【Introduction】AC/DC power supply designers are under constant competitive pressure to reduce cost, design time and form factor while improving energy efficiency and meeting a range of global electromagnetic compatibility (EMC) requirements. Converters must also be able to maintain energy efficiency and performance over a wide range of AC (and sometimes DC) input voltages, provide a wide operating temperature range, and ensure equipment and user safety with output short-circuit and overcurrent protection.
AC/DC power supply designers are under constant competitive pressure to reduce cost, design time and form factor while improving energy efficiency and meeting a range of global electromagnetic compatibility (EMC) requirements. Converters must also be able to maintain energy efficiency and performance over a wide range of AC (and sometimes DC) input voltages, provide a wide operating temperature range, and ensure equipment and user safety with output short-circuit and overcurrent protection.
Designing a flexible multi-application power supply is a time-consuming and difficult task that requires a specific set of skills. Even with these skills in-house, time to market can still be greatly increased. While there are off-the-shelf modules that meet specific performance specifications, designers must choose another module if design requirements change.
To address this, designers can use board mount AC/DC converters to meet core regulatory, footprint, and performance specifications while offering a high degree of customization to meet changing requirements.
This article discusses issues related to power design for low-power devices. It then introduces Mornsun’s small form factor AC/DC converters and shows how these converters can be easily customized to meet the requirements of various applications. This article shows how designers can minimize cost, increase energy efficiency, and reduce solution size by optimizing multi-application AC/DC converters, while ensuring user and equipment safety and achieving specific EMC levels.
Design Requirements for Low-Power Device Power Supplies
In terms of electromagnetic interference (EMI) and electromagnetic immunity (EMS), EMC requirements range from minimal filtering for some consumer applications to the need to meet CISPR32/EN55032 Class B level EMI (radiated) and IEC/EN61000 EMS (immunity). degree) level 4 (Table 1) for industrial systems and outdoor locations. In addition, these AC/DC converters must meet Class 6 efficiency standards, operate over a wide temperature range, have output short-circuit and overcurrent protection, and be compact and low-cost.
Table 1: By adding peripheral components, Mornsun’s AC/DC converters can be customized to meet the emissions and immunity requirements of various applications. (Table source: Mornsun)
Since it may be necessary to design or select a power supply for each application and its specific requirements, there are costs in design time, cost, and inventory management. Therefore, a more cost- and resource-efficient approach is to use standard power modules that are within the performance envelope of various applications and are easily optimized to meet the specific needs of each target application.
To take this approach, designers can choose to use Mornsun’s LS-R3 series of board-mounted AC/DC converters that meet a range of EMI and EMS requirements. The basic core board provides 3 to 10 W of power output and measures 28 x 14.73 x 11 millimeters (mm), which is 43 percent smaller than a converter of equivalent power (Figure 1).
Figure 1: The LS-R3 series of AC/DC converters are highly reliable flyback power stages that can be customized to meet a range of EMI/EMS levels. (Image credit: Mornsun)
The converter can be easily customized to meet various EMI/EMS specifications, up to CISPR32/EN55032 Class B EMI, IEC/EN61000-4-4 ±4 kilovolt (kV) electrical fast transient (EFT), and 4 ±2 kV surge for class EMS. By optimizing EMI/EMS levels, designers can implement multi-application AC/DC converters, minimizing cost and solution size.
The LS-R3 series is IEC/EN/UL62368 safety certified, complies with Class 6 energy efficiency standards, features output short-circuit and overcurrent protection, and a wide operating temperature range of -40°C to +85°C.
Multi-application customization starts with basic fusion and filtering needs. While the LS-R3 series has an input range of 85 to 305 V AC (or 70 to 430 V DC), individual applications operate at specific grid voltages such as 110, 230 or 277 V AC, which require corresponding ratings fuses (Table 2). For example, the Model LS05-13B12R3 has an output of 12 V DC and a current of 420 milliamps (mA), when used in equipment operating from a 277 V AC input, use Littelfuse’s 36911000000 fuse.
Table 2: The input range of the LS-R3 series is 85 to 305 V AC. The fuse selection is based on the actual grid voltage when using the power converter. (Table source: Mornsun)
As mentioned earlier, the LS-R3 series ranges from 3 W converters such as the LS03-13B03R3 with an output of 3.3 V DC, up to the 10 W LS10-13B24R3 with an output of 24 V DC. All three series (3, 5 and 10 W) are available with output voltages from 3.3 to 24 V DC. The design example discussed below is based on the LS05-13BxxR3 family of 5 W converters.
The basic design starts with a fuse, then adds a wirewound resistor such as Vishay’s 12 ohm (Ω), 3 W AC03000001209JAC00 to reduce inrush current and provide limited surge protection; an input capacitor such as Rubycon’s 450BXW22MEFR12.5X20 at 22 microfarads (μF), 450 V; and a basic output filter capacitor such as the Nichicon RS81C271MDN1, rated at 270 microfarads (μF) and 16 V (Figure 2).
This basic implementation is EMS Level 3 compliant, but cannot meet the more stringent EMI or EMS specifications and is only suitable for the most cost-sensitive designs and very basic performance needs. It operates from 85 to 305 V AC input, depending on fusing conditions, and produces an isolated DC output. With only minimal output filtering, it does not meet most EMI or EMS specifications, and the output ripple is relatively high.
