Pulse Transformer – An Overview

The transformer which handles voltages and currents in the form of pulse are called Pulse Transformer. Pulse Transformer is mostly used in Power Electronic circuits as an Isolating Transformer to isolate source and load. It is also used in television, radar, digital computers etc. The main functions of the Pulse Transformer are as below:

1)      For changing the amplitude of voltage pulse

2)      For inverting the polarity of the pulse

3)      For coupling different stages of pulse amplifier

4)      As an Isolation Transformer

It shall be noted here that the turn ratio of an Isolation Transformer is 1:1. The Pulse Transformer core is made of Ferrite.

The input voltage to a Pulse Transformer is of discontinued type as shown in figure below. The most and basic requirement of such transformer is that it should reproduce input voltage at its primary to secondary as accurately as possible.

Figure above shows the square input voltage pulse at the input terminals. The pulse with varies from fraction of microsecond to 25 microseconds. Normally enough time elapses before next pulse arrive at the input terminals. The waveform of output voltage can be determined by using the equivalent circuit of the pulse transformer. A typical output voltage waveform is shown in figure below. In the figure below, the pulse parameters are also shown and it must be noted.

Rise time here is the time interval required for the output to rise from 0.1 to 0.9 of its final value. The transformer response for to the flat top portion of the input voltage pulse can be determined using low frequency equivalent circuit. The output voltage seems to have downward tilt or drop off during pulse duration time. The output voltage cannot be flat as this would mean that DC is flowing through Transformer which is not possible. The drop off of the output voltage pulse can be minimized by using high magnetizing inductance for the transformer.

Again, when the input pulse is zero, the output pulse do not reduces to zero instantaneously because of magnetic energy stored in the inductance of the transformer rather there is some definite fall or decay time to reach the output voltage pulse to zero value.

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Pulse Transformers are quite small in size. To minimize the leakage inductance, there are few truns of primary and secondary but to have high magnetizing reactance, its core is either made of Ferrite or high permeability alloys like permalloy.