JP2019122132A - Isolated switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adopt a forward system capable of increasing the output and to improve the power factor in an isolated switching power supply of a one converter system. SOLUTION: A transformer having a primary coil and a secondary coil, a switching unit having a plurality of switching elements whose on / off control is controlled so as to conduct or cut off a current flowing through the primary coil by an input voltage, and the secondary coil. In an isolated switching power supply having a rectifying unit connected to the rectifying unit and a smoothing unit connected to the rectifying unit and including at least one reactor and a smoothing capacitor, a current flowing through the switching element to the primary coil is blocked. Therefore, it has a backflow prevention element connected in series between the switching element and the primary coil. [Selection diagram] Fig. 1

Description

  The present invention relates to a switching power supply, and more particularly to a single converter type isolated switching power supply having a power factor correction function.

  What used the power factor improvement circuit of patent documents 1-3 etc. for the input stage of the switching power supply which carries out electric power conversion of alternating current to direct current is known. The power factor correction circuit is, for example, a circuit that inputs a sine wave current that is as phase-matched as possible with respect to a sine wave input voltage. The power factor correction circuit is usually composed of a non-isolated boost converter. Therefore, it is general to configure one isolated switching power supply by a two-converter system in which an isolated DC / DC converter is disposed at the subsequent stage of the power factor correction circuit. There are a forward method and a flyback method as typical methods of the isolated DC / DC converter. The forward method is suitable for large output power supplies.

  On the other hand, as disclosed in Patent Documents 4 and 5, etc., there is also known a one-converter system which is an insulation type DC / DC converter having a power factor improvement function. The isolated DC / DC converter of the one converter system is configured in a flyback system that operates substantially the same as the boost converter. The flyback system has a power factor improvement function but is not suitable for a large output power source.

  Moreover, although the number of switching elements on the primary side of the isolated DC / DC converter may be one in principle, in order to increase the output and reduce the withstand voltage characteristics of the switching elements, a plurality of switchings as in Patent Document 6 etc. There are also known full bridge circuits and push-pull circuits made of elements.

Unexamined-Japanese-Patent No. 5-111246 JP 2007-37297 A JP, 2015-23722, A Unexamined-Japanese-Patent No. 5-236749 Japanese Patent Application Laid-Open No. 2002-300780 JP, 2015-70716, A

  Single-converter isolated DC / DC converters with good power factor (hereinafter referred to as isolated DC / DC converters in this specification are referred to as isolated switching power supplies) are usually flyback systems, but high power In order to achieve this, it is desirable to adopt a forward method. However, there are some problems with this. Those problems will be described with reference to FIG. 8 and FIG.

  FIG. 8 schematically shows an example of a conventional forward type isolated switching power supply. For example, an input voltage obtained by full-wave rectification of a sine wave alternating current is input to the input terminals 1 and 2. A switching unit formed of a full bridge circuit consisting of four MOSFETs (hereinafter simply referred to as "FET") Q1 to Q4 is disposed on the primary side of transformer T, and four diodes D1 to D4 are disposed on the secondary side of transformer T. Is arranged, and a smoothing circuit consisting of a reactor L and a capacitor C is arranged at the subsequent stage, and a direct current is output from both ends p and n of the capacitor C. The full bridge circuit is on / off controlled at a frequency higher than the frequency of the input AC. The arrowed solid line in the circuit of FIG. 8 schematically indicates the current during the on period of the switching element, and the dotted line with the arrow schematically indicates the current during the off period.

  FIG. 9 is a schematic timing diagram of the operation of the circuit of FIG. FIGS. 9A and 9B schematically show on / off of the FETs Q1 and Q4 in the first group and the FETs Q2 and Q3 in the second group. The FETs of the first group and the FETs of the second group are on / off controlled alternately at regular intervals of the same length and at regular intervals.

