Pulse Transformer - Operating Principles

8 Operating Principles:

Pulse Transformers
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Pulse transformer designers usually seek to minimize voltage droop, rise time, and pulse distortion. Droop is the decline of the output pulse voltage over the duration of one pulse. It is cause by the magnetizing current increasing during the time duration of the pulse.

To understand how voltage droop and pulse distortion occurs, one needs to understand the magnetizing ( exciting, or no-load ) current effects, load current effects, and the effects of leakage inductance and winding capacitance. The designer also needs to avoid core saturation and therefore needs to understand the voltage-time product.

The magnetic flux in a typical A.C. transformer core alternates between positive and negative values. The magnetic flux in the typical pulse transformer does not. The typical pulse transformer operates in an “unipolar” mode ( flux density may meet but does not cross zero ).

A fixed D.C. current could be used to create a biasing D.C. magnetic field in the transformer core, thereby forcing the field to cross over the zero line. Pulse transformers usually (not always) operate at high frequency necessitating use of low loss cores (usually ferrites).

Figure 1A shows the electrical schematic for a pulse transformer. Figure 1B shows an equivalent high frequency circuit representation for a transformer which is applicable to pulse transformers. The circuit treats parasitic elements, leakage inductances and winding capacitance, as lumped circuit elements, but they are actually distributed elements. Pulse transformers can be divided into two major types, power and signal.

 (Figure 1A) The electrical schematic for a pulse transformer. + click to enlarge and view full image +(Figure 1B) An equivalent high frequency circuit representation for a transformerwhich is applicable to pulse transformers.

An example of a power pulse transformer application would be precise control of a heating element from a fixed D.C. voltage source. The voltage may be stepped up or down as needed by the pulse transformer’s turns ratio. The power to the pulse transformer is turned on and off using a switch (or switching device) at an operating frequency and a pulse duration that delivers the required amount of power. Consequently, the temperature is also controlled. The transformer provides electrical isolation between the input and output. The transformers used in forward converter power supplies are essentially power type pulse transformers. There exists high-power pulse transformer designs that have exceeded 500 kilowatts of power capacity.

The design of “signal” type of pulse transformer focuses on the delivery of a signal at the output. The transformer delivers a “pulse-like” signal or a series of pulses. The turns ratio of the pulse transformer can be used to adjust signal amplitude and provide impedance matching between the source and load. Pulse transformers are often used in the transmittal of digital data and in the gate drive circuitry of transistors, F.E.T.s, S.C.R.s, and etc. In the latter application, the pulse transformers may be referred to as “gate transformers” or “gate drive transformers”. Signal type of pulse transformers handle relatively low levels of power. For digital data transmission, transformers are designed to minimized signal distortion. The transformers might be operated with a D.C. bias current. Many signal type pulse transformers are also categorized as wideband transformers. Signal type pulse transformers are frequently used in communication systems and digital networks.

Pulse transformer designs vary widely in terms of power rating, inductance, voltage level (low to high), operating frequency, size, impedance, bandwidth (frequency response), packaging, winding capacitance, and other parameters. Designers try to minimize parasitic elements such as leakage inductance and winding capacitance by using winding configurations which optimize the coupling between the windings.

Butler Winding can make (and has made) pulse transformers in a wide variety of shapes and sizes. This includes; various standard types of “core with bobbin” structures ( E, EP, EFD, PQ, POT, U and others ), toroids, and some custom designs. Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have experience with foil windings, litz wire windings, and perfect layering. For toroids, we can ( and have done ) sector winding, progressive winding, bank winding, and progressive bank winding. Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes two programmable automated machines and a taping machine for toroids. Butler winding has vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines. For more information on Butler Winding’s capabilities, click on our “capabilities” link.

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