Designers of AC/DC power supplies are under constant competitive pressure to reduce cost, design time, and form factor while improving efficiency and meeting a range of global electromagnetic compatibility (EMC) requirements. They must also maintain efficiency and performance across a wide range of AC (and sometimes DC) input voltages, operate over a wide temperature range, and ensure device and user safety with output short circuit and overcurrent protection.
Designing a flexible, multi-application power supply is a time-consuming and daunting task that requires a specific set of skills. Even with those skills in-house, it can still significantly increase time to market. While off-the-shelf modules are available with specific performance specifications, if requirements change, the designer has to opt for another module.
To resolve this issue, designers can use board mount AC/DC converters that meet the core regulatory, footprint, and performance specifications, while facilitating a high degree of customization to meet changing requirements.
This article discusses the issues around power supply design for low-power devices. It then introduces small-form-factor AC/DC converters from Mornsun and shows how they are easily customized to meet a variety of applications. It shows how, through these optimizations, designers can realize multi-application AC/DC converters that minimize cost, maximize efficiency and reduce solution size, all while ensuring user and device safety and delivering specific levels of EMC.
Design requirements for low-power power supplies
EMC requirements, in terms of both electromagnetic interference (EMI) and electromagnetic susceptibility (EMS), range from minimal filtering for some consumer applications to industrial systems and outdoor locations that need to meet CISPR32/EN55032 Class B level for EMI (emissions) and IEC/EN61000 EMS (immunity) Level 4 (Table 1). In addition, these AC/DC converters must meet Level 6 efficiency standards, operate over a wide temperature range, have output short circuit and overcurrent protection, and be compact and low cost.
Table 1: With the addition of peripheral components, AC/DC converters from Mornsun can be customized to meet a wide variety of application requirements for emissions and immunity. (Table source: Mornsun)
While it may be necessary to design or select a power supply for each application and its specific set of requirements, that comes at a cost in terms of design time, cost, and inventory management. A more cost and resource-effective route is to use a standard power supply module that is within the performance range of a wide variety of applications, and which is easily optimized to meet the specifics of each target application.
To start along this route, designers can turn to the LS-R3 series of board mount AC/DC converters from Mornsun that meet a range of EMI and EMS requirements. The basic core board is available with power outputs between 3 to 10 watts and measures 28 x 14.73 x 11 millimeters (mm), 43% smaller than converters of comparable power (Figure 1).
Figure 1: The LS-R3 series of AC/DC converters are highly reliable flyback power stages designed to be customized to meet a range of EMI/EMS levels. (Image source: Mornsun)
It’s easily customized to meet a variety of EMI/EMS specifications, up to CISPR32/EN55032 class B level for EMI, IEC/EN61000-4-4 ±4 kilovolts (kV) electrical fast transient (EFT), and ±2 kV surges for Level 4 EMS. By optimizing the EMI/EMS levels, designers can realize multi-application AC/DC converters that minimize cost and solution size.
The LS-R3 series carries IEC/EN/UL62368 safety approval and meets Level 6 efficiency standards, has output short circuit and overcurrent protection, and a wide operating temperature range of -40°C to +85°C.
Multi-application customization starts with basic fusing and filtering needs. While the LS-R3 series has an input range of 85 to 305 volts AC (or 70 to 430 volts DC), individual applications operate from specific grid voltages such as 110, 230, or 277-volts AC, which require corresponding fuse ratings (Table 2). For example, the model LS05-13B12R3 which has an output of 12 volts DC at 420 milliamperes (mA), can use the 36911000000 fuse from Littelfuse in devices operating from 277 volts AC inputs.
Table 2: The LS-R3 series has an input range of 85 to 305 volts AC. Fuse selection is based on the actual grid voltage where the power converter will be used. (Table source: Mornsun)
As mentioned, the LS-R3 series ranges from 3-watt converters such as the LS03-13B03R3 with an output of 3.3 volts DC, up to the 10 watt LS10-13B24R3 with an output of 24 volts DC. All three series—3, 5, and 10 watts—are available with models delivering output voltages from 3.3 to 24 volts DC. The design examples in the following discussion are based on the LS05-13BxxR3 series of 5-watt converters.
The basic design begins with the fuse and adds a wire wound resistor such as the 12 ohm (Ω), 3 watt AC03000001209JAC00 from Vishay to reduce inrush current and provide a limited level of surge protection; an input capacitor, like the 22 microfarad (μF), 450 volt 450BXW22MEFR12.5X20 from Rubycon; and a basic output filter capacitor, like the Nichicon RS81C271MDN1, rated for 270 microfarads (µF) and 16 volts (Figure 2).
This basic implementation meets EMS Level 3, but cannot meet more demanding EMI or EMS specifications, and is only intended for the most cost-sensitive designs with very basic performance needs. Depending on the fusing, it can operate with inputs from 85 to 305 volts AC and produces an isolated DC output. With minimal output filtering, it does not meet most EMI or EMS specifications and has relatively high output ripple.
Figure 2: This basic design rendering of a cost-sensitive design meets EMS Level 3 and comprises four external components (fuse, black component on left; input capacitor, black cylinder in the center; input resistor, to the left of the input capacitor; an output filter capacitor, white cylinder on the right). (Image source: Mornsun)
For designs that require more output filtering and a higher level of EMI performance, three additional components can be added (Figure 3). A “Y” capacitor, placed across the primary and secondary side of the converter, significantly reduces noise and improves EMI performance. Note: To meet IEC/EN60335 for home appliances, it may be necessary to add a second “Y” capacitor.
