JPH1155944A - Switching power supply equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a switching loss and increase efficiency, by controlling an equipment so that a synchronous rectifying switch may be turned off by a detection signal of a current detector and, after the voltage across a main switch becomes zero, the main switch may be turned off. SOLUTION: A control circuit CONT controls the timing of the turn-off of a synchronous rectifying switch SW2 using a detection signal dt of a current detector CDT. After a specified delay time from the rise of a driving signal P2 for turning the synchronous rectifying switch SW2 off, a main switch SW1 is turned off. Then, the main switch SW1 and the synchronous rectifying switch SW2 are switched in a zero voltage state. By this method, a switching loss of the switching power supply equipment can be reduced and the efficiency can be increased.

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 to which a synchronous rectification system is applied. Various configurations of a switching power supply device that converts an input voltage to a desired output voltage and supplies the output to a load in a stabilized manner have already been proposed and put to practical use. There is a demand for further improving the efficiency of such a switching power supply device.

[0002]

2. Description of the Related Art FIG. 9 is an explanatory diagram of a conventional flyback converter configuration, in which an input voltage Vin having a polarity shown in FIG.
The main switch SW turns on and off the primary winding N1 to apply the voltage. The voltage induced in the secondary winding N2 is rectified by the rectifying diode D, smoothed by the smoothing capacitor C2, and has the polarity shown in FIG. Output voltage Vo
ut is detected by the control circuit CONT, and compared with the set reference voltage, the ON period of the main switch SW is controlled by the drive signal P1 so that the error approaches zero. The main switch SW is constituted by a bipolar transistor, a field effect transistor, or the like.
Is an input side capacitor.

FIG. 10 is a diagram for explaining the operation of a conventional example.
2 is a current flowing through the secondary winding N2 of the transformer T, P1 is a drive signal of the main switch SW, In1 is a current flowing through the primary winding N1 of the transformer T, and Vsw1 is a main switch S
Shows the voltage applied to W. Toff indicates an off period of the main switch SW, and Ton indicates an on period of the main switch SW.

When the main switch SW is turned on by the high-level drive signal P1, the voltage Vsw1 applied to the main switch SW becomes zero. The current In1 due to the input voltage Vin flows through the primary winding N1 of the transformer T and is stored as excitation energy. At this time, the voltage induced in the secondary winding N2 of the transformer T is the reverse voltage of the rectifier diode D. Becomes Therefore, during the ON period Ton of the main switch SW, the current In2 of the secondary winding N2 becomes zero.

Next, when the main switch SW is turned off by the low-level drive signal P1, the current In1 flowing through the primary winding N1 of the transformer T becomes zero, and the voltage Vsw1 applied to the main switch SW becomes the input voltage Vin. To the flyback voltage generated in the primary winding N1 of the transformer T. The voltage induced in the secondary winding N2 of the transformer T is in the forward direction of the rectifying diode D. As a result, a current In2 flows through the secondary winding N2 of the transformer T via the rectifying diode D. Therefore, during the off-period Toff of the main switch SW, the current In flows through the secondary winding N2.
When the main switch SW is turned on, the induced voltage of the secondary winding N2 of the transformer T is inverted, so that the reverse voltage is applied to the rectifying diode D. And the current In2
Becomes zero.

FIG. 11 is an explanatory diagram of a conventional boost converter configuration and a buck-boost converter configuration.
(A) shows the main part of the switching power supply device of the boost converter configuration, C1 is a capacitor on the input side, L is a reactor, SW is a main switch, D is a diode, C2
Is a smoothing capacitor, CONT is a control circuit, Vin is an input voltage, and Vout is an output voltage.

A reactor L and a diode D are connected in series between an input terminal and an output terminal, and a connection point is connected to a main switch SW.
When the main switch SW is turned on by T, the input voltage Vin having the illustrated polarity is directly applied to the reactor L, a current flows, and the excitation energy is accumulated in the reactor L. Further, since the charging voltage of the smoothing capacitor C2 is applied as a reverse voltage to the diode D, it is prevented from discharging through the main switch SW in the ON state.

