JPH11196573A - Snubber circuit of switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a snubber circuit which is composed of a small number of components, needs no adjustment and exhibits sufficient counter electromotive force absorption characteristics. SOLUTION: The snubber circuit of a switching power supply in which a switching device Q1 connected in series to the primary winding Lp1 of a transformer T1 to which a DC voltage is applied is turned on and off to apply a current to the primary winding Lp1 intermittently and an AC voltage is induced in the secondary winding Ls is composed of the series circuit of a diode D1 and a capacitive varistor ZN. The series circuit is connected in parallel to the primary winding Lp1 so as to have the polarity of the diode D1 opposite to the polarity of a voltage applied to the primary winding Lp1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a snubber circuit used for various switching power supplies including a DC-DC converter, a chopper circuit (step-up / step-down system), a switching regulator, and the like.

[0002]

2. Description of the Related Art There are various types of switching power supplies as described above, such as a forward type and a flyback type. In each case, a current flowing through a primary winding of a transformer, which is an inductive element, is applied to the switching element. To cause a voltage (current) to be induced in the secondary winding. Therefore, a back electromotive voltage (surge reverse voltage) is generated at both ends of the primary winding of the transformer. To absorb this back electromotive voltage, a snubber circuit is provided.

FIGS. 5 to 7 show examples of a conventional switching power supply provided with a snubber circuit or a reverse voltage absorbing circuit in place of the snubber circuit. In the switching power supply of Conventional Example 1 shown in FIG. 5, a switching element (FET) Q1 is connected in series to a primary winding Lp1 of a high-frequency transformer 1, and power supply terminals 1 and 2 (1 are connected to the anode side) are connected to the series circuit. Terminal, 2 is the cathode side terminal)
By applying a DC voltage fed between the power supply and the DC voltage smoothed by the smoothing capacitor C1 and turning on / off (ON / OFF) the switching element Q1 by the control circuit 5, the current flowing through the primary winding Lp1 is interrupted.

Accordingly, the secondary winding L of the transformer T1
s, an AC voltage is induced in the rectifying diode D3.
Rectifying and smoothing is performed by a rectifying and smoothing circuit including a commutating diode D4, a choke coil L3, and a smoothing capacitor C3, and a DC voltage is output between the output terminals 3 and 4.

The control circuit 5 operates by a DC voltage supplied between the terminals Vsp and Vsn, and outputs the output terminals 3 and 4.
The DC voltage (output voltage) output during the detection is detected and compared with a preset constant voltage, and a pulse width-modulated according to the error is applied to the gate of the switching element Q1 to turn it on / off. The period ratio is controlled to keep the output voltage at the set voltage.

In such a switching power supply, since each winding of the transformer 1 is an inductive element, when a current flowing through the primary winding Lp1 is cut off, a high back electromotive voltage is generated at both ends of the primary winding Lp1. Generates noise. Further, since a high voltage is applied to the switching element Q1, a high withstand voltage is required, and there is a problem that resetting of the magnetic flux of the primary winding Lp1 is delayed.

For this reason, in the conventional example 1 shown in FIG.
A capacitor C is connected in parallel with the primary winding Lp1 of the transformer T1.
A snubber circuit 6 configured by connecting a diode D1 in series to a parallel circuit of the resistor 2 and the resistor R1 is connected to absorb the back electromotive voltage.

The switching power supply of the prior art 2 shown in FIG. 6 has the same configuration as the switching power supply of FIG. 5, except that a primary winding Lp1 and an absorption winding Lp2 are connected in series on the primary side as a transformer. Using the transformer T2 provided
The connection point between the primary winding Lp1 and the absorption winding Lp2 is connected to the anode side of the DC power supply, and the other end of the absorption winding Lp2 is connected to the cathode side of the DC power supply via the diode D2. The voltage absorption circuit 7 is constituted. The reverse voltage absorbing circuit 7 absorbs the reverse voltage at the rising portion below a desired clamp voltage.

Further, the switching power supply of the conventional example 3 shown in FIG. 7 is provided with both the snubber circuit 6 shown in FIG. 5 and the reverse voltage absorption circuit 7 shown in FIG. 6 to provide the primary winding Lp1 of the transformer T2. This is intended to more completely absorb the back electromotive voltage generated in the above.

