JP2006050858A - Synchronous rectifying circuit and switching power source using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a synchronous rectifying circuit capable of making an electric current flowing through a rectifying switch be zero or nearly zero when the rectifying switch is in an interrupted condition, and provide a switching power source using the same. <P>SOLUTION: A synchronous rectifying circuit has an electric current detecting means and a driving means for detecting electric currents as the thresholds of a first current value and a second current value. The driving means is configured to set a control signal at a first level if an electric current flowing through a rectifying switch is equal to a first current value or over, make a control signal change toward a second level if an electric current flowing through a rectifying switch falls between the first current value and the second curent value, and set a control signal at the second level if an electric current flowing through a rectifying switch is not greater than the second current value. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a synchronous rectifier circuit used for a switching power supply and the like, and a switching power supply using the same.

  In recent years, a synchronous rectifier circuit using a switching element such as a MOSFET instead of a diode as a rectifier has been frequently used to increase the efficiency of a switching power supply. FIG. 7 is a configuration diagram illustrating an example of a switching power supply that is a step-down converter using the conventional synchronous rectifier circuit disclosed in Patent Document 1. In FIG. The operation of the switching power supply using the conventional synchronous rectifier circuit will be described below with reference to FIG. In the switching power supply using the conventional synchronous rectifier circuit shown in FIG. 7, only the components necessary for the operation description in the configuration described in FIG. 1 of Patent Document 1 are left, and the other components are simplified. ing.

As shown in FIG. 7, the input power supply 101 that outputs the input voltage Vi is connected to the synchronous rectifier circuit 108 via the switch 102. The synchronous rectifier circuit 108 includes a rectifier switch 180 made of an N-channel MOSFET, a diode 181, and a hysteresis type comparator 182. In FIG. 7, 104 is an inductor, 105 is an output capacitor, 106 is a load, and 107 is a control circuit.
The switch 102 having one end connected to the input power supply 101 performs a switching operation that repeats conduction and interruption by the control circuit 107. The other end of the switch 102 is connected to one end of the inductor 104 and the synchronous rectifier circuit 108. The other end of the inductor 104 is connected to the output capacitor 105, and the output voltage Vo is supplied from the output capacitor 105 to the load 106. The control circuit 107 detects the output voltage Vo and drives and controls the switch 102 so that the output voltage Vo becomes a desired value.

  In the synchronous rectifier circuit 108, the hysteresis type comparator 182 that drives the rectifier switch 180 has a negative input terminal connected to the drain of the rectifier switch 180 and a positive input terminal grounded. The hysteresis type comparator 182 has two threshold voltages Vthl and Vthh. For example, when the drain voltage Vx of the rectifying switch 180 falls below Vthl = −0.20V, the hysteresis type comparator 182 outputs an H level signal to make the rectifying switch 180 conductive. To do. On the other hand, when the drain voltage Vx of the rectifying switch 180 exceeds Vthh = −0.05 V, an L level signal is output and the rectifying switch 180 is turned off.

The operation of the switching power supply, which is a step-down converter having the conventional synchronous rectifier circuit 108 configured as described above, will be described.
First, when the switch 102 is in a conductive state, the drain voltage Vx of the rectifier switch 180 of the synchronous rectifier circuit 108 becomes the input voltage Vi. Therefore, the hysteresis type comparator 182 outputs an L level signal and the rectifier switch 180 is cut off. It becomes a state. At this time, an increasing current flows in the order of the input power supply 101 → the switch 102 → the inductor 104 → the output capacitor 105 and the load 106 → the input power supply 101, stores magnetic energy in the inductor 104 and supplies power to the load 106.

  When the switch 102 is cut off, the voltage across the inductor 104 is inverted and the diode 181 is turned on. For this reason, the forward voltage of the diode 181 is generated in the negative direction in the drain voltage Vx of the rectifying switch 180, and the drain voltage Vx is lower than the threshold voltage Vthl. For this reason, the hysteresis type comparator 182 outputs an H level signal, and the rectifying switch 180 becomes conductive. Accordingly, a decreasing current flows through the rectifying switch 180 → the inductor 104 → the output capacitor 105 and the load 106 → the rectifying switch 180, and the magnetic energy of the inductor 104 is released and the power is supplied to the load 106. The rectifier switch 180 becomes conductive during most of the cut-off time of the switch 102 and the magnetic energy of the inductor 104 is released. However, the conduction voltage is lower than when the current flows through the diode 181, and the loss is reduced to the load 106. Power is supplied.

  Stable power is supplied to the load 106 by periodically repeating the switching operation of the switch 102 as described above. When the duty ratio δ is the ratio of the conduction time in one switching cycle of the switch 102, the relationship between the input voltage Vi and the output voltage Vo is approximately expressed by the following equation (1).

    Vo = δ · Vi (1)

The control circuit 107 can control the output voltage Vo by adjusting the duty ratio δ of the switch 102.
The above description is the operation in the case of the current continuous mode in which the current flowing through the inductor 104 is always zero or more.

Next, the operation in the current discontinuous mode in which the current flowing through the inductor 104 is zero will be described.
First, when the switch 102 is in a conductive state, the rectifier switch 180 of the synchronous rectifier circuit 108 is cut off, and current flows through the input power supply 101 → the switch 102 → the inductor 104 → the output capacitor 105 and the load 106 → the input power supply 101, In addition to storing magnetic energy in the inductor 104, power is supplied to the load 106.

  On the other hand, when the switch 102 is cut off, the diode 181 becomes conductive, and the forward voltage of the diode 181 is generated in the negative direction in the drain voltage Vx of the rectifying switch 180. For this reason, the hysteresis type comparator 182 outputs an H level signal and the rectifying switch 180 becomes conductive, and a decreasing current flows through the rectifying switch 180 → the inductor 104 → the output capacitor 105 and the load 106 → the rectifying switch 180, and the inductor The magnetic energy of 104 is released and power is supplied to the load 106.

Since the rectification switch 180 constituted by the MOSFET has a low resistance in the conductive state, the drain voltage Vx of the rectification switch 180 increases from the negative voltage as the current decreases due to the magnetic energy emission of the inductor 104. When the threshold voltage Vthh exceeds −0.05 V, the hysteresis type comparator 182 outputs an L level signal and the rectifying switch 180 is cut off. Thereafter, the current flowing through the inductor 104 becomes zero. At this time, since the switch 102 is in the cut-off state, the voltage of the inductor 104 vibrates. Thereafter, the switch 102 becomes conductive again and the above operation is repeated.
JP-A-10-215567

  In the switching power supply having the conventional synchronous rectifier circuit 108 configured as described above, the hysteresis comparator 182 outputs an L level signal after the drain voltage Vx of the rectifier switch 180 exceeds the threshold voltage Vthh. It is desirable that the current flowing through the inductor 104 becomes zero during the delay time until the rectifying switch 180 is cut off.

