JP2001218457A - Dc/dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC/DC converter capable of attaining miniaturization and high efficiency at a low cost. SOLUTION: This DC/DC converter is provided with an isolation transformer having a primary winding, a first secondary winding, and a second secondary winding, a primary switching element for energizing power from a power source to the primary winding intermittently, a series circuit of a capacitor and a secondary switching element, which is connected in parallel with the primary winding, a control circuit generating a control signal turning on/off the primary switching element and the secondary switching element alternately, a rectifier circuit for rectifying power generated on the first secondary winding and outputting the positive output to one end of the second secondary winding, and an output capacitor connected in parallel with the other end of the second secondary winding and the negative terminal of the rectifier circuit, wherein the isolation transformer is wound in such a direction that energy is transferred to the first secondary winding from the primary winding when the first switching element is turned on, and the primary winding, the first secondary winding, and the second secondary winding are wound around the same leg of the core of the isolation transformer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a DC / DC converter, and more particularly to an improvement of a structure of an insulating transformer constituting the DC / DC converter.
A small and efficient DC / DC converter is realized at low cost.

[0002]

2. Description of the Related Art In a power supply device such as a switching power supply, a DC / DC converter is used as a device for insulating a DC input voltage and supplying power to a load circuit. In the DC / DC converter having such a configuration, there are a forward type and a flyback type depending on the polarity difference between the primary winding and the secondary winding of the insulating transformer. For example, as a forward DC / DC converter, US Patent USP44
One such as disclosed in US Pat.

FIG. 7 is a circuit diagram showing an example of a forward type DC / DC converter.

In FIG. 1, a DC input voltage Vg is supplied to a capacitor C1 connected in series, a second switching element Q2 functioning as a sub-switching element, and a first switching element Q1 functioning as a main switching element.
Are applied to both ends. A primary winding Np of an insulating transformer T is connected to one end of the capacitor C1 and the source of the second switching element Q2, and both gates of the second switching element Q2 and the first switching element Q1 are connected to each other. Is connected to a control circuit 11 for generating a drive signal for driving the.

A rectifier circuit 22 composed of a forward diode D1 and a flywheel diode D2 is connected to the secondary winding Ns of the insulating transformer.
The output of the rectifier circuit 22 is connected to the smoothing capacitor C2 via the output choke L and to the load circuit 21.

In such a circuit, the control circuit 11 generates a control signal for alternately turning on and off the second switching element Q2 and the first switching element Q1.
DC input voltage V applied to the primary winding of the isolation transformer
g is turned on and off intermittently.

[0007] The induced electromotive force generated in the secondary winding Ns of the insulating transformer T is DC-smoothed by the rectifier circuit 22 and the smoothing capacitor C2 and supplied to the load circuit 21.

FIG. 8 is a circuit diagram showing an example of a flyback type DC / DC converter.

In the figure, the difference from the forward type DC / DC converter shown in FIG. 7 is that the polarities of the primary winding Np and the secondary winding Ns of the insulating transformer T are reversed. is there. Other configurations are the same as those of the circuit of FIG. 7, and it is possible to supply power to the load circuit 21 while insulating the DC input voltage Vg by the same principle.

[0010]

However, the conventional DC / DC converters shown in FIGS. 7 and 8 have the following problems.

In the forward type DC / DC converter shown in FIG. 7, the voltage Vs generated in the secondary winding is:
Suppose that the DC input voltage is Vg, and the turns ratio of the primary winding Np and the secondary winding Ns of the insulating transformer is n (n = Np / Ns). Vs = 1 / n × Vg (1) Ripple current I _PP flowing through output choke L
In the equation, if the switching period of the main switching element is T, the proportion of time during which the main switching element is on (hereinafter referred to as on-duty) is D, and the inductance of the output choke L is L, I _PP = Vs × D × T / L (2), and the output voltage Vo is expressed as: Vo = 1 / n × D × Vg (3)

Therefore, in the circuit of FIG. 7, for example, the output voltage Vo is 15 V, the switching period T of the main switching element is 100 kHz, and the DC input voltage Vg is 300 V.
When dc and the turns ratio n are 7, the on-duty D of the main switching element is 0.35, and the switching cycle T of the main switching element is 10 μs. At this time, if the ripple current I _PP is 1 A _PP , the inductance L is 160 μH from the equation (2).

The output choke L having an inductance of 160 μH is very large in the circuit of FIG. 7 as compared with the other elements constituting the output choke L. Further, when the output choke L is manufactured in a small size using a thin winding, the winding resistance increases, and as a result, the loss increases.

