A flyback switching power supply

Abstract

A flyback switching power supply, the power supply U DC The positive end + and N P1 The terminal with the same name is connected to the cathode of the absorption network 400, N P1 The opposite end is connected to the drain of Q1; the anode of 400 is connected to N P2 The different ends are connected, and the source of Q1 is connected to N P2 Name end, U DC The negative terminal of ‑; the gate of Q1 is connected to the control signal; it is characterized in that: N P1 and N P2 For bifilar parallel winding, also includes C1, one end of C1 and N P1 The opposite end is connected, and the other end of C1 is connected to N P2 Different names are connected. This is achieved: when Q1 is saturated and turned on, N P1 and N P2 Both are excited, when Q1 is turned off, the secondary side NS outputs energy, and the energy of the leakage inductance is determined by N P2 After 400 to achieve absorption, the power supply U DC Partial energy absorption, suitable for working at low voltage, improves the utilization rate and current density of the primary winding, so the power density is high, allowing a large leakage inductance between the primary and secondary windings, the primary winding is not connected in parallel on the circuit board, the circuit Board wiring is simple, and EMI is improved.

Description

technical field [0001] The invention relates to the field of switching power supplies, in particular to a flyback switching power supply. Background technique [0002] At present, switching power supplies are widely used. For occasions where the input power is below 75W and the power factor (PF, PowerFactor, also known as power factor) is not required, the flyback (Fly-back) switching power supply has attractive advantages: circuit topology Simple, wide input voltage range. Due to the small number of components, the reliability of the circuit is relatively high, so it is widely used. For convenience, many documents are also called flyback switching power supply, flyback power supply, Japan and Taiwan are also called flyback converter, flyback switching power supply, and flyback power supply. Common topologies for AC / DC converters such as figure 1 As shown, the prototype of this figure comes from page 60 of "Switching Power Converter Topology and Design" written by Dr. Zha...

AI Technical Summary

Problems solved by technology

In fact, the low-voltage DC / DC switching power supply adopts the flyback topology. This is because at low voltage, the input current of the flyback switching power supply is discontinuous, the ripple is large, and the requirements for the previous power supply equipment are relatively high. High; the output current is not continuous, the ripple is very large, and the capacity of the filter capacitor behind is required to be high; especially when the input voltage is low, because the excitation current becomes larger, the primary winding must be wound with multiple strands; The inductance of the side winding is also low, and it often happens that the calculated number of turns cannot be tiled from the left to the right of the wire slot that is full of the skeleton. Especially when the working voltage is high, the sandwich series winding method can be used. Downside, it is forced to adopt the sandwich parallel winding method. Since the two primary windings are not on the same layer, there is a leakage inductance between the two primary windings. This leakage inductance will cause losses, thereby reducing the efficiency of the switching power supply. , The problems caused by the leakage inductance between the two parallel primary windings: 1) During excitation, due to the existence of the leakage inductance, the induced voltage difference has a pressure difference on the leakage inductance, which causes a non-negligible loss, which is easier to understand: If the number of turns of the two parallel primary windings differs by one turn, it is equivalent to the existence of a short circuit between turns, but it is just a short circuit through the DC internal resistance of the two parallel primary windings. Relatively speaking, the loss has no real turns 2) When demagnetization occurs, that is, the rectifier diode on the secondary side is turned on, and the output filter capacitor is continuously charged. At this time, the reflected voltage is induced on the primary side, and the two parallel primary windings will induce unequal Due to the low internal resistance of the winding, the current induced by the unequal voltage is not small, which causes loss and large electromagnetic interference; 3) If the third winding is used for demagnetization, the third winding is the same as the two Which of the parallel primary windings is wound in parallel? Only two tertiary windings can be used, which are respectively wound in parallel with the two primary windings connected in parallel, and then connected in parallel to form a "third winding". voltage, causing loss and large electromagnetic interference

The problem caused by parallel winding of multi-strand wires is that it is inconvenient to wind and weld the multi-strand wires to the pins on the transformer skeleton. Since the pins are usually thinner, the multi-strand wires appear thicker in comparison, and it is difficult to wrap the leads well. feet, and the contact area between them is small, and the welding reliability is poor. Of course, the multi-strand wire can be divided into two strands, one of which is drawn from pins 1 and 3, and the other strand is drawn from pins 1 and 3. Pins 2 and 4 lead wires, pins 1, 2, 3, and 4 are arranged in order; or one of the large strands leads from pins 1 and 4, and the other large strand leads from pins 2 and 3; then in The circuit board is connected in parallel again, but in the actual circuit board layout design, there are still many problems encountered: in order to make the electromagnetic compatibility performance of the flyback power supply meet the standard, the circuit area of ​​the main power circuit should be small, and at the same time, the circuit area of ​​the demagnetization circuit The area is also small, and it is more difficult to route

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