Next, when the main switch SW is turned off, a voltage in a direction for maintaining the continuity of current is generated by the excitation energy stored in the reactor L, and this voltage is added to the input voltage Vin, and the diode D Is applied to the smoothing capacitor C2 and charged. Accordingly, the output voltage Vout having the illustrated polarity is equal to the input voltage Vin.
And the value obtained by adding the voltage of the reactor L to the above. This output voltage Vout is detected by the control circuit CONT, and the ON period of the main switch SW is controlled so that the output voltage Vout becomes a set constant output voltage Vout.

FIG. 11B shows a main part of a switching power supply having a buck-boost converter configuration.
(A) indicates the same name part, and a main switch SW and a diode D are provided between an input terminal and an output terminal.
Are connected in series, and a reactor L is connected to the connection point. The control circuit CONT detects the output voltage Vout having the polarity shown in FIG. Controls ON and OFF. When the main switch SW is turned on, the input voltage Vin having the illustrated polarity is applied to the reactor L, a current flows, and the excitation energy is accumulated. At this time, a reverse voltage is applied to the diode D.

When the main switch SW is turned off, a voltage is induced to maintain the continuity of the current flowing through the reactor L, and a forward voltage is applied to the diode D. The current flowing through the reactor L via the diode D charges the smoothing capacitor C2 to the illustrated polarity (the polarity opposite to that in the case of FIG. 11A), and the voltage at both ends becomes the output voltage Vout. The switching power supply device having this configuration can be either a step-up type or a step-down type.

FIG. 12 is an explanatory view of the configuration of a conventional synchronous rectification type flyback converter. The same reference numerals as in FIG. 9 denote the same parts, SW1 denotes a main switch, and SW2 denotes a synchronous rectification switch. Like the main switch SW in FIG. 9, the main switch SW1 is turned on and off by a drive signal P1 from the control circuit CONT, and is also a synchronous rectification switch SW2 connected to the secondary winding N2 of the transformer T. Are turned on and off by a drive signal P2 corresponding to a signal obtained by inverting the drive signal P1.

FIG. 13 is a diagram for explaining the operation of the conventional example.
Is a drive signal of the main switch SW1, P2 is a drive signal of the synchronous rectification switch SW2, In2 is a current flowing through the secondary winding N2 of the transformer T, and Vsw2 is a synchronous rectification switch SW.
2 shows the voltage applied. Also, Ton1 and Toff1 are the on and off periods of the main switch SW1,
2, Toff2 indicates an ON period and an OFF period of the synchronous rectification switch SW2.

The main switch SW1 is driven by the drive signal P1.
Is turned on, the synchronous rectifier switch SW2 is off, so that the current In2 flowing through the secondary winding N2 of the transformer T becomes zero, and the synchronous rectifier switch SW2 induces a current in the secondary winding N2 of the transformer T. A voltage is applied. Next, when the main switch SW1 is turned off, the synchronous rectification switch SW
At this time, the current In2 flows through the synchronous rectification switch SW2 due to the induced voltage of the secondary winding N2 of the transformer T, charges the smoothing capacitor C2, and outputs the output voltage Vout having the illustrated polarity. Is applied to a load (not shown).

[0014]

In a conventional example in which a rectifying diode D is connected to a secondary winding N2 of a transformer T,
When the main switch SW is turned on and turned off, a loss occurs due to the temporal overlap between the flowing current In1 and the applied voltage Vsw1. When a current flows in the forward direction of the rectifier diode D, a loss occurs because the forward voltage is not zero. In particular, in a switching power supply having a large output current capacity, there is a problem that the loss due to the forward voltage of the diode is large and the efficiency is reduced.

Therefore, a synchronous rectification type configuration as shown in FIG. 12 has been proposed. Synchronous rectification switch S in this case
W2 can set the forward voltage when the current flows through the secondary winding N2 of the transformer T to zero or a value close thereto.
Therefore, the loss can be reduced. However, the switching loss between the main switch SW1 and the synchronous rectification switch SW2 is relatively large, and it has been difficult to configure a highly efficient switching power supply. An object of the present invention is to improve efficiency by reducing switching loss in a synchronous rectification type.