  The operation of the switching power supply having the conventional synchronous rectifier circuit 108 will be described with reference to the operation waveform diagram of FIG. 8, (a) is the current IL flowing through the inductor 104 in the current discontinuous mode, (b) is the gate voltage Vg of the rectifier switch 180, (c) is the drain voltage Vx of the rectifier switch 180, and (d) is the rectifier. FIG. 6 is an operation waveform diagram showing a drain voltage Vx obtained by enlarging only the voltage on the vertical axis near the zero voltage of the drain voltage Vx of the switch 180. In FIG. 8, (A) on the left is a case where the delay time until the rectifying switch 180 is cut off is long and the current IL flowing through the inductor 104 is less than zero, and (B) on the right is that the rectifying switch 180 is This shows a case where the current IL flowing through the inductor 104 is larger than zero when it enters the cut-off state.

  As shown in (A) on the left side in FIG. 8, when the delay time until the rectifying switch 180 is cut off is long, the current flowing through the inductor 104 flows back below zero. This current stores magnetic energy in the inductor 104. Due to the magnetic energy stored in the inductor 104, when the rectifying switch 180 is cut off, the drain voltage Vx of the rectifying switch 180 rapidly increases and reaches the input voltage Vi. If the switch 102 is a semiconductor switch such as a MOSFET, a current flows backward to the input power supply 101 via the body diode. Under the relatively light load condition of the current discontinuous mode, such a backflow current increases the effective value of the switching current and causes efficiency degradation. Further, when the switch 102 and the synchronous rectifier circuit 108 are integrated on the one chip together with the control circuit 107 for miniaturization, the backflow current to the input via the body diode activates the parasitic element and causes malfunction. It can be a cause.

On the other hand, as shown in FIG. 8B on the right side, if the current IL flowing through the inductor 104 is larger than zero when the rectifying switch 180 is cut off, the diode 181 becomes conductive, and the drain voltage of the rectifying switch 180 is increased. A forward voltage of the diode 181 is generated in the negative direction at Vx. For this reason, the hysteresis type comparator 182 outputs an H level signal, and the rectifying switch 180 becomes conductive again.
During the delay time until the gate voltage Vg of the rectifying switch 180 becomes L level and the rectifying switch 180 is actually cut off, a current flows through the inductor 104, and the current IL of the inductor 104 flows back below zero. The problem when the current IL of the inductor 104 flows backward is as described above.

  However, in a switching power supply having a conventional synchronous rectifier circuit, there is a delay time from when the drain voltage Vx of the rectifier switch 180 exceeds the threshold voltage Vthh until the rectifier switch 180 is cut off, and therefore flows through the inductor 104. It was difficult to make the current zero or substantially zero. In particular, the delay time until the rectifying switch 180 is cut off is affected by the gate drive voltage of the rectifying switch 180, and thus is difficult to specify.

  The present invention provides a high-efficiency and high-reliability synchronization that enables the rectifier switch to be cut off when the current flowing through the inductor becomes zero or substantially zero, that is, when the current flowing through the rectifier switch becomes zero or substantially zero. An object is to provide a rectifier circuit and a switching power supply using the synchronous rectifier circuit.

In order to solve the above-described problem, a synchronous rectifier circuit according to the present invention includes a rectifier switch that is turned on or off according to a level of a control signal, as described in claim 1,
Current detection means for detecting a current flowing through the rectifying switch using a first current value and a second current value as threshold values;
Drive means for changing the level of the control signal in accordance with the output of the current detection means,
If the current flowing through the rectifying switch is greater than or equal to the first current value, the driving means sets the control signal to a first level, and the current flowing through the rectifying switch is equal to the first current value and the second current value. If it is between current values, the control signal is changed to the second level, and if the current flowing through the rectifying switch is equal to or less than the second current value, the control signal is set to the second level. It is configured as follows.
The synchronous rectifier circuit configured in this way can turn off the rectifier switch when the current flowing through the rectifier switch becomes zero or substantially zero. Become.

According to the synchronous rectifier circuit of the present invention, as described in claim 2, when the current flowing through the rectifier switch according to claim 1 is a second current value, the control signal is switched from conduction to cutoff in the rectification switch. You may comprise so that it may be more than a threshold value.
The synchronous rectifier circuit according to the present invention is the synchronous rectifier circuit according to the third aspect, wherein the current detection unit according to the first aspect includes a first comparison unit that compares a current flowing through the rectification switch with the first current value. And a second comparison means for comparing the current flowing through the rectifying switch with the second current value.

In the synchronous rectifier circuit according to the present invention, as described in claim 4, the rectifier switch of claim 3 is an N-channel MISFET, and a gate-source voltage is used as the control signal,
The first comparison means compares the drain-source voltage of the rectifying switch in a conductive state with a first threshold value corresponding to the first current value;
The second comparison unit may be configured to compare the drain-source voltage in the conductive state of the rectifying switch with a second threshold value corresponding to the second current value.

A synchronous rectifier circuit according to the present invention includes a DC voltage source having the positive terminal and the negative terminal according to claim 4, and the source terminal of the rectifying switch is connected to the negative terminal, as described in claim 5.
The drive means
If the drain-source voltage driven by the output of the first comparing means and the rectifying switch is in a conductive state is equal to or lower than a first threshold, the gate terminal of the rectifying switch is pulled up to the potential of the positive terminal. 1 auxiliary switch,
A current source circuit that is driven by the output of the first comparison means and discharges between the gate and the source of the rectifier switch if the drain-source voltage in the conductive state of the rectifier switch is equal to or higher than a first threshold;
If the drain-source voltage driven by the output of the second comparison means and the rectifier switch in the conductive state is equal to or higher than a second threshold value, the second rectifier switch pulls down the gate of the rectifier switch to the potential of the negative terminal. And an auxiliary switch.

The synchronous rectifier circuit according to the present invention is the synchronous rectifier circuit according to the sixth aspect, wherein the current source circuit according to the fifth aspect is configured such that the control signal maintains a gate threshold voltage or more between a gate and a source of the rectifier switch. You may comprise so that it may discharge.
The synchronous rectifier circuit according to the present invention is the synchronous rectifier circuit according to the seventh aspect, wherein the drive circuit according to the sixth aspect includes a third comparison unit that compares a gate-source voltage of the rectifier switch with a third threshold value. Have
The current source circuit comprises:
Driven by the output of the first comparison means and the output of the third comparison means, the drain-source voltage in the conductive state of the rectifier switch is equal to or higher than a first threshold, and the gate-source voltage is the first If the threshold is 3 or more, the rectifier switch may be configured to discharge between the gate and the source.

In the synchronous rectifier circuit according to the present invention, as described in claim 8, the rectifier switch of claim 3 is a P-channel MISFET, and a source-gate voltage is used as the control signal,
The first comparison means compares the drain-source voltage of the rectifying switch in a conductive state with a first threshold value corresponding to the first current value;
The second comparison unit may be configured to compare the drain-source voltage in the conductive state of the rectifying switch with a second threshold value corresponding to the second current value.

