Dear colleagues!!
I have already told you how to build a pulse transformer on a ferrite ring in my lessons. Now I’ll tell you how I make a transformer using a W-shaped ferrite core. For this, I use ferrites of suitable size from old “Soviet” equipment, old computers, televisions and other electrical equipment that I have lying around in the corner “on demand”.
For a UPS using a push-pull half-bridge generator circuit, the voltage on the primary winding of the transformer, according to the circuit, is 150 volts, under load we will take 145 volts. The secondary winding is made according to a full-wave rectification circuit with a midpoint.
See diagram.
I will give examples of calculation and manufacture of transformers for a small power UPS of 20 - 50 watts for this circuit. I use transformers of this power in switching power supplies for my LED lamps. The transformer diagram is below. It is necessary to pay attention that the W-core, folded from two halves, 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 powers 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. 
We will choose from ferrite cores No. 17, Ш - a shaped core Ш7.5 × 7.5. Cross-sectional area of the middle rod Sк = 56 mm.sq. = 0.56 cm2
Window Sо = 150 mm.sq. Rated power 200 watts.
The number of turns per 1 volt for 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 = n x 145 = 1.25 x 145 = 181.25. Let's take 182 turns.
When choosing the wire thickness for the windings, I proceeded from the “” table.
In my transformer, I used a wire with a diameter of 0.43 mm in the primary winding. (a wire with a large diameter does not fit in the window). It has a cross-sectional area S = 0.145 mm2. Allowable current (see table) I = 0.29 A.
The power of the primary winding will be: P = V x I = 145 x 0.29 = 42 watts.
A communication winding must be placed on top of the primary winding. It should produce a 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).
Current in the secondary winding, taking into account the efficiency (k=0.95) of the transformer: I = k xP/V = 0.95 x 42 watts / 30 volts = 1.33 A;
Let's select a wire for this current. I used a wire I had in stock with a diameter of 0.6 mm. Its S = 0.28 mm.sq.
The permissible 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 calculated current of 1.33 A.
The number of turns in each half-winding for a voltage of 30 volts: w2.1 = w2.2 = n x 30 = 1.25 x 30 = 37.5 vit.
Let's take 38 turns in each half-winding.
Transformer output power: Pout = V x I = 30 V x 1.12 A = 33.6 Watt, which, taking into account losses in the wire and core, is quite normal.
All windings: primary, secondary and communication winding fit perfectly into the window Sо = 150 mm2.
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 core “Sk” will double. Sk = 56 x 2 = 112 mm2 or 1.12 cm2
The window area will remain the same “So” = 150 mm2.
The indicator n (the number of turns per 1 volt) will decrease. n = 0.7 / Sk = 0.7 /1.12 = 0.63 vit./volt.
Hence, the number of turns in the primary winding of the transformer will be:
w1 = n x 145 = 0.63 x 145 = 91.35. Let's take 92 turns.
In the feedback winding w3, for 6 volts, there will be: w3 = n x v3 = 0.63 x 6 = 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.
Number of turns of secondary half-windings, each 30 volts: w2.1 = w2.2 = n x 30 = 0.63 x 30 = 18.9. Let's take 19 turns.
I used a wire for the primary winding with a diameter of 0.6 mm. : wire cross-section 0.28 mm2, current 0.56 A.
With this wire, the power of the primary winding will be: P1 = V1 x I = 145 V x 0.56 A = 81 Watts.
I wound the secondary winding with a wire with a diameter of 0.9 mm. 0.636 mm.sq. for a current of 1.36 amperes. For two half-windings, the current in the secondary winding is 2.72 amperes.
Secondary winding power P2 = V2 x I = 30 x 2.72 = 81.6 watts.
Wire with a diameter of 0.9 mm. a little big, fits with a large margin, that's not bad.
I use the wire for the windings at the rate of 2 A per square millimeter (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 mm2. and install a fan to cool the windings.
The general conclusion from these calculations is:
- when adding two identical Sh-shaped cores, the area “Sk” doubles with the same window area “So”.
- the number of turns in the windings (in comparison with the first option) changes.
- the primary winding w1 from 182 turns is reduced to 92 turns;
- the secondary winding w2 from 38 turns is reduced 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, and made a frame for them. 
It must be borne in mind that transformers, according to first and second For example, you can use it under a smaller load, down to 0 watts. The UPS maintains voltage quite well and stably.
Compare the appearance of transformers: example-1, with one core and example-2, with two folded cores. The actual sizes of transformers vary slightly.
The analysis of ferrite cores #18 and #19 is similar to the previous examples.
