KR102448319B1 - switching power supply - Google Patents

Abstract

In a flyback type switching power supply, the withstand voltage characteristics of the output diode are realized at low cost. In the switching power supply of the flyback type having a switching element (Q) connected to the transformer (T) and the primary coil (Np), between one end (a) of the secondary coil (Ns) and the first output terminal (3) The first rectifying means D1 and the second rectifying means D2 connected in series, the connection point b between the first rectifying means D1 and the second rectifying means D2, and the other end of the secondary coil Ns ( It has a third rectifying means (D3) connected between d) and a smoothing capacitor connected between the first output terminal (3) and the second output terminal (4) which is the other end (d) of the secondary coil (Ns), The first rectifying means and the second rectifying means are connected in a direction to be reverse biased with respect to the potential generated at one end of the secondary coil during the ON period of the switching element, and the third rectifying means is connected to the secondary coil during the ON period of the switching element. They are connected in a direction that becomes a forward bias with respect to a potential generated at the other end.

Description

switching power supply

The present invention relates to a flyback type switching power supply.

An insulated switching power supply that draws a desired DC power from a secondary coil by turning on/off DC power input to the primary coil of a transformer by a switching element is well known. The flyback method in an isolated switching power supply is also well known.

Figure 4 (A) is a basic circuit of the conventional flyback method. 4(B) to 4(F) are graphs schematically showing waveforms of voltage or current in each element of the circuit of FIG. 4(A).

Fig. 4(B) shows the waveform of the control voltage Vg of the switching element Q. In the flyback method, an excitation current flows through the primary coil Np during the ON period of the switching element Q, and magnetic energy is accumulated in the transformer T. In the secondary coil Ns, current does not flow because the output diode D is reverse-biased. During the off period of the switching element Q, a counter electromotive force that becomes a forward bias of the output diode D is generated in the secondary coil Ns, and a load current flows to the secondary coil Ns to release the accumulated magnetic energy, thereby causing the output diode. It is output through (D).

Fig. 4(C) shows the waveform of the current Is flowing through the secondary coil Ns. Fig. 4(D) shows the waveform of the voltage across the secondary coil Ns (the voltage between a-d with the point d as the reference potential) (Vs). Fig. 4(E) shows the waveform of the voltage across the capacitor C (the voltage between c-d with the point d as the reference potential), that is, the output voltage Vo. Fig. 4(F) shows the waveform of the voltage across the output diode D (the voltage between c-a with the point a as the reference potential) Vf (the forward voltage drop during the off period is ignored).

As shown in Fig. 4(E), since a large reverse bias voltage is applied to the output diode D during the ON period, it is necessary to ensure the withstand voltage. The magnitude of this reverse bias voltage is the sum of the secondary coil voltage (Vs) and the output voltage (Vo). Here, the secondary coil voltage (Vs) is (Ns/Np)·Vp. Ns/Np is the winding ratio of the transformer (T), and Vp is the primary coil voltage.

In the switching power supply constituting the AC/DC converter, the DC voltage after AC rectification on the primary side may become a high voltage of several hundred volts, and the reverse bias voltage applied to the output diode of the secondary side may also exceed 1,000 V. If a single output diode tries to secure the high voltage characteristic corresponding to this large reverse bias voltage, the device becomes expensive.

In addition, since a spike voltage superimposed on the reverse bias voltage is also applied to the output diode at the ON time at the start of the ON period, a withstand voltage characteristic in consideration of the spike voltage is required. Patent Document 1 proposes a secondary-side snubber circuit for suppressing a spike voltage generated in an output diode when on in a flyback-type switching power supply.

Japanese Patent Laid-Open No. 2010-88209 (Fig. 1, Fig. 2)

However, the snubber circuit on the secondary side presented in Patent Document 1 only suppresses the spike voltage at the time of ON, and the reverse bias voltage itself applied to the output diode throughout the ON period is not suppressed. Therefore, the output diode is still required to have a withstand voltage characteristic against a reverse bias voltage. Moreover, the snubber circuit of patent document 1 contains a coil, two diodes, and two capacitor|condensers, and there exists a problem of the enlargement of a component, and the cost increase by a snubber circuit.

