What is a Pulse Transformer

Pulse transformers are used for carrying pulses, often for driving circuits or carrying data whilst also providing galvanic or resistive isolation between two circuits.

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Pulse transformers are a form of transformer that are designed to carry pulses with a good level of fidelity rather than sine waves.

Most transformers are used to transfer power as in the case of line or mains transformers, and even audio transformers carry audio power, and RF transformers carry RF power. Instead pulse transformers are used to carry information using digital signals or pulses.

These digital signal or pulses may be used to carry data, or they may be used to drive a circuit that is controlled by pulses.

Typically the input and output signal or a pulse transformer is a rectangular wave of a few volts and it often has a frequency that is in excess of 100KHz.

The type of performance required for pulse transformers requires a slightly different design approach to that used for more standard transformers, although the basic concepts remain the same.

Understanding pulse transformers: the basics

The aim of a pulse transformer is to be able to carry the pulses from the input to the output with a minimum level of distortion.

To achieve this, the transformer must carry the waveform applied to it with a minimum level of voltage droop where the flat top of a pulse falls after it has passed through the pulse transformer.

The rise time must also be fast to accommodate the rising edges of the pulse and similarly it must accommodate the fall slope of the waveform as well.

The design of these transformers is not as straightforward as that of a standard AC transformer might be. For these transformers the flux will alternate backwards and forwards.

A typical pulse transformer operates in what is called a unipolar mode, with the pulses alternating between two levels, but with the same magnetisation level, i.e. not crossing the zero line.

Pulse transformers usually operate at high frequencies and this means that low loss cores need to be used and as a result, ferrites are widely employed as the cores for pulse transformers.

Pulse transformers can be divided into two main categories: power transformers and signal transformers.

• Power pulse transformer

As the name indicates, power pulse transformers are used to provide power to various items in the form of pulses. For example they may be used to provide a controlled level of power to a device which may be some form of heating element or possibly another item which is controlled by a digital power driver source, using pulse width or periodicity modulation.

For these transformers it is quite possible that the turns ratio will not be 1:1, but instead changed to provide the required voltage to the item being driven.

The transformer provides electrical resistive or galvanic isolation between the input and output circuits, enabling both circuits to be isolated from one another.

• Signal pulse transformer

As the title suggests, signal pulse transformers are used for carrying control signals and the power levels are not at all high. The power levels are typically quite low and the focus of these transformers is on providing a good low level pulse to the circuit that is being driven.

Ofte these pulse transformers may be used to drive other control circuits - possibly using FETs or SCRs and where isolation is required. Power FETs and SCRs may be controlling circuits that have high voltage levels and therefore it may be necessary to provide isolation between the control signal source and the circuit which is being controlled.

Pulse transformers may also be used various applications within communication systems and digital networks where it may be necessary to isolate different sections to improve signal integrity or generally provide isolation.

In many of these applications, the transformer needs to minimise the distortion levels, so this will be an important element of the specification.

Requirements for pulse transformers

The actual specifications and performance parameters for pulse transformers will depend largely upon the particular circuit and applications for which they are being used.

Parameters including size, inductance, impedance and a host of other figures will all vary according to the requirements.

Typically the bandwidth tends to be wide to enable the pulse shape to be accommodated with the minimum level of distortion.

Also the transformers typically minimise the levels of parasitic elements such as leakage inductance and winding capacitance by using winding configurations which optimise the coupling between the windings, while keeping the parasitic elements small. Often they only have the two windings as this will also minimise flux leakage.

Other common features include a core material that can accommodate high frequencies and often, although not always, their turns ratio is 1:1 as their main purpose is not to change the voltage, but to provide an isolation barrier. The signals are often brought to the right levels by the driver circuitry. In this way, standard transformers can be used.

Pulse transformers are a form of transformer that tends to be used more in commercial applications, and therefore it may not be as widely seen as other types. Nevertheless, pulse transformers are a very useful electronic component that is used in a good number of applications.