  FIGS. 9C, 9D, and 9E schematically show an example of the current flowing through the primary coil N1, the secondary coil N2 and the reactor L of the transformer T in the circuit of FIG. During the ON period of the first group FETs Q1 and Q4, the current i1 flows through the primary coil, and accordingly, the current i2 flows through the secondary coil N2 and is output through the reactor L. When the FETs Q1 and Q4 of the first group are turned off, the magnetic energy stored in the reactor L is output as the current i3 on the secondary side. Similarly, during the on period of the second group FETs Q2 and Q3, the current i4 flows through the primary coil N1, the current i5 flows through the secondary coil N2 and is output, and when it is off, the current i6 is output. This is a forward operation.

  Here, in the circuit of FIG. 8, a back electromotive force is generated in the primary coil N1 at the moment when the FETs Q1 and Q4 of the first group are turned off. Since this back electromotive force is a forward bias with respect to the body diodes of the FETs Q1 and Q4, a current iA shown in FIGS. 8 and 9 (c) flows. Similarly, at the moment when the FETs Q2 and Q3 of the second group are turned off, the current iB flows. The currents iA and iB play the role of magnetic reset of the transformer T. However, since the currents iA and iB are refluxes to the input side, they are energy not transmitted to the secondary side of the transformer, and it can be said that the power conversion efficiency is lowered.

  Furthermore, in the forward method, the secondary side currents i2 and i5 of the transformer T in the on period flow only when the electromotive force generated in the secondary coil N2 exceeds the voltage of the smoothing capacitor C. Therefore, no current is output in the range where the electromotive force of the secondary coil N2 is small, which degrades the power factor.

  The forward system is more suitable than the flyback system for the large output power source, but the flyback system is usually adopted in the isolated switching power supply of the one converter system because of the problems described above.

  From the above-mentioned present situation, the present invention aims at adopting a forward system capable of achieving a large output and achieving a good power factor in an isolated switching power supply of a one converter system.

In order to achieve the above object, the present invention provides the following configuration.
-The aspect of the present invention is
A transformer having a primary coil and a secondary coil;
A switching unit having a plurality of switching elements which are on / off controlled to conduct or cut off the current flowing to the primary coil by an input voltage;
A rectifying unit connected to the secondary coil;
An isolated switching power supply comprising: a smoothing unit connected to the rectifying unit and including at least one reactor and a smoothing capacitor.
In order to block the current flowing through the switching element to the primary coil when the switching element is turned off from the on state in the switching unit, the switching element is characterized by including a backflow prevention element connected in series to the switching element. .
In the above aspect, the plurality of switching elements in the switching unit includes a first group including at least one switching element and a second group including at least one switching element, the first group and the second group The switching elements are characterized in that they are on / off controlled alternately at regular intervals of the same length.
In the above aspect, the backflow prevention element is on the current path through which the current of the on period of the switching element of the first group flows exclusively, and on the current path along which the current of the switching element of the second group flows exclusively. Each is characterized by being connected in series.
In the above aspect, the switching unit is characterized in that any one of a full bridge circuit, a push-pull circuit, and a half bridge circuit is used as a basic configuration.
In the above aspect, current flows to the primary coil and forward current flows to the secondary coil during the on period of each of the switching elements of the first group and the second group, and the switching element is turned on to off When the current of the primary coil is cut off, the current due to the back electromotive force generated in the primary coil is blocked by the backflow prevention element and the back electromotive force generated in the secondary coil causes the secondary coil to It is characterized in that it is configured to flow a flyback current.
In the above aspect, the rectifier includes two pairs of series-connected positive and negative diodes, and the connection point of each pair of diodes connected in series is connected to each end of the secondary coil. Yes,
One end of each of two reactors is connected to the cathode of each of the two positive side diodes, and the other end of each of the two reactors is connected to the positive terminal of the smoothing capacitor, and
An anode of each of the two negative side diodes is connected to a negative end of the smoothing capacitor.
In the above aspect, the rectifier includes two pairs of series-connected positive and negative diodes, and the connection point of each pair of diodes connected in series is connected to each end of the secondary coil. Yes,
One end of each of two reactors is connected to the anode of each of the two negative side diodes, and the other end of each of the two reactors is connected to the negative end of the smoothing capacitor, and
A cathode of each of the two positive side diodes is connected to a positive terminal of the smoothing capacitor.

