WO2019117240A1

WO2019117240A1 - Insulated switching power supply

Info

Publication number: WO2019117240A1
Application number: PCT/JP2018/045850
Authority: WO
Inventors: 羽田　正二
Original assignee: Ntn株式会社
Priority date: 2017-12-13
Filing date: 2018-12-13
Publication date: 2019-06-20
Also published as: KR20200097722A; KR102525753B1; JP2019106809A; JP6942040B2

Abstract

This forward-type insulated switching power supply suppresses counter-electromotive force generated in a primary coil during the OFF time of a switching element, and comprises: a transformer (T); a switching element (Q); a first rectifying element (D1) and an inductor (L); and a second rectifying element (D2). The switching power supply has a capacitor (C2) connected between the other terminal of a secondary coil (Ns) and a negative electrode output end, and a third rectifying element (D3) connected between one end of the secondary coil and a negative electrode output end (n). During an ON period, discharge current of the capacitor flows to the secondary coil, the first rectifying element, and the inductor and is output. During an OFF period, the discharge current of the capacitor flows through the second rectifying element and the inductor and is output, and charging current also flows through the third rectifying element and the secondary coil.

Description

Isolated switching power supply

The present invention relates to a forward type isolated switching power supply capable of suppressing a surge voltage.

Insulated switching power supplies are known that use a transformer to insulate the input side from the output side (Patent Documents 1 to 5). When the input is an alternating voltage, generally, a DC / DC converter is disposed after the AC / DC conversion circuit. When the input is a DC voltage, it is directly input to the DC / DC converter. The DC / DC converter is also one of the isolated switching power supplies. There are a flyback method and a forward method as representative methods of the isolated switching power supply.

In the forward switching power supply, power is transmitted by the electromagnetic induction of the transformer during the on period of the switching element, current flows in the secondary coil, and is output through the inductor, and magnetic energy is accumulated in the inductor. Then, during the off period of the switching element, a current flows and is output through the free wheeling diode so as to release the magnetic energy of the inductor.

Japanese Patent Laid-Open No. 7-31150 JP-A-8-331860 JP 2002-10632 A JP 2005-218224 A JP 2007-37297 A

In the isolated switching power supply, a high back electromotive force ("electromotive force" and "back electromotive force" in this specification are used in the meaning of voltage), ie, surge voltage, to the primary coil of the transformer at the moment when the switching element is turned off. Is generated and applied to the switching element. For this reason, it has been necessary to use a high withstand voltage switching element or to provide a snubber circuit or the like for processing a back electromotive force.

Further, in the general forward system, since no current flows in the transformer during the off period, magnetic saturation occurs unless the magnetic energy stored in the transformer is reset during the on period. A snubber circuit or the like was also required to reset this magnetic energy. In the point that the energy processed by the snubber circuit or the like is not transmitted to the secondary side, the power transmission efficiency of the switching power supply is reduced.

In view of the above problems, it is an object of the present invention to suppress the back electromotive force generated in the transformer when the switching element is off and to suppress the magnetic energy stored in the transformer in the on period in the forward type insulation switching power supply. It is to be able to transmit to the secondary side.

In order to achieve the above object, one aspect of the isolated switching power supply of the present invention has the following configuration.
An aspect of the present invention is a transformer having a primary coil and a secondary coil, a switching element connected in series to the primary coil of the transformer and controlled on / off by a control signal, and a first connected in series to the secondary coil. A forward type isolated switching power supply comprising: a rectifying element and an inductor; and a second rectifying element connected in parallel to the secondary coil and the first rectifying element connected in series;
(A) a capacitor connected between a connection point of the secondary coil and the second rectifying element, and a negative electrode output end;
(B) A third rectifying element connected between a connection point between the secondary coil and the first rectifying element, and a negative electrode output end,
(C) During the on period of the switching element, a current that discharges the capacitor flows to and outputted from the secondary coil, the first rectifying element, and the inductor, and
(D) During the off period of the switching element, a current discharging the capacitor flows through the second rectifying element and the inductor and is output while a current charging the capacitor is the third rectifying element and the second rectifying element It is characterized by flowing through the next coil.
In the above aspect, one end of the third rectifying element is connected to an intermediate point of the secondary coil instead of being connected to a connecting point of the secondary coil and the first rectifying element. Is preferred.

According to the present invention, in an isolated switching power supply, it is possible to suppress the back electromotive force generated in the primary coil of the transformer when the switching element is off, that is, the surge voltage, and reduce the withstand voltage required for the switching element. In addition, since the magnetic energy stored in the transformer can be transmitted to the secondary side when the switching element is turned on, the power transfer efficiency is improved.

