TPI type transformers

  Pulse power transformers (TPI) are used in pulsed power supply devices for household and office equipment with intermediate conversion of the supply voltage of 127 or 220 V at a frequency of 50 Hz into rectangular pulses with a repetition rate of up to 30 kHz, made in the form of modules or power supplies: BP, MP-1, MP-2, MP-Z, MP-403, etc. The modules have the same circuit and differ only in the type of pulse transformer used and the rating of one of the capacitors at the filter output, which is determined by the features of the model in which they are used.
     Powerful TPI transformers for switching power supplies are used for decoupling and transferring energy to secondary circuits. Energy storage in these transformers is undesirable. When designing such transformers, as a first step, it is necessary to determine the range of oscillations of the magnetic induction of the DW in the steady state. The transformer should be designed to operate at the highest possible value of the LI, which allows having a smaller number of turns in the magnetizing winding, increasing the rated power and reducing the leakage inductance In practice, the value of LI can be limited either by the saturation induction of the core Bs, or losses in the magnetic circuit of the transformer.
     In most full-bridge, half-bridge, and full-wave (balanced) midpoint circuits, the transformer is driven symmetrically. In this case, the value of the magnetic induction changes symmetrically with respect to the zero of the magnetization characteristic, which makes it possible to have a theoretical maximum value of the DR equal to the doubled value of the saturation induction Bs. In most single-cycle circuits, used, for example, in single-cycle converters, the magnetic induction fluctuates completely within the first quadrant of the magnetization characteristic from the residual induction Br to the saturation induction Bs, limiting the theoretical maximum of the LW to the value (Bs - BR). This means that if the DV is not limited by losses in the magnetic circuit (usually at frequencies below 50 ... 100 kHz), single-ended circuits will require a large transformer with the same output power.
     In voltage-fed circuits (which include all buck regulator circuits), in accordance with Faraday's law, the value of DV is determined by the product of the "volt-second" on the primary winding. In steady state, the volt-second product on the primary winding is set to a constant level. The amplitude of the fluctuations of the magnetic induction is thus also constant.
     However, with the usual method of controlling the duty cycle, which is used by most microcircuits for switching regulators, at start-up and during a sharp increase in the load current, the value of DV can reach twice the value in the steady state. Therefore, so that the core does not saturate during transients, the steady-state value of DV should be twice less than the theoretical maximum However, if a microcircuit is used that allows you to control the value of the "volt-second" product (circuits with tracking the disturbance of the input voltage), then the maximum value of the "volt-second" product is fixed at a level slightly higher than the steady-state value. allows you to increase the value of DV and improves the performance of the transformer.
     Saturation induction value Bs for most ferrites for strong magnetic fields such as 2500NMS exceeds 0.3 T. In push-pull voltage-fed circuits, the magnitude of the increment in the induction of the LW is usually limited to 0.3 T. With an increase in the excitation frequency to 50 kHz, the losses in the magnetic circuit approach the losses in the wires. An increase in losses in the magnetic circuit at frequencies above 50 kHz leads to a decrease in the value of the DW.
      In single-ended circuits without fixing the "volt-second" product for cores with (Bs - Br) equal to 0.2 T, and taking into account transients, the steady-state value of DV is limited to only 0.1 T. Losses in the magnetic circuit at a frequency of 50 kHz will be insignificant due to the small range of fluctuations in magnetic induction. In circuits with a fixed value of the "volt-second" product, the value of DV can take values up to 0.2 T, which makes it possible to significantly reduce the overall dimensions of the pulse transformer.
       In current-powered power supply circuits (step-up converters and current-controlled step-down regulators on coupled inductors), the DV value is determined by the volt-second product at the secondary winding at a fixed output voltage. Since the “volt-second” product at the output does not depend on changes in the input voltage, the circuits supplied with current can operate with a DV value close to the theoretical maximum (if the core loss is not taken into account), without the need to limit the value of the “volt-second” product. ...
      At frequencies above 50. The 100 kHz LW value is usually limited by the losses in the magnetic circuit.
      The second step in the design of high-power transformers for switching power supplies is to make the correct choice of the type of core that will not saturate at a given volt-second product and will provide acceptable losses in the magnetic circuit and windings To do this, you can use an iterative calculation process, however, the formulas below ( 3 1) and (3 2) allow you to calculate the approximate value of the product of the core areas SoSc (the product of the area of the core window So and the cross-sectional area of the magnetic circuit Sc) Formula (3 1) is applied when the LW value is limited by saturation, and formula (3.2) - when the LW value is limited by losses in the magnetic circuit, in doubtful cases, both values are calculated and the largest of the reference data tables is used for various cores, the type of core is selected for which product So Sc exceeds the calculated value.


    Pvx = Pout / l = (output power / efficiency);
    K is a coefficient that takes into account the degree of use of the core window, the area of the primary winding and the design factor (see table 3 1); fp - operating frequency of the transformer
   For most ferrites for strong magnetic fields, the hysteresis coefficient is KTo = 4 • 105, and the eddy current loss factor is Ktue = 4 • 1010.
   In formulas (3.1) and (3.2), it is assumed that the windings occupy 40% from the area of the core window, the ratio between the areas of the primary and secondary windings corresponds to the same current density in both windings, equal to 420 A / cm2, and that the total losses in the magnetic circuit and windings lead to a temperature difference in the heating zone by 30 ° C with natural cooling.
   As a third step in the design of high-power transformers for switching power supplies, it is necessary to calculate the windings of the pulse transformer.
   Table 3.2 shows unified power supply transformers of the TPI type used in television receivers.
     The winding data of TPI-type transformers operating in switching power supplies of stationary and portable television receivers are shown in Table 3. 3 The schematic electrical diagrams of TPI transformers are shown in Figure 3.1


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