All our calculations are theoretical estimates. In fact, it is quite difficult to obtain such power from a UPS on transformers of these sizes. The design features of the switching power supply circuits 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 winding wires 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 size and weight of the UPS on ferrite is significantly less.
The procedure for assembling a ferrite Ш - transformer.
Whether ready-made or assembled, a new frame is made to fit the dimensions of the core.
See how to make "" here. Although this article talks about a frame for a transformer with a steel core, the description is quite suitable for our case.
The frame must be placed on a wooden frame. Winding of the transformer is done manually.
The primary winding is first wound onto the frame. The first row is filled turn by turn, then a layer of thin paper, varnished cloth, then the second row of wire, etc. A thin PVC tube is placed at the beginning and end of the wire (insulation from the installation wire can be used) to stiffen the wire so that it does not break off.
Two layers of paper are applied on top of the primary winding (inter-winding insulation), then you need to wind the turns of the communication winding w3. Winding w3 has few turns, and therefore it is placed at the edge of the frame. Then the turns of the secondary winding are applied. Here it is advisable to proceed in such a way that the turns of the secondary winding w2 are not located on top of the turns w3. Otherwise, malfunctions of the switching power supply may occur.
Winding is carried out with two wires at once (two half-windings), turn to turn in a row, then a layer of paper or tape and a second row of two wires. There is no need to put a PVC tube on the ends of the wire, because The wire is thick and will not break. The finished frame is removed from the mandrel and placed on a ferrite core. First check the core for any play.
If the frame is tight on the core, be very careful, the ferrite breaks very easily. A broken core can be glued together. I glue with PVA glue, followed by drying.
The assembled ferrite transformer is secured at the end with tape for strength. It is necessary to ensure that the ends of the core halves coincide without gap or shift.
Various types of transformers are widely used in electronics and electrical engineering. This makes it possible to use electronic systems in many areas of production and economic activity. Therefore, along with the basic calculations, the calculation of the pulse transformer becomes of great importance. These devices are important elements that are used in all circuits of modern power supplies.
Purpose and operation of a pulse transformer
Pulse transformers are used in communication systems and various automatic devices. Their main function is to make changes in the amplitude and polarity of pulses. The main condition for the normal operation of these devices is considered to be minimal distortion of the signals they transmit.
The principle of operation of a pulse transformer is as follows: when rectangular voltage pulses with a certain value are received at its input, an electric current gradually appears in the primary winding and its strength further increases. This state, in turn, leads to a change in the magnetic field in the secondary winding and the appearance of an electromotive force. In this case, the signal is practically not distorted, and small current losses do not affect anything.
When the transformer reaches its design power, a negative part of the pulse necessarily appears. It is quite possible to minimize its impact by installing a simple diode in the secondary winding. As a result, at this point the pulse will also be as close as possible to a rectangular configuration.
The main difference between a pulse transformer and other similar technical systems is its extremely unsaturated operating mode. A special alloy is used to manufacture the magnetic core, which ensures high magnetic field throughput.
Calculation of initial data and selection of device elements
First of all, it is necessary to correctly select the most suitable magnetic circuit. Universal designs include armor cores with W-shaped and cup-shaped configurations. Setting the required gap between the parts of the core makes it possible to use them in any switching power supplies. However, if a half-bridge push-pull converter is being assembled, you can get by with a conventional ring magnetic core. When calculating, it is necessary to take into account the outer diameter of the ring (D), the inner diameter of the ring (d) and the height of the ring (H).
There are special reference books on magnetic cores, where the dimensions of the ring are presented in the KDxdxH format.
Before calculating a pulse transformer, it is necessary to obtain a certain set of initial data. First you need to decide on the supply voltage. There are some difficulties here due to possible... Therefore, for calculations, the maximum value of 220 V + 10% is taken, to which special coefficients are applied:
- The amplitude value is: 242 V x 1.41 = 341.22 V.
- Next 341.22 - 0.8 x 2 = 340 V minus the voltage drop across the rectifier.
The value of induction and frequency is determined using tables:
1. Manganese-zinc ferrites.
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Ferrite grade |
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2. Nickel-zinc ferrites.
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Ferrite grade |
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Cutoff frequency at tgδ ≤ 0.1, MHz |
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Magnetic induction B at Hm = 800 A/m, T |
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Winding pulse transformers
When winding pulse transformers, it is necessary to take into account the features of these devices. First of all, you should pay attention to the uniform distribution of the winding along the entire perimeter of the magnetic core. Otherwise, there will be a significant reduction in the power of the device, and in some cases, its failure.
