Calculation and modeling of a high-frequency transformer as part of a single-cycle flyback converter. What is a pulse transformer and how to calculate it? Overall power of the ferrite ring

Figure 1 - Here is such a 2 kW transformer for the H-bridge, we will end up with

Great battle or what material to choose?

Why is it so good and is it really better than ferrite?

Figure 2 - Hysteresis cycles of ferromagnets: 1) hard cycle, 2) soft cycle

2) the material must have the highest possible saturation induction, which will either reduce the dimensions of the core, or, while maintaining them, increase the power.

Saturation

The phenomenon of transformer saturation is that, despite the increase in current in the winding, the magnetic flux in the core, having reached a certain maximum value, then practically does not change.
In a transformer, saturation mode leads to the fact that the transfer of energy from the primary to the secondary winding is partially stopped. Normal operation of the transformer is possible only when the magnetic flux in its core changes in proportion to the change in current in the primary winding. To fulfill this condition, it is necessary that the core is not in a state of saturation, and this is possible only when its volume and cross section are not less than a well-defined value. Therefore, the greater the power of the transformer, the larger its core must be.

4) the properties of the material should not change significantly under external influence: mechanical forces (compression or tension), changes in temperature and humidity.

Now consider the properties of ferrite and how it meets the above requirements.

Figure 3 - Magnetization curve at a frequency of 20 kHz at different temperatures

And finally, the most important point - on the chart above, we saw a parameter that will determine almost everything - saturation induction. For ferrite, it is usually taken as 0.39 T. It is worth remembering that under different conditions - this parameter will change. It depends both on the frequency and on the operating temperature and on other parameters, but special emphasis should be placed on the first two.

Conclusion: ferrite nishtyak! perfect for our purposes.

A few words about alsifera and how it differs

Conclusion: alsifer is better than ferrite, they didn’t lie to me in this uncle.

Battle result: anyone reading the description above will say alsifer give us! And rightly so ... but try to find an alsifer core with an overall power of 10 kW? Here, usually a person comes to a standstill, it turns out they are not particularly on sale, and if there is, then order directly from the manufacturer and the price will scare you.
It turns out that we use ferrite, especially if we evaluate it as a whole, then it loses very little ... ferrite is evaluated relative to alsifer in "8 out of 10 parrots."

I wanted to turn to my beloved matan, but I decided not to do this, because. I think +10,000 characters to the article is redundant. I can only recommend a book with very good calculations by B. Semenov "Power Electronics: From Simple to Complex". I don’t see the point of retelling his calculations with some additions.

So, we proceed to the calculation and manufacture of the transformer

To make a transformer, we need the following set:

A)
Figure 4 - Ferrite core E70 / 33/32 made of 3C90 material (a slightly better analogue of N87)

B)
Figure 5 - Frame for E70/33/32 core (whichever is larger) and D46 sprayed iron choke

The overall power of such a transformer is 7.2 kW. We need such a margin to provide starting currents 6-7 times higher than the nominal ones (600% according to the specification). Such starting currents are true only for asynchronous motors, but everything must be taken into account!
Suddenly, a certain throttle “surfaced”, it will be needed in our further scheme (as many as 5 pieces) and therefore I decided to show how to wind it.

Next, you need to calculate the winding parameters. I use a program from a well-known friend in certain circles Starichok51 . A man with great knowledge and always ready to teach and help, for which I thank him - at one time he helped to take the right path. The program is called Excellent IT 8.1 .

I give an example of a calculation for 2 kW:


Figure 6 - Calculation of a pulse transformer for a 2 kW step-up bridge circuit

How to calculate:

2) Highlighted in blue.
a) set 400V, because this is a margin for voltage feedback and a minimum of 342V is required to cut the sine
b) rated current. We choose from consideration 2400 W / 220 (230) V = 12A. As you can see, everywhere I take a margin of at least 20%. This is what any self-respecting manufacturer of quality equipment does. In the USSR, such a reserve was a reference 25% even for the most difficult conditions. Why 220 (230) V is the output voltage of an already pure sine.
c) minimum current. It is selected from real conditions, this parameter affects the size of the output inductor, so the larger the minimum current, the smaller the inductor, and therefore the cheaper device. Again, I chose the worst option 1A, this is the current for 2-3 light bulbs or 3-4 routers.
d) drop on diodes. Because we will have high-speed diodes (ultra-fast) at the output, then the drop on them is 0.6V in the worst conditions (temperature is exceeded).
e) wire diameter. I once bought a 20 kg copper coil for such a case and just with a diameter of 1 mm. Here we put the one that you have. I do not advise you to set only more than 1.18 mm, because the skin effect starts to show

