JP2020137320A - Step-down converter - Google Patents

Abstract

To provide a non-insulated step-down converter of a tapped inductor system, in which a magnetic saturation is hardly generated, and a residual flux hardly remains.SOLUTION: A step-down converter is provided in which an input voltage is input to between a reference potential end and an input end, and an output voltage is input to between the reference potential end and an output end, the step-down converter includes: a first switching element in which one end is connected to the input end of the step-down converter; a second switching element in which one end is connected to the input end of step-down converter; and a transformer arranged between the other end of the first switching element and the other end of the second switching element. The transformer includes: a central tap connected to the output end; and a first coil and a second coil connected in the central tap. The first coil has a first intermediate tap, and the second coil has a second intermediate tap, and a first rectification element connected to the first intermediate tap and the reference potential end and a second rectification element connected to the second intermediate tap and the reference potential end.SELECTED DRAWING: Figure 1

Description

The present invention relates to a DC / DC converter, particularly a non-isolated buck converter.

In a non-isolated buck converter, a tapped inductor type converter that utilizes a trans-acting action by providing an intermediate tap in the reactor and connecting a commutation diode to the intermediate tap is known (Patent Documents 1, 2, etc.). ). By adopting the tapped inductor method, it is possible to set the duty ratio in an appropriate range with respect to a desired input / output voltage ratio and / or output current range. As one of the effects, it is possible to avoid the operation at an extremely small duty ratio which requires steep switching of the switching element. Further, by adopting the tapped inductor method in the step-down converter in which a large current is expected, the amount of current flowing through the switching element can be reduced and the load on the switching element can be reduced.

JP-A-2008-248974 Patent No. 6380623

In a normal tapped inductor type converter, the inductance of the transformer is increased so that a large current can be output at the intermediate tap. However, there are problems that magnetic saturation is likely to occur and the counter electromotive voltage due to the residual magnetic flux is also increased.

An object of the present invention is to provide a tapped inductor type non-insulated step-down converter in which magnetic saturation is unlikely to occur and residual magnetic flux is unlikely to remain.

In order to achieve the above object, the present invention provides the following configurations.
An aspect of the present invention is in a buck converter in which an input voltage is input between a reference potential end and an input end and an output voltage is output between the reference potential end and the output end.
A first switching element whose one end is connected to the input end,
A second switching element whose one end is connected to the input end,
It has a transformer arranged between the other end of the first switching element and the other end of the second switching element.
The transformer has a central tap connected to the output end, a first coil and a second coil connected by the central tap, and the first coil includes a first intermediate tap. And the second coil is provided with a second intermediate tap and
A first rectifying element connected between the first intermediate tap and the reference potential end, and a second rectifying element connected between the second intermediate tap and the reference potential end. It is characterized by having.
-In the above embodiment, it is preferable that the first coil and the second coil are loosely coupled to each other and are symmetrically configured with respect to the central tap.
-In the above embodiment, the first coil is composed of a first partial coil and a second partial coil that are connected by the first intermediate tap and are tightly coupled to each other.
It is preferable that the second coil is composed of a third partial coil and a fourth partial coil that are connected by the second intermediate tap and are tightly coupled to each other.
-In the above embodiment, the turns ratio of the first partial coil to the second partial coil is the same as the turns ratio of the third partial coil to the fourth partial coil. It is suitable.
In the above aspect, it is preferable that the first switching element and the second switching element are on / off controlled by control signals having the same duty ratio but different phases by 180 °.

INDUSTRIAL APPLICABILITY According to the present invention, a tapped inductor type non-insulated buck converter that is less likely to cause magnetic saturation and less likely to leave residual magnetic flux is realized.

FIG. 1 is a diagram schematically showing a circuit example of a tapped inductor type step-down converter of the present invention. 2 (a) and 2 (b) are diagrams schematically showing the current flows of modes I and II in the circuit of FIG. 2 (a) and 2 (b) are diagrams schematically showing the current flow of mode III and mode IV in the circuit of FIG. FIG. 4 is a timing diagram schematically showing a waveform example of the current shown in FIGS. 2 and 3.

Hereinafter, embodiments of a tapped inductor type non-insulated buck converter according to the present invention will be described with reference to the drawings. In the following, for example, a buck converter having a relatively large input / output voltage ratio that steps down 300 V DC to 12 V DC will be described. However, the scope of application of the present invention is not limited to such cases.

