JP2012205356A - Rectification switch unit, rectification circuit, and switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rectification switch unit, a rectification circuit, and a switching power supply device capable of performing synchronization rectification even when an output voltage is high, and having an excellent cost performance.SOLUTION: A rectification switch unit 1 has: a normally-off MOSFET 6; a body diode Db whose anode is connected with a source of the normally-off MOSFET 6, and whose cathode is connected with a drain of the normally-off MOSFET 6; a normally-on MOSFET 5 whose source is connected with the drain of the normally-off MOSFET 6, and whose gate is connected with the source of the normally-off MOSFET 6; a sensor 2 whose one input terminal is connected with the drain, and that receives a threshold voltage -Vth from a voltage source 50 at the other input terminal and outputs a signal indicating a comparison result of a voltage at the one input terminal with the threshold voltage -Vth; and a control circuit 30 outputting a signal instructing to turn on and off the normally-off MOSFET 6 to the normally-off MOSFET 6.

Description

  The present invention relates to a rectifying switch unit that performs synchronous rectification, a rectifying circuit, and a switching power supply device.

  One method for rectifying current is a synchronous rectification method (hereinafter referred to as synchronous rectification). In synchronous rectification, the direction of the current is adopted in a circuit that changes according to time, and the direction of the current and the on / off of the rectifying element are synchronized. That is, when the current flows in a desired direction, the rectifying element is turned on to allow the current to flow. On the other hand, when the current flows in a direction opposite to the desired direction, the rectifying element is turned off to block the current.

  Such synchronous rectification has been conventionally applied to a switching power supply device having a relatively low output voltage. The relatively low output voltage described here is, for example, a voltage of about 12V to 24V. In one of the synchronous rectifications applied to the switching power supply device described above, a drain-source voltage of a FET (field-effect transistor) which is a rectifying element is monitored. Thereby, it is detected whether or not the direction of the drain-source voltage of the FET is negative (the source-drain voltage is positive) and exceeds a predetermined threshold voltage. Then, the FET is turned on / off according to the detection result. Thereby, synchronous rectification was performed.

  In the switching power supply described above, the output voltage is relatively low. Therefore, when the FET is turned off, the voltage applied to the detection circuit which is a circuit for monitoring the drain-source voltage is also low. Therefore, the detection circuit does not require a high breakdown voltage.

  On the other hand, conventionally, it has been said that a switching power supply having a relatively high output voltage has no merit of employing synchronous rectification. The relatively high output voltage described here is, for example, a voltage of about 200V to 500V.

  However, in recent years, super on-resistance devices (devices with extremely low on-resistance) such as wide band gap devices have been put into practical use. With the practical application of ultra-on-resistance devices, it has become possible to expect the effect that power loss can be reduced by adopting synchronous rectification in a switching power supply device having a relatively high output voltage.

  However, if the above-described synchronous rectification for monitoring the drain-source voltage of the FET, which is a voltage substantially equal to the output voltage, is used as it is in a switching power supply device having a relatively high output voltage, the detection circuit needs to have a high breakdown voltage. Occurs.

  Here, Patent Document 1 discloses the following voltage detection circuit as a detection circuit in the above-described switching power supply device having a relatively high output voltage. That is, Patent Document 1 discloses a voltage detection circuit that does not require the detection circuit to have a high withstand voltage even when voltage detection is performed at a detection point where a voltage having a large potential difference is output.

JP 2009-278718 A (published November 26, 2009)

  FIG. 6 is a circuit diagram of a conventional switching power supply device 101. The switching power supply device 101 of FIG. 6 is the switching power supply device of FIG. In the switching power supply apparatus 101 of FIG. 6, when the current rectification is performed when the output voltage Vo is a high voltage of about 200 V to 500 V, the following problems 1 and 2 occur.

(Problem 1)
When the switching element SW52 is turned off, the voltage at the connection point between the switching element SW52 and the resistor R51 rises steeply from zero to a value approximately double the output voltage Vo. This high voltage is divided by the resistor R51 and the resistor R52. In this case, it is necessary to increase the resistance value of the resistor R51 in order to avoid a decrease in power efficiency and an increase in cost.

  The resistor R51 and the capacitor Ca constitute an RC integrating circuit, and the time constant is R51 × Ca. Since the resistance value of the resistor R51 is high, the time constant is increased. As a result, the waveform of the voltage input to the detection circuit 51 is rounded (that is, the time required until the voltage input to the detection circuit 51 rises). Therefore, the response speed of the entire voltage detection circuit including the detection circuit 51 is reduced.

(Problem 2)
In order to solve the above-described problem 1, consider a case where the resistance value of the resistor R51 is reduced to reduce the time constant. In this case, the current flowing through the resistor R51 and the power consumption of the resistor R51 increase. Thereby, (1) power efficiency falls. At the same time, (2) it is necessary to use a resistor with a high rated power consumption (power capacity) as the resistor R51, and the cost increases.