Skin effect

Skin effect - the effect of reducing the amplitude of electromagnetic waves as they penetrate deep into the conducting medium. As a result of this effect, for example, high-frequency alternating current, when flowing through a conductor, is not distributed uniformly over the cross section, but mainly in the surface layer.
If we speak not like Google, but in my collective farm language, then if we take a large-section conductor, then it will not be used completely, because. currents at a higher frequency flow over the surface, and the center of the conductor will be "empty"

3) Highlighted in green. Everything is simple here - we plan to have a “full bridge” topology and select it.

4) Highlighted in orange. There is a process of choosing the core, everything is intuitive. A large number of standard cores are already in the library, like ours, but if anything can be added by entering the dimensions.

5) Highlighted in purple. Output parameters with calculations. I highlighted the coefficient in a separate window. filling the window, remember - no more than 0.35, and preferably no more than 0.3. All the necessary values ​​​​are also given: the number of turns for the primary and secondary windings, the number of wires of a previously specified diameter in the "braid" for winding.
The parameters for further calculation of the output inductor are also given: inductance and voltage ripple.

Now you need to calculate the output inductor. It is needed to smooth out the ripples, as well as to create a "uniform" current. The calculation is carried out in the program of the same author and it is called DrosselRing 5.0. I will give the calculation for our transformer:


Figure 7 - Calculation of the output choke for a step-up DC-DC converter

In this calculation, everything is simpler and clearer, it works on the same principle, the output data: the number of turns and the number of wires in the braid.

Manufacturing stages

We wind on the frame the required number of turns in 2 wires with a diameter of 1 mm. We remember the direction of winding, but rather we mark it with a marker on the frame.

Stage 2:

Figure 9 - Isolate the secondary winding

We isolate the secondary winding with a fluoroplastic tape 1 mm thick, such insulation can withstand at least 1000 V. We also additionally impregnate it with varnish, this is another + 600V to the insulation. If there is no fluoroplastic tape, then we isolate it with an ordinary plumbing foam in 4-6 layers. This is the same fluoroplast, only 150-200 microns thick.

Stage 3:

Figure 10 - We begin to wind the primary winding, solder the wires to the frame
We wind in one direction with the secondary winding!

Stage 4:

Figure 11 - We display the tail of the primary winding

It winds the winding, we isolate it with the same fluoroplastic tape. It is also desirable to soak with varnish.

Stage 5:


Figure 12 - We impregnate with varnish and solder the "tail". Winding is finished
Stage 6:

Figure 13 - We complete the winding and insulation of the transformer with a keeper tape with final impregnation in varnish

Kiper tape

Kiper tape - cotton (less often silk or semi-silk) braid made of kiper fabric with a width of 8 to 50 mm, twill or diagonal weave; severe, bleached or one-colored. The material of the tape is highly dense due to the weave, it is thicker than its closest counterpart - calico tape - due to the use of thicker threads.
Thanks wikipedia.

Stage 7:

Figure 14 - This is what the finished version of the transformer looks like

A gap of 0.15 mm is set during the bonding process by inserting a suitable film between the halves of the core. The best option is a film for printing. The core is glued together with a moment glue (good) or epoxy. The 1st option is for centuries, the 2nd one allows, in which case, to disassemble the transformer without damage, for example, if you need to rewind another winding or add turns.

Choke winding

Now, by analogy, it is necessary to wind the choke, of course, winding on a toroidal core is more difficult, but this option will be more compact. We have all the data from the program, the material of the core is atomized iron or permalloy. The saturation induction of this material is 0.55 T.

Stage 1:


Figure 15 - We wrap the ring with fluoroplastic tape

This operation allows you to avoid the case with a breakdown of the winding on the core, this is rare, but we are for quality and do it for ourselves!

