JP2019193474A - Tapped inductor type switching power supply - Google Patents

Abstract

To reduce the burden on a switching element in a tapped inductor type switching power supply, and can deal with a wide range of fluctuations in output voltage and output current quickly and stably.SOLUTION: A switching power supply includes a reactor N that has an intermediate tap T at the connection point between the end of a primary coil N1 and the start of a secondary coil N2, and in which the intermediate tap T is connected to the output terminal 2, a switching element Q that is on/off-controlled to conduct or block a current path between the input terminal 1 and the starting end of the primary coil N1, a first rectifying element D1 that conducts a current flowing from the ground end e to the starting end of the primary coil N1, a second rectifying element D2 that conducts a current flowing from the ground end e to the end of the secondary coil N2, a smoothing capacitor C connected between the output terminal 2 and the ground terminal e, and a switch element S capable of conducting or blocking a current path between the ground end e and the end of the secondary coil N2.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a switching power supply, and more particularly to a tapped inductor type switching power supply.

  FIG. 7A shows a basic circuit of a non-insulated step-down converter that is one of the switching power supplies. In the non-insulated converter, the output voltage Vo can be controlled by changing the duty ratio D (= Ton / T), which is the ratio of the ON time Ton of the switching element to one period T. In the case of a step-down converter, D = Vo / Vin. Therefore, as shown in FIG. 7B, when the input voltage Vin applied during the on-time Ton is averaged over one period T, an output voltage Vo (shown by a broken line) is obtained.

  When the ratio (Vo / Vin) of the output voltage Vo to the input voltage Vin becomes small (for example, 400V is converted to 12V), the duty ratio D needs to be extremely reduced. Such a situation requires a steep switching operation for a switching element such as a MOSFET, resulting in a problem that the switching loss increases or the control becomes difficult. In the range where the duty ratio is close to 1, the input voltage Vin is almost directly applied to the load, which is not preferable in the case of a load with a low voltage specification. The range of the duty ratio D suitable for control is, for example, about 0.2 to 0.8.

  In order to cope with this problem, a tapped inductor type step-down converter disclosed in Patent Document 1 is known. A tapped inductor type step-down converter can be stepped down to a smaller voltage with the same duty ratio by providing an intermediate tap in the reactor and connecting a commutation diode to the intermediate tap.

  In Patent Document 2, in a tapped inductor type switching power supply, the second diode is connected to the end of the secondary coil and the output is taken out from the intermediate tap, so that the transformer acts as a primary coil and a secondary coil. In addition, a large current output can be obtained during the off period. As a result, it is possible to cope with a steep current rise of the load.

Japanese Patent Laying-Open No. 2005-101406 (FIG. 5, 0026, 0027) JP 2007-68278 A

  The tapped inductor type switching power supply of Patent Document 2 is not sufficient to respond quickly and stably to a wide range of fluctuations in output current.

  In light of the above, the present invention aims to reduce the burden on the switching element in a tapped inductor type switching power supply and to be able to respond quickly and stably to a wide range of fluctuations in output current. .

In order to achieve the above object, the present invention provides the following configurations.
The aspect of the present invention is a tapped inductor type switching power supply that converts DC power applied between an input end and a ground end and outputs the output between the output end and the ground end.
A reactor including a primary coil and a secondary coil, and having an intermediate tap at a connection point between a terminal end of the primary coil and a starting end of the secondary coil, and the intermediate tap connected to the output end;
A switching element that is on / off controlled to conduct or block a current path between the input end and the starting end of the primary coil;
A first rectifying element that conducts a current flowing from the ground end to the starting end of the primary coil;
A second rectifying element for conducting a current flowing from the ground end to the end of the secondary coil;
A smoothing capacitor connected between the output terminal and the ground terminal;
And a switch element capable of conducting or interrupting a current path between the ground terminal and the terminal end of the secondary coil.
In the above aspect, when the duty ratio in the on / off control of the switching element is in a range larger than a predetermined value, the switch element is turned on, and in a range smaller than the predetermined value, the switch element is turned off. Is preferred.

  The tapped inductor type switching power supply according to the present invention can reduce the burden on the switching element and can quickly and stably cope with a wide range of fluctuations in the output current. As a result, for example, it is possible to cope with applications in which a large current several times that of a load in a state where a normal output current is flowing is required to flow quickly.

