Pulse transformer

Pulse transformer (IT) - a transformer designed to convert current and voltage of pulse signals with minimal distortion of the original pulse shape at the output.

Content

Description

Pulse transformers designed to transform short pulses with minimal distortion and operating in transient mode are used in various pulse devices ^[1] ^[2] . Pulse transformers allow you to change the level and polarity of the generated voltage or current pulse, coordinate the resistance of the pulse generator with the load resistance, separate the potentials of the source and receiver of pulses, receive pulses from several generators at several separate loads, create feedback in the circuit circuits of the pulse device. A pulse transformer can also be used as a converter element , for example a differentiating transformer .

The generation of powerful pulses of modern parameters is impossible without the use of high-voltage pulse transformers. The resulting shape of the output pulses is largely determined by the properties of IT, especially with a large transformation ratio. The use of output boosting IT makes it possible to sharply reduce the size, weight, and cost of generating devices ^[3] , although it negatively affects the shape of quasi-rectangular pulses, increasing the relative duration of the front, slice, and vertex unevenness. In this regard, the transformation coefficient of modern output IT with a pulse duration of units and tens of microseconds increases to 10 - 20 or more.

The most widespread are IT, transforming pulses, similar in shape to rectangular, which have a steep front and a constant voltage of the pulse peak necessary for a wide class of loads. A rectangular pulse should be transformed with small distortions, the duration of the pulse front should be much shorter than the pulse duration, and transients during transformation of the front and peak of the pulse should be considered separately. Equivalent IT circuits during a separate consideration of transients are simplified and allow you to establish a relationship between the parameters of equivalent circuits and the design parameters of IT and find such relationships between them that satisfy the requirements for the duration of the front and the bevel of the pulse peak ^[4]

Equivalent Schemes

The transformation of the pulse front with small distortions is achieved with small values of the leakage inductance and the distributed capacitance of the transformer, which decrease with a decrease in the number of turns of the windings and the cross section of the IT magnetic circuit. At the same time, to transform the peak of the pulse with a small decay, one should strive to increase the magnetization inductance of the transformer, which increases with the number of turns and the cross section of the magnetic circuit.

Satisfying several requirements at the same time when calculating IT will require a compromise solution. It should be adopted depending on the significance of a particular requirement.

IT calculations are based on an approximate equivalent circuit with lumped parameters. The inductive effect and losses in the wires of the windings can be taken into account using the well-known T-shaped equivalent circuit.

Equivalent T-shaped pulse transformer circuit

Circuit Parameters:

$L_{\mu }$ ${\ displaystyle L _ {\ mu}}$ - the magnetization inductance of the transformer, taking into account the energy storage in the main stream of the mutual induction of the magnetic circuit when voltage is applied to the primary winding. A magnetizing current is connected to the stream in the core, flowing along the primary winding;

$L_{s1},L_{s2}$ ${\ displaystyle L_ {s1}, L_ {s2}}$ - scattering inductance of the windings, taking into account the energy storage in the scattering flux associated with the flow of the load current through the windings;

$R_{1},R_{2}$ ${\ displaystyle R_ {1}, R_ {2}}$ - the active resistances of the wires of the windings, taking into account losses when the load current flows through them;

$R_{B}$ ${\ displaystyle R_ {B}}$ - equivalent resistance, taking into account the energy loss in the magnetic circuit for hysteresis and eddy currents .

Along with the energy storage in magnetic fields, as well as losses in the wires of the windings in IT, it is necessary to take into account the energy storage in electric fields between the winding and the magnetic circuit and between the layers of the windings. This energy is taken into account by introducing three containers that form a U-shaped structure: $C_{1}$ ${\ displaystyle C_ {1}}$ - capacity of the primary winding, $C_{2}$ ${\ displaystyle C_ {2}}$ - capacity of the secondary winding, $C_{1,2}$ ${\ displaystyle C_ {1,2}}$ - capacity between windings.

The resulting equivalent IT scheme is described by a high-order equation, which complicates the analysis in general:

Sixth-order equivalent IT scheme

However, without introducing a noticeable error, we can simplify the scheme if we keep in mind the following:

1. The magnetizing current is usually a small part of the load current, and therefore its effect on the scattering flux can be neglected. This allows you to move from a T-shaped circuit from inductive branches to a L-shaped circuit.