In the case of winding the wire with your own hands, a “turn to turn” winding is used, made in one layer. Based on this technical characteristic, the calculation of the pulse transformer is also carried out in terms of determining the required number of turns. The diameter of the wire used for winding must be selected in such a way that the entire wire fits exactly into one layer, and the number of turns in this case will coincide with the calculated data. The difference between and the result obtained using the formula can be from 10 to 20%, which allows you to make a winding without paying attention to the exact number of turns.
There is a formula to perform the calculations: W = n(D - 10 S - 4 d) / d, wherein W-is the number of turns in the primary winding, n- constant value equal to 3.1416, D- internal diameter of the magnetic circuit ring, S- thickness of the insulating gasket, d- diameter of the insulated wire. The maximum tolerance for errors in calculations ranges from -5 to +10% depending on the density of the wires.
And yet I was invited! Now things will go more quickly with the articles. Initially, I wanted to focus on the circuit design of some block for the next part, but what are you waiting for? But then I remembered my school youth and the great problem that I faced - how to make a beast device unknown to me at that time - pulse transformer
. Ten years have passed and I understand that many (and not just beginners) radio amateurs, electronics engineers and students have such difficulties - they are simply afraid of them, and as a result they try to avoid powerful switching power sources (further IIP).
After these thoughts, I came to the conclusion that the first topic should be about the transformer and nothing else! I would also like to make a reservation: what I mean by the concept of “powerful SMPS” is power from 1 kW and above, or in the case of amateurs, at least 500 W.
Figure 1 - This is the kind of 2 kW transformer we will eventually get for the H-bridge
The Great Battle or Which Material to Choose?
Once upon a time, having introduced pulse technology into my arsenal, I thought that transformers could only be made using ferrite, which was available to everyone. Having assembled the first designs, the first thing I decided to do was present them to the judgment of more experienced comrades and very often heard the following phrase: “Your shitty ferrite is not the best material for an impulse generator.”. I immediately decided to find out from them what alternative could be opposed to it and they told me - alsifer or whatever they call it sindust.
Why is it so good and is it really better than ferrite?
First, you need to decide what an almost ideal material for a transformer should be able to do:
1) must be soft magnetic, that is, it is easy to be magnetized and demagnetized
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, the saturation mode leads to the fact that the transfer of energy from the primary winding to the secondary winding partially stops. Normal operation of a 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 only possible when its volume and cross-section are not less than a certain value. Therefore, the greater the power of the transformer, the larger its core should be.
3) the material must have the lowest possible losses due to magnetization reversal and Foucault currents
4) the properties of the material should not change significantly under external influences: mechanical forces (compression or tension), changes in temperature and humidity.
Now let's look at the properties of ferrite and how well it meets the requirements presented above.
Ferrite is a semiconductor, which means it has its own high electrical resistance. This means that at high frequencies, eddy current losses (currents Foucault) will be quite low. It turns out that at least one condition from the list above has already been met. Go ahead…
Ferrites can be thermally stable or unstable, but this parameter is not decisive for the SMPS. The important thing is that ferrites work stably in the temperature range from -60 to +100 o C, and this is for the simplest and cheapest brands.
Figure 3 - Magnetization curve at a frequency of 20 kHz at different temperatures
And finally, the most important point - in the graph above we saw a parameter that will determine almost everything - saturation induction. For ferrite it is usually taken as 0.39 Tesla. It is worth remembering that under different conditions this parameter will change. It depends on both frequency and operating temperature and other parameters, but special emphasis should be placed on the first two.
Conclusion: ferrite is good! perfect for our purposes.
A few words about alsifer and how it differs
1) alsifer works in a slightly wider range of temperatures: from -60 to +120 o C - is it suitable? Even better than ferrite!
2) the coefficient of losses due to hysteresis for alsifers is constant only in weak fields (at low power), in a powerful field they increase very strongly - this is a very serious disadvantage, especially at powers of more than 2 kW, so it loses here.
3) saturation induction up to 1.2 Tesla!, 4 times more than ferrite! - the main parameter is already ahead, but not everything is so simple... Of course, this advantage will not go anywhere, but point 2 weakens it very much - definitely a plus.
Conclusion: Alsifer is better than ferrite, this guy didn’t lie to me.
Result of the battle:
Anyone who reads the description above will say give us Alsifer! And rightly so... but try to find an alsifer core with an overall power of 10 kW? Here usually a person comes to a dead end, it turns out that they are not really on sale, and if they are, then they are ordered 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, it loses very little... ferrite is estimated relative to alsifer at "8 out of 10 parrots."
