JP2015231267A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compact and low cost switching power supply in which circuit components can be prevented from being destroyed due to heat.SOLUTION: A switching power supply includes a chopper circuit including a transistor 2, a reflux diode 3, a reactor 4, and a control IC 10 for turning the transistor 2 on/off, a start-up circuit 8 including a transistor 23 and generating a power supply voltage VCC1 for starting up the control IC 10 based on an input voltage Vi, a power supply circuit 9 generating a power supply voltage VCC2 for operating the control IC 10 steadily based on the voltage between terminals of a reactor 5 coupled electromagnetically with the reactor 4, and a heatsink 30 for heating the transistors 2, 23.

Description

  The present invention relates to a switching power supply device, and more particularly to a switching power supply device that converts a first DC voltage into a second DC voltage by turning on / off a switching element.

  For example, Patent Document 1 discloses a switching power supply device that includes a switching element, a freewheeling diode, a reactor, and a control circuit that turns on / off the switching element, and generates a second DC voltage by stepping down the first DC voltage. Has been. The control circuit is driven by the first DC voltage.

  Patent Document 2 discloses a switching power supply device that uses a primary winding of a transformer as a reactor. In this switching power supply, after the control circuit is activated by the activation circuit, power is supplied to the control circuit from the secondary winding of the transformer, and the control circuit operates in a steady manner.

  Further, Patent Document 3 discloses a switching system including an AC / DC converter that converts an AC voltage into a first DC voltage, a DC / DC converter that converts a first DC voltage into a second DC voltage, and a heat sink. A power supply is disclosed. The heat sink is provided in common to the switching element of the AC / DC converter and the switching element of the DC / DC converter, and dissipates heat generated by the two switching elements.

JP 2012-016136 A Japanese Utility Model Publication No. 3-18684 Japanese Patent Laid-Open No. 11-356047

  In the switching power supply device of Patent Document 2, for example, when the output terminals are short-circuited, the voltage of the secondary winding of the transformer is lower than the voltage required for the steady operation of the control circuit, and the control circuit is intermittently operated by the starting circuit. Are operated. Normally, the startup circuit is designed so that only an instantaneous current at the time of startup flows. Therefore, if the control circuit is operated repeatedly, circuit components constituting the startup circuit may generate heat and be destroyed. As a countermeasure against this, it is conceivable to separately provide a heat sink for the starting circuit, but there is a problem that the apparatus is increased in size and cost.

  Therefore, a main object of the present invention is to provide a small-sized and low-cost switching power supply apparatus that can prevent circuit components from being destroyed by heat.

  A switching power supply device according to the present invention includes a first switching element, a free wheel diode, a first reactor, and a control circuit for turning on / off the first switching element, and a first DC voltage applied to an input terminal Is provided in common to the chopper circuit that converts the first DC voltage into the second DC voltage and outputs it to the output terminal, and the first and second circuit components that generate heat exclusively from each other in the switching power supply device. And a heat sink for cooling the circuit components.

  In the switching power supply device according to the present invention, since one heat sink is provided in common to the first and second circuit components that generate heat exclusively from each other, the circuit components can be prevented from being destroyed by heat, It is possible to reduce the size and cost of the apparatus.

1 is a circuit block diagram showing a configuration of a switching power supply device according to Embodiment 1 of the present invention. FIG. 2 is a circuit diagram illustrating a configuration of a startup circuit illustrated in FIG. 1. It is a figure which shows the heat sink for cooling two transistors shown in FIG. 1 and FIG. It is a circuit diagram which shows the structure of the starting circuit of the switching power supply device by Embodiment 2 of this invention. It is a figure which shows the principal part of the switching power supply apparatus by Embodiment 3 of this invention. It is a figure which shows the principal part of the switching power supply apparatus by Embodiment 4 of this invention.

[Embodiment 1]
1 is a circuit block diagram showing a configuration of a switching power supply apparatus according to Embodiment 1 of the present invention. 1, this switching power supply device includes input terminals T1 and T2, output terminals T3 and T4, an input capacitor 1, an N-channel MOS transistor 2 (first switching element), a freewheeling diode 3, and a reactor 4 (first reactor). ), A reactor 5 (second reactor), a rectifier diode 6, an output capacitor 7, a starting circuit 8, an IC power supply circuit 9 (power circuit), a switching control IC 10 (control circuit), and a gate drive circuit 11.