In view of the above problems, it is an object of the present invention to use a diode having a low withstand voltage characteristic by appropriately distributing the reverse bias voltage applied to the conventional output diode to a plurality of diodes in a flyback type switching power supply. . In addition, in this case, the enlargement of the parts and the increase in the number of parts are avoided as much as possible.

In order to achieve the above object, the present invention provides the following configurations. In addition, the code|symbol in parentheses is a code|symbol in the drawing mentioned later, and is attached for reference.

A form of the present invention is a flyback type switching power supply having a transformer (T) and a switching element (Q) connected to the primary coil (Np) of the transformer (T),

first rectifying means (D1) and second rectifying means (D2) connected in series between one end (a) of the secondary coil (Ns) of the transformer and the first output terminal (3);

a third rectifying means (D3) connected between the connection point (b) of the first rectifying means (D1) and the second rectifying means (D2) and the other end (d) of the secondary coil (Ns);

and a smoothing capacitor (C) connected between the first output terminal (3) and the second output terminal (4), which is the other end (d) of the secondary coil (Ns);

The first rectifying means (D1) and the second rectifying means (D2) are reverse biased with respect to the potential generated at one end (a) of the secondary coil (Ns) during the on period of the switching element (Q). connected in the direction

The third rectifying means (D3) is connected in a forward bias direction with respect to the potential generated at the other end (d) of the secondary coil (Ns) during the ON period of the switching element (Q). .

- In the above aspect, it is preferable that the first rectifying means D1, the second rectifying means D2, and the third rectifying means D3 are each a diode.

In the flyback type switching power supply, the present invention is applied to a conventional output diode by adding second and third diodes between the secondary coil and the conventional output diode as a secondary side component in addition to the conventional output diode. It is possible to appropriately distribute the reverse bias voltage during the ON period. As a result, since the withstand voltage characteristic of each diode can be made low, a low-cost element can be used, and the cost of a switching power supply can be reduced.

1A is a circuit diagram schematically showing a configuration example of an embodiment of a switching power supply according to the present invention, and FIG.
FIG. 2 is a graph schematically showing a waveform of voltage or current of each element in an on period and an off period in the circuit shown in FIG. 1, ie, temporal change.
Fig. 3 is a diagram schematically showing the potential relationship between points in the on period and the off period in the circuit shown in Fig. 1;
Fig. 4(A) is a basic circuit of a conventional flyback system, and (B) to (F) are graphs schematically showing waveforms of voltage or current in each element of the circuit of (A).

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of the switching power supply by this invention is described in detail, referring drawings.

The switching power supply according to the present invention is an insulated type that converts power between a pair of input terminals and a pair of output terminals through a transformer, and has a flyback method as a basic configuration. A DC power is supplied between a pair of input terminals. The supplied DC power may be the output of any other DC power supply, or the output after rectification of the AC power supply or the output smoothed after rectification. Accordingly, the input DC voltage is unipolar except for the case of a constant voltage, but includes a case of a varying voltage. For example, it is a pulsating flow after alternating current rectification, a square wave, a triangular wave, etc. A load is connected to the pair of output terminals.

<Configuration of switching power supply>

1A is a circuit diagram schematically showing a configuration example of an embodiment of a switching power supply according to the present invention, and FIG. In this circuit, DC power is supplied between the input terminal (1) and the input terminal (2). That is, a direct voltage is applied. In addition, DC power is output between the output terminal 3 and the output terminal 4 . Hereinafter, the input voltage at which the input terminal 1 becomes a positive potential is applied to the input terminal 2 which is the reference potential on the input side, and the voltage at which the output terminal 3 becomes a positive potential with respect to the output terminal 4 which is the reference potential on the output side is output A case where it becomes an example will be described.

This circuit has a transformer T including a primary coil Np and a secondary coil Ns. The winding start terminal of each coil is indicated by a black circle (the black circle indicates the polarity of the coil). Regarding the coil, when "one end" and "the other end" are referred to, both the case of "winding start terminal" and "winding end terminal" and the case of "winding end terminal" and "winding start terminal" are included. .