I wanted to turn to my favorite matan, but decided not to do so, because... I consider +10,000 characters to the article excessive. 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 in retelling his calculations with some additions.
And so we proceed to the calculation and manufacture of the transformer
First of all, I would like to immediately remember a very serious point - the gap in the core. It can “kill” all the power or add another 30-40%. I want to remind you what we do transformer for H-bridge, and it refers to forward converters (forward in bourgeois). This means that the gap should ideally be 0 mm.
Once, while studying for a 2-3 course, I decided to assemble a welding inverter and turned to the topology of Kemppi inverters. There I saw a gap of 0.15 mm in the transformers. I wondered what it was for. I didn’t approach the teachers, but instead called the Russian representative office of Kemppi! What to lose? To my surprise, I was connected to a circuit engineer and he told me several theoretical points that allowed me to “crawl” beyond the 1 kW ceiling.
In short - a gap of 0.1-0.2 mm is simply necessary! This increases the rate of demagnetization of the core, which allows more power to be pumped through the transformer. The maximum effect of such a feint with the ears of the gap was achieved in the topology "oblique bridge", there the introduction of a gap of 0.15 mm gives an increase of 100%! In our H-bridge this increase is more modest, but I think 40-60% is not bad either.
To make a transformer we need the following kit:
A) 
Figure 4 - Ferrite core E70/33/32 made of 3C90 material (slightly better analogue of N87)
b) 
Figure 5 - Frame for core E70/33/32 (the larger one) and choke D46 made of atomized iron
The overall power of such a transformer is 7.2 kW. We need such a reserve to provide starting currents 6-7 times higher than the rated ones (600% according to technical specifications). It’s true that such starting currents only occur in asynchronous motors, but everything needs to be taken into account!
Suddenly, a certain choke “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 enormous knowledge and always ready to teach and help, for which I thank him - at one time he helped me take the right path. The program is called - Excellent IT 8.1 .
Here is an example of a calculation for 2 kW:
Figure 6 - Calculation of a pulse transformer using a bridge circuit for 2 kW step-up
How to calculate:
1) Highlighted in red. These are the input parameters that are usually set by default:
a) maximum induction. Remember for ferrite it is 0.39 T, but our transformer operates at a fairly high frequency, so the program sets 0.186 itself. This is saturation induction in the very worst conditions, including heating up to 125 degrees
b) conversion frequency, it is set by us and how it is determined in the diagram will be in the following articles. This frequency should be from 20 to 120 kHz. If less, we will hear the trance and whistle, if higher, then our switches (transistors) will have large dynamic losses. And even expensive IGBT switches operate up to 150 kHz
c) coefficient filling the window is an important parameter, because the space on the frame and core is limited, you should not make it more than 0.35, otherwise the windings will not fit
d) current density - this parameter can be up to 10 A/mm 2. This is the maximum current that can flow through a conductor. The optimal value is 5-6 A/mm 2 - under severe operating conditions: poor cooling, constant operation at maximum load, etc. 8-10 A/mm 2 - can be set if your device is perfectly ventilated and several coolers cost over 9000.
e) food at the entrance. Because we calculate the transformer for DC->DC 48V to 400V, then we set the input voltage as in the calculation. Where did the figure come from? In a discharged state, the battery produces 10.5V, further discharging will reduce the service life, multiply by the number of batteries (4 pcs) and get 42V. Let's take 40V with a reserve. 48V is taken from the product 12V * 4 pcs. 58V is taken from the consideration that in a charged state the battery has a voltage of 14.2-14.4V and, by analogy, multiply by 4.
2) Highlighted in blue.
a) set 400V, because this is a reserve for voltage feedback and for cutting a sine wave a minimum of 342V is required
b) rated current. We choose from consideration 2400 W / 220 (230) V = 12A. As you can see, everywhere I take a reserve of at least 20%. This is what any self-respecting manufacturer of quality equipment does. In the USSR, such a reserve was the standard 25%, even for the most difficult conditions. Why is 220 (230) V the voltage at the output of a pure sine wave?
c) minimum current. Selected from real conditions, this parameter affects the size of the output choke, so the higher the minimum current, the smaller the choke, and therefore the cheaper the 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 ultra-fast diodes at the output, then the drop across them will be 0.6V in the worst conditions (the temperature is exceeded).
d) 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 you have. I just don’t recommend setting it to more than 1.18 mm, because... the skin effect will begin to affect
Skin effect
Skin effect is the effect of reducing the amplitude of electromagnetic waves as they penetrate deep into a conducting medium. As a result of this effect, for example, high-frequency alternating current when flowing through a conductor is not distributed evenly over the cross-section, but mainly in the surface layer.