  The isolated switching power supply according to the present invention is a switching power supply of a one-converter system, and realizes a large output by adopting a forward system, and transfers power from the primary side to the secondary side without waste. The power efficiency can also be improved by increasing the conversion efficiency and further enabling the output of current even in the range where the electromotive force of the secondary coil is small.

FIG. 1 is a view schematically showing a circuit example of the embodiment of the isolated switching power supply of the present invention, in which (a) shows the current during the on period of the first group FET, and (b) shows the first group FET The current during the off period is also shown. FIG. 2 shows the current during the on period of the second group FET and the current during the off period of the second group FET in the circuit example of FIG. FIG. 3 is a diagram schematically showing an example of a timing chart of each current shown in FIG. 1 and FIG. FIG. 4 is a view schematically showing another configuration example of the primary side of the transformer of the switching power supply according to the present invention. FIG. 5 is a view schematically showing still another configuration example of the primary side of the transformer of the switching power supply according to the present invention. FIG. 6 is a view schematically showing still another structural example of the secondary side of the transformer of the switching power supply according to the present invention. FIG. 7 is a view schematically showing a preferred example of the transformer in the present invention. FIG. 8 is a diagram schematically showing an example of a conventional forward type isolated switching power supply. FIG. 9 is a schematic timing diagram of the circuit of FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings shown as examples. The isolated switching power supply of the present invention targets the one-converter system. The typical input voltage is a rectified sinusoidal voltage. However, the isolated switching power supply of the present invention functions in the same manner when the input voltage is a square wave or triangular wave other than a sine wave, or a constant voltage.

(1) First Embodiment (1-1) Circuit Example of the First Embodiment FIG. 1A is a view schematically showing a circuit example of the embodiment of the insulating switching power supply of the present invention. FIG. 1A also shows the current during the on period of the first group of FETs, which will be described later. The same or similar reference numerals are given to the same or similar components as the conventional forward type isolated switching power supply of FIG. 8 described above.

  In the isolated switching power supply of FIG. 1, an input voltage obtained by full-wave rectifying an alternating voltage is input to input terminals 1 and 2. The AC voltage here is, for example, a sine wave having a frequency of about several Hz to several tens Hz generated by 50 Hz or 60 Hz of the system power supply or various power generation devices. However, the waveform of the input voltage is not limited to a sine wave, and can be an arbitrary waveform of positive potential.

  The transformer T is a forward transformer in which the primary coil N1 and the secondary coil N2 are wound in the same polarity (the winding start end of the coil is indicated by a black circle). The primary side of the transformer T is provided with a plurality of switching elements, in this case, MOSFETs, which are on / off controlled to conduct or cut off the current flowing through the primary coil N1 by the input voltage.

  In the configuration example of FIG. 1, the switching unit includes four N-channel MOSFETs (hereinafter simply referred to as "FET") Q1 to Q4 as switching elements, and basically constitutes a full bridge circuit. The full bridge circuit is suitable for a high output switching power supply. Furthermore, the voltage applied to each FET is half that of a single FET switching portion. The FET Q1 and the FET Q4 constitute a first group which is simultaneously on / off controlled, and the FET Q2 and the FET Q3 constitute a second group which is simultaneously on / off controlled. Each FET is on / off controlled by a control voltage applied to its gate. The frequency of the control voltage is several tens kH to several hundreds kH higher than the frequency of the input AC.