[0016]

The switching power supply according to the present invention comprises (1) an input capacitor C1 connected between input terminals and a smoothing capacitor C2 connected between output terminals.
And a diode-connected main switch SW1 for turning on / off the current by the input voltage Vin, a diode-connected synchronous rectification switch SW2 for turning on / off the charging current to the smoothing capacitor C2, and an output voltage Vout. Main switch SW1 and synchronous rectifier switch S
In a switching power supply including a control circuit CONT for controlling W2, a capacitor C3 connected in parallel to the main switch SW1 and a smoothing capacitor C2 using excitation energy of a reactor such as a transformer T during a period when the synchronous rectification switch SW2 is on. A current detector CDT for detecting a current flowing in the reverse direction after the charging current flows to the main switch SW1 and the synchronous rectifying switch S
W2 is turned on, the synchronous rectification switch SW2 is turned off by the detection signal dt of the current detector CDT, and then the main switch SW1 is turned on after the voltage across the main switch SW1 becomes zero. And

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory diagram of a first embodiment of the present invention, and shows a case where the present invention is applied to a flyback converter configuration. C1 to C3 are capacitors, D1 and D2 are diodes, and SW1 is A main switch, SW2 is a synchronous rectification switch, T is a transformer, N1 is a primary winding, N2 is a secondary winding, CDT is a current detector, and CONT is a control circuit.

A diode D1 and a capacitor C3 are connected in parallel with the main switch SW1, and a synchronous rectification switch S
A diode D2 is connected in parallel with W2. In this case, the diode D1 is connected to the polarity at which the input voltage Vin is applied as a reverse voltage, and the diode D2 is connected to the output voltage Vo.
ut is connected to the polarity applied as a reverse voltage. The current In2 flows in the secondary winding N2 of the transformer T in the direction of the arrow, but the current flowing in the direction opposite to the arrow is detected by the current detector CDT, and the detection signal dt is sent to the control circuit CON.
Add to T.

The control circuit CONT detects the output voltage Vout, controls on / off of the main switch SW1 by the drive signal P1, and controls the synchronous rectification switch SW1.
2 is controlled by the drive signal P2, and the output voltage Vo is controlled even when the input voltage Vin and the load current fluctuate.
ut is fixed. At this time, the turn-off timing of the synchronous rectification switch SW2 is controlled using the detection signal dt from the current detector CDT, and the synchronous rectification switch SW2 is controlled.
The drive signal P1 for turning on the main switch SW1 is set to a high level "H" after a predetermined delay time Td4 from the fall of the drive signal P2 for turning off the switch. Thereby, the switching control of the main switch SW1 and the synchronous rectification switch SW2 can be performed in the zero voltage state, and the switching loss can be reduced.

FIG. 2 is a diagram for explaining the operation of the first embodiment of the present invention, in which the current In2 of the secondary winding N2 in FIG.
The drive signals P2 and P1, the current In1 of the primary winding N1, the current Ir1 flowing through the diode D1 and the main switch SW1, and the current Isw flowing through the main switch SW1
1 shows an example of a waveform of an applied voltage Vsw1 of the main switch SW1.

When the drive signal P1 is changed from the high level "H" to the low level "L" and the drive signal P2 is changed from the low level "L" to the high level "H", the main switch S
W1 is off, the synchronous rectification switch SW2 is on, and the current I that flows during the on-period Ton1 of the main switch SW1 is
A voltage is induced in the secondary winding N2 of the transformer T by the excitation energy accumulated by n1, and a current In2 flows through the synchronous rectification switch SW2 to charge the smoothing capacitor C2.

The current In1 of the primary winding N1 of the transformer T
Continuously suppresses the surge voltage at the time of turn-off by continuously flowing through the capacitor C3 even after the main switch SW1 is turned off, and the current In1 becomes zero after the period of Td3. Also, the ON period Ton2 of the synchronous rectification switch SW2
The current In2 decreases to zero as the excitation energy of the transformer T is released, and thereafter flows in the opposite direction via the synchronous rectification switch SW2 due to the discharge of the charge of the smoothing capacitor C2. This reverse current value I
The detection signal dt detected by the current detector CDT is applied to the control circuit CONT, whereby the drive signal P2 is set to the low level “L” and the synchronous rectification switch SW2
Is turned off.

