Rutronik News

What Needs To Be Considered When Developing a Switched-Mode Power Supply Unit?


Switched-mode power supply units have replaced traditional linear PSUs in many applications. The transformer plays a key role in their development.

When compared against linear power supply units, switched-mode PSUs offer a number of benefits:

  • Greater efficiency
  • Greater effectiveness
  • Lower size and weight
  • Reduced need for raw materials
  • Greater environmental compatibility

However, low-quality switched-mode PSUs may give rise to voltage drops that cause the device to cease functioning. To prevent this, a number of things should be considered in their development.

The varying nature of the topologies of switched-mode power supply units usually requires the use of a transformer to isolate the output voltage from the input voltage. Transformers vary in terms of

  • construction
  • size
  • the properties of the material from which the ferrite core is made.

These properties, alongside the operating temperature and frequency, largely determine the effectiveness of the PSU.


Wah Hing        Rutronik Item        Core        Power / Voltage
903-2935-00        FETRA1622        EF20        10W / 12V
903-2936-00        FETRA1623        EF20        20W / 12V
903-2937-00        FETRA1624        EF20        12W / 5V
903-2938-00        FETRA1625        EF20        20W / 5V
903-2939-00        FETRA1630        EF25        30W / 12V
903-2940-00        FETRA1627        EF20        5W / 5V
903-2941-00        FETRA1628        EF20        10W / 3,3V
903-2942-00        FETRA1629        EF20        5W / 3,3V
903-2943-00        FETRA1631        EF25        20W / 5V

Transformers are developed with ferrite cores, because ferromagnetic material offers ideal properties for the switching frequencies – it has a much better magnetic conductivity than air, for instance, allowing for a stronger magnetic flux. Even so, it becomes saturated as of certain magnetic flux densities, resulting in a reduction in magnetic conductivity, which in turn causes non-linear transmission.


Ferrite cores that are developed as flyback converters have an air gap in the middle limb, which stores much of the field energy required on the consumer end.


Such ferrite cores are used for the development of Wah Hing transformers, for example. They also have input and output coils with excellent magnetic coupling, which enables them to output the energy stored with the primary current to be discharged through the secondary output when the transistor is switched off. 


A Cost-Effective Alternative to Transformers from Familiar Manufacturers

For modern applications with switching controller ICs, the wired, horizontal EF20 and EF25-size transformers from Wah Hing offer the appropriate input voltage range of 85V to 265V and power range of 5W to 30W. The transformers offer output voltages of 3.3V, 5V and 12V, with other power classes available beyond that. 

With these characteristics, Wah Hing’s models represent a cost-effective alternative to a transformer from a renowned German manufacturer, and even so, developers do not need to compromise on quality in the process.


Customized Transformers for Special-Purpose Power Supply Units

Depending on the application, a power supply unit must satisfy various quality criteria, including a specified switch-on current, low energy consumption in standby mode, or precisely defined buffer times. This is usually not possible to achieve with a standard transformer – customizations are necessary for this purpose. Such customized transformers are only economically viable to develop in production quantities of 10,000 upwards. Wah Hing has specialized precisely in this field and performs various modifications, with the most commonly requested including:


  • special sizes, especially small transformers
  • different input and output voltages
  • coil designs with special turn ratios or multiple output coils
  • individualized isolation classes
  • a higher degree of isolation.


With conventional functional isolation, the coil wire is merely coated in paint. With base isolation, the input and output coils are isolated from one another by an isolating barrier. With reinforced isolation, this separation is even implemented in the form of at least two separate isolating barriers. This is necessary with switched-mode power supply units used in high-grade applications such as medical engineering.