JP2005117852A - Switching power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly efficient switching power supply device which suppresses radiation noise. <P>SOLUTION: A snubber circuit 8 is composed of a first diode 10, a first resistor 12, a series circuit of a first capacitor 11, a second diode 14, and a second capacitor 13 at both ends of a primary winding 3 of a transformer 2. Consequently, dV/dt on both ends of a switching element 4 during turning-off is suppressed low. The radiation noises can be reduced with high efficiency in order to regenerate the energy to a secondary side. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a switching power supply device that can suppress radiation noise generated from a power supply with high efficiency.

  In a conventional switching power supply device that takes a DC voltage as an input and outputs a DC voltage using a transformer, the switching element connected in series to the transformer is turned on and off to obtain a desired output voltage, but the switching element is turned off. In order to prevent the surge voltage generated by the leakage inductance of the primary winding of the transformer from being applied to the switching element, a snubber circuit is provided between both ends of the primary winding of the transformer and both ends of the switching element.

  A typical configuration of the snubber circuit is a CRD snubber circuit as shown in FIG. Since the CRD snubber circuit consumes surge energy absorbed by the capacitor 102 by the discharge resistor 103 used to suppress the breakdown voltage of the switching element or less, the efficiency cannot be increased so much, and the leakage inductance of the transformer 2 and It is well known that relatively high radiation noise is generated because high frequency ringing as shown in FIG. 11 occurs due to resonance with the parasitic capacitance of the transformer 2 and the switching element 4.

  Further, as a snubber circuit method for suppressing the radiation noise, a method is described in which a resistor 104 is provided in parallel with the diode 101 of the snubber circuit instead of connecting the discharge resistor 103 in parallel with the capacitor 102 of the snubber circuit. (For example, refer to Patent Document 1).

FIG. 12 shows a conventional switching power supply device described in Patent Document 1. In FIG. As shown in FIG. 12, a snubber circuit including a diode 101, a capacitor 102, and a resistor 103, a transformer, and a switching element are included.
JP 2000-1973370 A

  However, the conventional configuration has a problem that the energy stored in the capacitor 102 at the time of turn-off is consumed through the resistor 104 every turn-on, so that the efficiency cannot be increased so much.

  The present invention solves the above-described conventional problems, and an object thereof is to provide a switching power supply device that can suppress unnecessary radiation while achieving high efficiency with a simple configuration, particularly at a light load. And

  In order to solve the above-described conventional problem, a switching power supply apparatus according to the present invention includes a first diode connected to both ends of a primary winding of a transformer in a direction to absorb a surge voltage generated when the switching element is turned off. A series circuit composed of one capacitor and a first resistor, a second capacitor connected in parallel to the first diode, and a second diode connected in parallel to the first resistor. A snubber circuit is provided.

By charging the second capacitor at the time of turn-on, the switching element voltage at which the surge energy starts to be absorbed at the time of turn-off is lowered in the snubber circuit, and further, the second diode is affected by the first resistance. Since the surge energy is absorbed, dV / dt of the switching element voltage is also reduced, so that radiation noise can be reduced. At the same time, since the energy stored in the first capacitor is not released every time the turn-on is performed, a low-loss power source can be obtained.

  In addition, the switching power supply device of the present invention sets the ON time of the switching element at the minimum time ratio to the first capacitor and the first capacitor so that the second capacitor can be sufficiently charged at the time of turn-on even at the minimum time ratio. This is smaller than 2.2 times the time constant of the resistance.

  As a result, since the second capacitor is almost fully charged within the turn-on time, it is possible to absorb the surge voltage generated immediately after the turn-off in the snubber circuit, and to suppress the radiation noise generated from the power source to a low level. Can do.

  The switching power supply device of the present invention can increase efficiency at a light load while suppressing radiation noise with a simple configuration.

