JP2661180B2 - Flyback type DC-DC converter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flyback type DC-DC converter that raises (or lowers) a DC voltage by a predetermined voltage.

[Related Art] Conventionally, in a DC-DC converter, when a diode of a rectifier circuit connected to a secondary winding is turned off, a reverse recovery current temporarily flows due to movement of a carrier of the diode, and the movement of the carrier is completed. Later, the reverse voltage blocking function works on the diode, the reverse voltage applied to the diode rises sharply, and spike noise may occur. A series circuit (CR snubber) of a capacitor and a resistor is connected in parallel to the diode. By doing so, spike noise is suppressed.

However, these CRT snubbers have some effect when the above switching frequency is low, but have problems in terms of power loss, heat generation and efficiency, and when the switching frequency is high, it is not possible to mount a CR snubber. Met.

Therefore, in recent years, in order to obtain a sufficient spike noise suppression effect even when the switching frequency is high, as disclosed in the Institute of Electrical Engineers of Japan research group material MAG-87-129, it is connected in series with a diode. It has been considered to provide a saturable reactor as a magnetic snubber.

[Problems to be Solved by the Invention] However, when a saturable reactor is connected in series to a rectifying element such as a diode as described above, the primary winding of a transformer is energized as in a forward converter or a push-pull converter. In a so-called on / off type DC-DC converter in which a rectifier circuit on the secondary winding side sometimes operates, it is possible to suppress the occurrence of spike noise by suppressing the reverse recovery current from flowing through the rectifying element. However, a flyback type (so-called on / off type) DC-DC converter in which the rectifier circuit on the secondary winding operates when the primary winding of the transformer is de-energized has a good effect. There was a problem that can not be.

This is because in a flyback type DC-DC converter, the saturable reactor becomes a load on the secondary side when the power supply to the primary winding is cut off, and the flyback voltage rises abnormally, and the primary side switching element This is because abnormalities may occur.

Therefore, in a conventional flyback type DC-DC converter, as disclosed in Japanese Patent Application Laid-Open No. 62-262659, there is no snubber for suppressing a reverse current flowing through a rectifying element (diode) on the secondary side. It is currently not fitted.

Therefore, an object of the present invention is to make it possible to obtain the effect of a magnetic snubber by a saturable reactor even in a flyback type DC-DC converter.

[Means for Solving the Problems] In order to achieve the above object, the invention according to claim 1 is a switching element for interrupting connection between a transformer and a primary winding of the transformer and a DC power supply. And, connected to the secondary winding of the transformer, when the switching element cuts off the connection between the primary winding of the transformer and the DC power supply, the current flows by the magnetic energy stored in the transformer, and the capacitor A flyback type DC-DC converter comprising: a main winding of a saturable reactor in series with the rectifier, and a secondary winding of the transformer. In parallel, when the switching element connects the primary winding of the transformer and the DC power supply, a current flows through the control winding of the saturable reactor through the secondary winding, and the saturable reactor The magnetic Energizing circuit which is characterized in that the provided.

In a second aspect of the present invention, in a flyback type DC-DC converter, a main winding of a saturable reactor is provided in series with the rectifying element, and a primary winding or a DC of the transformer is provided. In parallel with the power supply, when the switching element connects the primary winding of the transformer and the DC power supply, a current flows from the DC power supply to the control winding of the saturable reactor, and the saturable reactor is An energizing circuit for magnetic saturation is provided.

[Operation] The present invention configured as described above (claims 1 and 2)
In the flyback type DC-DC converter, when the switching element is turned on and the primary winding of the transformer is energized, magnetic energy is stored in the gap of the transformer,
Thereafter, when the switching element is turned off and the energization of the primary winding is cut off, a current flows through the rectifying element due to the magnetic energy stored in the gap, and the capacitor is charged.

Further, in the present invention, the main winding of the saturable reactor is provided in series with the rectifying element, but the saturable reactor is magnetically saturated by the energizing circuit when the switching element is turned on.

