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Solving Design Problems with Flat Pack Transformers

Magnetic components are often viewed as constraints to power supply innovation. Today, market forces and research are driving power supplies to new levels of efficiency and power density. However, along with all this innovation comes a new level of circuit complexity complete with its own issues of reliability and MTBF. In many applications, proven 50/60Hz low profile flat pack transformers will provide very adequate power for "on board" power supply applications.

A flat pack transformer is a PC board-mounted electromagnetic device operating from a 110/220AC 50/60Hz power source. It is designed to provide a wide range of isolated 50/60Hz AC output voltages for a number of applications:

  • Energizing coils of electromagnetic devices.
  • Energizing power contactors.
  • Energizing relays.
  • Energizing timers or control circuits.
  • Providing AC input voltages for DC power supplies.

Flat pack transformers have a long history of reliable service in the field. They are made from a proven design, constructed with UL recognized materials, and use a split bobbin. Also, they are non-concentrically wound with primaries and secondaries placed side-by-side — making them different from conventional transformers which have secondaries that are wound concentrically around the primaries. The split bobbin winding reduces capacitive coupling and eliminates the need for costly electrostatic shielding. These transformers are offered with a dual secondary option with single or dual primaries. The flat pack transformers are wire wound on 2 dual-channel nylon bobbins. This design is rated for class B (130°C.) operation. The bobbins are then assembled with a stack of thin magnetic steel laminations which form the electromagnetic path. Molded-in terminals within the bobbin provide the input and output connections. The flat pack transformers are 100 percent hipot tested and achieve an insulation factor of 2000VAC between the primary and secondary windings (isolation) and achieve 1500VAC between the primary to laminated core and secondary to laminated core.

Power Capabilities
The power capabilities can vary. The Hytronics flat pack transformers are available with a power capacity of 2.5 to 24VA and are RoHS-compliant.

The reliability benefits for these devices arise from the excellent isolation that they provide from input to output along with their simple "low parts count" construction. In addition to the excellent reliability, they can also be easily designed to meet an extremely wide variation of output voltages. This output voltage range can be anything from 110/220VAC down to a fraction of a volt.

While some of the newer high frequency transformer designs can achieve small sizes, a price is often paid in terms of additional/new problems encountered. These problems entail such things as EMI emissions at high frequencies, since there is some difficulty in achieving proper shielding. Additional FCC and UL requirements are often imposed on the high-frequency transformers because of these shielding problems. By way of comparison, the 50/60 flat pack transformers only produce 50/60Hz (low frequency) low level emissions. When used to provide power to DC power supplies, the ripple components will have an emission of only 100 to 120Hz ripple for a full wave system.

The electrical demand for smaller-size power devices is driving legislation worldwide to provide new constraints on their manufacturing and performance. This often results in a complex mix of reliability issues and increased costs. Simpler proven reliability magnetic devices, such as like the Hytronics flat pack transformers, can fill a valuable niche for many of these circuit applications.

Contact: Hytronics Corp., Div. of Electro Technik Industries, Inc., P.O. Box 18802,
Clearwater, FL 33762 727-536-7861 fax: 727-535-3508 Web:


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