Stage 2:

Figure 16 - We wind the required number of turns and isolate

In this case, the number of turns will not fit in one winding layer, therefore, after winding the first layer, it is necessary to insulate and wind the second layer with subsequent insulation.
IIP

Switching power supplies, increasingly common in amateur radio practice due to their high efficiency, small size and weight, usually require the calculation of one or more (according to the number of stages) transformers. This is dictated by the fact that the values ​​\u200b\u200bof the number of turns and their diameter given in the literature often do not coincide with the desired output data of the assembled or designed power source, or the ferrite rings or transistors available to the radio amateur do not correspond to those given in the circuit.
A simplified methodology for calculating transformers of switching power supplies was given in the literature. The general procedure for calculating the switching power supply transformer is as follows:
1. Calculate (in W) the power used by the transformer
Fig.=1,ЗРн, where Рн is the power consumed by the load.
2. Select a toroidal ferrite magnetic core that satisfies the condition Rgab> Fig., where Rgab. - overall power of the transformer, W, calculated as:

Where D is the outer diameter of the ferrite ring, cm; d is the inner diameter; h is the height of the ring; f is the converter operation frequency, Hz; Vmax - the maximum value of induction (in Tesla), which depends on the brand of ferrite and is determined from the reference book.
3. Given the voltage on the primary winding of the transformer
U1 is determined with rounding up
the number of its turns:

For a half-bridge converter U1 = Upit / 2-UKENas, where Upit is the supply voltage of the converter, UKENas is the saturation voltage of the collector - emitter of transistors VT1, VT2.
4. Determine the maximum current of the primary winding (in A):

Where η is the efficiency of the transformer (usually 0.8).
5. Determine the diameter of the primary wire (in mm):

6. Find the number of turns and the wire diameter of the output (secondary) winding:

M.A. Shustov; “Practical circuitry. Voltage converters”; Altex-A, 2002

Dear colleagues!!

I already talked about how to build a pulse transformer on a ferrite ring in my lessons. Now I'll tell you how I make a transformer on a W - shaped ferrite core. For this, I use ferrites of suitable size from old “Soviet” equipment, old computers, from TVs and other electrical equipment, which I have lying around in the corner “on demand”.

For a UPS according to the two-stroke half-bridge generator scheme, the voltage on the primary winding of the transformer, according to the scheme, is 150 volts, under load we will take 145 volts. The secondary winding is made according to the scheme of full-wave rectification with a midpoint.
See diagram.

I will give examples of the calculation and manufacture of transformers for a low-power UPS of 20 - 50 watts for this circuit. I use transformers of such power in switching power supplies for my LED lamps. Transformer diagram below. It is necessary to pay attention that, folded from two halves, W - the core does not have a gap. A magnetic core with a gap is used only in single-cycle UPSs.

Here are two examples of calculating a typical transformer for different needs. In principle, all transformers for different capacities have the same calculation method, almost the same wire diameters and the same winding methods. If you need a transformer for a UPS with a power of up to 30 watts, then this is the first calculation example. If you need a UPS with a power of up to 60 watts, then the second example.

First example.

Let's choose from ferrite cores No. 17, Ш - shaped core Ш7.5 × 7.5. The cross-sectional area of ​​the middle rod Sk = 56 mm.sq. \u003d 0.56 cm.sq.
Window So = 150 mm2 Estimated power 200 watts.
The number of turns per 1 volt of this core will be: n = 0.7 / Sk = 0.7 / 0.56 = 1.25 turns.
The number of turns in the primary winding of the transformer will be: w1 \u003d n x 145 \u003d 1.25 x 145 \u003d 181.25. Let's take 182 turns.
When choosing the thickness of the wire for the windings, I proceeded from the table "".
In my transformer, I used, in the primary winding, a wire with a diameter of 0.43 mm. (a wire with a large diameter does not fit in the window). It has a cross-sectional area S = 0.145 mm2. Permissible current (see table) I = 0.29 A.
The power of the primary winding will be: P \u003d V x I \u003d 145 x 0.29 \u003d 42 watts.
A coupling winding must be placed on top of the primary winding. It should output voltage v3 = 6 volts. The number of turns will be: w3 = n x v3 = 1.25 x 6 = 7.5 turns. Let's take 7 turns. Wire diameter 0.3 - 0.4 mm.
Then the secondary winding w2 is wound. The number of turns of the secondary winding depends on the voltage we need. The secondary winding, for example at 30 volts, consists of two equal half-windings, w3-1 and w3-2).
The current in the secondary winding, taking into account the efficiency (k \u003d 0.95) of the transformer: I \u003d k x P / V \u003d 0.95 x 42 watts / 30 volts \u003d 1.33 A;
Let's select a wire for this current. I used the wire that I had in stock, with a diameter of 0.6 mm. Its S = 0.28 mm2
The allowable current of each of the two half-windings is I = 0.56 A. Since these two secondary half-windings work together, the total current is 1.12 A, which is slightly different from the rated current of 1.33 A.
The number of turns in each half-winding for a voltage of 30 volts: w2.1 \u003d w2.2 \u003d n x 30 \u003d 1.25 x 30 \u003d 37.5 vit.
Let's take 38 turns in each half-winding.
Power at the output of the transformer: Pout \u003d V x I \u003d 30 V x 1.12 A \u003d 33.6 watts, which, taking into account losses in the wire and core, is quite normal.