Since the switching power supply of the present invention has a flyback circuit as a basic configuration, the transformer T has a gap formed in the core.

One end (winding start terminal in this example) of the primary coil Np is connected to the input terminal 1 . The drain of the switching element Q, which is an N-channel FET, is connected to the other end of the primary coil Np (the winding end terminal in this example), and the source is connected to the input terminal 2 . A pulse voltage having a predetermined switching frequency and duty ratio is input as a control voltage Vg to the gate, which is the control terminal of the switching element Q. In this case, when the control voltage Vg is positive with respect to the source (input terminal 2) potential, the switching element Q is turned on, and the current path between the primary coil Np and the input terminal 2 conducts. . When the control voltage Vg is zero, the switching element Q is turned off, and the current between the primary coil Np and the input terminal 2 is cut off.

As the switching element Q, other than the FET, for example, a switching element such as an IGBT or a bipolar transistor may be used.

A first diode D1 and a second diode D2 are connected in series between one end a (the winding end terminal in this example) of the secondary coil Ns and the output terminal 3 . The directions of the first diode D1 and the second diode D2 are connected in a direction that becomes a reverse bias when a negative potential is generated at the point a, which is one end of the secondary coil Ns. That is, in the first and second diodes D1 and D2, the anode is connected to one end (a) side of the secondary coil Ns, and the cathode is connected to the output terminal 3 side. One end a of the secondary coil Ns becomes a negative potential with respect to the point d which is the other end of the secondary coil Ns during the ON period of the switching element Q, and becomes a positive potential during the OFF period. During the off period, the first and second diodes D1 and D2 are forward biased.

In addition, a third diode D3 is connected between the point b, which is the connection point between the first diode D1 and the second diode D2, and the point d, which is the other end of the secondary coil Ns. The direction of the third diode D3 is connected in a direction that becomes a forward bias when a positive potential is generated at the other end d of the secondary coil Ns. That is, in the third diode D3, the anode is connected to the other end d of the secondary coil Ns, and the cathode is connected to the connection point b. The other end d of the secondary coil Ns becomes a positive potential with respect to one end a of the secondary coil Ns during the ON period of the switching element, and becomes a negative potential during the OFF period. During the off period, the third diode D3 is reverse biased.

Here, the "direction in forward bias" merely indicates the polarity of the connection, and does not necessarily mean that forward current flows when forward bias is established.

Each of the diodes D1, D2, and D3 is a rectifying means that enters a conduction state when a forward bias voltage is applied, and becomes a cutoff state when a reverse bias voltage is applied. It is assumed that the rectifying means includes a rectifying device or a rectifying circuit equivalent to a diode in addition to the diode which is a rectifying element.

A smoothing capacitor C is connected between the output terminal 3 and the output terminal 4 . A load is connected between these output terminals 3 and 4 . The point d, which is the other end of the secondary coil Ns, is common to the output end 4 and has the same potential. The d-point potential is a reference potential on the secondary side of the transformer T.

As another configuration example of the switching power supply of the present invention, the input terminal 1 may apply an input voltage that becomes a negative potential to the input terminal 2 serving as a reference potential on the primary side. In this case, the polarity of each diode on the secondary side and the polarity of the smoothing capacitor are reversed.

<Switching power supply operation>

FIG. 2 is a graph schematically showing a waveform of voltage or current of each element in an on period and an off period in the circuit shown in FIG. 1, ie, temporal change. In Fig. 2, the horizontal axis of the voltage graph indicates a point serving as a reference for the potential of the voltage.

FIG. 3 is a diagram schematically showing an example of a potential state in an ON period and an OFF period at each point of the circuit shown in FIG. 1 . 3 , a black arrow indicates a reverse bias voltage of the diode, and a white arrow indicates a forward bias voltage of the diode. Hereinafter, the operation will be described with reference to FIG. 1 as well.

