JPH1169802A - Synchronous rectification circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the loss of a synchronous rectification circuit with improved efficiency. SOLUTION: This circuit has an input-side capacitor C1, provided connectively between input terminals, a smoothing capacitor C2 provided connectively between output terminals, a main switch SW1 for switching on or off the current caused by an input voltage Vin, a synchronous rectification switch SW2 connected in parallel with a diode D2 and for switching on or off the charging current of the smoothing capacitor C2, a control circuit CONT for controlling the on/off of the main switch SW1, by sensing an output voltage Vout, and a current detector CDT for detecting the current flowing through a parallel circuit comprising the synchronous rectification switch SW2 and the diode D2, which switches on the synchronous rectification switch SW2, when the detected current by it exceeds a preset value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a synchronous rectifier circuit with improved efficiency. A synchronous rectifier circuit for reducing a loss due to a rectifier diode is known, and further improvement in efficiency is demanded.

[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. , The value obtained by adding 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 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, C
ONT 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 the buck-boost converter configuration. The same reference numerals as those in FIG. 11A denote the same parts, and a main switch SW and a main switch SW are provided between an input terminal and an output terminal. A diode D is connected in series, and a reactor L is connected to the connection point.
The ONT detects the output voltage Vout having the illustrated polarity,
On / off of the main switch SW is controlled so that the set voltage is obtained. 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 a conventional buck converter configuration and a forward converter configuration. FIG. 12A shows a main part of the buck converter configuration. A capacitor C1 is connected between input terminals and a capacitor is connected between output terminals. Has a configuration in which a smoothing capacitor C2 is connected, a main switch SW and a reactor L are connected in series between an input terminal and an output terminal, and a diode D is connected to the connection point. The connection is made such that when the main switch SW is turned on, the input voltage Vin of the illustrated polarity has the polarity applied as the reverse voltage.

The control circuit CONT detects the output voltage Vout having the illustrated polarity and controls the ON / OFF of the main switch SW so that the output voltage Vout reaches the set voltage. When the main switch SW is turned on, the input voltage Vin is applied to the smoothing capacitor C connected to the output terminal via the reactor L.
2 and the load. At this time, the voltage VL applied to the reactor L becomes VL = Vin−Vout, the reactor L is excited according to this voltage VL, and the smoothing capacitor C2 is charged.

When the main switch SW is turned off, the voltage induced by the characteristic of maintaining the continuity of the current flowing through the reactor L has a forward polarity with respect to the diode D. Therefore, the charging of the smoothing capacitor C2 and the supply of the load current are continued. In this configuration, the excitation energy stored in reactor L is equal to input voltage Vi.
n and a difference between the output voltage Vout and a step-down switching power supply device.

FIG. 12B shows a main part of a forward converter configuration, in which a main switch SW is connected to a primary winding N1 of a transformer T, a capacitor C1 is connected to an input terminal, and a main circuit is controlled by a control circuit CONT. SW is turned on and off to turn on and off an input voltage Vin having a polarity shown in the figure applied to the primary winding N1 of the transformer T.

When the main switch SW is turned on, the induced voltage of the secondary winding N2 has a forward polarity for the diode Da and a reverse polarity for the diode Db. Exciting energy is accumulated in the reactor L as a charging current and a load current of the smoothing capacitor C2 via Da and the reactor L. Further, a voltage of the illustrated polarity at both ends of the smoothing capacitor C2 becomes the output voltage Vout. The control circuit CONT detects the output voltage Vout, compares the output voltage Vout with the set reference voltage, and controls the ON period of the main switch SW by pulse width control or the like so that an error is reduced to zero.

When the main switch SW is turned off, the voltage of the induced voltage in the secondary winding N2 of the transformer T is inverted, so that the voltage is applied to the diode Da in the reverse direction and to the diode Db in the forward direction. However, the voltage applied to the diode Db is blocked by the diode Da. In the reactor L, a forward voltage is induced in the diode Db by the stored excitation energy in order to maintain the continuity of the current. Therefore, the charging current and the load current of the smoothing capacitor C2 are supplied.

