JP6037972B2 - 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 switching element having a first electrode connected to a first input terminal and controlled to be turned on / off so that a second DC voltage becomes a target voltage; and an anode serving as a second switching element A backflow prevention element connected to the electrode and configured to flow current only in a direction from the anode to the cathode; a reactor having one terminal connected to the cathode of the backflow prevention element and the other terminal connected to the first output terminal; And a capacitor connected between the second output terminal and an anode connected to the second input terminal and the second output terminal, and output from the other terminal of the reactor when the switching element is turned off. The current is obtained by a reflux diode back to one terminal of the reactor.

  Another step-down chopper circuit according to the present invention steps down a DC voltage applied between the first and second input terminals and outputs a second DC voltage between the first and second output terminals. A step-down chopper circuit having an anode connected to a first input terminal, a cathode connected to a first output terminal, and a backflow prevention element for flowing a current only in a direction from the anode to the cathode; A capacitor connected between the two output terminals, a reactor having one terminal connected to the second output terminal, a first electrode connected to the other terminal of the reactor, and a second electrode connected to the second input terminal A switching element that is connected and controlled to be turned on / off so that the second DC voltage becomes a target voltage, and an anode connected to the other terminal of the reactor and the other of the reactor when the switching element is turned off It is obtained by a reflux diode passing a current output from the child to the first output terminal.

  In the step-down chopper circuit according to the present invention, since the backflow prevention element is provided in the path through which the reverse current flows in the switching element, the reverse current can be prevented from flowing in the switching element, the efficiency is lowered, and the configuration is complicated. It will never be.

1 is a circuit block diagram showing a configuration of a step-down chopper circuit according to a first embodiment of the present invention. 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. It is a circuit block diagram which shows the structure of the step-down chopper circuit by Embodiment 4 of this invention. It is a circuit block diagram which shows the structure of the step-down chopper circuit by Embodiment 5 of this invention. It is a circuit block diagram which shows the structure of the step-down chopper circuit by Embodiment 6 of this invention. It is a circuit block diagram which shows the structure of the step-down chopper circuit by Embodiment 7 of this invention. It is a circuit block diagram which shows the structure of the step-down chopper circuit by Embodiment 8 of this invention. It is a circuit block diagram which shows the structure of the step-down chopper circuit by Embodiment 9 of this invention.

[Embodiment 1]
As shown in FIG. 1, the step-down chopper circuit according to the first embodiment of the present invention includes input terminals T1, T2, output terminals T3, T4, an input capacitor 1, an N-channel MOS transistor 2 (switching element), a freewheeling diode 3, A backflow prevention 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 anode of the freewheeling diode 3 is connected to the input terminal T2 and the output terminal T4, and the cathode is connected to the source of the N-channel MOS transistor 2. The anode of the backflow prevention diode 4 is connected to the source of the N channel MOS transistor 2 (the cathode of the freewheeling diode 3). One terminal of the reactor 5 is connected to the cathode of the backflow prevention diode 4, and the other terminal is connected to the 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. Drive circuit 9 drives the gate of N-channel MOS transistor 2 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 transistor 2 so that the DC voltage Vo between the output terminals T3 and T4 becomes the target voltage.

  Next, the operation of this step-down chopper circuit will be described. When control signal CNT is set to “H” level, N-channel MOS transistor 2 is turned on, N-channel MOS transistor 2, backflow prevention diode 4, reactor 5, output capacitor 6 and load 11 from the positive electrode of DC power supply 10. A current flows through a path leading to the negative electrode of the DC power supply 10 through the parallel connection body. 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 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, current flows through a path reaching one terminal of the reactor 5 (terminal on the input terminal T1 side) via the freewheeling diode 3 and the backflow prevention diode 4. 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 in which no current flows through the reactor 5 (a period in which the reactor current becomes 0) occurs. During this period, power is supplied from the output capacitor 6 to the load 11.

  An operation mode having a period in which the reactor current does not flow in one cycle in which the N-channel MOS transistor 2 repeats on and off is called a discontinuous mode, and an operation mode having no period in which the reactor current does not flow is called a continuous mode. . 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 3 blocks current when the N-channel MOS transistor 2 is turned on, and flows current in a direction to release electromagnetic energy stored in the reactor 5 while 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 whether or not the current flowing through the reactor 5 becomes zero during the OFF period 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 state where the reactor current becomes zero continues, the next N-channel MOS transistor 2 starts to be turned on.

