Fairchild offline flyback PWM with integrated MOSFETs is an integral part of a power supply system design. Our devices will not meet your requirements unless all other parts that make the system also do their job. The transformer is an important component that is often seen as a black box that is someone else's problem. Designing the right transformer requires consideration in these six areas:
- Safety standards
- Design requirements
- IC selection
- How to make it for manufacturability and allowing flexibility to the manufacturer
- How to verify if they have a good design
- Ensuring a good size to cost to performance balance
What safety standards matter?
When designing an offline flyback power supply, integrating the IC with the other components can be overwhelming. Integrating the transformer properly is more than finding the turns-ratio to match with your input and output(s). “I thought a Flyback transformer is just transferring energy across a safety isolation barrier.” “I guess there is more to understand.” Not knowing the safety standards will usually mean more iterations than necessary when working with a transformer manufacturer. Not knowing the safety standards can lead to unwanted surprises of non-conformance at the qualifying labs and sending a product back to design phase rather than qualification before mass production.
Understanding the requirements that govern transformer design and manufacturing will shorten design cycles to finished products using Fairchild parts in Power Supply WebDesigner (PSW). Just knowing the safety standards required for your design will be enough to allow Wurth Electronics Midcom to design the right part the first time over 90% of the time.
Knowing safety standard required during product definition or design definition will ensure unnecessary design iteration and cost. Below you will find the most common standards used in the industry to define the safety requirements of offline transformer. Wurth Electronics Midcom is very familiar with these standards and typically knowing which standard you are trying to meet and the insulation type is enough to get you what you need.
Transformer safety standard to use
EN 50470-1: AC Electricity metering equipment
EN 60935: Fixed inductors for electromagnetic interference suppression
EN 61347-2-13: LED module lamp control gear for both AC and DC applications
IEC 60335-1: Safety for household and similar electrical appliances
IEC/EN/UL 60601-1: Medical electrical equipment
IEC 61558-2-16: Safety for switch mode power supply units and transformers for switch mode power supply units
IEC 60950-1: Safety for Information technology equipment
UL1310: Standard for Class 2 Power Units
From Wurth Electonics Midcom
You will be asked to define which safety standard you are using. Depending on your application, grounding method, as well as power supply housing your level of insulation can vary. Typical insulation definitions used in power supplies include:
Functional: Ensures operation of the device only and no level of protection
Basic: Provides a single layer of protection and a second is usually required elsewhere
Reinforced: The most common for offline and provide the need two layers of protection
While not all standards define an insulation level, i.e. UL1310 does not, all standards do require to meet a certain dielectric withstand voltage. While dielectric withstand voltage is sufficient for functional insulation designs, it is not sufficient for Basic or Reinforced — other transformer design consideration must be made.
The working voltage is used in defining how the transformer needs to be built to meet the standards. Typically this is the input voltage on the power supply for an offline power supply. Other power supplies such as cell balancing applications can have high (cell) voltages that includes the input. This will affect your working voltage for safety considerations.
To find more information:
Fairchild — go visit Power Supply WebDesigner
Wurth — go visit here
- Link to the EDN article: Transformer design: For which safety standards should you be concerned?