  FIG. 7 is a circuit diagram of a conventional switching power supply apparatus 102. The switching power supply device 102 in FIG. 7 is the switching power supply device in FIG. 1 of Patent Document 1, and includes a voltage detection circuit 113 corresponding to the voltage detection circuit including the detection circuit 51 in FIG. A part of the correspondence of the components between the voltage detection circuit of the switching power supply apparatus 101 in FIG. 6 and the voltage detection circuit 113 of the switching power supply apparatus 102 in FIG. 7 is shown below.

  First, the switching element SW102 in FIG. 7 corresponds to the switching element SW52 in FIG. Second, the resistor R101 in FIG. 7 corresponds to the resistor R51 in FIG. Third, the resistor R102 in FIG. 7 corresponds to the resistor R52 in FIG. Fourth, the capacitance Ca in FIG. 7 corresponds to the capacitance Ca in FIG. Fifth, the detection circuit 121 in FIG. 7 corresponds to the detection circuit 51 in FIG.

  Although the voltage detection circuit 113 is a circuit in which the influence of the response speed, which is the problem 1 described above, is relatively small, the integration circuit composed of the resistor R101 and the capacitor Ca cannot be eliminated. For this reason, when it is going to apply to the switching power supply device whose output voltage is about 200V to 500V, the influence of the response speed cannot be ignored. In addition, when the resistance value of the resistor R101 is reduced, the above problem 2 occurs.

  There is also a new problem that the SW 121 needs to have a high breakdown voltage instead of the detection circuit 121 having a low breakdown voltage.

  The present invention has been made in view of the above-described conventional problems, and the purpose thereof is to perform synchronous rectification even when the output voltage is high, and a rectification switch unit having excellent cost performance, A rectifier circuit and a switching power supply device are provided.

  In order to solve the above problems, a rectifying switch unit of the present invention includes a first switching element that rectifies a current flowing between a source and a drain, a diode having an anode connected to the source, and a cathode connected to the drain. , A normally-on second switching element whose source is connected to the drain of the first switching element and whose gate is connected to the source of the first switching element; one input terminal connected to the drain; and the other input A comparator that outputs a signal indicating a comparison result between the voltage of the one input terminal and the predetermined threshold voltage, and a signal indicating the comparison result; A control unit that outputs a signal instructing on and off of the first switching element to the first switching element. And wherein the door.

  According to the invention, when the voltage applied between the source of the first switching element and the drain of the second switching element rises from zero, the gate-source voltage of the second switching element is positive. A voltage is applied. Since the second switching element is normally on, the threshold voltage has a negative value. In other words, the applied positive voltage exceeds this, and the second switching element becomes conductive. Then, the diode becomes conductive and the current flowing through the diode increases from zero, so that the voltage drop due to the diode generated at both ends of the diode increases from zero. As a result, the absolute value of the voltage input to the one input terminal at a certain time becomes higher than the absolute value of the predetermined threshold voltage, and the logic of the signal output from the comparison unit is inverted.

  The output signal of the comparison unit whose logic is inverted is converted to a level at which the first switching element is turned on by the control unit, and input to the first switching element. As a result, the gate-source voltage of the first switching element becomes a high level, so that the first switching element is turned on.

  When the first switching element is turned on, a source current can flow from the source of the first switching element to the drain of the second switching element.

  Next, consider a case where the first switching element and the second switching element are turned on and a source current flows from the source of the first switching element to the drain of the second switching element. In this case, the source current flows through the on-resistance of the first switching element. Therefore, a source-drain voltage is generated which is a voltage in which the source of the first switching element is positive and the drain of the first switching element is negative.

  The source-drain voltage increases as the source current increases. On the other hand, the source-drain voltage decreases as the source current decreases.

  When the source current decreases to near zero and the absolute value of the voltage input to the one input terminal becomes lower than the absolute value of the threshold voltage input to the other input terminal, the output from the comparison unit The logic of the signal to be inverted is inverted. The output signal of the comparison unit whose logic is inverted is converted to a level at which the first switching element is turned off by the control unit, and is input to the first switching element. As a result, the gate-source voltage of the first switching element becomes a low level, and thus the first switching element is turned off. When the drain-source voltage of the first switching element rises and the gate-source voltage of the second switching element falls below the threshold voltage (negative value), the second switching element is also turned off. The source current is blocked (cut off) by turning off the first and second switching elements.

  As described above, in the rectifying switch unit, the timing at which the source current flows from the source of the first switching element toward the drain of the second switching element is synchronized with the ON of the first and second switching elements. Synchronous rectification can be performed.

  More specifically, in the synchronous rectification, when the absolute value of the source-drain voltage exceeds the absolute value of the threshold voltage, the first and second switching elements are turned on. On the other hand, when the absolute value of the source-drain voltage is lower than the absolute value of the threshold voltage, the first and second switching elements are turned off.

