JPH05316730A - Switching power source - Google Patents

Abstract

PURPOSE:To reduce switching loss of a switching power source by connecting a second switching element to be turned ON only at the time of generating a flyback voltage in series with a surging absorption capacitor. CONSTITUTION:A second switching element 6 is so controlled that a current direction is conducted in both directions at the time of generating a flyback voltage to charge/discharge a surging absorption capacitor 2. Thus, ringing component of a surge voltage can be attenuated in both rising and falling directions. When energy is discharged by ON of the element 6, the flyback voltage is lowered, and the element 6 is turned OFF. A first switching element 3 is turned ON by an applied voltage of OV. As a result, abrupt flow of a current by charges remaining in a circuit 4, the elements 3, 6 is avoided. Thus, noise and heating loss are suppressed, and switching loss can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device using a switching element.

[0002]

2. Description of the Related Art Recently, electronic devices have been remarkably miniaturized, and small and highly efficient switching power supply devices have been widely used for these electronic devices. Further, miniaturization of electronic devices further requires miniaturization of switching power supply devices, and in response to the needs, miniaturization by high frequency has been attempted. That is, attention has been paid to the fact that components such as a transformer and a capacitor can be reduced by increasing the switching frequency, and the switching power supply device has been miniaturized.

However, as the frequency increases, there are problems that must be solved, such as an increase in switching loss (increase in heat generation of the switching element), an increase in switching noise (increase in heat generation loss in the noise absorption circuit element), and these are devices. If the size of the device is reduced, a large problem of heat concentration is brought about, so that the size of the device is limited in the end.

As a means for solving such a problem, research and development of a resonance converter and a partial resonance converter are under way. Although the resonant converter has been put to practical use in a large-capacity power supply, it has not been put to practical use in general because of problems such as an increase in the number of parts and a large stress on switching elements and transformers. The partial resonance converter is a method of increasing the frequency by utilizing the characteristics of the resonance converter, such as low switching loss and low noise, in the switching part of the existing power supply, and is the method that is currently receiving the most attention. However, although various circuits have been proposed by various manufacturers based on the concept of the resonance converter and the partial resonance converter, few proposals consider the cost that can be applied to the actual design of the control circuit, which is a key point.

Regarding this partial resonance converter system,
In a typical conventional circuit, for example, as shown in FIGS. 13 and 14, the switching element SW1 generated in the transformer T is generated.
As a circuit to suppress the application of surge voltage to the
A so-called snubber circuit using a resistor R (FIG. 13) or C, R and D (diode) (FIG. 14) is used.

[0006]

However, as shown in FIG.
In the C / R snubber circuit shown in (1), there is a problem that the heat loss of the resistor R is large, and if the resistance value of the resistor R is reduced in order to suppress the heat loss, a start failure tends to occur.
After all, the resistance value cannot be made so small, and it can be applied only to a switching power supply device having a small capacity.

The CR / D snubber circuit shown in FIG. 14 has the following problem in addition to the problem that the heat loss of the resistor R is large. That is, when the switching element SW1 is turned on and off, a voltage is intermittently and repeatedly applied to the transformer T from a power source (not shown), but the switching element SW1 is turned off and the voltage V
When is applied, surging tends to occur as shown in FIG. The ringing component when the voltage rises becomes noise. The C / R / D snubber circuit tries to suppress the ringing component at the time of voltage rise to a small level. However, although it has an effect in the voltage rising direction, it has no effect in the voltage rising direction, and as shown in FIG. It has only a perfect waveform. That is, since the surge (ringing) voltage is absorbed in one direction, the noise absorption effect is small.

In addition, the conventional C / R snubber circuit and C / R /
The switching power supply device using the D snubber circuit also has the following problems. For example, FIG. 17 shows a more detailed circuit configuration of a switching power supply device using a CR / D snubber circuit. In such a circuit, the switching element SW1 is turned on while voltage remains in the circuit or element. Then, the switching element SW1 consumes power. Therefore, as shown in the actual operation switching waveform of the circuit shown in FIG. 17 in FIG. 18 (V _DS is the applied voltage of the switching element SW1, I _D is the current flowing through the switching element SW1), when the switching element SW1 is turned on,
The electric current that has remained in the circuit or element causes a sudden current to flow,
It causes switching loss (SL1 portion in FIG. 18). In addition, as described above, there is a noise generation and a heat generation loss in the snubber circuit for absorbing the noise (see FIG. 1).
SL2 part in 8.). FIG. 19 is a schematic diagram showing this in a more understandable manner. Such switching losses SL1 and SL2 increase as the frequency increases and the number of repetitions of switching increases.