That is, in the DC-DC converter according to the first aspect, the energizing circuit connected in parallel to the secondary winding of the transformer controls the saturable reactor via the secondary winding when the switching element is turned on. The saturable reactor is magnetically saturated by passing a current through the winding, and in the DC-DC converter according to claim 2, an energizing circuit connected in parallel to the primary winding of the transformer or the DC power supply is switched. When the element is turned on, a current flows from the DC power supply to the control winding of the saturable reactor, thereby magnetically saturating the saturable reactor.

Therefore, in the present invention, when the switching element is turned off and the rectifying element operates, the main winding does not become a load,
A current flows through the rectifier in the same manner as in a conventional device having no saturable reactor, and the capacitor is charged.

On the other hand, when the rectifier turns off after the capacitor is charged, reverse recovery current tends to flow through the rectifier due to carrier movement inside the rectifier, but the main winding of the saturable reactor reacts to this reverse recovery current. Therefore, the reverse current is suppressed because the reactor has a large inductance.

Therefore, according to the present invention, the reverse recovery current flowing when the rectifying element is turned off can be suppressed, and the occurrence of an abnormal reverse voltage across the rectifying element and spike noise can be prevented.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, the flyback type DC-
As in the conventional DC converter, the connection between the transformer 2 and the primary winding L1 of the transformer 2 and the DC power supply 4 is interrupted and connected to the primary winding.
A transistor TR as a switching element for switching between energization and non-energization of L1, and a forward direction with respect to a voltage generated in the secondary winding L2 of the transformer 2 when the transistor TR is switched from on to off, Secondary winding L2 of transformer 2
And a capacitor C1 that is charged with a high voltage generated in the secondary winding L2 via the diode D1.

Further, in this embodiment, as shown in FIG. 2, a main winding is attached to a toroidal core TC made of amorphous, ferrite or the like.
A saturable reactor 6 formed by winding the control winding l1 and the control winding l2 and having magnetic characteristics (BH characteristics) as shown in FIG. 3 is provided, and as shown in FIG. l1 is connected in series between the diode D1 and the capacitor C1, and the control winding l2 is connected in parallel to the secondary winding L2 of the transformer 2 via the diode D2 and the resistor R1.

The diode D2 is provided in the forward direction with respect to the voltage generated in the secondary winding L2 of the transformer 2 when the switching element TR is on, and the control winding l2 is energized when the primary winding L1 is energized. The saturable reactor 6 can be magnetically saturated. The direction in which the control winding l2 is energized is set so that the inductance of the main winding l1 decreases with respect to the forward current of the diode D1 due to the magnetic saturation of the saturable reactor 6.

In the flyback type DC-DC converter of the present embodiment configured as described above, as shown in FIG. 4, first, the transistor TR is turned on, and the primary winding L1 of the transformer 2 is connected to the DC power supply 4. Then, the primary current I1 flows through the primary winding L1 and accumulates magnetic energy in the gap of the transformer 2. Then, a part of the magnetic energy flows as a current through the secondary winding L2 of the transformer 2, the diode D2, and the resistor R1 to the control winding l2 of the saturable reactor 6. At this time, the operating point of the saturable reactor shifts to a point a of the magnetic characteristic shown in FIG. 3, and the saturable reactor 6 is magnetically saturated.

Next, when the transistor TR is switched from on to off, the secondary current I2 starts to flow through the secondary winding L2 due to the magnetic energy stored in the gap of the transformer 2. At this time, the saturable reactor 6 is already magnetically saturated, and the inductance of the main winding l1 is sufficiently small. Therefore, the main winding l1 does not become a load, and the diode D1 as in the conventional device having no saturable reactor. Is charged via the capacitor C1. At this time, the current of the control winding l2 is cut off, but the secondary current I2 flows through the main winding l1.
Move to the point b in FIG.

