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Transformer-Based UPS

In the traditional transformer-based UPS (uninterruptible power supply), the power flows via the rectifier, transformer, an inverter to the output to deliver the critical load (double conversion mode). In double conversion mode, the battery is continuously kept fully charged until a power outage is experienced at which time the battery feeds the inverter, which then delivers continuous power to the critical load through the transformer.


The role of the static bypass is to serve as an emergency path that is switched in when double conversion path experiences issues such as too high a temperature, overload or output short circuit.  The maintenance bypass switch (MBPS) links input to output and allows the UPS to be bypassed so that the unit may be serviced.


In general, transformer-based UPS systems are highly robust and excel at providing the highest capacities and availability while simplifying external and internal voltage management and fault current control.


For high-power enterprise data centers and other critical applications, a state-of-the-art transformer-based UPS can still provide an edge over a transformerless UPS since they provide integrated fault management and galvanic isolation as well as greater compatibility with critical power distribution system requirements. Even more important is that technology developments and configuration options allow the latest transformer-based designs to operate at higher efficiencies compared to previous designs, making them more comparable to the transformer-free models in terms of efficiency.



Transformer-Free UPS

The operational principle of a transformer-free UPS is the same as that of a transformer-based UPS except that the transformer-free UPS utilizes an insulated-gate bipolar transistor (IGBTs) which deals with high voltages, therefore, eliminating the requirement for a step-up transformer after the inverter. This industry standard increases energy efficiency is seen as a measured increase from 90 to 96%.  Equally important, the transformer-free UPSs have a smaller footprint and are lighter, thus reducing investment and running costs.


As a general consideration, 200 kW power is the threshold below which space, weight and cost advantages of transformer-free UPS systems outweigh the robustness and higher capacity capabilities of transformer-based systems. These under-200 kW applications exhibit high efficiency and excellent input power conditioning because of the transformer-free design.  Because of the modular construction and thus scalability of the transformer-free UPS, these UPS can avoid over-provisioning of power while maintaining operational efficiency.



Benefits of a Transformerless UPS
  • Energy efficiency


  • Reduced Physical Size and Weight

Because they lack a transformer, the transformerless UPS can be sized smaller compared to a transformer UPS. This makes it a suitable choice for small data centers and the space freed by a transformer-based UPS could be used to add another module to accommodate more ICT load.


The Downside of the Transformerless UPS:


  • Service and Maintainability

a UPS without a transformer passes more fault current to the critical load and can’t clear and isolate more internal faults as opposed to a transformer-based UPS.


  • Power Strength Limitations

The Transformer free design is also limited to UPS modules under 300 kVA, requiring several units to be paralleled together in order to achieve redundancy at a larger KW size.


Staco Energy Products designs both transformer free and the traditional UPS with transformers.  Please visit their web site at to view their entire line of UPS and other power conditioning equipment.  For more information please contact us at Peninsula Technical Sales phone number 650-965-3636 or visit us at




Peninsula Technical Sales represents electronic equipment manufacturers and is proud to offer our services online and to the following cities and their surrounding areas: San Francisco, Santa Clara, San Jose, Fremont, Sacramento, Milpitas, and Santa Rosa.