FIG. 13 is an explanatory diagram of the configuration of a conventional synchronous rectification type flyback converter. The same reference numerals as those 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. 14 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. Vs indicates a surge voltage generated when the synchronous rectification switch SW2 is turned off.

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).

[0020]

SUMMARY OF THE INVENTION Diode D for rectification
In the conventional example using the diode, the loss due to the diode D is large when the output current capacity is large. Therefore, a synchronous rectifier circuit as shown in FIG. 12 has been proposed. This synchronous rectifier circuit is a synchronous rectifier switch S at the timing when a current flows in the forward direction of the rectifying diode D.
W2 is turned on, and its drive signal P2 and
Since the drive signal P1 of the main switch SW1 and the drive signal P1 are merely in inverted phases, it is not possible to guarantee operation at an appropriate timing. For example, the synchronous rectification switch SW2 and the main switch SW1 are simultaneously turned on. Then, there is a problem that the loss increases. The present invention
It is an object of the present invention to reduce the loss by controlling the synchronous rectifier switch on and off at an optimum timing with a simple configuration.

[0021]

According to the present invention, there is provided a synchronous rectifier circuit comprising: (1) an input-side capacitor C1 connected between input terminals;
, A smoothing capacitor C2 connected between the output terminals, a main switch SW1 for turning on / off a current based on the input voltage Vin, and a diode D2 for connecting / discharging a charging current to the smoothing capacitor C2. SW2 and a control circuit CONT that controls the main switch SW1 by detecting the output voltage Vout, detects a current flowing through the synchronous rectifier switch SW2 and the diode D2, and, when the current value exceeds a set value, the synchronous rectifier switch A current detector CDT for turning on SW2 is provided.

FIG. 1 is an explanatory view of a first embodiment of the present invention, wherein 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, C1 Is an input-side capacitor, C2 is a smoothing capacitor, D2 is a diode, CONT is a control circuit, CDT is a current detector, Vin is an input voltage, and Vout is an output voltage.

The control circuit CONT detects the output voltage Vout and controls on / off of the main switch SW1 by the drive signal P1. When the main switch SW1 is turned on, a current In1 flows through the primary winding N1 of the transformer T due to the input voltage Vin. When the main switch SW1 is turned off, the voltage induced in the secondary winding N2 of the transformer T causes the current Id via the diode D2.
2 (= In2) flows and is supplied to the charging of the smoothing capacitor C2 and the load. This current is detected by the current detector CDT.

When the current Id2 (= In2) exceeds the set value, the detection signal dt becomes "1", thereby turning on the synchronous rectification switch SW2. Therefore, the transformer T
The current In2 flowing through the secondary winding N2 becomes the current Isw2 flowing through the synchronous rectification switch SW2, and the current Id2 flowing through the diode D2 becomes zero.
The loss due to the forward voltage of 2 is zero. Then, when the current Id2 due to the excitation energy accumulated in the transformer T gradually decreases and becomes equal to or less than the set value, the detection signal dt becomes “0” and the synchronous rectification switch SW2 is turned off.

FIG. 2 is a diagram for explaining the operation of the first embodiment of the present invention. In FIG. 2, the drive signal P1, the current In1,
5 shows exemplary waveforms of In2, a detection signal dt, and currents Id2 and Isw2. Also, Ton1 and Toff1 are the on and off periods of the main switch SW1, and Ton2 and Toff2.
Indicates an ON period and an OFF period of the synchronous rectification switch SW2.

When the drive signal P1 from the control circuit CONT is "1", the main switch SW1 is turned on, and the current In1 flows through the primary winding N1 of the transformer T. At this time, the synchronous rectification switch SW2 is off, and the voltage induced in the secondary winding N2 of the transformer T has a polarity opposite to that of the diode D2, and the current In2 does not flow.