A synchronous rectifier circuit according to the present invention includes a DC voltage source having a positive electrode terminal and a negative electrode terminal according to claim 8, and a source of the rectifying switch is connected to the positive electrode terminal. The drive means
First driven to pull down the gate terminal of the rectifying switch to the potential of the negative terminal if driven by the output of the first comparing means and the drain-source voltage in the conducting state of the rectifying switch is greater than or equal to a first threshold. Auxiliary switch
A current source circuit that is driven by the output of the first comparison means and discharges between the gate and the source of the rectifying switch if the drain-source voltage in the conductive state of the rectifying switch is equal to or lower than a first threshold;
If the drain-source voltage driven by the output of the second comparing means and the rectifying switch is in a conducting state is equal to or lower than a second threshold, the second rectifying switch pulls up the gate of the rectifying switch to the potential of the positive terminal. And an auxiliary switch.

According to a synchronous rectifier circuit of the present invention, as described in claim 10, in the current source circuit of claim 9, the source-gate of the rectifier switch is connected between the source and gate of the rectifier switch so that the control signal maintains a gate threshold voltage or more. You may comprise so that it may discharge.
The synchronous rectifier circuit according to the present invention is the synchronous rectifier circuit according to the eleventh aspect, wherein the driving unit according to the tenth aspect includes a third comparing unit that compares a source-gate voltage of the rectifying switch with a third threshold value. Have
The current source circuit comprises:
Driven by the output of the first comparison means and the output of the third comparison means, the drain-source voltage in the conductive state of the rectifier switch is less than or equal to a first threshold, and the source-gate voltage is third If it is more than the threshold value, the source-gate of the rectifying switch may be discharged.

A switching power supply according to the present invention, as described in claim 12, a DC voltage source having a positive terminal and a negative terminal,
A switch having one end connected to the positive terminal of the DC voltage source and the other end connected to a load via an inductor and smoothing means;
A switching power supply comprising a synchronous rectifier circuit connected to the other end of the switch,
The synchronous rectification circuit is a rectification switch that is turned on or off according to the level of the control signal;
Current detection means for detecting a current flowing through the rectifying switch using a first current value and a second current value as threshold values;
Drive means for changing the level of the control signal in accordance with the output of the current detection means,
If the current flowing through the rectifying switch is greater than or equal to the first current value, the driving means sets the control signal to a first level, and the current flowing through the rectifying switch is equal to the first current value and the second current value. If the current is between the current values, the control signal is changed toward the second level, and if the current flowing through the rectifying switch is equal to or lower than the second current value, the control signal is set to the second level. It may be configured.
The switching power supply according to the present invention configured as described above uses a high-efficiency and highly reliable synchronous rectifier circuit that can turn off the rectifier switch when the current flowing through the inductor becomes zero or substantially zero. A switching power supply can be provided.

  The synchronous rectifier circuit according to the present invention is configured to keep the control signal to the rectifier switch at a low level until the vicinity of the threshold voltage at which the rectifier switch can be kept on when the current flowing through the rectifier switch is equal to or less than a predetermined value. Therefore, when the current flowing through the rectifying switch becomes zero or substantially zero, the rectifying switch can be cut off. As a result, the synchronous rectifier circuit according to the present invention has a configuration that can prevent the current of the rectifier switch from flowing backward. Therefore, by applying this synchronous rectifier circuit to a switching power supply, high efficiency and reliability can be achieved. A power supply device having high performance can be provided.

  Preferred embodiments of a synchronous rectifier circuit according to the present invention and a switching power supply using the synchronous rectifier circuit will be described below with reference to the accompanying drawings.

<< First Embodiment >>
FIG. 1 is a circuit configuration diagram showing a switching power supply which is a step-down converter using a synchronous rectifier circuit according to a first embodiment of the present invention. In FIG. 1, an input power source 1 that outputs an input voltage Vi is connected to a switch 2 and a synchronous rectifier circuit 3. One end of the switch 2 is connected to the input power source 1 that is a DC voltage source, and the other end is connected to one end of the inductor 4 and the synchronous rectifier circuit 3. The switch 2 performs a switching operation that repeats conduction and interruption by the control circuit 7. The other end of the inductor 4 is connected to the output capacitor 5, and the output voltage Vo is supplied from the output capacitor 5 to the load 6. The control circuit 7 detects the output voltage Vo, and drives and controls the switch 2 so that the output voltage Vo becomes a desired value. The synchronous rectifier circuit 3 is configured to receive a signal from the control circuit 7 and to turn on the rectifier switch 30 of the synchronous rectifier circuit 3 only when the switch 2 is in the cut-off state.

  In FIG. 1, a synchronous rectifier circuit 3 includes a rectifier switch 30 composed of an N-channel MISFET, a first voltage source 31 that outputs a first threshold voltage Vth1, a second voltage source 32 that outputs a second threshold voltage Vth2, A first comparator 33, which is a first comparison unit that compares the drain voltage Vx of the rectifying switch 30 with the first threshold voltage Vth1, and a second comparator that compares the drain voltage Vx of the rectifying switch 30 with the second threshold voltage Vth2. Comparator 34, a P-channel MISFET, a switch 35 driven by the first comparator 33, a current source circuit 36 activated by the first comparator 33, an N-channel MISFET The switch 37 is driven by the second comparator 34. In the first embodiment, the first voltage source 31, the second voltage source 32, the first comparator 33, and the second comparator 34 constitute DC detection means, and the switch 35, The current source circuit 36 and the switch 37 constitute drive means.

  The first threshold voltage Vth1 is set to −0.07V, for example, and the second threshold voltage Vth2 is set to −0.02V. If the drain voltage Vx of the rectifying switch 30 is equal to or lower than the first threshold voltage Vth1, the switch 35 is in a conducting state, the current source circuit 36 is in an inactive state, the switch 37 is in a blocking state, and the rectifying switch 30 is in a conducting state. . If the drain voltage Vx of the rectifying switch 30 is not less than the first threshold voltage Vth1 and not more than the second threshold voltage Vth2, the switch 35 is in the cut-off state, the current source circuit 36 is in the active state, and the switch 37 is in the cut-off state. The gate voltage of the switch 30 gradually decreases. If the drain voltage Vx of the rectifying switch 30 is equal to or higher than the second threshold voltage Vth2, the switch 35 is in the cut-off state, the current source circuit 36 is in the active state, the switch 37 is in the conductive state, and the rectifier switch 30 is in the cut-off state.