  Here, since the connection between the first switching element and the second switching element is a cascode connection, only a voltage in the order of the threshold voltage of the second switching element is applied between the drain and source of the first switching element. Not. That is, the voltage input to the one input terminal is much lower than the high voltage applied between the source of the first switching element and the drain of the second switching element. Therefore, it is not necessary to use a high breakdown voltage and expensive sensor as the comparison unit.

  Therefore, synchronous rectification can be performed even when the output voltage is high, and a rectification switch unit having excellent cost performance can be provided.

  Further, the rectifying switch unit does not include the RC integrating circuit included in the conventional rectifying circuit. Therefore, the problem ((Problem 1) of [Problem to be Solved by the Invention]) that has occurred in the conventional rectifier circuit does not occur.

  That is, in the conventional rectifier circuit, when the resistance value of the electric resistance in the RC integration circuit is increased, the time constant is increased, resulting in the voltage waveform being distorted and the response speed being lowered. However, in the rectifier circuit, the voltage waveform is not reduced by integration, so that the response speed does not decrease.

  Further, the rectifying switch unit is not configured to divide the voltage input to the one input terminal by a resistor. Thus, the problem ((Problem 2) of [Problem to be Solved by the Invention]) that has occurred in the conventional rectifier circuit does not occur.

  That is, in the conventional rectifier circuit, in order to reduce the time constant which has been a problem in (Problem 1), by reducing the resistance value of the resistor, the power consumption of the resistor is increased and the power efficiency is decreased. , And increased costs. However, in the rectifying switch unit, there is no reduction in power efficiency due to current flowing through the resistor, and no increase in cost due to the use of a resistor with high rated power consumption (power capacity).

  The rectifying switch unit may further include a constant voltage diode having an anode connected to a source of the first switching element and a cathode connected to a drain of the first switching element.

  As a result, even if a transient high voltage is applied between the drain and source of the first switching element, a current flows through the constant voltage diode, thereby reducing the voltage between the drain and source of the first switching element. Can be clamped. Therefore, the first switching element can be protected from a transient high voltage.

  The rectifying switch unit further includes a diode array composed of a plurality of diodes connected in series, and an anode of a diode located at a final end of the plurality of diodes connected in series is the first switching element. The cathode of the diode located at the forefront among the plurality of diodes connected to the drain and connected in series may be connected to the source of the first switching element.

  Accordingly, even if a transient high voltage is applied between the drain and source of the first switching element, all the diodes in the diode row are turned on, and the voltage between the drain and source of the first switching element. Can be clamped. Therefore, the first switching element can be protected from a transient high voltage.

  In the rectifying switch unit, the comparison unit selects either the first voltage or a second voltage different from the first voltage as a threshold voltage input to the other input terminal in accordance with an instruction from the control unit. The control unit outputs a signal instructing selection of the first voltage to the comparison unit when the first switching element is turned on from the off state, and the first switching element is turned on. When the signal is turned off, a signal instructing selection of the second voltage may be output to the comparison unit.

  When the first switching element is turned on and when the first switching element is turned off, the value of the predetermined threshold voltage is individually set, thereby turning on and off the first switching element. It is possible to control with higher accuracy.

  In the rectifying switch unit, one input terminal includes two first and second comparison units connected to the drain of the first switching element, and the other input terminal of the first comparison unit has a threshold voltage. The second voltage different from the first voltage is input as the threshold voltage to the other input terminal of the second comparison unit, and the control When the first switching element is turned on from off, the output unit applies the output logic inversion of the first comparison unit to output a signal to the first switching element, and the first switching element is turned on from off Then, the output logic inversion of the second comparison unit may be applied to output a signal to the first switching element.

  When the first switching element is turned on and when the first switching element is turned off, the value of the predetermined threshold voltage is individually set, thereby turning on and off the first switching element. It is possible to control with higher accuracy.

  In the rectifying switch unit, the first switching element and the second switching element are metal / oxide / semiconductor field effect transistors. Thereby, the current flowing between the source and drain of each switching element can be rectified.

  Since the rectifier circuit of the present invention includes any one of the rectifier switch units described above, the rectifier circuit is superior in cost performance to the conventional rectifier circuit.

  Since the switching power supply of the present invention includes the rectifier circuit, the switching power supply is superior in cost performance to the conventional switching power supply.

  As described above, the rectifying switch unit according to the present invention includes the first switching element that rectifies the current flowing between the source and the drain, the diode having the anode connected to the source, the cathode connected to the drain, and the source. A normally-on second switching element connected to the drain of the first switching element and having a gate connected to the source of the first switching element; one input terminal connected to the drain; and the other input terminal to A comparison unit that receives a predetermined threshold voltage from a voltage source and outputs a signal indicating a comparison result between the voltage of the one input terminal and the predetermined threshold voltage, and the signal corresponding to the signal indicating the comparison result. And a control unit that outputs a signal instructing on and off of one switching element to the first switching element.

  Therefore, it is possible to perform synchronous rectification even when the output voltage is high, and to provide a rectifier switch unit, a rectifier circuit, and a switching power supply device that are excellent in cost performance.