  According to a first aspect of the present invention, a series comprising a first diode, a first capacitor, and a first resistor connected to both ends of a primary winding of a transformer in a direction to absorb a surge voltage generated when the switching element is turned off. By providing a snubber circuit comprising a circuit, a second capacitor connected in parallel to the first diode, and a second diode connected in parallel to the first resistor, dV / dt at turn-off And a part of the generated surge voltage is transmitted to the secondary side of the transformer, so that radiation noise can be suppressed with high efficiency.

  According to a second aspect of the present invention, in particular, in the switching power supply device according to the first aspect of the invention, the switching element corresponding to the minimum time ratio in the switching power supply, wherein the capacity of the first capacitor is 10 times or more that of the second capacitor Is set to be longer than 2.2 times the time constant composed of the second capacitor and the first resistor, so that the charging of the second capacitor is almost completed within the turn-on time. Therefore, the dV / dt at both ends of the switching element at the time of turn-off can be suppressed low, and radiation noise can be further suppressed.

  According to a third aspect of the present invention, in the switching power supply device of the first or second aspect, by providing the second resistor in parallel with the first capacitor, a part of the generated surge voltage is consumed by the second resistor. Therefore, the peak value of the surge voltage at both ends of the switching element can be suppressed.

  According to a fourth invention, in the switching power supply device of the first or second invention, a surge voltage generated by providing a second resistor in parallel with a series circuit of the first capacitor and the first resistor. Is consumed by the first resistor and the second resistor, the peak value of the surge voltage across the switching element can be suppressed.

  According to a fifth invention, in the switching power supply device according to any one of the first to fourth inventions, by providing a third capacitor in parallel with the switching element, dV / dt of the switching element is further reduced. Noise can be further reduced.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
FIG. 1 is a block diagram showing a configuration of a switching power supply apparatus according to the first embodiment of the present invention.

  In FIG. 1, the switching power supply of the first embodiment includes a DC power supply 1 obtained by rectifying and smoothing a commercial power supply, a primary winding 3 of a transformer 2 connected between both ends of the DC power supply 1, and a power transistor. A series circuit with a switching element 4 composed of a MOSFET and a MOSFET, a secondary winding 5 of the transformer 2, a rectifying / smoothing circuit 6 connected to the secondary winding 5, and a control for turning on / off the switching element 4. This is a flyback type power source composed of a portion 7 and a snubber circuit 8 connected to both ends of the primary winding 3.

  The control unit 7 is connected to the output side of the rectifying / smoothing circuit 6 on the secondary side and drives the switching element 4 while changing the on / off time ratio based on the voltage detection unit 9 that detects the output voltage. Thus, the output voltage on the secondary side is kept almost constant.

  The snubber circuit 8 includes a series circuit of a first diode 10, a first capacitor 11, and a first resistor 12 connected in a direction to absorb a surge voltage generated when the switching element 4 is turned off, and a first diode 10. The second capacitor 13 is connected in parallel to the first resistor 12, and the second diode 14 is connected in parallel to the first resistor 12.

  Further, the first capacitor 11 can keep the voltage applied to the capacitor in a steady operation substantially constant, and the series combined capacity with the second capacitor 13 is the capacity of the second capacitor 13 alone. The capacity is necessary and sufficient so as not to be so small.

  The operation and action of the switching power supply configured as described above will be described below along the behavior of the switching power supply according to the present invention from turn-on to turn-off.

  FIG. 2 shows an example in which the leakage inductance viewed from the primary side of the transformer 2 is 10 μH, the first capacitor 11 is 0.1 μ, the second capacitor 13 is 220 pF, and the first resistor 12 is 2 kΩ. The waveform of each part is shown.

  When the switching element 4 is turned on, the DC capacitor 1 charges the second capacitor 13 through the first resistor 12 through the first capacitor 11. FIG. 2 shows a waveform for a relatively light load. Since the ON time of the switching element 4 is relatively short, the charging of the second capacitor 13 is not yet completed immediately before the switching element 4 is turned off. .