In recent years, power supplies used in portable information devices and the like, which have exploded in popularity, are required to be miniaturized due to the installation space problem, and are efficient power supplies capable of suppressing generation of ripple current. Is required. Furthermore, portable information devices and the like are generally set at inexpensive prices, and are required to be manufactured at low cost. Therefore, the DC / DC converter used for such a power supply needs to satisfy all the above-mentioned conditions.

On the other hand, in the circuit shown in FIG. 7, since the inductor size of the output choke L becomes large, D
There is a problem that it is difficult to reduce the size of the C / DC converter. Further, when the number of turns is increased by using a thin winding in order to reduce the size of the output choke L, there is a problem that the winding resistance increases and the loss increases.

The flyback type DC shown in FIG.
In the / DC converter, the voltage Vs generated in the secondary winding Ns is expressed by the same equation as the above equation (1), and the ripple current I _PP flowing through the output choke L and the output voltage Vo
Is the on-duty D of the first switching element Q1 in the above equations (2) and (3), and the time ratio (hereinafter referred to as off-duty) D ′ ( D ′ = 1−D).

In this case, the DC input voltage V
g is set to 300 Vdc, the turns ratio n is set to 7, and the ripple current I _{PP is set} to 1 A _PP , and the inductance L of the output choke is calculated to be 278 μH. Increase.

On the other hand, US Pat.
At 64, as shown in FIG. 9, an inductor winding NL is inserted between the rectifier circuit 22 and the output capacitor C1, and this inductor winding NL is connected to the windings Np and Ns of the insulating transformer T1.
In this configuration, it is possible to reduce the ripple current, to reduce the reactance ratio between the secondary winding Ns and the inductor winding NL and the on-duty of the main switching element Q1. It is disclosed that when D is configured to be equal, the ripple current can be reduced to zero.

However, in this case, the secondary winding Ns
In order to adjust the reactance ratio between the secondary winding Ns and the inductor winding NL to be equal to the on-duty D of the main switching element, the coupling coefficient between the secondary winding Ns and the inductor winding NL is about 0.6. A large leakage inductance is required, and in order to secure such a large leakage inductance, it is necessary to wind both windings on separate legs of the core.

Such a winding structure is difficult to realize with a structure using a general-purpose insulating transformer and an inductor winding, and a transformer having a complicated core shape as shown in FIG. 10 is required. The insulation transformer shown in FIG. 1 has a primary winding Np and a secondary winding Ns wound on the center leg of the core, a winding of the inductor winding NL wound on another leg, and a core gap GA.
P1 to P3 adjust the bond between them. Further, it is necessary to adjust the core gaps GAP1 to GAP3 to different values.

Therefore, in the DC / DC converter shown in FIG. 9, two winding points of the insulating transformer are required, and three core gaps GAP1 to GAP3 must be adjusted to different values. There has been a problem that the shape of the insulating transformer becomes complicated and the manufacturing cost increases.

An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to provide a DC / DC converter realizing a small size and high efficiency at a low cost.

[0023]

According to the first aspect of the present invention, there is provided an insulation device having a primary winding, a first secondary winding, and a second secondary winding. A first transformer for intermittently supplying power from a transformer and a power supply to the primary winding;
A switching element, a series circuit of a capacitor and a second switching element connected in parallel to the primary winding, and a control signal for alternately turning on and off the first switching element and the second switching element. A control circuit, a rectifier circuit for rectifying power generated in the first secondary winding and outputting an output of one polarity to one end of the second secondary winding; An output capacitor connected in parallel to the other end of the winding and an output terminal of the other polarity of the rectifier circuit, wherein the insulating transformer is connected to the first winding from the primary winding when the first switching element is on. The primary winding, the first secondary winding, and the second secondary winding are wound around the same leg of the core of the insulating transformer. It is characterized by the following.

According to a second aspect of the present invention, in the first aspect, an inductance element is inserted between the second secondary winding and the output capacitor. Things.

According to the third aspect of the present invention, an insulating transformer having a primary winding, a first secondary winding, and a second secondary winding, and an electric power from a power supply is intermittently transmitted to the primary winding. A first switching element for supplying current to the first winding, a series circuit of a capacitor and a second switching element connected in parallel to the primary winding, and the first switching element and the second switching element are alternately turned on and off. A control circuit that generates a control signal; a rectifier circuit that rectifies power generated in the first secondary winding and outputs an output of one polarity to one end of the second secondary winding; An output capacitor connected in parallel to the other end of the second secondary winding and an output terminal of the other polarity of the rectifier circuit, wherein the insulating transformer is configured to switch the primary winding when the first switching element is off. To the first secondary winding Wherein the primary winding, the first secondary winding, and the second secondary winding are wound on the same leg of the core of the insulating transformer. Is what you do.