Next, the operation of the switching power supply which is a step-down converter using the synchronous rectifier circuit 3 of the first embodiment will be described.
First, when the switch 2 is in a conducting state by the control circuit 7, the rectifying switch 30 is in a cut-off state. At this time, an increasing current flows in the order of input power source 1 → switch 2 → inductor 4 → output capacitor 5 and load 6 → input power source 1 to store magnetic energy in the inductor 4 and supply power to the load 6. When the switch 2 is cut off, the voltage across the inductor 4 is inverted, and the body diode of the rectifying switch 30 becomes conductive. For this reason, the forward voltage of the diode is generated in the negative direction in the drain voltage Vx of the rectifying switch 30 and is lower than the first threshold voltage Vth1, so that the rectifying switch 30 becomes conductive. Accordingly, a decreasing current flows through the rectifier switch 30 → the inductor 4 → the output capacitor 5 and the load 6 → the rectifier switch 30 to release the magnetic energy of the inductor 4 and supply power to the load 6. Since the rectifying switch 30 is in a conducting state for most of the cutoff time of the switch 2 and the conducting voltage is lowered, power supply to the load 6 is performed with low loss. Stable power is supplied to the load 6 by periodically repeating the switching operation of the switch 2 as described above. When the ratio of the conduction time in one switching cycle of the switch 2 is the duty ratio δ, the relationship between the input voltage Vi and the output voltage Vo is approximately expressed by the following equation (2).

    Vo = δ · Vi (2)

  Therefore, the output voltage Vo can be controlled by the control circuit 7 adjusting the duty ratio δ of the switch 2. The above is the operation in the case of the current continuous mode in which the current flowing through the inductor 4 is always zero or more.

Next, the operation in the current discontinuous mode in which the current flowing through the inductor 4 is zero will be described with reference to the operation waveform diagram shown in FIG.
2, (a) is the current IL flowing through the inductor 4 in the current discontinuous mode, (b) is the gate voltage Vg of the rectifying switch 30, (c) is the drain voltage Vx of the rectifying switch 30, and (d) is the rectifying switch. A drain voltage Vx obtained by enlarging the vicinity of the zero voltage of 30 drain voltages Vx only with respect to the voltage on the vertical axis is shown.

First, in the time domain 1 shown in FIG. 2, when the switch 2 is in a conducting state, the rectifying switch 30 is cut off, and the input power source 1 → the switch 2 → the inductor 4 → the output capacitor 5 and the load 6 → the input power source 1 A current flows, storing magnetic energy in the inductor 4 and supplying power to the load 6.
In the time domain 2 shown in FIG. 2, when the switch 2 is cut off, the body diode of the rectifying switch 30 becomes conductive, the drain voltage Vx of the rectifying switch 30 falls below the first threshold voltage Vth1, and the first comparator 33 Puts the switch 35 in the conducting state and inactivates the current source circuit 36, and the second comparator 34 puts the switch 37 in the cut-off state. That is, the rectifier switch 30 is turned on when the input voltage Vi is applied to the gate. In this state, a decreasing current flows through the rectifier switch 30 → the inductor 4 → the output capacitor 5 and the load 6 → the rectifier switch 30 to release the magnetic energy of the inductor 4 and supply power to the load 6. Since the rectifying switch 30 that is a MISFET has a low resistance in the conductive state, the drain voltage Vx of the rectifying switch 30 increases from the negative voltage as the current decreases. At a light load such as the current discontinuous mode, the current value is small, and the drain voltage Vx of the rectifying switch 30 exceeds the first threshold voltage Vth1 = −0.07 V at the start of conduction of the rectifying switch 30 or immediately thereafter.

In the time region 3 shown in FIG. 2, the drain voltage Vx of the rectifying switch 30 exceeds the first threshold value Vth1 = −0.07 V, the switch 35 is cut off, and the current source circuit 36 is activated. As a result, the gate voltage Vg of the rectifying switch 30 decreases.
When the drain voltage Vx of the rectifying switch 30 exceeds the second threshold voltage Vth2 = −0.02 V in the time domain 4 shown in FIG. 2, the switch 37 is turned on and the rectifying switch 30 is turned off. For this reason, the current flowing through the inductor 4 becomes zero, but since the switch 2 is also in the cut-off state, the voltage of the inductor 4 vibrates. Eventually, the switch 2 becomes conductive again and the above operation is repeated.

  In the time region 3 in which the gate voltage of the rectifying switch 30 decreases in the above operation, the gate voltage is decreased to the vicinity of the gate threshold voltage Vgth for maintaining the conducting state of the rectifying switch 30. Thus, by reducing the gate voltage Vg of the rectifying switch 30 to the vicinity of the gate threshold voltage Vgth, the time until the rectifying switch 30 is cut off in the time domain 4 is It is time to lower the gate voltage Vg to the gate threshold voltage Vgth. That is, the delay time from when the drain voltage Vx of the rectifying switch 30 exceeds the second threshold voltage Vth2 to when the rectifying switch 30 is cut off is influenced by the gate drive voltage determined by the input voltage Vi as in the prior art. Without a short period of fluctuation. The inductance of the inductor 4 is L, the delay time from when the drain voltage Vx of the rectifying switch 30 exceeds the second threshold voltage Vth2 until the rectifying switch 30 is cut off is Td, and the on-resistance of the rectifying switch 30 is Ron And Since the current IL flowing through the inductor 4 decreases with a slope −Vo / L, by setting the second threshold voltage Vth2 to an appropriate value close to zero, as shown in the following equation (3), the inductor When the current IL flowing through 4 becomes zero or substantially zero, the rectification switch 30 can be cut off.

    Vth2 ＝ Ron ・ Vo ・ Td / L (3)

From Expression (3), if the output voltage Vo is stabilized, the second threshold voltage Vth2 can be set to a substantially constant value.
Since the synchronous rectifier circuit of the present invention operates effectively in the current discontinuous mode, the first threshold voltage Vth1 is set to the current peak value of the inductor 4 when the current discontinuous mode is set. It is good to set below the value multiplied by the ON resistance. That is, if the switching period is T, the cutoff time Toff of the switch 2 is Toff = T · (Vi−Vo) / Vi, and therefore Vth1 ≦ Ron · Vo · T · (Vi−Vo) / Vi / L Conditions are obtained. Since the right side of this conditional expression is maximum when Vo = Vi / 2, the first threshold voltage Vth1 may be set as shown in the following expression (4).

    Vth1 = Ron · T / L / 4 (4)

  Note that the current value Ig of the current source circuit 36 is set so that the gate voltage Vg of the rectifier switch 30 is reduced to the vicinity of the gate threshold voltage Vgth during the conduction time of the synchronous rectifier circuit 3 in the current discontinuous mode. Since the conduction time of the synchronous rectifier circuit 3 is Ton · (Vi−Vo) / Vo, if the gate capacitance of the rectifier switch 30 is Cg and the gate voltage when the rectifier switch 30 is turned on is Vg0, the current source circuit 36 The current value Ig is expressed by the following equation (5).

    Ig = Cg ・ (Vg0−Vgth) ・ Vo / (Vi−Vo) / Ton (5)

  The above setting differs depending on the control operation in the current discontinuous mode. For example, in the current discontinuous mode, when the switch 2 off period is controlled by limiting the current peak value of the switch 2 to a predetermined value, (Vi−Vo) · Ton is constant, so Vg0 is fixed. As a result, the current value Ig of the current source circuit 36 can also be set to a fixed value. Otherwise, the current value Ig of the current source circuit 36 may be corrected so as to be inversely proportional to the difference voltage (Vi−Vo) · Ton between the input voltage Vi and the output voltage Vo from the equation (5).

