JP2014180084A - Synchronous rectification switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a synchronous rectification switching power supply in which each element can be protected, with high efficiency, even if the output voltage is abnormal.SOLUTION: A synchronous rectification switching power supply 100 comprises: a first switching element 1 connected in series with a power supply line 31 on which an input voltage Vin is applied; a coil 3 having one terminal connected with the output end of the first switching element 1; a second switching element 2 connected between the output end of the first switching element 1 and a ground terminal; a capacitor 4 connected between the other end of the coil 3 and the ground terminal; a control unit 5 for controlling the switching state of the first switching element 1 and second switching element 2, so that the input voltage Vin ensures a desired output voltage Vout; and an instruction unit 6 for instructing the control unit 5 to open the second switching element 2, when a current flowing through the second switching element 2 reaches a preset value.

Description

  The present invention relates to a step-down synchronous rectification switching power supply.

  Conventionally, a switching power supply has been used to efficiently step down a predetermined voltage. As a technique related to such a switching power supply, there are those described in Patent Documents 1 and 2 shown below.

  Patent Document 1 describes a synchronous rectification step-down switching power supply. This step-down switching power supply adjusts the output voltage so that the feedback circuit that feeds back the divided output voltage opens or shorts, and even if an abnormality occurs, the expected output voltage is properly maintained. Another circuit for dividing the voltage is provided.

  The switching power supply described in Patent Document 2 describes a diode-type step-down switching power supply. This switching power supply is provided with two circuits for dividing the output voltage in order to reduce the fluctuation of the output voltage, and selectively switches according to the fluctuation of the output voltage.

JP 2011-30390 A JP 2000-134914 A

  In the switching power supply described in Patent Document 1, for example, when a voltage higher than the input voltage is applied to the output voltage from the outside, the low-side switching element is continuously closed. For this reason, an overcurrent continues to flow through the switching element, and the switching element may be destroyed.

  On the other hand, since the switching power supply described in Patent Document 2 is a diode rectification type, the diode is not destroyed even when a voltage higher than the input voltage is continuously applied to the output voltage as described above. However, since it is a diode rectification type, it is not as efficient as a switching power supply as compared with a synchronous rectification type.

  In view of the above problems, an object of the present invention is to provide a synchronous rectification switching power supply capable of protecting each element even when the output voltage is abnormal with high efficiency.

To achieve the above object, the characteristic configuration of the synchronous rectification switching power supply according to the present invention is
A first switching element connected in series to a power supply line to which an input voltage is applied;
A coil having one terminal connected in series to the output end of the first switching element;
A second switching element connected between an output terminal of the first switching element and a ground terminal;
A capacitor connected between the other terminal of the coil and a ground terminal;
A control unit for controlling an open / close state of the first switching element and the second switching element so that the input voltage becomes an intended output voltage;
An instruction unit that instructs the control unit to open the second switching element when a current flowing through the second switching element reaches a preset value; is there.

  With such a characteristic configuration, when the output voltage becomes abnormal and the current flowing through the second switching element reaches a preset value larger than the current value in the steady state, the second switching is automatically performed. Since the element can be opened, it is possible to prevent an overcurrent from flowing through the second switching element. Therefore, it becomes possible to protect each element which comprises a synchronous rectification type switching power supply. In addition, when the output voltage is normal, high efficiency can be achieved because it is configured by a synchronous rectification type.

  The indicating unit includes a second switching element and a third switching element constituting a current mirror circuit, and a resistor connected in series between the third switching element and a ground terminal, It is preferable to instruct the control unit when the voltage between the terminals of the resistor becomes a preset voltage.

  With such a configuration, the current flowing through the second switching element can be appropriately monitored by the current mirror circuit. In addition, by appropriately selecting the size of the third switching element, the detection current flowing through the third switching element can be reduced with respect to the current flowing through the second switching element. Therefore, the efficiency of the synchronous rectification switching element is not greatly deteriorated.