[0010]

However, it was difficult to completely absorb the back electromotive voltage generated in the primary winding Lp1 only by the snubber circuit 6 shown in FIG. Also, FIG.
When the reverse voltage absorbing circuit 7 shown in FIG. 7 or a combination circuit of the reverse voltage absorbing circuit 7 and the snubber circuit 6 is provided, the clamp voltage changes depending on the degree of coupling between the primary winding Lp1 and the absorbing winding Lp2. In doing so, it was necessary to adjust the number of windings of the absorbing winding Lp2 in order to clamp it to a desired voltage.

Therefore, there is a problem that not only the manufacturing cost of the transformer T2 is increased, but also a mounting space for providing the absorbing winding Lp2 is required. The circuit shown in FIG. 7 also has a disadvantage that the number of components constituting the circuit for absorbing the back electromotive force is increased.

The present invention has been made in view of such conventional problems, and has as its object to provide a snubber circuit having a small number of components, requiring no adjustment, and exhibiting sufficient back electromotive force absorption characteristics. I do.

[0013]

According to the present invention, in order to achieve the above object, a switching element is connected in series to a primary winding of a transformer to which a DC voltage is applied as described above,
A snubber circuit provided in a switching power supply for interrupting a current flowing through the primary winding by turning on / off the switching element and inducing an AC voltage in a secondary winding of a transformer includes a diode and a capacitive varistor. And the series circuit is connected in parallel with the primary winding of the transformer so that the direction of the diode is opposite to the voltage applied to the primary winding. Things.

In this snubber circuit, if a resistor is connected in parallel with the above-mentioned capacitive varistor, a counter electromotive voltage equal to or lower than a certain voltage can be absorbed. Further, when an inductive element is connected in series to one or both ends of the capacitive varistor, high frequency noise superimposed on the back electromotive voltage generated in the primary winding can be absorbed.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. First, FIG. 1 is a circuit diagram of a switching power supply provided with a snubber circuit according to a first embodiment of the present invention. This switching power supply has the same circuit configuration as the conventional switching power supply described with reference to FIG. 5, and has a snubber circuit 8 according to the present invention instead of the snubber circuit 6.

This snubber circuit is a series circuit of a diode D1 and a capacitive varistor ZN. The series circuit is connected in parallel to a primary winding Lp1 of a transformer T1, which is an inductive element, and the direction of the diode D1 is one. The connection is made in a direction opposite to the voltage applied to the next winding Lp1.

In this embodiment, one end of a primary winding Lp1 of a transformer T1, which is an inductive element, is connected to the anode side of a DC power supply supplied between the power supply terminals 1 and 2, and the other end is connected to an FET.
, And the source (or emitter) of the switching element Q1 is connected to the cathode side. And
The pulse width modulated pulse output from the control circuit 5 is applied to the gate (or base) of the switching element Q1.
And the switching element Q1 is turned on / off to interrupt the current flowing through the primary winding Lp1 of the transformer T1 and to induce an AC voltage in the secondary winding Ls.

Therefore, the anode of the diode D1 constituting the snubber circuit 8 is connected to the connection point between the primary winding Lp1 and the drain of the switching element Q1, and the cathode of the diode D1 is connected to one end of the capacitive varistor ZN. ,
The other end of the capacitive varistor ZN is connected to the anode side (power supply terminal 1 side) of the DC power supply.

The feature of the snubber circuit 8 is that the capacitive varistor ZN is connected to the capacitor Cz like the equivalent circuit shown in FIG.
And a series circuit of a diode D1 and this capacitive varistor ZN are connected in parallel to the primary winding Lp1 of the transformer T1, so that an effective snubber can be realized with a small number of parts. That is, a circuit is configured. By the snubber circuit 8, the back electromotive voltage generated in the primary winding Lp1 of the transformer T1 which is an inductive element can be reduced to a certain voltage or less. As a result, noise can be suppressed and the voltage applied to the switching element Q1 can be reduced. it can.

However, when the back electromotive voltage generated in the primary winding Lp1 is lower than a certain voltage, the back electromotive voltage cannot be suppressed, so that another snubber circuit is required. That is, there is an effect as a back electromotive force absorption circuit having a certain voltage or more.

FIG. 3 is a circuit diagram of a switching power supply provided with a snubber circuit according to a second embodiment of the present invention. In this embodiment, a snubber circuit 9 is connected in parallel to the primary winding Lp1 of the transformer T1 instead of the snubber circuit 8 in FIG.

