pcb transformerpcb transformerpcb transformer

Image Credit: Allied Electronics | Tamura Corporation of America | Digi-Key Corporation

 

Board mount transformers are transformers designed to be embedded in or mounted on computer boards. The purpose of a transformer, in general, is to transfer power from one device or circuit to another. Normally this is done by means of a process called induction. Board mount transformers are also called PCB transformers, PCB mount transformers or circuit board transformers.

 

How Transformers Work

A transformer transfers (and many times converts) power from circuit to another. In practical applications it is used to lower or to increase the voltage or current of a signal.  A transformer consists of two non-interacting inductive circuits (windings) called the primary and the secondary, respectively. When a voltage is applied to the primary winding a magnetic field is generated that induces another magnetic field in the secondary winding, which in turn generates a voltage in the secondary. The value of the induced voltage in the secondary depends on the ratio of the number of winding turns in the secondary to the number of turns of the primary. The process is depicted below:

 

transformer diagram of operation

Image Credit: BillC - Wikipedia

 

Transformer Composition

current coil transformerBoard mount transformers consist of primary and secondary coil wires, a metal core, an outer casing, mounting brackets, and connecting terminals. They are normally made of light materials in order to mount on a computer board or on a chassis. Some of these transformers are actually integrated circuits manufactured using general semiconductor fabrication techniques.

 

 

 

Image Credit: Ohio Semitronics, Inc.

 

Performance Specifications

 There are a few key parameters that are important to specify when selecting board mount transformers:

  • Primary voltage rating - the range of voltages or the maximum voltage that the transformer can handle as an input, measured in volts (V). Transformers used several nominal voltages need more than on primary winding.
  • Secondary voltage rating - the range of voltages or the maximum voltage that the transformer can output, measured in volts (V). A transformer may provide more than one secondary voltage.
  • Power rating - the maximum power in both watts (W) and volt-amps (VA) that the transformer is able to deliver.
    • Real power rating (or wattage) refers to the output power range in watts the transformer is capable of producing.
    • Imaginary power (VA) rating is the sum of the volts-amps rating for all of the secondary windings.

Another specification to keep in mind is operating temperature, which describes the full range of temperatures the transistor can effectively operate at without danger of failure or damage.

 

Applications

Because the design and characteristics of transformers depend on their use, specifying applications is very important when selecting transformers. For instance, a medical transformer has to have a very large isolation to protect patients and health personnel from electric shocks; a power transformer should be rated to handle large amounts of power.

 

Form Factor

The form factor is important because these transformers are embedded in computer boards and the space available is restricted. Therefore, knowing the size of the transformer is essential. For board mount transformers, the following form factors are the most prevalent:

  • Connectorized devices attach with coaxial or other types of connectors.

Connectorized transformer

Image Credit: Emhiser Tele-Tech, Inc.

  • Flat pack (FPAK) devices have gull wing or flat leads on two-or-four sides. They provide exceptional thermal and electrical performance and are available in cavity-up and cavity-down configurations. Flat pack devices are typically ceramic, hermetic, and surface-mounted. They are available in a variety of body sizes and pin counts.

Flat pack transformerFPAK transformer

Image Credit: MPS Industries

 

  • Surface mount technology (SMT) adds components to a printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board. SMT components have a flat surface that is soldered to a flat pad on the face of the PCB. The PCB pad is typically coated with a paste-like formulation of solder and flux. With careful placement, SMT components on solder paste remain in position until elevated temperatures, usually from an infrared oven, melt the paste and solder the component leads to the PCB pads.  

Surface mount technology SMT transformer

Image Credit: Digi-key Corporation

  • Through hole technology (THT) mounts components on a printed circuit board (PCB) by inserting component leads through holes in the board, and then soldering the leads in place on the opposite side of the board.

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Image Credit: Digi-key Corporation

  • Waveguide assemblies are used in microwave waveguide systems. They consist of a hollow, metallic conductor with a rectangular, elliptical, or circular cross-section. Some conductors contain solid or gaseous dielectric materials.

Waveguide assembly transformer

Image Credit: Allen Avionics, Inc.

 

Types

There are many types of transformers with a form factor suitable to be mounted on a computer board. Some of the most significant types are listed here:

  • Audio transformers are ideally suited for voice and data transmission. The frequency response of these transformers typically ranges from 300 Hz to 20 kHz.
  • Autotransformers are a special type of power transformer that only has one winding. By tapping or connecting at certain points along the winding, different voltages can be obtained. Autotransformers are generally used in low-power applications.
  • Balun transformers are used to interface a balanced line (two conductors carrying equal currents in opposite directions) to an unbalanced line (one conductor carrying current and a ground).  Its name is derived from the terms applied to BALanced to UNbalanced lines.
  • Current sense transformers are generally used to detect and measure current. There are two basic types of current sense transformers: those designed and used to precisely measure current in accurate instrumentation applications, and others designed and used in switch-mode power converter type applications. 
  • Flyback transformers (FBT)or line output transformers (LOPT) are transformers that are designed to produce a very high output voltage (normally in the order of kilovolts) from a relatively low input voltage. Flyback is a topology that uses the flayback - also known as kickback - of an inductor to convert the input voltage to the transformer to a desired output voltage.  This is done by first storing the input energy (in the inductor) produced by the input voltage (this is called the charging cycle), and then transferring this energy (this is called the discharge cycle) to the output. They are commonly used to generate high voltage to drive CRT's.
  • Interface transformers are used in communications applications where the signal is isolated.
  • Lighting transformers are used to provide suitable power to lights and lighting systems.
  • Medical transformers are used in medical applications such as biomedical equipment in hospitals. There are many strict safety rules, guidelines and laws governing the design, construction and use  of these transformers.
  • Power transformers convert one voltage to another at high power level. Power transformers are used in electronic circuits and come in many different types and applications. Power transformers are sometimes considered to be those with ratings of 300 volt-amperes and below. These transformers normally provide power to the power supply of an electronic device, such as in power amplifiers in audio receivers.
  • RF transformers are low-power devices normally used for impedance matching that operate over the radio frequency or microwave spectrum.
  • Resonant transformers operate at the resonant frequency of one of its coils (the primary or the secondary). They are used for arc welding, discharge lamps, etc.
  • Switch mode transformers or switching transformers are used in switching power supplies and DC-DC converters. These transformers provide the energy storage element to transfer energy from input-to-output in discrete packets that are required in switching power supplies, regulators, or converters.

References

 

EEM Buyer's Guide - Transformers

 

eHow - Electrical Transformers Tutorial