According to a fourth aspect of the present invention, in the third aspect of the invention, an inductance element is inserted between the second secondary winding and the output capacitor. Things.

According to the invention described in claim 5, an insulating transformer having a primary winding, a first secondary winding, and a second secondary winding, and power from a power supply intermittently transmitted to the primary winding. And a push-pull type primary winding control circuit for rectifying power generated in the first secondary winding and outputting an output of one polarity to one end of the second secondary winding. A rectifier circuit, an output capacitor connected in parallel with the other end of the second secondary winding and an output terminal of the other polarity of the rectifier circuit, wherein the primary winding, the first secondary winding, and the The secondary winding of No. 2 is wound around the same leg of the core of the insulating transformer.

According to a sixth aspect of the present invention, in the fifth aspect, an inductance element is inserted between the second secondary winding and the output capacitor. Things.

According to a seventh aspect of the present invention, in the first to sixth aspects of the present invention, the rectifier circuit includes an M
It is characterized in that a synchronous rectifier circuit composed of an OSFET is used.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1A is a circuit diagram showing an embodiment of a DC / DC converter according to the present invention, and FIG.
It is a block diagram of the insulation transformer T2 used for this. In addition,
In the figure, the same components as those of the conventional example are denoted by the same reference numerals, and description thereof will be omitted.

In the figure, the DC / D described in the conventional example is used.
The difference from the C converter is the configuration of the insulating transformer T2. The insulating transformer T2 used in the DC / DC converter according to the present invention has a core gap G as shown in FIG.
The primary winding Np,
The secondary winding Ns and the inductor winding Nl are wound in an overlapping manner. Further, in the figure, the winding of the insulating transformer T2 is wound so as to form a forward type configuration.

The operation of the DC / DC converter having such a configuration will be described with reference to an equivalent circuit shown in FIG. 2 for easy understanding.

In FIG. 2, (a) shows a DC having the above configuration.
This is an equivalent circuit of a period during which the first switching element Q1 of the / DC converter is in the ON state (at this time, the second switching element Q2 is in the OFF state; this period is hereinafter referred to as D period). , (B) show DC /
This is an equivalent circuit of a period during which the first switching element Q1 of the DC converter is in the off state (at this time, the second switching element Q2 is in the on state; this period is hereinafter referred to as a D 'period). .

In the drawing, the number of turns of the primary winding of the insulating transformer T2 is Np, the number of turns of the secondary winding Ns is N, the coupling coefficient between them is M, the number of turns of the inductor is NL, and the number of turns of the insulating transformer T2 is 2 The coupling coefficient between the secondary winding and the inductor winding is M ', the inductance of the output choke is L, the on-duty of the first switching element Q1 is D, the off-duty is D', and the voltage between the terminals of the output capacitor C2 is Vc. If the output voltage is Vo, D + D '= 1 (4) D * Vg = D' * Vc (5) Vo = M / n * Vg * D (6) n = Np / Ns (7) n ' = Ns / NL (8) holds.

The voltage generated at the inductor winding NL is VL, the voltage generated at the output choke L is VL ', and the voltage generated at the inductor winding NL and the output choke L is VL.
Then, in the D period, the relational expression of Vl = VL + VL '(9) M / n * Vg-Vl = Vo (10) holds. Equation (10) can be modified as follows: M / n * Vg−M * M ′ / (n * n ′) * Vg−VL ′ = Vo (11)

In the period D ', the relational expression of Vl = Vo (12) holds. Equation (12) is given by M * M '/ (n * n') * Vc + VL '= Vo (13) VL' = Vo-M * M '/ (n * n') * Vc (14) And can be transformed.

Here, the equation (11) in the D period can be transformed using the above basic equation into the following formula: VL '= M / n * Vg * D'-M * M' / (n * n ') * Vg ( 15), and the expression (14) in the D ′ period is as follows: VL ′ = M / n * Vg * DM * M ′ / (n * n ′) * (D / D ′) * Vg (16)

Here, in the circuit of FIG. 2, since the condition that the ripple current becomes zero is when VL ′ = 0, the ripple current becomes zero when D ′ = M ′ / n ′ (17) Become.

D ′ is the output voltage Vo,
The input voltage Vgz is determined by the DC input voltage Vg, the turns ratio n of the isolation transformer, and the coupling M. When these parameters are determined, the input voltage Vgz becomes zero ripple (the on-duty at this time is Dz).
Is uniquely determined.