It is a block diagram of the rectification switch unit which concerns on embodiment of this invention. It is a figure for demonstrating ON of normally-off MOSFET in the rectifier switch unit which concerns on embodiment of this invention. It is a figure for demonstrating OFF of normally-off MOSFET in the rectifier switch unit which concerns on embodiment of this invention. It is a block diagram of the rectifier switch unit which concerns on embodiment of this invention which used the diode row | line | column comprised by the several diode connected in series instead of the Zener diode. 1 is a circuit diagram of a switching power supply device to which a rectifier circuit according to an embodiment of the present invention is applied. It is a circuit diagram of the conventional switching power supply device. It is a circuit diagram of the other conventional switching power supply device. It is a block diagram of the rectification switch unit which concerns on embodiment of this invention provided with two comparison parts.

  An embodiment of the present invention will be described with reference to FIGS. 1 to 5 and FIG.

(Configuration of switching power supply device 103 and rectifier circuit 20)
FIG. 5 is a circuit diagram of the switching power supply apparatus 103 to which the rectifier circuit 20 according to the embodiment of the present invention is applied. In the switching power supply 103 of FIG. 5, the circuit on the primary side of the transformer has the same configuration as the conventional switching power supply. The rectification switch unit 1 described below is applied to the rectifier circuit 20 on the secondary side of the transformer.

(Configuration of rectifying switch unit 1)
FIG. 1 is a configuration diagram of a rectifying switch unit 1 according to the present embodiment. The rectifying switch unit 1 includes a sensor 2 (comparison unit), a signal processing circuit 3, and a driver 4. The sensor 2 is a comparator, for example, and outputs a signal indicating a comparison result between the voltage at the inverting input terminal and the voltage at the non-inverting input terminal. The signal processing circuit 3 and the driver 4 constitute a control circuit 30 (control unit). The control circuit 30 outputs, to the gate of the normally-off MOSFET 6, a signal that instructs on (turn-on) and off (turn-off) of a normally-off MOSFET 6, which will be described later, according to the signal indicating the comparison result.

  The rectifying switch unit 1 includes a normally-on MOSFET (metal-oxide-semiconductor field-effect transistor) 5 (second switching element).

  Furthermore, the rectifying switch unit 1 includes an N-channel normally-off MOSFET 6 (first switching element). Furthermore, the rectifying switch unit 1 includes a Zener diode Dz (constant voltage diode). The rectifying switch unit 1 includes the voltage source 50, but instead of including the voltage source 50, the rectifying switch unit 1 may include two voltage sources 51 and 52 and a switch 7 for selecting one of them. The normally-on MOSFET 5 and the normally-off MOSFET 6 rectify the current flowing between the source and the drain, respectively.

  Further, the rectifying switch unit 1 is provided with an input node in and an output node out. The normally-off MOSFET 6 has a body diode Db (diode).

  In the rectifying switch unit 1 of FIG. 1, the input node in, the source of the normally-off MOSFET 6, the anode of the Zener diode Dz, the anode of the body diode Db, the gate of the normally-on MOSFET 5, the reference point of the voltage source 50, and the sensor The two reference points are connected to each other.

  The non-inverting input terminal (+) of the sensor 2 is connected to the input of the voltage source 50 (or any one of the voltage sources 51 and 52 selected by the switch 7). When the input of the voltage source 50 is connected, the threshold voltage −Vth (predetermined threshold voltage) is input. When the input of the voltage source 51 is connected, the threshold voltage −Vth1 (first voltage) is input, and when the input of the voltage source 52 is connected, the threshold voltage −Vth2 (second voltage) is input. Each threshold voltage will be described later in the section of (Threshold Voltage).

  The inverting input terminal (−) of the sensor 2 is connected to the source of the normally-on MOSFET 5, the drain of the normally-off MOSFET 6, the cathode of the Zener diode Dz, and the cathode of the body diode Db.

  The output of the sensor 2 is connected to the input of the signal processing circuit 3. The output of the signal processing circuit 3 is connected to the input of the driver 4. The output of the driver 4 is connected to the gate of the normally-off MOSFET 6. When the rectifying switch unit 1 includes the voltage sources 51 and 52, the output of the signal processing circuit 3 is also connected to the control input of the switch 7.

  The drain of normally-on MOSFET 5 is connected to output node out.

(Operation of rectifying switch unit 1)
The operation of the rectifying switch unit 1 will be described below. In the rectifying switch unit 1 according to the present embodiment, a voltage between the input node in and the output node out is referred to as a voltage Vds.

  Further, a voltage between the input node “in” and the inverting input terminal (−) of the sensor 2 is referred to as a voltage Vm. That is, the voltage Vm is a voltage across the Zener diode Dz and a drain-source voltage of the normally-off MOSFET 6.

  Furthermore, although the following description is a case where the threshold voltage is switched, the threshold voltage may be fixed at -Vth as shown in FIG.