・Operation during the ON period

Fig. 2A shows the control voltage Vg of the switching element Q connected to the primary coil Np. When the control signal Vg is turned on, the current path of the switching element Q conducts and a DC voltage is applied to one end of the primary coil Np, so that one end of the primary coil Np has a positive potential and the other end is negative. be comforted As a result, an excitation current flows through the primary coil Np, and magnetic energy is accumulated in the transformer T.

Fig. 2(B) shows a change in the voltage Vs of the secondary coil Ns. That is, the change in the potential of the point a, which is one end of the secondary coil Ns, where the potential of the point d, which is the other end of the secondary coil Ns, is used as the reference potential is shown. When the switching element Q is turned on, which is the start of the on period, the potential of the point a of the secondary coil Ns is changed from a positive potential to a negative potential. The voltage Vs of the secondary coil Ns in the ON period is expressed by the following equation.

Vs = (Ns/Np) Vp

(Ns/Np: winding ratio of transformer (T), Vp: voltage of primary coil (Np))

2(C) shows the change in the output voltage Vo between the output terminal 3 and the output terminal 4 . That is, the change in the potential of the point c, which is the positive end of the smoothing capacitor C, with the potential of the point d as the reference potential is shown. The potential at point c is always a positive potential. Accordingly, during the on period, the first diode D1 and the second diode D2 are reverse biased, and no current flows. Therefore, no current flows also in the third diode D3, whose cathode is connected to the connection point b of these diodes D1 and D2.

Fig. 2(D) shows a change in the current Is flowing through the secondary coil Ns. During the on period, the current Is is zero. In the on period, the electric charge charged in the smoothing capacitor C is supplied as a current to the load, and therefore the output voltage Vo decreases in the on period as shown in Fig. 2(C).

The total reverse bias voltage applied to the first diode D1 and the second diode D2 during the on period is expressed by the following equation.

Vf1+Vf2=Vs+Vo

(Vf1: reverse bias voltage applied to the first diode D1)

(Vf2: reverse bias voltage applied to the second diode D2)

Conventionally, a reverse bias voltage of Vf1 + Vf2 was applied to one output diode. However, in the present invention, since the first diode D1 and the second diode D2 are distributed and applied, each diode has lower withstand voltage characteristics compared to the prior art. can be done by

The cathode of the third diode D3 is connected to the connection point b of the first diode D1 and the second diode D2, and the anode thereof is connected to the reference potential d point. The potential does not drop below the d-point potential. The d-point potential is a reference potential on the secondary side. Accordingly, when the potential of the connection point b becomes almost equal to the potential of the d point, the output voltage Vo is applied to the first diode D1 and the secondary coil voltage Vs is applied to the second diode D2. becomes The output voltage Vo and the secondary coil voltage Vs are usually different values.

2(E) and 2(F) show the voltages applied to the first diode D1 and the second diode D2 during the ON period, respectively. As shown in Fig. 2(G), the voltage applied to the third diode D3 during the ON period is almost zero.

3(A) and 3(B) schematically show examples of potential states at points a to d on the secondary side of the transformer at the on time (at the start of the on period) and at the end of the on period, respectively.

Immediately after turning on as shown in Fig. 3A, the potential at point a, which is one end of the secondary coil Ns, inverts and drops from the positive potential at the end of the OFF period to the negative potential with respect to the reference potential at point d. The potential of the point c at the output stage 3 is high because the smoothing capacitor C is charged during the off period. At this time, the maximum reverse bias voltage (Vs+Vo) is applied to the first and second diodes D1 and D2. Vf1 is applied to the first diode D1 and Vf2 is applied to the second diode D2.

At the end of the ON period shown in Fig. 3B, the smoothing capacitor C is discharged and the potential at the point c at the output terminal 3 is lowered. Since the potential of the point b does not become lower than the potential of the point d by the third diode D3, the reverse bias voltage Vs+Vo is appropriately distributed between the first diode D1 and the second diode D2 without large bias. do.

・Operation during off period

When the control signal Vg of the switching element Q is turned off, the current path of the switching element Q is cut off and the current flowing through the primary coil Np is lost. Thereby, a counter electromotive force is generated in the primary coil Np and the secondary coil Ns.

