirements such as higher efficiency, better control of heat generation, downsizing in terms of both size and part count, more extensive protection functions, and reducing the risk of obsolescence. This also holds true for a range of applications, including everything from industrial lighting and air conditioners to PLCs and manufacturing equipment.

These design requirements can be met through the right choice of AC-DC converters, which includes those that combine features such as utilizing power control ICs. This is opposed to a discrete component approach, using switching systems instead of conventional transistors and high voltage SiC MOSFETs in place of Si MOSFETs.

The example provided earlier is just a single application of ROHM’s new BM2Pxxx series of high-efficiency industrial AC/DC Converter ICs comprised of 24 models to meet a variety of industrial auxiliary power supply needs up to 100W. These power control ICs have built-in 1700V SiC MOSFETs, use a QR switching system, and are encapsulated within a compact surface mount package. Also available is the BM2SC123FP2-EVK-001 evaluation board using ROHM’s AC/DC BM2SC123FP2-LBZ IC to support engineers through various stages of the design process.