In the present invention, attention is paid to the problems in the prior art as described above, the heat generation loss and the generated noise can be suppressed to a small level by the partial resonance system, and the switching loss as a whole can be suppressed, and the circuit is simple. Therefore, it is an object of the present invention to provide a circuit configuration of a switching power supply device which can realize low cost and can be used for actual design.

[0010]

A switching power supply device according to the present invention, which is directed to the above object, includes a voltage surging absorbing capacitor on a primary side of a transformer and a voltage intermittently applied to the primary side of the transformer. The first to turn on and off
In the switching power supply device having the switching element of, the primary side of the transformer is connected between the surging absorption capacitor and the first switching element, and is turned on only when a flyback voltage is generated on the primary side of the transformer. A second switching element is connected in series with the surging absorbing capacitor.

The circuit of this switching power supply device has a second
The field effect transistor (hereinafter, F
Called ET. ) Is used to set the polarity and gate voltage so that the second switching element is turned on when the flyback voltage is generated by directly controlling the on-off control of the second switching element from the switching transformer. It can be easily constructed by directly applying the voltage between them.
That is, in the conventional switching power supply device of the ringing choke type, the snubber circuit can be simply configured by a circuit in which the surging absorption capacitor is simply replaced with a circuit for performing on / off control by the FET.

[0012]

In such a switching power supply device,
The second switching element is turned on when the flyback voltage is generated. Since the second switching element is connected in series to the surging absorption capacitor, when the flyback voltage is generated, the second switching element is turned on to make the current direction bidirectionally conductive, enabling charging and discharging of the surging absorption capacitor. become. The generation of the surge voltage including the ringing component that causes noise depends on the flyback voltage at the moment when the first switching element is turned off. Therefore, when the flyback voltage is generated, the current direction becomes bidirectional conduction. By controlling the switching element to enable charging / discharging of the surging absorbing capacitor, the ringing component of the surge voltage can be attenuated in both the rising and falling directions of the ringing voltage by charging / discharging the capacitor. As a result, noise is suppressed to be small and heat generation loss is also suppressed to be small.

When the energy stored when the second switching element is turned on is released, the flyback voltage drops, and the second switching element turns off eventually. At this time, due to the parasitic capacitance of the second switching element, the charge charged in the surging absorption capacitor tries to return to the transformer side, so that the voltage polarity becomes opposite to that when the capacitor is charged, and accordingly the first Electric charges are also extracted from the switching element side of. That is, the operation of pushing down the voltage applied to the first switching element is performed. Then, the voltage applied to the first switching element is completely reduced to zero, and the first switching element becomes 0
It can be turned on in the applied voltage state of V. As a result, when the first switching element is turned on, a situation in which a current suddenly flows due to the electric charge remaining in the circuit or the element is avoided, and the switching loss is reduced.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a circuit of a switching power supply device according to an embodiment of the present invention, and FIG. 2 shows a main part thereof. In FIG. 1, a voltage surging absorption capacitor 2 (C) is provided on the primary side of a transformer 1, and a first switching element 3 (SW1) that is repeatedly turned on and off to intermittently apply a voltage to the primary side of the transformer. ) And are provided. Switching element 3 (SW1) is
On / off control is performed by the control circuit 4 using the forward voltage of the transformer 1. Reference numeral 5 denotes the secondary side of the transformer 1, and a voltage of a certain frequency, which is induced by on / off control of voltage application on the primary side, is applied to the load.

A primary side of the transformer 1 is connected between the surging absorbing capacitor 2 (C) and the first switching element 3 (SW1), and is turned on only when a flyback voltage is generated on the primary side of the transformer 1. The second switching element 6 (SW2) is connected in series with the surging absorption capacitor 2 (C). The second switching element 6 (SW2) is composed of an FET with a built-in diode. The main part of the primary side of the transformer 1 in FIG. 1 is as shown in FIG.