Adding a Pi filter significantly reduces output ripple. This can be accomplished using the output capacitor in the basic design and adding an electrolytic capacitor, such as Rubycon’s 35THV47M6.3X8, rated for 47 μF and 35 volts, along with an inductor.
Figure 3: This second-level design adds a “Y” capacitor (blue component) across the primary and secondary side to further reduce noise and EMI, and incorporates a Pi filter (designed by taking the original white output capacitor and adding an electrolytic capacitor (black component in upper right) and an inductor (grey component below the electrolytic capacitor)) to reduce output ripple. (Image source: Mornsun)
Designs that require Class A or Class B levels of EMI and Level 4 EMS performance can also be realized using the LS-R3 core printed circuit (pc) boards (Figure 4). Placing a differential-mode inductor on the input enables the design to meet Class A EMI requirements.
Figure 4: Class A EMI limitations can be met with the addition of a differential-mode input inductor (L1, black cylinder on lower part of pc board); Class B EMI can be met with the further addition of an “X” capacitor (CX1, yellow component, left of center), and Level 4 EMS can be met by adding a varistor (MOV1, blue component top left) at the AC input. (Image source: Mornsun)
Class B EMI limitations can be met with the addition of an “X” capacitor such as TDK’s B32671Z6104K000, a 0.1 μF, 630 volt, radial film device. Level 4 EMS performance can be achieved by inserting a varistor, such as TDK’s B72214S0351K101 metal oxide varistor (MOV).
A complete circuit that can meet EMI (CISPR32/EN55032) Class B level, EMS (IEC/EN61000) EFT ±4 kV, and a surge of ±2 kV is shown in Figure 5.
circuit diagram that can meet EMI (CISPR32/EN55032) Class B level” alt=”How to Easily Optimize AC/DC Converters to Meet a Wide Range of EMC Requirements”>Figure 5: Shown is a complete circuit diagram that can meet EMI (CISPR32/EN55032) Class B level, EMS (IEC/EN61000) EFT ±4 kV and a surge of ±2 kV. (Image source: Mornsun)
In Figure 5, CY2 is the second Y capacitor mentioned earlier that is necessary to meet IEC/EN60335 for home appliances. LDM is the differential inductor.
Board layout considerations
Once the application-specific design based on the LS-R3 has been completed, it’s time to lay out the pc board for the peripheral components. The LS-R3 core pc board meets the requirements of IEC/EN61558, IEC/EN60335, and IEC/EN/UL62368, and is designed to plug into the pc board containing the peripheral components.
Two key considerations for the peripheral component pc board are the specification of the correct dimensions and weight for the copper traces, and the need for adequate creepage and clearance distances to meet safety requirements.
The minimum copper trace width, thickness, and weight need to be calculated based on the required current carrying capacity and the maximum allowable temperature rise in the copper. The IPC 2221A “generic standard on printed board design” provides information on the requirements for organic printed board design, including how to calculate the copper specifications.
When specifying creepage and clearance distances, IEC 60335-1 or IEC 60950-1 needs to be considered. Household and similar electrical appliances with rated voltages up to 250 volts AC, single-phase, are covered by IEC 60335-1, while information technology (IT) equipment is covered by IEC 60950-1.
Clearance is the distance between two conductive parts through air. IEC 60950-1 is the more restrictive standard, requiring 4.0 mm of clearance when considering reinforced insulation and a working voltage of between 150 and 300 volts, while the IEC 60335-1 requirement is 3.5 mm.
Creepage is the shortest distance between two conductive parts following along a surface. In this case, IEC 60335-1 is more restrictive and requires 8.0 mm of creepage for reinforced insulation when the working voltage is between 250 and 300 volts, while IEC 60950-1 requires only 6.4 mm creepage. Both standards call for 5 mm of creepage if the working voltage is between 200 and 250 volts.
Pre-engineered pc boards
A custom-designed pc board for the peripheral components may be required, but when the packaging requirements are less restrictive, Mornsun offers pre-engineered pc board layouts. For the design examples shown, based on the LS05-13BxxR3 series, Mornsun offers 11 pre-engineered pc board layouts: two for the basic design, three that meet Class B EMI and Level 3 EMS, three for Class A EMI and Level 4 EMS, and three for Class B EMI and Level 4 EMS.
Each of the peripheral pc board layouts also comes with a bill of materials (BOM) optimized for the mechanical requirements of that pc board. For example, for the LS05-13BxxR3 solution described above that meets Level 4 EMS and Class B EMI, designers can select from three pre-engineered peripheral pc board layouts (with corresponding BOMs):
- Optimized for minimum height: 48.5 mm long, 32.2 mm wide, and 17 mm high
- Optimized for nearly equal length and width measurements: 40.5 mm long, 37.5 mm wide, and 23 mm high
- Optimized for minimum width: 55 mm long, 25.3 mm wide, and 23 mm high
Designers of power supplies across a range of applications and EMC levels face similar cost, efficiency, size, and time-to-market challenges. To compete effectively and minimize inventory, designers need to be able to take a pre-designed core module that is 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 satisfy a wide variety of EMC (both EMI and EMS) requirements, up to Class B EMI and Level 4 EMS performance. Additionally, the availability of pre-engineered pc boards guarantees the use of the correct copper trace dimensions and the needed creepage and clearance distances.