  One of the elements affected by the difference between the continuous mode and the discontinuous mode is the freewheeling diode 3. As the freewheeling diode 3, a Schottky diode or a fast recovery diode is used, and the proper use of these is determined by the breakdown voltage. When the breakdown voltage required for the freewheeling diode 3 is less than 200 V, a Schottky diode is mainly used as the freewheeling diode 3. Further, when the withstand voltage required for the freewheeling diode 3 exceeds 200V, a fast recovery diode is used as the freewheeling diode 3.

  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 with a breakdown voltage exceeding 200V, so a fast recovery diode must be used as the freewheeling diode 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 during which the current of the freewheeling diode 3 becomes zero, reverse loss occurs even when the next N-channel MOS transistor 2 is turned on, that is, when reverse current flows through the freewheeling diode 3. do not do. For this reason, the loss of the return diode 3 can be reduced by adopting the discontinuous mode.

  The discontinuous mode has advantages for the freewheeling diode 3, but also has disadvantages in the entire circuit. When the return 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 (free oscillation voltage) centered on the output voltage value is supplied to the return diode 3 Between the anode and the cathode.

  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 shown in the first embodiment, by connecting the backflow prevention diode 4 in the forward direction between the source of the N-channel MOS transistor 2 and one terminal of the reactor 5, the parasitic capacitor 2c of the N-channel MOS transistor 2 and The reverse flow path accompanying series resonance with the reactor 5 is interrupted, and the reverse flow 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.

  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.

[Embodiment 2]
FIG. 2 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. Referring to FIG. 2, this step-down chopper circuit is different from the step-down chopper circuit of FIG. 1 in that the cathode of freewheeling diode 3 is connected to the cathode of backflow prevention diode 4.

  Next, the operation of this step-down chopper circuit will be described. When control signal CNT is set to “H” level, N-channel MOS transistor 2 is turned on, N-channel MOS transistor 2, backflow prevention diode 4, reactor 5, output capacitor 6, and load 11 from the positive electrode of DC power supply 7. A current flows through a path leading to the negative electrode of the DC power supply 10 through the parallel connection body. As a result, the output capacitor 6 is charged and electromagnetic energy is stored in the reactor 5.

  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, and a current flows through a path reaching one terminal of the reactor 5 (terminal on the input terminal T1 side) via the freewheeling diode 3. Thus, the addition of the backflow prevention diode 4 does not hinder the original operation of the step-down chopper circuit.

  Also in the discontinuous mode, as in the first embodiment, the backflow prevention diode 4 is connected at a position where the series resonance between the parasitic capacitor 2c of the N-channel MOS transistor 2 and the reactor 5 is cut off. Can be prevented.

  Further, since the backflow prevention diode 4 is not arranged in the return path, no loss occurs in the backflow prevention diode 4 during the return period, and the power loss is reduced as compared with the first embodiment.

[Embodiment 3]
The conditions for selecting the operation mode of the step-down chopper circuit as a continuous mode or a discontinuous mode depend on input / output voltage specifications and current consumption. Even with the same input / output voltage specifications and current consumption, it varies depending on the inductance value of the reactor 5.

  When the inductance value of the reactor 5 is L, the current flowing through the reactor 5 is IL, the input voltage is Vi, the output voltage is Vo, and the on-time of the N-channel MOS transistor 2 is Ton, IL = (Vi−Vo) X Ton / L.

  Here, if (Vi−Vo) / IL is constant, the ON time Ton of the N-channel MOS transistor 2 is proportional to the inductance value L of the reactor 5. That is, in order to enter the discontinuous mode, the on-time Ton can be shortened, that is, the inductance value L can be reduced.

  If (Vin−Vout) × Ton is constant, the current IL flowing through the reactor 5 is inversely proportional to the inductance value L of the reactor 5. Here, the reactor loss is classified into iron loss and copper loss. Of these, the copper loss may be obtained by adding the resistance of the cross-sectional area of the copper wire of the reactor 5 to the square of the current flowing through the reactor 5. Therefore, if inductance value L of reactor 5 becomes small and other conditions relating to reactor 5 do not change, the loss of reactor 5 increases and the temperature of reactor 5 rises.