The current that has begun to flow through the secondary winding L2 gradually decreases as the magnetic energy decreases, and the operating point of the saturable reactor 6 shifts to the point a shown in FIG. 3 again. When I2 becomes a slightly negative value (Io), it shifts to the point c. At this time, in the conventional device having no saturable reactor, as shown by a dotted line in FIG.
The reverse recovery current of D1 flows, and spike noise is generated at the time of the recovery. In this embodiment, since the saturable reactor 6 is provided, the operating point of the saturable reactor is set as shown in FIG. In the period from point c to point e, the reverse recovery current is greatly relaxed, so that the loss due to the reverse recovery current and the generation of noise can be suppressed.

Here, in order to suppress the reverse recovery current of the diode D1 for the time To as shown in FIG. 4, the time (holding time) until the operating point of the saturable reactor 6 shifts from the point c to the point e shown in FIG. Time) T must be greater than or equal to To
The coercive force Ho and the saturation magnetic flux density Bo shown in FIG.
Must be determined.

First, the coercive force Ho and the saturation magnetic flux density Bo
Determined from TC characteristics.

Further, when the operating point of the saturable reactor 6 shifts from point c to point e in FIG. 3, the reverse current Io flowing through the main winding l1 and the coercive force
The relationship with Ho is given by the following equation (1): Ho <Io + Nm / l (1) (where l is the average magnetic path length of the toroidal core TC) and the saturation magnetic flux density Bo of the saturable reactor and the holding time The relationship of T is given by the following equation (2).

E + T <2 + Nm + Bo + A (2) (where, E: applied voltage, A: cross-sectional area of toroidal core TC) Therefore, in this embodiment, by adding predetermined conditions to the above equations (1) and (2), The number of turns Nm of the winding l1, the average magnetic path length l of the toroidal core TC and the cross-sectional area A are determined so that the reverse recovery current of the diode D1 can be completely suppressed (that is, T ≧ To). ing.

Next, while the transistor TR is on, the control winding
To energize l2 and magnetically saturate the saturable reactor, the following equations (3) and (4) must be satisfied: Es + Ts ≦ 2 + Ns + Bo + A (3) Ho <Is + Ns / l (4) (However, Es: voltage induced in the secondary winding when the transistor TR is on, Ts: transistor TR on time, Is: energizing current of the control winding, Ns: number of turns of the control winding l2) These (3) and ( From equation (4), the following equation (5) is obtained.

Es + Ts / 2 + Bo + A ≧ Ns> Ho + 1 / Is (5) On the other hand, the conduction current Is is obtained by setting the resistance value of the resistor R1 to Rs and the control winding.
Assuming that the inductance of l2 is Ls, it can be described as the following equation (6).

It is desirable that the conduction current Is be as small as possible in consideration of the current consumption for saturating the saturable reactor.

Therefore, in the present embodiment, first, the number of turns Ns of the control winding l2 is set to be the largest within a range satisfying the above equation (5), and then Is is set to an accurate value. The number of turns Ns of the control winding l2 and the resistor R1 are set by a procedure such as determining the resistance value Rs of the resistor R1.

As described above, in the present embodiment, since the saturable reactor is magnetically saturated when the transistor TR is turned on, a magnetic snubber can be applied to a flyback type DC-DC converter, and the diode D1 can be used. The reverse recovery current can be greatly reduced, and the loss of the rectifying element and the spike noise can be reduced.

Here, in the above embodiment, an energizing circuit including the control winding l2 of the saturable reactor 6, the diode D2, and the resistor R1 is
Connected in parallel with the secondary winding L2 of the transformer 2, the transistor TR
In the above description, the saturable reactor 6 is magnetically saturated by the current flowing through the secondary winding L2 of the transformer 2 when the power supply is turned on (that is, the saturable reactor 6 is applied). For example, as shown in FIG. Item 2 is applied, and an energizing circuit including the control winding l2, the diode D2, and the resistor R1 is connected to the transformer 2
May be connected in parallel to the primary winding L1 to flow a current directly from the DC power supply 4 to the control winding l2 of the saturable reactor 6 when the transistor TR is on, thereby magnetically saturating the saturable reactor 6. . In this case, the control winding l2
Is supplied directly from the DC power supply 4. Therefore, in a DC-DC converter for boosting the power supply voltage,
It is possible to use a resistor having a small withstand voltage as the resistor, and it is possible to produce the resistor at a lower cost.