(Rectification switch unit 1 is on)
First, turning on of the rectifying switch unit 1 will be described with reference to FIG. FIG. 2 is a view for explaining turning on of the rectifying switch unit 1 according to the present embodiment.

  In the rectifying switch unit 1 of FIG. 2, the normally-off MOSFET 6 is off. The threshold voltage input to the non-inverting input terminal (+) of the sensor 2 is switched from the threshold voltage −Vth2 to the threshold voltage −Vth1 after the normally-off MOSFET 6 is turned off. The threshold voltage is switched based on a signal input from the output of the signal processing circuit 3 (the output of the control circuit 30) to the control input of the switch 7.

  Here, when the voltage applied between the input node in and the output node out of the rectifying switch unit 1 rises from zero, the gate-source voltage of the normally-on MOSFET 5 has a positive voltage exceeding the threshold voltage (negative value). The normally-on MOSFET 5 becomes conductive. Then, the body diode Db becomes conductive, and the current If flowing through the body diode Db starts to increase. As a result, the voltage drop Vf generated at both ends of the body diode Db due to the body diode Db gradually increases. As a result, the voltage −Vf input to the inverting input terminal (−) of the sensor 2 becomes the non-inverting input of the sensor 2. It becomes lower than the threshold voltage −Vth1 input to the terminal (+). That is, −Vf <−Vth1 (Vf> Vth1 for absolute value comparison). Then, the level of the signal output from the sensor 2 is inverted. For example, in this case, the inversion is performed from the low level to the high level, but the inversion input terminal and the non-inversion input terminal may be exchanged to invert from the high level to the low level. The output signal of the sensor 2 whose level is inverted is converted to an appropriate level by the control circuit 30 (that is, by the signal processing circuit 3 and the driver 4), and then input to the gate of the normally-off MOSFET 6. As a result, the gate-source voltage Vgs of the normally-off MOSFET 6 becomes a high level, so that the normally-off MOSFET 6 is turned on.

  The signal processing in the signal processing circuit 3 includes AND (logical product), OR (logical sum), etc., of the signal output from the sensor 2 and other signals. However, the signal processing may be omitted as necessary.

  When the normally-off MOSFET 6 is turned on, a current can flow between the source and drain of the normally-off MOSFET 6. In the rectifying switch unit 1, when the normally-off MOSFET 6 is turned on, the two MOFETs are simultaneously turned on. Therefore, a current (source current Is in FIG. 3 to be described later) flows through a path from the input node in → between the source and drain of the normally-off MOSFET 6 → between the source and drain of the normally-on MOSFET 5 → the output node out. Therefore, in the rectifying switch unit 1, the source current Is that is input from the input node in and output from the output node out can be rectified by the normally-on MOSFET 5 and the normally-off MOSFET 6.

(Rectification switch unit 1 off)
Next, turning off of the rectifying switch unit 1 will be described with reference to FIG. FIG. 3 is a diagram for explaining turning off of the rectifying switch unit 1 according to the present embodiment.

  As described above, when both the normally-on MOSFET 5 and the normally-off MOSFET 6 are turned on in the rectifying switch unit 1, the source current Is flows.

  Here, a case where the normally-off MOSFET 6 is turned on and a source current flows between the source and drain of the normally-off MOSFET 6 is considered. In this case, the source current Is flows through the ON resistance of the normally-off MOSFET 6. Therefore, a source-drain voltage Vsd, which is a voltage in which the source of the normally-off MOSFET 6 is positive and the drain of the normally-off MOSFET 6 is negative, is generated.

  The source-drain voltage Vsd increases as the source current Is increases. On the other hand, the source-drain voltage Vsd decreases as the source current Is decreases.

In the rectifying switch unit 1 of FIG. 3, the normally-off MOSFET 6 is on. The threshold voltage input to the non-inverting input terminal (+) of the sensor 2 is switched from the threshold voltage −Vth1 to the threshold voltage −Vth2 after the normally-off MOSFET 6 is turned on.
The threshold voltage is switched based on a signal input from the output of the signal processing circuit 3 (the output of the control circuit 30) to the control input of the switch 7.

  When the source current Is decreases to near zero, the voltage −Vsd, which is the voltage obtained by inverting the polarity of the source-drain voltage Vsd and is input to the inverting input terminal (−) of the sensor 2, is the non-inverting input of the sensor 2. It becomes higher than the threshold voltage −Vth2 input to the terminal (+). That is, −Vth2 <−Vsd (Vth2> Vsd for absolute value comparison). Then, the level of the signal output from the sensor 2 is inverted. For example, in this case, the inversion is performed from the high level to the low level, but the inversion input terminal and the non-inversion input terminal may be exchanged to invert from the low level to the high level. The output signal of the sensor 2 whose level is inverted is converted to an appropriate level by the control circuit 30 (that is, by the signal processing circuit 3 and the driver 4), and then input to the gate of the normally-off MOSFET 6. As a result, the gate-source voltage Vgs of the normally-off MOSFET 6 becomes low level, so that the normally-off MOSFET 6 is turned off. Then, when the drain-source voltage of the normally-off MOSFET 6 rises and the gate-source voltage of the normally-on MOSFET 5 falls below the threshold voltage, the normally-on MOSFET 5 is also turned off. When both the normally-off MOSFET 6 and the normally-on MOSFET 5 are turned off, the source current Is flowing from the input node in to the output node out is blocked (cut off).