FIG. 1 is a diagram schematically showing a circuit example of a tapped inductor type switching power supply according to the present invention. FIG. 2A is a diagram illustrating a current during an on period in the on mode of the switch element, and FIG. 2B is a diagram illustrating a current during an off period in the circuit of FIG. FIG. 3 is a diagram showing voltage and current waveforms at different intermediate tap positions in the circuit of FIG. FIG. 4 is a graph showing the relationship between the duty ratio, output voltage, and output current at different intermediate tap positions in the circuit of FIG. 5A shows the current during the ON period in the OFF mode of the switch element in the circuit of FIG. 1, and FIG. 5B shows the current during the OFF period. FIG. 6 is a graph showing the relationship between the duty ratio and the output current in the case of FIG. FIG. 7A is a diagram schematically showing a basic step-down converter, and FIG. 7B is a diagram showing the relationship between the input / output voltage and the duty ratio.

  Embodiments of a tapped inductor type switching power supply according to the present invention will be described below with reference to the drawings. In the following, description will be made assuming a step-down converter having a relatively large input / output voltage ratio, for example, stepping down DC 400V to DC 12V. However, the application target of the present invention is not limited to such a case.

(1) Circuit Configuration FIG. 1 is a diagram schematically showing a circuit example of a tapped inductor type switching power supply according to the present invention.

  The switching power supply shown in FIG. 1 is a non-insulated step-down converter that converts DC power applied between an input terminal 1 and a ground terminal e and outputs it between the output terminal 2 and the ground terminal e. A DC input voltage Vin is applied between the input terminal 1 and the ground terminal e. In the current path between the input terminal 1 and the reactor N, a switching element Q is connected in series. The switching element Q is ON / OFF controlled so as to conduct or block the current flowing through the reactor N by the input voltage Vin.

  Here, the switching element Q is an n-channel MOSFET, and has a drain connected to the input terminal 1 and a source connected to one end of the reactor N. A PWM signal which is an on / off control voltage is applied to the gate. The PWM signal has a predetermined duty ratio. The PWM signal is generated by a control unit (illustrated picture) including a PWMIC and the like. The control unit detects the input voltage Vin, the output voltage Vo, and / or the output current Io, and controls the duty ratio of the PWM signal according to them. The Zener diode Z connected between the gate and source of the FET is for protection against high voltage.

  In the reactor N, a primary coil N1 and a secondary coil N2 are wound around a core with the same polarity, and are magnetically coupled to each other. The winding start end of each coil is indicated by a black circle. Here, the side close to the input end in each of the primary coil N1 and the secondary coil N2 is referred to as a start end, and the opposite side is referred to as a termination end. Reactor N performs a substantially transformer operation. The magnetic coupling between the primary coil N1 and the secondary coil N2 may be tightly coupled or loosely coupled with a leakage inductance.

  The starting end of the primary coil N1 is connected to the switching element Q. The end of the primary coil N1 and the start of the secondary coil N2 are connected, and an intermediate tap T is provided at the connection point. The turn ratio between the primary coil N1 and the secondary coil N2 is set as necessary, and the position of the intermediate tap T is determined based on the ratio. The voltage at the intermediate tap T is denoted by reference sign Vt.

  A load (not shown) is connected between the output terminal 2 and the ground terminal e. A DC output voltage Vo is applied to the load, and a DC output current Io flows. The intermediate tap T is connected to the output end 2. Further, a smoothing capacitor C is connected between the output terminal 2 and the ground terminal e.

  The voltage Vt at the intermediate tap T and the current output from the intermediate tap T fluctuate in time with the switching element Q being turned on and off, but are smoothed by the smoothing capacitor C and output as the DC voltage Vo and the DC current Io. Therefore, the time average of the voltage Vt at the intermediate tap T is the output voltage Vo. Further, the time average of the current output from the intermediate tap T is the output current Io.

  Further, the first rectifying element D1 is connected to the current path between the ground terminal e and the starting end of the primary coil N1. The first rectifying element D1 conducts a current flowing from the ground end e to the starting end of the primary coil N1, and interrupts the current in the opposite direction. Here, the first rectifying element D1 is a diode, and has an anode connected to the ground terminal e and a cathode connected to the starting end of the primary coil N1.

  Further, the second rectifying element D2 is connected to the current path between the ground terminal e and the end of the secondary coil N2. The second rectifying element D2 conducts the current flowing from the ground end e to the end of the secondary coil N2, and interrupts the current in the opposite direction. Here, the second rectifying element D2 is a diode, and has an anode connected to the ground end e and a cathode connected to the end of the secondary coil N2.

