JP2002078339A - Dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the noise of a DC-DC converter and to improve the efficiency of the DC-DC converter. SOLUTION: A series circuit composed of a switching device Q1 and the primary winding 8 of a transformer 2 is connected to a DC power supply 1. Second and third diodes D2 and D3 and second and third switching devices Q2 and Q3, comprising a synchronous rectifier circuit, pare connected to the secondary winding 9 of the transformer 2. The second and third switching devices Q2 and Q3 are driven by the voltage of the secondary winding. A surge absorption circuit 15a is connected to the primary winding 8. The surge absorption circuit 15a consists of a series circuit, having a long accumulation time and comprising a diode 21, a resistor 20, and a capacitor 17.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a DC-DC converter for supplying DC power to a load, that is, a DC-DC converter.

[0002]

2. Description of the Related Art A typical conventional DC-DC converter includes a switching element connected to a DC power supply via a primary winding of a transformer, and an on / off switching element. And a rectifier circuit connected to the secondary winding of the transformer, and a smoothing circuit connected to the rectifier circuit. In this type of DC-DC converter, the rectifier circuit is constituted by a diode or a Schottky barrier diode. However, the voltage drop at the diode becomes about 0.5 to 0.8 V, which causes a decrease in the efficiency of the DC-DC converter. This reduction in efficiency becomes remarkable especially when the output voltage is relatively low, such as 2 V, 3.3 V, or 5 V. In order to solve this kind of problem, it is known to configure a synchronous rectifier circuit by adding a switch element to the rectifier circuit as shown in FIG.

The DC-DC converter shown in FIG.
A DC-DC converter, for example, a DC power supply 1 composed of a rectifying and smoothing circuit, a transformer 2, a first switching element Q1 as a main switching element, and a synchronous rectifying circuit 3 , A smoothing circuit 4, a control circuit 5, a voltage detection circuit 6, a snubber capacitor C, and a first diode D1. The transformer 2 has primary and secondary windings 8, 9 wound around a magnetic core 7 and electromagnetically coupled to each other. The first switching element Q1 composed of an FET has a drain electrode D and a source electrode S as first and second main terminals, and a gate electrode G as a control terminal. One terminal of the first switching element Q1, ie, a drain electrode D, is connected to one terminal 1a of the DC power supply 1 through a primary winding 8 having inductance, and the other terminal, ie, the source of the first switching element Q1, is a source. The electrode S is connected to the other terminal 1b of the DC power supply 1. The synchronous rectifier circuit 3 is connected to one end 9a and the other end 9b of the secondary winding 9, and includes second and third switching elements Q2 and Q3 for synchronous rectification, each of which is composed of an FET, and second and third diodes. D2 and D3. A drain electrode, which is a first main terminal of the second switching element Q2, is connected to the other end 9b of the secondary winding 9, and a gate as this control terminal is provided.
The third electrode is connected to one end 9 a of the secondary winding 9. Third
The drain electrode as the first main terminal of the switching element Q3 is connected to one end of the secondary winding 9, and the source electrode as the second main terminal is connected to the second main terminal of the second switching element Q2. The gate electrode as a control terminal is connected to the other end 9 b of the secondary winding 9. The second and third diodes D2 and D3 are the second diodes.
And the third switching element Q2, Q3. The direction of the second diode D2 is determined so as to be forward-biased by the voltage obtained on the secondary winding 9 during the ON period of the first switching element Q1. The direction of the third diode D3 is the direction of the first switching element Q1.
Is determined to be reverse-biased by the voltage obtained at the secondary winding 9 during the ON period of the power supply. First, second and third
Diodes D1, D2 and D3 are built-in or parasitic diodes formed on the same semiconductor substrate as the first, second and third switching elements Q1, Q2 and Q3. Primary winding 8
The polarity of the secondary winding 9 is set as shown by a black circle in FIG. Accordingly, the second switching element Q2 and the second diode D2 connected to the secondary winding 9 conduct during the ON period of the first switching element Q1.