  As described above, the rectifying switch unit 1 performs synchronous rectification, which is rectification in which the timing at which the source current Is flows from the source of the normally-off MOSFET 6 to the drain of the normally-on MOSFET 5 and the on-state of the normally-off MOSFET 6 and the normally-on MOSFET 5 are synchronized. Can be done.

  More specifically, in the synchronous rectification, when the absolute value of the source-drain voltage Vsd exceeds the absolute value of the threshold voltage Vth (Vth1), the normally-off MOSFET 6 and the normally-on MOSFET 5 are turned on. On the other hand, when the absolute value of the source-drain voltage Vsd falls below the absolute value of the threshold voltage Vth (Vth2), the normally-off MOSFET 6 and the normally-on MOSFET 5 are turned off.

  Here, since the connection between the normally-off MOSFET 6 and the normally-on MOSFET 5 is a cascode connection, only a voltage in the same order as the threshold voltage of the normally-on MOSFET 5 is applied between the drain and source of the normally-off MOSFET 6. That is, the voltage input to the inverting input terminal is much lower than the high voltage (that is, the voltage Vds in FIG. 2) applied at the time of OFF between the input node in and the output node out. Therefore, it is not necessary to use a high pressure resistant and expensive sensor as the sensor 2.

  Therefore, synchronous rectification can be performed even when the output voltage is high, and the rectifier switch unit 1, the rectifier circuit 20, and the switching power supply device 103 that are excellent in cost performance can be provided.

  Further, the rectifying switch unit 1 does not include the RC integrating circuit that the conventional rectifying circuit has. Therefore, the problem ((Problem 1) of [Problem to be Solved by the Invention]) that has occurred in the conventional rectifier circuit does not occur.

  That is, in the conventional rectifier circuit, when the resistance value of the electric resistance in the RC integration circuit is increased, the time constant is increased, resulting in the voltage waveform being distorted and the response speed being lowered. However, in the rectifying switch unit 1, the voltage waveform is not reduced by integration, so that the response speed does not decrease.

  Furthermore, the rectifying switch unit 1 does not divide the voltage input to the inverting input terminal (−) of the sensor 2 by the resistance. Thus, the problem ((Problem 2) of [Problem to be Solved by the Invention]) that has occurred in the conventional rectifier circuit does not occur.

  That is, in the conventional rectifier circuit, the power consumption of the resistor R51 is increased by reducing the resistance value of the resistor R51 in order to reduce the time constant which has been a problem in (Problem 1). , And increased costs. However, in the rectifying switch unit 1, a reduction in power efficiency due to a current flowing through the resistor and an increase in cost due to the use of a resistor with high rated power consumption (power capacity) do not occur.

(Diode array 8)
In the rectifying switch unit 1 of FIG. 1, a Zener diode Dz is used to clamp the voltage Vm. As a result, even if a transient high voltage is applied between the drain and source of the normally-off MOSFET 6, the current flows through the Zener diode Dz, whereby the voltage peak between the drain and source of the normally-off MOSFET 6 can be suppressed. Therefore, the normally-off MOSFET 6 can be protected from a transient high voltage.

  However, in the rectifying switch unit 1 according to the present embodiment, the component that protects against a transient high voltage is not limited to the Zener diode Dz.

  That is, in the rectifying switch unit 1 of FIG. 1, the voltage Vm is clamped by using a diode array 8 (multistage series diode, see FIG. 4) composed of a plurality of diodes connected in series instead of the zener diode Dz. Also good. In this case, the anode of the diode 8e located at the final end among the plurality of diodes connected in series is connected to the drain of the normally-off MOSFET 6. At the same time, among the plurality of diodes connected in series, the cathode of the diode 8 f located at the forefront is connected to the source of the normally-off MOSFET 6. As a result, even if a transient high voltage is applied between the drain and source of the normally-off MOSFET 6, all the diodes in the diode array 8 are turned on, and the voltage peak between the drain and source of the normally-off MOSFET 6 can be suppressed. I can do it. Therefore, the normally-off MOSFET 6 can be protected from a transient high voltage.

(Threshold voltage)
The threshold voltage in the rectifying switch unit 1 according to the present embodiment will be described below.

  First, the threshold voltage of the rectifying switch unit 1 may be a fixed threshold voltage −Vth. In this case, the rectifying switch unit 1 may include the voltage source 50.

  Next, the threshold voltage of the rectifying switch unit 1 may be the threshold voltage −Vth1 input to the non-inverting input terminal when the normally-off MOSFET 6 is turned on. Similarly, the threshold voltage of the rectifying switch unit 1 may be the threshold voltage −Vth2 input to the non-inverting input terminal when the normally-off MOSFET 6 is turned off. In this case, the rectifying switch unit 1 may include two voltage sources 51 and 52 and a switch 7 for selecting them.

