JP2004040923A - Switching power circuit and switching regulator equipped therewith - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power circuit which suppressess common mode noise. <P>SOLUTION: This switching power circuit is equipped with a switching circuit 1 which switches DC voltage into AC voltage having specified frequency, and a high frequency transformer T which converts the AC voltage switched by this switching circuit 1 into a specified voltage value. This is provided with first and second resonant circuits 2 and 3 connected to both ends of the primary winding of the high frequency transformer T, between the switching circuit 1 and the high frequency transformer T. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a switching power supply circuit and a switching regulator including the power supply circuit.
[0002]
[Prior art]
Conventionally, in a switching power supply circuit of a switching regulator, a DC voltage obtained by rectifying and smoothing an AC voltage from a commercial AC power supply is converted into an AC voltage having a high frequency by switching with a switching element. Then, a desired DC voltage is generated from the AC voltage using a high-frequency transformer, and the DC voltage is used, for example, as an internal power supply voltage of an electronic device.
[0003]
FIG. 4 is a diagram illustrating an example of the switching power supply circuit. According to this circuit diagram, a switching circuit 11 is connected to an input voltage E which is obtained by rectifying a commercial AC power supply to be DC. The switching circuit 11 is configured by the first to fourth switching elements SW11 to SW14 having a full bridge configuration. Diodes D11 to D14 and capacitors C11 to C14 are connected in parallel to both ends of each of switching elements SW11 to SW14. The diodes D11 to D14 may be built in the switching elements SW11 to SW14 themselves, and the capacitors C11 to C14 are configured as junction capacitances and / or inter-terminal capacitances of the switching elements SW11 to SW14 themselves. Is also good.
[0004]
A primary winding of the high-frequency transformer T is connected to a midpoint between the first switching element SW11 and the second switching element SW12 and a midpoint between the third switching element SW13 and the fourth switching element SW14. A rectifier circuit 14 constituted by a rectifier diode bridge is connected to the secondary winding of the high-frequency transformer T. The rectifying circuit 14 is connected to a smoothing circuit 15 including a choke coil L11 and a capacitor C15.
[0005]
In such a switching power supply circuit, the first and fourth switching elements SW11 and SW14 are linked to each other, and the second and third switching elements SW12 and SW13 are linked to each other, and are turned on and off alternately. By this operation, a rectangular wave AC voltage is applied to the primary winding of the high-frequency transformer T, as shown in FIGS.
[0006]
[Problems to be solved by the invention]
However, in the above-described circuit configuration, when the load is light, the first to fourth switching elements SW11 are changed by the input capacitance of the primary winding of the high-frequency transformer T and the capacitance components of the capacitors C11 to C14 connected in parallel to the switching elements SW11 to SW14. The load on SW14 becomes capacitive. For this reason, for example, when the second and third switching elements SW12 and SW13 are turned on, the electric charges that have been stored in the capacitors C12 and C13 up to now suddenly pass through the second and third switching elements SW12 and SW13, respectively. To discharge.
[0007]
Therefore, as shown in FIG. 5 (c), the second and third switching elements SW12, SW13, it will flow steep spike current _{I S.} The steep spike current I _S not only increases the switching loss of the first to fourth switching elements SW11 to SW14, but also causes noise.
[0008]
At the time of heavy load, for example, during the on-period of the first and fourth switching elements SW11 and SW14, the current (magnetic) energy stored in the leakage inductance component of the primary winding of the high-frequency transformer T is reduced by the first and fourth currents. When the four switching elements SW11 and SW14 are turned off, they are emitted through diodes (flywheel diodes) D12 and D13, and the diodes D12 and D13 are turned on. Therefore, the voltage between the terminals of the second and third switching elements SW12 and SW13 can be reduced (to almost zero). Then, in this state, by turning on the second and third switching elements SW12, SW13, it is possible to avoid the spike current _{I S} as shown in FIG. 5 (c).