  In the synchronous rectifier circuit 3 of the first embodiment configured as described above, when the current flowing through the rectifier switch 30 is equal to or lower than a predetermined value, the rectifier switch 30 has a threshold voltage near the threshold voltage that can maintain the ON state. Since the gate voltage Vg, which is a control signal, is maintained at a low level, the rectifier switch 30 can be cut off when the current flowing through the rectifier switch 30 is zero or substantially zero. Since the synchronous rectifier circuit 3 according to the first embodiment is configured to prevent the current of the rectifier switch 30 from flowing backward, by applying the synchronous rectifier circuit 3 to a switching power supply, high efficiency and high reliability are achieved. A power supply device can be provided.

<< Second Embodiment >>
FIG. 3 is a circuit configuration diagram of a switching power supply that is a step-down converter using the synchronous rectifier circuit according to the second embodiment of the present invention. In the second embodiment, components having the same functions and configurations as those of the first embodiment shown in FIG. 1 are given the same numbers, and descriptions thereof are omitted. The synchronous rectifier circuit 3A of the second embodiment is different from the configuration of the synchronous rectifier circuit 3 of the first embodiment in that a reference voltage source 38, a third comparator 39, and an AND gate 40 are added. It is. In the synchronous rectifier circuit 3A of the second embodiment, the third comparator 39 compares the gate voltage Vg of the rectifier switch 30 with the voltage of the reference voltage source 38, and the output of the third comparator 39 and the second comparator 39 The output of the first comparator 33 is input to the AND gate 40, and the output of the AND gate 40 activates the current source circuit 36. The reference voltage output from the reference voltage source 38 is set to a voltage slightly higher than the gate threshold voltage Vgth. Further, the current value Ig of the current source circuit 36 is set to be large so that the gate voltage Vg of the rectifying switch 30 falls to the vicinity of the gate threshold voltage Vgth within the conduction time of the synchronous rectifier circuit 3A.

Hereinafter, the operation in the current discontinuous mode in the switching power supply according to the second embodiment will be described with reference to FIG.
FIG. 4 is an operation waveform diagram of the main part of the step-down converter using the synchronous rectifier circuit 3A of the second embodiment shown in FIG. 4, (a) is the current IL flowing through the inductor 4, (b) is the gate voltage Vg of the rectifier switch 30, and (c) is enlarged only near the zero voltage of the drain voltage Vx of the rectifier switch 30 with respect to the vertical axis voltage. The drain voltage Vx is shown.

First, in the time domain 1 shown in FIG. 4, the switch 2 is in a conductive state and the rectifying switch 30 is in a cut-off state, and a current flows from the input power source 1 to the output through the switch 2 and the inductor 4.
In the time domain 2, when the switch 2 is cut off, the body diode of the rectifying switch 30 becomes conductive, and the drain voltage Vx of the rectifying switch 30 is lower than the first threshold voltage Vth1. As a result, the first comparator 33 brings the switch 35 into a conducting state and the current source circuit 36 in an inactive state, and the second comparator 34 puts the switch 37 in a cut-off state. That is, the rectifier switch 30 is turned on when the input voltage Vi is applied to the gate. For this reason, a current flows to the output via the rectifying switch 30 and the inductor 4, and this current decreases. As the current decreases, the drain voltage Vx of the rectifying switch 30 increases from the negative voltage.

In the time domain 3, the drain voltage Vx of the rectifying switch 30 exceeds the first threshold voltage Vth1 = −0.07 V, the switch 35 is cut off, the current source circuit 36 is activated, and the gate voltage Vg of the rectifying switch 30 is It goes down.
In the time domain 4, when the gate voltage Vg of the rectifying switch 30 approaches the gate threshold voltage Vgth and reaches the voltage of the reference voltage source 38, the output of the third comparator 39 becomes L level, and the current source circuit 36 is inactive. It becomes. For this reason, the decrease in the gate voltage Vg of the rectifying switch 30 is stopped almost at the gate threshold voltage Vgth.

  When the drain voltage Vx of the rectifying switch 30 exceeds the second threshold voltage Vth2 = −0.02 V in the time domain 5, the switch 37 is turned on and the rectifying switch 30 is turned off. As a result, the current flowing through the inductor 4 becomes zero, but the voltage of the inductor 4 vibrates because the switch 2 is also in the cut-off state. Eventually, the switch 2 becomes conductive again and the above operation is repeated.

  As described above, the switching power supply according to the second embodiment has the current source circuit 36 in the current discontinuous mode even when the input / output voltage and the initial gate voltage Vg0 when the rectifier switch 30 is turned on greatly fluctuate. By being deactivated, the gate voltage Vg is configured to stop decreasing near the gate threshold voltage Vgth. Accordingly, when the drain voltage Vx of the rectifying switch 30 reaches the second threshold voltage Vth2, the gate voltage Vg is always near the gate threshold voltage Vgth. That is, in the switching power supply according to the second embodiment, the timing for turning off the rectifying switch 30 can be set when the current flowing through the inductor 4 is zero or substantially zero.

  In the synchronous rectifier circuit 3A of the second embodiment configured as described above, when the current flowing through the rectifier switch 30 is equal to or less than a predetermined value, the rectifier switch 30 has a threshold voltage around the threshold voltage that can maintain the ON state of the rectifier switch 30. Since the gate voltage Vg, which is a control signal, is maintained at a low level, the rectifier switch 30 can be cut off when the current flowing through the rectifier switch 30 is zero or substantially zero. Since the synchronous rectifier circuit 3A of the second embodiment is configured to prevent the current of the rectifier switch 30 from flowing backward, applying the synchronous rectifier circuit 3A to a switching power supply enables high efficiency and reliability. A highly efficient power supply device can be provided.

<< Third Embodiment >>
In the first embodiment and the second embodiment described above, the synchronous rectifier circuit having a rectifier switch composed of an N-channel MISFET has been described. However, even if the rectifier switch is a P-channel MISFET, the present invention is similarly applied. The inventive synchronous rectifier circuit can be constructed. In the third embodiment, a synchronous rectifier circuit having a rectifier switch composed of a P-channel MISFET will be described.

