Pulse transformer can also be called switching transformer or short for high frequency transformer. In the traditional design of high-frequency transformer, due to the limitation of magnetic core material, its working frequency is low, generally around 20kHz. With the continuous development of power supply technology, miniaturization, high frequency and high power of power supply system have become an eternal research direction and development trend. Therefore, the study of higher frequency power transformer is the key factor to reduce the volume of power system and improve the output power ratio of power supply.
With the continuous expansion of application technology, the application of switching power supply is more and more extensive, but the main technology and energy consumed in making switching power supply are the components of switching transformer.
The difference between switching transformer and ordinary transformer is as follows:
(1) The power supply voltage is not sine wave, but AC square wave, and the current in the primary winding is non-sine wave.
(2) The working frequency of the transformer is relatively high, usually in tens of hertz, or even as high as hundreds of thousands of hertz. When determining the material and loss of magnetic core, the need of high frequency operation and the influence of higher harmonics in magnetic core must be considered.
(3) The winding circuit is complex, and most of them have center taps. This not only increases the size of the primary winding and the volume and weight of the transformer, but also changes the distribution relationship of the winding in the window of the magnetic core.
Figure 1 Schematic diagram of switching power supply
This paper introduces a DC-DC converter as shown in figure 1. The input voltage is 24V DC, and the output voltage is 5V and 12V respectively. The output current of each channel should be above1a. The core device is the high-performance single-ended output current-controlled pulse width modulator chip UC3842 produced by Unitrode Company of the United States, and the highest working frequency can reach 200kHz. According to the excellent electromagnetic properties of Zn-Mn ferrite alloy, the design and matters needing attention of high frequency switching power supply transformer with working frequency of 100kHz are introduced through specific examples.
Selection of transformer core and determination of working point
2. 1 Selection of magnetic core materials
It can be seen from the performance requirements of the transformer that the traditional thin-strip silicon steel is difficult to meet the design requirements of the transformer in terms of frequency and use environment. The material of magnetic core can only be considered from permalloy, ferrite, cobalt-based amorphous alloy and ultra-microcrystalline alloy. Amorphous permalloy and cobalt-based alloy are expensive, about several times as much as ferrite materials, but the saturation magnetic induction intensity Bs is not very high and the processing technology is complicated. Considering that the required power output is not high, about 30W, based on the performance comparison of several materials, we still choose zinc-manganese ferrite material with high saturation magnetic induction intensity Bs, good temperature stability, low price and convenient processing, and choose EI28 as the frame for winding pulse transformer in this example.
2.2 Determination of working point
According to relevant data, the output power of EC35 is 50W, and the saturation magnetic induction intensity is about 2000Gs. The value of the bought magnetic core is not accurate because of the monthly magnetic induction intensity provided by the manufacturer. You can roughly test it in the way provided in Figure 2. Connect the voltage regulator to the primary coil and observe the output voltage waveform of the secondary coil with an oscilloscope. Gradually increase the input voltage of the primary coil from small to large until the waveform displayed by the oscilloscope suddenly changes. At this point, the magnetic core is saturated, according to the formula:
U = 4.44fn 1 φ m, the φ m value at power frequency can be inferred. In the case of low requirements, the number of turns of the primary coil can be roughly estimated according to the calculated φ m.
Fig. 2 Schematic diagram of working point test
3 Calculation of main parameters of transformer
The single-ended flyback converter in this example is widely used in the design of low-power switching power supply, and the multi-channel output is convenient. Single-ended flyback power supply has two working modes: continuous current mode and intermittent current mode. The former is suitable for low power and whether the secondary diode has reverse recovery, but the peak current of MOS tube is relatively large. The peak current of the latter MOS transistor is relatively small, but there is a reverse recovery problem of the secondary diode, so it is necessary to add an absorption circuit to the diode. These two working modes can be selected according to actual needs, and the latter is adopted in this paper.
When designing a transformer, the following problems need to be considered: frequency f (H2) of frequency converter; Primary voltage U 1(V), secondary voltage U2 (v); Secondary current I2 (a); N2 winding line parameter n1; Temperature rise τ (℃); Winding relative voltage drop u; Environmental temperature τ HJ (℃); Insulation material density γ z (g/cm3)
1) Select the iron core according to the output power of the transformer, and the value of the selected iron core should be equal to or greater than the given value.
2) Number of turns per volt winding
( 1)
ST is the cross-sectional area of the iron core; KT is the filling coefficient of the window;
3) Primary winding potential
E 1=U 1( 1- ) (2)
4) Number of turns of primary winding
W 1=W0El (3)
5) potential of secondary winding
E2i=U2i ( 1+ ) (4)
6) Number of turns of secondary winding
W2i=W0E2i (5)
7) Primary winding current
(6)
8) Secondary winding current
(7)
Where n 1 and n2 are the turns of each layer of the primary winding and the secondary winding, respectively.
9) Wire diameter of primary winding
(8)
10) secondary winding wire diameter
(9)
Where j is the current density.