  Here, when the switching element 4 is turned off, the second capacitor 13 is continuously charged via the first resistor 12, and the capacitor voltage rises. The voltage across the switching element 4 is such that the current flowing through the primary winding 3 together with the charging current flows into the parasitic capacitance of the switching element 4 as a resonance current with the leakage inductance of the transformer 2. The voltage begins to rise (point A in FIG. 2).

Thereafter, the voltage of the second capacitor 13 continues to rise, and the sum of the voltage of the second capacitor 13 and the voltage of the first capacitor 11 is the voltage across the switching element 4, the voltage of the DC power supply 1, and When it becomes equal to the sum of the voltage drop in the first resistor 12, the voltage of the second capacitor 13 reaches a peak (point B in FIG. 2). Thereafter, since the second capacitor 13 starts to discharge, the direction of the current flowing through the second capacitor 13 is reversed.

  In a state where the second capacitor 13 is discharged, the voltage across the switching element 4 is a resonance determined by the combined capacitance of the second capacitor 13 and the parasitic capacitance of the switching element 4 and the leakage inductance of the primary winding 3. It becomes a phenomenon. Since the resonance frequency is lower than that immediately after the turn-off, the time change rate dV / dt of the voltage across the switching element 4 is further reduced compared to that immediately after the turn-off.

  As described above, the second capacitor 13 is charged up to the sum of the voltages of the DC power supply 1 and the first capacitor 11 at the maximum, but has a relatively high initial voltage immediately before the turn-on. The loss generated in the first resistor 12 due to charging / discharging of the second capacitor 13 is extremely small as compared with the conventional circuit.

  When more time elapses and the voltage of the second capacitor 13 becomes equal to the forward voltage VF of the first diode 10, the first diode 10 is turned on (point C in FIG. 2).

  When the first diode 10 is turned on, a resonance phenomenon determined by the parallel combined capacitance of the first capacitor 11 and the parasitic capacitance of the switching element 4 and the leakage inductance of the transformer 2 occurs. Since the first capacitor 11 is set to a very large capacitance compared to the parasitic capacitance, the frequency of ringing that occurs is greatly reduced.

  If the capacitance of the first capacitor 11 is set to a sufficiently large value, the voltage across the switching element 4 is completely clamped after the first diode 10 is turned on (not shown). )

  As described above, the switching power supply according to the first embodiment has a resonant operation between the leakage inductance of the transformer 2 and the parasitic capacitance of the switching element 4 immediately after the turn-off. Next, similarly, the leakage inductance and the second capacitor 13 and the resonant operation of the parallel combined capacitance of the parasitic capacitance, and finally the resonant operation of the leakage inductance and the first capacitor 11 and gradually both ends of the switching element 4 over three stages, and hence the primary winding of the transformer 2 3 suppresses steep voltage fluctuations at both ends.

  Further, during the charging period of the first capacitor 11 immediately after the turn-off, a part of surge energy generated by the leakage inductance of the transformer 2 is regenerated to the DC power source 1 via the first capacitor 11.

  When the time further elapses thereafter, the current flowing through the first capacitor 11 turns in the opposite direction, and ringing at a resonance frequency determined by the leakage inductance of the transformer 2 and the combined capacitance of the first capacitor 11 and the second capacitor 13. Occurs. In this period, power is regenerated to the secondary side via the transformer 2, so that the efficiency is high.

  At the same time, since a parallel circuit of the first resistor 12 and the second diode 14 is inserted in this current path, energy is supplied when a current flows in a direction in which the second diode 14 is reverse-biased. Since it is consumed, the ringing amplitude decays relatively quickly.

  Therefore, unlike the voltage waveform at the time of the conventional CRD snubber shown in FIG. 12, the ringing does not continuously occur for a long period, and the ringing is suppressed to be extremely small.

As described above, the switching power supply device of the present invention can suppress the voltage rise gradient dV / dt at both ends of the switching element 4 at the time of turn-off and ringing that occurs immediately thereafter without increasing the loss at the time of turn-on. As a result, noise radiated from the switching power supply can be suppressed to a low level.