If we speak not like Google, but in my collective farm language, then if you take a conductor with a large cross-section, it will not be fully used, because currents at a higher frequency flow along the surface, and the center of the conductor will be “empty”
3) Highlighted in green. Everything is simple here - we plan a “full bridge” topology and select it.
4) Highlighted in orange. The core selection process takes place, 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. The coefficient was highlighted 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.
Parameters for further calculation of the output choke are also given: inductance and voltage ripple.
Now you need to calculate the output choke. It is needed to smooth out 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 ThrottleRing 5.0. Here is the calculation for our transformer:
Figure 7 - Calculation of the output choke for a boost DC-DC converter
In this calculation, everything is simpler and clearer, it works on the same principle, the output data is: the number of turns and the number of wires in the braid.
Manufacturing stages
Now we have all the data for manufacturing the transformer and inductor.
The main rule for winding a pulse transformer is that all windings, without exception, must be wound in one direction!
Stage 1:
Figure 8 - Winding process of the secondary (high-voltage) winding
We wind the required number of turns of 2 wires with a diameter of 1 mm onto the frame. We remember the direction of winding, or better yet, mark it with a marker on the frame.
Stage 2:
Figure 9 - Isolate the secondary winding
We insulate the secondary winding with fluoroplastic tape 1 mm thick, this 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 insulate it with ordinary plumbing foam in 4-6 layers. This is the same fluoroplastic, 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 - Drawing out the tail of the primary winding
He wraps the winding and insulates it with fluoroplastic tape. It is also advisable to impregnate it with varnish.
Stage 5:
Figure 12 - We impregnate with varnish and solder the “tail”. Winding of windings is completed
Stage 6:
Figure 13 - We complete the winding and insulation of the transformer with keeper tape with final impregnation in varnish
Keeper 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; harsh, bleached or plain-dyed. The tape material has a high density due to the weave, it is thicker than its closest analogue - plain tape - due to the use of thicker threads.
Thanks to Wikipedia.
Stage 7:
Figure 14 - This is what the finished version of the transformer looks like
A gap of 0.15 mm is established during the gluing process by inserting a suitable film between the core halves. The best option is printing film. The core is glued together with instant glue (good) or epoxy resin. The 1st option is forever, the 2nd allows you to disassemble the transformer without damage if something happens, for example, if you need to wind another winding or add more turns.
Choke winding
Now, by analogy, you need to wind the inductor; of course, winding it on a toroidal core is more difficult, but this option will be more compact. All the data we have is from the program, the core material is atomized iron or permalloy. The saturation induction of this material is 0.55 Tesla.
Stage 1:
Figure 15 - Wrap the ring with fluoroplastic tape
This operation allows you to avoid the case of breakdown of the winding on the core, this happens rarely, but we do it for ourselves for quality!
Stage 2:
Figure 16 - Wind the required number of turns and insulate
In this case, the number of turns will not fit into one winding layer, so after winding the first layer, it is necessary to insulate and wind the second layer, followed by insulation.
Stage 3:
Figure 17 - Insulate after the second layer and impregnate with varnish
Epilogue
I hope my article will teach you the process of calculating and manufacturing a pulse transformer, as well as give you some theoretical concepts about its operation and the materials from which it is made. I tried not to load this part with unnecessary theory, keep everything to a minimum and focus exclusively on practical aspects. And most importantly, on the key features that affect performance, such as clearance, winding directions, etc.
To be continued...
Switching power supplies, which are increasingly found 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 cascades) transformers. This is dictated by the fact that the values of the number of turns and their diameter given in the literature often do not coincide with the desired output data of the power source being assembled or designed, or the ferrite rings or transistors available to the radio amateur do not correspond to those given in the circuit.
A simplified method for calculating transformers of switching power supplies has been presented in the literature. The general procedure for calculating a switching power supply transformer is as follows:
1. Calculate (in W) the used power of 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 Pgab>Fig., where Pgab. — overall power of the transformer, W, calculated as:
Where D is the outer diameter of the ferrite ring, cm; d—inner diameter; h is the height of the ring; f is the operating frequency of the converter, Hz; Bmax is the maximum value of induction (in Tesla), which depends on the grade of ferrite and is determined from the reference book.