In addition, as shown in FIG. 6, for example, the control winding l2 of the saturable reactor 6 is connected to a resistor R1 and a transistor TRx.
May be directly connected to the DC power supply 4 via the. In this case, the energization control of the control winding l2 can be performed more efficiently.

[Effects of the Invention] As described in detail above, in the flyback type DC-DC converter of the present invention, the main winding of the saturable reactor is provided in series with the rectifying element, and the saturable reactor is turned on when the switching element is turned on. Since it is configured to be magnetically saturated, it is possible to operate the rectifier as it is when the switching element is turned off, and to suppress the reverse recovery current flowing when the rectifier is turned off. As a result, according to the present invention, a magnetic snubber can be applied to a flyback type DC-DC converter, the reverse recovery current of the diode D1 is greatly reduced, the loss of the rectifying element is reduced, and spike noise is reduced. Can be reduced.

Further, in the present invention, by connecting an energizing circuit for magnetically saturating the saturable reactor in parallel with the secondary winding of the transformer, the saturable state is established via the secondary winding when the switching element is turned on. By supplying a current to the control winding of the reactor (Claim 1) or by connecting an energizing circuit in parallel with the primary winding of the transformer or the DC power supply, the DC power supply can be used when the switching element is turned on. The current is caused to flow directly to the control winding of the saturation reactor (claim 2). For this reason, as an energizing circuit for magnetically saturating the saturable reactor, for example, as described in the above embodiment, a diode or a transistor that allows current to flow in one direction through the control winding of the saturable reactor when the switching element is turned on. By using a rectifying element such as described above and a current limiting resistor, the configuration can be made extremely simple.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing the entire configuration of a flyback type DC-DC converter according to an embodiment, FIG. 2 is an explanatory diagram showing the configuration of a saturable reactor, and FIG. 3 shows the magnetic characteristics of the saturable reactor. FIG. 4 is a time chart for explaining the operation of the embodiment, and FIGS. 5 and 6 are electric circuit diagrams each showing another configuration example of the flyback type DC / DC converter. 2 Transformer 4 DC power supply 6 Saturable reactor L1 Primary winding L2 Secondary winding l1 Main winding l2 Control winding C1 Capacitor D1 …… Diode (rectifier) D2 …… Diode, R1… Resistor TR …… Transistor

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasutaka Nakamori 1-1-1 Showa-cho, Kariya-shi, Aichi Japan Inside Denso Co., Ltd. (56) References JP-A-1-303056 (JP, A)

Claims (2)

(57) [Claims] 1. A transformer, a switching element for interrupting connection between a primary winding of the transformer and a DC power supply, and a secondary winding of the transformer, wherein the switching element is connected to a primary winding of the transformer. A rectifying element that supplies a current by magnetic energy stored in the transformer to charge a capacitor when the connection between the line and the DC power supply is interrupted. The main winding of the saturable reactor is provided in series with the element, and the switching element connects the primary winding of the transformer and the DC power supply in parallel with the secondary winding of the transformer. A current flowing through the control winding of the saturable reactor through the secondary winding to magnetically saturate the saturable reactor. . 2. A transformer, a switching element for interrupting connection between a primary winding of the transformer and a DC power supply, and a secondary winding of the transformer, wherein the switching element is connected to a primary winding of the transformer. A rectifying element that supplies a current by magnetic energy stored in the transformer to charge a capacitor when the connection between the line and the DC power supply is interrupted. The main winding of the saturable reactor is provided in series with the element, and the switching element connects the primary winding of the transformer and the DC power supply in parallel with the primary winding of the transformer or the DC power supply. A current flowing from the DC power supply to the control winding of the saturable reactor to magnetically saturate the saturable reactor, and a flyback type DC-DC converter.