[0004] The smoothing circuit 4 includes a smoothing reactor 10 and a smoothing capacitor 11. The smoothing capacitor 11 is connected in parallel to the third switching element Q3 for commutation via the smoothing reactor 10. Although the third switching element Q3 is included in the synchronous rectification circuit 3 here, it can be considered as a part of the smoothing circuit 4. A pair of output terminals 12 and 13 connected to a smoothing capacitor 11
The load 14 is connected therebetween. The voltage detection circuit 6 detects a voltage between the pair of output terminals 12 and 13 and sends the voltage to the control circuit 5. The voltage detection circuit 6 generally includes a voltage dividing resistor for detecting an output voltage, a reference voltage source, and an error amplifier. The detected value of the output voltage obtained from the voltage dividing resistor and the reference voltage of the reference voltage source Are input to the error amplifier, and the output of the error amplifier becomes a voltage detection signal or voltage feedback control. Control circuit 5
Forms a control signal for keeping the voltage between the output terminals 12 and 13 constant, and thereby the first switching element Q
Turns 1 on and off. As is apparent from FIG. 2 schematically showing the control circuit 5 of FIG. 1, the control circuit 5 includes a sawtooth generator 5a, a comparator 5b, and a drive circuit 5c, and has a frequency of, for example, about 20 to 150 kHz. The sawtooth voltage and the output voltage of the voltage detection circuit 6 of FIG. 1 on the line 6a are compared to generate a square wave pulse, and a control signal including this pulse is transmitted via the drive circuit 5c to the gate electrode G of the switching element Q1.
Send to The voltage detection circuit 6 and the control circuit 5 are generally optically coupled.

[0005] in FIG. 3 (A) gate of the first switching element Q1 in Fig. 1 - DOO control signal _{V G1, (B) (D} ) (E) is first, second and third switching elements Q1 , Q2, Q3 between the main terminals, ie, the drain-source voltages V _Q1 , V _Q2 ,
V _Q3 , (C) indicates the voltage V 9 of the secondary winding 9. This figure 3
As is clear from FIG. 3, when the first switching element Q1 is turned on by the control signal of FIG. 3A at t0 to t1 and t5 to t6, the first switching element Q1 is turned on, and the main terminal voltage V _Q1 is turned on.
Becomes a value close to zero as shown in FIG. During the on-periods t0 to t1 and t5 to t6 of the first switching element Q1, the voltage V9 of the secondary winding 9 is generated in the positive direction as shown in FIG. D2 is in a forward bias state, and the second switching element Q2 is turned on by the voltage V9 of the secondary winding 9 from the N-channel insulated gate field effect transistor. Therefore, t0 to t
1, the main terminal voltage V _Q2 of the second switching element Q2 in the ON period Ton of t5~t6 is a value close to zero as shown in Figure 3 (D). Further, since the third switching element Q3 and the third diode D3 are in the reverse bias state during the on-period Ton between t0 and t1 and between t5 and t6, the voltage V between the main terminals of the third switching element Q3 is changed. _Q3 is Fig. 3 (E)
As shown in FIG. 7, the voltage becomes a high value substantially corresponding to the voltage V9 of the secondary winding 9.

When the first switching element Q1 is turned off at time t1, the flyback voltage Vf generated in the primary winding 8 due to the release of the energy stored in the transformer 2 during the ON period Ton and the voltage of the power supply 1 Voltage E of the sum with Es
s + Vf is applied to the first switching element Q1.
Since a soft switching or snubber capacitor C is connected in parallel to the first switching element Q1, the surge voltage is absorbed by this capacitor C.
A sharp increase in the voltage of the capacitor C, that is, the voltage V _Q1 between the terminals of the first switching element Q 1 is suppressed, and this voltage V _Q1
Gradually rises during the period from t1 to t3 as shown in FIG. When the charging of the capacitor C is completed at the time t3, t3
During the period t4, the capacitor C is discharged, and a part of the energy of the capacitor C is regenerated to the power supply 1. Discharging of the capacitor C ends at time t4 when the voltage V _Q1 becomes equal to the voltage Es of the power supply 1. Voltage V _Q1 of the first switching element Q1 subsequent t4~t5 section is kept at the power supply voltage Es. The voltage applied to the primary winding 8 during the off period Toff from t1 to t5 is the voltage of the difference between the voltage VQ1 of the first switching element _Q1 and the power supply voltage Es, and becomes a voltage that changes in a sine wave shape. . As a result, the voltage V9 of the secondary winding 9 becomes
As shown in (C), it changes sinusoidally in the period from t1 to t5. The OFF period Toff second diode - since the de D2 and the second switching element Q2 is reverse biased, the terminal voltage V _{Q 2} is of the secondary winding 9 as shown in FIG. 3 (D) The value corresponds to the voltage V9. Off period Tof
In f, a current flows through the circuit of the reactor 10, the capacitor 11, and the third diode D3 or the switching element Q3 due to the release of the energy stored in the smoothing reactor 10. In the off period Toff, the third diode D3
Turns into a conductive state irrespective of the voltage V9 of the secondary winding 9, but the third switching element Q3 turns on depending on the voltage V9 of the secondary winding 9. That is, the secondary winding 9
The threshold voltage of the gate-source voltage at which the voltage V9 in the opposite direction of the third switching element Q3 can be turned on crosses the threshold voltage _Vth during the period from t2 to t4. Q3 turns on. The voltage V9 of the secondary winding is from t1 to t2.
And the threshold _Vth is not reached in the section from t4 to t5. In this section, the third switching element Q3 is kept off and only the third diode D3 conducts. As shown in FIG. 3 (E), the third state when only the diode D3 is conducting.
The value of the inter-terminal voltage V _Q3 of the switching element Q3 is Va
(Approximately 0.6 V), and the value of the inter-terminal voltage V _Q3 when the third switching element Q 3 is turned on is Vb.
a is higher than Vb. Therefore, in the sections from t1 to t2 and t4 to t5, the effects of reducing the voltage drop and the loss by the synchronous rectification cannot be obtained. Further, since the charge remains in the capacitor C at the time when the first switching element Q1 is turned on, it is discharged at the time when the first switching element Q1 is turned on, resulting in a loss.