The snubber circuit 9 has a configuration in which the resistor R1 is connected in parallel with the capacitive varistor ZN in addition to the configuration of the snubber circuit 8 in FIG. The feature of this snubber circuit 9 is that, in addition to the features of the first embodiment, a resistor R
By 1 the counter electromotive voltage below a certain voltage can be suppressed. That is, the back electromotive voltage generated in the primary winding Lp1, which is an inductive element, can be sufficiently suppressed. As a result, noise can be suppressed and the voltage applied to the switching element Q1 can be further reduced. Further, the primary winding Lp1
Can be reset quickly.

FIG. 4 is a circuit diagram of a switching power supply provided with a snubber circuit according to a third embodiment of the present invention. In this embodiment, a snubber circuit 10 is provided instead of the snubber circuit 9 of the second embodiment shown in FIG.

The snubber circuit 10 has the same structure as the snubber circuit 9 shown in FIG.
Inductive elements L1 and L2, such as coils, are connected in series to both ends, respectively, and the other end of the inductive element L1 is connected to the anode side of the DC power supply, and the other end of the inductive element L2 is connected to the cathode of the diode. doing.

The feature of the snubber circuit 10 is that, in addition to the features of the second embodiment, high frequency noise superimposed on the back electromotive voltage generated in the primary winding Lp1 of the transformer T1 can be absorbed. Thereby, the back electromotive voltage of the primary winding Lp1 can be stably and sufficiently suppressed.

It is effective to connect an inductive element in series only to one end of the capacitive varistor ZN constituting the snubber circuit. Further, even if an inductive element such as a coil is connected in series to one or both ends of the capacitive varistor ZN in the snubber circuit 8 (FIG. 1) of the first embodiment, the effect of absorbing high-frequency noise can be obtained.

[0027]

According to the present invention, the back electromotive voltage generated in the primary winding of the transformer of the switching power supply can be reduced to a certain voltage or less by the capacitive varistor.
The back electromotive voltage can be sufficiently suppressed without providing an absorption winding in the transformer or using a large number of components. Since the winding for absorption is not used, it is not necessary to adjust the number of windings, and the circuit configuration has a small variation in the clamp voltage, so that the performance can be improved and the cost can be reduced.

According to the second aspect of the present invention, it is possible to further absorb a counter electromotive voltage equal to or less than a certain voltage, thereby suppressing noise and reducing the voltage applied to the switching element, thereby reducing the magnetic flux of the primary winding. Reset can be quicker. According to the third aspect of the present invention, high frequency noise superimposed on the back electromotive voltage can be further absorbed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a switching power supply provided with a snubber circuit according to a first embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of a capacitive varistor used in the present invention.

FIG. 3 is a circuit diagram of a switching power supply provided with a snubber circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram of a switching power supply provided with a snubber circuit according to a third embodiment of the present invention.

FIG. 5 is a circuit diagram showing an example of a conventional switching power supply provided with a snubber circuit.

FIG. 6 is a circuit diagram showing another example of a switching power supply provided with a conventional reverse voltage absorption circuit.

7 is a circuit diagram of a switching power supply provided with the snubber circuit 6 of FIG. 5 and the reverse voltage absorption circuit 7 of FIG.

[Explanation of symbols]

 1: Power supply terminal (anode side) 2: Power supply terminal (cathode side) 3, 4: Output terminal 5: Control circuit 8, 9, 10: Snubber circuit T1, T2: Transformer Lp1: Primary winding Ls: Secondary winding Line Q1: Switching element D1 to D4: Diode ZN: Capacitive varistor C1 to C3: Capacitor R1: Resistance L1, L2: Inductive element

Claims (3)

[Claims] 1. A switching element is connected in series to a primary winding of a transformer to which a DC voltage is applied, and a current flowing through the primary winding is intermittently turned on / off by the switching element. A switching power supply is provided for inducing an AC voltage in the secondary winding. The switching power supply comprises a series circuit of a diode and a capacitive varistor. The series circuit is connected in parallel to the primary winding of the transformer, and the direction of the diode is changed. A snubber circuit connected in a direction opposite to a voltage applied to the primary winding. 2. The snubber circuit according to claim 1, wherein a resistor is connected in parallel with said capacitive varistor. 3. The snubber circuit according to claim 1, wherein an inductive element is connected in series to one or both ends of the capacitive varistor.