[0009]
However, even in this case, the voltage waveform applied to the primary winding of the high-frequency transformer T is a rectangular waveform. A rectangular wave voltage contains many harmonic components and can be a major cause of noise.
[0010]
Here, if the voltage and current applied to the high-frequency transformer T are sine-wave (or cosine-wave), the harmonic components are small and the generation of noise can be suppressed. Therefore, as shown in FIG. 6, there has been proposed a switching power supply circuit in which a resonance circuit 12 including a capacitor C16 and a coil L12 is inserted into one end of a primary winding of a high-frequency transformer T.
[0011]
In this switching power supply circuit, the switching frequency of the switching circuit 11 is set equal to or slightly higher than the resonance frequency of the resonance circuit 12 to shift the phase of the current flowing through the primary winding of the high frequency transformer T. As shown in FIGS. 7 (c) and 7 (d), the generation of spike current can be avoided, and the voltage waveform applied to the high-frequency transformer T can be made sinusoidal (or cosine). Therefore, it is possible to realize a switching power supply circuit in which the generation of high frequency components is suppressed and the influence of noise on other circuits is suppressed.
[0012]
However, in the switching power supply circuit shown in FIG. 6, the voltage waveform applied to one end (see point A in FIG. 6) of the primary winding of the high-frequency transformer T in which the resonance circuit 12 is provided is as shown in FIG. As shown in (2), the waveform is a waveform in which a rectangular wave and a sine wave (or cosine wave) are superimposed. On the other hand, since one end on the opposite side (see point B in FIG. 6) is directly connected to the third and fourth switching elements SW13 and SW14, the voltage waveform is as shown in FIG. As shown, it is a rectangular wave.
[0013]
Here, since the normal mode component of the voltage applied to the primary winding of the high-frequency transformer T is a difference between the voltage waveforms applied to both ends of the primary winding of the high-frequency transformer T, a sine wave (or cosine wave) ), And the current waveform flowing through the high-frequency transformer T also has a sine wave shape (or a cosine wave shape). However, since the common mode component of the voltage applied to the primary winding of the high-frequency transformer T is the average of the voltages applied to both ends of the primary winding of the high-frequency transformer T, as shown in FIG. In this case, the waveform has about half the amplitude of the waveform at the point A, and a rectangular wave and a sine wave (or a cosine wave) are superimposed.
[0014]
In the high-frequency transformer T, usually, between the primary winding and the secondary winding, between the primary winding and the core (not shown), or between the primary winding and the case (not shown). Capacitive couplings exist between them. Then, a high-frequency component of the voltage causes a current to flow through the capacitive coupling.
[0015]
The capacitive coupling usually exists in a distributed manner in a portion such as the beginning of the primary winding of the high-frequency transformer T, the end of the primary winding, or the middle of the primary winding. Therefore, the magnitude of the current flowing through the capacitive coupling tends to increase due to the common mode voltage of the primary winding of the high frequency transformer T.
[0016]
That is, the current flows from the primary winding to the secondary winding of the high-frequency transformer T (see I _C1 and I _C2 ), as shown in the circuit diagram of FIG. This is so-called common mode noise, which is fed back through the capacitance component or the insulation resistance.
[0017]
Therefore, in the switching power supply circuit shown in FIGS. 6 and 9, although a spike current in the switching power supply circuit shown in FIG. 4 can be avoided, a favorable DC current is generated by the common mode noise component applied to the primary winding of the high frequency transformer T. There is a problem that voltage cannot be supplied to the load.
[0018]
The present invention has been conceived under such circumstances, and has as its object to provide a switching power supply circuit capable of suppressing common mode noise.
[0019]
DISCLOSURE OF THE INVENTION
In order to solve the above problems, the present invention takes the following technical measures.
[0020]
A switching power supply circuit provided according to a first aspect of the present invention includes a switching circuit that switches a DC voltage to convert it to an AC voltage having a predetermined frequency, and converts the AC voltage switched by the switching circuit into a predetermined voltage value. And a transformer connected between both ends of the primary winding of the transformer, between the switching circuit and the transformer. .