Instead of the snubber capacitor C shown in FIG.
It is conceivable to provide a snubber circuit, that is, a surge absorbing circuit 15 shown in FIG. The surge absorbing circuit 15 includes a diode 16, a surge absorbing capacitor 17, and a resistor 18. The surge absorbing capacitor 17 is a diode 16
Is connected in parallel to the primary winding 8. Resistance 18
Are connected in parallel to the surge absorbing capacitor 17. The diode 16 is connected in such a direction as to be forward biased by a voltage generated in the primary winding 8 when the switching element Q3 is turned off. In FIG. 4, components other than the surge absorbing circuit 15 are formed in the same manner as in FIG.

During normal operation of the DC-DC converter,
The surge absorbing capacitor 17 is charged to the polarity shown in FIG. When the switching element Q1 is turned off,
Since the voltage V1 of the primary winding 8 becomes higher than the voltage Vc of the surge absorbing capacitor 17, the diode 16 becomes conductive and the surge voltage is absorbed by the capacitor 17.
When the diode 16 is conducting, the voltage V1 of the primary winding 8 is clamped by the surge absorbing capacitor 17.
Thereafter, when the voltage V1 of the primary winding 8 becomes lower than the voltage Vc of the surge absorbing capacitor 17, the diode 16 becomes non-conductive. Since the discharge current of the surge absorbing capacitor 17 flows through the resistor 18, the voltage Vc of the capacitor 17 gradually decreases, but does not become lower than the voltage V1 of the primary winding 8.

The primary winding 8 has an inductance L and a parasitic capacitance, that is, a stray capacitance C, as shown by a broken line in FIG.
1, and the switching element Q1 also has a stray capacitance C2. In the following description, the sum of stray capacitances C1 +
C2 is simply referred to as stray capacitance C. The inductance L is the sum of the leakage inductance and the excitation inductance,
The leakage inductance is equivalently connected in series with the primary winding 8, and the exciting inductance is equivalently connected in parallel with the primary winding 8. The stray capacitance C is equivalently connected in parallel with the inductance L of the primary winding 8. As a result, an LC resonance circuit, that is, a ringing circuit is formed. The stray capacitance C is much smaller than the capacitance of the surge absorbing capacitor 17 and the output smoothing capacitor 11. The resonance frequency f0 of the LC resonance circuit is 1 / {2π (LC)}, that is, 1 / {2π (LC) ^1/2 }. Since the inductance L of the LC resonance circuit is given by the primary winding 8, the voltage between the drain and source of the switching element Q1 is equal to the sum of the voltage Es of the power supply 1 and the flyback voltage generated in the primary winding 8. Become. When the switching element Q1 is turned off, the diode 1
When the diode 16 is turned off after the surge voltage is turned on and the surge voltage is absorbed by the surge absorbing capacitor 17, ringing is generated by the LC circuit, and t1 to t1 in FIG.
The drain-source voltage _VDS takes a relatively high value as shown in the period t2 and after time t1 in FIG. The operation when the switching element Q1 is turned off will be described in more detail with reference to FIG. Waveform V _DS in FIG. 6 is an enlarged view of a part of the waveform of the V _DS of Figure 5, Id is diode - shows the de 16 current. Once-off control, the drain-source - - switching element Q1 is t1 deterministic 6 becomes a higher voltage with di voltage - scan voltage V _DS Gasa. However, diode has a slight delay as shown in t2 to t5 - since de 16 of the current Id flows, the drain-source - scan voltage V _DS is limited. The current Id of the diode 16 flows in the positive direction during the interval from t2 to t3,
It flows in the reverse direction in the section from t3 to t5. The section from t3 to t5 is the reverse recovery time trr, and the section from t3 to t4 is the accumulation time ts. t
The section from 4 to t5 is the time required for the depletion layer to spread so that the reverse blocking capability is restored at the pn junction of the diode 16, and is generally indicated by td. Since the diode 16 can be regarded as an on state until the accumulation time ts ends in an electric circuit, the ringing circuit of the LC is connected to the capacitor 1 via the diode 16 until t4 in FIG.
7 in parallel. As a result, L until time t4
Ringing by C is prevented. However, ringing starts after t4. At time t6, if the ringing voltage is going to be higher than the voltage of the capacitor 17, t6
It is suppressed again as shown in the period from t7 to t7. In FIG. 4, the diode 16 does not turn on after t7, ringing occurs without a surge absorption effect, and this level gradually decreases. When ringing occurs as described above, this becomes high-frequency noise and interferes with external circuits. When the withstand voltage between the drain and the source of the switching element Q1 is low, the switching element Q1 is broken by ringing. Since the DC power supply 1 of FIG. 4 generally includes a rectifying and smoothing circuit connected to an AC power supply, it is necessary to provide a noise removal filter having a relatively high impedance on the AC input line in order to remove the high-frequency noise described above. As a result, the efficiency of the entire power supply device is reduced, the cost is increased, and the external dimensions are increased. Further, the circuit of FIG. 4 has a problem similar to that of the circuit of FIG. 1 because a flyback voltage cannot be obtained during the entire off period of the first switching element Q1.