  The operation of the rectifying switch unit 1 when there are two types of threshold voltages (−Vth1 and −Vth2) will be described below.

  In the rectifying switch unit 1 having two types of threshold voltages, the control circuit 30 outputs a signal for switching the threshold voltage from the threshold voltage −Vth2 to the threshold voltage −Vth1 after turning off the normally-off MOSFET 6. To the control input of the switch 7.

  Similarly, after the normally-off MOSFET 6 is turned on, the control circuit 30 outputs a signal for switching the threshold voltage from the threshold voltage -Vth1 to the threshold voltage -Vth2 from the output of the signal processing circuit 3 to the control input of the switch 7.

  As described above, in the rectifying switch unit 1, the voltage source may output the threshold voltage −Vth 1 and the threshold voltage −Vth 2 different from the threshold voltage −Vth 1, that is, the two voltage sources 51 and 52. The two voltage sources 51 and 52 output one of the threshold voltage −Vth1 and the threshold voltage −Vth2 as the threshold voltage −Vth as selected by the switch 7 in accordance with an instruction from the outside. The control circuit 30 may output a signal (that is, a signal for selecting the voltage source 51) that instructs the switch 7 to output the threshold voltage −Vth <b> 1 after outputting a signal that instructs the normally-off MOSFET 6 to be turned off. Similarly, the control circuit 30 outputs a signal instructing the output of the threshold voltage −Vth2 (ie, a signal for selecting the voltage source 52) to the switch 7 after outputting a signal instructing to turn on the normally-off MOSFET 6. Good.

  When the normally-off MOSFET 6 is turned on and when the normally-off MOSFET 6 is turned off, the threshold voltage values are individually set, whereby the on / off of the normally-off MOSFET 6 can be controlled with higher accuracy.

(Rectifier switch unit 1 ′ having two comparison units)
FIG. 8 is a configuration diagram of a rectifying switch unit 1 ′ according to an embodiment of the present invention that includes two comparison units. The rectifying switch unit 1 ′ includes sensors 2 and 2 ′ (first and second comparison units) whose one input terminal is connected to the drain of the normally-off MOSFET 6. The threshold voltage −Vth1 is input as the threshold voltage to the other input terminal of the sensor 2, and the threshold voltage different from the threshold voltage −Vth1 is input to the other input terminal of the sensor 2 ′ as the threshold voltage. The voltage -Vth2 is input.

  When the normally-off MOSFET 6 is switched from OFF to ON, the control circuit 30 applies the output logic inversion of the sensor 2 and outputs a signal to the normally-off MOSFET 6. Further, when the normally-off MOSFET 6 is switched from on to off, the output logic inversion of the sensor 2 ′ is applied to output a signal to the normally-off MOSFET 6.

  When the normally-off MOSFET 6 is turned on and when the normally-off MOSFET 6 is turned off, the threshold voltage value is individually set, so that the on / off of the normally-off MOSFET 6 can be controlled with higher accuracy.

[Application example]
The application of the rectifying switch unit 1 to the rectifying circuit and the switching power supply will be described below. As described above, FIG. 5 is a circuit diagram of the switching power supply device 103 to which the rectifier circuit 20 according to the embodiment of the present invention is applied. The circuit on the primary side of the transformer has the same configuration as the conventional switching power supply device, and the rectifying switch unit 1 described above is applied to the rectifier circuit 20 on the secondary side of the transformer.

  In the switching power supply 103 of FIG. 5, one end of the transformer secondary winding is connected to the output node out of the rectifier switch unit 1. Further, the positive terminal of the output capacitor is connected to the other end of the transformer secondary winding. Furthermore, the negative terminal of the output capacitor and the input node in of the rectifying switch unit 1 are connected.

  The voltage across the output capacitor is the output voltage Vo.

  The voltage across the transformer secondary winding is the voltage Vtr across the transformer. The voltage Vtr at both ends has a positive direction with respect to the other end of the transformer secondary winding.

  When the both-end voltage Vtr is positive, the voltage Vds in the rectifying switch unit 1 is the sum of the output voltage Vo and the both-end voltage Vtr.

  The both-end voltage Vtr is a voltage substantially equal to the output voltage Vo.

  Therefore, when the output voltage Vo is a low voltage of about 24V, the voltage Vds of the rectifying switch unit 1 is about 50V.

  On the other hand, when the output voltage Vo is a high voltage of about 250V, the voltage Vds of the rectifying switch unit 1 is about 500V.

  Here, it returns to the rectification switch unit 1 of FIG.

  The voltage Vm, which is a voltage between the input node in and the inverting input terminal (−) of the sensor 2, has an upper limit of about 10 V (same order as the threshold voltage of the normally-on MOSFET 5), including the transient state. is there. This is because normally-off MOSFET 6 and normally-on MOSFET 5 are cascode-connected.

