JP6033199B2 - Step-down chopper circuit - Google Patents

Description

  The present invention relates to a step-down chopper circuit, and more particularly to a step-down chopper circuit that steps down a first DC voltage and outputs a second DC voltage.

  2. Description of the Related Art Conventionally, there is known a step-down chopper circuit that includes a switching element, a reactor, and a free wheel diode, and generates a desired DC voltage by stepping down a DC voltage. In such a step-down chopper circuit, depending on the operating conditions, a resonance phenomenon may occur due to the parasitic capacitor and the reactor of the switching element, and a current may flow back to the switching element due to the resonance voltage generated in the reactor. When the current flows backward through the switching element, the DC power supply in the previous stage of the step-down chopper circuit deteriorates in performance and malfunctions, and the power loss of the switching element increases.

  As a countermeasure, there is a method in which a series connection body of a resistance element and a diode is connected in parallel to a switching element or a reactor to attenuate a resonance voltage generated in the reactor (for example, refer to Patent Documents 1 and 2).

  Further, there is a method in which a diode is connected to the current path, and the switching element is turned off when a reverse bias voltage is applied to the diode (see, for example, Patent Document 3).

JP 2002-95245 A JP 2007-236128 A JP-A-1-185161

  However, the step-down chopper circuits of Patent Documents 1 and 2 have a problem that efficiency is reduced because power is consumed by the resistance element.

  Further, in Patent Document 3, it is necessary to provide a circuit for detecting that a reverse bias voltage is applied to the diode, and there is a problem that the circuit configuration becomes complicated.

  Therefore, a main object of the present invention is to provide a step-down chopper circuit that can prevent a reverse current from flowing through a switching element and has a high efficiency and a simple configuration.

The step-down chopper circuit according to the present invention steps down the first DC voltage applied between the first and second input terminals and outputs the second DC voltage between the first and second output terminals. A first switching element which is a chopper circuit, the first electrode is connected to the first input terminal, and is controlled to be turned on / off so that the second DC voltage becomes the target voltage, and the first electrode A second switching element connected to the second electrode of the first switching element and controlled to be turned on / off together with the first switching element; one terminal connected to the second electrode of the second switching element; A reactor having the other terminal connected to the first output terminal, a capacitor connected between the first and second output terminals, and an anode connected to the second input terminal and the second output terminal, And the second switch Grayed element is that a reflux diode back to one terminal of the reactor the current output from the other terminal of the reactor when it is turned off. The second switching element includes a first parasitic diode having an anode and a cathode connected to the first and second electrodes of the second switching element, respectively. The cathode of the freewheeling diode is connected to the anode of the first parasitic diode.

  In the step-down chopper circuit according to the present invention, the first and second switching elements are connected in series, and the second switching element is connected so that the parasitic diode prevents the reverse current, so that the reverse current is connected to the first switching element. Can be prevented from flowing, and the efficiency is not lowered and the configuration is not complicated.

1 is a circuit block diagram showing a configuration of a step-down chopper circuit according to a first embodiment of the present invention. 3 is a time chart illustrating an operation of a step-down chopper circuit as a comparative example of the first embodiment. FIG. 2 is a diagram for describing on / off control of N-channel MOS transistors 2 and 3 shown in FIG. 1. It is a circuit block diagram which shows the structure of the step-down chopper circuit by Embodiment 2 of this invention. It is a circuit block diagram which shows the structure of the step-down chopper circuit by Embodiment 3 of this invention.

[Embodiment 1]
As shown in FIG. 1, the step-down chopper circuit according to the first embodiment of the present invention has input terminals T1, T2, output terminals T3, T4, an input capacitor 1, an N-channel MOS transistor 2 (first switching element), N A channel MOS transistor 3 (second switching element), a freewheeling diode 4, a reactor 5, an output capacitor 6, a feedback circuit 7, a control circuit 8, and a drive circuit 9 are provided.