As shown in Fig. 2(B) , the potential of the point a, which is one end of the secondary coil Ns, becomes a positive potential with respect to the potential of the point d. The voltage Vs of the secondary coil Ns in the OFF period is expressed by the following equation.

Vs=Vo-(Vf1+Vf2)

The first diode D1 and the second diode D2 are forward biased to conduct. As shown in Fig. 2(D), a current Is flows through the secondary coil Ns and is supplied to the load. Further, as shown in Fig. 2(C), the smoothing capacitor C is also charged by a part of the current Is.

Since the forward voltages of Vf1 and Vf2 of the first diode D1 and the second diode D2 are about -1 V, as shown in Figs. 2(E) and 2(F), the first and second diodes in the OFF period are shown in Figs. The voltage across (D1, D2) is usually negligible.

In the third diode D3, since the potential of the point b becomes a positive potential with respect to the potential of the point d, a reverse bias voltage is applied as shown in Fig. 2(G).

3(C) and 3(D) schematically show examples of potential states at points a to d on the secondary side of the transformer at the time of OFF (at the start of the OFF period) and at the end of the OFF period, respectively.

Immediately after the turn-off shown in Fig. 3C, the potential of the point a, which is one end of the secondary coil Ns, inverts and rises from the negative potential at the end of the on period to the positive potential with respect to the d-point potential. The potential of the point c at the output stage 3 becomes the lowest potential due to the discharge of the smoothing capacitor C during the ON period. At this time, the first diode D1 and the second diode D2 conduct, so that the voltage across each end becomes only the forward voltage. In Fig. 3C, each forward voltage is exaggerated. The third diode D3 is blocked because the reverse bias voltage is applied.

At the end of the off period shown in Fig. 3D, the smoothing capacitor C is charged and the potential of the point c at the output terminal 3 is rising.

In the off period, a reverse bias voltage is applied to the third diode D3, but as shown in FIGS. 3C and 3D, the withstand voltage characteristics required for the third diode D3 are the first and It can be seen that the second diodes D1 and D2 are as good as those of the second diodes D1 and D2.

The switching power supply of the present invention has a flyback method as a basic configuration, and by adding two diodes (D2, D3) in addition to the conventional output diode (D1), the reverse bias voltage applied during the on period is applied to the diode. (D1, D2) can be appropriately dispersed.

In particular, when a withstand voltage of 1,000 V or more is required, it is cheaper to use three diodes with low withstand voltage than to use one diode with high withstand voltage.

In addition, the effect of dispersing the spike voltage generated in the secondary coil Ns at the moment of turning on is also obtained in the two diodes D1 and D2.

T : transformer Np : primary coil
Ns : Secondary coil 1, 2 : Input terminal
3, 4: Output stage Q: Switching element
D1, D2, D3: Diode C: Smoothing Capacitor

Claims (2)

In the flyback type switching power supply having a transformer (T) and a switching element (Q) connected to the primary coil (Np) of the transformer (T),
first rectifying means (D1) and second rectifying means (D2) connected in series between one end (a) of the secondary coil (Ns) of the transformer and the first output terminal (3);
a third rectifying means (D3) connected between the connection point (b) of the first rectifying means (D1) and the second rectifying means (D2) and the other end (d) of the secondary coil (Ns);
and a smoothing capacitor (C) connected between the first output terminal (3) and the second output terminal (4), which is the other end (d) of the secondary coil (Ns);
The first rectifying means (D1) and the second rectifying means (D2) are reverse biased with respect to the potential generated at one end (a) of the secondary coil (Ns) during the on period of the switching element (Q). connected in the direction
The third rectifying means (D3) is connected in a direction that is forward biased with respect to the potential generated at the other end (d) of the secondary coil (Ns) during the ON period of the switching element (Q),
In the ON period of the switching element (Q), current does not flow in any of the first rectifying means (D1), the second rectifying means (D2), and the third rectifying means (D3) switching power. The method of claim 1,
The switching power supply, characterized in that each of the first rectifying means (D1), the second rectifying means (D2) and the third rectifying means (D3) is a diode.

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