By turning off the synchronous rectification switch SW2, the current In2 becomes zero, and the induced voltage of the primary winding N1 of the transformer T causes the electric charge of the capacitor C3 to move to the input side capacitor C1 side during the period of Td1 in the negative direction of the current In1. And the voltage Vsw1 applied to the main switch SW1 becomes zero when the discharge is completed. After that, the current In1 flows through the diode D1. In this case, the capacitor C3 is charged for absorbing a surge voltage when the main switch SW1 is turned off, and the input side capacitor C1 is charged when the synchronous rectification switch SW2 is turned off.
Since the charge is fed back in the direction, the capacitor C3 has a lossless configuration.

Next, after the voltage Vsw1 across the main switch SW1 becomes zero, the drive signal P
1 is set to a high level “H”. As a result, the main switch SW1 is turned on, and the currents Isw1, Ir1, I1
n1 flows in the directions of the arrows. Control circuit CONT
Is an on-period Ton for stabilizing the output voltage Vout
After one, the drive signal P1 is set to a low level "L" and the drive signal P2 is set to a high level "H" to turn off the main switch SW1 and turn on the synchronous rectification switch SW2.

As described above, the main switch SW1 is
A capacitor C3 connected in parallel and a synchronous rectifier switch S
By turning off W2 and then turning on after a period of Td1 + Td2 = Td4, the applied voltage Vsw
When 1 is at zero voltage, it can be turned off and turned on, and switching loss can be reduced.
Since there is no loss due to charging and discharging of the capacitor C3, the efficiency of the switching power supply can be improved.

FIGS. 3A and 3B are explanatory diagrams of the switches. FIG. 3A shows the main switch SW1 and the synchronous rectification switch SW described above.
2 shows a configuration of a switch corresponding to No. 2, having terminals a and b and a control terminal c, and controlling the on / off between the terminals a and b by applying a drive signal to the control terminal c. The diode d is connected in parallel.

Such a configuration can be easily realized by the field effect transistor shown in FIG. 1B, and the diode d corresponds to a parasitic diode of the field effect transistor as shown by a dotted line. Note that the figure shows a case of an n-channel field-effect transistor, which turns on when a drive signal applied to a gate corresponding to the control terminal c is at a high level “H” and turns off when the drive signal is at a low level “L”. Note that another switching element such as a p-channel field-effect transistor can be used. When a switching element having no parasitic diode is used, diodes are connected in parallel.

FIG. 4 is an explanatory view of the current detector, and FIG.
Indicates the case where the current transformer CT is used.
2 is a resistor, and Dc is a diode. For example, in a case where the detection signal dt is output when a current flows in the direction of the arrow and applied to the current detector CDT in FIG. 1, the current in the negative direction of the current In2 in FIG. The detection signal dt is not output for the current In2, but the detection signal dt is output when the current Ik in the negative direction is detected. Note that a configuration is possible in which a detection signal dt having a polarity corresponding to the positive and negative directions of the flowing current is output.

FIG. 3B shows the case where the operational amplifier OPA is used, and R3 to R7 are resistors. When a current flows through the resistor R3 in the direction of the arrow, a detection signal dt corresponding to the voltage across the resistor R3 is output from the operational amplifier OPA. Therefore, when applied to the current detector CDT in FIG. 1, the current in the negative direction of the current In2 in FIG. 2 is set to the arrow direction, and the detection signal dt is not output for the current In2 in the positive direction, but the current in the negative direction is output. A detection signal dt is output when Ik is detected.

FIG. 5 is an explanatory diagram of a control circuit according to the first embodiment of the present invention. Reference numerals 11 to 13 denote comparators, 14 denotes a flip-flop, 15 denotes a current conversion circuit, 16 denotes a constant current circuit,
17 is an intermittent constant current circuit, 18 is a delay circuit, 19 is a transistor, 20, 21, and 22 are AND circuits, 23 is an inverter, V1, V2, and V3 are reference voltages, Coff, and Con.
Is a capacitor, and Roff is a resistor.