  Input terminals T1 and T2 are connected to the positive electrode and the negative electrode of DC power supply 10, respectively. The DC power supply 10 outputs a DC voltage Vi between the input terminals T1 and T2. A load 11 is connected between the output terminals T3 and T4. The input terminal T2 and the output terminal T4 are connected to each other. This step-down chopper circuit steps down a DC voltage Vi applied between input terminals T1 and T2 to generate a desired DC voltage Vo, and outputs the DC voltage Vo between output terminals T3 and T4. The load 11 is driven by the output voltage Vo of the step-down chopper circuit.

  The input capacitor 1 is connected between the input terminals T1 and T2, and stabilizes the input DC voltage Vi. The drain of the N channel MOS transistor 2 is connected to the input terminal T1. N-channel MOS transistor 2 includes a parasitic diode 2d having an anode connected to the source and a cathode connected to the drain, and a parasitic capacitor 2c connected between the source and drain.

  The source of N channel MOS transistor 3 is connected to the source of N channel MOS transistor 2. N-channel MOS transistor 3 includes a parasitic diode 3d having an anode connected to the source and a cathode connected to the drain, and a parasitic capacitor 3c connected between the source and drain. The gates of N channel MOS transistors 2 and 3 are connected to each other. N channel MOS transistor 3 is provided to prevent reverse current from flowing through N channel MOS transistor 2.

  The anode of the freewheeling diode 4 is connected to the input terminal T2 and the output terminal T4, and the cathode thereof is connected to the drain of the N-channel MOS transistor 3 (the cathode of the parasitic diode 3d).

  Reactor 5 has one terminal connected to the drain of N-channel MOS transistor 3 and the other terminal connected to output terminal T3. The output capacitor 6 is connected between the output terminals T3 and T4, and smoothes and stabilizes the DC voltage Vo between the output terminals T3 and T4.

  The feedback circuit 7 detects the voltage Vo between the output terminals T3 and T4 and supplies a signal indicating the detected value to the control circuit 8. The control circuit 8 generates the control signal CNT based on the signal from the feedback circuit 7 so that the output voltage Vo becomes the target voltage. The control signal CNT alternately becomes “H” level and “L” level in a predetermined switching cycle. The ratio of the time for switching to the “H” level and the switching period is called the duty ratio. The control circuit 8 adjusts the duty ratio of the control signal CNT so that the output voltage Vo becomes the target voltage. Drive circuit 9 drives the gates of N-channel MOS transistors 2 and 3 in response to control signal CNT.

  The feedback circuit 7, the control circuit 8, and the drive circuit 9 constitute a switching control unit and turn on / off the N-channel MOS transistors 2 and 3 so that the DC voltage Vo between the output terminals T3 and T4 becomes the target voltage. Let

  Here, the reason why the N-channel MOS transistor 3 is provided will be described. 2A to 2D are time charts showing the operation of a step-down chopper as a comparative example. In the step-down chopper as a comparative example, the N-channel MOS transistor 3 is removed, and the source of the N-channel MOS transistor 2 is connected to the cathode of the free-wheeling diode 4. 2A shows the control signal CNT, FIG. 2B shows the drain-source voltage VDS of the N-channel MOS transistor 2, FIG. 2C shows the current IL flowing through the reactor 5, and FIG. (D) shows the input current Ii.

  2A to 2D, when the control signal CNT is set to the “H” level, the N-channel MOS transistor 2 is turned on, and from the positive electrode of the DC power supply 10, the N-channel MOS transistor 2, the reactor 5, and A current flows through a path reaching the negative electrode of the DC power supply 10 via the parallel connection body of the output capacitor 6 and the load 11. As a result, the output capacitor 6 is charged and electromagnetic energy is stored in the reactor 5. At this time, the current IL flowing through the reactor 5 increases linearly with time.