  In addition, about the control part 7 of the switching power supply device of this invention, a self-excitation type or a separate excitation type may be sufficient. Further, the transformer 2 may be provided with a tertiary winding, and the duty ratio of the switching element 4 may be adjusted by the output voltage obtained by smoothing the output, or each of the secondary windings 5 may be smoothed. The duty ratio may be adjusted according to the output voltage obtained in this way.

  As described above, in the present embodiment, by providing the snubber circuit 8 described above, the dV / dt of the switching element 4 is suppressed with low loss, so that radiation noise is suppressed with high efficiency. Can do.

(Embodiment 2)
The configuration of the switching power supply device according to the second embodiment of the present invention is the same as the configuration of FIG.

  In the switching power supply according to the second embodiment of the present invention, the capacity of the first capacitor 11 is set to be one digit or more larger than the capacity of the second capacitor 13. Further, this is a separately-excited power supply in which the minimum on-time corresponding to the minimum time ratio of the switching power supply is set to be longer than 2.2 times the time constant composed of the second capacitor 13 and the first resistor 12. If the load is lighter than that, intermittent oscillation is performed to stop the oscillation.

  FIG. 3 shows voltage and current waveforms of each part in the switching power supply device according to the second embodiment of the present invention. The basic operation is the same as in the first embodiment.

  Generally, in the CR charging circuit, 90% or more of charging is completed when a time more than 2.2 times the time constant elapses. Therefore, in the second embodiment, the second charging is performed within the turn-on period. This is different in that the charging of the capacitor 13 is almost completed.

  Therefore, immediately before the turn-off, since the voltage sum of the first capacitor 11 and the second capacitor 13 is substantially equal to the DC voltage 1 on the input side, the snubber circuit 8 immediately after the turn-off. Surge voltage is absorbed. Therefore, it is suppressed to be lower than dV / dt immediately after the turn-off.

  As described above, in the snubber circuit 8, by setting the minimum on-time to be larger than 2.2 times the time constant of the second capacitor 13 and the first resistor 12, dV / dt is set immediately after the turn-off. By being suppressed, radiation noise generated from the power supply is further reduced.

(Embodiment 3)
FIG. 4 is a block diagram showing the configuration of the switching power supply device according to the third embodiment of the present invention.

  In FIG. 4, a second resistor 15 is connected to the snubber circuit 8 in parallel with the first capacitor 11.

  Regarding the switching power supply configured as described above, the basic operation and action are the same as those of the first embodiment, but the first capacitor 11 is connected to the first capacitor 11 by the second resistor 15 immediately after the turn-off. Since a part of the absorbed surge energy is discharged, the voltage peak value of the switching element 4 is lowered.

  FIG. 5 shows voltage waveforms at both ends of the switching element 4 in the switching power supply according to the third embodiment.

  As shown in FIG. 5, the energy is always discharged by the second resistor 15 with respect to the leakage inductance of the primary winding 3 of the transformer 2 and the amplitude of the resonance voltage composed of the first capacitor 11 and the second capacitor 13. Therefore, it attenuates faster.

  With the above operation, by adding the second resistor 15 to the snubber circuit 8, radiation noise generated from the power source is further reduced.

(Embodiment 4)
FIG. 6 is a block diagram showing a configuration of a switching power supply apparatus according to the fourth embodiment of the present invention.

  In FIG. 6, a second resistor 15 is connected to the snubber circuit 8 in parallel with a series circuit of a first capacitor 11 and a first resistor 12.

  Regarding the switching power supply configured as described above, the basic operation and action are the same as those of the third embodiment, but a part of the surge energy absorbed by the first capacitor 11 is the first. Is discharged by the combined resistance of the resistor 12 and the second resistor 15, the voltage applied to the second resistor 15 can be reduced, and as a result, the heat generation of the second resistor 15 can be further suppressed. Therefore, there is an effect that it is possible to use a lower watt type resistor.