  Input terminals T1 and T2 are connected to the positive electrode and the negative electrode of DC power supply 12, respectively. The DC power supply 12 outputs a DC voltage Vi between the input terminals T1 and T2. A load 13 is connected between the output terminals T3 and T4. The input terminal T2 and the output terminal T4 are connected to each other. N-channel MOS transistor 2, freewheeling diode 3, reactor 4, switching control IC 10, and gate drive circuit 11 step down DC voltage Vi (first DC voltage) applied between input terminals T1 and T2 to obtain a desired voltage. A step-down chopper circuit that generates a DC voltage Vo (second DC voltage) and outputs the DC voltage Vo between the output terminals T3 and T4 is configured. The load 13 is driven by the output voltage Vo of the switching power supply device.

  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 2a. The anode and cathode of parasitic diode 2a are connected to the source and drain of N-channel MOS transistor 2, respectively.

  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. One terminal of the reactor 4 is connected to the source of the N-channel MOS transistor 2, and the other terminal is connected to the output terminal T3.

  The reactor 5 is electromagnetically coupled to the reactor 4. For example, a primary winding and a secondary winding of a transformer may be used as the reactors 4 and 5, respectively. One terminal of the reactor 5 is connected to the input terminal T 2, and the other terminal is connected to the anode of the rectifier diode 6. The cathode of the rectifier diode 6 is connected to the input terminal of the IC power supply circuit 9. The voltage between the terminals of the reactor 5 is rectified by the rectifier diode 6 and applied to the input terminal of the IC power supply circuit 9.

  The starting circuit 8 generates a power supply voltage VCC1 based on the DC voltage Vi applied between the input terminals T1 and T2, and supplies the power supply voltage VCC1 to the power supply terminal 10a of the switching control IC. The IC power supply circuit 9 generates a power supply voltage VCC2 based on the power supplied from the reactor 5 via the diode 6, and supplies the power supply voltage VCC2 to the power supply terminal 10b of the switching control IC10. At startup, the power supply voltage VCC1 is higher than the power supply voltage VCC2, and during steady operation, the power supply voltage VCC2 is higher than the power supply voltage VCC1.

  When the voltage generated by the reactor 5 and the diode 6 is sufficiently stable and can be used as the power supply voltage of the switching control IC 10, the IC power supply circuit 9 is deleted and the cathode of the rectifier diode 6 is connected. You may connect directly to the power supply terminal 10b of the switching control IC10.

  The switching control IC 10 is driven by the higher one of the power supply voltages VCC1 and VCC2, supplies a control signal CNT to the gate of the N-channel MOS transistor 2 via the gate drive circuit 11, and outputs between the output terminals T3 and T4. The N-channel MOS transistor 2 is turned on / off so that the DC voltage Vo becomes the target voltage. The output capacitor 7 is connected between the output terminals T3 and T4, and stabilizes the DC voltage Vo between the output terminals T3 and T4.

  Next, the operation of this switching power supply device will be described. When the DC voltage Vi is applied between the input terminals T1 and T2, power is supplied to the starting circuit 8 to generate the power supply voltage VCC1. When the power supply voltage VCC1 generated by the activation circuit 8 reaches the activation voltage of the switching control IC 10, the switching control IC 10 starts outputting the control signal CNT.

  When control signal CNT is set to “H” level, N-channel MOS transistor 2 is turned on, from the positive electrode of DC power supply 12 through N-channel MOS transistor 2, reactor 4, and output capacitor 7 and load 13 connected in parallel. Thus, a current flows along the path to the negative electrode of the DC power supply 12. As a result, the output capacitor 7 is charged and electromagnetic energy is stored in the reactor 4.