  When the control signal CNT is set to the “L” level, the N-channel MOS transistor 2 is turned off, the electromagnetic energy of the reactor 5 is released, and the output capacitor 6 is output from the other terminal of the reactor 5 (terminal on the output terminal T3 side). And a load 11 connected in parallel, a current flows through a path that reaches the one terminal of the reactor 5 (terminal on the input terminal T1 side) via the freewheeling diode 4. A state in which current flows through such a path is generally called reflux. In this case, the current IL flowing through the reactor 5 decreases linearly with time.

  When the release of the electromagnetic energy of the reactor 5 is completed, a period Tr during which no current flows through the reactor 5 (a period in which the reactor current is zero) Tr occurs. In this period Tr, power is supplied from the output capacitor 6 to the load 11.

  An operation mode having a period Tr in which the reactor current does not flow is called a discontinuous mode in one cycle in which the N-channel MOS transistor 2 is repeatedly turned on and off, and an operation mode having no period Tr in which the reactor current does not flow is a continuous mode. Call it. In the control circuit 8, a predetermined switching cycle is usually set in advance. When the sum of the on time and the off time reaches a predetermined switching period, control circuit 8 turns on N channel MOS transistor 2 again, and thereafter repeats such on / off control.

  Here, the element loss in the continuous mode and the discontinuous mode will be described. The free-wheeling diode 4 blocks the current during the period Ton when the N-channel MOS transistor 2 is turned on, and flows the current in the direction of releasing the electromagnetic energy stored in the reactor 5 during the period Toff when the N-channel MOS transistor 2 is turned off.

  It can be said that the difference between the continuous mode and the discontinuous mode is a difference between whether or not there is a period Tr in which the current IL flowing through the reactor 5 is zero during the OFF period Toff of the N-channel MOS transistor 2. That is, in the continuous mode, the next N-channel MOS transistor 2 starts to be turned on before all energy stored in the reactor 5 is released. In contrast, in the discontinuous mode, all the energy stored in the reactor 5 is released, and after the reactor current IL becomes zero, the next N-channel MOS transistors 2 and 3 are turned on.

  One of the elements affected by the difference between the continuous mode and the discontinuous mode is the free wheel diode 4. As the freewheeling diode 4, a Schottky diode or a fast recovery diode is used. When the breakdown voltage required for the freewheeling diode 4 is less than 200V, a Schottky diode is mainly used as the freewheeling diode 4. Further, when the withstand voltage required for the freewheeling diode 4 exceeds 200V, a fast recovery diode is used as the freewheeling diode 4.

  Further, the loss of the diode is roughly classified into a forward loss and a reverse loss. The forward loss refers to a loss in a direction in which a diode originally flows current, and is obtained by a product of a voltage and a current between the anode and the cathode of the diode. On the other hand, the reverse loss is a loss when the diode originally flows in the direction of blocking current. In the case of an ideal diode, current does not flow in the reverse direction, but in an actual diode, when the direction of the current flowing in the forward direction is reversed, the current cannot be cut off quickly, and the current is constant. Time current flows. This period is called reverse recovery time. The reverse loss is determined by the product of the reverse voltage and reverse current applied to the diode.

  In the Schottky diode, there is almost no reverse loss because there is no carrier accumulation that causes reverse time to occur. On the other hand, reverse loss occurs in the fast recovery diode. In general, there is no Schottky barrier diode whose breakdown voltage exceeds 200V, and therefore, a fast recovery diode must be used as the free wheel diode 4 of the step-down chopper circuit for high voltage. Further, since the reverse loss is obtained by the product of the voltage and current applied to the diode, the higher the voltage, the larger the reverse loss, causing thermal destruction.

  This reverse loss is a loss that occurs when the polarity is reversed from the period in which the forward current flows, and when the forward current does not flow in advance, suddenly applying a reverse voltage, There is no reverse loss. Therefore, in the case of a step-down chopper circuit that handles a high voltage, the discontinuous mode is actively adopted by utilizing this characteristic of the diode. In the discontinuous mode, since there is a period in which the current of the freewheeling diode 4 becomes zero, reverse loss occurs even at the timing when the next N-channel MOS transistor 2 is turned on, that is, when the reverse current flows through the freewheeling diode 4. do not do. For this reason, the loss of the return diode 4 can be reduced by adopting the discontinuous mode.