Figure 2: Basic design renderings of this cost-sensitive design conforming to EMS Level 3 standards, including four external components (fuse, the black component on the left; input capacitor, the black cylinder in the middle; input resistor, located in the The left side of the input capacitor; the output filter capacitor, which is the white cylinder on the right). (Image credit: Mornsun)
For designs that require more output filtering and higher levels of EMI performance, three additional components can be added (Figure 3). A “Y” capacitor across the primary and secondary sides of the converter greatly reduces noise and improves EMI performance. NOTE: To meet the IEC/EN60335 standard for household appliances, a second “Y” capacitor may need to be added.
Adding a Pi filter can significantly reduce the output ripple. This can be achieved by using the output capacitor in the base design and adding an electrolytic capacitor (such as Rubycon’s 35THV47M6.3X8 rated at 47 μF and 35 V) and an Inductor.
Figure 3: This secondary design adds a “Y” capacitor (blue component) on the primary and secondary sides to further reduce noise and EMI, and a Pi filter to reduce output ripple (the filter The controller was designed by removing the original white output capacitor and adding an electrolytic capacitor (black component on the upper right) and an inductor (gray component below the electrolytic capacitor). (Image credit: Mornsun)
Designs requiring Class A or Class B EMI levels and Class 4 EMS performance can also be implemented using the LS-R3 core printed circuit board (Figure 4). Placing a differential mode inductor at the input enables the design to meet Class A EMI requirements.
Figure 4: Add a differential mode input inductor (L1, black cylinder under the PC board) to meet Class A EMI limits; add another “X” capacitor (CX1, yellow component, left of center) to meet Class B EMI ; and add a varistor (MOV1, blue component on the upper left) at the AC input to meet the 4-level EMS. (Image credit: Mornsun)
Class B EMI limits can be met by adding an “X” capacitor, such as TDK’s B32671Z6104K000, a 0.1 μF, 630 V radial thin-film device. Level 4 EMS performance can be achieved by inserting a varistor, such as TDK’s B72214S0351K101 Metal Oxide Varistor (MOV).
The complete circuit shown in Figure 5 can meet EMI (CISPR32/EN55032) Class B levels, EMS (IEC/EN61000) EFT ±4 kV, and ±2 kV surge.
Figure 5: The complete circuit diagram shown meets EMI (CISPR32/EN55032) Class B levels, EMS (IEC/EN61000) EFT ±4 kV, and ±2 kV surge. (Image credit: Mornsun)
In Figure 5, CY2 is the second Y-type capacitor mentioned earlier and is a necessary component to meet the IEC/EN60335 requirements for household appliances. LDM is a differential inductor.
Board Layout Considerations
Once the specific application design based on the LS-R3 is complete, it is time to lay out the peripheral components on the PC board. The LS-R3 core PC board complies with IEC/EN61558, IEC/EN60335 and IEC/EN/UL62368 requirements for insertion into PC boards containing peripheral components.
Two key considerations for peripheral components PC boards are correct size and weight specifications for copper traces, and the need for adequate creepage and clearance distances to meet safety requirements.
The minimum copper wire width, thickness and weight need to be calculated based on the required ampacity and the maximum allowable temperature rise of the copper. IPC 2221A, “General Standard for Printed Circuit Board Design,” provides information on design requirements for organic printed circuit boards, including how to calculate copper specifications.
When specifying creepage and clearance distances, IEC 60335-1 or IEC 60950-1 needs to be considered. Household appliances and similar appliances rated up to 250 V single-phase alternating current are covered by IEC 60335-1, and information technology (IT) is covered by IEC 60950-1.
The gap distance is the distance through the air between two conductive parts. IEC 60950-1 is the more stringent standard, requiring a 4.0 mm gap for reinforced insulation at working voltages of 150 to 300 V, while IEC 60335-1 requires 3.5 mm.
Creepage distance is the shortest distance along a surface between two conductive parts. In this case, IEC 60335-1 is more stringent, requiring 8.0 mm of creepage distance for reinforced insulation at working voltages of 250 to 300 V, while IEC 60950-1 requires only 6.4 mm of creepage distance. Both standards require a creepage distance of 5 mm if the operating voltage is between 200 and 250 V.
Prefabricated PC Board
While peripheral components may require custom-designed PC boards, prefabricated PC board layouts from Mornsun can be used when packaging requirements are less stringent. For the design example shown, Mornsun offers 11 prefabricated PC board layouts based on the LS05-13BxxR3 family: two for the base design, three for Class B EMI and Class 3 EMS, and three for Class A EMI and Class 4 EMS , and three meet Class B EMI and Class 4 EMS.
Each peripheral PC board layout also has a bill of materials (BOM) optimized for the mechanical requirements of that PC board. For example, for the above Class 4 EMS and Class B EMI compliant LS05-13BxxR3 solution, designers can choose from three prefabricated peripheral PC board layouts (with corresponding BOMs).
Minimum height optimized: 48.5 mm long, 32.2 mm wide, 17 mm high
Almost equal length and width dimension optimization: length 40.5 mm, width 37.5 mm, height 23 mm
Optimized minimum width: 55 mm long, 25.3 mm wide and 23 mm high
Power designers across a variety of applications and EMC levels face similar cost, energy efficiency, size, and time-to-market challenges. To compete effectively and minimize inventory, designers need to be able to take pre-engineered core modules that can be easily customized to meet specific requirements.
As shown, the LS-R3 series of board mount multi-application AC/DC converters facilitate this rapid customization to meet various EMC (including EMI and EMS) requirements, up to Class B EMI and Class 4 EMS performance. In addition, the availability of prefabricated PC boards ensures that the correct copper wire size and required creepage and clearance distances are used.
(, Source: China Power Grid, Author: Jeff Shepard)