  In another example, elements other than diodes can be used as the first and second rectifying elements D1 and D2. For example, when a synchronous rectification method is employed, a switching element such as an FET can be used.

  Although not shown, a capacitor may be connected in parallel with the second rectifying element D2. By connecting a capacitor, when the second rectifying element D2 is for low frequency, it is possible to supply a current from the capacitor promptly by compensating for the response delay. When the second rectifying element D2 is for high frequency use, a capacitor is not necessary.

  Further, the switch element S is connected in series with the second rectifying element D2 in the current path between the ground terminal e and the end of the secondary coil N2. The switch element S can selectively conduct or cut off the current path between the ground end e and the end of the secondary coil N2. The switch element S may be any element that can automatically or manually open and close the current path. The switch element S may be a mechanical switch such as a relay, or may be a switching element such as an FET or a transistor. In the example of FIG. 1, the switch element S is inserted between the ground terminal e and the second rectifying element D2, but instead of the second rectifying element D2 and the end of the secondary coil N2. It may be inserted in between.

  The switch element S is for stopping the function of the secondary coil N2 as necessary. When the switch element S is closed, that is, when the switch element S is in the on mode, the circuit performs an operation in which the primary coil N1 and the secondary coil N2 of the reactor N cooperate. On the other hand, when the switch element S is opened, that is, when the switch element S is in the off mode, the circuit is operated only by the primary coil N1 of the reactor N. In that case, the configuration is the same as that of the basic step-down converter shown in FIG.

(2) Circuit Operation (2-1) Operation of Switch Element S in On Mode With reference to FIG. 2, the operation of switch element S in the on mode in the circuit of FIG. 1 will be described. 2A shows the current during the ON period of the switching element Q, and FIG. 2B shows the current during the OFF period of the switching element Q by solid lines with arrows.

<Operation of switching element Q during ON period>
When the switching element Q is turned on, the current i1 flows from the input terminal 1 to the primary coil N1 by the input voltage Vin, and is output from the intermediate tap T to the output terminal 2. Since the starting end of the primary coil N1 has a positive potential, the diode D1 is reverse-biased and no current flows. The primary coil N1 is excited by the current i1 to accumulate magnetic energy.

  On the other hand, as a result of the current i1 flowing through the primary coil N1, an electromotive force is generated by mutual induction in the magnetically coupled secondary coil N2. Since this electromotive force is in the direction in which the end of the secondary coil N2 becomes a negative potential, the diode D2 is forward biased and the current i2 flows. The current i2 flows through the secondary coil N2, leaves the intermediate tap T, and is output to the output terminal 2. The current i2 can be said to be a forward current due to the mutual induction of the reactor N. The secondary coil N2 is excited by the current i2 to accumulate magnetic energy.

  Therefore, during the ON period of the switching element Q, the sum of the current i1 and the current i2 contributes to the output current Io.

The ratio between the input voltage Vin and the voltage Vt at the intermediate tap T is the turn ratio of the primary coil N1 and the secondary coil N2. On the other hand, the current i1 and the current i2 are the reciprocals of the turns ratio of the primary coil N1 and the secondary coil N2. Therefore, the following equation holds.
Vin / Vt = (N1 + N2) / N2
i1 / i2 = N2 / (N1 + N2)

  For example, when the turn ratio N1 / N2 is 9, the combination is Vin = 400V, Vt = 40V, i1 = 1A, i2 = 9A. As the position of the intermediate tap T approaches the end of the secondary coil N1, the voltage Vt becomes smaller than the input voltage Vin, while the current i2 becomes larger than the current i1. That is, by changing the position of the intermediate tap T, the ratio of the current i1 and the current i2 changes even if the sum of the current i1 and the current i2 is the same. Decreasing the current i1 means that the burden on the switching element Q through which the current i1 flows is reduced.

The output voltage Vo is obtained by the following equation as a time average of the voltage Vt at the intermediate tap T.
Vo = D · Vt = D · Vin · N2 / (N1 + N2)
(D is the duty ratio)

<Operation of switching element Q during off period>
When the switching element Q is turned off, the current i1 of the primary coil N1 due to the input voltage Vin is cut off, and a back electromotive force is generated in the primary coil N1. Due to this counter electromotive force, the starting end of the primary coil N1 becomes a negative potential, so that the diode D1 becomes forward biased, and the current i3 passes from the diode D1 through the primary coil N1 and is output from the intermediate tap T and output to the output end 2. As a result, the magnetic energy accumulated during the ON period is released.