  The FET Q1 is connected in series between the positive input terminal 1 and one end of the primary coil N1. Between the other end of the primary coil N1 and the negative input terminal 2, a backflow preventing diode D1 and a FET Q4 are connected in series. Unlike the basic full bridge circuit, a backflow prevention diode D1 is inserted. The polarity of the backflow prevention diode D1 is reverse to that of the body diodes of the FET Q1 and the FET Q4.

  Similarly, the FET Q2 is connected in series between the positive input terminal 1 and the other end of the primary coil N1. Between the one end of the primary coil N1 and the negative input terminal 2, a backflow preventing diode D2 and a FET Q3 are connected in series. Unlike the basic full bridge circuit, the backflow prevention diode D2 is inserted. The polarity of the backflow prevention diode D2 is reverse to that of the body diode of the FET Q2 and the FET Q3.

  As another example, the backflow prevention diode D1 can be inserted in series at any position on the current path between point a → FET Q1 → primary coil N1 → FET Q4 → b, in addition to the position shown in FIG. Similarly, the backflow prevention diode D2 can be inserted in series at any position on the current path of point a → FET Q 2 → primary coil N 1 → FET Q 3 → b other than the position shown in FIG.

  In other words, the backflow prevention diode D1 is inserted in series on the current path on which the on-period current of the first group FET flows exclusively, while the backflow prevention diode D2 is a current on which the on-period current of the second group FET flows only It is preferred to be inserted on the road. In either case, the polarity of the backflow prevention diodes D1 and D2 is opposite to that of the body diode of each FET.

  Even when a diode is inserted in the current path between the input terminal 1 and the point a or between the input terminal 2 and the point b, there is a backflow prevention function, but the backflow prevention inserted in these current paths Diodes are disadvantageous in that they cause recovery current when the FET is turned off. However, when a fast recovery diode (FRD), a Schottky diode using a silicon carbide (SIC), or the like is used, it is also possible to insert a backflow prevention diode in the current path of these input lines.

  On the secondary side of the transformer T, a rectifying unit and a smoothing unit at the subsequent stage are disposed. The rectifying unit in the configuration example of FIG. 1 has a configuration based on a bridge rectifier circuit including four diodes D3 to D6. The first pair of positive side diode D3 and negative side diode D5 are connected in series, and the second pair of positive side diode D4 and negative side diode D6 are connected in series. The connection point of the first pair of diodes D3 and D5 connected in series is connected to one end of the secondary coil N2, and the connection point of the second pair of diodes D4 and D6 connected in series is the other of the secondary coil N2. Connected to the end.

  Unlike the normal bridge rectifier circuit, in the configuration example of FIG. 1, the cathodes of the positive side diodes D3 and D4 are not connected to each other. The cathode of one positive side diode D3 is connected to one end of the first reactor L1, and the cathode of the other positive side diode D4 is connected to one end of the second reactor L2. The inductances of the first reactor L1 and the second reactor L2 have substantially the same value.

  The other end of each of the first reactor L1 and the second reactor L2 is connected to the positive terminal of the smoothing capacitor C, and this point is also the positive output terminal p.

  The anode of each of the two negative diodes D5 and D6 is connected to the negative terminal of the smoothing capacitor C, which is also the negative output terminal n.

(1-2) Operation of First Embodiment The operation of the circuit of the first embodiment will be described with reference to FIGS. 1 to 3.
FIG. 1A shows the current during the on period of the FETs of the first group in a solid line with arrows, and FIG. 1B shows the current in the off periods of the FETs of the first group in dotted lines with an arrow.
FIG. 2A shows the current in the on period of the FETs of the second group in a solid line with arrows, and FIG. 2B shows the current in the off period of the FETs of the second group in dotted lines with arrows.
FIG. 3 is a diagram schematically showing an example of a timing chart of each current shown in FIG. 1 and FIG.

