KR102525753B1 - Isolated switching power supply - Google Patents

Abstract

The present invention relates to an isolated switching power supply of a forward type, and is configured to suppress counter electromotive force generated in a primary coil of a transformer when a switching element is turned off. The device includes a transformer (T), a switching element (Q), a first rectifying element (D1) and an inductor (L), and a second rectifying element (D2), and the other end of the secondary coil (Ns) and the negative output terminal It has a capacitor (C2) connected between, and a third rectifying element (D3) connected between one end of the secondary coil and the cathode output terminal (n), and the discharge current of the capacitor during the on period is discharged from the secondary coil, the first rectification While being output through the element and the inductor, the discharge current of the capacitor flows through the second rectifying element and the inductor during the off period, and the charging current is configured to flow through the third rectifying element and the secondary coil.

Description

Isolated switching power supply

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

Background Art [0002] Insulated switching power supply devices in which an input side and an output side are insulated using a transformer are known (Patent Documents 1 to 5). When the input is an AC voltage, a DC/DC converter is usually placed after the AC/DC conversion circuit. If the input is a DC voltage, it is directly input to the DC/DC converter. A DC/DC converter is also an isolated switching power supply. Representative methods of the isolated switching power supply include a flyback method and a forward method.

In a forward-type switching power supply, electric power is transmitted by electromagnetic induction of a transformer during an on-period of a switching element, so that current flows in a secondary coil and is output through an inductor, while magnetic energy is accumulated in the inductor. And, during the off period of the switching element, current flows through the freewheeling diode and is output so as to release the magnetic energy of the inductor.

Patent Document 1: Japanese Unexamined Patent Publication No. 7-31150 Patent Document 2: Japanese Unexamined Patent Publication No. 8-331860 Patent Document 3: Japanese Patent Laid-Open No. 2002-10632 Patent Document 4: Japanese Patent Laid-Open No. 2005-218224 Patent Document 5: Japanese Patent Laid-Open No. 2007-37297

In an isolated switching power supply, at the moment when the switching element is turned off, a high back electromotive force is applied to the primary coil of the transformer (in this specification, "electromotive force" and "back electromotive force" are used in the meaning of voltage), that is, surge voltage. This occurs and is applied to the switching element. To this end, it was necessary to use a switching element with a high breakdown voltage or to install a snubber circuit for processing counter electromotive force.

In addition, in the case of a general forward method, since current does not flow through the transformer during the off period, magnetic saturation occurs unless the magnetic energy accumulated in the transformer is reset during the on period. Even for resetting such magnetic energy, a snubber circuit or the like was required. Since the energy processed in the snubber circuit is not transferred to the secondary side, the power transfer efficiency of the switching power supply is reduced.

In view of the above problems, an object of the present invention is to suppress counter electromotive force generated in a transformer when a switching element is turned off in a forward type isolated switching power supply, while transferring magnetic energy accumulated in the transformer to the secondary side during the on period. is to make it possible.

In order to achieve the above object, an isolated switching power supply device according to an aspect of the present invention has the following configuration.

Forward-type isolated switching power supply device according to the present invention includes a transformer having a primary coil and a secondary coil; a switching element controlled on/off by a control signal serially connected to the primary coil of the transformer; a first rectifying element and an inductor connected in series to the secondary coil; A second rectifying element connected in parallel to the secondary coil and the first rectifying element connected in series;

(a) a capacitor connected between the connection point between the secondary coil and the second rectifying element and the cathode output terminal;

(b) a third rectifying element connected between a connection point between the secondary coil and the first rectifying element and a cathode output terminal;

(c) During the on-period of the switching element, the current discharging the capacitor flows through the secondary coil, the first rectifying element, and the inductor, and is output.

(d) During the off period of the switching element, the current discharging the capacitor flows through the second rectifying element and the inductor, and the current charging the capacitor flows through the third rectifying element and the secondary coil.

In the above aspect, it is preferable that one end of the third rectifying element is connected to a middle point of the secondary coil instead of being connected to a connection point between the secondary coil and the first rectifying element.

According to the present invention, in an isolated switching power supply device, counter-electromotive force, that is, surge voltage generated in the primary coil of a transformer when the switching element is turned off is suppressed, thereby reducing the voltage resistance required for the switching element. . In addition, since the magnetic energy stored in the transformer can be transferred to the secondary side when the switching element is turned on, power transfer efficiency is improved.

