KR20200097722A - Isolated switching power supply - Google Patents

Abstract

The present invention relates to a forward type isolated switching power supply device, and is configured to suppress a back 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), the other end of the secondary coil (Ns) and the cathode output terminal and 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 the secondary coil, the first rectification. While being outputted to 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 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.

Insulation type switching power supply devices are known in which an input side and an output side are insulated using a transformer (Patent Documents 1 to 5). When the input is an AC voltage, a DC/DC converter is generally arranged behind 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. Representative methods of isolated switching power supplies include a flyback method and a forward method.

In the forward switching power supply device, power is transferred by electromagnetic induction of a transformer during the on period of the switching element, so that current flows through the secondary coil and is output through the inductor, and magnetic energy is accumulated in the inductor. In addition, during the off period of the switching element, a current flows through the freewheeling diode to discharge the magnetic energy of the inductor and is output.

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

In the isolated switching power supply, at the moment the switching element is turned off, a high back electromotive force (“electromotive force” and “back electromotive force” in this specification are used in the sense of voltage), that is, a surge voltage. Is generated and applied to the switching element. To this end, it was necessary to use a switching element of high withstand voltage or to install a snubber circuit to handle back EMF.

In addition, in the case of the general forward system, since no current flows through the transformer during the off period, magnetic saturation occurs if the magnetic energy accumulated in the transformer is not reset during the on period. A snubber circuit or the like was required to reset the magnetic energy. Since the energy processed by the snubber circuit or the like is not transferred to the secondary side, the efficiency of power transfer of the switching power supply is lowered.

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

Insulated switching power supply device of the forward type according to the present invention comprises 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; Having the secondary coil connected in series and a second rectifying element connected in parallel to the first rectifying element,

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

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

(c) the current discharging the capacitor during the ON period of the switching element flows to the secondary coil, the first rectifying element and the inductor, and is output,

(d) In the OFF period of the switching element, a current discharging the capacitor flows through the second rectifying element and the inductor, and a 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 an intermediate 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 the isolated switching power supply device, the back electromotive force, that is, the surge voltage generated in the primary coil of the 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 accumulated 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 the circuit configuration of the first embodiment for the isolated switching power supply device of the present invention.
2A and 2B show current flows in the on-period and off-period of the circuit shown in Fig. 1, respectively.
3A and 3B are diagrams schematically showing an example of a potential relationship with respect to the on period and the off period of the secondary side of the transformer of the circuit shown in FIG. 1, respectively.
4 is a diagram schematically showing an example of the circuit configuration of the second embodiment of the isolated switching power supply device of the present invention, and also shows the flow of current in the off period.

An embodiment of an isolated switching power supply device according to the present invention will be described below with reference to the drawings showing the embodiment. 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 DC/DC converter to which a DC voltage is input as an embodiment. On the other hand, the isolated switching power supply device according to the present invention functions in the same way even when a voltage of any waveform such as a pulsating current, a square wave, or an alternating current in which the voltage is fluctuating is input in addition to a DC having a constant voltage. It is a power conversion device capable of outputting a DC voltage.

(1) First embodiment

(1-1) Circuit configuration of the first embodiment

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

The circuit configuration of the first embodiment will be described with reference to FIG. 1. This circuit is an isolated switching power supply that electrically insulates the input side and the output side by a transformer, and is based on the forward method. The transformer T includes a primary coil Np and a secondary coil Ns. Transformer (T) has the same polarity (winding start end) of the primary and secondary coils. It is preferable that the transformer T has a degree of coupling as high as possible, that is, closely couples the primary coil Np and the secondary coil Ns.

In the drawing, the winding start ends of each coil are indicated by black circles. In the present specification, when the coil is referred to as "one end" and "the other end", both the case corresponding to the "winding start end" and the "winding end", and the case corresponding to the "winding end" and "winding start end" respectively Includes. In the following description, for each coil, the starting end of the winding is referred to as one end, and the end of the winding is referred to as the other end.

The input voltage is applied between a pair of terminals consisting of the first input terminal 1 and the 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 controlled on/off to communicate or cut off the current path including the primary coil Np. Basically, the switching element Q needs to be connected in series 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 of a predetermined frequency and duty ratio. In the illustrated example, the switching element Q is an n-channel MOSFET (hereinafter referred to as "FETQ"), 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.

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

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

Like a general forward-type circuit, a first diode D1, which is a rectifier diode, and an inductor L are connected in series 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 secondary coil Ns and the first diode D1 connected in series. 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. Further, 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 the connection point (a) of the secondary coil (Ns) and the second diode (D2) (the other end of the secondary coil [Ns]), and between the cathode 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 cathode output terminal (n). have. 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 connection point b between the secondary coil Ns and the first diode D1. .

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

In this circuit, the diode is preferably capable of high-speed operation with a small forward voltage drop. Further, 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 that generates a control signal Vg for the switching element Q. For example, the controller detects an input voltage and/or an output voltage, determines a duty ratio of the control signal Vg according to the detected voltage, and generates a control signal Vg of a predetermined high-frequency pulse accordingly. PWMIC can be used as the main part of such a 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 in the on-period and off-period, respectively, with a solid line or a dotted line (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 in the on period and the off period, respectively.