  On the other hand, a counter electromotive force is also generated in the secondary coil N2, but when the current i3 flows through the primary coil N1, an action is maintained in which the current in the same direction as the current i2 in the ON period is maintained, and the current i4 flows. The current i4 is output from the diode D2 through the secondary coil N2 and from the intermediate tap T to the output terminal 2. As a result, the magnetic energy accumulated during the ON period is released.

  Therefore, during the OFF period of the switching element Q, the sum of the current i3 and the current i4 contributes to the output current Io.

<Relationship between duty ratio, voltage and current>
Next, the relationship among the position of the intermediate tap T, the duty ratio, and the voltage and current at each location on the circuit will be described with reference to FIG.

Each of the reactor N portions in the circuit of FIG. 1 is schematically shown in the rectangular frames of FIGS. (A) and (B) are examples of different positions of the intermediate tap T. In (A), there intermediate tap T is substantially in the center of the reactor N, the number of turns of the primary coil N1 _A and the secondary coil N2 _A is approximately the same. In (B), the intermediate tap T is at a position on the right side, and the number of turns of the secondary coil N2 _B is smaller than the number of turns of the primary coil N1 _B.

In FIG. 3, the on period Ton is replaced with the duty ratio D. In (A), when the duty ratio D is small (a1) (a2) (a3) and when the duty ratio D is large (a4) (a5) (a6), the input voltage Vin, the intermediate tap voltage Vt _A, and the output The waveforms of voltage Vo _A and currents i1 _{A to} i4 _A are schematically shown. The output voltage Vo _A is a time average of one cycle of the intermediate tap voltage Vt _A and is indicated by a broken line.

In the case of (A), the duty ratio D becomes larger, the period of the intermediate tap voltage Vt _A is prolonged by the input voltage Vin is applied longer. As a result, the output voltage Vo _A, which is the time average of the intermediate tap voltage Vt _A , also increases (compare (a2) and (a5)). Similarly, when the duty ratio D increases, the peak values of the current i1 and the current i2 during the on-period increase, thereby increasing the current i3 and the current i4 (compare (a3) and (a6)).

  Also in the case of (B), when the duty ratio D is small (b1) (b2) (b3) and when the duty ratio D is large (b4) (b5) (b6), the relationship between each voltage and each current is ( The same as in the case of A).

Next, (A) and (B) are compared. Compared to (A), the turn ratio of the secondary coil to the primary coil is smaller in (B). As a result, the intermediate tap voltage Vt _B is smaller than the intermediate tap voltage Vt _A (compare (a2) and (b2) or compare (a5) and (b5)). The current i1 _B and the current i3 _B are smaller than the current i1 _A and the current i3 _A , while the current i2 _B and the current i4 _B are larger than the current i2 _A and the current i4 _A by the decrease ((( a3) and (b3) are compared, or (a6) and (b6) are compared).

  Thus, the output voltage Vo becomes smaller as the position of the intermediate tap T is changed to reduce the number of turns of the secondary coil N2. That is, when the duty ratio D is the same, the output voltage Vo can be further reduced as the number of turns of the secondary coil N2 is reduced. It can also be seen that the currents i1 and i3 decrease and the currents i2 and i4 increase as the position of the intermediate tap T is changed to reduce the number of turns of the secondary coil N2. That is, as the number of turns of the secondary coil N2 is decreased, the burden on the switching element Q through which the current i1 flows can be reduced.

4A shows the output voltages Vo _A and Vo _B when the duty ratio D is changed between 0 and 1 at the position (A) and the position (B) of the intermediate tap T shown in FIG. FIG. 4B is a graph showing changes in the output current Io. The output current Io is a time average of one cycle in which the currents i1 to i4 are combined. Assume that the output current Io having the same magnitude is output at each of the position (A) and the position (B).

As shown in FIG. 4 (a), when outputting a predetermined output voltage Vox, than the duty ratio D _A at the position of the intermediate tap T (A), is possible to increase the duty ratio D _B at the position (B) it can. When the duty ratio D is an extremely small value, on / off control of the switching element may be difficult. However, the duty ratio D is an appropriate value (for example, a range of 0.2 to 0.8, or 0.5, for example). In the case of (front and back), on / off control is easy.