<Operation by on / off control of first group FET>
Please refer to FIG. 1 (a) and FIG. When the FETs Q1 and Q4 of the first group are turned on, a current i1 flows through the primary coil N1 (see FIG. 3C). The current i1 flows in the forward direction of the backflow prevention diode D1. As a result of the current i1 flowing through the primary coil, an electromotive force (meaning a voltage) is generated in the secondary coil N2 by mutual induction, and a current i2 flows. The current i2 flows in the path of the diode D6 → secondary coil N2 → diode D3 → reactor L1 and is output from the output terminal p and supplied to the load (see FIG. 3 (d) (e)). The current i2 is a forward current due to mutual induction of the transformer T. The first reactor L1 is excited by the current i2 to store magnetic energy. Since the diodes D4 and D5 are reverse biased, no current flows.

  Here, the smoothing capacitor C in the steady state is charged at a substantially constant voltage except for the ripple fluctuation. Therefore, the current i2 flows only when the electromotive force of the secondary coil N2 exceeds the voltage of the smoothing capacitor. When a sine wave input voltage is applied to the input terminals 1 and 2, for example, the electromotive force of the secondary coil N2 is also small in the range where the input voltage is small, so the forward current i2 is not output to the output terminals p and n. This is a feature of the forward operation, which reduces the power factor. This is the reason why the forward method has not been conventionally adopted as a one converter having a power factor improvement function.

  Next, FIG. 1 (b) and FIG. 3 will be referred to. When the FETs Q1 and Q4 of the first group are turned off, the current i1 of the primary coil N1 is cut off and a back electromotive force is generated in the primary coil N1 and the secondary coil N2. The back electromotive force of the primary coil N1 is forward biased to the body diodes of the FETs Q1 and Q4, but no current flows because the backflow prevention diode D1 is inserted in the current path. That is, unlike the normal full bridge circuit of FIG. 8 described above, the reflux to the input side is blocked. As a result, in the normal full bridge circuit, the energy returned to the input side remains in the transformer T.

  On the other hand, due to the back electromotive force of the secondary coil N2, the winding start end has a low potential and the winding end has a high potential. Since the first reactor L1 tries to maintain the current in the same direction as the on period, the current i3 flows through the path of diode D5 → diode D3 → reactor L1 and is output to the output terminal p and supplied to the load (FIG. 3) (D) (e)). Thereby, the magnetic energy accumulated in the first reactor L1 in the on period is released. The diode D6 is reverse biased.

  Furthermore, due to the back electromotive force of the secondary coil N2, the diode D4 becomes forward biased, and a current ifb flows along the path of the diode D5 → secondary coil N2 → diode D4 → reactor L2, and is output to the output terminal p and supplied to the load. (See FIG. 3 (d) (f)). The current ifb is not due to the mutual induction of the transformer T, but is due to the release of the magnetic energy retained in the transformer T because the reflux is prevented by the backflow preventing diode D1 on the primary side. Therefore, the current ifb can be said to be a flyback current.

  The flyback current ifb is different from the forward current i2, and the output terminal p has a magnitude according to the magnitude of the back electromotive force even when the input voltage is small and the back electromotive force generated in the secondary coil N2 is small. Output. Therefore, for example, in the case of a sinusoidal input voltage, the power factor is improved by the flyback current ifb flowing even in the range where the input voltage is small. Although this circuit is basically of the forward type, it also has a power factor correction function since the flyback current ifb can also flow. This is realized by providing the backflow prevention diode D1 on the primary side.

  In the normal full bridge circuit of FIG. 8, the reflux to the input side at the off time plays the role of the magnetic reset of the transformer T. However, in this circuit, the flyback current to the secondary coil instead of the reflux to the input side The magnetic reset of the transformer T is performed by the flow of ifb. Accordingly, magnetic saturation of the transformer T can be avoided also in this circuit.

<Operation by on / off control of FET in second group>
Next, FIG. 2 (a) and FIG. 3 will be referred to. The second group of FETs Q2 and Q3 are turned on at regular intervals after the first group of FETs Q1 and Q4 are turned off. When the FETs Q2 and Q3 of the second group are turned on, a current i4 flows through the primary coil N1 (see FIG. 3C). The current i4 flows in the forward direction of the backflow prevention diode D2. The direction of the current i4 in the primary coil N1 is opposite to the current i1 during the on period of the first group of FETs.