1 is a diagram schematically showing an example of a circuit configuration of a first embodiment for an isolated switching power supply device of the present invention.
2A and 2B show the flow of current for an on period and an off period of the circuit shown in FIG. 1, respectively.
3A and 3B are views schematically illustrating an example of a potential relationship for an on period and an off period of the secondary side of a transformer in the circuit shown in FIG. 1, respectively.
4 is a diagram schematically showing a circuit configuration example of a second embodiment of an isolated switching power supply device of the present invention, showing the flow of current for an off period together.

An embodiment of an isolated switching power supply device according to the present invention will be described below with reference to drawings showing embodiments. In the drawings of each embodiment, the same reference numerals are assigned to the same or similar components.

Hereinafter, an isolated switching power supply device according to the present invention will be described using a case of a DC/DC converter to which a DC voltage is input as an example. On the other hand, the isolated switching power supply device according to the present invention functions the same even when a voltage of any waveform, such as a pulsating current with a fluctuating voltage, a square wave, or an alternating current, is input, in addition to direct current with a constant voltage. It is a power conversion device capable of outputting a direct current voltage.

(1) First Embodiment

(1-1) Circuit configuration of the first embodiment

1 is a diagram schematically showing an example of a circuit configuration of a first embodiment for an isolated switching power supply device of the present invention.

Referring to Fig. 1, the circuit configuration of the first embodiment will be described. This circuit is an isolated switching power supply that electrically insulates the input and output sides by a transformer, and is based on the forward method. The transformer T includes a primary coil Np and a secondary coil Ns. In the transformer T, the polarity of the primary coil and the secondary coil (winding start end) are in the same direction. In the transformer T, the degree of coupling is preferably as high as possible, that is, the primary coil Np and the secondary coil Ns are closely coupled.

In the drawing, the winding start end of each coil is indicated by a black circle. In this specification, "one end" and "the other end" of a coil correspond to both "winding start end" and "winding end", and "winding end" and "winding start end" respectively. includes 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 a first input terminal (1) and a second input terminal (2). One end of the primary coil (Np) of the transformer (T) is connected to the first input terminal (1). Here, the second input terminal 2 is an input-side reference potential terminal.

One end of the 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 second input terminal (2). The switching element Q has a control stage. The control stage is on/off controlled to connect or block the current path including the primary coil Np. Basically, the switching element Q only needs to be serially connected to the primary coil Np.

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

Further, as switching elements other than FETs, for example, IGBTs or bipolar transistors can also be used.

A positive output terminal (p) and a negative output terminal (n), which are a pair of output terminals for outputting DC voltage, are installed on the secondary side of the transformer T. Here, the cathode output terminal (n) is a secondary side reference potential terminal. An output voltage is applied and an output current is supplied to a load (not shown) connected between the positive output terminal p and the negative output terminal n.

Like the general forward type circuit, a first diode D1 which is a rectifier diode and an inductor L are connected in series with respect to the secondary coil Ns of the transformer T. The anode of the first diode D1 is connected to one end of the secondary coil Ns, while the cathode of the first 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). In addition, a second diode D2, which is a freewheeling diode, is connected in parallel to the series-connected secondary coil Ns and the first diode D1. The anode of the second diode D2 is connected to the other end of the secondary coil Ns, while the cathode of the second diode D2 is connected to the cathode of the first diode D1. In addition, a first capacitor (C1), which is a smoothing capacitor, is connected between the positive output terminal (p) and the negative output terminal (n).

In the present invention, the second capacitor (C2) is between the connection point (a) of the secondary coil (Ns) and the second diode (D2) (the other end of the secondary coil [Ns]) and the negative output terminal (n). is connected to In addition, the third diode D3 is connected between the connection point b of the secondary coil Ns and the first diode D1 (one end of the secondary coil [Ns]) and the negative output terminal n there is. The anode of the third diode D3 is connected to the negative output terminal n, while the cathode of the third diode D3 is connected to the secondary coil Ns and the junction point b of the first diode D1 .

In addition, a fourth diode D4 is installed, the anode of which is connected to the cathode output terminal (n) while the cathode is connected to the other end of the secondary coil (Ns).

In this circuit, it is preferable that the diode is capable of high-speed operation while having a small forward voltage drop. Also, instead of the diode, a rectifying element composed of an element or circuit having the same function may be used.

Although not shown, it is preferable to have a control unit for generating a control signal (Vg) for the switching element (Q). For example, the controller detects the input voltage and/or the output voltage, determines the duty ratio of the control signal Vg according to the detected voltage, and generates the control signal Vg of a predetermined high frequency pulse accordingly. A PWMIC can be used as the main part of this control unit.