2. Since electric energy is proportional to the square of the voltage, its main part is stored in the winding of a higher voltage. Therefore, the U-shaped circuit of capacitive elements is replaced by one equivalent capacitance connected in parallel with a higher voltage winding.

3. The number of turns of IT windings is small and, therefore, can be neglected when calculating the most important electrical characteristics of the resistance of the windings, assuming $R_{1}=R_{2}=0$ ${\ displaystyle R_ {1} = R_ {2} = 0}$ . Winding resistance is taken into account when determining losses.

As a result of these simplifications, the front is analyzed on the basis of an equivalent 2nd-order circuit with concentrated inductance and capacitance, determined from energy considerations:

Equivalent 2nd-order Front Formation Scheme

Although it is convenient for mathematical description, it does not fully reflect the processes occurring during the transmission of the pulse, since it is believed that most of the electrical energy of the parasitic capacitance is stored in the higher voltage winding.

Meanwhile, the use of such a scheme is unacceptable with the commensurability of the reduced capacitances of the windings, which include parasitic capacitances of the load and the generator, since it is impossible to give preference to any of the capacities. In addition, with a sharp difference in the given capacities, when, it would seem, you can limit yourself to one of them, it is possible to form a front with spurious oscillations superimposed on the front itself, and not on the top. Such oscillations should be excluded, for example, in the case of pulsed modulation of high-power magnetron generators. But the second-order scheme not only does not allow us to determine the conditions for their appearance, but even excludes their very existence. In the works of the above authors, this type of distortion of the front of a rectangular pulse is absent. Therefore, it is necessary at least to take into account the separation of the capacitances of the windings by the scattering inductance. Therefore, it is preferable to consider the equivalent circuit of the 3rd order, as was done in ^[5] :

Equivalent 3rd-order Front Formation Scheme

$L$ ${\ displaystyle L}$ - leakage inductance;

$R$ ${\ displaystyle R}$ - resistance of the windings, including the reduced resistance of the secondary winding;

$R_{i}$ ${\ displaystyle R_ {i}}$ - resistance of the pulse generator;

$C_{1}$ ${\ displaystyle C_ {1}}$ - equivalent capacity of the primary winding, including the output capacity of the generator;

$C_{2}$ ${\ displaystyle C_ {2}}$ - equivalent reduced secondary winding capacity including stray load capacitance.

Types of pulse transformers

All constructive schemes can be reduced to four main ^[2] :

Rod
Armored
Armored rod
Toroidal

Sources

↑ Mathanov P.N., Gogolitsyn L.Z. Calculation of pulse transformers. - Energy, 1980.
↑ ¹ ² Vdovin S. S. Design of pulse transformers 2nd ed. reslave. and add. - Energoatomizdat. Leningra. Department, 1991 .-- 208 p. with. - ISBN 5-283-04484-X .
↑ Kashtanov V.V., Saprygin A.V. Possibilities of reducing the mass and dimensions of powerful micro-millisecond pulse modulators // Issues of Applied Physics. - 1997 .-- T. 3 . - S. 75 - 78 .
↑ Yitzhoki Y. S. Pulse devices. - Sov. Radio, 1959.- 729 p.
↑ Kashtanov V.V. Analysis of the front of the output pulses of the transformer. - Radio engineering, 1995.- T. 12. - S. 38 - 40.

[1] Mathanov P.N., Gogolitsyn L.Z. Calculation of pulse transformers. - Energy, 1980.

[Vdovin-2] ¹ ² Vdovin S. S. Design of pulse transformers 2nd ed. reslave. and add. - Energoatomizdat. Leningra. Department, 1991 .-- 208 p. with. - ISBN 5-283-04484-X .

[Kashtanov-3] Kashtanov V.V., Saprygin A.V. Possibilities of reducing the mass and dimensions of powerful micro-millisecond pulse modulators // Issues of Applied Physics. - 1997 .-- T. 3 . - S. 75 - 78 .

[Ickhoki-4] Yitzhoki Y. S. Pulse devices. - Sov. Radio, 1959.- 729 p.

[Kashtanov2-5] Kashtanov V.V. Analysis of the front of the output pulses of the transformer. - Radio engineering, 1995.- T. 12. - S. 38 - 40.