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Does UPS technology have Transformer or not output transformer?

"Just a matter of choice!"

This article attempts to provide an answer to the common dilemma in choosing the UPS category, which may be related to both the seller or the buyer:   Which UPS Technology should choose - Has variable Pressure or not transformer?


Static UPSs for AC loads were introduced about half a century ago. In particular, dual conversion technology has dominated the market for a long time and is still considered to be the best quality energy supply for the load (in terms of safety and electrical parameters). In this configuration, the AC power supply gives the normal load generated by UPS.

Figure 1: Single UPS diagram with double conversion technology

One of the main modules of static UPS is a DC / AC converter, often called an Inverter. Traditional transformer (T) UPS technology is based on a switching module (usually an IGBT bridge), a transformer and an LC filter circuit (Figure 2). Common L inductors are included right in the transformer.

To obtain the sine voltage at the output of the inverter block, the usual technique is to modulate the PWM pulse width, but the vector technique (vectorial) or step pulse, 3-level IGBT is also common. use.

Figure 2: Inverter circuit with 3-phase transformer

The main advantage of transformer technology (T):

  •             - Load isolation

  •             -  Prevent DC current to load

  •             -  Low voltage harmonic distortion for all types of loads

  •             -  Good response to dynamic loads

  •             -  Good short-circuit protection

  •             -  Can operate compatible with wide DC voltage range (DC bus)

  •             -  Only need a range of battery (less wiring, lower voltage)

  •             -  Large supply capacity range (1 MVA or more)

  •             -  Synchronize well with the primary Bypass circuit on inverter transformer, ensure complete load isolation.

  •             -  Do not create electric current on neutral wire at UPS input

  •             - The  number of components in the UPS is low


  •             -  Size is relatively large
  •             -  High cost
  •             -  Losses are greater than those without transformers, especially with low power range
  •             -  Difficult to comply with stringent requirements in some cases requires the input power factor correction compensation circuit (PFC)

            -  In the 90s of the last century, the "Transformer less" (TL) technology began to appear, initially with single-phase UPS, small-capacity UPS, and then three phases. Non-transformer based UPS (TL) can use different rectifier input configuration, but the inverter block is quite similar.

Inverters design is based on a static switching module (spherical), an inductor on each phase (L1, 2, 3) and a filter capacitor C (Figure 2). PWM and DCM modulation technologies (DisContinuous Mode) are the most used technique to achieve sine voltage at the output. In general, switching frequency used in TL technology is usually higher (10-20 kHz) compared to T technology (<10 kHz).


mach inverter

Form. 3: 3-phase inverter circuit without output transformer


The main advantage of TL technology is:


  •             -  Compact size
  •             -  Low price (if not special requirements for harmonic THD)
  •             -  Low loss
  •             -  Relatively easy to implement input power factor correction compensation circuit (PFC)
  •             -  Low noise level (caused by switching element)



  •             -  Download without isolation
  •             -  The potential DC load on the load
  •             -  Difficult to restrict relatively high DC currents on neutral wires
  •             - The  number of components in the UPS is high
  •             -  Use at least 2 ranges of battery
  •             -  Design capacity is limited




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