In Figs. 3A and 3B, the vertical direction corresponds to the high and low electric potential, and the secondary-side reference potential (potential at the negative output terminal [n]) is indicated by a thick line. The voltage across the secondary coil (Ns) of the transformer (T), the inductor (L), and the capacitors (C1 and C2) is indicated by two arrows. In addition, the starting end of the winding for the secondary coil Ns is indicated by a black circle.

In addition, with the exception of the transient operation at the time of starting and stopping of this circuit, the operation when the present circuit is in a steady state will be described. In the normal state, the capacitor C1 is charged with an almost constant voltage at both ends except for ripple-like fluctuations. Since the charge/discharge current of the 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 of each diode is ignored.

(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 in the ON period, the FETQ is turned on and a current path is communicated. As shown in FIG. 2A, the input current i1 by the input voltage flows through the following path through the primary coil Np of the transformer T. 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 through 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 is forward direction. It becomes a bias, and a current i2 flows in the next path.

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

The current i2 flows as a current that discharges the capacitor C2. Accordingly, the current i2 is a current that outputs the electric energy accumulated in the capacitor C2 and the electric energy by mutual induction of the transformer T to the output terminal p. In addition, as 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 cut off to become reverse biased. Since the fourth diode D4 is parallel to the capacitor C2, it is cut off as long as the potential of one end of the capacitor C2 (the connection point with the secondary coil Ns) is a positive (+) potential. The fourth diode D4 is a tool for not making the potential of one end of the capacitor C2 a negative potential, and is not essential.

As indicated by the dotted line in the potential relationship diagram of Fig. 3A, since the capacitor C2 discharges during the ON period, the voltage at both ends thereof decreases.

In addition, 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, the current that discharges the capacitor C2 among the currents flowing in the off period is schematically shown by a solid line and a charging current by a dotted line.

[Off period: primary side]

On the primary side of the transformer T, when the control signal Vg is released, the FETQ 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. Accordingly, 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 increased by the voltage of the capacitor C2 and is substantially suppressed. As a result, the back electromotive force generated in the primary coil Np at the moment when it is turned off, that is, a surge The voltage also decreases. A voltage obtained by adding the input voltage and the back electromotive force generated in the primary coil Np is applied to the switching element Q (between the drain and source in the case of FET). Accordingly, in this circuit, the pressure resistance required for the switching element Q is reduced, and 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 also reduced, the size of the transformer T can be reduced.

[Off period: secondary side]

As shown in the potential relationship of FIG. 3B, when the off period is reached, the potential relationship between the secondary coil Ns of the transformer T and the both ends of the inductor L is reversed. The first diode D1 becomes reverse biased and is 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 second diode D2 and the third diode D3 communicate with each other, so that each current i3 and i4 flows in the next path.

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

-Current (i4): negative output terminal (n) → third diode (D3) → secondary coil (Ns) → capacitor (C2)

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

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

Further, if the current i4 is sufficiently larger than the current i3, the capacitor C2 is configured to be charged in the off period. As a result, the voltage at both ends of the capacitor C2 is recovered, as indicated by a dotted line in the potential relationship diagram of Fig. 3B.

As described above, since the presence of the capacitor C2 suppresses the electromotive force generated in the secondary coil Ns, the back electromotive force generated in the secondary coil Ns is also reduced, thereby reducing the withstand voltage required for 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 the circuit configuration of the second embodiment of the isolated switching power supply device of the present invention, and also shows the current flow in the off period.

The circuit of the second embodiment will be mainly described different from the first embodiment. In the circuit of Fig. 4, one end of the third diode D3 is not connected to a connection point between the secondary coil Ns and the first diode D1. On the other hand, one end of the third diode D3 is connected to the middle point of the secondary coil Ns. The meaning of the intermediate point here is not the meaning of the dividing point, and the position is set as necessary. A tap is installed at the set middle point, and one end of the third diode D3 is connected.

In the circuit of Fig. 4, the current flowing in the ON period of the FETQ is the same as in the first embodiment shown in Fig. 2A. Also, the current i3 for discharging the capacitor C2 with respect to the current flowing in the off period is the same as in the first embodiment shown in Fig. 2B.

In the circuit of Fig. 4, the path of the current i4 for charging the capacitor C2 flowing in the off period is different from that in the first embodiment. As shown, the current i4 passes only a part of the secondary coil Ns. Accordingly, the current i4 has a smaller resistance than when passing through the entire 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 of a size capable of sufficiently charging the capacitor C2 in the off period.

1 input
2 input terminal (reference terminal on the input side)
p positive output stage
n Negative output terminal (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 first capacitor (smoothing capacitor)
C2 second capacitor

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 insulated switching power supply device comprising;
(a) a connection point between the secondary coil and the second rectifying element, and a capacitor connected between the cathode output terminal,
(b) having a connection point between the secondary coil and the first rectifying element, and a third rectifying element connected between the cathode output terminal,
(c) the current discharging the capacitor during the ON period of the switching element flows to the secondary coil, the first rectifying element and the inductor, and is output,
(d) a forward method, characterized in that the current discharging the capacitor flows through the second rectifying element and the inductor during the off period of the switching element, and the current charging the capacitor flows through the third rectifying element and the secondary coil. Isolated switching power supply. The method of claim 1,
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.

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