3. Having set the voltage on the primary winding of the transformer
U1 is determined by 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 winding wire (in mm):
6. Find the number of turns and diameter of the wire of the output (secondary) winding:
M.A. Shustov; “Practical circuit design. Voltage converters"; "Altex-A", 2002
For a ring core there is no need to make a frame and make a winding device. The only thing you have to do is make a simple shuttle.
The picture shows a ferrite magnetic core M2000NM.
The standard size of the ring magnetic core can be identified by the following parameters.
D is the outer diameter of the ring.
d – internal diameter of the ring.
H – ring height.
In reference books on ferrite magnetic cores, these dimensions are usually indicated in the following format: K D x d x H.
Example: K28x16x9
Return to top to menu.
Obtaining initial data for simple calculation of a pulse transformer.
Supply voltage.
I remember when our power grids had not yet been privatized by foreigners, I built a switching power supply. The work dragged on until night. During the last tests, it suddenly turned out that the key transistors began to get very hot. It turned out that the network voltage jumped to 256 Volts at night!
Of course, 256 Volts is too much, but you shouldn’t rely on GOST 220 +5% –10% either. If you choose 220 Volts +10% as the maximum network voltage, then:
242 * 1.41 = 341.22V(we count the amplitude value).
341.22 – 0.8 * 2 ≈ 340V(subtract the drop on the rectifier).
Induction.
We determine the approximate value of induction from the table.
Example: M2000NM – 0.39T.
Frequency.
The generation frequency of a self-excited converter depends on many factors, including the size of the load. If you choose 20-30 kHz, you are unlikely to make a big mistake.
Limit frequencies and induction values of widespread ferrites.
Manganese-zinc ferrites.
Parameter | Ferrite grade |
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Nickel-zinc ferrites.
Parameter | Ferrite grade |
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Cutoff frequency at tg δ ≤ 0.1, MHz | ||||||
Magnetic induction B at Hm = 800 A/m, T |
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How to choose ferrite ring core?
You can select the approximate size of a ferrite ring using a calculator for calculating pulse transformers and a guide to ferrite magnetic cores. Both can be found in the “Additional Materials”.
We enter the data of the proposed magnetic core and the data obtained in the previous paragraph into the calculator form to determine the overall power of the core.
You should not choose ring dimensions close to the maximum load power. It is not so convenient to wind small rings, and you will have to wind a lot more turns.
If there is enough free space in the body of the future design, then you can choose a ring with a obviously larger overall power.
switching power supplies on ferrite rings http://www. ferrite /user_files/File/...literature8.zip diagram for the article:
Throttle calculation (article) http://valvolodin. na...ms/throttle. html
Calculation of chokes using MLT resistors (prog) - http://rf. *****/s3/r-dros. html
Program for calculating high-frequency transformers and chokes - http://www. /...gramm/5/3.shtml
Program for calculating a pulse transformer - http://www. /...gramm/5/2.shtml
AC chokes for electronic equipment - http://dmitriks. naro...ooks/dptra. djvu
Calculation of chokes and coils book - http://depositfiles....files/mcckejoig
Transformers and chokes 1.1 on archive. ***** -
Optimal design of power high-frequency ferromagnetic devices - http://dmitriks. naro...oks/opsvfu. djvu
"Pulse sources of secondary power supply in household radio equipment" - http://dmitriks. naro...books1/iip. djvu
at 494 http://focus. /...1d/slva001d. pdf
TRANSFORMERS AND CHOCKS FOR PULSE POWER SUPPLIES - http://members. kern....ouz/chokes. html
http://www. /ser2800.cfm
Selection and calculation of the anode choke design - http://qrx. *****/hams/r_and. htm
Calculation of the inductance of a choke with a magnetic gap - http://www. gerelo. dp...ras_indukt. html
Calculation of transformer and choke - http://enginee-ru. uc...oad/
http://enginee-ru. uc.../load/
automaticallyq on-line calculator
http://schmidt-walte...smps_e. html#Abw
.
