JP6845672B2 - Switching power supply - Google Patents

Description

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

An isolated switching power supply that extracts a desired DC power from a secondary coil by turning the DC power input to the primary coil of a transformer on and off by a switching element is well known. Flyback schemes for isolated switching power supplies are also well known.

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

FIG. 4B shows a waveform of the control voltage Vg of the switching element Q. In the flyback method, an exciting current flows through the primary coil Np and magnetic energy is stored in the transformer T during the ON period of the switching element Q. No current flows through the secondary coil Ns because the output diode D has a reverse bias. 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, a load current flows in the secondary coil Ns so as to release the stored magnetic energy, and the output diode It is output through D.

FIG. 4C shows the waveform of the current Is flowing through the secondary coil Ns. FIG. 4D shows a waveform of voltage across the secondary coil Ns (voltage between a and d with point d as a reference potential) Vs. FIG. 4 (E) shows the waveform of the voltage across the capacitor C (the voltage between cd 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 ca 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 secure a 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 turns ratio of the transformer T, and Vp is the primary coil voltage.

In the switching power supply that constitutes an AC / DC converter, the DC voltage after AC rectification on the primary side may be as high as several hundred volts, and the reverse bias voltage applied to the output diode on the secondary side also exceeds 1000V. Sometimes. If one output diode is used to secure high withstand voltage characteristics corresponding to this large reverse bias voltage, the cost of the element becomes high.

Further, at the time of ON, which is the start of the ON period, the spike voltage superimposed on the reverse bias voltage is also applied to the output diode, so that the withstand voltage characteristic in consideration of the spike voltage is required. Patent Document 1 presents a secondary side snubber circuit for suppressing a spike voltage generated in an output diode when it is turned on in a flyback type switching power supply.

Japanese Unexamined Patent Publication No. 2010-88209 (FIGS. 1 and 2)

However, the secondary side snubber circuit presented in Patent Document 1 only suppresses the spike voltage at the time of turning on, and does not suppress the reverse bias voltage itself applied to the output diode over the entire turning on period. Therefore, the output diode is still required to have a withstand voltage characteristic against a reverse bias voltage. Further, the snubber circuit of Patent Document 1 includes a coil, two diodes, and two capacitors, and has problems of increasing the size of parts and increasing the cost due to the snubber circuit.

In view of the above problems, an object of the present invention is to use a diode having a low withstand voltage characteristic in a flyback type switching power supply by appropriately dispersing the reverse bias voltage applied to the conventional output diode among a plurality of diodes. Is to be. In addition, in that case, it is necessary to avoid an increase in the size of parts and an increase in the number of parts as much as possible.

In order to achieve the above object, the present invention provides the following configurations. The reference numerals in parentheses are the reference numerals in the drawings described later and are provided for reference.

An aspect 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).
The first rectifying means (D1) and the second rectifying means (D2) connected in series between one end (a) of the secondary coil (Ns) of the transformer and the first output end (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). )When,
It has a smoothing capacitor (C) connected between the first output end (3) and the second output end (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 orientation,
It said third rectifying means (D3) is connected in a direction to become forward biased with respect to the potential occurring at the other end (d) of the secondary coil (Ns) to the ON period of the switching element (Q), and ,
During the ON period of the switching element (Q), no current flows through any of the first rectifying means (D1), the second rectifying means (D2), and the third rectifying means (D3). And.
-In the above embodiment, it is preferable that the first rectifying means (D1), the second rectifying means (D2), and the third rectifying means (D3) are diodes, respectively.

According to the present invention, in a flyback type switching power supply, a second and a third diode are added between the flyback type switching power supply and the secondary coil in addition to the conventional output diode as a component on the secondary side. The reverse bias voltage during the on-period applied to the diode can be appropriately dispersed. As a result, the withstand voltage characteristic of each diode can be lowered, so that a low-cost element can be used and the cost of the switching power supply can be reduced.

FIG. 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. 1B is a circuit diagram in which each reference numeral for explanation is added to the circuit diagram of (A). is there. FIG. 2 is a graph schematically showing the waveform, that is, the temporal change of the voltage or current of each element of the on period and the off period in the circuit shown in FIG. FIG. 3 is a diagram schematically showing the potential relationship at each point of the on period and the off period in the circuit shown in FIG. FIG. 4A is a conventional flyback type basic circuit, and FIGS. 4B to 4F are graphs schematically showing voltage or current waveforms in each element of the circuit of (A).