In addition, another characteristic configuration of the synchronous rectification switching power supply according to the present invention for achieving the above object is as follows:
A first switching element connected in series to a power supply line to which an input voltage is applied;
A coil having one terminal connected in series to the output end of the first switching element;
A second switching element connected between an output terminal of the first switching element and a ground terminal;
A capacitor connected between the other terminal of the coil and a ground terminal;
A control unit for controlling an open / close state of the first switching element and the second switching element so that the input voltage becomes an intended output voltage;
Instructing the control unit to open the second switching element when the voltage of the output terminal that outputs the output voltage becomes a preset voltage value higher than the intended output voltage. And an instruction unit.

  With such a characteristic configuration, when the output voltage becomes abnormal and the voltage at the output terminal becomes a preset voltage value higher than the intended output voltage, the second switching element is automatically opened. Since it can be in a state, it is possible to prevent an overcurrent from flowing through the second switching element. Therefore, it becomes possible to protect each element which comprises a synchronous rectification type switching element. In addition, when the output voltage is normal, high efficiency can be achieved because it is configured by a synchronous rectification type.

  Further, at least three resistors for setting the output voltage to the desired output voltage are provided in series between the output terminal and a ground terminal, and the indicating unit includes the at least three resistors. Of these, it is preferable to instruct the control unit when the voltage between the terminals of the resistor near the ground terminal becomes a preset voltage.

  With such a configuration, among the at least three resistors, the terminal voltage of the resistor closer to the ground terminal fluctuates in conjunction with the output voltage, so that the second switching is immediately performed according to the fluctuation of the output voltage. The element can be opened. Therefore, it is possible to reliably protect the elements constituting the synchronous rectification switching element.

It is a circuit diagram of a synchronous rectification switching power supply. It is a waveform of each part at the time of starting of a synchronous rectification type switching power supply, and a steady state. It is a waveform of each part at the time of abnormality of a synchronous rectification type switching power supply. It is a circuit diagram of the synchronous rectification type switching power supply concerning other embodiments. It is a circuit diagram of the synchronous rectification type switching power supply concerning other embodiments.

  The synchronous rectification switching power supply according to the present invention is configured to have a function of preventing the low-side switching element from being destroyed even if the operation becomes abnormal. Hereinafter, the synchronous rectification switching power supply (hereinafter referred to as “switching power supply”) 100 of the present embodiment will be described in detail. FIG. 1 is a schematic diagram showing the configuration of the switching power supply 100. As shown in FIG. 1, the switching power supply 100 is configured to include the first switching element 1, the second switching element 2, the coil 3, the capacitor 4, the control unit 5, and the instruction unit 6.

  The first switching element 1 is connected in series to a power supply line 31 to which an input voltage Vin is applied. The input voltage Vin is a voltage before being stepped down by the switching power supply 100 and is applied from the input terminal 30. A power supply line 31 is connected to the input terminal 30, and the first switching element 1 is connected to the power supply line 31. Therefore, the first switching element 1 corresponds to a so-called “high-side switching element”. In the present embodiment, an n-type MOS-FET is used as the first switching element 1, and the drain terminal of the first switching element 1 is connected to the power supply line 31.

  One terminal of the coil 3 is connected to the output end of the first switching element 1. The output terminal of the first switching element 1 corresponds to the source terminal of an n-type MOS-FET used as the first switching element 1. As the coil 3 according to the present embodiment, a coil composed of a primary winding is used. For this reason, it has a pair of terminal. One terminal of the pair of terminals is connected to the source terminal of the first switching element 1.

  The second switching element 2 is connected between the output terminal of the first switching element 1 and the ground terminal. The output terminal of the first switching element 1 is a source terminal of the n-type MOS-FET as described above, and is a terminal to which one terminal of the coil 3 is connected. The ground terminal is a ground terminal. The second switching element 2 corresponds to a so-called “low-side switching element”. In the present embodiment, an n-type MOS-FET is used as the second switching element 2, the drain terminal of the second switching element 2 is connected to the source terminal of the first switching element 1, and the source terminal of the second switching element 2 Is connected to the ground terminal.