Calculation of low-power power transformers and filter chokes
http://*****/book/krizeSN. zip
characteristics and program for calculating inductances on metal powder
Micrometals cores - http://www. /
Materials - http://www. ferrite /
Program for coils - http://*****/nuke/modules/Downloads/pub.../l_%20meter. zip
Ring cores: Amidon ferrite rings - http://www. *****/...rrite_Cores. htm
Knowledge Library: http://www. /library. asp
Calculation programs: http://www. mag-inc. c...re/software. asp
Transformers and chokes for switching power supplies - http://www. *****/~slash/st8.html
More materials and calculations - http://*****sgates....ocore. php? pg=12
imp cores and their calculation - http://www. /default. asp
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CORE SATURATION
If a large current flows through the core coil, the magnetic material of the core may become saturated. When the core is saturated, its relative magnetic permeability decreases sharply, which entails a proportional decrease in inductance. The decreased inductance causes a further accelerated increase in the current through the CI, etc. In most SMPS, core saturation is extremely undesirable and can lead to the following negative phenomena:
the increased level of losses in the core material and the increased level of ohmic losses in the winding wire lead to an unreasonably low efficiency of the SMPS;
additional losses cause overheating of the CI, as well as nearby radio components
strong magnetic fields in the core, combined with its decreased magnetic permeability, are a much stronger source of interference and interference in small-signal SMPS circuits and other devices compared to normal operation;
the rapidly increasing current through the CI causes shock current overloads of the SMPS switches, increased ohmic losses in the switches, their overheating and premature failure;
Abnormally large CI pulse currents lead to overheating of the electrolytic capacitors of the power filters, as well as an increased level of noise emitted by the wires and traces of the SMPS printed circuit board.
The list can be continued, but it is already clear that operating the core in saturation mode should be avoided. Ferrites enter saturation if the magnetic induction flux density value exceeds 300 [mT] (millitesla), and this value does not depend so much on the grade of ferrite. That is, 300 [mT] is, as it were, an innate property of ferrites; other magnetic materials have different saturation threshold values. For example, transformer iron and powdered iron are saturated at approximately 1 [T], meaning they can operate in much stronger fields. More precise values of the saturation threshold for different ferrites are shown in Table 5.
The magnetic flux density in the core is calculated using the following formula:
(8) B = 1000 * µ0 * µe * I * N / le [mT]
where µ0 is the absolute magnetic permeability of vacuum, 1.257*10-3 [µH/mm]
µe - relative magnetic permeability of the core (not to be confused with the permeability of the core material!)
I - current through the winding, [A]
N - number of turns in the winding
le - length of the average magnetic line of the core, [mm]
A simple transformation of formula (8) will help find the answer to the practical question - what is the maximum current that can pass through the inductor before the core enters saturation:
(9) Imax = 0.001 * Bmax * le / (µ0 * µe * N) [A]
where Bmax is the table value for the core material used, instead of which you can use the value 300 [mT] for any power ferrites
For cores with a gap, it is convenient to substitute expression (4) here; after abbreviations we obtain:
(10) Imax = 0.001 * Bmax * g / (µ0 * N) [A]
At first glance, the result is quite paradoxical: the maximum current through a CI with a gap is determined by the ratio of the size of the gap to the number of turns of the winding, and does not depend on the size and type of the core. However, this apparent paradox is easily explained. The ferrite core conducts the magnetic field so well that the entire drop in magnetic field strength occurs in the gap. In this case, the magnitude of the magnetic induction flux, the same for both the gap and the core, depends only on the thickness of the gap, the current through the winding and the number of turns in the winding, and should not exceed 300 [mT] for ordinary power ferrites.
To answer the question of what size the total gap g must be introduced into the core so that it can withstand the given current without saturation, we transform expression (10) to the following form:
(11) g = 1000 * µ0 * I * N / Bmax [mm]
To more clearly show the effect of the gap, we give the following example. Let's take an E30/15/7 core without a gap, 3C85 ferrite, magnetic permeability µe = 1700. Let's calculate the number of turns required to obtain an inductance of 500 [µH]. The core, according to the table, has AL = 1.9 [µH], using formula (7) we get a little more than 16 turns. Knowing the effective core length le = 67 [mm], using formula (9) we calculate the maximum operating current, Imax = 0.58 [A].
Now let’s insert a gasket with a thickness of 1 [mm] into the core; the gap will be g = 2 [mm]. The effective magnetic permeability will decrease; after simple calculations using formulas (5) and (7), we find that to obtain an inductance of 500 [μH], 125 turns must be wound. Using formula (10), we determine the maximum CI current; it has increased to 3.8 [A], that is, more than 5 times!
This leads to a practical recommendation for readers who design chokes themselves. To get an inductor that operates at the highest possible current, fill the core completely with wire, and then insert as much clearance into the core as possible. If the test calculation turns out that the inductor has an excessive current reserve, then select a smaller core size, or at least reduce the number of turns in the winding to reduce copper losses, and at the same time reduce the core gap. It is important to emphasize that this recommendation does not apply to transformers in which the current through the primary winding consists of two components: the current transmitted to the secondary winding and a small current that magnetizes the core (magnetizing current).