When the drive signal P1 is set to "0", the main switch SW1 is turned off. Thereby, the transformer T
, A voltage is induced in the secondary winding N2, the current flows in the forward direction with respect to the diode D2, and the current Id2 flows. This current Id2
Detector CDT flows through the secondary winding N2.
Then, when it is detected that the current Id2 (= In2) exceeds the set value Ik, the detection signal dt is set to “1”.

When the detection signal dt becomes "1", the synchronous rectification switch SW2 is turned on and the current Isw2 (= In
2) flows, and the current Id2 flows through the diode D2.
Becomes zero. Current I flowing through secondary winding N2 of transformer T
n2 decreases as the excitation energy is released, and when it becomes equal to or less than the set value Ik, the detection signal dt becomes “0”. As a result, the synchronous rectification switch SW2 is turned off, and the current Isw flowing through the synchronous rectification switch SW2 is
2 becomes zero and the current I 2 flowing through the diode D2 is
d2.

When the drive signal P1 becomes "1",
When the main switch SW1 is turned on, the current In1 flows in the primary winding N1 of the transformer T, and a voltage is induced in the secondary winding N2 of the transformer T, and the voltage has a polarity opposite to that of the diode D2. Current Id flowing through diode D2
2 becomes zero.

Therefore, the control circuit CONT outputs the output voltage V
The synchronous rectification switch SW2 can be realized by a configuration similar to that of the conventional example that outputs a drive signal P1 for controlling on / off of the main switch SW1 by detecting the output signal out. The ON / OFF control is performed, and the ON period Ton2 is automatically set after the periods Td1 and Td2 before and after the OFF period Toff1 of the main switch SW1, and synchronous rectification can be performed.
Then, the main switch SW1 and the synchronous rectification switch SW
2 is not turned on at the same time, the loss can be reduced.

FIG. 3 is an explanatory view of the current detector, and FIG.
Denotes a main part of the current detector, CT denotes a current transformer,
R1 and R2 are resistors, Dc is a diode, and (B) is an operation explanatory diagram. The current In2 flowing through the secondary winding N2 of the transformer T has a characteristic of increasing according to the reactance of the secondary winding N2 and decreasing according to the emission speed of the excitation energy, and the current transformer CT is proportional to the current In2. A voltage is induced to output a voltage Vc via a rectifier circuit including the resistors R1 and R2 and the diode Dc.
Instead of using the current transformer CT, it is also possible to use a resistor and output a voltage drop corresponding to the value of the current In2 flowing through the resistor as the above-described voltage Vc.

The voltage Vc is directly applied to the synchronous rectification switch SW.
In addition to the above, using the threshold value of the drive characteristics of the synchronous rectification switch SW2, the synchronous rectification switch SW2 is turned on by the voltage Vc exceeding the set value, or the voltage Vc is compared with the set value Vk for detection. A configuration for outputting the signal dt can be employed. For example, a comparator that compares the voltage Vc with the set value Vk or a gate circuit that uses the set value Vk as a threshold can be used. In FIG. 3B, the voltage V
This shows a case where the detection signal dt is obtained by comparing c with the set value Vk, and the period when the detection signal dt is “1” is the ON period Ton2 of the synchronous rectification switch SW2.

FIG. 4 is an explanatory diagram of the switch. FIG. 4A shows a configuration in which a diode d is connected in parallel to a switch having terminals a and b and a control terminal c.
1 and the synchronous rectification switch SW2.
Such a switch can be realized by a field-effect transistor shown in FIG. In this case, the diode d indicated by the dotted line is a parasitic diode of the field effect transistor. Although the case of an n-channel field-effect transistor is shown, a p-channel field-effect transistor can be used. Further, it is also possible to use a switching element such as a transistor having another configuration.