(1-2) Operation of the first embodiment

The operation of the first embodiment will be described with reference to FIGS. 2 and 3 . 2A and 2B schematically show the flow of current for an on period and an off period with a solid line or a dotted line, respectively (the arrow indicates the direction of the current). 3A and 3B are diagrams schematically showing an example of a potential relationship for each component on the secondary side of the transformer T during an on period and an off period, respectively.

In FIGS. 3A and 3B , the vertical direction corresponds to the level of the potential, and the secondary-side reference potential (potential of the cathode output terminal [n]) is indicated by a thick line. The voltages across the secondary coil (Ns) of the transformer (T), the inductor (L), and the capacitors (C1 and C2) are indicated by two arrows. In addition, the winding start end of the secondary coil Ns is indicated by a black circle.

In addition, with the exception of transient operations at startup and stop of this circuit, operations in the case where this circuit is in a steady state will be described. In a normal state, the capacitor C1 is charged with an almost constant voltage across the capacitor excluding ripple-like fluctuations. Since the charge/discharge current of capacitor C1 is insignificant, it is ignored in the following description. It is assumed that the capacitor C2 is also charged and discharged in a steady state. Also, the forward voltage drop across each diode is neglected.

(1-2-1) Operation of the primary and secondary sides of the transformer (T) during the on period

[On Period: Primary Side]

On the primary side of the transformer T, when the control signal Vg is turned on during the on period, the FETQ is turned on and the current path is connected. As shown in FIG. 2A, the input current i1 by the input voltage flows through the primary coil Np of the transformer T through the following path. Magnetic energy is accumulated in the transformer T by being excited by the input current i1.

- Input current (i1): input terminal (1) → primary coil (Np) of transformer (T) → FETQ → input terminal (2)

[Off period: secondary side]

As shown in FIG. 2A, as the input current i1 flows in the primary coil Np of the transformer T, an electromotive force due to mutual induction is generated in the secondary coil Ns, while the diode D1 moves forward. As a bias, current i2 flows through the following path.

- Current (i2): negative output terminal (n) → capacitor (C2) → secondary coil (Ns) of transformer (T) → first diode (D1) → inductor (L) → positive output terminal (p)

Current i2 flows as a current that discharges capacitor C2. Therefore, the current i2 is a current that outputs the electric energy stored in the condenser C2 and the electric energy due to the mutual induction of the transformer T to the output terminal p. In addition, when the current i2 flows through the inductor L, the inductor L is excited and thus magnetic energy is accumulated.

As can be seen from the potential relationship diagram of FIG. 3A, the second diode D2 and the third diode D3 are blocked to become reverse biased. Since the fourth diode D4 is in parallel with the condenser C2, it is blocked as long as the potential of one end of the condenser C2 (the connection point with the secondary coil [Ns]) is positive (+) potential. The fourth diode D4 is a tool for not turning the potential of one end of the capacitor C2 into a negative (-) potential, and is not essential.

As indicated by a dotted line in the potential relationship diagram of FIG. 3A, since the capacitor C2 is discharged during the on period, the voltage across its ends decreases.

Also, as shown in FIG. 3A, the electromotive force generated in the secondary coil Ns is increased by the voltage across the capacitor C2. This means that the electromotive force generated in the secondary coil Ns is small (suppressed) compared to the case of the general forward method without the capacitor C2.

(1-2-2) Operation of the primary and secondary sides of the transformer (T) during the off period

In FIG. 2B, among the currents flowing in the off period, the current for discharging the capacitor C2 is schematically represented by a solid line and the charging current by a dotted line.

[Off Period: Primary Side]

On the primary side of transformer T, when control signal Vg is released, FETQ is also turned off and the switch is opened. The current path of the primary coil Np of the transformer T is blocked, and the current becomes zero. Accordingly, counter electromotive force is generated in the primary coil Np and the secondary coil Ns of the transformer T, respectively.

As described above, as a result of the electromotive force of the secondary coil Ns during the on period being increased by the voltage of the condenser C2 and substantially suppressed, the counter electromotive force generated in the primary coil Np at the moment it is turned off, that is, surge voltage also decreases. A voltage obtained by adding the input voltage and the counter electromotive force generated in the primary coil Np is applied to the switching element Q (between drain and source in case of FET). Therefore, in this circuit, the voltage resistance required for the switching element Q is reduced, and at the same time, the processing capacity of the snubber circuit or the like can be reduced. Similarly, since the possibility of magnetic saturation of the transformer T is reduced, the size of the transformer T can be reduced.