All windings: primary, secondary and communication winding fit perfectly in the window So = 150 mm.kv.

The secondary winding can thus be designed for any voltage and current, within a given power.

Second example.
Now let's experiment. Let's add two identical cores No. 17, W 7.5 x 7.5.

In this case, the cross-sectional area of ​​the magnetic circuit "Sk" will double. Sk \u003d 56 x 2 \u003d 112 mm.sq. or 1.12 cm.sq.
The area of ​​the window will remain the same "So" = 150 mm2. The indicator n (the number of turns per 1 volt) will decrease. n \u003d 0.7 / Sk \u003d 0.7 / 1.12 \u003d 0.63 vit./volt.
Hence, the number of turns in the primary winding of the transformer will be:
w1 \u003d n x 145 \u003d 0.63 x 145 \u003d 91.35. Let's take 92 turns.

In the feedback winding w3, for 6 volts, it will be: w3 \u003d n x v3 \u003d 0.63 x 6 \u003d 3.78 turns. Let's take 4 turns.
Let us take the voltage of the secondary winding, as in the first example, equal to 30 volts.
The number of turns of the secondary half-windings, each 30 volts: w2.1 \u003d w2.2 \u003d n x 30 \u003d 0.63 x 30 \u003d 18.9. Let's take 19 turns.
I used a wire for the primary winding with a diameter of 0.6 mm. : wire section 0.28 mm2, current 0.56 A.
With this wire, the power of the primary winding will be: P1 \u003d V1 x I \u003d 145 V x 0.56 A \u003d 81 watts.
I wound the secondary winding with a wire with a diameter of 0.9 mm. 0.636 mm2 for a current of 1.36 amperes. For two half-windings, the current in the secondary winding is 2.72 amperes.
The power of the secondary winding P2 \u003d V2 x I \u003d 30 x 2.72 \u003d 81.6 watts.
Wire with a diameter of 0.9 mm. a little big, fits with a large margin, it's not bad.

I use a wire for windings at the rate of 2 A per millimeter square (this way it heats up less, and the voltage drop across it will be less), although all “factory” transformers are wound at the rate of 3 - 3.5 A per mm square. and install a fan to cool the windings.
The general conclusion from these calculations is:
- when adding two identical W - shaped cores, the area "Sk" doubles with the same area of ​​the window "So".
- the number of turns in the windings (in comparison with the first option) changes.
- primary winding w1 from 182 turns is reduced to 92 turns;
- the secondary winding w2 is reduced from 38 turns to 19 turns.

This means that in the same “So” window, with a decrease in the number of turns in the windings, it is possible to place a thicker wire of the windings, that is, to double the real power of the transformer.

I wound such a transformer, with folded cores No. 17, made a frame for them.

It must be borne in mind that transformers first and second for example, it can be used under a smaller load, up to 0 watts. UPS is quite good and stable voltage.

Compare the appearance of transformers: example-1, with one core and example-2, with two folded cores. The actual dimensions of the transformers vary slightly.

The analysis of ferrite cores #18 and #19 is similar to the previous examples.
All our performed calculations are theoretical estimates. In fact, it is quite difficult to get such power from a UPS on transformers of these sizes. The features of the construction of the circuits of the switching power supplies themselves come into force. Scheme.
The output voltage (and therefore the output power) depends on many factors:
- capacity of the network electrolytic capacitor C1,
- containers C4 and C5,
- power drops in the wires of the windings and in the ferrite core itself;
- power drops on the key transistors in the generator and on the output rectifier diodes.
The overall efficiency "k" of such switching power supplies is about 85%.
This figure is still better than that of a rectifier with a steel core transformer, where k = 60%. Despite the fact that the dimensions and weight of the UPS on ferrite are significantly smaller.