  When the control signal CNT is set to the “L” level, the N-channel MOS transistor 2 is turned off, the electromagnetic energy stored in the reactor 4 is released, and the other terminal (terminal on the output terminal T3 side) of the reactor 4 is released. In addition, a current flows through a path connecting the parallel connection body of the output capacitor 7 and the load 13 and one terminal of the reactor 4 (terminal on the input terminal T1 side) via the freewheeling diode 3. A state in which current flows through such a path is generally called reflux. In this example, the free-wheeling diode 3 is used as a circuit component for forming the free-wheeling path. However, a switching element that is turned on / off by an external control signal may be used for the purpose of improving the efficiency of the circuit.

  The control signal CNT is a signal having a constant frequency, and an “H” level period and an “L” level period are provided for each period. The ratio between the time of “H” level in each cycle and the time of one cycle is called the duty ratio. Increasing the duty ratio increases the output voltage Vo, and decreasing the duty ratio decreases the output voltage Vo. Therefore, the output voltage Vo can be adjusted to a desired target voltage by adjusting the duty ratio. However, Vo ≦ Vi.

  Further, when the N channel MOS transistor 2 is turned on / off by the control signal CNT and the current flowing through the reactor 4 increases or decreases, an AC voltage is generated between the terminals of the reactor 5. The AC voltage generated between the terminals of the reactor 5 is rectified by the rectifier diode 6 and converted into the power supply voltage VCC2 of the switching control IC 10 by the IC power supply circuit 9.

  At this time, by setting the power supply voltage VCC2 generated by the IC power supply circuit 9 higher than the power supply voltage VCC1 generated by the startup circuit 8, the output of the startup circuit 8 can be stopped. . If there is a concern about the backflow of current to the starting circuit 8, a diode may be inserted between the output node of the starting circuit 8 and the power supply terminal 10 a of the switching control IC 10.

  FIG. 2 is a circuit diagram showing a configuration of the starting circuit 8. In FIG. 2, activation circuit 8 is a constant voltage circuit, and includes resistance elements 21 and 22, an N channel MOS transistor 23 (second switching element), and a Zener diode 24. One terminals of resistance elements 21 and 22 are both connected to input terminal T 1, and the other terminals thereof are connected to the gate and drain of N channel MOS transistor 23, respectively.

  The source of N channel MOS transistor 23 is connected to output node 8 a of activation circuit 8. N channel MOS transistor 23 includes a parasitic diode 23a. The anode and cathode of the parasitic diode 23a are connected to the source and drain of the N-channel MOS transistor 23, respectively. Zener diode 24 has an anode connected to input terminal T 2 and a cathode connected to the gate of N-channel MOS transistor 23.

  When the Zener voltage of the Zener diode 24 is higher than the sum of the voltage of the output node 8a and the threshold voltage of the N-channel MOS transistor 23, the transistor 23 is turned on, and the resistance element 22 and the transistor 23 from the input terminal T1. Current flows through the output node 8a. When the Zener voltage of Zener diode 24 is lower than the sum of the voltage of output node 8a and the threshold voltage of N channel MOS transistor 23, transistor 23 is turned off.

  The power supply terminals 10a and 10b of the switching control IC 10 are conductive inside the switching control IC 10. When the output voltage VCC2 of the IC power supply circuit 9 becomes higher than the output voltage VCC1 of the start circuit 8, the gate-source voltage of the N channel MOS transistor 23 of the start circuit 8 becomes higher than the threshold voltage of the N channel MOS transistor 23. And the N-channel MOS transistor 23 is turned off. For this reason, the switching control IC 10 is driven by VCC1 when VCC1> VCC2, and the switching control IC10 is driven by VCC2 during steady operation when VCC2> VCC1.

  The output voltage VCC1 of the starting circuit 8 is determined by the Zener voltage of the Zener diode 24, and the maximum output current is limited by the resistance element 22. Usually, a voltage stabilizing capacitor is connected to the power supply terminal 10a of the switching control IC10. The time from when the DC voltage Vi is applied between the input terminals T1 and T2 until the switching control IC 10 is activated is determined by the time constant between the capacitance value of the voltage stabilizing capacitor and the resistance value of the resistance element 22.