It is an object of the present invention to provide a DC device capable of preventing or suppressing ringing when a switching element is turned off and capable of reducing power loss.
-To provide a DC converter.

[0011]

SUMMARY OF THE INVENTION In order to solve the above problems and to achieve the above object, the present invention relates to a DC-DC converter for supplying DC power to a load, comprising a DC power supply for supplying a DC voltage. And a main switching element connected between one end and the other end of the DC power supply for repeatedly turning on and off the DC voltage, and having first and second main terminals and a control terminal. A primary winding connected between one end and the other end of the DC power supply via the main switching element; and a secondary winding electromagnetically coupled to the primary winding. The secondary winding includes a transformer having an inductance and a stray capacitance, a synchronous rectification switching element for converting the voltage of the secondary winding into a DC voltage, and a diode. The switching element is based on the voltage of the secondary winding. A synchronous rectifier circuit formed so as to be driven, a control circuit for controlling on / off of the main switching element, and absorbing a surge voltage applied to the main switching element when the main switching element is turned off. A surge absorbing capacitor connected in parallel to the primary winding, a direction in which the main switching element is kept off when the main switching element is on, and is forward biased when the main switching element is turned off. The primary winding based on an inductance of the primary winding and a stray capacitance which is electrically distributed in parallel with the inductance, and which is serially connected to the surge absorbing capacitor, Has a storage time longer than の of the cycle of the oscillating voltage generated in the main switching element and shorter than the minimum off period of the main switching element. A rectifier diode, it is intended according to the DC-DC converter, characterized in that and a both series resistor connected in series to the said rectifier diode and the surge absorption capacitor.

It is possible to connect a discharging parallel resistor in parallel with the surge absorbing capacitor. Further, as described in claim 3, the parallel resistor can be connected in parallel to a series circuit of the surge absorbing capacitor and the series resistor. Further, another rectifier diode having a shorter storage time than a rectifier diode having a longer storage time can be connected in parallel to the series resistor. Further, as described in claim 5, the series resistor and the rectifier diode can be housed in the same enclosure. Further, as described in claim 6, the surge absorbing circuit can be connected in parallel to the primary winding of the transformer. Further, a surge absorbing circuit can be connected in parallel to the main switching element.
The storage time is set to 125 ns to 7 μs.
It is desirable that the value be in the range of

[0013]

According to the invention of each claim, the following effects can be obtained. (1) The on-control period of the synchronous rectification switching element can be lengthened by adjusting the capacitance and resistance of the surge absorbing capacitor, and the efficiency can be improved. (2) A high voltage (surge voltage) generated in the primary winding when the main switching element is controlled to be turned off causes a current to flow through the rectifier diode to the surge absorbing capacitor, thereby absorbing the surge voltage. Thereafter, the rectifier diode is in a reverse-biased state. However, the rectifier diode has a relatively long accumulation time and maintains a conductive state despite the reverse-biased state. Therefore, the state where the surge absorbing capacitor is connected in parallel to the primary winding is maintained for a relatively long period. As a result, the surge absorbing capacitor is connected in parallel to the stray capacitance via the diode, and ringing due to the inductance of the primary winding and the stray capacitance is suppressed or prohibited. As a result, generation of noise due to ringing is suppressed, and destruction of the switching element due to ringing is prevented. (3) Since the charge of the surge absorbing capacitor after the surge is absorbed is discharged through the primary winding, the power is regenerated on the output side or the power supply side, and the efficiency is improved.