[0021]
According to a preferred embodiment, the resonance circuit includes a plurality of resonance circuits, each of which is connected in series to both ends of a primary winding of the transformer.
[0022]
According to another preferred embodiment, the switching circuit includes a plurality of switching elements forming a bridge circuit, and each of the resonance circuits includes a capacitor and a coil connected to a connection end of the bridge circuit. In series.
[0023]
According to another preferred embodiment, the switching circuit includes a plurality of switching elements forming a bridge circuit, and the resonance circuit includes a plurality of capacitors each having one end connected to a connection end of the bridge circuit. And a coupling type coil connected to both ends of these capacitors and both ends of the primary winding of the transformer.
[0024]
According to another preferred embodiment, a resonance frequency of the resonance circuit is set to at least a minimum operation frequency of switching in the switching circuit.
[0025]
According to the present invention, since a plurality of resonance circuits are respectively connected in series to both ends of the primary winding of the transformer, the voltage waveform in the primary winding of the transformer can be made symmetrical. The common mode voltages input to the primary winding can cancel each other. Therefore, the common mode noise component input to the primary winding of the transformer can be suppressed.
[0026]
A switching regulator provided by the second aspect of the present invention includes the switching power supply circuit provided by the first aspect of the present invention.
[0027]
Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be specifically described with reference to the drawings.
[0029]
FIG. 1 is a block diagram showing a switching power supply circuit according to the present invention. This switching power supply circuit is applied to a switching regulator capable of outputting a stable power supply voltage, and includes a switching circuit 1, a first resonance circuit 2, a second resonance circuit 3, a high-frequency transformer T, a rectifier circuit 4, and a smoothing circuit 5. Have.
[0030]
The switching circuit 1 switches an input voltage E as a DC voltage obtained by, for example, rectifying a commercial AC power supply to convert the input voltage E into an AC voltage having a predetermined frequency (for example, 50 kHz to 200 kHz). The switching circuit 1 is configured such that the first to fourth switching elements SW1 to SW4 are configured as a full bridge circuit at both ends of the input terminal IN to which the DC voltage E is input.
[0031]
Specifically, first and second switching elements SW1 and SW2 are connected in series to both ends of the input terminal IN, and third and fourth switching elements SW3 and SW4 are connected in parallel to these. Diodes (flywheel diodes) D1 to D4 and capacitors C1 to C4 are connected in parallel to both ends of the switching elements SW1 to SW4, respectively. Note that the diodes D1 to D4 may be built in the switching elements SW1 to SW4 themselves, and the capacitors C1 to C4 are configured as junction capacitances and / or inter-terminal capacitances of the switching elements SW1 to SW4 themselves. Is also good.
[0032]
The first resonance circuit 2 resonates the output of the switching circuit 1 and makes the output waveform sine (or cosine). The first resonance circuit 2 includes a series resonance circuit including a capacitor C5 having one end connected to the midpoint between the first and second switching elements SW1 and SW2, and a coil L1 connected in series to the other end of the capacitor C5. Is done.
[0033]
Similarly to the first resonance circuit 2, the second resonance circuit 3 resonates the output of the switching circuit 1 and makes the output waveform sine (or cosine). The second resonance circuit 3 includes a capacitor C6 having one end connected to the middle point between the third and fourth switching elements SW3 and SW4, and a coil L2 connected in series to the other end of the capacitor C6.
[0034]
The high-frequency transformer T boosts the voltage value input to the primary winding to a predetermined voltage value and outputs the voltage from the secondary winding. In the high-frequency transformer T, a first resonance circuit 2 is connected to one end of a primary winding (see point A in FIG. 1), and a second resonance circuit is connected to one end on the opposite side of the primary winding (see point B in FIG. 1). Circuit 3 is connected.