  The discontinuous mode has advantages for the freewheeling diode 4, but also has disadvantages for the entire circuit. When the N-channel MOS transistor 3 is not provided, when the reflux period ends, a series resonance circuit is formed by the parasitic capacitor 2c between the source and drain of the N-channel MOS transistor 2 and the reactor 5, and the resonance voltage centered on the output voltage value. (Free oscillation voltage) is generated between the anode and the cathode of the reflux diode 4.

  At this time, when the source potential of the N-channel MOS transistor 2 rises due to the influence of the resonance voltage and the source potential becomes higher than the drain potential (the output voltage Vi of the DC power supply 10), the parasitic diode 2d of the N-channel MOS transistor 2 Current flows back to the input side via In the state where a reverse flow occurs and a current flows through the parasitic diode 2d of the N-channel MOS transistor 2, the source potential is the sum of the output voltage Vi of the DC power supply 10 and the forward drop voltage of the parasitic diode 2d. To be clamped.

  Backflow to the input side is likely to occur when the relationship between the input voltage Vi and the output voltage Vo is Vo ≧ Vi / 2. The backflow of the input current may cause performance degradation and circuit malfunction of the DC power supply 10 connected to the input side, and further increase the power loss of the N-channel MOS transistor 2.

  Therefore, as in the first embodiment, N channel MOS transistor 3 for preventing backflow is connected between the source of N channel MOS transistor 2 and one terminal of reactor 5 so that parasitic diode 3d is in the forward direction. As a result, the backflow path associated with the series resonance between the parasitic capacitor 2c of the N-channel MOS transistor 2 and the reactor 5 is cut off, and backflow to the input side is prevented. As a result, performance degradation and circuit malfunction of the DC power supply 10 connected to the input side can be prevented, and loss of the N-channel MOS transistor 2 can be reduced.

  Here, when the N-channel MOS transistor 3 is inserted, in the loss during the on period Ton, the forward loss, which is the integral of the forward voltage Vf of the parasitic diode 3d and the input current Ii, increases from the conventional level. By supplying the gate signal supplied to the N-channel MOS transistor 2 also to the gate of the N-channel MOS transistor 3, the N-channel MOS transistor 3 is also turned on while the N-channel MOS transistor 2 is turned on.

  The forward current causes a conduction loss due to the product of the on-resistance Ron of the N-channel MOS transistor 3 and the square of the input current Ii. The conduction loss of the N-channel MOS transistor is much higher than the conduction loss of the diode. It is generally known to be small. Conduction using a MOS transistor is called a synchronous rectification method. By adopting this synchronous rectification method, efficiency reduction due to the addition of the N-channel MOS transistor 3 can be minimized.

  Further, since parasitic capacitor 3c also exists in N channel MOS transistor 3, the capacitance component that was only parasitic capacitor 2c of conventional N channel MOS transistor 2 becomes a series connection body of parasitic capacitors 2c and 3c. If the free vibration voltage generated by the reactor 5 and the parasitic capacitor is constant, the parasitic capacitor 3c is added, so that the free vibration component that has conventionally appeared at both ends of the parasitic capacitor 2c is distributed to the parasitic capacitors 2c and 3c. Is done.

  For example, when the same element as N-channel MOS transistor 2 is used as N-channel MOS transistor 3, the free oscillation voltage appearing at the source of N-channel MOS transistor 2 is half that of the conventional one. Here, in the drive circuit 9, since the reference ground is the source of the N-channel MOS transistor 2 and is different from the ground on the load 11 side, it is necessary to provide the drive circuit 9 with an insulating circuit. Compared with the load 11 side, the source of the N-channel MOS transistor 2 insulated by the insulation circuit is unstable and easily affected by external noise. If the free oscillation voltage appearing at the source is large, the source is not turned on. There is a risk of causing a malfunction that turns on.