The detection signal dt from the current detector CDT and the reference voltage V1 are compared by the comparator 11, and the output voltage V
out is input to the current conversion circuit 15 and converted into a current Io. The AND circuit 21 outputs a drive signal P1 to be applied to the main switch SW1, and the AND circuit 22 outputs a drive signal P2 to be applied to the synchronous rectification switch SW2. The constant current circuit 16 outputs a constant current Ion, and the intermittent constant current circuit 17 outputs a constant current Ioff when the output signal Vc of the comparator 12 is “0”, and stops when the output signal Vc is “1”. The flip-flop 14 outputs the output signal Vc of “1” of the comparator 12.
And reset by the output signal Vd of “1” of the AND circuit 20. When the output terminal Q is "1", the transistor 19 is turned on, and the delay circuit 18 and the inverter 23 are connected to the output terminal * Q.

FIG. 6 is a diagram for explaining the operation of the control circuit.
2 shows an example of the signal of each section, and dt ′ represents a current detection signal proportional to the current In2 shown in FIG. Comparator 1
The reference voltage V1 added to 1 corresponds to the value indicated as Ik of the negative current In2 in FIG. In the latter half of the on-period Ton2 of the synchronous rectification switch SW2, the positive current I2 that charges the smoothing capacitor C2 due to the emission of the excitation energy of the transformer T flows, and then becomes zero. Current I in the direction
n2 flows, and the detection signal dt 'of the current In2 in the negative direction.
Exceeds the reference voltage V1, the output signal Va of the comparator 11
Becomes "1".

When the output signal Va of the comparator 11 is "0", the output signal Vd of the AND circuit 20 becomes "0".
When the output signal Va of the comparator 11 is “1” and the output signal Vb of the comparator 13 is “1”, the output signal Vd of the AND circuit 20 becomes “1”. The flip-flop 14 is reset by the output signal Vd “1”. Therefore, the output terminal Q is "0" and the output terminal * Q is "1".

When the output terminal Q of the flip-flop 14 becomes "0", the transistor 19 is turned off. When the output terminal * Q becomes "1", the output signal Ve of the inverter 23 becomes "0". , And circuit 2
2, the drive signal P2 becomes low level "0", and the synchronous rectification switch SW2 is turned off. As a result, the current In2 becomes zero, so that the detection signal dt 'is also kept at the zero level.

The delay circuit 18 is for providing a delay time of Td4, and may be, for example, a time constant circuit composed of a resistor and a capacitor. In this case, when the output terminal * Q of the flip-flop 14 becomes “1”, the output signal of the delay circuit 18 rises according to the time constant. When the output signal exceeds the threshold voltage Vth of the AND circuit 21, the drive signal P1 becomes high. Level "1" and main switch S
W1 is turned on. That is, after turning off the synchronous rectification switch SW2, after a period of Td4, the main switch SW1 is turned off.
Can be turned on.

The intermittent constant current circuit 17 outputs the constant current Ioff and charges the capacitor Coff when the output signal Vc of the comparator 12 is "0".
Since ff rises sharply and immediately exceeds the reference voltage V3, the output signal Vb of the comparator 13 becomes "1". The capacitor Con has a constant current Io from the constant current circuit 16.
n and the current Io proportional to the output voltage Vout, the terminal voltage Von becomes equal to the output voltage Vo.
It rises with a slope proportional to ut. And the reference voltage V2
, The output signal Vc of the comparator 12 becomes “1”, the flip-flop 14 is set, and the intermittent constant current circuit 17 stops outputting the constant current Ioff.

The output terminal Q is set to "1" by the setting of the flip-flop 14, the transistor 19 is turned on and the capacitor Con is rapidly discharged, and its terminal voltage Von
Quickly becomes zero. Since the output terminal * Q becomes "0" and the output signal Ve of the inverter 23 becomes "1", the high level "1" drive signal P2 is applied from the AND circuit 22 to the synchronous rectification switch SW2. 21 from the main switch SW.
Added to 1. In this case, a transistor is connected in parallel with the capacitor constituting the delay circuit 18, and for example, it is turned on when the output signal Ve of the inverter 23 is "1".
A configuration for rapidly discharging the capacitor can be added.