  When the normally-off MOSFET 6 is turned off, the drain-source voltage rises. The drain-source voltage of the normally-off MOSFET 6 is a voltage obtained by inverting the gate-source voltage of the normally-on MOSFET 5 (source-gate voltage). Therefore, the normally-on MOSFET 5 is also turned off at the moment when the magnitude (absolute value) of this voltage reaches the threshold voltage of the normally-on MOSFET 5. As a result, even if the output voltage Vo is a high voltage of about 250V, the upper limit of the voltage Vm is about 10V, which is the same order as the threshold voltage of the normally-on MOSFET 5.

  The constant voltage Vz of the Zener diode Dz, which is a diode for protecting the normally-off MOSFET 6 and is a constant voltage diode, is about 10V. The breakdown voltage of the normally-off MOSFET 6 is 40V to 50V.

  As described above, even when the rectifying switch unit 1 according to the present embodiment is applied to the switching power supply device 103 whose output voltage is a high voltage, it is not necessary to increase the withstand voltage of the sensor 2. Therefore, it is possible to provide a switching power supply device that is superior in cost performance to conventional switching power supply devices.

  Needless to say, the switching power supply apparatus to which the rectifying switch unit 1 according to the present embodiment is applied is not limited to the switching power supply apparatus 103. The rectifying switch unit 1 according to the present embodiment can be applied to a switching power supply (including not only a DCDC converter but also an inverter) that requires a high output voltage and synchronous rectification.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  The rectifying switch unit of the present invention can be applied to a switching power supply device. In particular, the output voltage is a high voltage of about 250 V, and can be suitably used for a switching power supply device in which synchronous rectification is performed.

20 Rectifier circuit 1,1 ′ Rectifier switch unit 2 Sensor (comparison unit, first comparison unit)
2 'sensor (second comparison unit)
3 signal processing circuit 4 driver 5 normally-on MOSFET (second switching element)
6 Normally-off MOSFET (first switching element)
7 Switch 8 Diode array 8e Diode located at the final end 8f Diode located at the forefront 30 Control circuit (control unit)
50 to 52 Voltage source 103 Switching power supply Db Body diode (diode)
Dz Zener diode (constant voltage diode)
If current Is source current Vds voltage Vf voltage drop Vgs gate-source voltage Vm voltage Vo output voltage Vsd source-drain voltage Vtr voltage across terminal Vz constant voltage in input node out output node −Vf voltage −Vsd voltage −Vth threshold voltage ( Predetermined voltage)
-Vth1 threshold voltage (first voltage)
-Vth2 threshold voltage (second voltage)

Claims (8)

A first switching element for rectifying a current flowing between the source and the drain;
A diode having an anode connected to the source and a cathode connected to the drain;
A normally-on second switching element having a source connected to the drain of the first switching element and a gate connected to the source of the first switching element;
One input terminal is connected to the drain, and a predetermined threshold voltage is input from the voltage source to the other input terminal, and a signal indicating a comparison result between the voltage of the one input terminal and the predetermined threshold voltage is output. A comparison unit to
A rectifying switch unit comprising: a control unit that outputs a signal that instructs on and off of the first switching element to the first switching element in accordance with a signal indicating the comparison result.   2. The rectifying switch unit according to claim 1, further comprising a constant voltage diode having an anode connected to a source of the first switching element and a cathode connected to a drain of the first switching element. It further includes a diode array composed of a plurality of diodes connected in series,
Among the plurality of diodes connected in series, the anode of the diode located at the final end is connected to the drain of the first switching element,
2. The rectifying switch unit according to claim 1, wherein a cathode of a diode located at the forefront among the plurality of diodes connected in series is connected to a source of the first switching element. The comparison unit is capable of selecting either a first voltage or a second voltage different from the first voltage as a threshold voltage input to the other input terminal according to an instruction from the control unit,
The control unit
When the first switching element is turned on from off, the comparator outputs a signal instructing selection of the first voltage,
4. The rectification according to claim 1, wherein when the first switching element is turned from on to off, a signal instructing selection of the second voltage is output to the comparison unit. 5. Switch unit. One input terminal comprises two first and second comparison units connected to the drain of the first switching element,
A first voltage is input as a threshold voltage to the other input terminal of the first comparison unit,
A second voltage different from the first voltage is input as a threshold voltage to the other input terminal of the second comparison unit,
When the first switching element is turned on from off, the control unit applies the output logic inversion of the first comparison unit to output a signal to the first switching element, and the first switching element is turned on. 4. The rectifier according to claim 1, wherein when the signal is turned off, a signal is output to the first switching element by applying an output logic inversion of the second comparison unit. 5. Switch unit.   6. The rectifying switch unit according to claim 1, wherein the first switching element and the second switching element are metal / oxide / semiconductor field effect transistors.   A rectifier circuit comprising the rectifier switch unit according to claim 1.   A switching power supply comprising the rectifier circuit according to claim 7.

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