The order of assembly of the ferrite W - transformer.

It is used ready-made or assembled - a new frame is made to fit the dimensions of the core.
How to make "" see here. Although this article talks about a frame for a steel core transformer, the description is quite suitable for our case.
The frame must be placed on a wooden mandrel. The winding of the transformer is done manually.
The primary winding is first wound onto the frame. Turn to turn, the first row is filled, then a layer of thin paper, varnished cloth, then the second row of wire, etc. A thin PVC tube is put on the beginning and end of the wire (insulation from the mounting wire can be used) to stiffen the wire so that it does not break off.
Two layers of paper are applied over the primary winding (interwinding insulation), then it is necessary to wind the turns of the coupling winding w3. Winding w3 has few turns, and therefore it is placed on the edge of the frame. Then turns of the secondary winding are applied. Here it is desirable to act in such a way that the turns of the secondary winding w2 are not located on top of the turns w3. Otherwise, malfunctions in the switching power supply may occur.
Winding is carried out immediately with two wires (two half-windings), turn to turn in a row, then a layer of paper or adhesive tape and the second row of two wires. You can not put a PVC tube on the ends of the wire, because. The wire is thick and won't break. The finished frame is removed from the mandrel and put on a ferrite core. Pre-check the core for a gap.
If the frame is tight on the core, be very careful, ferrite breaks very easily. Broken core can be glued. I glue with PVA glue, followed by drying.
The assembled ferrite transformer, for strength, is pulled together at the end with tape. It is necessary to ensure that the ends of the halves of the core coincide without a gap and shift.

Various types of transformer equipment are used in electronic and electrical circuits, which are in demand in many areas of economic activity. For example, pulse transformers (hereinafter referred to as IT) are an important element installed in almost all modern power supplies.

Design (types) of pulse transformers

Depending on the shape of the core and the placement of coils on it, IT are produced in the following designs:

• rod;
  
• armored;
  
• toroidal (does not have coils, the wire is wound on an insulated core);
  
• armored rod;
  

The figures show:

• A - a magnetic circuit made of transformer steel grades made using the technology of cold or hot rolled metal (with the exception of a toroidal core, it is made of ferrite);
• B - coil of insulating material
• C - wires that create an inductive connection.

Note that electrical steel contains few additives of silicon, since it causes power loss from the effect of eddy currents on the circuit of the magnetic circuit. In IT of toroidal design, the core can be made from rolled or ferrimagnetic steel.

Plates for a set of an electromagnetic core are selected in thickness depending on the frequency. With an increase in this parameter, it is necessary to install plates of smaller thickness.

Principle of operation

The main feature of pulse type transformers (hereinafter referred to as IT) is that they are supplied with unipolar pulses with a constant current component, and therefore the magnetic circuit is in a state of constant bias. The schematic diagram for connecting such a device is shown below.

As you can see, the connection diagram is almost identical with conventional transformers, which cannot be said about the timing diagram.

The primary winding receives pulse signals having a rectangular shape e (t), the time interval between which is quite short. This causes an increase in the inductance during the interval t u , after which its decline is observed in the interval (T-t u).

The induction drops occur at a rate that can be expressed in terms of the time constant by the formula: τ p =L 0 /R n

The coefficient describing the difference of the inductive difference is determined as follows: ∆V=V max - V r

• B max - the level of the maximum value of induction;
• In r - residual.

More clearly, the difference in inductions is shown in the figure showing the shift of the operating point in the IT magnetic circuit.

As can be seen in the timing diagram, the secondary coil has a voltage level U 2 in which there are reverse surges. This is how the energy accumulated in the magnetic circuit manifests itself, which depends on the magnetization (parameter i u).

The current pulses passing through the primary coil are trapezoidal in shape, since the load and linear currents (caused by core magnetization) are combined.

The voltage level in the range from 0 to t u remains unchanged, its value e t =U m . As for the voltage on the secondary coil, it can be calculated using the formula:

wherein:

• Ψ is the flux linkage parameter;
• S is a value that displays the cross section of the magnetic core.

Considering that the derivative characterizing the changes in the current passing through the primary coil is a constant value, the increase in the level of induction in the magnetic circuit occurs linearly. Based on this, it is permissible, instead of the derivative, to introduce the difference of indicators made after a certain time interval, which allows you to make changes to the formula:

in this case, ∆t will be identified with the parameter t u , which characterizes the duration with which the input voltage pulse flows.