Hereinafter, embodiments of the switching power supply according to the present invention will be described in detail with reference to the drawings.

The switching power supply according to the present invention is an isolated type that performs power conversion between a pair of input ends and a pair of output ends via a transformer, and has a flyback method as a basic configuration. DC power is supplied between the pair of input ends. The DC power supplied may be the output of another arbitrary DC power supply, the output after rectification of the AC power supply, or the output smoothed after rectification. Therefore, the input DC voltage includes not only the case of a constant voltage but also the case of a unipolar but fluctuating voltage. For example, pulsating current after AC rectification, square wave, triangular wave, etc. A load is connected to the pair of output ends.

<Configuration of switching power supply>
FIG. 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. 1B is a circuit diagram in which each reference numeral for explanation is added to the circuit diagram of (A). is there. In this circuit, DC power is supplied between the input terminal 1 and the input terminal 2. That is, a DC voltage is applied. Further, DC power is output between the output end 3 and the output end 4. In the following, an input voltage at which the input end 1 is a positive potential is applied to the input end 2 which is a reference potential on the input side, and the output end 3 is a positive potential with respect to the output end 4 which is a reference potential on the output side. The case where the voltage is output will be described as an example.

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 (black circle indicates the polarity of the coil). The terms "one end" and "the other end" of the coil include both the case of referring to the "winding start terminal" and the "winding end terminal" and the case of referring to the "winding end terminal" and the "winding start terminal".

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

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

As the switching element Q, a switching element such as an IGBT or a bipolar transistor may be used in addition to the FET.

A first diode D1 and a second diode D2 are connected in series between one end a (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 causes a reverse bias when a negative potential is generated at 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 end 3 side. One end a of the secondary coil Ns has a negative potential with respect to point d, which is the other end of the secondary coil Ns, during the on period of the switching element Q, and has a positive potential during the off period. During the off period, the first and second diodes D1 and D2 are forward biased.

Further, 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 has a positive potential with respect to one end a of the secondary coil Ns during the on period of the switching element, and has a negative potential during the off period. During the off period, the third diode D3 is reverse biased.

Here, the "direction in which the forward bias occurs" merely indicates the polarity of the connection, and does not mean that the forward current always flows when the forward bias occurs.

The diodes D1, D2, and D3 are rectifying means that are in a conductive state when a forward bias voltage is applied and are in a cutoff state when a reverse bias voltage is applied. In addition to the diode which is a rectifying element, the rectifying means shall include a rectifying device or a rectifying circuit equivalent to the diode.

A smoothing capacitor C is connected between the output end 3 and the output end 4. A load is connected between these output terminals 3 and 4. The d point, 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 the reference potential on the secondary side of the transformer T.

As another configuration example of the switching power supply of the present invention, an input voltage at which the input end 1 becomes a negative potential may be applied to the input end 2 which is the reference potential on the primary side. In that case, the polarities of each diode on the secondary side and the smoothing capacitor are reversed.

<Operation of switching power supply>
FIG. 2 is a graph schematically showing the waveform, that is, the temporal change of the voltage or current of each element of the on period and the off period in the circuit shown in FIG. In FIG. 2, the horizontal axis of the voltage graph indicates a point that serves as a reference for the potential of the voltage.

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

-Operation during the on period FIG. 2 (A) 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 is conducted, a DC voltage is applied to one end of the primary coil Np, and one end of the primary coil Np becomes a positive potential and the other end becomes a negative potential. As a result, an exciting current flows through the primary coil Np, and magnetic energy is stored in the transformer T.

FIG. 2B shows a change in the voltage Vs of the secondary coil Ns. That is, the change in the a point potential at one end of the secondary coil Ns with the d point potential at the other end of the secondary coil Ns as the reference potential is shown. The a point potential of the secondary coil Ns changes from a positive potential to a negative potential when the switching element Q is turned on, which is the start of the on period. The voltage Vs of the secondary coil Ns during the on period is expressed by the following equation.
Vs = (Ns / Np) · Vp
(Ns / Np: Transformer T turns ratio, Vp: Primary coil Np voltage)

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

FIG. 2D shows a change in the current Is flowing through the secondary coil Ns. During the on period, the current Is is zero. During the on period, the electric charge charged in the smoothing capacitor C is supplied to the load as a current, so that the output voltage Vo decreases during 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, the reverse bias voltage of Vf1 + Vf2 is applied to one output diode, but in the present invention, since it is distributed and applied to the first diode D1 and the second diode D2, each diode is lower than the conventional one. It can have pressure resistance characteristics.