  FIG. 5 is a circuit configuration diagram of a switching power supply that is a step-up converter using the synchronous rectifier circuit according to the third embodiment of the present invention. In FIG. 5, 11 is an input power source that outputs an input voltage Vi, 12 is a switch, 13 is a synchronous rectifier circuit, 14 is an inductor, 15 is an output capacitor, 16 is a load, and 17 is a control circuit. The inductor 14 has one end connected to the input power supply 11 and the other end connected to the switch 12 and the synchronous rectifier circuit 13. The other end of the switch 12 is grounded, and the switch 12 performs a switching operation that repeats conduction and interruption by the control circuit 17. The other end of the synchronous rectifier circuit 13 is connected to the output capacitor 15, and the output voltage Vo is supplied from the output capacitor 15 to the load 16. The control circuit 17 detects the output voltage Vo and drives and controls the switch 12 so that the output voltage Vo becomes a desired value. The synchronous rectifier circuit 13 receives the signal from the control circuit 17 and makes the rectifier switch 50 conductive only when the switch 12 is in the cut-off state.

  In FIG. 5, a synchronous rectifier circuit 13 includes a rectifier switch 50 made of a P-channel MISFET, a first voltage source 51 that outputs a first threshold voltage Vth1, and a second voltage source 52 that outputs a second threshold voltage Vth2. The first comparator 53, which is a first comparator for comparing the drain voltage Vx of the rectifying switch 50 with the first threshold voltage Vth1, and the first comparator 53 for comparing the drain voltage Vx of the rectifying switch 50 with the second threshold voltage Vth2. A second comparator 54 which is a second comparison means, a switch 55 which is an N-channel MISFET and is driven by the first comparator 53, a current source circuit 56 which is activated by the first comparator 53, and a P-channel The switch 57 is a MISFET and is driven by the second comparator 54. In the second embodiment, the first voltage source 51, the second voltage source 52, the first comparator 53, and the second comparator 54 constitute a DC detecting means, and the switch 55, The current source circuit 56 and the switch 57 constitute driving means.

The first threshold voltage Vth1 is set to 0.07V, for example, and the second threshold voltage Vth2 is set to 0.02V. If the drain voltage Vx of the rectifying switch 50 is equal to or higher than the sum voltage (Vo + Vth1) of the output voltage Vo and the first threshold voltage Vth1, the switch 55 is conductive, the current source circuit 56 is inactive, and the switch 57 is cut off. Thus, the rectifying switch 50 becomes conductive.
If the drain voltage Vx of the rectifying switch 50 is equal to or lower than (Vo + Vth1) and is equal to or higher than the sum voltage (Vo + Vth2) of the output voltage Vo and the second threshold voltage, the switch 55 is cut off, the current source circuit 56 is activated, and the switch 57 Is cut off, and the source-gate voltage of the rectifying switch 50 gradually decreases.
If the drain voltage Vx of the rectifying switch 50 is equal to or lower than (Vo + Vth2), the switch 55 is cut off, the current source circuit 56 is activated, the switch 57 is turned on, and the rectifying switch 50 is cut off.

The operation of the switching power supply according to the third embodiment configured as described above will be described.
First, when the switch 12 is in a conducting state by the control circuit 17, the rectifying switch 50 is in a cut-off state. At this time, an increasing current flows through the input power source 11 → the inductor 14 → the switch 12 → the input power source 11. When the switch 12 is cut off, the voltage across the inductor 14 is inverted, and the body diode of the rectifying switch 50 becomes conductive. For this reason, a voltage obtained by adding the forward voltage of the diode to the output voltage Vo is generated in the drain voltage Vx of the rectifying switch 50. Since the drain voltage Vx exceeds (Vo + Vth1), the rectifying switch 50 becomes conductive. Therefore, a decreasing current flows through the input power supply 11 → the inductor 14 → the rectifier switch 50 → the output capacitor 15 and the load 16 → the input power supply 11 to supply power to the load 16. Since the rectifying switch 50 is in a conductive state during most of the cutoff time of the switch 12 and the conductive voltage is lowered, power is supplied to the load 16 with low loss.

  Stable power is supplied to the load 16 by periodically repeating the switching operation of the switch 12 as described above. When the duty ratio δ is the ratio of the conduction time in one switching cycle of the switch 12, the relationship between the input voltage Vi and the output voltage Vo is approximately expressed by the following equation (6).

    Vo = Vi / (1-δ) (6)

  Therefore, the control circuit 17 can control the output voltage Vo by adjusting the duty ratio δ of the switch 12. The above operation is an operation when the current IL flowing through the inductor 14 is always referred to as a current continuous mode in which the current IL is zero or more.

Next, the operation in the current discontinuous mode in which the current flowing through the inductor 14 is zero will be described.
First, when the switch 12 is in a conducting state, the rectifying switch 50 is cut off, and a current flows through the input power source 11 → the inductor 14 → the switch 12 → the input power source 11. When the switch 12 is cut off, the body diode of the rectifying switch 50 becomes conductive, and the drain voltage Vx of the rectifying switch 50 generates a voltage obtained by adding the forward voltage of the diode to the output voltage Vo. Therefore, since the drain voltage Vx exceeds (Vo + Vth1), the first comparator 53 brings the switch 55 into a conducting state and the current source circuit 56 into an inactive state, and the second comparator 54 cuts off the switch 57. State. That is, the rectifying switch 50 is turned on when the output voltage Vo is applied between the source and the gate. At this time, a decreasing current flows through the input power supply 11 → the inductor 14 → the rectifier switch 50 → the output capacitor 15 and the load 16 → the input power supply 11 to supply power to the load 16. Since the rectifying switch 50 which is a MISFET in the conductive state has a low resistance, the drain voltage Vx of the rectifying switch 50 decreases from a potential exceeding the output voltage Vi as the current decreases. At a light load such as the current discontinuous mode, the current value is small, and the drain voltage Vx of the rectifying switch 50 falls below (Vo + Vth1) at the time of starting the conduction of the rectifying switch 50 or immediately thereafter. As a result, the switch 55 is cut off, the current source circuit 56 is activated, and the source-gate voltage of the rectifying switch 50 decreases. Eventually, when the drain voltage Vx of the rectifying switch 50 falls below (Vo + Vth2), the switch 57 is turned on and the rectifying switch 50 is turned off. As a result, the current flowing through the inductor 14 becomes zero, but the voltage of the inductor 14 oscillates because the switch 12 is also cut off. Eventually, the switch 12 becomes conductive again, and the above operation is repeated.

  In the above operation, when the gate voltage Vg of the rectifying switch 50 is lowered, the gate voltage Vg is lowered to the vicinity of the gate threshold voltage Vgth at which the rectifying switch 50 is maintained in the conductive state. Thus, by lowering the gate voltage Vg to the vicinity of the gate threshold voltage Vgth, the time until the rectifier switch 50 is cut off is that the switch 57 lowers the source-gate voltage of the rectifier switch 50 below the gate threshold voltage Vgth. It will be time to let That is, the delay time from when the drain voltage Vx of the rectifying switch 50 falls below (Vo + Vth2) to when the rectifying switch 50 is cut off is not affected by the gate drive voltage determined by the input voltage Vi as in the prior art. It will be a short time with little fluctuation. The inductance of the inductor 14 is L, the delay time from when the drain voltage Vx of the rectifying switch 50 exceeds (Vo + Vth2) until the rectifying switch 50 is cut off is Td, and the on-resistance of the rectifying switch 50 is Ron. Since the current IL flowing through the inductor 14 decreases with a slope (Vi−Vo) / L, the second threshold voltage Vth2 is set to an appropriate value close to zero as shown in the following equation (7). Thus, when the current flowing through the inductor 14 becomes zero or substantially zero, the rectifying switch 50 can be cut off.