To calculate the area of ​​the pulse with which the voltage is formed in the secondary winding of IT, it is necessary to multiply both parts of the previous formula by t u. As a result, we will come to an expression that allows us to obtain the main IT parameter:

U m x t u =S x W 1 x ∆V

Note that the value of the pulse area directly depends on the parameter ∆В.

The second most important value that characterizes the operation of IT is the induction drop, it is influenced by such parameters as the cross section and magnetic permeability of the core of the magnetic circuit, as well as the number of turns on the coil:

Here:

• L 0 - induction difference;
• µ a is the magnetic permeability of the core;
• W 1 - the number of turns of the primary winding;
• S is the cross-sectional area of ​​the core;
• l cp - length (perimeter) of the core (magnetic circuit)
• B r is the value of the residual induction;
• In max - the level of the maximum value of the induction.
• H m - Magnetic field strength (maximum).

Considering that the IT inductance parameter completely depends on the magnetic permeability of the core, the calculation must be based on the maximum value of µ a, which is shown by the magnetization curve. Accordingly, for the material from which the core is made, the level of the parameter B r , which reflects the residual induction, should be minimal.

Video: a detailed description of the principle of operation of a pulse transformer

Based on this, a tape made of transformer steel is ideal for the role of the IT core material. You can also use permalloy, in which such a parameter as the coefficient of squareness is minimal.

Ferrite alloy cores are ideal for high-frequency IT because this material has low dynamic losses. But because of its low inductance, it is necessary to make IT of large sizes.

Calculation of a pulse transformer

Consider how it is necessary to calculate IT. Note that the efficiency of the device is directly related to the accuracy of the calculations. As an example, let's take a conventional converter circuit that uses a toroidal type IT.

First of all, we need to calculate the IT power level, for this we use the formula: P \u003d 1.3 x P n.

The value of R n displays how much power the load will consume. After that, we calculate the overall power (P gb), it should be no less than the load power:

Parameters required for calculation:

• S c - displays the cross-sectional area of ​​the toroidal core;
• S 0 - the area of ​​​​its window (as a hint, this and the previous value are shown in the figure);

• B max is the maximum peak induction, it depends on which brand of ferromagnetic material is used (the reference value is taken from sources describing the characteristics of ferrite grades);
• f is a parameter characterizing the frequency with which the voltage is converted.

The next step is to determine the number of turns in the primary winding Tr2:

(results are rounded up)

The value of U I is determined by the expression:

U I \u003d U / 2-U e (U is the voltage supply to the converter; U e is the voltage level supplied to the emitters of transistor elements V1 and V2).

We proceed to the calculation of the maximum current passing through the primary winding of IT:

The parameter η is equal to 0.8, this is the efficiency with which our converter must work.

The diameter of the wire used in the winding is calculated by the formula:

If you have problems with determining the main IT parameters, you can find thematic sites on the Internet that allow you to calculate any pulse transformers online.

Various types of transformer equipment are used in electronic and electrical circuits, which are in demand in many areas of economic activity. For example, pulse transformers (hereinafter referred to as IT) are an important element installed in almost all modern power supplies.

Design (types) of pulse transformers

Depending on the shape of the core and the placement of coils on it, IT are produced in the following designs:

• rod;
  
• armored;
  
• toroidal (does not have coils, the wire is wound on an insulated core);
  
• armored rod;
  

The figures show:

• A - a magnetic circuit made of transformer steel grades made using the technology of cold or hot rolled metal (with the exception of a toroidal core, it is made of ferrite);
• B - coil of insulating material
• C - wires that create an inductive connection.

Note that electrical steel contains few additives of silicon, since it causes power loss from the effect of eddy currents on the circuit of the magnetic circuit. In IT of toroidal design, the core can be made from rolled or ferrimagnetic steel.

Plates for a set of an electromagnetic core are selected in thickness depending on the frequency. With an increase in this parameter, it is necessary to install plates of smaller thickness.

Principle of operation

The main feature of pulse type transformers (hereinafter referred to as IT) is that they are supplied with unipolar pulses with a constant current component, and therefore the magnetic circuit is in a state of constant bias. The schematic diagram for connecting such a device is shown below.

As you can see, the connection diagram is almost identical with conventional transformers, which cannot be said about the timing diagram.