  In order to shorten the startup time of the switching control IC 10, the capacitance value of the voltage stabilizing capacitor is reduced or the resistance value of the resistance element 22 is reduced to increase the maximum current that can be supplied by the startup circuit 8. The loss of the N-channel MOS transistor 23 is the product of the current flowing through the transistor 23 and the voltage applied between the drain and source of the transistor 23. Therefore, the resistance value of the resistance element 22 is reduced to shorten the startup time. In this case, since the heat generation amount of the N-channel MOS transistor 23 increases, a heat sink is necessary.

  Further, when the output node 8a of the starting circuit 8 and the input terminal T2 are short-circuited, the maximum current that can be supplied from the starting circuit 8 continues to flow through the transistor 23. Therefore, a heat sink can be attached to the transistor 23 for thermal protection. desirable. However, in recent years, since the switching power supply device is required to be reduced in size and cost, the heat sink cannot be easily attached. Therefore, the object of the first embodiment is to provide thermal protection to the N-channel MOS transistor 23 of the startup circuit 8 without increasing the number of heat sinks.

  As shown in FIG. 3, in the first embodiment, one heat sink 30 is provided in common to the N channel MOS transistors 2 and 23. The packages of the N-channel MOS transistors 2 and 23 are fixed to the heat sink 30 by screws 31 and 32, respectively. The heat sink 30 is formed of a metal having high thermal conductivity (for example, aluminum or copper). The heat generated in the transistors 2 and 23 is conducted to the heat sink and is dissipated from the heat sink into the air.

  Since the output voltage of the starting circuit 8 is lower than the starting voltage of the switching control IC 10 or when the output node 8a of the starting circuit 8 is short-circuited to the input terminal T1, the control signal CNT is fixed at the “L” level. No current flows through the N channel MOS transistor 2, and the N channel MOS transistor 2 does not generate heat.

  On the other hand, when the Zener voltage of Zener diode 24 is higher than the sum of the voltage of output node 8a and the threshold voltage of N channel MOS transistor 23, N channel MOS transistor 23 of starting circuit 8 is turned on. A current flows through the N-channel MOS transistor 23 and the N-channel MOS transistor 23 generates heat.

  On the other hand, during the steady operation, the period during which the control signal CNT is at the “H” level and the period at which the control signal CNT is at the “L” level are alternately repeated, and the N-channel MOS transistor 2 is repeatedly turned on / off. As a result, current intermittently flows in N channel MOS transistor 2 and N channel MOS transistor 2 generates heat.

  On the other hand, since the output voltage VCC2 of the IC power supply circuit 9 is set higher than the output voltage VCC1 of the starting circuit 8, the N-channel MOS transistor 23 has a gate-source voltage of the N-channel MOS transistor 23. The voltage becomes lower than the threshold voltage, and the N-channel MOS transistor 23 is turned off and does not generate heat.

  That is, the N channel MOS transistor 2 and the N channel MOS transistor 23 generate heat exclusively. Therefore, the heat sink 30 functions as a heat radiator for the N channel MOS transistor 23 at the time of startup, and functions as a heat radiator for the N channel MOS transistor 2 at the time of steady operation.

  When heat sinks are individually attached to the N channel MOS transistor 2 and the N channel MOS transistor 23, a heat sink that allows the maximum heat generation amount of the N channel MOS transistor 2 and a heat sink that allows the maximum heat generation amount of the N channel MOS transistor 23 are provided. There is a need.

  On the other hand, in the first embodiment, the heat sink 30 is designed to allow a larger heat generation amount between the maximum heat generation amount of the N-channel MOS transistor 2 and the maximum heat generation amount of the N-channel MOS transistor 23. Therefore, the number of heat sinks can be reduced, the size of the device can be reduced, and the cost can be reduced as compared with the case where heat sinks are individually attached to the N-channel MOS transistors 2 and 23.

  Since N channel MOS transistor 2 and N channel MOS transistor 23 generate heat exclusively, the package of the N channel MOS transistor that does not generate heat acts as a part of the radiator. For this reason, heat dissipation improves from the heat sink 30 single-piece | unit.

  Further, since the allowable current of the N channel MOS transistor 23 is increased by thermally protecting the N channel MOS transistor 23, the output current of the start circuit 8 can be increased and the time until the switching control IC 10 is started is shortened. be able to.