(1) Circuit Configuration FIG. 1 is a diagram schematically showing a circuit example of the tapped inductor type step-down converter of the present invention.

The buck converter of FIG. 1 is a power conversion circuit that converts DC power. A DC input voltage is input between the input terminal 1 and the reference potential end 3, and a DC output voltage is output between the output terminal 2 and the reference potential end 3. The reference potential end 3 is common to the input side and the output side.

In this example, the reference potential end 3 has a high potential with respect to the input end 1 and the output end 2. Assuming that the reference potential end 3 is the ground potential, both the input end 1 and the output end 2 have negative potentials. The input voltage is high voltage and the output voltage is low voltage.

As another example, a circuit configuration in which the input end 1 and the output end 2 have positive potentials can be used.

A load 5 is connected between the output end 2 and the reference potential end 3. An output voltage is applied to the load 5, and an output current flows.

A first switching element S1 and a second switching element S2 are provided. Each of the switching elements S1 and S2 has a current path and a control end, and the current path is conducted or cut off by a control signal applied to the control end. One end of the current path of each of the switching elements S1 and S2 is connected to the input end 1, respectively. A transformer TR is arranged between the other end of the current path of the switching element S1 and the other end of the current path of the switching element S2.

The switching elements S1 and S2 are n-channel MOSFETs as an example here. The source of each FET is connected to the input end 1. One end of the coil of the transformer TR is connected to the drain of one FET, and the other end of the coil of the transformer TR is connected to the drain of the other FET. A bipolar transistor or an IGBT can be used instead of the FET.

For example, control voltages v1 and v2 by PWM signals are applied to the gate of each FET. The PWM signal has a predetermined duty ratio. The PWM signal is generated by a control unit (not shown) including, for example, a PWM IC or the like. The control unit detects, for example, an input voltage, an output voltage and / or an output current, and controls the duty ratio of the PWM signal according to them.

The transformer TR has a coil wound around a magnetic core. Three taps are provided in the middle of the coil.

The center tap Tc is connected to the output end 2. The coil of the transformer TR includes a first coil N1 and a second coil N2 connected by a central tap Tc. The first coil N1 and the second coil N2 are configured symmetrically with respect to the central tap Tc, and more specifically, mirror image symmetrically. Preferably, the first coil N1 and the second coil N2 are loosely coupled.

Further, the first coil N1 has an intermediate tap T1, and the first partial coil n1 and the second partial coil n2 are connected by the intermediate tap T1. Preferably, the partial coil n1 and the partial coil n2 are tightly coupled. Similarly, the second coil N1 has an intermediate tap T2, and the third partial coil n3 and the fourth partial coil n4 are connected by the intermediate tap T2. Preferably, the partial coil n3 and the partial coil n4 are tightly coupled.

To make the two coils loosely coupled to each other, for example, by arranging the two coils apart from each other, all the magnetic flux generated in one coil does not pass through the other coil, and a part of the magnetic flux becomes leakage flux. To. Further, in order to tightly couple the two coils to each other, for example, by arranging the two coils in close contact with each other, all the magnetic flux generated in one coil passes through the other coil.

Since the first coil N1 and the second coil N2 are symmetrical with respect to the central tap Tc, the first coil N1 and the second coil N2 have the same number of turns, and the turns ratio of the partial coil n1 and the partial coil n2 , The turns ratio of the partial coil n3 and the partial coil n4 is the same. That is, the partial coil n1 and the partial coil n3 have the same number of turns, and the partial coil n2 and the partial coil n4 have the same number of turns.

Further, the first rectifying element D1 is connected between the intermediate tap T1 and the reference potential end 3, and the second rectifying element D2 is connected between the intermediate tap T2 and the reference potential end 3. The rectifying elements D1 and D2 are diodes here. In the diode D1, the anode is connected to the intermediate tap T1 and the cathode is connected to the reference potential end 3. In the diode D2, the anode is connected to the intermediate tap T2 and the cathode is connected to the reference potential end 3.

The rectifying elements D1 and D2 are not limited to diodes, and may be replaced with elements or circuits having the same function.

Further, a smoothing capacitor 1 is connected between the input end 1 and the reference potential end 3, and a smoothing capacitor 2 is connected between the output end 2 and the reference potential end 3.

