Pulse transformers

Pulse transformers are used in communication devices, automation, computer technology, when working with short pulses, to change their amplitude and polarity, remove the permanent component, etc.

One of the main requirements for pulse transformers is the minimum distortion of the transmitted signal shape, which occurs due to the influence of leakage currents, capacitive connections between windings and turns, eddy currents.

Assume that the input of an ideal transformer (without losses and capacitances) receives rectangular voltage pulses (Fig. 1, a) of duration I with period T. The time constant of the primary winding of the transformer — the time during which the current reaches a stationary value (Fig. 1, b) is equal to: T1 = L1/r1 , where L1 — inductance of the primary winding, G.

A current appears and begins to increase in the primary winding, the curve of which is shown in fig. 1b. This will cause exactly the same change in the magnetic flux, which in turn will lead to the EMF in the secondary winding, which in idle mode is equal to ti2 (Fig. 1, b).

The negative part of the pulse is "cut off" by switching on the diode in the secondary circuit of the transformer. This produces a pulse that is close to rectangular on the secondary side of the transformer.

Rice. 1. Curves of voltages and currents in a pulse transformer

It should be noted that T.1 >T, i.e. the time constant of the primary winding must be greater than the pulse duration. If — on the contrary, T.1 < T the result is negative — the shape of the pulse will be far from rectangular.

To make the pulse shape even more rectangular, the pulse transformer has its own characteristics: it works in unsaturated mode, the magnetic circuit of the pulse transformer must have a small residual induction. Therefore, it is made of soft magnetic material (low coercive force), with increased magnetic permeability.

Rice. 2. Pulse transformers

Sometimes, to reduce residual induction, the magnetic circuit of a pulse transformer is designed with an air gap. To reduce stray capacitance and leakage currents, windings are tried to be made with the least number of turns.