JP4910190B2 - Soft switching circuit of self-excited switching power supply - Google Patents

Description

  The present invention relates to a switching power supply, and more particularly to a soft switching technique.

  Utility model registration No. 2560208 devised by the present applicant is known as a conventional means of soft switching technology of a switching power supply. This device comprises a circuit as shown in FIG. In FIG. 3, 31a is a primary winding, 32 is a switch element, 31b is a positive feedback winding, and 33 is a snubber capacitor. The switch element 32 is repeatedly turned on and off by the voltage generated in the positive feedback winding 31b. Reference numeral 34 inserted in series between the positive feedback winding 31 b and the control electrode of the switch element 32 is a saturable inductor. The saturable inductor 34 exhibits a high impedance when a pulse voltage is applied to both ends thereof. However, the saturable inductor 34 functions as a delay circuit element because it is saturated with a predetermined time and is close to a short state.

  The switch element 32 is turned on when the combined value of the voltage of the positive feedback winding 31b and the voltage of the capacitor 36 exceeds the threshold value of the control electrode of the switch element 32, but the capacitor 36 is turned on while the switch element 32 is off. Is charged with a voltage at which the control electrode side has a positive potential, so that the voltage of the positive feedback winding 31b becomes equal to the voltage of the positive feedback winding 31b before the side with the dot mark in FIG. The composite value of the voltage of the capacitor 36 exceeds the threshold value of the control electrode of the switch element.

  Although the voltage waveform of the positive feedback winding 31b is similar to the voltage waveform of the primary winding 31a, the voltage of the snubber capacitor 33 is the lowest due to resonance between the snubber capacitor 33 and the primary winding 31a. Is a half period from the start of resonance. If the switch element 32 is turned on when the voltage of the snubber capacitor is lowest, the loss due to the discharge of the charge of the snubber capacitor 33 is reduced, so that even if the capacity of the snubber capacitor 33 is increased, the loss is smaller than that of the conventional method. In addition, the effect of reducing the switching noise is produced by increasing the capacitance.

  On the other hand, the combined value of the voltage of the positive feedback winding 31b and the voltage of the capacitor 36 exceeds the threshold value of the control electrode of the switch element 32 before the snubber capacitor voltage becomes the lowest due to resonance. Therefore, the voltage of the control electrode is made to exceed the threshold value after a half period of resonance by the delay effect of the saturable inductor 34.

  As a result, when the switch element 32 is turned on, the voltage of the snubber capacitor 33 is the lowest.

  In the case of the conventional method of FIG. 3, since the voltage is charged in the capacitor 36 during the off period, the combined value exceeds the threshold before the voltage of the positive feedback winding 31b becomes a positive potential on the dot mark side. . If the capacitor 36 is not provided, the switch element 32 is not turned on until the voltage of the positive feedback winding exceeds the threshold value, so that a soft switch is possible without a delay due to the saturable inductor 34.

  However, in a self-excited switching power supply, a voltage must be applied to the control electrode by some method in order to start self-excited oscillation. Therefore, a voltage is usually applied from the DC power supply to the control electrode via a resistor. The resistor is called a starting resistor, and is required only at the time of starting, and a high resistance value is selected to suppress power loss after starting. Therefore, if the capacitor 36 is in a short state, the current applied to the control electrode by the starting resistor flows to the ground via the resistor 35 and does not reach the threshold value.

  An object of the present invention is to provide means for enabling soft switching even when there is no delay due to a conventional saturable inductor, because the capacitor 36 works only at the time of start-up and becomes short-circuited after the start-up.

  To achieve the above object, the present invention provides a first switching element having a primary winding and a control electrode connected in series thereto, a positive feedback winding for self-excited oscillation, a positive feedback winding, In a self-excited switching power supply device having a series circuit composed of a resistor and a first capacitor connected between control electrodes of a first switch element and a snubber capacitor, a diode and a second capacitor at both ends of a positive feedback winding A second switch element having a control electrode was connected in parallel to the first capacitor, and the voltage of the second capacitor was applied to the control electrode of the second switch element.

  In the present invention, although it is necessary to add the second switch element, there is an advantage in terms of cost because the saturable inductor can be omitted. In addition, the saturable inductor was inserted in series between the positive feedback winding and the control electrode, so that it was difficult to start, but the disadvantage was eliminated.

  FIG. 1 is a circuit diagram showing an embodiment of the present invention. FIG. 2 is a waveform diagram of each part of FIG.

  In FIG. 1, the voltage of the control electrode of the first switch element 12 rises due to the current flowing through the starting resistor 17. Since it is before oscillation, the voltage of the second capacitor 2 is zero and the second switch element 3 is in the off state. The first switch element 12 is turned on and oscillation is started. When oscillation starts, the voltage of the second capacitor 2 rises. When the threshold value of the control electrode of the second switch element 3 is exceeded, the second switch element 3 is turned on and the capacitor 15 does not work. . That is, only the resistor 14 exists between the control electrode of the first switch element 12 and the positive feedback winding 11b.

FIG. 2 shows the voltage waveform of the positive feedback winding 11b, the current waveform of the first switch element 12, and the voltage waveform of the first switch element 12 from the top.
Since the first switch element 12 does not turn on until the voltage of the positive feedback winding reaches the threshold value of the control electrode of the first switch element 12, the first switch element 12 is turned on near the half cycle of resonance. After the half cycle, the voltage of the snubber capacitor 13 is the lowest, so that the loss due to the discharge of the charge of the snubber capacitor 13 is reduced when the first switch element 12 is turned on.

  Compared to conventional saturable inductors, it is cheaper and its shape is small, so the possibility of use is high.

  It is a circuit diagram which shows the Example of this invention.   It is a wave form diagram of each part of FIG.   It is a circuit diagram which shows an example of a conventional system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Diode 2 2nd capacitor | condenser 3 2nd switch element 11 Transformer 11a Primary winding 11b Positive feedback winding 11c Secondary winding 12 1st switch element 13 Snubber capacitor 14 Resistance 15 1st capacitor 16 DC power supply 17 Starting resistor 18 Diode 19 Capacitor 20 Load 21 Error amplifier 22 Reference voltage source 31 Switching transformer 31a Primary winding of switching transformer 31b Positive feedback winding of switching transformer 31c Secondary winding of switching transformer 32 Switching transistor 33 Snubber capacitor 34 Possible Saturation inductor 35 Resistor 36 Capacitor 37 DC power supply 38 Diode 39 Smoothing capacitor 40 Load resistance 41 Error amplifier 42 Reference voltage source

Claims (1)

  A primary winding of the transformer, a first switch element having a control electrode connected in series to the primary winding, and a positive coil wound around the transformer to cause the first switch element to self-oscillate Absorbs a surge voltage generated in the primary winding, a feedback circuit, a series circuit including a resistor and a first capacitor connected between the positive feedback winding and the control electrode of the first switch element. In a self-excited switching power supply device including a snubber capacitor, a second circuit having a control circuit in parallel with the first capacitor, wherein a series circuit including a diode and a second capacitor is connected to both ends of the positive feedback winding. A self-excited switching power supply device, characterized in that a switch element is connected and the voltage of the second capacitor is applied to a control electrode of the second switch element.

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