  As a result of the current i4 flowing through the primary coil, an electromotive force is generated in the secondary coil N2 by mutual induction, and a current i5 flows. The current i5 flows in a path of the diode D5 → secondary coil N2 → diode D4 → reactor L2 and is output from the output terminal p and supplied to the load (see FIGS. 3 (d) and 3 (f)). The current i5 is a forward current due to mutual induction of the transformer T. As a result, the second reactor L2 is excited to accumulate magnetic energy. Since the diodes D3 and D6 are reverse biased, no current flows.

  As described above, when a sine wave input voltage is applied to the input terminals 1 and 2, for example, the electromotive force of the secondary coil N2 is also small in the small range of the input voltage, so the forward current i5 is not output.

  Next, FIG. 2 (b) and FIG. 3 will be referred to. When the FETs Q2 and Q3 of the second group are turned off, the current i4 of the primary coil N1 is cut off, and a back electromotive force is generated in the primary coil N1 and the secondary coil N2. The back electromotive force of the primary coil N1 is forward biased to the body diodes of the FETs Q2 and Q3, but no current flows because the backflow preventing diode D2 is inserted in the current path. As a result, the energy stays in the transformer T.

  On the other hand, due to the back electromotive force of the secondary coil N2, the winding start end has a high potential and the winding end has a low potential. Since the second reactor L2 tries to maintain the current during the on period, the current i6 flows through the path of the diode D6 → the diode D4 → the reactor L2 and is output to the output terminal p and supplied to the load (FIG. 3 (d)) (F)). Thereby, the magnetic energy accumulated in the second reactor L2 in the on period is released. The diode D5 is reverse biased.

  Furthermore, due to the back electromotive force of the secondary coil N2, the diode D3 becomes forward biased, and a current ifb flows along the path of the diode D6 → secondary coil N2 → diode D3 → reactor L1 and is output to the output terminal p and supplied to the load. (See FIG. 3 (d) (e)). The current ifb is not due to the mutual induction of the transformer T, but is a flyback current due to the release of magnetic energy that is retained in the transformer T because the reflux is blocked by the backflow prevention diode D2 on the primary side. As described above, since the current ifb is output regardless of the magnitude of the back electromotive force of the secondary coil N2, the power factor is improved.

  In (e) and (f) of FIG. 3, the continuous mode in which the end portion of the flyback current ifb flowing through the first reactor L1 and the second reactor L2 overlaps the rising portion of the forward current i2 and i5 of the next on period. It has become. This is an example, and a discontinuous mode in which the portions do not overlap may be used.

(2) Another Embodiment of Transformer Primary Side FIG. 4 shows another example of the configuration of the switching part on the primary side in the circuit of FIG. Although the secondary side circuit is not illustrated, it may be the same as the configuration example of FIG. The configuration example of FIG. 4 is based on a push-pull circuit. In the push-pull circuit, the primary coil is divided into two parts of a first primary coil N11 and a second primary coil N12. In this case, the first group of the plurality of FETs includes only the FET Q1, and the second group includes only the FET Q2. The FET Q1 and the FET Q2 are on / off controlled alternately at constant intervals and with the same length of on period. Between the FETs and the primary coils N11 and N12, anti-backflow diodes D1 and D2 connected in series in the reverse direction to the body diodes of the FETs are inserted. The circuit operation is the same as the circuit of FIG. 1 on both the primary side and the secondary side.