As you can see, the gap in the throttle core plays an extremely important role. However, not all cores allow the insertion of spacers. The ring cores are made one-piece, and instead of “adjusting” the equivalent magnetic permeability with a gap, you have to choose a ring with a certain ferrite magnetic permeability. This explains the fact that there is a wide variety of types of magnetic materials used by the industry to make rings, while split cores for SMPS, where it is easy to introduce a gap, are almost always made of ferrites with high magnetic permeability. The most common types of rings for SMPS are: with low permeability (within 50...200) for chokes, and with high permeability (1000 or more) for transformers.
Powdered iron turns out to be the most preferred material for annular one-piece chokes cores operating at high bias currents. The permeability of powdered iron is usually in the range of 40...125, most often there are rings made of materials with a permeability of 50...80. Table 6 provides reference data for Philips iron powder ring cores.
It is not difficult to check whether the core enters saturation during operation of the SMPS; it is enough to use an oscilloscope to monitor the shape of the current flowing through the CI. The current sensor can be a low-resistance resistor or a current transformer. A CI operating in normal mode will have a geometrically regular triangular or sawtooth current shape. If the core is saturated, the current shape will be bent.
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Magnetic field induction inside the toroid:
B=m*m0*N*I/Lavg,
m0 - magnetic constant = 4*pi*10^(-7),
N - number of turns,
I - current in the winding,
Toroid inductance:
L=m*m0*N^2*S/Lavg,
where m is the magnetic permeability of ferrite,
m0 - magnetic constant,
N - number of turns,
S is the cross-sectional area of ferrite,
Lср - length of the center line of the ferrite ring.
Active winding resistance (excluding skin effect):
R=p*Lп/S,
where p is the resistivity of copper (0.017 Ohm*m),
Lп - winding wire length,
Sp - cross-sectional area of the wire.
I calculate the throttle in the following order:
1) We identify the parameters of the ferrite ring: magnetic permeability m, center line length Lср, cross-sectional area S, saturation induction Bm. The last parameter can be found in a reference book for a well-known brand of ferrite, or on the website of the ferrite manufacturer.
2) We set the required inductance of the inductor L.
3) Knowing the parameters L, m, Lav, S, we calculate the required number of turns N.
4) We determine the maximum current consumption of load I and take it with a 10-15% margin.
5) Knowing the parameters m, Lav, S, I, N, we calculate the induction B inside the ferrite. If it turns out to be greater than 0.8Bm, then the ring is not suitable for the task at hand; it is necessary to select a ring either with a larger cross-section or with a higher saturation induction.
6) If the induction does not exceed 0.8Bm, we determine whether the choke satisfies us in terms of power dissipation. To do this, we set the maximum power dissipated at the inductor (Pm = 0.5-2W depending on the size of the ring).
7) Based on the given power Pm and current consumption I, we determine the active resistance of the winding wire R.
8) We select the wire with which we are going to wind (0.8-1mm for winding in one wire, 0.5-0.6mm for winding in several wires).
9) Knowing the cross-section of the wire(s) Spr and their active resistance R, we calculate the maximum length of the wire(s) Lpr.
10) We wind one turn of wire around the ring and determine its length Lв. Add 1-2mm to the angular displacement of the wire when winding.
11) Based on the found maximum wire length Lpr and the length of one turn Lv, we calculate the permissible number of turns Nadd.
12) If Nadd turns out to be less than the previously calculated number of turns N, it is necessary to use a wire with a larger cross-section, or wind it in several wires.
13) If Nadd>=N, we evaluate the possibility of winding the calculated number of turns. To do this, measure the internal diameter of the ring d and see if the inequality holds:
pi*(d-Spr)>=N*dpr,
where Spr is the cross-sectional area of the wire to be wound,
dpr - diameter of the wire to be wound.
14) If the inequality does not hold, then it is necessary to wind in 2 or more layers. For small rings with an internal diameter of up to 8mm, I personally do not recommend winding in several layers. In this case, it is better to take a larger ring, or with greater magnetic permeability.
From the site - _http://www. /comment/112509
Tips for designing buck converters - http://peljou.../enews/2007/8/7
Program for calculating transformers and chokes 6mV - http://brwbr. /...e=s2-Droselprog
Mack R. Switching power supplies. Theoretical foundations of design and guidance for practical application
is on the website - http://www. electrotechnika. info/index. php?...down&id=177
Another article - http://www. ferrite /site/page-Trancf...tori_i_drocceli
Design of switching power supplies (SMPS). - http://megaohm. people...S/smps_rus. html