According to the second and third aspects of the present invention, the degree of freedom in adjusting the discharge of the surge absorbing capacitor is increased.
Further, according to the invention of claim 4, surge absorption immediately after turn-off can be quickly performed by removing the influence of series resistance. According to the fifth aspect of the present invention, the number of components can be reduced by integrating the series resistor and the rectifier diode, and the cost and the size can be reduced.

[0015]

Next, an embodiment of the present invention will be described with reference to FIGS. However, in FIGS.
6 are denoted by the same reference numerals, and description thereof will be omitted. 7 to 14, the same reference numerals are given to parts common to each other, and only one of them will be described in detail, and the other description will be omitted. In the following description, FIGS. 1 to 5 will be referred to as needed.

[0016]

First Embodiment A forward type DC-DC converter according to a first embodiment shown in FIG. 7 is provided with an improved surge absorbing circuit 15a instead of the surge absorbing circuit 15 of the DC-DC converter shown in FIG. The other components are the same as those in FIG. 4, for example, a DC power supply 1 composed of a rectifying and smoothing circuit,
A transformer 2, a first switching element Q1 as a main switching element, a synchronous rectifier circuit 3, and a smoothing circuit 4.
, A control circuit 5, a voltage detection circuit 6, and a surge absorption circuit 15a.

FIG. 7 shows an improved surge absorbing circuit 15a.
Has the same configuration as that of FIG. 4 except that a resistor 20 is added to the surge absorbing circuit 15 of FIG. 4 and the rectifier diode 16 of FIG. 4 is replaced with a rectifier diode 21 having a long accumulation time ts.

The resistor 20 consumes the energy of the ringing of the voltage of the primary winding 8.
1 and the surge absorbing capacitor 17 are connected in series. Therefore, this resistor 20 is called a series resistor. The resistance value of the series resistor 20 is 10 to 10 when the voltage Es of the DC power supply 1 is about 140 V to 280 V.
It is set to be about 330Ω, which is about 47Ω in the embodiment of FIG. The discharging resistor 18 connected in parallel to the series circuit of the surge absorbing capacitor 17 and the series resistor 20 is preferably set to a value larger than the series resistor 20. Note that this resistor 18 is referred to as a parallel resistor to distinguish it from the series resistor 20. This parallel resistance 1
Although it is possible to omit 8, it is desirable to provide it in order to increase the degree of freedom in setting the discharge of the surge absorbing capacitor 17.

The rectifier diode 21 of the present embodiment is similar to that of FIG.
Like the diode 16, the switching element Q1 is connected between the winding 8 and the surge absorbing capacitor 17 so as to be forward-biased by the voltage V1 of the primary winding 8 when the switching element Q1 is turned off. Therefore, a series circuit of the rectifier diode 21, the series resistor 20, and the surge absorbing capacitor 17 is connected in parallel to the primary winding 8.

The accumulation time ts of the minority carrier of the rectifier diode 21 is determined in the state where the surge absorbing circuit 15a is not provided.
When the first switching element Q1 is off, it has a value longer than 1/2 of the period T1 of the oscillating voltage generated in the voltage of the primary winding 8 and shorter than the minimum off-period of the first switching element Q1. The oscillation voltage generated in the primary winding 8 when the first switching element Q1 is off is shown in FIGS. 5 and 6, and the inductance L of the primary winding 8 when the switching element Q1 is off is shown in FIG. And the total C of the stray capacitance C1 and the stray capacitance C2 of the switching element Q1, and has a frequency sufficiently higher than the on / off frequency of the first switching element Q1. The inductance L of the primary winding 8 corresponds to the sum of the leakage inductance and the exciting inductance. The preferable storage time of the diode 21 is t1 to t2 shown in FIG.
This is a period during which ringing occurs due to LC resonance. Since the ringing frequency is about 4 MHz, the ringing period is about 2.5 μs, the off period is about 7 μs, and the ringing cycle is about 250 ns, the accumulation time of the diode 21 is in the range of 125 ns to 7 μs, more preferably 125 ns. About 500 ns is desirable. This diode 2
The storage time of 1 is, for example, 300 ns, which is much longer than the storage time (about 60 ns) of the rectifier diodes D2 and D3 and the diode 16 of FIG.