The primary winding receives pulse signals having a rectangular shape e (t), the time interval between which is quite short. This causes an increase in the inductance during the interval t u , after which its decline is observed in the interval (T-t u).

The induction drops occur at a rate that can be expressed in terms of the time constant by the formula: τ p =L 0 /R n

The coefficient describing the difference of the inductive difference is determined as follows: ∆V=V max - V r

• B max - the level of the maximum value of induction;
• In r - residual.

More clearly, the difference in inductions is shown in the figure showing the shift of the operating point in the IT magnetic circuit.

As can be seen in the timing diagram, the secondary coil has a voltage level U 2 in which there are reverse surges. This is how the energy accumulated in the magnetic circuit manifests itself, which depends on the magnetization (parameter i u).

The current pulses passing through the primary coil are trapezoidal in shape, since the load and linear currents (caused by core magnetization) are combined.

The voltage level in the range from 0 to t u remains unchanged, its value e t =U m . As for the voltage on the secondary coil, it can be calculated using the formula:

wherein:

• Ψ is the flux linkage parameter;
• S is a value that displays the cross section of the magnetic core.

Considering that the derivative characterizing the changes in the current passing through the primary coil is a constant value, the increase in the level of induction in the magnetic circuit occurs linearly. Based on this, it is permissible, instead of the derivative, to introduce the difference of indicators made after a certain time interval, which allows you to make changes to the formula:

in this case, ∆t will be identified with the parameter t u , which characterizes the duration with which the input voltage pulse flows.

To calculate the area of ​​the pulse with which the voltage is formed in the secondary winding of IT, it is necessary to multiply both parts of the previous formula by t u. As a result, we will come to an expression that allows us to obtain the main IT parameter:

U m x t u =S x W 1 x ∆V

Note that the value of the pulse area directly depends on the parameter ∆В.

The second most important value that characterizes the operation of IT is the induction drop, it is influenced by such parameters as the cross section and magnetic permeability of the core of the magnetic circuit, as well as the number of turns on the coil:

Here:

• L 0 - induction difference;
• µ a is the magnetic permeability of the core;
• W 1 - the number of turns of the primary winding;
• S is the cross-sectional area of ​​the core;
• l cp - length (perimeter) of the core (magnetic circuit)
• B r is the value of the residual induction;
• In max - the level of the maximum value of the induction.
• H m - Magnetic field strength (maximum).

Considering that the IT inductance parameter completely depends on the magnetic permeability of the core, the calculation must be based on the maximum value of µ a, which is shown by the magnetization curve. Accordingly, for the material from which the core is made, the level of the parameter B r , which reflects the residual induction, should be minimal.

Video: a detailed description of the principle of operation of a pulse transformer

Based on this, a tape made of transformer steel is ideal for the role of the IT core material. You can also use permalloy, in which such a parameter as the coefficient of squareness is minimal.

Ferrite alloy cores are ideal for high-frequency IT because this material has low dynamic losses. But because of its low inductance, it is necessary to make IT of large sizes.

Calculation of a pulse transformer

Consider how it is necessary to calculate IT. Note that the efficiency of the device is directly related to the accuracy of the calculations. As an example, let's take a conventional converter circuit that uses a toroidal type IT.

First of all, we need to calculate the IT power level, for this we use the formula: P \u003d 1.3 x P n.

The value of R n displays how much power the load will consume. After that, we calculate the overall power (P gb), it should be no less than the load power:

Parameters required for calculation:

• S c - displays the cross-sectional area of ​​the toroidal core;
• S 0 - the area of ​​​​its window (as a hint, this and the previous value are shown in the figure);

• B max is the maximum peak induction, it depends on which brand of ferromagnetic material is used (the reference value is taken from sources describing the characteristics of ferrite grades);
• f is a parameter characterizing the frequency with which the voltage is converted.

The next step is to determine the number of turns in the primary winding Tr2:

(results are rounded up)

The value of U I is determined by the expression:

U I \u003d U / 2-U e (U is the voltage supply to the converter; U e is the voltage level supplied to the emitters of transistor elements V1 and V2).

We proceed to the calculation of the maximum current passing through the primary winding of IT:

The parameter η is equal to 0.8, this is the efficiency with which our converter must work.

The diameter of the wire used in the winding is calculated by the formula:

If you have problems with determining the main IT parameters, you can find thematic sites on the Internet that allow you to calculate any pulse transformers online.