[0035]
The rectifier circuit 4 is for rectifying the output of the secondary winding of the high-frequency transformer T, and includes four diodes D5 to D8 configured as a diode bridge circuit.
[0036]
The smoothing circuit 5 is connected to the rectifier circuit 4 and includes an LC circuit including a choke coil L3 and a capacitor C7. The output of the smoothing circuit 5 is provided as a power supply voltage from an output terminal OUT to a load (not shown) such as an electronic device.
[0037]
Next, the operation of the switching power supply circuit will be described. In this switching power supply circuit, an AC voltage from a commercial AC power supply is rectified by a rectifier circuit (not shown), and obtained. (For example, 50 kHz to 200 kHz).
[0038]
More specifically, the first and fourth switching elements SW1 and SW4 are linked to each other, and the second and third switching elements SW2 and SW3 are linked to each other, and are turned on and off alternately. Due to this operation, the first and second resonance circuits 2 and 3 provided at both ends of the primary winding of the high-frequency transformer T respectively have a rectangular wave shape as shown in FIGS. The alternating voltage is applied alternately.
[0039]
Here, for example, during the ON periods of the first and fourth switching elements SW1 and SW4, current (magnetic) energy is stored in the leakage inductance of the coils L1 and L2 and the primary winding of the high-frequency transformer T. When the first and fourth switching elements SW1 and SW4 are turned off, the stored energy flows through the diodes D2 and D3, and the diodes D2 and D3 are turned on. Therefore, the voltage between the terminals of the second and third switching elements SW2 and SW3 (capacitors C2 and C3) can be reduced (to substantially zero). By turning on the second and third switching elements SW2 and SW3 in this state, generation of a spike current can be prevented.
[0040]
The output of the switching circuit 1 is input to the first and second resonance circuits 2 and 3, and both ends of the primary winding of the high-frequency transformer T (see points A and B) by the first and second resonance circuits 2 and 3. 2) is a waveform in which a rectangular wave and a sine wave (or cosine wave) are superimposed as shown in FIGS. 2 (a) and 2 (b).
[0041]
These voltage waveforms are substantially symmetrical waveforms because the first and fourth switching elements SW1 and SW4 and the second and third switching elements SW2 and SW3 are alternately turned on and off. The common mode component of the voltage applied to the primary winding of the high frequency transformer T is an average of the voltage waveform applied to both ends of the primary winding of the high frequency transformer T. In this case, the common mode voltage component is In the primary winding of the high-frequency transformer T, as shown in FIG. Therefore, in this switching power supply circuit, it is possible to suppress the generation of the common mode noise transmitted through the capacitive coupling generated between the transformer and the coil.
[0042]
Further, since the output of the switching circuit 1 is converted into a sine wave (or cosine wave) by the first and second resonance circuits 2 and 3, it is possible to suppress a harmonic component of the current flowing through the high-frequency transformer T. Therefore, switching noise and loss of the first to fourth switching elements SW1 to SW4 can be suppressed.
[0043]
Returning to FIG. 1, the AC voltage supplied to the primary winding of the high-frequency transformer T is transformed into a predetermined voltage value and output from the secondary winding. The voltage output from the secondary winding is rectified by the rectifier circuit 4, smoothed by the smoothing circuit 5, and supplied to a load (not shown) as a power supply voltage.
[0044]
When the constants for setting the respective resonance frequencies of the first and second resonance circuits 2 and 3 are the same, the common mode voltage can be minimized. However, even if these constants have some variation, when one of the first and second resonance circuits 2 and 3 is provided only at one end of the primary winding of the high-frequency transformer T as in the related art. As compared with (for example, the circuit shown in FIG. 6), a sufficient canceling effect on the common mode voltage can be obtained.
[0045]
The switching characteristics of the switching circuit 1 of the switching power supply circuit are the same as those of a conventional circuit (for example, the circuit shown in FIG. 4). That is, the phase of the current flowing through the primary winding of the high-frequency transformer T can be shifted by setting the switching frequency of the switching circuit 1 to be equal to or slightly higher than the resonance frequencies of the first and second resonance circuits 2 and 3. can, it is possible to obtain a spike current I _S is avoided characteristic as shown in FIG.