  That is, if the discontinuous mode is adopted as a measure for reducing the reverse loss of the freewheeling diode 3, the inductance value L of the reactor 5 can be reduced, but the temperature of the reactor 5 is increased. Therefore, in the third embodiment, the reactor 5 is divided into a plurality of parts in order to reduce the temperature rise of the reactor 5.

  FIG. 3 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. Referring to FIG. 3, this step-down chopper is different from the step-down chopper of FIG. 1 in that reactor 5 is replaced with two switching transformers 20 and 21, and diode 22 and capacitor 23 are added.

  The primary winding 20a of the switching transformer 20 and the primary winding 21a of the switching transformer 21 are connected in series between the cathode of the backflow prevention diode 4 and the output terminal T3. The first terminals (terminals marked with ●) of the primary windings 20a and 21a of the switching transformers 20 and 21 are directed to the output terminal T3 side. That is, in the third embodiment, the reactor 5 is configured by the primary windings 20a and 21a of the two switching transformers 20 and 21, and the temperature rise of the reactor 5 is suppressed.

  The secondary winding 20b of the switching transformer 20 and the secondary winding 21b of the switching transformer 21 are connected in series between the anode of the diode 22 and the terminals T2 and T4. The first terminals (terminals marked with ●) of the secondary windings 20 b and 21 b of the switching transformers 20 and 21 are directed to the anode side of the diode 22. The cathode of the diode 22 is connected to the power supply node 7 a of the feedback circuit 7 and the power supply node 8 a of the control circuit 8. Capacitor 23 is connected between power supply nodes 7a and 8a and terminals T2 and T4. Therefore, the AC voltage generated in the secondary windings 20b and 21b is rectified by the diode 22, smoothed and stabilized by the capacitor 23, and becomes the DC power supply voltage of the feedback circuit 7 and the control circuit 8.

  In the third embodiment, since the reactor 5 is configured by connecting the primary windings 20a and 21a of the two switching transformers 20 and 21 in series, an increase in temperature of the reactor 5 can be suppressed.

  Further, since the AC voltage appearing in the secondary windings 20b and 21b of the switching transformers 20 and 21 is rectified and smoothed to generate the DC power supply voltage for the feedback circuit 7 and the control circuit 8, the feedback circuit 7 and the control circuit 8 are externally supplied. There is no need to supply a DC power supply voltage to the power supply.

  In addition, since the DC power supply voltage of the feedback circuit 7 and the control circuit 8 is shared by the two switching transformers 20 and 21, the number of windings of the switching transformer can be reduced and the switching transformer can be downsized.

  In the third embodiment, the primary windings 20a and 21a of the two switching transformers 20 and 21 are connected in series to form the reactor 5, but the primary windings of three or more switching transformers are connected in series. Then, the reactor 5 may be configured. In this case, if the same type of switching transformer is used, the cost can be reduced.

[Embodiment 4]
4 is a circuit block diagram showing a configuration of a step-down chopper circuit according to a fourth embodiment of the present invention, which is compared with FIG. Referring to FIG. 4, this step-down chopper is different from the step-down chopper of FIG. 3 in that secondary windings 20b and 21b of switching transformers 20 and 21 are disconnected, diodes 24, 26 and 27 and capacitors. 25 is added.

  The first terminal (terminal marked with ●) of the secondary winding 20b of the switching transformer 20 is connected to the anode of the diode 22, and the second terminal of the secondary winding 20b is connected to the terminals T2 and T4. The capacitor 23 is connected between the cathode of the diode 22 and the terminals T2 and T4. The cathode of diode 22 is connected to the anode of diode 26, and the cathode of diode 26 is connected to power supply nodes 7a and 8a.

  The first terminal (terminal marked with ●) of the secondary winding 21b of the switching transformer 21 is connected to the anode of the diode 24, and the second terminal of the secondary winding 21b is connected to the terminals T2 and T4. The capacitor 25 is connected between the cathode of the diode 24 and the terminals T2 and T4. The cathode of diode 24 is connected to the anode of diode 27, and the cathode of diode 27 is connected to power supply nodes 7a and 8a.

  The AC voltage generated in the secondary winding 20 b of the switching transformer 20 is rectified and smoothed by the diode 22 and the capacitor 23 to be converted into a DC voltage, and is supplied to the power supply nodes 7 a and 8 a via the diode 26. The AC voltage generated in the secondary winding 21 b of the switching transformer 21 is rectified and smoothed by the diode 24 and the capacitor 25 to be converted into a DC voltage, and is supplied to the power supply nodes 7 a and 8 a via the diode 27.