The diode 21 has a low value when the forward voltage VF rises when a forward current flows in a stepwise manner. As a characteristic of the diode 21, the peak value at the time of a forward rise when a current of 10 mA flows in a step-like manner is 6.4V. A diode SARS01 manufactured by Sanken Electric Co., Ltd. can be used as the diode 21 satisfying the accumulation time ts and the rising characteristics of the forward current.

Next, referring to FIGS. 8 and 9, the DC of FIG.
-The operation of the DC converter will be described. DC-DC of FIG.
The converter operates similarly to the DC-DC converter of FIG. 4 except for the operation of the surge absorbing circuit 15a. That is, the gate-source voltage V _GS of the switching element Q1 is changed as shown in FIG.
As shown in (A), the switching element Q1 is turned on and off by intermittently raising the level, and power is supplied to the capacitor 11 and the load 14 during the on-period Ton. The adjustment of the output voltage by the voltage detection circuit 6 and the control circuit 5 is performed similarly to the DC-DC converter of FIGS.

The switching element Q1 is, for example, t1 in FIG.
, The surge voltage is generated in the primary winding 8 based on the release of the energy stored in the transformer 2.
Since the diode 21 is turned on, the surge voltage is suppressed by the capacitor 17, and the drain-source voltage V _DS of the switching element Q1 does not become so high. When the voltage of the capacitor 17 increases due to the absorption of the surge voltage,
A reverse voltage is applied to the diode 21. Since a minority carrier of a forward current flowing during absorption of a surge voltage is accumulated in the diode 21, the diode 21 maintains a conductive state even when a reverse voltage is applied, and the diode 21 remains in the state of t3 to t5 in FIG.
As shown in the figure, the current Id of the diode 21 flows in the reverse direction. In FIG. 9, t3 to t4 are the accumulation times ts, and t3
The time td from 4 to t5 is the time when the depletion layer spreads at the pn junction of the diode 21. In the period of the accumulation time ts,
The primary winding 8 and the stray capacitance C such as the switching element Q1 are connected in parallel to the capacitor 17 via the diode 21 and the vibration energy absorbing resistor 20, so that a high frequency resonance circuit can be formed by LC. It is blocked, and a resonance circuit with a frequency sufficiently lower than this is formed. As a result, the voltage of the primary winding 8 does not ring, and the drain-source voltage V _DS of the switching element 3 becomes t t in FIG.
At a point in time, the surge voltage reaches a very low level and then falls with a slope, and becomes a substantially constant value shortly before the point in time t2. Immediately after t1 in FIG. 8, the drain-source voltage V
_DS that low resistance and the voltage V _F at the time of the rise of the forward current is low diode - is due to the use of de 21. If the storage time ts of the diode 21 is set to a relatively short value of about 150 ns, the drain voltage
The source-to-source voltage V _DS and the current Id of the diode 21 are shown in FIG.
Changes as shown in FIG. In this case, ringing occurs at a low level, but the ringing is improved as compared with the prior art of FIG. As described above, if high-frequency noise due to ringing does not occur, interference with external circuits is reduced. Further, in the case where the power supply 1 is constituted by a rectifying / smoothing circuit, it is not necessary to connect a filter for removing noise due to ringing to the input AC line,
It is possible to improve the efficiency, reduce the size, and reduce the cost of the entire power supply device. In the present embodiment, by adjusting the capacitance of the surge absorbing capacitor 17 and the value of the resistor 18, FIG.
As shown in (B), the flat flyback voltage Vf is t1
.About.t2 during almost the entire off-period Toff. Therefore, as shown in FIG. 8C, the absolute value of the voltage V9 of the secondary winding 9 is almost equal to t1 of the off-period Toff.
From time t2 to time threshold V of the third switching element Q3
higher than _th . In the period from t2 to t3, the voltage V9 of the secondary winding 9 becomes lower than the threshold value _Vth , so that the third switching element Q3 is turned off, and the current flows only through the third diode D3. . Accordingly, the voltage V _Q3 between the terminals of the third switching element Q3 in the section between t1 and t2 in FIG.
As shown in (E), the value becomes slightly higher than zero, and t1 to
The voltage V _{Q3 in the} section t3 has a value slightly higher than the voltage in the section t1 to t2. As is clear from the comparison between FIG. 3E and FIG. 8E, the third period in the off-period Toff of the present embodiment.
Since the ON period of the switching element Q3 is longer than that in FIG. 3, the effect of synchronous rectification is favorably obtained, and the loss is reduced.