  The difference between the threshold voltages of the MOS transistors is at most several volts. However, in a high voltage step-down chopper, the free oscillation voltage may be several hundred volts, and the free oscillation of several volts is applied to the source of the N-channel MOS transistor 2. Even when a voltage is applied, it may cause a malfunction. However, by connecting the N-channel MOS transistor 3 in series with the N-channel MOS transistor 2, not only can reverse current be prevented, but also the free oscillation voltage appearing at the source of the N-channel MOS transistor 2 can be reduced. Therefore, the risk of malfunction of the N-channel MOS transistor 2 due to free vibration can be reduced.

  Further, if the same type of component as the N channel MOS transistor 2 is used as the N channel MOS transistor 3 and the same control signal CNT is supplied to the gates of the N channel MOS transistors 2 and 3, the ON / OFF control can be performed almost simultaneously. This can be easily realized without increasing the types of components of the conventional step-down chopper circuit.

  Further, since no resistive element is used as in Patent Documents 1 and 2, the efficiency does not decrease. Further, since a circuit for detecting that a reverse bias voltage is applied to the diode is not provided as in Patent Document 3, the circuit configuration is not complicated.

  In the first embodiment, the reactor 5 is used as an electromagnetic energy storage element. However, the present invention is not limited to this, and a primary winding of a switching transformer or the like may be used as an electromagnetic energy storage element.

  The voltage is applied to the N-channel MOS transistor 3 only when a reverse current flows in the free oscillation period Tr in the discontinuous mode, that is, the sum of the output voltage Vo and the free oscillation voltage is greater than the input voltage Vi. Only high. For this reason, the N-channel MOS transistor 2 requires a withstand voltage at least equivalent to or higher than the input voltage Vi, whereas the N-channel MOS transistor 3 uses a high withstand voltage product equivalent to the input voltage Vi. There is no need, and a low withstand voltage product is sufficient. In general, the on-resistance of the N-channel MOS transistor increases in proportion to the withstand voltage. Therefore, if a low-voltage product can be used, the conduction loss during the on-period Ton of the N-channel MOS transistor 3 is reduced accordingly. It becomes possible.

  Even when an element different from N channel MOS transistor 2 is used as N channel MOS transistor 3, it is not necessary to perform special on / off control. In the first place, the purpose of providing the N-channel MOS transistor 3 is to prevent the backflow of current by the parasitic diode 3d in the off-period Toff, particularly in the free oscillation period Tr. There is no need to control.

  3A and 3B show the gate charge amount Qg (nC) of the drain-source voltage VDS (V) and the gate-source voltage VGS (V) when the N-channel MOS transistor is on and off, respectively. It is a figure which shows dependency. 3A and 3B, the turn-on voltage and turn-off voltage of this N channel MOS transistor are both about 5V, and the threshold voltage VT is about 5V.

  Even if an element having a threshold voltage VT lower than that of N channel MOS transistor 2 is used as N channel MOS transistor 3, input current Ii does not flow until N channel MOS transistor 2 enters an on operation. Conversely, even when an element having a threshold voltage VT higher than that of the N channel MOS transistor 2 is used as the N channel MOS transistor 3, only the period during which the current path is switched from the parasitic diode 3d to the N channel MOS transistor 3 is delayed. There is no big difference.

  Similarly, when the N-channel MOS transistor 3 enters the off operation earlier than the N-channel MOS transistor 2, the current path is simply switched from the N-channel MOS transistor 3 to the parasitic diode 3d for a moment. Even when the N-channel MOS transistor 3 enters the OFF operation later than the N-channel MOS transistor 2, the OFF operation can be sufficiently completed during the reflux mode, so that the free oscillation period is not hindered. That is, the gate signal can be shared between the N channel MOS transistor 2 and the N channel MOS transistor 3.

[Embodiment 2]
FIG. 4 is a circuit block diagram showing the configuration of the step-down chopper circuit according to the second embodiment of the present invention, which is compared with FIG. The step-down chopper is different from the step-down chopper of FIG. 1 in that the cathode of the freewheeling diode 4 is connected to the anode of the parasitic diode 3 d of the N-channel MOS transistor 3.