  In the fourth embodiment, the same effect as in the third embodiment can be obtained, and any one of the rectifier circuit including the diode 22 and the capacitor 23 and the rectifier circuit including the diode 24 and the capacitor 25 has failed. Also, a DC power supply voltage can be supplied to the power supply nodes 7a and 8a.

[Embodiment 5]
5 is a circuit block diagram showing a configuration of a step-down chopper circuit according to a fifth embodiment of the present invention, which is compared with FIG. Referring to FIG. 5, the step-down chopper is different from the step-down chopper of FIG. 1 in that a diode 28 and a resistance element 29 are added.

  The cathode of the diode 28 is connected to the cathode of the backflow prevention diode 4. The resistance element 29 is connected between the anode of the diode 28 and the output terminal T3. The diode 28 and the resistance element 29 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.

[Embodiment 6]
6 is a circuit block diagram showing a configuration of a step-down chopper circuit according to a sixth embodiment of the present invention, which is compared with FIG. Referring to FIG. 6, this step-down chopper is different from the step-down chopper of FIG. 1 in that N-channel MOS transistor 2 and reactor 5 are provided on the negative electrode side of DC power supply 10 to perform low-side switching.

  That is, the source of the N-channel MOS transistor 2 is connected to the input terminal T2, and the drain thereof is connected to the output terminal T4 via the reactor 5. The anode of the backflow prevention diode 4 is connected to the input terminal T1, and its cathode is connected to the output terminal T3. The anode of the freewheeling diode 3 is connected to the drain of the N-channel MOS transistor 2, and its cathode is connected to the anode of the backflow prevention diode 4. Since the other configuration is the same as that of the step-down chopper circuit of FIG. 1, the description thereof will not be repeated.

  Next, the operation of this step-down chopper circuit will be described. During the period when control signal CNT is at “H” level, N-channel MOS transistor 2 is turned on, the parallel connection body of reverse current prevention diode 4, output capacitor 6 and load 11, reactor 5, and N channel from the positive electrode of DC power supply 10. A current flows through the MOS transistor 2 through a path reaching the negative electrode of the DC power supply 1. 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 transistor 2 is turned off, the electromagnetic energy of the reactor 5 is released, and the freewheeling diode 3, the backflow prevention diode 4, and the output from one terminal of the reactor 5 A current flows through a path reaching the other terminal of the reactor 5 through the parallel connection body of the capacitor 6 and the load 11. 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. Therefore, the voltage Vo between the output terminals T3 and T4 can be controlled to a desired target voltage by adjusting the duty ratio of the control signal CNT.

  Also in this step-down chopper circuit, when there is no backflow prevention diode 4, series resonance occurs due to the parasitic capacitor 2 c and the reactor 5 of the N-channel MOS transistor 2 during the current discontinuous period, and current flows back to the DC power supply 10. There is a case. However, in this step-down chopper circuit, the backflow prevention diode 4 is provided in a path (a path from the drain of the transistor 2 to the source of the transistor 2 via the reactor 5, the output capacitor 6, and the DC power supply 11) forming a series resonance circuit. Since it is connected, the reverse flow path associated with the series resonance can be cut off, and the reverse flow of current can be prevented.

  In the drive circuit 9 of the first to fifth embodiments, 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. On the other hand, in the sixth embodiment, since the source of the N-channel MOS transistor 2 is shared with the ground on the load 11 side, it is not necessary to provide the drive circuit 9 with an insulating circuit. Therefore, the drive circuit 9 can be configured at a lower cost.

  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.

[Embodiment 7]
FIG. 7 is a circuit block diagram showing a configuration of the step-down chopper circuit according to the seventh embodiment of the present invention, and is compared with FIG. Referring to FIG. 7, this step-down chopper is different from the step-down chopper of FIG. 6 in that the cathode of freewheeling diode 3 is connected to the cathode of backflow prevention diode 4.