FIG. 5 is an explanatory diagram of a second embodiment of the present invention, showing a main part of the configuration of a boost converter, wherein the same reference numerals as in FIG. 1 denote parts having the same names, and L denotes a reactor. A configuration in which a reactor L, a current detector CDT, and a synchronous rectification switch SW2 are connected in series between an input terminal and an output terminal, and a main switch SW1 is connected to a connection point between the reactor L and the current detector CDT. The control circuit CONT detects the output voltage Vout, controls the ON / OFF of the main switch SW1 by the drive signal P1, and detects the current flowing through the diode D2 and the synchronous rectification switch SW2 by the current detector CDT. Then, the synchronous rectification switch SW2 is turned on by the detection signal dt when a current exceeding the set value flows.

Accordingly, when the current flowing through the reactor L and the diode D2 exceeds the set value after the main switch SW1 is turned off, the synchronous rectification switch SW2
Is turned on, and when the current due to the emission of the excitation energy stored in the reactor L decreases and becomes less than the set value,
The synchronous rectification switch SW2 is turned off, and then the main switch S2 is driven by the drive signal P1 from the control circuit CONT.
W1 is turned on. That is, it is possible to avoid that the main switch SW1 and the synchronous rectification switch SW2 are simultaneously turned on.

FIG. 6 is an explanatory diagram of a third embodiment of the present invention, showing a main part of a buck-boost converter configuration.
1 and 5 denote the same parts. A configuration in which a main switch SW1, a current detector CDT, and a synchronous rectification switch SW2 are connected in series between an input terminal and an output terminal, and a reactor L is connected to a connection point between the main switch SW1 and the current detector CDT. Control circuit C
The ONT controls the ON / OFF of the main switch SW1 by the drive signal P1 by detecting the output voltage Vout, and turns on the synchronous rectification switch SW2 by the detection signal dt when a current exceeding the set value is detected by the current detector CDT. And

Therefore, after the main switch SW1 is turned off, when the current due to the excitation energy stored in the reactor L flows through the diode D2 and the current detector CDT detects that the current exceeds the set value, the current detector CDT detects that The synchronous rectification switch SW2 is turned on by the detection signal dt. Therefore, the diode D2 is connected to the synchronous rectification switch SW2.
, And the loss due to the forward voltage of the diode D2 becomes zero.

When the current flowing through the reactor L decreases below the set value, the detection signal dt from the current detector CDT becomes "0" and the synchronous rectification switch SW2 is turned off. Then, the driving signal P from the control circuit CONT
1, the main switch SW1 is turned on, and the above operation is repeated to stabilize the output voltage Vout.

FIG. 7 is an explanatory view of a fourth embodiment of the present invention, showing a main part of the buck converter configuration, and the same reference numerals as those in FIGS. 1, 5 and 6 denote parts having the same names. The main switch SW1 and the reactor L are connected in series between the input terminal and the output terminal, and the synchronous rectification switch SW2 and the current detector CDT are connected at the connection point.

The control circuit CONT detects the output voltage Vout and controls the on / off of the main switch SW1 by a drive signal P1, and when the main switch SW1 is turned off, it is connected via the reactor L and the diode D2. The current flowing is detected by a current detector CDT, and a synchronous rectification switch SW is detected by a detection signal dt when the current exceeds a set value.
2 is turned on. Thereby, the diode D2 is bypassed by the synchronous rectification switch SW2, and the loss due to the forward voltage of the diode D2 becomes zero.

When the current supplied to the smoothing capacitor C2 and the load side decreases due to the release of the excitation energy stored in the reactor L and becomes equal to or less than the set value, the synchronous rectification switch SW2 is turned off. After that, the control circuit CON
The main switch SW1 is turned on by the drive signal P1 from T, and the above-described operation is repeated, so that the output voltage Vou
t is fixed.

FIG. 8 is an explanatory diagram of the fifth embodiment of the present invention, showing the main part of the configuration of the forward converter. The main switch SW and the diodes Da and Db in the conventional forward converter configuration of FIG. 12B correspond to a configuration in which the main switch SW1, the synchronous rectification switches SW3 and SW2, and the diodes D3 and D2 are used. . A current detector CDT is connected in series with the synchronous rectification switch SW2.