  In the first embodiment, the case where two N-channel MOS transistors 2 and 23 share one heat sink 30 has been described. However, the present invention is not limited to this, and any combination with different operating conditions during maximum heat generation may be used. Any combination may be used.

  For example, one heat sink 30 may be shared by the freewheeling diode 3 and the N channel MOS transistor 23, or one heat sink 30 may be shared by the rectifier diode 6 and the N channel MOS transistor 23.

  Further, the IC power supply circuit 9 is constituted by the constant voltage circuit shown in FIG. 2, and the N channel MOS transistor 23 of the starter circuit 8 and the N channel MOS transistor 23 of the IC power supply circuit 9 share one heat sink 30. May be.

  Further, the number of circuit components commonly attached to one heat sink 30 is not limited to two, and may be three or more.

[Embodiment 2]
FIG. 4 is a circuit diagram showing the main part of the switching power supply device according to Embodiment 2 of the present invention, which is compared with FIG. Referring to FIG. 4, this switching power supply device is different from the switching power supply device of the first embodiment in that starting circuit 8 is replaced with starting circuit 35.

  The start circuit 35 is a constant current circuit in which the connection order of the resistance element 22 of the start circuit 8 and the N-channel MOS transistor 23 is changed. The drain of the transistor 23 is connected to the input terminal T1, and the source thereof is connected to the output node 8a via the resistance element 22.

  When a DC voltage Vi is applied between the input terminals T1 and T2, a current flows from the input terminal T1 to the input terminal T2 via the resistance element 21 and the Zener diode 24. As a result, a Zener voltage is generated between the cathode and anode of the Zener diode 24, and the “H” level is input to the gate of the N-channel MOS transistor 23 to turn on the transistor 23.

  The output voltage of the starter circuit 35 is determined by the Zener voltage of the Zener diode 24, and the maximum output current is a voltage obtained by subtracting the gate-source voltage of the N-channel MOS transistor 23 from the Zener voltage generated in the Zener diode 24. The value is divided by the resistance value. The drain of the N channel MOS transistor 23 is connected to the input terminal T 1 and the drain of the N channel MOS transistor 2. Since other configurations and operations are the same as those in the first embodiment, description thereof will not be repeated.

  In the second embodiment, since the potentials of the drains of N channel MOS transistor 2 and N channel MOS transistor 23 are equal, it is necessary to consider the insulation of circuit components attached to heat sink 30 (ie, N channel MOS transistors 2 and 23). There is no. For example, the N-channel MOS transistors 2 and 23 in a package that is not fully molded (insulated) can be used, the design can be simplified, and the range of component selection is widened.

  In general, a package that is not fully molded has a lower thermal resistance between the package and the air than a fully molded package, and further there is no need to consider insulation. Circuit components can be attached to the heat sink 30 using small grease. For this reason, the heat sink 30 can be reduced in size.

[Embodiment 3]
FIG. 5 is a diagram showing a main part of a switching power supply device according to Embodiment 3 of the present invention, and is compared with FIG. Referring to FIG. 5, this switching power supply device is different from the switching power supply device of the first embodiment in that the heat sink 30 is removed and the N channel MOS transistor 2 and the N channel MOS transistor 23 are fixed back to back. The points are fixed to each other by 36.

  Since N channel MOS transistor 2 and N channel MOS transistor 23 generate heat exclusively from each other, when N channel MOS transistor 2 is generating heat, N channel MOS transistor 23 functions as a heat sink and N channel MOS transistor 23 generates heat. N channel MOS transistor 2 functions as a heat sink when it is in operation. Since other configurations and operations are the same as those in the first embodiment, description thereof will not be repeated.

  In the third embodiment, when one of the N-channel MOS transistor 2 and the N-channel MOS transistor 23 is generating heat, the other is used as a heat sink. Therefore, compared to the case where each of the transistors 2 and 23 is mounted alone, The transistors 2 and 23 can be effectively cooled, and the allowable heat loss can be increased.

  When the starting circuit 8 is configured by the constant voltage circuit shown in FIG. 2, it is necessary to insulate the N channel MOS transistor 2, the channel MOS transistor 23, and the fixing screw 36.