  Next, the operation of this step-down chopper circuit will be described. During the period when control signal CNT is at “H” level, N-channel MOS transistor 2 is turned on, the parallel connection body of reverse current prevention diode 4, output capacitor 6 and load 11, reactor 5, and N channel from the positive electrode of DC power supply 10. A current flows through the MOS transistor 2 through a path reaching the negative electrode of the DC power supply 1. 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 transistor 2 is turned off, the electromagnetic energy of the reactor 5 is released, and the free-wheeling diode 3, the output capacitor 6, and the load 11 are discharged from one terminal of the reactor 5. Current flows through a path reaching the other terminal of the reactor 5 through the parallel connection body. In this case, the current flowing through the reactor 5 decreases linearly with time. Therefore, the voltage Vo between the output terminals T3 and T4 can be controlled to a desired target voltage by adjusting the duty ratio of the control signal CNT.

  In addition, since the backflow prevention diode 4 is connected in the path forming the series resonance circuit (the path from the drain of the transistor 2 to the source of the transistor 2 via the reactor 5, the output capacitor 6, and the DC power supply 11), the series resonance Therefore, the backflow path associated with the current can be cut off and the backflow of current can be prevented.

  Further, since the backflow prevention diode 4 is not arranged in the return path, no loss occurs in the backflow prevention diode 4 during the return period, and the power loss is reduced as compared with the sixth embodiment.

[Embodiment 8]
FIG. 8 is a circuit block diagram showing the configuration of the step-down chopper circuit according to the eighth embodiment of the present invention, which is compared with FIG. Referring to FIG. 8, this step-down chopper is different from the step-down chopper of FIG. 6 in that reactor 5 is replaced with two switching transformers 20 and 21, and diodes 22 and 24 and capacitors 23 and 25 are added. It is.

  The primary winding 20a of the switching transformer 20 and the primary winding 21a of the switching transformer 21 are connected in series between the output terminal T4 and the drain of the N-channel MOS transistor 2. The first terminals (terminals marked with ●) of the primary windings 20 a and 21 a of the switching transformers 20 and 21 are directed to the drain of the N-channel MOS transistor 2. That is, in the eighth embodiment, the reactor 5 is configured by the primary windings 20a and 21a of the two switching transformers 20 and 21, and the temperature rise of the reactor 5 is suppressed.

  The first terminal (terminal marked with ●) of the secondary winding 20b of the switching transformer 20 is connected to the anode of the diode 22, and the second terminal of the secondary winding 20b is connected to the output terminal T4. . The cathode of the diode 22 is connected to the power supply node 7 a of the feedback circuit 7. The capacitor 23 is connected between the power supply node 7a and the output terminal T4. The AC voltage generated in the secondary winding 20b is rectified by the diode 22, smoothed and stabilized by the capacitor 23, and becomes the DC power supply voltage of the feedback circuit 7.

  The first terminal (terminal marked with ●) of the secondary winding 21b of the switching transformer 21 is connected to the anode of the diode 24, and the second terminal of the secondary winding 21b is connected to the input terminal T2. . The cathode of the diode 24 is connected to the power supply node 8 a of the control circuit 8. The capacitor 25 is connected between the power supply node 8a and the input terminal T2. The AC voltage generated in the secondary winding 21b is rectified by the diode 24, smoothed and stabilized by the capacitor 25, and becomes the DC power supply voltage of the control circuit 8.

  In the eighth embodiment, since the reactor 5 is configured by connecting the primary windings 20a, 21a of the two switching transformers 20, 21 in series, an increase in temperature of the reactor 5 can be suppressed.

  Further, since the AC voltage appearing in the secondary windings 20b and 21b of the switching transformers 20 and 21 is rectified and smoothed to generate the DC power supply voltage for the feedback circuit 7 and the control circuit 8, the feedback circuit 7 and the control circuit 8 are externally supplied. There is no need to supply a DC power supply voltage to the power supply.

  In addition, since the DC power supply voltage of the feedback circuit 7 and the control circuit 8 is shared by the two switching transformers 20 and 21, the number of windings of the switching transformer can be reduced and the switching transformer can be downsized.

  In the eighth embodiment, the primary windings 20a and 21a of the two switching transformers 20 and 21 are connected in series to form the reactor 5. However, the primary windings of three or more switching transformers are connected in series. Then, the reactor 5 may be configured. In this case, if the same type of switching transformer is used, the cost can be reduced.

[Embodiment 9]
FIG. 9 is a circuit block diagram showing the configuration of the step-down chopper circuit according to the ninth embodiment of the present invention, which is compared with FIG. Referring to FIG. 9, the step-down chopper is different from the step-down chopper of FIG. 6 in that a diode 28 and a resistance element 29 are added.