In the DC-DC converter of FIG. 7, while the diode 21 is conducting for the accumulation time, the capacitor 1
7, a current in the opposite direction to the current in the on-period Ton flows through the closed circuit of the resistor 20, the diode 21, and the primary winding 8. Therefore, the energy released from the capacitor 17 is regenerated to the secondary winding 9 side, which contributes to an improvement in efficiency. That is, it is possible to regenerate the primary winding 8 without discharging the entire capacitor 17 via the resistor 18.

[0025]

Second Embodiment A DC according to a second embodiment shown in FIG.
-DC converter, the surge absorbing circuit 15a of the DC-DC converter of FIG. 7 is transformed into a surge absorbing circuit 15b,
The other parts are formed in the same manner as in FIG. The surge absorbing circuit 15b of FIG. 11 is formed in the same manner as FIG. 7, except that the parallel resistor 18 of the surge absorbing circuit 15a of FIG. However, the series resistor 20 is formed integrally with the diode 21 as shown in FIG.

The operation of the surge absorbing circuit 15b in which the connection positions of the resistors 18 and 20 are modified as shown in FIG. 12 is substantially the same as that of the surge absorbing circuit 15a of FIG. it can.

In the second embodiment, the resistance 20
12 and the diode 21 are housed in the same resin sealing body 23 as an enclosure as shown in FIG.
The size and cost of the DC converter can be reduced. In the composite component 24 of FIG. 12, a resistor 20 formed of a resistor chip and a diode 21 formed of a semiconductor chip are joined by a brazing material 25, one terminal 26 is joined to the resistor 20 by a brazing material 27, and the other terminal 28 Is joined to the diode 21 with a brazing material 29.

[0028]

Third Embodiment A DC according to a third embodiment shown in FIG.
The -DC converter is provided with a surge absorbing circuit 15c which is a modification of the surge absorbing circuit 15a of FIG. 7, and has the same configuration as that of FIG. The surge absorbing circuit 15 of FIG.
“c” corresponds to a circuit obtained by adding a second rectifier diode 16a to the surge absorbing circuit 15a of FIG. That is, the surge absorbing circuit 15c of FIG. 13 has a series circuit of the first rectifier diode 21, the series resistor 20, and the capacitor 17 as in FIG. However, the parallel resistor 18 is directly connected in parallel with the capacitor 17 as in FIG. The second rectifier diode 16a is connected to the series resistor 20 in parallel. The second rectifier diode 16a has a shorter accumulation time ts than the first rectifier diode 21, and has the same electrical characteristics as the conventional rectifier diode 16 of FIG.

When the switching element Q1 is turned off in the DC-DC converter of FIG.
Of the first and second rectifier diodes 21, 1
6a conducts, and a surge current flows to the capacitor 17 through these. Therefore, the second rectifier diode 16a functions as a bypass for the series resistor 20. When the capacitor 17 absorbs the surge voltage and the voltage Vc increases, the first and second rectifier diodes 21 and 16a enter a reverse bias state. The second rectifier diode 16a is turned off in a relatively short time because the accumulation time is short, but the first rectifier diode 21 is long in accumulation time because the accumulation time is short.
It is kept in the ON state, and the capacitor 17 is turned on as in the case of FIG.
And a series circuit of a resistor 20 and a first rectifier diode 21 are connected in parallel to the primary winding 8 to prevent ringing of the voltage V1 of the primary winding 8. Therefore, the third embodiment has the same effect as that of the first embodiment, and further has an effect that the surge can be promptly absorbed by the bypass effect of the second rectifier diode 16a.

[0030]

Fourth Embodiment The DC-DC converter according to the fourth embodiment shown in FIG.
The DC converter has a surge absorbing circuit 15d connected in parallel with the first switching element Q1, and the other configuration is the same as that of FIG. That is, the DC-DC converter of FIG. 14 has a surge absorbing capacitor 17 connected in parallel to the primary winding 8 via the power supply 1. Even if the connection point of the surge absorbing circuit 15d is modified as shown in FIG. 14, the surge absorbing circuit 15d of FIG. 14 is the same as the surge absorbing circuit 15a of FIG. Since the relationship is the same as that of FIG. 7 in terms of AC, the same operation and effect as in the first embodiment can be obtained. In the case of FIG.
During the storage time of the diode 21 after the capacitor 17 absorbs the surge voltage, the capacitor 17 is connected in parallel to the series circuit of the power supply 1 and the primary winding 8 so that the primary winding 8 The ringing voltage by LC is suppressed. As a modification of the DC-DC converters of FIGS. 11 and 13, the surge absorbing circuits 15b and 15c here can be connected in parallel to the first switching element Q1 as in FIG.