The synchronous rectification switch SW2 is turned on by the drive signals P1 and P2, and the main switch SW1 is turned on.
Is turned off. Therefore, when the output voltage Vout increases, the voltage rise of the capacitor Con becomes faster, and the on-period Ton1 of the main switch SW1 becomes shorter. Conversely, when the output voltage Vout decreases, the voltage rise of the capacitor Con becomes slower. The output voltage Vout can be made constant by increasing the ON period Ton1 of the switch SW1.

When the output signal Vc of the comparator 12 becomes "1", the intermittent constant current circuit 17 stops outputting the constant current Ioff, so that the capacitor Coff is connected to the resistor Ro.
Discharge via ff. Therefore, the terminal voltage Voff
Decreases in accordance with the discharge time constant, and when the voltage falls below the reference voltage V3, the output signal Vb of the comparator 13 becomes "0". As described above, when the detection signal dt 'of the current In2 exceeds the reference voltage V1 and increases in the positive direction, the output signal Va of the comparator 11 becomes "1", whereby the driving signal P2 becomes low level. 0, and thereafter, after a period of Td4, the drive signal P1 becomes high level "1". That is, after turning off the synchronous rectification switch SW2, the main switch SW1 can be turned on after a period of Td4.

FIG. 7 is an explanatory diagram of the second embodiment of the present invention, showing a case where the present invention is applied to a switching power supply device having a boost converter configuration, wherein an input terminal to which an input voltage Vin is applied and an output voltage Vout are applied to a load ( (Not shown) between the reactor L and the current detector CDT.
And a synchronous rectifier switch SW2 are connected in series, a diode D2 is connected in parallel with the synchronous rectifier switch SW2, and a main switch SW1 is connected to a connection point between the current detector CDT and the synchronous rectifier switch SW2. A diode D1 and a capacitor C3 are connected in parallel with the main switch SW1, an input-side capacitor C1 is connected to an input terminal,
A smoothing capacitor C2 is connected to the output terminal, and a control circuit C
The ONT detects the output voltage Vout and outputs the main switch SW by the detection signal dt from the current detector CDT.
1 and a drive signal P2 for controlling on / off of the synchronous rectification switch SW2.

In this embodiment, the control circuit CO
NT detects the output voltage Vout and outputs the main switch S
W1 is turned off, the synchronous rectifier switch SW2 is turned on, and the smoothing capacitor C2 is charged by the positive current by the exciting energy of the reactor L. After the exciting energy is released, the current becomes zero. Discharge from C2 and C3 to the input side capacitor C1 causes a current in the negative direction to flow. The current in the negative direction is detected by the current detector CDT, and the detection signal dt is applied to the control circuit CONT, whereby the synchronous rectification switch SW
2 is turned off, and then the main switch SW1 is turned on after a period of Td4 as described above. Thus, as in the above-described embodiment, the main switch S
Since W1 can perform switching in a zero voltage state, switching loss can be reduced.

FIG. 8 is an explanatory diagram of the third embodiment of the present invention, showing a case where the present invention is applied to a switching power supply device having a buck-boost converter configuration. The same reference numerals as in FIG. 7 denote the same names. The current detector CDT connected in series with the reactor L detects a current flowing in the reactor L in the negative direction while the synchronous rectification switch SW2 is on, and applies the detection signal dt to the control circuit CONT. The control circuit CONT thereby controls the synchronous rectification switch SW2
A drive signal P2 for turning off the main switch SW1 is output after a period of Td4 has elapsed. Further, the main switch SW1 is turned off and the synchronous rectification switch SW2 is turned on so as to control the ON period of the main switch SW1 so that the output voltage Vout is constant. Therefore, the main switch SW1 can perform switching in the zero voltage state, and thus the switching loss can be reduced.