    Vth2 = Ron ・ (Vo−Vi) ・ Td / L (7)

  However, according to the above equation (7), in order to obtain the ideal second threshold voltage Vth2, it is only necessary to correct so as to be proportional to the input / output voltage difference (Vo−Vi). For example, two resistors connected in series are installed between the input and output, and the connection point potential is used as (Vo + Vth2). When the resistance values of the two resistors are r1 and r2, the following equation (8) is obtained.

    Vth2 = (Vo−Vi) ・ r1 / (r1 + r2) (8)

  Therefore, from the equations (7) and (8), each resistance value may be set as r1 / (r1 + r2) = Ron · Td / L.

  Since the synchronous rectifier circuit according to the present invention operates effectively in the current discontinuous mode, the first threshold voltage Vth1 is set to the current peak value of the inductor 14 when the current discontinuous mode is set. It is good to set below the value which multiplied 50 ON resistance. That is, assuming that the switching period is T, the cutoff time Toff of the switch 12 is Toff = T · Vi / Vo, so that the condition Vth1 ≦ Ron · T · Vi / L is obtained. Since the right side of this conditional expression is maximum when the input voltage is maximum, that is, Vi = Vimax, the first threshold voltage Vth1 may be set as shown in the following expression (9).

    Vth1 ＝ Ron ・ T ・ Vimax / L (9)

  The current value Ig of the current source circuit 56 is set so that the source-gate voltage of the rectifier switch 50 is reduced to around the gate threshold voltage Vgth during the conduction time of the synchronous rectifier circuit 13 in the current discontinuous mode. Since the conduction time of the synchronous rectifier circuit 13 is Ton · Vi / (Vo−Vi), assuming that the gate capacitance of the rectifier switch 50 is Cg and the gate voltage when the rectifier switch 50 is turned on is Vg0, the current value Ig is (10)

    Ig = Cg ・ (Vg0−Vgth) ・ (Vo−Vi) / Vi / Ton (10)

  The setting for the current value Ig differs depending on the control operation in the current discontinuous mode. For example, in the current discontinuous mode, when the current peak value of the switch 12 is limited to a predetermined value and the off period of the switch 12 is controlled, Vi · Ton is constant, so the gate voltage Vg0 at turn-on is fixed. As a result, the current value Ig of the current source circuit 56 is equal to the current value Ig of the current source circuit 56 so as to be inversely proportional to the difference voltage (Vo−Vi) between the input voltage Vi and the output voltage Vo according to the equation (10). You only have to make corrections. Alternatively, as in the above-described second embodiment, when the current value Ig is set sufficiently large and the source-gate voltage of the rectifying switch 50 reaches the vicinity of the gate threshold voltage Vgth, the current source circuit 56 is deactivated. May be.

The operation in the current discontinuous mode in the switching power supply according to the third embodiment will be described below with reference to FIG.
FIG. 6 is an operation waveform diagram of the main part of the boost converter using the synchronous rectifier circuit 13 of the third embodiment shown in FIG. 6A is a current IL flowing through the inductor 14, FIG. 6B is a source-gate voltage Vg of the rectifying switch 50, and FIG. 6C is a voltage around the zero voltage of the drain voltage Vx of the rectifying switch 50 with respect to the vertical axis voltage. Only the enlarged drain voltage Vx is shown.

First, in the time domain 1 shown in FIG. 6, the switch 12 is in a conducting state and the rectifying switch 50 is in a cut-off state, and current flows from the input power supply 1 to the switch 12 via the inductor 14.
In the time domain 2, when the switch 12 is cut off, the rectifier switch 50 is turned on, and the drain voltage Vx of the rectifier switch 50 is generated by adding the forward voltage of the diode to the output voltage Vo. The drain voltage Vx exceeds the voltage (Vo + Vth1) obtained by adding the first threshold voltage Vth1 to the output voltage Vo. For this reason, the first comparator 53 brings the switch 55 into a conducting state and the current source circuit 56 into an inactive state, and the second comparator 54 puts the switch 57 into a cut-off state. That is, the rectifying switch 50 is turned on when the output voltage Vo is applied between the source and the gate. For this reason, a current flows to the output via the inductor 14 and the rectifying switch 50, and this current decreases. As the current decreases, the drain voltage Vx of the rectifying switch 50 decreases.

In the time domain 3, when the drain voltage Vx of the rectifying switch 50 falls below the voltage (Vo + Vth1), the switch 55 is cut off, the current source circuit 56 is activated, and the source-gate voltage of the rectifying switch 50 decreases. Go.
In the time domain 4, when the drain voltage Vx of the rectifying switch 50 falls below the voltage (Vo + Vth2) obtained by adding the second threshold voltage Vth2 to the output voltage Vo, the switch 57 is turned on and the rectifying switch 50 is turned off. To do. As a result, the current flowing through the inductor 14 becomes zero, but the switch 12 is also cut off, so the voltage of the inductor 14 vibrates. Eventually, the switch 12 becomes conductive again and the above operation is repeated.

  As described above, the switching power supply according to the third embodiment includes the current source circuit 56 in the current discontinuous mode even when the input / output voltage and the initial gate voltage Vg0 when the rectifier switch 50 is turned on greatly fluctuate. By being deactivated, the source-gate voltage Vg is configured to stop decreasing near the gate threshold voltage Vgth. Accordingly, when the drain voltage Vx of the rectifying switch 30 reaches the voltage (Vo + Vth2) obtained by adding the second threshold voltage Vth2 to the output voltage Vo, the source-gate voltage Vg is always in the vicinity of the gate threshold voltage Vgth. That is, in the switching power supply of the third embodiment, the timing for turning off the rectifying switch 50 can be set when the current flowing through the inductor 14 is zero or substantially zero.

  In the synchronous rectifier circuit 13 of the third embodiment configured as described above, when the current flowing through the rectifier switch 50 is less than or equal to a predetermined value, the rectifier switch 50 is in the vicinity of a threshold voltage that can maintain the ON state of the rectifier switch 50. Since the source-gate voltage Vg which is a control signal is maintained at a high level, the rectifier switch 50 can be cut off when the current flowing through the rectifier switch 50 becomes zero or substantially zero. Since the synchronous rectifier circuit 13 of the third embodiment is configured to prevent the current of the rectifying switch 50 from flowing backward, applying the synchronous rectifier circuit 13 to a switching power supply enables high efficiency and reliability. A highly efficient power supply device can be provided.

  The synchronous rectifier circuit of the present invention is a highly efficient and reliable power supply device when used as a rectifier circuit such as a switching power supply, and is useful in the field of switching power supplies.