  Next, the operation of this step-down chopper circuit will be described. When control signal CNT is set to “H” level, N channel MOS transistors 2 and 3 are turned on, and N channel MOS transistors 2 and 3, reactor 5, output capacitor 6 and load 11 are connected from the positive electrode of DC power supply 10. A current flows through a path that reaches the negative electrode of the DC power supply 10 via the parallel connection. As a result, the output capacitor 6 is charged and electromagnetic energy is stored in the reactor 5. At this time, the current flowing through the reactor 5 increases linearly with time.

  When the control signal CNT is set to the “L” level, the N-channel MOS transistors 2 and 3 are turned off, the electromagnetic energy of the reactor 5 is released, and output from the other terminal (terminal on the output terminal T3 side) of the reactor 5 A current flows in a path that reaches one terminal of the reactor 5 (terminal on the input terminal T1 side) via the parallel connection body of the capacitor 6 and the load 11, the return diode 4, and the parasitic diode 3d. A state in which current flows through such a path is generally called reflux. In this case, the current flowing through the reactor 5 decreases linearly with time.

  When the release of the electromagnetic energy of the reactor 5 is completed, a period Tr during which no current flows through the reactor 5 (a period in which the reactor current IL is 0) occurs. In this period Tr, power is supplied from the output capacitor 6 to the load 11.

  Here, since the N-channel MOS transistor 3 is connected in a path (a path from the drain of the transistor 2 to the source of the transistor 2 through the DC power supply 10, the output capacitor 6, and the reactor 5) forming the series resonance circuit, The reverse flow path associated with the series resonance is interrupted, and the reverse current flow can be prevented.

  Here, by inserting the N-channel MOS transistor 3, the loss of the integration of the forward voltage Vf and the input current of the parasitic diode 3d increases as compared with the loss in the ON period Ton and the return period. However, by supplying the gate signal supplied to the N-channel MOS transistor 2 also to the N-channel MOS transistor 3, the N-channel MOS transistor 3 is also turned on while the N-channel MOS transistor 2 is turned on.

  Therefore, the current flowing in the forward direction causes a loss of the square of the on-resistance Ron of the N-channel MOS transistor 3 and the integration of the input current, but the MOS transistor conduction loss is much smaller than the diode conduction loss. Generally known. A method of causing the reflux current to flow through the MOS transistor is called a synchronous rectification method. By adopting this synchronous rectification method, a reduction in efficiency due to the addition of the N-channel MOS transistor 3 can be minimized.

  In addition to the path from the drain of the transistor 2 described above to the source of the transistor 2 via the DC power supply 10, the output capacitor 6, and the reactor 5, the path that forms the series resonant circuit includes the parasitic capacitor 4 c of the freewheeling diode 4. There is also a path from the output capacitor 6 and the reactor 5 to the cathode of the freewheeling diode 4. In FIG. 4, the cathode of the freewheeling diode 4 is connected to the anode of the parasitic diode 3 d of the N-channel MOS transistor 3. Therefore, the N-channel MOS transistor 3 can prevent not only the resonance of the reactor 5 and the parasitic capacitor 2c of the N-channel MOS transistor 2, but also the resonance of the reactor 5 and the free-wheeling diode 4 due to the parasitic capacitor 4c.

  Further, by using an element having a lower threshold voltage than N channel MOS transistor 2 as N channel MOS transistor 3, loss can be reduced. In the VGS characteristic of FIG. 3, a low constant voltage means that the voltage is turned on earlier in the transition period from the off period to the on period, and is turned off later in the transition period from the on period to the off period. Means.