The detailed transformer design method and calculation are quite complicated. Referring to the empirical formula, this paper designs the high frequency transformer in this converter according to the following steps.
3. 1 Determine the transformation ratio of the transformer
According to the relation of output voltage U0
( 10)
The conversion ratio is
( 1 1)
Where UD is the DC voltage output by the rectifier.
In this example, ud = 24v, f is 100kHz, and tON is 0.5; n=2 .
3.2 Calculate the current in the primary coil
Assume that the output DC voltage U0 = 12V and 5V, and the load current is I0 = La, then the output power.
P0=P 1+P2+P3=29W
The efficiency η of switching power supply is generally between 60 and 90%. In this case, η = 0.65 and the input power is
The average primary current is
Assuming that the initial current of the primary coil is zero, the current center in the primary coil linearly increases from zero to the peak value I 1P within the conduction period tON of the switch tube.
3.3 Calculate the number of turns of the primary winding N 1.
The minimum inductance L 1 of the primary winding is
According to the magnitude of output power P, choose a suitable magnetic core, which can be annular, EI-shaped or can-shaped. In this example, EI28 is adopted. The power of this kind of magnetic core can reach 60W at F = 50KHz, and the output power can reach 90W at F = 100KHz.
Where Ilp—— is the peak current of the primary coil, a;
L1-primary inductance, h;
S—— cross-sectional area of magnetic core, mm2；;
BM —— Maximum magnetic flux density of magnetic core, t.
3.4 Calculate the number of turns N2 of the secondary winding.
That is, 12V makes 5 turns and 5V makes 3 turns respectively.
3.5 Estimation of Feedback Winding N3
The determination of the number of turns of the feedback winding requires that the saturation conduction of the switching elements can be ensured without causing excessive losses. According to the requirements of UC3842, the output voltage of the feedback winding should be about 13V. Therefore,
3.6 Selection of steel wire diameter
According to the estimation of input and output, the average current value of the primary coil should be allowed to reach 2A.
1) primary winding
The wire diameter of primary winding can be round copper wire with D = 0.80 mm and cross-sectional area of 0.5027mm2.
2) Secondary winding
According to the output current of each group, the wire diameter of the secondary winding can be the same as that of the primary winding, and it can be solved by multi-strand parallel winding. In order to facilitate the winding of the coil, a wire with a larger wire diameter can also be selected. Because of the high working frequency, skin effect should be considered.
3.7 coil winding and insulation
The most important problem in winding a switching transformer is to find a way to make the primary and secondary coils closely coupled together, so as to reduce the leakage inductance of the transformer, because the leakage inductance is too large, which will cause a large spike pulse and thus break down the switching tube. Therefore, the distance between the primary coil and the secondary coil should be as close as possible when winding the high frequency transformer coil.
Specifically, the following methods can be adopted:
(1) Double parallel winding method
The enameled wires of the primary and secondary coils are wound in parallel, which is the so-called double-wire parallel winding. In this way, the distance between the primary line and the secondary line is the smallest, and the leakage inductance can be minimized. However, this winding method is not easy to wind, and the voltage resistance between two wires is low.
(2) layer by layer winding method
In order to overcome the shortcomings of the parallel winding method, such as low voltage resistance and difficulty in winding, the primary and secondary layered winding method is adopted, that is, 1, 3 and 5 rows of odd layers are used to wind the primary winding, and 2, 4 and 6 rows of even layers are used to wind the secondary winding. This winding method can still maintain the coupling between the primary and secondary, and can also pad insulating paper between the primary and secondary to improve the insulation degree.
(3) Interlayer winding method
The secondary winding is wound in the middle of the primary winding, and the primary winding is wound twice. This winding method only adds a joint in the primary winding, which is simple in process and convenient for mass production.
In this example, in order to reduce the influence of distributed parameters, the primary adopts the structure of double-wire parallel winding connection, and the secondary adopts the way of sectional winding series connection, which is called stacked winding method. Reduce the voltage difference between windings and improve the reliability of transformer. In the aspect of transformer insulation, composite fiber insulation paper with high electrical strength and low dielectric loss should be selected as far as possible to improve the insulation strength and corona resistance between primary and secondary. In this case, since high voltage is not involved, the insulation problem does not need special consideration.
4 conclusion
Winding pulse transformer is an important work in the manufacture of switching power supply, and it also takes a lot of time and energy in the design and manufacture process. The transformer is well done, and the whole design and production work is over 70%. Failure to do so may lead to vibration stop, howling or unstable output voltage and low load capacity. When that temperature of the transform rises
(1) Even if the input voltage is the largest, the main switching device has the longest conduction time, and the transformer core will not be saturated;
(2) The coupling between the primary coil and the secondary coil is better and the leakage inductance is smaller;
(3) The skin effect of high-frequency switching transformer will increase the resistance of the conductor, so the current density should be reduced. In general, the maximum magnetic flux density at work depends on the secondary coil.
( 12)
(4) Generally speaking, when using ferrite core E 128, Bm should be controlled below 3kg.