As a result of observing the actual operation of the switching power supply device circuit of FIG. 1, the switching waveform as shown in FIG. 3 was obtained. V _DS is the first switching element 3 (SW1)
The applied voltage, I _D , of the first switching element 3 (SW
The current flowing through 1) is shown. The switching waveform and the circuit operation will be described below. The first
If the switching element 3 (SW1) and the second switching element 6 (SW2) are turned on at the same time, the switching element may be destroyed. However, the first switching element 3 (SW1) is turned on when the forward voltage of the transformer 1 is generated. Therefore, since the second switching element 6 (SW2) is turned on only when the flyback voltage is generated, both switching elements are not turned on at the same time.

Next, the circuit operation and the obtained waveform will be described with reference to FIGS. FIG. 4 shows the circuit of FIG. 1 more schematically for the sake of explanation. First, when the input voltage V _IN is applied from the power source 7, the first switching element 3 (SW1) is turned on, the current I ₁ flows, and the magnetic energy is stored in the transformer 1 (T). At this time, the second switching element 6 (SW2) is in the off state. That is, the control voltage V _GS for the second switching element 6 (SW2) from the winding NP2 of the transformer 1 is in the reverse bias voltage applied state.

Next, the control circuit 4 turns off the first switching element 3 (SW1). The stored magnetic energy causes flyback voltage generation, and is discharged to the secondary side as a current I ₂ from the secondary winding NS1 of the transformer 1.

When this flyback voltage is generated, the second switching element 6 (SW2) is turned on. The control circuit can be easily configured by setting the polarity and the gate voltage so that the NP2 is turned on when the flyback voltage is generated, and directly applying it between the source and gate of the second switching element 6 (SW2). First switching element 3
The surge voltage (flyback voltage + surge voltage) generated at the instant when (SW1) is turned off is charged into the surging absorption capacitor 2 (C) through the built-in diode of the second switching element 6 (SW2), and is a sudden voltage. Absorb the rising edge of. Further, FIG. 5 shows a conventional snubber circuit, and FIG.
As shown in comparison with the circuit of this embodiment, in this embodiment, since the second switching element 6 (SW2) is turned on, the current direction becomes conductive in both directions. The capacitor 2 (C) can be charged and discharged. As a result, FIG. 7 compares the surge (ringing) voltage waveform (cut at a certain level only in the voltage rising direction) by the conventional snubber circuit, and FIG. 8 compares the surge (ringing) voltage waveform by the circuit of this embodiment. As shown in the figure, in this embodiment, the ringing waveform of V _DS is suppressed to be small in both the rising and falling directions of the voltage. Therefore, noise can be suppressed small. Further, in the circuit of the present invention, since the resistor R is not essentially required, there is no heat loss due to the resistor R, and the heat loss is extremely low. Therefore, miniaturization and high efficiency can be easily achieved even at high frequencies. A switching power supply device is obtained.

When the stored energy continues to be released, the flyback voltage drops, the flyback voltage generated in NP2 also drops, and the second switching element 6 (SW2) turns off eventually. At this time, as shown in FIG. 9, since the electric charge charged in the capacitor 2 (C) tries to return to NP1 due to the parasitic capacitance of the second switching element 6 (SW2), the NP1 operates the capacitor 2 (C). becomes reverse voltage and when the charge, further V _DS from the state of V _DS <V _iN drops to 0V, the first switching element 3 (S
The charge accumulated in the parasitic capacitance of W1) is extracted. A so-called resonance phenomenon between the coil L and the capacitor C occurs. Figure 1
As shown in 0 and 11, between the capacitor 2 (C) and NP1, the voltage polarity of NP1 when charging the capacitor 2 (C) (FIG. 10) and when discharging from the capacitor 2 (C) The voltage polarity of NP1 (FIG. 11) is opposite to each other.

When the above-mentioned charge extraction phenomenon, that is, the action of lowering V _DS further progresses, V _DS becomes lower than 0 V, and V of the diode with the first switching element 3 (SW1) built-in.
Above F, current flows through this diode.
In the current I _D waveform of FIG. 3, the negative current portion (A)
It is because of this effect that Therefore, the first
The applied voltage V _DS of the switching element 3 (SW1) is surely lowered to 0V.