  On the other hand, when the starting circuit 8 is configured by the constant current circuit shown in FIG. 4, since the drain potentials of the N channel MOS transistor 2 and the channel MOS transistor 23 are equal, it is not necessary to consider their insulation. For this reason, it is generally possible to fix the transistors 2 and 23 by applying grease having a smaller thermal resistance than that of the heat dissipation sheet, thereby further increasing the allowable heat loss.

[Embodiment 4]
FIG. 6 is a diagram showing a main part of a switching power supply device according to Embodiment 4 of the present invention. In FIG. 6, the N channel MOS transistors 2 and 23 are mounted on the printed circuit board 40 so that the area where the packages of the N channel MOS transistors 2 and 23 are in contact with the common pattern 41 on the printed circuit board 40 is the largest. The pattern 41 is formed of a metal having high thermal conductivity (for example, aluminum or copper).

  By making the pattern 41 thick, heat generated in the N-channel MOS transistors 2 and 23 is easily conducted to the pattern 41. Further, by widening the pattern 41, the heat dissipation of the heat conducted from the N-channel MOS transistors 2 and 23 to the pattern 41 is improved, and the pattern 41 functions as a radiator.

  Since the N-channel MOS transistor 2 and the N-channel MOS transistor 23 generate heat exclusively as described above, the total area of the pattern is smaller than when connected to the pattern individually for the purpose of heat dissipation, and as a result, the printed circuit board 40 can be reduced in size. Can be realized.

  In the case of using the starting circuit 8 composed of the constant voltage circuit shown in FIG. 2, the package of the N channel MOS transistors 2 and 23 needs to be fully molded (insulated). On the other hand, when the starting circuit 35 configured by the constant current circuit shown in FIG. 4 is used, since the potentials of the drains of the N-channel MOS transistors 2 and 23 are equal, the packages of the transistors 2 and 23 need to be insulated. No, the range of parts selection is widened.

  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, 2,23 N-channel MOS transistor, 3 freewheeling diode, 4,5 reactor, 6 rectifier diode, 7 output capacitor, 8,35 start-up circuit, 9 for IC Power supply circuit, 10 switching control IC, 11 gate drive circuit, 12 DC power supply, 13 load, 21, 22 resistance element, 24 Zener diode, 30 heat sink, 31, 32, 36 screw, 40 printed circuit board, 41 pattern.

Claims (6)

A switching power supply,
A first switching element, a free-wheeling diode, a first reactor, and a control circuit for turning on / off the first switching element, and converts the first DC voltage applied to the input terminal into a second DC voltage A chopper circuit that outputs to the output terminal,
A switching power supply comprising a heat sink that is provided in common to the first and second circuit components that generate heat exclusively from each other of the switching power supply, and that cools the first and second circuit components. A start circuit for generating a first power supply voltage for starting the control circuit based on the first DC voltage;
A power supply circuit that includes a second reactor that is electromagnetically coupled to the first reactor, and that generates a second power supply voltage for causing the control circuit to operate steadily based on a voltage across the terminals of the second reactor. Prepared,
The first circuit component generates heat when the control circuit is activated,
The switching power supply device according to claim 1, wherein the second circuit component generates heat during a steady operation of the control circuit. The startup circuit includes a resistance element and a second switching element connected in series between the input terminal and a power supply terminal of the control circuit,
The switching power supply device according to claim 2, wherein the first circuit component is the second switching element, and the second circuit component is the first switching element. The startup circuit includes a second switching element and a resistance element connected in series between the input terminal and a power supply terminal of the control circuit,
One terminals of the first and second switching elements are connected to each other;
The switching power supply device according to claim 2, wherein the first circuit component is the second switching element, and the second circuit component is the first switching element. The first and second circuit components are in contact with each other;
The switching power supply device according to any one of claims 1 to 4, wherein when one of the first and second circuit components generates heat, the other circuit component serves as the heat sink. . The first and second circuit components are mounted on a printed circuit board pattern;
The switching power supply device according to any one of claims 1 to 4, wherein the pattern serves as the heat sink.

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