[Off period: secondary side]

As shown in the potential relationship of FIG. 3B, in the off period, the potential relationship between the secondary coil Ns of the transformer T and both ends of the inductor L reverses. The first diode D1 is reverse biased and blocked. Meanwhile, the second diode D2 and the third diode D3 communicate with each other because they are forward biased.

As shown in FIG. 2B, the currents i3 and i4 flow through the following paths when the second diode D2 and the third diode D3 communicate with each other.

- Current (i3): negative output terminal (n) → capacitor (C2) → inductor (L) → positive output terminal (p)

- Current (i4): cathode output terminal (n) → 3rd diode (D3) → secondary coil (Ns) → capacitor (C2)

Current i3 is the current that discharges capacitor C2. Therefore, the current i3 is a current that outputs the electrical energy stored in the condenser C2 and the magnetic energy stored in the inductor L as electric power.

On the other hand, the current i4 is the current that charges the condenser C2. As the current i4 flows through the secondary coil Ns to the condenser C2, the magnetic energy accumulated during the on period of the transformer T is released and stored as electrical energy in the condenser C2. Thus, the self-reset of transformer T is promoted. 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 method, the magnetic energy stored in the transformer (T) is consumed in the snubber circuit of the primary side, but in the present invention, the magnetic energy of the transformer (T) is converted into electrical energy of the capacitor (C2). In addition, since the electrical energy stored in the condenser C can be output as a discharge current, power transfer efficiency can be improved.

Also, if the current i4 is sufficiently greater than the current i3, the capacitor C2 is configured to be charged during the off period. As a result, the voltage across the capacitor C2 is restored, as indicated by dotted lines in the potential relationship diagram of FIG. 3B.

As described above, since the electromotive force generated in the secondary coil Ns is suppressed due to the presence of the condenser C2, the counter electromotive force generated in the secondary coil Ns is also reduced, thereby reducing the voltage resistance required of the diode D1. .

(2) First Embodiment

(2-1) Circuit configuration of the second embodiment

The second embodiment is a modified example of the first embodiment. 4 is a diagram schematically showing an example of a circuit configuration of a second embodiment of an isolated switching power supply device of the present invention, together with showing the flow of current during an off period.

Regarding the circuit of the second embodiment, the main differences from the first embodiment will be explained. In the circuit of FIG. 4, one end of the third diode D3 is not connected to the connection point between the secondary coil Ns and the first diode D1. In contrast, one end of the third diode D3 is connected to the middle point of the secondary coil Ns. The meaning of the middle point here is not the meaning of a bisecting point, and its location is set as needed. A tap is installed at the set middle point, and one end of the third diode (D3) is connected.

The current flowing during the on-period of the FETQ in the circuit of FIG. 4 is the same as that of the first embodiment shown in FIG. 2A. Also, the current i3 for discharging the condenser C2 with respect to the current flowing during the off period is the same as that of the first embodiment shown in FIG. 2B.

In the circuit of Fig. 4, the path of the current i4 that charges the condenser C2 flowing during the off period is different from that of the first embodiment. As shown, 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 entirety of the secondary coil Ns, resulting in a larger current. By appropriately setting the position of the intermediate point, it is possible to secure a current i4 sufficient to sufficiently charge the condenser C2 during the off period.

1 input
2 input terminal (reference terminal on the input side)
p positive output stage
n negative output stage (reference potential on the output side)
T transformer
Np primary coil
Ns secondary coil
Q switching element (FET)
D1, D2 D3, D4 rectifying element (diode)
C1 1st capacitor (smoothing capacitor)
C2 2nd condenser

Claims (2)

A transformer having a primary coil and a secondary coil; a switching element controlled on/off by a control signal serially connected to the primary coil of the transformer; a first rectifying element and an inductor connected in series to the secondary coil; In the forward-type isolated switching power supply including a second rectifying element connected in parallel with respect to the secondary coil and the first rectifying element connected in series,
(a) a capacitor connected between the connection point between the secondary coil and the second rectifying element and the cathode output terminal;
(b) a third rectifying element connected between a connection point between the secondary coil and the first rectifying element and a cathode output terminal;
(c) During the on-period of the switching element, the current discharging the capacitor flows through the secondary coil, the first rectifying element, and the inductor, and is output.
(d) During the off period of the switching element, the current discharging the capacitor flows through the second rectifying element and the inductor, and the current charging the capacitor flows through the third rectifying element and the secondary coil. Isolated switching power supplies. According to claim 1,
A forward type isolated switching power supply device, characterized in that one end of the third rectifying element is connected to the middle point of the secondary coil instead of being connected to the connection point between the secondary coil and the first rectifying element.

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