Again the first switching element 3 (SW1)
However, the current I _D does not flow immediately because the above voltage drop is fast. Then, the first switching element 3
Since (SW1) is turned on, a current flows out through NP1, the operation returns to that shown in FIG. 4, and the same operation is repeated thereafter.

The waveforms of the voltage V _DS and the current I _D obtained by the above operation are schematically shown in FIG. 12 for comparison with the conventional waveforms shown in FIG. As shown in FIG. 12, the first
The switching loss when the switching element 3 (SW1) is turned on disappears, and the first switching element 3 (SW1)
The ringing waveform at the voltage rising portion at the time of off is also suppressed to a small fluctuation, and the heat generation loss and generated noise can be suppressed to be small.

[0024]

As described above, according to the switching power supply device of the present invention, the second switching element, which is turned on only when the flyback voltage is generated, is connected in series to the surging absorption capacitor, and no resistor is required. In addition, since the surging voltage can be absorbed in both directions, there is no heat loss due to resistance, and a circuit with extremely low heat loss and low noise can be configured as a whole. Therefore, it is possible to obtain a switching power supply device of small size, high efficiency and low noise even at high frequencies.

Further, after the applied voltage of the first switching element is surely lowered to 0V, the first switching element can be turned on to allow the current to flow, so that the switching loss in this portion can be eliminated. A highly efficient switching power supply device with extremely small switching loss can be realized. And the switching power supply device of the present invention,
The higher the frequency, the more effective it is.

Furthermore, a resistor is not required, and a simple circuit configuration in which a second switching element composed of an FET is connected in series to a surging absorbing capacitor instead of the conventional snubber circuit is sufficient. Further downsizing and cost reduction can be achieved.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a schematic circuit diagram showing a main part of the circuit of FIG.

FIG. 3 is a switching waveform diagram in which an actual operation of the device of FIG. 1 is observed.

FIG. 4 is a schematic circuit diagram schematically showing the circuit of FIG.

FIG. 5 is a schematic circuit diagram showing a current flow direction of a conventional snubber circuit.

6 is a schematic partial circuit diagram showing the direction of current flow in the circuit of FIG.

FIG. 7 is a waveform diagram of a voltage applied to a first switching element when a conventional snubber circuit is used.

8 is a waveform diagram of a voltage applied to the first switching element when the circuit of FIG. 6 is used.

FIG. 9 is a schematic circuit diagram schematically showing the circuit of FIG. 1 and showing an operation state different from that of FIG.

10 is a schematic circuit diagram showing resonance of L and C in the circuit of FIG.

11 is a schematic circuit diagram showing a state of reverse voltage polarity to that of FIG. 10 regarding resonance of L and C in the circuit of FIG.

12 is a schematic waveform diagram of the voltage and current applied to the first switching element in the circuit of FIG.

FIG. 13 is a circuit diagram of a conventional snubber circuit.

FIG. 14 is a circuit diagram of another conventional snubber circuit.

FIG. 15 is a waveform diagram showing voltage surging that generally tends to occur in a switching power supply device.

16 is a voltage waveform diagram when the snubber circuit of FIG. 14 is used.

17 is a circuit diagram of a switching power supply device using the snubber circuit of FIG.

FIG. 18 is a switching waveform diagram in which an actual operation of the switching power supply device of FIG. 17 is observed.

FIG. 19 is a schematic waveform diagram of the voltage and current applied to the first switching element, schematically showing the waveform of FIG. 18.

[Explanation of symbols]

1 Transformer 2 Surging Absorption Capacitor 3 First Switching Element 4 Control Circuit 5 Secondary Side of Transformer 6 Second Switching Element 7 Power Supply V _DS Applied Voltage of First Switching Element I _D Current of First Switching Element

Claims (3)

[Claims] 1. A switching power supply device having a voltage surging absorption capacitor on a primary side of a transformer, and a first switching element which is repeatedly turned on and off for intermittently applying a voltage to the primary side of the transformer, A second switching element, which is connected to the primary side of the transformer and is turned on only when a flyback voltage is generated on the primary side of the transformer, is provided between the surging absorption capacitor and the first switching element. A switching power supply device characterized by being connected in series to a capacitor. 2. The switching power supply device according to claim 1, wherein the second switching element is a field effect transistor. 3. The first switching element is turned on after the applied voltage is reduced to 0V.
Or the switching power supply device of 2.