FIG. 1 is a diagram schematically showing a circuit configuration example of a first embodiment of the isolated switching power supply of the present invention. FIGS. 2 (a) and 2 (b) show the flow of current during the on period and the off period in the circuit shown in FIG. 1, respectively. FIGS. 3A and 3B schematically show an example of the potential relationship between the on period and the off period on the transformer secondary side of the circuit shown in FIG. 1, respectively. FIG. 3 is a diagram schematically showing a circuit configuration example of a second embodiment of the isolated switching power supply of the present invention, and also shows the flow of current during the off period.

Hereinafter, embodiments of the isolated switching power supply according to the present invention will be described with reference to the drawings showing the embodiments. In the drawings of each embodiment, the same or similar components are denoted by the same reference numerals.

In the following, the isolated switching power supply of the present invention will be described by way of an example of a DC / DC converter to which a DC voltage is input. However, the isolated switching power supply according to the present invention functions similarly even if a voltage of any waveform is inputted, such as a pulsating current or a rectangular wave whose voltage fluctuates, or an alternating current other than a direct current having a constant voltage. It is a power converter capable of outputting a voltage.

(1) First Embodiment (1-1) Circuit Configuration of First Embodiment FIG. 1 is a view schematically showing a circuit configuration example of a first embodiment of the isolated switching power supply of the present invention. is there.

The circuit configuration of the first embodiment will be described with reference to FIG. This circuit is an insulated switching power supply in which the input side and the output side are electrically isolated by a transformer, and is based on the forward system. The transformer T includes a primary coil Np and a secondary coil Ns. In the transformer T, the polarities (winding start ends) of the primary coil and the secondary coil are in the same direction. It is preferable that the transformer T have the coupling degree as high as possible, that is, the primary coil Np and the secondary coil Ns be closely coupled.

In the figure, the winding start end of each coil is shown by a black circle. In the present specification, when the coil is referred to as "one end" and "the other end", the coil corresponds to the "roll start end" and the "roll end", and the coil end corresponds to the "roll end" and the "roll start end", respectively. Shall be included. In the following description, for each coil, the winding start end is referred to as one end, and the winding end is referred to as the other end.

An input voltage is applied between a pair of terminals consisting of an input end 1 and an input end 2. One end of the primary coil Np of the transformer T is connected to the input end 1. Here, the input end 2 is the input side reference potential end.

One end of a switching element Q is connected to the other end of the primary coil Np of the transformer T. The other end of the switching element Q is connected to the input end 2. The switching element Q has a control end. The control end is on / off controlled to conduct or shut off the current path including the primary coil Np. Basically, the switching element Q may be connected in series to the primary coil Np.

The control end of the switching element Q is controlled by a control signal Vg. The control signal Vg is, for example, a PWM signal having a pulse waveform of a predetermined frequency and a duty ratio. In the illustrated example, the switching element Q is an n-channel MOSFET (hereinafter referred to as "FET Q"), one end is a drain, the other end is a source, and the control end is a gate. In this case, the control signal Vg is a voltage signal.

For example, an IGBT or a bipolar transistor can be used as a switching element other than the FET.

On the secondary side of the transformer T, a positive electrode output end p and a negative electrode output end n, which are a pair of output ends from which a DC voltage is output, are provided. Here, the negative electrode output end n is the secondary side reference potential end. An output voltage is applied to a load (not shown) connected between the positive electrode output end p and the negative electrode output end n, and an output current is supplied.

Similar to a general forward type circuit, a first diode D1 which is a rectifying diode and an inductor L are connected in series to a secondary coil Ns of a transformer T. The anode of the diode D1 is connected to one end of the secondary coil Ns, and the cathode of the diode D1 is connected to one end of the inductor L. The other end of the inductor L is connected to the positive output terminal p. Furthermore, a second diode D2 which is a freewheeling diode is connected in parallel to the series-connected secondary coil Ns and the diode D1. The anode of the diode D2 is connected to the other end of the secondary coil Ns, and the cathode of the diode D2 is connected to the cathode of the diode D1. Furthermore, a first capacitor C1, which is a smoothing capacitor, is connected between the positive electrode output end p and the negative electrode output end n.

Further, in the present invention, the second capacitor C2 is connected between the connection point a (the other end of the secondary coil Ns) of the secondary coil Ns and the diode D2 and the negative output end n. Furthermore, the third diode D3 is connected between a connection point b (one end of the secondary coil Ns) of the secondary coil Ns and the diode D1 and the negative output end n. The anode of the diode D3 is connected to the negative output terminal n, and the cathode of the diode D3 is connected to the connection point b of the secondary coil Ns and the diode D1.

Furthermore, a fourth diode D4 is provided, the anode of which is connected to the negative output terminal n, and the cathode of which is connected to the other end of the secondary coil Ns.

The diode in this circuit preferably has a small forward voltage drop and operates at high speed. Note that, instead of the diode, a rectifying element formed of an element or a circuit having the same function can be used.