[0046]
FIG. 3 is a diagram illustrating a switching power supply circuit according to a modification of the present embodiment. In this switching power supply circuit, a third resonance circuit 6 is provided instead of the first and second resonance circuits 2 and 3 shown in FIG.
[0047]
The third resonance circuit 6 is provided between the switching circuit 1 and the high-frequency transformer T and has one end connected to the middle point of the first and second switching elements SW1 and SW2, and the other end connected to the third end. And a capacitor C9 connected to the midpoint of the fourth switching elements SW3 and SW4, and a coupling coil L4 connected in series to the other ends of the capacitors C8 and C9.
[0048]
Specifically, one end of one winding of the coupling type coil L4 is connected to the other end of the capacitor C8. One end of the primary winding of the high-frequency transformer T is connected to the other end of the one winding of the coupling type coil L4. The other end of the capacitor C9 is connected to one end of the other winding of the coupling coil L4. The other end of the other winding of the coupled coil L4 is connected to one end of the primary winding of the high-frequency transformer T on the opposite side.
[0049]
The same operation and effect as those of the switching power supply circuit shown in FIG. Further, since the coupling type coil L4 has a structure in which a winding is wound around the same core, parts can be reduced in weight and size as compared with the switching power supply circuit shown in FIG. Can be.
[0050]
Of course, the scope of the present invention is not limited to the above embodiment. For example, the switching regulator to which the switching power supply circuit is applied may be a so-called module type or an on-board type.
[Brief description of the drawings]
FIG. 1 is a diagram showing a switching power supply circuit according to the present invention.
FIG. 2 is a waveform chart in the switching power supply circuit shown in FIG.
FIG. 3 is a diagram showing a modification of the switching power supply circuit shown in FIG. 2;
FIG. 4 is a diagram showing a conventional switching power supply circuit.
FIG. 5 is a waveform chart in the conventional switching power supply circuit shown in FIG.
FIG. 6 is a diagram showing another conventional switching power supply circuit.
FIG. 7 is a waveform diagram in another conventional switching power supply circuit shown in FIG.
FIG. 8 is a waveform diagram of another conventional switching power supply circuit shown in FIG.
FIG. 9 is a diagram showing another conventional switching power supply circuit.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Switching circuit 2 1st resonance circuit 3 2nd resonance circuit C1-C9 Capacitor D1-D4 Diode L1-L3 Coil L4 Coupling type coil SW1-SW4 Switching element T High frequency transformer

Claims (6)

A switching power supply circuit comprising: a switching circuit that switches a DC voltage to convert it into an AC voltage having a predetermined frequency; and a transformer that converts the AC voltage switched by the switching circuit into a predetermined voltage value.
A switching power supply circuit, wherein a resonance circuit connected to both ends of a primary winding of the transformer is provided between the switching circuit and the transformer. 2. The switching power supply circuit according to claim 1, wherein the plurality of resonance circuits are connected in series to both ends of a primary winding of the transformer. 3. The switching circuit comprises a plurality of switching elements constituting a bridge circuit,
The switching power supply circuit according to claim 2, wherein each of the resonance circuits is configured by a series circuit including a capacitor and a coil connected to a connection end of the bridge circuit. The switching circuit comprises a plurality of switching elements constituting a bridge circuit,
The resonance circuit includes a plurality of capacitors each having one end connected to a connection end of the bridge circuit, and coupling coils connected to the other ends of these capacitors and both ends of the primary winding of the transformer. The switching power supply circuit according to claim 1. The switching power supply circuit according to claim 1, wherein a resonance frequency of the resonance circuit is set to at least a minimum operation frequency of switching in the switching circuit. A switching regulator, comprising the switching power supply circuit according to claim 1.

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