(2) Circuit operation The operation of the circuit shown in FIG. 1 will be described with reference to FIGS. 2 to 4. 2 (a) and 2 (b) are diagrams schematically showing the current flows of modes I and II in the circuit of FIG. 3 (a) and 3 (b) are diagrams schematically showing the current flow of mode III and mode IV in the circuit of FIG. FIG. 4 is a timing diagram schematically showing an example of the waveform of the current shown in FIGS. 2 and 3.

FIGS. 4A and 4B schematically show waveforms of one cycle of control voltages v1 and v2 of switching elements S1 and S2. The control voltages v1 and v2 are PWM signals having the same duty ratio and having phases different from each other by 180 °. The switching elements S1 and S2 are provided with a dead time so that they are not turned on at the same time. Therefore, the duty ratio of the PWM signal is less than 50%. Therefore, the operation of the circuit of FIG. 1 is roughly divided into the four modes I, II, III, and IV shown in FIG. Mode II and mode IV are dead times when both the switching elements S1 and S1 are turned off.

<Operation of mode I>
FIG. 2A shows the operation when the switching element S1 is turned from off to on and the switching element S2 is off in the circuit of FIG. The current flowing through the circuit is indicated by a solid line with an arrow (same for other figures).

When the switching element S1 is turned on, an input voltage is applied, so that the current i1 flows in the following path (see FIGS. 4C and 4D). The diode D1 has a reverse bias.
・ Reference potential end 3 → load 5 → output end 2 → center tap Tc → first coil N1 → input end 1

When the current i1 flows through the first coil N1, an electromotive voltage due to mutual induction is generated in the second coil N2, and the diode D2 becomes a forward bias. As a result, the current i2 flows in the following path (see FIG. 4 (f)).
-Reference potential end 3-> load 5-> output end 2-> center tap Tc-> partial coil n4 of second coil N2-> diode D2-> reference potential end 3

The electromotive voltage generated in the partial coil n4 is proportional to the turns ratio of the first coil N1 and the partial coil n4. As described above, the first coil N1 and the second coil N2 have the same number of turns. Also, the turns ratio of the partial coils n1 and n2 and the turns ratio of the partial coils n3 and n4 are the same. Assuming that each code represents the number of turns, N1 = N2 and n1 / n2 = n3 / n4.

For example, when the turns ratio (n1 / n2) of the partial coils n1 and n2 in the first coil N1 is 9, the turns ratio (n3 / n4) of the partial coils n3 and n4 in the second coil N2 is also 9. At this time, the turns ratio (N1 / n4) of the first coil N1 and the partial coil n4 is 10. When the voltage applied to the first coil N1 is 400V, the electromotive voltage generated in the partial coil n4 is 40V. Since the current i1 and the current i2 flow in inverse proportion to the turns ratio, when the current i1 is 1A, the current i2 is 10A.

The current i1 corresponds to the primary current in the so-called forward type switching power supply, and the current i2 corresponds to the forward current induced by the primary current. An output current that is a combination of the current i1 and the current i2 is supplied to the load 5. The current i2, which is a part of the output current, flows through the partial coil n4 of the second coil N2, so that the current i1 flowing through the switching element S1 can be reduced.

The position of the intermediate tap T2 in the second coil N2 is set by the circuit design. By changing the position of the intermediate tap T2, the distribution of the current i1 and the current i2 can be changed. As a result, the amount of current flowing through the switching element S1 can be set. By reducing the amount of current of the switching element S1, the on-period of the control signal can be extended, and more stable switching operation can be performed. Moreover, the maximum output current can be set larger. Further, the output voltage setting can be changed by changing the position of the intermediate tap T2.

Since the configurations of the first coil N1 and the second coil N2 are symmetrical with respect to the central tap Tc, the intermediate tap T1 and the intermediate tap T2 are also in symmetrical positions. Therefore, the positions of the intermediate tap T1 and the intermediate tap T2 are set at the same time.

Magnetic energy is stored in the transformer TR when the current i1 flows, but it is difficult for magnetic energy to be stored in the core due to the flow of the current i2 corresponding to the forward current.

Further, since the symmetrical first coil N1 and the second coil N2 are preferably loosely coupled, the current i2 does not flow in a rush and does not become an excessive current.

<Operation of mode II>
FIG. 2B shows the operation when the mode I ends in the circuit of FIG. 1 and the switching element S1 is turned from on to off. The switching element S2 remains off.