Although not shown, it is preferable to have a control unit that generates a control signal Vg for the switching element Q. As an example, the control unit detects an input voltage and / or an output voltage, determines a duty ratio of the control signal Vg based on the detected voltage, and generates a control signal Vg of a predetermined high frequency pulse based thereon. PWMIC can be used as a main part of such a control part.

(1-2) Operation of the First Embodiment The operation of the first embodiment will be described with reference to FIG. 2 and FIG. FIGS. 2 (a) and 2 (b) schematically show the flow of current in the on and off periods, respectively, as a solid line or a dotted line (the arrow indicates the direction of the current). FIGS. 3A and 3B are diagrams schematically showing an example of the potential relationship between the components on the secondary side of the transformer T in the on period and the off period, respectively.

In FIGS. 3A and 3B, the vertical direction corresponds to the level of the potential, and the secondary side reference potential (the potential of the negative electrode output end n) is indicated by a thick line. The voltage across the secondary coil Ns of the transformer T, the inductor L, the capacitor C1 and the capacitor C2 is indicated by double arrows. Further, with regard to the secondary coil Ns, the winding start end is indicated by a black circle.

The transient operation at the start and stop of the circuit is an exception, and the operation when the circuit is in the steady state will be described. In the steady state, the capacitor C1 is charged at a substantially constant voltage across it, except for ripple fluctuations. Since the charge and discharge current in the capacitor C1 is slight, it will be ignored in the following description. It is assumed that the charge / discharge operation in the steady state is performed also for the capacitor C2. Also ignore the forward voltage drop of each diode.

(1-2-1) Operation of the primary side and secondary side of the transformer T in the on period [on period: primary side]
On the primary side of the transformer T, when the control signal Vg is turned on in the on period, the FET Q is turned on and the current path is conducted. As shown in FIG. 2A, an input current i1 according to the input voltage flows through the primary coil Np of the transformer T in the following path. Magnetic energy is stored in the transformer T by being excited by the input current i1.
· Input current i1: input end 1 → primary coil Np of transformer T → FET Q → input end 2

[On period: Secondary side]
As shown in FIG. 2A, when the input current i1 flows through the primary coil Np of the transformer T, an electromotive force due to mutual induction is generated in the secondary coil Ns, the diode D1 becomes forward biased, and the current flows in the following path i2 flows.
Current i2: negative electrode output terminal n → capacitor C2 → secondary coil Ns of transformer T → diode D1 → inductor L → positive electrode output terminal p

The current i2 flows as a current for discharging the capacitor C2. Therefore, the current i2 is a current that outputs the electrical energy stored in the capacitor C2 and the electrical energy due to mutual induction of the transformer T to the output terminal p. Further, when the current i2 flows to the inductor L, the inductor L is excited and magnetic energy is accumulated.

As can be understood from the potential relationship diagram of FIG. 3A, the diode D2 and the diode D3 are blocked because they are reverse biased. Since the diode D4 is in parallel with the capacitor C2, it is cut off as long as the potential at one end of the capacitor C2 (the connection point with the secondary coil Ns) is a positive potential. The diode D4 is a clipper for preventing the potential at one end of the capacitor C2 from being a negative potential, and is not essential.

As indicated by a dotted line in the potential relationship diagram of FIG. 3A, the capacitor C2 discharges during the on period, so the voltage across the capacitor C2 decreases.

Further, as shown in FIG. 3A, the electromotive force generated in the secondary coil Ns is boosted by the voltage across the capacitor C2. This means that the electromotive force generated in the secondary coil Ns is smaller (suppressed) as compared to the general forward system without the capacitor C2.

(1-2-2) Operation of primary side and secondary side of the transformer T in the off period In FIG. 2 (b), of the current flowing in the off period, the current discharging the capacitor C2 is indicated by the solid line, It is schematically shown by dotted lines.

[Off period: Primary side]
On the primary side of the transformer T, when the control signal Vg is turned off, the FET Q is also turned off and the switch is opened. The current path of the primary coil Np of the transformer T is cut off, and the current becomes zero. Thereby, back electromotive force is generated in the primary coil Np and the secondary coil Ns of the transformer T, respectively.

As described above, the electromotive force of the secondary coil Ns in the on period is boosted by the voltage of the capacitor C2 and substantially suppressed, so that the back electromotive force generated in the primary coil Np at the moment it is turned off is small. Become. A voltage obtained by adding the back electromotive force generated in the input voltage and the primary coil Np is applied to the switching element Q (between the drain and the source in the case of the FET). Therefore, in the present circuit, the withstand voltage required for the switching element Q is reduced, and the processing capacity of the snubber circuit can be reduced. Similarly, since the possibility of magnetic saturation of the transformer T also decreases, the size of the transformer T can be reduced.