  FIG. 7 shows voltage waveforms across the switching element 4 of the switching power supply according to the second embodiment.

  As shown in FIG. 7, the first resistor 12 and the second resistor 15 are also used for the leakage inductance of the primary winding 3 of the transformer 2 and the amplitude of the resonance voltage composed of the first capacitor 11 and the second capacitor 13. Because the energy is always discharged by the series resistance, the decay is faster.

  With the above operation, by adding the second resistor 15 to the snubber circuit 8, radiation noise generated from the power source is further reduced.

(Embodiment 5)
FIG. 8 is a block diagram showing a configuration of a switching power supply apparatus according to the fifth embodiment of the present invention.

  In FIG. 8, a third capacitor 16 is connected to the switching element 4 in parallel.

  FIG. 9 shows the voltage waveform across the switching element in the fifth embodiment.

  As shown in FIG. 9, the third capacitor 16 suppresses the voltage increase of the voltage across the switching element 4 at the time of turn-off, so that the dV / dt is lowered. Further, since the resonance frequency with the leakage inductance is determined by the combined capacitance of the first capacitor 11 and the second capacitor 13, it is further lowered.

  With the above operation, by connecting the third capacitor 16 in parallel with the switching element 4, radiation noise generated from the power source is further reduced.

  As described above, the switching power supply device according to the present invention can suppress radiation noise with a low loss with a simple configuration, and thus can be applied to applications such as power supply circuits for televisions, VTRs, and air conditioners. it can.

The block diagram which shows the structure of the switching power supply device in Embodiment 1 of this invention Waveform diagram of each part of the switching power supply in Embodiment 1 of the present invention Waveform diagram of each part of the switching power supply in Embodiment 2 of the present invention The block diagram which shows the structure of the switching power supply device in Embodiment 3 of this invention. Voltage waveform diagram across switching element in Embodiment 3 of the present invention The block diagram which shows the structure of the switching power supply device in Embodiment 4 of this invention. Voltage waveform diagram across switching element in Embodiment 4 of the present invention The block diagram which shows the structure of the switching power supply device in Embodiment 5 of this invention. Voltage waveform diagram across switching element in Embodiment 5 of the present invention Block diagram showing the configuration of a conventional switching power supply device Voltage waveform diagram across switching elements in a conventional switching power supply Block diagram showing the configuration of a conventional switching power supply device

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 DC power supply 2 Transformer 3 Primary winding 4 Switching element 5 Secondary winding 6 Rectification smoothing circuit 7 Control part 8 Snubber circuit 9 Voltage detection part 10 1st diode 11 1st capacitor | condenser 12 1st resistance 13 2nd Capacitor 14 Second diode 15 Second resistor 16 Third capacitor

Claims (5)

DC power supply, a series circuit of a primary winding and a switching element of a transformer connected between one end and the other end of the DC power supply, a secondary winding of the transformer, and a secondary winding connected to the secondary winding A switching power supply device that supplies a DC voltage, and is generated at both ends of the primary winding of the transformer when the switching element is turned off. A series circuit including a first diode, a first capacitor, and a first resistor connected in a direction to absorb a surge voltage; a second capacitor connected in parallel to the first diode; A switching power supply device comprising a snubber circuit including a second diode connected in parallel to one resistor. In the snubber circuit, the capacitance of the first capacitor is 10 times or more that of the second capacitor, and the minimum on-time of the switching element corresponding to the minimum duty in the switching power supply is set to the second capacitor and the first resistor. 2. The switching power supply device according to claim 1, wherein the switching power supply device is set to be longer than 2.2 times a time constant consisting of: The switching power supply device according to claim 1, wherein the snubber circuit includes a second resistor in parallel with the first capacitor. 3. The switching power supply device according to claim 1, wherein the snubber circuit includes a second resistor in parallel with a series circuit of the first capacitor and the first resistor. The switching power supply according to any one of claims 1 to 4, wherein a third capacitor is connected in parallel with the switching element.

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