When the switching element S1 is turned from on to off, a counter electromotive voltage is generated in the transformer TR. The switching element S2 remains off. Due to the counter electromotive voltage, the diode D1 becomes forward biased and the diode D2 becomes reverse biased. As a result, the current i3 flows in the following path (see FIG. 4D).
-Reference potential end 3-> load 5-> center tap Tc-> partial coil n2 of the first coil N1-> diode D1-> reference potential end 3

The current i3 corresponds to the flyback current of the transformer TR. The current i3 releases the magnetic energy stored in the transformer TR in mode I.

<Operation of mode III and mode IV>
In mode III and mode IV, operations symmetrical to those of mode I and mode II described above are performed, respectively.

As shown in FIG. 3A, in mode III, the switching element S2 is turned from off to on. The switching element S1 remains off. In this case, the diode D2 becomes a forward bias due to the input voltage, and the current i4 flows in the following path (see FIGS. 4 (e) and 4 (f)).
・ Reference potential end 3 → load 5 → center tap Tc → second coil N2 → switching element S2 → input end 1

When the current i4 flows, an electromotive voltage due to mutual induction is generated, the diode D1 becomes a forward bias, and the current i5 flows in the following path (see FIG. 4D).
・ Reference potential end 3 → load 5 → center tap Tc → partial coil n2 of the first coil N1 → diode D1 → reference potential end 3

Mode III currents i4 and i5 flow symmetrically with mode I currents i1 and i2, respectively.

As shown in FIG. 3B, in mode IV, the switching element S2 is turned from on to off. The switching element S1 remains off. In this case, a counter electromotive voltage is generated, the diode D2 becomes a forward bias, and the current i6 flows in the following path (FIG. 4 (f)).
・ Reference potential end 3 → load 5 → center tap Tc → partial coil n4 of the second coil N2 → diode D2 → reference potential end 3

The mode IV current i6 flows symmetrically with the mode II current i3.

The characteristics of the operation described for modes I and II described above also apply to modes III and IV as they are. By repeating modes I and II and modes III and IV, input voltages are alternately applied to the transformer TR with polarities opposite to each other. As a result, the transformer TR is less likely to be magnetically saturated. This also makes it possible to realize a circuit in which a large current can easily flow.

It is possible to adjust the output current by changing the duty ratio of the control signals of the switching elements S1 and S2, thereby adjusting the amount of power transmitted from the input side to the output side.

The tapped inductor type step-down converter of the present invention described above is not limited to the illustrated configuration example, and can be variously modified within a range in line with the gist of the present invention.

1 Input end 2 Output end 3 Reference potential end 5 Load S1, S2 Switching element (MOSFET)
TR transformer N1 1st coil n1 1st partial coil n2 2nd partial coil N2 2nd coil n3 3rd partial coil n4 4th partial coil D1, D2 Rectifying element (diode)
C1, C2 smoothing capacitor

Claims (5)

In a buck converter in which an input voltage is input between a reference potential end and an input end and an output voltage is output between the reference potential end and the output end.
A first switching element whose one end is connected to the input end,
A second switching element whose one end is connected to the input end,
It has a transformer arranged between the other end of the first switching element and the other end of the second switching element.
The transformer has a central tap connected to the output end, a first coil and a second coil connected by the central tap, and the first coil includes a first intermediate tap. And the second coil is provided with a second intermediate tap and
A first rectifying element connected between the first intermediate tap and the reference potential end, and a second rectifying element connected between the second intermediate tap and the reference potential end. A buck converter characterized by having. The buck converter according to claim 1, wherein the first coil and the second coil are loosely coupled to each other and are configured symmetrically with respect to the central tap. The first coil comprises a first partial coil and a second partial coil that are connected by the first intermediate tap and are tightly coupled to each other.
The step-down according to claim 1 or 2, wherein the second coil includes a third partial coil and a fourth partial coil that are connected by the second intermediate tap and are tightly coupled to each other. converter. Claim 1 is characterized in that the turns ratio of the first partial coil to the second partial coil is the same as the turns ratio of the third partial coil to the fourth partial coil. The step-down converter according to any one of. The buck converter according to any one of claims 1 to 4, wherein the first switching element and the second switching element are on / off controlled by control signals having the same duty ratio and 180 ° different phases. ..

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