  The cathode of the diode 28 is connected to the output terminal T4. Resistance element 29 is connected between the drain of anode N-channel MOS transistor 2 of diode 28. The diode 28 and the resistance element 29 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.

  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 N channel MOS transistor, 3 freewheeling diode, 4 backflow prevention diode, 5 reactor, 6 output capacitor, 7 feedback circuit, 8 control circuit, 9 drive circuit, 10 DC power supply, 11 load, 20, 21 switching transformer, 20a, 21a primary winding, 20b, 21b secondary winding, 22, 24, 26-28 diode, 23, 25 capacitor, 29 resistance 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 switching element having a first electrode connected to the first input terminal and controlled to be turned on / off so that the second DC voltage becomes a target voltage;
A backflow prevention element having an anode connected to the second electrode of the switching element and flowing a current only in a direction from the anode toward the cathode;
A reactor having one terminal connected to the cathode of the backflow prevention element and the other terminal connected to the first output terminal;
A capacitor connected between the first and second output terminals;
A freewheeling diode having an anode connected to the second input terminal and the second output terminal, and returning a current output from the other terminal of the reactor to the one terminal of the reactor when the switching element is turned off; Prepared ,
The reactor includes a plurality of primary windings connected in series,
A plurality of secondary windings that respectively constitute a plurality of transformers with the plurality of primary windings;
A rectifier circuit that rectifies an alternating voltage appearing in the plurality of secondary windings to generate a power supply voltage;
A feedback circuit that receives the power supply voltage from the rectifier circuit, detects a voltage between the first and second output terminals, and outputs a signal indicating a detected value;
A step-down chopper circuit, further comprising: a control circuit that receives the power supply voltage from the rectifier circuit and outputs a control signal for controlling on / off of the switching element based on a signal indicating the detection value . The rectifier circuit includes a plurality of first diodes that rectify currents flowing through the plurality of secondary windings, respectively.
A plurality of capacitors each charged with current rectified by the plurality of first diodes;
A plurality of second diodes each having an anode connected to the plurality of capacitors,
2. The step-down chopper circuit according to claim 1, wherein a cathode of each of the plurality of second diodes is connected to a power supply wiring that supplies the power supply voltage to the control circuit and the feedback circuit. A step-down chopper circuit that steps down a DC voltage applied between first and second input terminals and outputs a second DC voltage between first and second output terminals,
A backflow preventing element having an anode connected to the first input terminal, a cathode connected to the first output terminal, and a current flowing only in a direction from the anode toward the cathode;
A capacitor connected between the first and second output terminals;
A reactor having one terminal connected to the second output terminal;
A switching element in which the first electrode is connected to the other terminal of the reactor, the second electrode is connected to the second input terminal, and is controlled to be turned on / off so that the second DC voltage becomes a target voltage When,
A reflux diode that has an anode connected to the other terminal of the reactor and causes a current output from the other terminal of the reactor to flow to the first output terminal when the switching element is turned off ;
The reactor includes a plurality of primary windings connected in series,
A plurality of secondary windings that respectively constitute a plurality of transformers with the plurality of primary windings;
A rectifying circuit for rectifying an AC voltage appearing in the plurality of secondary windings;
A feedback circuit that receives power supply from the rectifier circuit, detects a voltage between the first and second output terminals, and outputs a signal indicating a detection value;
A step-down chopper circuit, further comprising: a control circuit that receives power supply from the rectifier circuit and outputs a control signal for controlling on / off of the switching element based on a signal indicating the detection value . The rectifier circuit is
A first diode for rectifying a current flowing in a first secondary winding of the plurality of secondary windings;
A first capacitor charged with a current rectified by the first diode;
A second diode for rectifying a current flowing in a second secondary winding of the plurality of secondary windings;
A second capacitor charged with a current rectified by the second diode;
The feedback circuit receives a charging voltage of the first capacitor as a power supply voltage;
The step-down chopper circuit according to claim 3, wherein the control circuit receives a charging voltage of the second capacitor as a power supply voltage. The cathode of the freewheeling diode is connected to the anode of the backflow prevention device, the step-down chopper circuit according to claim 1 or claim 3. Furthermore, a snubber circuit connected in parallel to the reactor,
The snubber circuit, the cathode includes one and a diode connected to the terminals of the reactor, the anode and a resistor element connected to the other terminal of the reactor of the diode, the step-down of claim 1 or claim 3 Chopper circuit.

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