The control circuit CONT detects the output voltage Vout and controls the main switch SW1 to be turned on and off by the drive signal P1, and controls the synchronous rectifier switch SW3 to be turned on and off by the drive signal P3 having the same phase. . When the main switch SW1 and the synchronous rectifier switch SW3 are turned on, the voltage induced in the secondary winding N2 of the transformer T causes the charging current of the smoothing capacitor C2 and the current to the load via the synchronous rectifier switch SW3 and the reactor L. Flows. At this time, since the diode D3 is bypassed by the synchronous rectification switch SW3, the loss due to the forward voltage of the diode D3 can be avoided.

When the main switch SW1 and the synchronous rectification switch SW3 are turned off, a voltage of the opposite polarity is induced in the secondary winding N2, so that the secondary winding N2 is blocked by the diode D3 and the excitation stored in the reactor L. Due to the energy, a current flows through the reactor L and the diode D2. The current flowing through the diode D2 is detected by the current detector CDT, and the synchronous rectification switch SW2 is turned on by the detection signal dt when the current value exceeds the set value. Therefore, the diode D2 is bypassed by the synchronous rectification switch SW2, and the loss due to the forward voltage can be avoided.

When the current decreases due to the emission of the excitation energy of the reactor L and falls below the set value, the synchronous rectification switch SW2 is turned off. After that, the control circuit CONT
Turns on the main switch SW1 and the synchronous rectification switch SW3 in response to the drive signals P1 and P3, and repeats the above operation to stabilize the output voltage Vout.

The present invention can be applied not only to the above-described embodiments but also to various types of switching power supply devices. The control circuit CONT can apply various known configurations for detecting the output voltage Vout and controlling on / off of the main switch SW1.

[0047]

As described above, according to the present invention, the main switch S for turning on / off the current by the input voltage Vin is used.
W1 and a synchronous rectifying switch SW connected in parallel with a diode D2 for turning on and off a charging current to a smoothing capacitor C2.
The control circuit CONT detects the output voltage Vout to control the on / off of the main switch SW1, and detects the current flowing through the synchronous rectification switch SW2 and the diode D2 by the current detector CDT to set the voltage. When the value exceeds the value, the synchronous rectification switch SW2 is turned on, so that synchronous rectification can be performed with the configuration of the control circuit CONT as in the conventional example, and loss due to the rectifying diode can be reduced. Further, the main switch SW1 and the synchronous rectification switch S
W2 is controlled so as not to be turned on at the same time, and the occurrence of loss can be avoided. Thereby, there is an advantage that 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 current detector.

FIG. 4 is an explanatory diagram of a switch.

FIG. 5 is an explanatory diagram of a second embodiment of the present invention.

FIG. 6 is an explanatory diagram of a third embodiment of the present invention.

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

FIG. 8 is an explanatory diagram of a fifth 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 diagram of a conventional buck converter configuration and a forward converter configuration.

FIG. 13 is an explanatory diagram of a configuration of a conventional synchronous rectification type flyback converter.

FIG. 14 is a diagram illustrating the operation of a conventional example.

[Explanation of symbols]

 SW1 Main switch SW2 Synchronous rectification switch D2 Diode CONT Control circuit CDT Current detector C1 Input side capacitor C2 Smoothing capacitor T Transformer

Claims (1)

[Claims] An input-side capacitor connected between input terminals, a smoothing capacitor connected between output terminals, a main switch for turning on and off a current based on an input voltage, and a charging current for the smoothing capacitor turned on. A synchronous rectifier switch connected in parallel with a diode to be turned off, and a control circuit for detecting the output voltage and controlling the main switch, and detects a current flowing through the synchronous rectifier switch and the diode to exceed a set value. A synchronous rectifier circuit comprising: a current detector that turns on the synchronous rectifier switch when the current value is reached.