Further, if the off duty D 'at the time of zero ripple is set to D'z, D'z = M' / n '(18), and the voltage generated in the output choke L at a point other than the zero ripple point at this time VL ′ = M / n * Vg * (D′−D′z) (19), and VL ′ * D * T / L (20) becomes the ripple current.

At this time, a DC magnetic flux corresponding to NL * Io is biased to the transformer, but the core gap G is set so that the core is not saturated according to the biased DC magnetic flux.
Design AP4.

Therefore, if the values of the insulating transformer T2 and each inductor are determined so that the above relational expression is satisfied, an operating point at which zero ripple occurs can be obtained. Generally, when windings are formed by superposing each winding on the same foot of the core 30, M and M 'have values close to 1, and the operating point is determined by n'.

By determining the parameters and operating conditions of the isolation transformer T2 in FIG. 1 according to the procedure described above, the DC / DC that can make the ripple current zero can be obtained.
A DC converter can be realized.

When the above parameters deviate from the operating point due to various factors, it is necessary to suppress the ripple current by the output choke L. For example, when M and M 'are set to 1 and Vg = 300 V, In the case of n = 7, T = 10us, Vo = 15V, D = 0.35, if n '= 1.5, the inductance of the output choke L at which the refresh current becomes 1Ap-p is 2.5uH, which is compared with the conventional example. And a value of 1/64.

Therefore, in the DC / DC converter of the present invention, it is possible to make the ripple current zero by using a very small output choke as compared with the conventional example.

In the case of a flyback type DC / DC converter, the relationship between the period D and the period D ', and the condition where Vg and Vc are switched and the condition of zero ripple is obtained, D = M' / n '(21) . If D at the time of zero ripple is Dz, VL '= M / n * (D / D') * Vg * (D-Dz) (22)

In this case, the DC magnetic flux biased to the transformer is equivalent to (Ns-NL) * Io, and the parameter is designed in consideration of the saturation magnetic flux density of the core. In order to reduce the biased DC magnetic flux, Ns-NL is preferably set to be close to zero.

It should be noted that the foregoing description has been directed to specific preferred embodiments for the purpose of illustration and illustration of the invention. Therefore, the present invention is not limited to the above-described embodiment, but includes many more changes and modifications without departing from the essence thereof.

For example, the configuration of the DC / DC converter of the present invention is a flyback type DC / D converter having a configuration as shown in FIG.
The present invention is also applicable to a C converter, a half-bridge DC / DC converter having a configuration as shown in FIG. 4, and a full-bridge DC / DC converter having a configuration as shown in FIG.
The present invention is also applicable to a DC / DC converter and a push-pull type DC / DC converter as shown in FIG. A push-pull type DC as shown in FIG.
In the case of the / DC converter, the condition for zero ripple is M '/ n' = 1-2D (23).

Further, the rectifier circuit constituted by the forward diode D1 and the flywheel diode D2 is not limited to the one in which the cathodes of the diodes D1 and D2 are interconnected as in the embodiment, and the anode is connected to the anode. They may be interconnected.

In the embodiment, the output choke constituting the smoothing circuit is connected to the cathode of the diode D2. However, the same effect can be obtained by connecting the output choke to the anode of the diode D2.

Further, a synchronous rectifier circuit using a MOSFET may be used as the rectifier circuit. Since the MOSFET has a low on-resistance, the loss can be significantly improved as compared with a rectifier circuit using a diode, and a more efficient power supply can be realized.
For example, when a comparison is made assuming a power supply with an output current of 4 A, when the forward voltage drop of the diode is 0.5 V, 2 W
(= 4 * 0.5). In contrast, MOSF
When the ON resistance of ET is 20mΩ, the loss is 0.32W
(= 16 * 0.02), which can be reduced to about 0.5 W even when the loss such as the parasitic capacitance is added, so that a significant improvement in efficiency can be realized.

[0053]

As is apparent from the above description,
According to the present invention, the following effects can be obtained. According to the first and second aspects of the present invention, in the forward type active clamp DC / DC converter, the ripple current can be reduced to zero by using an inexpensive insulating transformer having a simple structure. In addition, when an insulating transformer is manufactured in accordance with the design conditions for reducing the ripple current to zero, there is no need to add an output choke, so that a DC / DC converter that simultaneously realizes miniaturization, low cost, and high efficiency. Can be provided.

Further, the insulating transformer used here is easy to manufacture because there are only one winding point and one air gap. Also, since such transformers have already been standardized and distributed as general-purpose transformers,
It can be obtained at low cost.