The present invention is not limited to the above-described embodiment, but can be applied to a synchronous rectification type switching power supply having another configuration.
Is applicable to each of the above-described embodiments. The control circuit CONT has a positive current that charges the smoothing capacitor C2 due to the emission of the excitation energy of the transformer T or the reactor L. It becomes zero at the end of the release of the excitation energy, and thereafter, the current detector CDT detects that a negative current flows due to the discharge current of the capacitor, and turns off the synchronous rectification switch SW2 using the detection signal dt. Control is performed so that the main switch SW1 is turned on after the delay time of the period Td4 is set and the voltage between both ends of the main switch SW1 becomes zero. The main switch SW1 is turned on based on the timing at which the current Ir1 reverses from the negative direction to the positive direction. It is also possible to adopt a configuration for controlling the so that.

[0044]

As described above, the present invention provides a main switch SW1 in which an input-side capacitor C1, a smoothing capacitor C2, a diode D1 and a capacitor C3 are connected in parallel, and a synchronous rectifier switch in which a diode D2 is connected in parallel. SW2, current detector CDT, and control circuit CON
T and the input voltage Vi by the main switch SW1.
n to turn on and off the current caused by the switch n
, The charging current to the smoothing capacitor C2 is turned on and off, the excitation energy accumulated in the reactor such as the transformer T during the on-period of the main switch SW1, the main switch SW1 is turned off, and the synchronous rectification switch SW2 is turned on. At the same time, the smoothing capacitor C2 is charged with the current, and after the current zero due to the end of the release of the excitation energy, the current flowing in the reverse direction is detected by the current detector CDT, and the control signal CONT is detected by the detection signal dt.
Turns off the synchronous rectification switch SW2, and then sets Td
After the period of 4, the main switch SW1 is turned on, and the capacitor C3 connected in parallel to the main switch SW1 operates as described above for absorbing surge voltage, has no loss due to charging and discharging, and Since the switching operation can be performed when the voltage applied to the main switch SW1 is zero, there is an advantage that the switching loss can be reduced and the efficiency can be improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a first embodiment of the present invention.

FIG. 2 is an operation explanatory diagram of the first embodiment of the present invention.

FIG. 3 is an explanatory diagram of a switch.

FIG. 4 is an explanatory diagram of a current detector.

FIG. 5 is an explanatory diagram of a control circuit according to the first embodiment of the present invention.

FIG. 6 is an explanatory diagram of the operation of the control circuit.

FIG. 7 is an explanatory diagram of a second embodiment of the present invention.

FIG. 8 is an explanatory diagram of a third embodiment of the present invention.

FIG. 9 is an explanatory diagram of a conventional flyback converter configuration.

FIG. 10 is an operation explanatory diagram of a conventional example.

FIG. 11 is an explanatory diagram of a conventional boost converter configuration and a buck-boost converter configuration.

FIG. 12 is an explanatory view of a configuration of a conventional synchronous rectification type flyback converter.

FIG. 13 is an operation explanatory diagram of a conventional example.

[Explanation of symbols]

 SW1 Main switch SW2 Synchronous rectification switch CDT Current detector CONT Control circuit C1 Input side capacitor C2 Smoothing capacitor C3 Capacitor D1, D2 Diode T Transformer P1, P2 Drive signal dt Detection signal

Claims (1)

[Claims] 1. An input-side capacitor connected between input terminals, a smoothing capacitor connected between output terminals, a diode-parallel-connected main switch for turning on and off a current by an input voltage, and a switch connected to the smoothing capacitor. In a switching power supply device including a diode parallel-connected synchronous rectification switch for turning on / off a charging current and a control circuit for detecting an output voltage to control the main switch and the synchronous rectification switch, A connected capacitor, a current detector for detecting a current flowing in a reverse direction after a charging current to the smoothing capacitor by exciting energy of the reactor flows while the synchronous rectification switch is on, and turning off the main switch. And the synchronous rectifier switch is turned on, and the detection signal of the current detector And a control circuit configured to turn on the main switch after the voltage at both ends of the main switch becomes zero after the synchronous rectification switch is turned off.