Question

What are the safety standards for SMPS design?

Answer 1

When designing a Switched-Mode Power Supply (SMPS), ensuring safety is crucial to protect both the end-user and the equipment. There are several safety standards and guidelines that need to be followed during the design process. These standards cover various aspects like electrical safety, electromagnetic compatibility (EMC), and environmental safety. Below are the key safety standards for SMPS design:

1. Electrical Safety Standards

These standards ensure that the SMPS does not pose any electrical hazards (e.g., electric shock, fire, etc.).

1. IEC 60950-1 / IEC 62368-1: These are international safety standards for IT equipment, audio/video equipment, and similar devices. They define the requirements for electrical insulation, creepage, clearance distances, and protection against electrical shock.
   

  - IEC 60950-1: Older standard that was used for electrical safety, mainly for telecommunications and data equipment.
  - IEC 62368-1: A more modern standard that replaces IEC 60950-1 and is intended to cover both safety and risk-based approaches in electrical appliances, including SMPS designs.

1. UL 60950-1 (US Standard): Similar to IEC 60950, this standard is widely used in the U.S. for electrical and electronic equipment.
   

1. IEC 61010-1: This standard applies to electrical equipment used for measurement, control, and laboratory use. SMPS designs used in such equipment need to adhere to this standard.
   

  

1. Isolation and Creepage/ Clearance Distances: SMPS circuits typically involve high-voltage areas and low-voltage areas that must be electrically isolated from each other. Safety standards define minimum distances for creepage (distance along the surface of an insulator) and clearance (air distance between conductive parts) to prevent electric shock and arc formation.
   

1. Overvoltage Protection: SMPS designs often include surge protection features to safeguard the device against voltage spikes. This includes MOVs (Metal Oxide Varistors) or TVS (Transient Voltage Suppressors).
   

2. Electromagnetic Compatibility (EMC)

These standards ensure that the SMPS does not emit excessive electromagnetic interference (EMI) and is immune to external EMI, which could affect its operation.

1. EN 55022 / CISPR 22: This standard specifies limits for radio frequency interference (RFI) from electrical equipment. SMPS must meet limits for radiated and conducted emissions.
   

  

1. IEC 61000 series: This series deals with electromagnetic compatibility and defines immunity and emission limits to ensure that the device does not disturb other electronic equipment and is also immune to external electrical noise. Key sub-standards:
   

  - IEC 61000-4-2: Electrostatic discharge (ESD) immunity.
  - IEC 61000-4-3: Radiated electromagnetic fields immunity.
  - IEC 61000-4-4: Electrical fast transients (EFT) immunity.
  - IEC 61000-4-5: Surge immunity.

3. Thermal Safety

SMPS components can generate significant heat, especially during high power operation. Overheating can cause damage or even fire hazards, so it's essential to follow thermal design standards:

1. UL 94: This standard defines the flammability of plastic materials. SMPS designers should use materials that are rated for high fire resistance (e.g., V-0 rated materials).
   

1. Thermal Runaway Protection: Adequate cooling and heat dissipation are necessary to prevent thermal runaway in components such as capacitors, transistors, or ICs.
   

1. Over-temperature Protection: Many SMPS designs integrate over-temperature protection circuits (e.g., thermal shutdown) to disconnect the power supply in case of excessive temperature.
   

4. Power Factor and Efficiency Standards

1. IEC 61000-3-2: This standard covers harmonic emissions for power systems. It requires SMPS to maintain a low total harmonic distortion (THD) to minimize pollution of the power grid.
   

  

1. Energy Efficiency Standards: Energy-efficient designs reduce electrical waste and improve the longevity and safety of the components.
   

  - Level VI / Energy Star: SMPS designs, especially for consumer electronics, must comply with energy efficiency standards like Energy Star, which require a minimum efficiency level during operation.

5. Protection Features

SMPS designs must include various protection mechanisms to enhance safety and reliability:
  

1. Overcurrent Protection (OCP): Prevents excessive current that could damage the power supply and connected circuits.
   

1. Overvoltage Protection (OVP): Ensures the output voltage does not exceed a certain threshold, protecting connected devices.
   

1. Short-circuit Protection (SCP): Protects the SMPS from damage due to a short circuit at the output.
   

1. Under-voltage Lockout (UVLO): Ensures the SMPS does not operate below a safe voltage threshold, preventing malfunction.
   

6. Environmental and RoHS Compliance

1. RoHS (Restriction of Hazardous Substances): SMPS designs must comply with RoHS regulations to minimize the use of harmful substances (e.g., lead, mercury, cadmium).
   

1. WEEE (Waste Electrical and Electronic Equipment): Ensures that the device is designed with the end-of-life disposal process in mind to reduce environmental impact.
   

7. Safety Testing and Certification

After designing an SMPS, manufacturers must test the device to verify compliance with the applicable safety standards. Certification agencies such as UL, TÜV, and CSA often perform these tests. Once certified, the device will display a mark (e.g., UL mark, CE mark) showing it has passed the required safety standards.

Conclusion

Incorporating safety standards into the SMPS design is essential to ensure not only the protection of the device but also the safety of users and the environment. Standards such as IEC, UL, and RoHS provide a clear set of requirements for electrical, electromagnetic, and environmental safety. Adhering to these standards will help ensure that your SMPS is both reliable and safe to use.