1 is a circuit configuration diagram showing a step-down converter using a synchronous rectifier circuit according to a first embodiment of the present invention. Operation waveform diagram of main part in current discontinuous mode of step-down converter using synchronous rectifier circuit of first embodiment The circuit block diagram which shows the step-down converter using the synchronous rectification circuit of 2nd Embodiment which concerns on this invention Operation waveform diagram of main part in current discontinuous mode of step-down converter using synchronous rectifier circuit of second embodiment Circuit configuration diagram of a boost converter using a synchronous rectifier circuit according to a third embodiment of the present invention Operation waveform diagram of main part in current discontinuous mode of boost converter using synchronous rectifier circuit of third embodiment Circuit configuration diagram showing a step-down converter using a conventional synchronous rectifier circuit Operation waveform diagram of main part in current discontinuous mode of step-down converter using conventional synchronous rectifier circuit

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Input power supply 2 Switch 3 Synchronous rectifier circuit 4 Inductor 5 Output capacitor 6 Load 7 Control circuit 30 Rectifier switch 31 1st voltage source 32 2nd voltage source 33 1st comparator 34 2nd comparator 35 Switch 36 Current Source circuit 37 switch

Claims (12)

A rectifying switch that is turned on or off according to the level of the control signal;
Current detection means for detecting a current flowing through the rectifying switch using a first current value and a second current value as threshold values;
Drive means for changing the level of the control signal in accordance with the output of the current detection means,
If the current flowing through the rectifying switch is greater than or equal to the first current value, the driving means sets the control signal to a first level, and the current flowing through the rectifying switch is equal to the first current value and the second current value. If it is between current values, the control signal is changed to the second level, and if the current flowing through the rectifying switch is equal to or less than the second current value, the control signal is set to the second level. A synchronous rectifier circuit configured as described above.   2. The synchronous rectifier circuit according to claim 1, wherein when the current flowing through the rectifier switch is a second current value, the control signal is configured to be equal to or greater than a threshold value from conduction to cutoff in the rectification switch.   The current detecting means compares the first current value flowing through the rectifying switch with the first current value, and the second comparing means compares the current flowing through the rectifying switch with the second current value. And a synchronous rectifier circuit according to claim 1. The rectifying switch is an N-channel MISFET, and a gate-source voltage is used as the control signal.
The first comparison means compares the drain-source voltage in the conductive state of the rectifying switch with a first threshold value corresponding to the first current value,
4. The synchronous rectifier circuit according to claim 3, wherein the second comparison unit is configured to compare a drain-source voltage in a conductive state of the rectifier switch with a second threshold value corresponding to the second current value. 5. A DC voltage source having a positive terminal and a negative terminal, and connecting a source terminal of the rectifying switch to the negative terminal,
The drive means
If the drain-source voltage driven by the output of the first comparing means and the rectifying switch is in a conductive state is equal to or lower than a first threshold, the gate terminal of the rectifying switch is pulled up to the potential of the positive terminal. 1 auxiliary switch,
A current source circuit that is driven by the output of the first comparison means and discharges between the gate and the source of the rectifier switch if the drain-source voltage in the conductive state of the rectifier switch is equal to or higher than a first threshold;
If the drain-source voltage driven by the output of the second comparison means and the rectifier switch in the conductive state is equal to or higher than a second threshold value, the second rectifier switch pulls down the gate of the rectifier switch to the potential of the negative terminal. The synchronous rectifier circuit according to claim 4, further comprising an auxiliary switch.   The synchronous rectifier circuit according to claim 5, wherein the current source circuit is configured to discharge between the gate and the source of the rectifier switch so that the control signal maintains a gate threshold voltage or more. The driving circuit includes a third comparison unit that compares a gate-source voltage of the rectifying switch with a third threshold value;
The current source circuit is:
Driven by the output of the first comparison means and the output of the third comparison means, the drain-source voltage in the conductive state of the rectifier switch is equal to or higher than a first threshold, and the gate-source voltage is the first The synchronous rectifier circuit according to claim 6, wherein the synchronous rectifier circuit is configured to discharge between the gate and the source of the rectifier switch when the threshold value is 3 or more. The rectifier switch is a P-channel MISFET, and a source-gate voltage is used as the control signal.
The first comparison means compares the drain-source voltage in the conductive state of the rectifying switch with a first threshold value corresponding to the first current value,
4. The synchronous rectifier circuit according to claim 3, wherein the second comparison unit is configured to compare a drain-source voltage in a conductive state of the rectifier switch with a second threshold value corresponding to the second current value. 5. A DC voltage source having a positive electrode terminal and a negative electrode terminal, connecting a source of the rectifying switch to the positive electrode terminal;
First driven to pull down the gate terminal of the rectifying switch to the potential of the negative terminal if driven by the output of the first comparing means and the drain-source voltage in the conducting state of the rectifying switch is greater than or equal to a first threshold. Auxiliary switch
A current source circuit that is driven by the output of the first comparison means and discharges between the gate and the source of the rectifying switch if the drain-source voltage in the conductive state of the rectifying switch is equal to or lower than a first threshold;
If the drain-source voltage driven by the output of the second comparing means and the rectifying switch is in a conducting state is equal to or lower than a second threshold, the second rectifying switch pulls up the gate of the rectifying switch to the potential of the positive terminal. The synchronous rectifier circuit according to claim 8, further comprising: an auxiliary switch.   The synchronous rectifier circuit according to claim 9, wherein the current source circuit is configured to discharge between a source and a gate of the rectifier switch so that the control signal maintains a gate threshold voltage or more. The driving means includes third comparing means for comparing a source-gate voltage of the rectifying switch and a third threshold value,
The current source circuit comprises:
Driven by the output of the first comparison means and the output of the third comparison means, the drain-source voltage in the conductive state of the rectifier switch is less than or equal to a first threshold, and the source-gate voltage is third The synchronous rectifier circuit according to claim 10, wherein the synchronous rectifier circuit is configured to discharge between the source and the gate of the rectifier switch when the threshold value is equal to or greater than the threshold value. A DC voltage source having a positive terminal and a negative terminal;
A switch having one end connected to the positive terminal of the DC voltage source and the other end connected to a load via an inductor and smoothing means;
A switching power supply comprising a synchronous rectifier circuit connected to the other end of the switch,
The synchronous rectification circuit is a rectification switch that is turned on or off according to the level of the control signal;
Current detection means for detecting a current flowing through the rectifying switch using a first current value and a second current value as threshold values;
Drive means for changing the level of the control signal in accordance with the output of the current detection means,
If the current flowing through the rectifying switch is greater than or equal to the first current value, the driving means sets the control signal to a first level, and the current flowing through the rectifying switch is equal to the first current value and the second current value. If it is between current values, the control signal is changed toward the second level, and if the current flowing through the rectifying switch is equal to or lower than the second current value, the control signal is set to the second level. Configured switching power supply.

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