Since 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 of the connection point b is the d point. It does not drop below the potential. The d point potential is the reference potential on the secondary side. Therefore, when the potential of the connection point b becomes substantially the same as 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. The output voltage Vo and the secondary coil voltage Vs usually have different values.

FIGS. 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 time of turning on (at the beginning of the on period) and at the end of the on period, respectively.

Immediately after it is turned on as shown in FIG. 3 (A), the a point potential, which is one end of the secondary coil Ns, reverses and drops from the positive potential at the end of the off period to the negative potential with respect to the d point reference potential. The c-point potential of the output terminal 3 is the maximum potential when 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 c-point potential of the output terminal 3 is lowered. Since the b-point potential does not fall below the d-point potential due to the third diode D3, the reverse bias voltage Vs + Vo is appropriately dispersed in the first diode D1 and the second diode D2 without a large deviation.

-Operation during the 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 disappears. As a result, a counter electromotive force is generated in the primary coil Np and the secondary coil Ns.

As shown in FIG. 2B, the a point potential at one end of the secondary coil Ns becomes a positive potential with respect to the d point potential. The voltage Vs of the secondary coil Ns during the off period is expressed by the following equation.
Vs = Vo- (Vf1 + Vf2)

The first diode D1 and the second diode D2 have a forward bias and conduct with each other. As shown in FIG. 2D, a current Is flows through the secondary coil Ns and is supplied to the load. Further, as shown in FIG. 2C, the smoothing capacitor C is also charged by a part of the current Is.

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

Since the b-point potential of the third diode D3 is a positive potential with respect to the d-point potential, 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 it is turned off as shown in FIG. 3C, the a point potential at one end of the secondary coil Ns reversely rises from the negative potential at the end of the on period to the positive potential with respect to the d point potential. The c-point potential of the output terminal 3 is the lowest potential due to the smoothing capacitor C being discharged during the on period. At this time, the first diode D1 and the second diode D2 are conductive, and the voltage across each is only the forward voltage. In FIG. 3C, each forward voltage is exaggerated. The third diode D3 is cut off because a reverse bias voltage is applied.

At the end of the off period shown in FIG. 3D, the smoothing capacitor C is charged and the c-point potential of the output terminal 3 rises.

A reverse bias voltage is applied to the third diode D3 during the off period, but as shown in FIGS. 3C and 3D, the withstand voltage characteristic required for the third diode D3 is the first. It can be seen that the same degree as that of the 1st and 2nd diodes D1 and D2 is sufficient.

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

In particular, when a withstand voltage of 1000 V or more is required, it is possible to reduce the cost by using three diodes having a low withstand voltage characteristic rather than using one diode having a high withstand voltage characteristic.

In addition, the spike voltage generated in the secondary coil Ns at the moment of turning on can also be similarly dispersed in the two diodes D1 and D2.

T transformer Np primary coil Ns secondary coil 1, 2 input end 3, 4 output end Q switching element D1, D2, D3 diode C smoothing capacitor

Claims (2)

In 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).
The first rectifying means (D1) and the second rectifying means (D2) connected in series between one end (a) of the secondary coil (Ns) of the transformer and the first output end (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). )When,
It has a smoothing capacitor (C) connected between the first output end (3) and the second output end (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 orientation,
It said third rectifying means (D3) is connected in a direction to become forward biased with respect to the potential occurring at the other end (d) of the secondary coil (Ns) to the ON period of the switching element (Q), and ,
During the ON period of the switching element (Q), no current flows through any of the first rectifying means (D1), the second rectifying means (D2), and the third rectifying means (D3). Switching power supply. The switching power supply according to claim 1, wherein the first rectifying means (D1), the second rectifying means (D2), and the third rectifying means (D3) are diodes, respectively.

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