  Even if the N-channel MOS transistor 3 is turned on before the N-channel MOS transistor 2, the input current Ii does not flow until the N-channel MOS transistor 2 is turned on, so that the operation is not hindered. Similarly, when the N channel MOS transistor 3 enters the OFF operation later than the N channel MOS transistor 2, it means that the synchronous current mode, that is, a period in which the conduction loss is small continues. Even if the N-channel MOS transistor 2 is turned off and shifts to the return period, if the N-channel MOS transistor 3 is in the synchronous rectification mode, the loss in the return period is reduced accordingly. There is no problem if an element is selected as the N-channel MOS transistor 3 so that the complete turn-off timing is before the start of the free oscillation period Tr.

[Embodiment 3]
FIG. 5 is a circuit block diagram showing the configuration of the step-down chopper circuit according to the third embodiment of the present invention, which is compared with FIG. The step-down chopper is different from the step-down chopper of FIG. 1 in that a diode 20 and a resistance element 21 are added.

  The cathode of diode 20 is connected to the cathode of parasitic diode 3 d of N-channel MOS transistor 3. The resistance element 21 is connected between the anode of the diode 20 and the output terminal T3. The diode 20 and the resistance element 21 constitute a snubber circuit connected in parallel to the reactor 5. Even if free vibration occurs due to the wiring capacity of the reactor 5, the free vibration can be quickly converged by the snubber circuit.

  In the first to third embodiments, a backflow prevention diode may be connected in parallel to the parasitic diode 3d of the N-channel MOS transistor 3. Since the characteristics of the parasitic diode 3d are not good compared to the Schottky diode and the fast recovery diode used as the freewheeling diode 4, the period during which the parasitic diode 3d is conducted by adding a backflow prevention diode. Loss can be further reduced.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  T1, T2 input terminal, T3, T4 output terminal, 1 input capacitor, 2, 3 N channel MOS transistor, 4 freewheeling diode, 5 reactor, 6 output capacitor, 7 feedback circuit, 8 control circuit, 9 drive circuit, 10 DC power supply , 11 load, 20 diode, 21 resistive element.

Claims (6)

A step-down chopper circuit that steps down a first DC voltage applied between first and second input terminals and outputs a second DC voltage between first and second output terminals,
A first switching element having a first electrode connected to the first input terminal and on / off controlled so that the second DC voltage becomes a target voltage;
A second switching element having a first electrode connected to a second electrode of the first switching element and being on / off controlled together with the first switching element;
A reactor having one terminal connected to the second electrode of the second switching element and the other terminal connected to the first output terminal;
A capacitor connected between the first and second output terminals;
The anode is connected to the second input terminal and the second output terminal, and the current output from the other terminal of the reactor when the first and second switching elements are turned off is the one terminal of the reactor. And a return diode to return to
The second switching element, viewed contains a first parasitic diode having an anode and a cathode connected to the first and second electrode of the second switching element,
A step-down chopper circuit in which a cathode of the freewheeling diode is connected to an anode of the first parasitic diode .   2. The step-down chopper circuit according to claim 1, wherein the first switching element includes a second parasitic diode having an anode and a cathode connected to second and first electrodes of the first switching element, respectively. The first and second switching elements include first and second N-channel MOS transistors, respectively.
The drain of the first N channel MOS transistor is connected to the first input terminal, the source thereof is connected to the source of the second N channel MOS transistor, and one terminal of the reactor is connected to the second N channel. 3. The step-down chopper circuit according to claim 2, wherein the step-down chopper circuit is connected to a drain of a MOS transistor, and the other terminal of the reactor is connected to the first output terminal.   4. The device according to claim 1, wherein a breakdown voltage between the first and second electrodes of the second switching element is lower than a breakdown voltage between the first and second electrodes of the first switching element. 5. 2. A step-down chopper circuit according to item 1.   The step-down chopper circuit according to claim 1, wherein a threshold voltage of the second switching element is lower than a threshold voltage of the first switching element. Furthermore, a snubber circuit connected in parallel to the reactor,
The snubber circuit is
A diode having a cathode connected to one terminal of the reactor;
The step-down chopper circuit according to any one of claims 1 to 5 , comprising a resistance element connected to an anode of the diode and the other terminal of the reactor.

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