[Off period: Secondary side]
As shown in the potential relationship diagram of FIG. 3B, in the off period, the potential relationship between both ends of each of the secondary coil Ns of the transformer T and the inductor L is reversed. The diode D1 is reverse biased and cut off. On the other hand, the diode D2 and the diode D3 become conductive because they are forward biased.

As shown in FIG. 2B, the conduction of the diode D2 and the diode D3 causes the current i3 and the current i4 to flow in the following paths, respectively.
· Current i3: negative electrode output terminal n → capacitor C2 → inductor L → positive electrode output terminal p
Current i4: negative electrode output terminal n → diode D3 → secondary coil Ns → capacitor C2

The current i3 is a current for discharging the capacitor C2. Therefore, the current i3 is a current that outputs the electric energy stored in the capacitor C2 and the magnetic energy stored in the inductor L as electric power.

On the other hand, the current i4 is a current for charging the capacitor C2. The current i4 flows through the secondary coil Ns to the capacitor C2, so that the magnetic energy stored in the on period of the transformer T is released and stored as the electrical energy of the capacitor C2. This promotes the magnetic reset of the transformer T. As a result, since the possibility of magnetic saturation of the transformer T is reduced, the size of the transformer T can be reduced.

In the normal forward system, the magnetic energy stored in the transformer T is consumed by the snubber circuit or the like on the primary side, but in the present invention, the magnetic energy of the transformer T is converted to the electric energy of the capacitor C2. Then, since the electrical energy stored in the capacitor C can be output as a discharge current, the power transfer efficiency can be improved.

If the current i4 is sufficiently larger than the current i3, the capacitor C2 is charged in the off period. As a result, as shown by a dotted line in the potential relationship diagram of FIG. 3B, the voltage across the capacitor C2 is recovered.

As described above, the presence of the capacitor C2 suppresses the electromotive force generated in the secondary coil Ns, thereby reducing the back electromotive force generated in the secondary coil Ns, thereby reducing the withstand voltage required for the diode D1. Be done.

(2) Second Embodiment (2-1) Circuit Configuration of Second Embodiment The second embodiment is a modification of the first embodiment. FIG. 4 is a diagram schematically showing a circuit configuration example of a second embodiment of the isolated switching power supply of the present invention, and also shows the flow of current during the off period.

Regarding the circuit of the second embodiment, mainly the points different from the first embodiment will be described. In the circuit of FIG. 4, one end of the diode D3 is not connected to the connection point between the secondary coil Ns and the diode D1. Instead, one end of the diode D3 is connected to the midpoint of the secondary coil Ns. The meaning of the middle point here is not the meaning of the bisecting point, and its position is set as needed. A tap is provided at the set middle point, and one end of the diode D3 is connected.

In the circuit of FIG. 4, the current flowing in the on period of the FET Q is the same as that of the first embodiment shown in FIG. 2 (a). Further, the current i3 which discharges the capacitor C2 is the same as that of the first embodiment shown in FIG.

In the circuit of FIG. 4, the path of the current i4 charging the capacitor C2 flowing in the off period is different from that of the first embodiment. As shown, the current i4 passes through only a portion of the secondary coil Ns. Therefore, the current i4 has a smaller resistance than when passing through the entire secondary coil Ns, and becomes a larger current. By appropriately setting the position of the middle point, it is possible to secure current i4 of a size that can sufficiently charge capacitor C2 in the off period.

1 input end 2 input end (input side reference end)
p Positive output terminal n Negative output terminal (Output side reference potential)
T transformer Np Primary coil Ns Secondary coil Q Switching element (FET)
D1, D2 D3, D4 Rectifying element (diode)
C1 First capacitor (smoothing capacitor)
C2 second capacitor

Claims

A transformer having a primary coil and a secondary coil, a switching element connected in series to the primary coil of the transformer and controlled on / off by a control signal, a first rectifying element and an inductor connected in series to the secondary coil; A forward switching power supply comprising: said secondary coil connected in series; and a second rectifying element connected in parallel to said first rectifying element,
(A) a capacitor connected between a connection point of the secondary coil and the second rectifying element, and a negative electrode output end;
(B) A third rectifying element connected between a connection point between the secondary coil and the first rectifying element, and a negative electrode output end,
(C) During the on period of the switching element, a current that discharges the capacitor flows to and outputted from the secondary coil, the first rectifying element, and the inductor, and
(D) During the off period of the switching element, a current discharging the capacitor flows through the second rectifying element and the inductor and is output while a current charging the capacitor is the third rectifying element and the second rectifying element An isolated switching power supply characterized by flowing through the following coil.
Instead of being connected to a connection point between the secondary coil and the first rectification element, one end of the third rectification element is connected to an intermediate point of the secondary coil. The isolated switching power supply according to claim 1.