According to the second aspect of the present invention, when the structure of the insulating transformer deviates from the design condition for reducing the ripple current to zero, the ripple current can be suppressed by adding an output choke. . The output choke to be added at this time may be very small,
This does not hinder miniaturization of the C / DC converter.

According to the third and fourth aspects of the present invention, it is possible to achieve the same effect as described above in a flyback type active clamp DC / DC converter.

According to the fifth and sixth aspects of the present invention, the same effects as those described above can be realized in a push-pull DC / DC converter.

According to the present invention, a power supply with higher efficiency can be realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a DC / DC converter according to the present invention.

FIG. 2 is an equivalent circuit diagram of the DC / DC converter according to the present invention.

FIG. 3 is a circuit diagram showing another embodiment of the DC / DC converter according to the present invention.

FIG. 4 is a circuit diagram showing another embodiment of the DC / DC converter according to the present invention.

FIG. 5 is a circuit diagram showing another embodiment of the DC / DC converter according to the present invention.

FIG. 6 is a circuit diagram showing another embodiment of the DC / DC converter according to the present invention.

FIG. 7 is a circuit diagram showing an example of a conventional DC / DC converter.

FIG. 8 is a circuit diagram showing an example of a conventional DC / DC converter.

FIG. 9 is a circuit diagram showing an example of a conventional DC / DC converter.

FIG. 10 is a diagram showing a configuration of a conventional insulating transformer of a DC / DC converter.

[Explanation of symbols]

 Reference Signs List 11 control circuit 21 additional circuit Vg input DC voltage C1, C2 capacitor Q1 first switching element Q2 second switching element D2, D2 diode L output choke T, T1, T2 isolation transformer

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H730 AA14 BB23 BB25 BB27 BB43 BB57 DD04 DD43 EE02 EE03 EE08 EE72 EE76 ZZ16

Claims (7)

[Claims] 1. An insulating transformer having a primary winding, a first secondary winding, and a second secondary winding, and a first switching device for intermittently supplying power from a power supply to the primary winding. An element, a series circuit of a capacitor and a second switching element connected in parallel to the primary winding, and a control circuit for generating a control signal for turning on and off the first switching element and the second switching element alternately. And rectifying the power generated in the first secondary winding;
A rectifier circuit for outputting the one-polarity output to one end of the second secondary winding; and an output connected in parallel to the other end of the second secondary winding and the other polarity output terminal of the rectifier circuit. A capacitor, wherein the insulating transformer is wound in a direction in which energy is transmitted from a primary winding to a first secondary winding when the first switching element is in an on state; A DC / DC converter, wherein the first secondary winding and the second secondary winding are wound on the same leg of the core of the insulating transformer. 2. The DC / DC converter according to claim 1, wherein an inductance element is inserted between said second secondary winding and said output capacitor. 3. An insulating transformer having a primary winding, a first secondary winding, and a second secondary winding, and a first switching device for intermittently supplying power from a power supply to the primary winding. An element, a series circuit of a capacitor and a second switching element connected in parallel to the primary winding, and a control circuit for generating a control signal for turning on and off the first switching element and the second switching element alternately. And rectifying the power generated in the first secondary winding;
A rectifier circuit for outputting the one-polarity output to one end of the second secondary winding; and an output connected in parallel to the other end of the second secondary winding and the other polarity output terminal of the rectifier circuit. A capacitor, wherein the insulating transformer is wound in a direction in which energy is transmitted from a primary winding to a first secondary winding when the first switching element is in an off state. A DC / DC converter, wherein the first secondary winding and the second secondary winding are wound on the same leg of the core of the insulating transformer. 4. The DC / DC converter according to claim 3, wherein an inductance element is inserted between said second secondary winding and said output capacitor. 5. An isolation transformer having a primary winding, a first secondary winding, and a second secondary winding, and a push-pull type in which power from a power supply is intermittently supplied to the primary winding. A primary winding control circuit, and rectifying power generated in the first secondary winding;
A rectifier circuit for outputting the one-polarity output to one end of the second secondary winding; and an output connected in parallel to the other end of the second secondary winding and the other polarity output terminal of the rectifier circuit. A DC / DC converter comprising: a capacitor; wherein the primary winding, the first secondary winding, and the second secondary winding are wound on the same leg of the core of the insulating transformer. 6. The DC / DC converter according to claim 5, wherein an inductance element is inserted between said second secondary winding and said output capacitor. 7. The DC / DC converter according to claim 1, wherein a synchronous rectifier circuit composed of a MOSFET is used as the rectifier circuit.