As shown in FIG. 4B, when a predetermined output current Iox is output, the current i1 _B at the position (B) can be made smaller than the current i1 _{A at} the position (A) of the intermediate tap T. Thereby, the burden of the switching element Q through which the current i1 flows can be reduced.

(2-2) Operation of Switch Element S in Off Mode With reference to FIG. 5, the operation of the switch element S in the off mode in the circuit of FIG. 1 will be described. FIG. 5A shows the current during the ON period of the switching element Q, and FIG. 5B shows the current during the OFF period of the switching element Q by a solid line with an arrow.

<Operation of switching element Q during ON period>
When the switching element Q is turned on, the current i1 flows from the input terminal 1 to the primary coil N1 by the input voltage Vin, and is output from the intermediate tap T to the output terminal 2. Since the starting end of the primary coil N1 has a positive potential, the diode D1 is reverse-biased and no current flows. The primary coil N1 is excited by the current i1 to accumulate magnetic energy. The secondary coil N2 does not flow because the current path is interrupted.

  Therefore, only the current i1 contributes to the output current Io during the ON period of the switching element Q.

<Operation of switching element Q during off period>
When the switching element Q is turned off, the current i1 of the primary coil N1 due to the input voltage Vin is cut off, and a back electromotive force is generated in the primary coil N1. Due to this counter electromotive force, the starting end of the primary coil N1 becomes a negative potential, so that the diode D1 becomes forward biased, and the current i3 passes from the diode D1 through the primary coil N1 and is output from the intermediate tap T and output to the output end 2. As a result, the magnetic energy accumulated during the ON period is released. The secondary coil N2 does not flow because the current path is interrupted.

  Therefore, only the current i3 contributes to the output current Io during the OFF period of the switching element Q.

  In the circuit of FIG. 1, the operation of the switch element S in the off mode is the same as the operation of a normal step-down converter (see FIG. 7) using only the primary coil N1 as a reactor.

  FIG. 6 is a graph showing an example of a change in the output current Io when the duty ratio D is changed between 0 and 1 in the circuit of FIG. FIG. 6 shows a case where the switch element S is turned on in a range larger than the predetermined duty ratio Ds and the switch element S is turned off in a smaller range.

  In the on mode (D> Ds) of the switch element S, the currents i1 and i3 flowing through the primary coil N1 and the currents i2 and i4 flowing through the secondary coil N2 are added, and an output current Io is obtained as a time average thereof. On the other hand, in the off mode (D <Ds) of the switch element S, the output current Io is obtained as a time average of only the currents i1 and i3 flowing through the primary coil N1.

  As shown in FIG. 6, when the predetermined output current Iox is output, the duty ratio D (on) is extremely small if the switch element S is in the on mode. The duty ratio D (off) can be set to a large value. Thereby, the switching loss of the switching element Q can be reduced, and switching control becomes easy.

  As described above, with the tapped inductor type switching power supply of the present invention, the switching element can be controlled within the range of the duty ratio that enables stable on / off control even in the case of a small output current. Thus, according to the present invention, a wide range of current control is possible. Furthermore, the switching loss of the switching element is reduced.

  The tapped inductor type switching power supply of the present invention described above is not limited to the illustrated configuration example, and various modifications are possible within the scope of the gist of the present invention.

1 Input terminal 2 Output terminal e Ground terminal Q Switching element (MOSFET)
N reactor N1 primary coil N2 secondary coil D1, D2 rectifier element (diode)
C Smoothing capacitor S Switch element

Claims (2)

In a tapped inductor type switching power supply that converts DC power applied between an input end and a ground end and outputs between the output end and the ground end.
A reactor including a primary coil and a secondary coil, and having an intermediate tap at a connection point between a terminal end of the primary coil and a starting end of the secondary coil, and the intermediate tap connected to the output end;
A switching element that is on / off controlled to conduct or block a current path between the input end and the starting end of the primary coil;
A first rectifying element that conducts a current flowing from the ground end to the starting end of the primary coil;
A second rectifying element for conducting a current flowing from the ground end to the end of the secondary coil;
A smoothing capacitor connected between the output terminal and the ground terminal;
A tapped inductor type switching power supply comprising: a switch element capable of conducting or blocking a current path between the ground end and the end of the secondary coil.   2. The tapped inductor type switching power supply according to claim 1, wherein the switch element is cut off in a range in which a duty ratio in on / off control of the switching element is smaller than a predetermined value.

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