[0031]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) The switching elements Q1, Q2, and Q3 are not limited to FETs, but can be semiconductor switches such as bipolar transistors. (2) Separate the second and third diodes D2 and D3 from the second and third switching elements Q2 and Q3.
Can be (3) A tertiary winding can be provided in the transformer 2 to form a power supply for the control circuit 5. (4) Switching element Q for performing current feedback control
A current detection resistor can be connected in series with 1. (5) The control circuit 5 can be modified to change the on / off control mode of the switching element Q1. Further, a self-excited DC-DC converter can be used. (6) The power supply 1 can be a battery.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a conventional DC-DC converter.

FIG. 2 is a block diagram schematically showing a control circuit of FIG. 1;

FIG. 3 is a waveform diagram schematically showing voltages of respective parts in FIG.

FIG. 4 is a circuit diagram showing another conventional DC-DC converter.

FIG. 5 is a waveform diagram schematically showing voltages of respective parts in FIG.

[6] Some of the V _DS of Fig. 4 and diode - is a waveform diagram showing a current of de 16.

FIG. 7 is a circuit diagram showing a DC-DC converter according to the first embodiment.

FIG. 8 is a waveform diagram schematically showing voltages at respective parts in FIG. 7;

[9] Some of the V _DS of Fig. 8 and in FIG. 1 diode - is a waveform diagram showing a current of de 21.

FIG. 10 is a waveform diagram showing V _DS and Id when the accumulation time of the diode 21 of FIG. 1 is shortened, similarly to FIG.

FIG. 11 is a circuit diagram illustrating a DC-DC converter according to a second embodiment.

FIG. 12 is a cross-sectional view schematically showing a composite element of the diode and the resistor of FIG. 11;

FIG. 13 is a circuit diagram illustrating a DC-DC converter according to a third embodiment.

FIG. 14 is a circuit diagram showing a DC-DC converter according to a fourth embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Power supply 2 Transformer Q1, Q2, Q3 Switching element 17 Surge absorption capacitor 18, 20 Resistance 21 Diode

Claims (8)

[Claims] 1. A DC-DC converter for supplying DC power to a load.
A DC converter for supplying a DC voltage, connected between one end and the other end of the DC power supply for repeatedly turning on and off the DC voltage;
A main switching element having a main terminal and a control terminal, and a primary winding and the primary winding connected between one end and the other end of the DC power supply via the main switching element. A transformer having an electromagnetically coupled secondary winding and the primary winding having an inductance and a stray capacitance; and a synchronous rectifier for converting a voltage of the secondary winding into a DC voltage. A synchronous rectifier circuit including a switching element and a diode, wherein the synchronous rectifying switching element is formed so as to be driven by the voltage of the secondary winding; and for controlling on / off of the main switching element. A surge absorbing capacitor connected in parallel to the primary winding to absorb a surge voltage applied to the main switching element when the main switching element is turned off; The switching element is kept in a non-conductive state when in an on state, is connected in series to the surge absorbing capacitor with a direction of being forward biased when the main switching element is turned off, and has a primary winding. On the basis of the inductance and the stray capacitance distributed in parallel with the inductance, the period of the oscillation voltage generated in the primary winding is longer than の and the minimum off period of the main switching element. A DC-DC converter comprising: a rectifier diode having a short storage time; and a series resistor connected in series to both the surge absorbing capacitor and the rectifier diode. 2. The DC-DC converter according to claim 1, further comprising a discharging parallel resistor connected in parallel with said surge absorbing capacitor. 3. The DC-DC according to claim 2, wherein the parallel resistor is connected in parallel to a series circuit of the surge absorbing capacitor and the series resistor.
converter. 4. The rectifier diode further comprises another rectifier diode having a shorter storage time than the storage time of the rectifier diode,
3. A DC-DC converter according to claim 2, wherein said another rectifier diode is connected in parallel with said series resistor. 5. The rectifier diode according to claim 1, wherein the series resistor is housed in the same enclosure as the rectifier diode.
Or the DC-DC converter according to 2. 6. The capacitor according to claim 1, wherein the surge absorbing capacitor comprises:
6. The DC-DC converter according to claim 1, wherein the DC-DC converter is connected in parallel to the next winding without passing through the DC power supply. 7. The surge absorbing capacitor according to claim 1, wherein
The DC-DC converter according to any one of claims 1 to 5, wherein the DC-DC converter is connected in parallel to the next winding via the DC power supply and connected in parallel to the main switching element. . 8. The DC-DC according to claim 1, wherein the accumulation time is a value in a range of 125 ns to 7 μs.
converter.