  FIG. 5 is still another configuration example of the switching unit on the primary side in the circuit of FIG. Although the secondary side circuit is not illustrated, it may be the same as the configuration example of FIG. The configuration example of FIG. 5 is based on a half bridge circuit. In a half bridge circuit, the first group of FETs includes only FET Q1 and the second group includes only FET Q2. The FET Q1 and the FET Q2 are on / off controlled alternately at constant intervals and with the same length of on period. Between each FET and the primary coil, reverse blocking diodes D1 and D2 connected in series in the reverse direction to the body diode of each FET are inserted. The circuit operation is the same as the circuit of FIG. 1 on both the primary side and the secondary side.

(3) Another Embodiment of Transformer Secondary Side FIG. 6 shows another example of the configuration of the rectifying part and the smoothing part on the secondary side in the circuit of FIG. The rectifying unit in the configuration example of FIG. 6 also has a configuration based on a bridge rectifier circuit including four diodes D3 to D6 as in FIG. The first pair of positive side diode D3 and negative side diode D5 are connected in series, and the second pair of positive side diode D4 and negative side diode D6 are connected in series. The connection point of the first pair of diodes D3 and D5 connected in series is connected to one end of the secondary coil N2, and the connection point of the second pair of diodes D4 and D6 connected in series is the other of the secondary coil N2. Connected to the end.

  In the configuration example of FIG. 6, the anodes of the negative side diodes D5 and D6 are not connected to each other unlike a normal bridge rectifier circuit. The anode of one negative side diode D5 is connected to one end of the first reactor L1, and the anode of the other negative side diode D6 is connected to one end of the second reactor L2. The inductances of the first reactor L1 and the second reactor L2 have substantially the same value.

  The other end of each of the first reactor L1 and the second reactor L2 is connected to the negative terminal of the smoothing capacitor C, and this point is also the negative output terminal n.

  Further, the cathode of each of the two positive side diodes D3 and D4 is connected to the positive terminal of the smoothing capacitor C, and this point is also the positive output terminal p.

  The secondary side circuit of the isolated switching power supply according to the present invention can be variously configured in addition to the circuits shown in FIG. 1 and FIG. It is preferable that the secondary side circuit is basically configured in the forward mode. The forward current may flow in the on period, the reactor current for releasing the magnetic energy of the external reactor may flow in the off period, and the flyback current may flow due to the back electromotive force generated in the secondary coil. . The conventional forward secondary side circuit shown in FIG. 8 can also be employed. The flyback current flowing through the secondary coil is due to the release of the magnetic energy held in the transformer T by providing the backflow prevention diode in the switching section on the primary side.

(4) Configuration of Transformer FIG. 7 is a cross-sectional view showing a preferred example of the transformer T used in the isolated switching power supply of the present invention.

  The core of the transformer T is basically an EI-type core formed by butting an I-shaped core 20 with an E-shaped core 10 having a central leg 11 and two outer legs 12, 12. It is preferable to provide gaps 14 and 14 between the end faces of the two outer legs 12 and 12 and the I-shaped core 20 respectively. By providing the gaps 14, the magnetic permeability can be reduced to avoid magnetic saturation.

  Furthermore, the primary coil N1 and the secondary coil N2 are lap-wound on the central leg 11 so as to be tightly coupled. There is no gap between the central leg 11 and the I-shaped core 20. Generally, the magnetic flux is likely to leak at the gap, but in this transformer T, there is no gap where the magnetic flux is likely to leak around the primary coil N1 and the secondary coil N2, and the gaps 14, 14 are located far from each coil It is provided. Thereby, close coupling between the primary coil N1 and the secondary coil N2 can be secured, and high power conversion efficiency can be maintained.

(5) Summary The isolated switching power supply of the present invention is basically applicable to a large output by having a forward operation, and the power factor is improved by the flow of the flyback current. Therefore, it is suitable as a high power, one converter type isolated switching power supply. The power factor correction function can be provided, the circuit configuration can be simplified, and the safety is ensured because of the isolation type.

  As shown in FIGS. 1, 4 and 5, by alternately switching the two MOSFET groups on the primary side of the transformer, a large output can be obtained as compared to the case of switching a single switching element. In each embodiment described above, the switching element in the switching unit may be an IGBT or a bipolar transistor other than the MOSFET.

  Further, as the backflow prevention diode provided in the switching unit is for the purpose of preventing reflux to the input side, an FET may be used. In that case, the backflow prevention FET is on / off controlled at the same timing as the corresponding FET targeted for the reflux prevention, but the body diode of the backflow prevention FET is connected in the reverse direction to that of the corresponding FET. In the present specification, these backflow prevention diodes and backflow prevention FETs are collectively referred to as “backflow prevention elements”.

  Also, as shown in FIGS. 1 and 6, on the secondary side of the transformer, by dividing the reactor in the normal forward system into two parallel circuits, the current supply capacity is increased and the drive load on the primary side is reduced. Be done. The diode in the circuit configuration on the secondary side can be replaced with a rectifying element having an equivalent function, and such a rectifying element is also included in the concept of the diode.

  The insulating switching power supply according to the present invention described above is not limited to the illustrated configuration example, and various modifications can be made within the scope of the present invention.

p Positive output terminal n Negative output terminal T transformer N1, N11, N12 Primary coil N2 Secondary coil Q1, Q2, Q3, Q4 MOSFET
D1, D2 Backflow prevention diode D3 to D6 Diode L1 First reactor L2 Second reactor C Smoothing capacitor

Claims (7)

A transformer having a primary coil and a secondary coil;
A switching unit having a plurality of switching elements which are on / off controlled to conduct or cut off the current flowing to the primary coil by an input voltage;
A rectifying unit connected to the secondary coil;
An isolated switching power supply comprising: a smoothing unit connected to the rectifying unit and including at least one reactor and a smoothing capacitor.
In order to block the current flowing through the switching element to the primary coil when the switching element is turned off from the on state in the switching unit, the switching element is characterized by including a backflow prevention element connected in series to the switching element. Isolated switching power supply.   The plurality of switching elements in the switching unit are composed of a first group including at least one switching element and a second group including at least one switching element, and the switching elements of the first group and the second group are: 2. The isolated switching power supply according to claim 1, wherein on / off control is performed alternately at regular intervals with the same length of on period.   The backflow prevention element is connected in series on the current path on which the current of the on period of the switching elements of the first group exclusively flows, and on the current path on which the current of on periods of the switching elements of the second group exclusively flows. An isolated switching power supply according to claim 2, characterized in that:   The switching power supply according to any one of claims 1 to 3, wherein the switching unit basically has any one of a full bridge circuit, a push-pull circuit, and a half bridge circuit.   During the on period of each of the switching elements of the first group and the second group, a current flows through the primary coil and a forward current flows through the secondary coil, and the switching element is switched from on to off and the primary coil When the current is cut off, the current due to the back electromotive force generated in the primary coil is blocked by the backflow prevention element, and the back electromotive force generated in the secondary coil causes a flyback current to flow in the secondary coil The insulated switching power supply according to any one of claims 1 to 4, which is configured as follows. The rectifying unit includes two pairs of series-connected positive and negative diodes, and a connection point of each pair of diodes connected in series is connected to each end of the secondary coil,
One end of each of two reactors is connected to the cathode of each of the two positive side diodes, and the other end of each of the two reactors is connected to the positive terminal of the smoothing capacitor, and
The isolated switching power supply according to any one of claims 1 to 5, wherein the anode of each of the two negative side diodes is connected to the negative terminal of the smoothing capacitor. The rectifying unit includes two pairs of series-connected positive and negative diodes, and a connection point of each pair of diodes connected in series is connected to each end of the secondary coil,
One end of each of two reactors is connected to the anode of each of the two negative side diodes, and the other end of each of the two reactors is connected to the negative end of the smoothing capacitor, and
The isolated switching power supply according to any one of claims 1 to 5, wherein a cathode of each of the two positive side diodes is connected to a positive terminal of the smoothing capacitor.

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