  The capacitor 4 is connected between the other terminal of the coil 3 and the ground terminal. As described above, one terminal of the pair of terminals of the coil 3 is connected to the source terminal of the first switching element 1. For this reason, the other terminal of the coil 3 corresponds to the terminal not connected to the source terminal of the first switching element 1. The capacitor 4 is connected between this terminal and the ground terminal.

  The controller 5 controls the open / close state of the first switching element 1 and the second switching element 2 so that the input voltage Vin becomes the desired output voltage Vout. To make the input voltage Vin the intended output voltage Vout means that the switching power supply 100 steps down the input voltage Vin to a predetermined output voltage Vout that is set in advance. The open / close state refers to an open state (OFF state) or a closed state (ON state). Therefore, the control unit 5 opens or closes the first switching element 1 and the second switching element 2 so that the input voltage Vin becomes the predetermined output voltage Vout set in advance. To control.

  Note that the first switching element 1 and the second switching element 2 are not closed at least simultaneously. Also, in order to prevent a through current from flowing from the input terminal 30 via the first switching element 1 and the second switching element 2, the first switching element 1 and the second switching element 2 are simultaneously opened. You may control to have "dead time". In the present embodiment, such a control unit 5 controls ON-DUTY of the first switching element 1 and the second switching element 2 by PWM control.

  The control unit 5 according to the present embodiment includes a reference voltage generation unit 51, a soft start circuit 52, a triangular wave generation unit 53, an error amplifier 54, a Low-selector 55, a comparator 56, an inverter 57, and an AND gate 58. As shown in FIG. 2A, when the power supply voltage input to the control unit 5 reaches a predetermined voltage (a threshold voltage at which the control unit 5 is activated) Va, a reference voltage set in advance from the reference voltage generation unit 51. At the same time as Vref is output (see FIG. 2B), the soft start circuit 52 is activated and outputs a voltage rising at a predetermined slope (output ss in FIG. 2C). Further, the triangular wave Tw is output from the triangular wave generating unit 53 at a preset cycle (see FIG. 2D).

  In this case, the output voltage Vout of the switching power supply 100 does not reach the intended voltage (see FIG. 2G), and the feedback voltage FB1 also rises in the same manner as the output voltage Vout (see FIG. 2B). ). Therefore, as shown in FIG. 2C, the output Err of the error amplifier 54 outputs a predetermined voltage close to the power supply voltage of the error amplifier 54 until the feedback voltage FB1 reaches the reference voltage Vref. Therefore, from the Low-selector 55, as shown in FIG. 2D, the smaller one of the output Err of the error amplifier 54 and the output ss of the soft start circuit 52 is output Lo-sel.

  In the comparator 56, the triangular wave Tw from the triangular wave generator 53 is input to the non-inverting terminal, and the output Lo-sel of the Low-selector 55 is input to the inverting terminal. For this reason, the output Comp1 of the comparator 56 is as shown in FIG. Here, the output Comp1 of the comparator 56 is input to the inverter 57, and the output INV of the inverter 57 is input to the gate terminal of the n-type MOS-FET as the first switching element 1 (see FIG. 2 (f)). ).

  On the other hand, the output Comp 2 from the instruction unit 6 described later is input to the AND gate 58 together with the output Comp 1 of the comparator 56. Although details will be described later, the output Comp2 of the instructing unit 6 becomes a High signal when the switching power supply 100 is activated and during a steady operation. Therefore, the output AND of the AND gate 58 when the switching power supply 100 is activated and during steady operation is the same as the output Comp1 of the comparator 56 shown in FIG. This output AND is input to the gate terminal of the n-type MOS-FET as the second switching element 2.

  When the output voltage Vout reaches a desired voltage, the control unit 5 shifts to a steady operation. In other words, the output Err of the error amplifier 54 determined according to the feedback voltage FB1 and the reference voltage Vref and the triangular wave Tw from the triangular wave generator 53 are input to the gate terminals of the first switching element 1 and the second switching element 2, respectively. Signal is output.

  Further, when output fluctuation occurs within the range assumed as t1 and t2 in FIG. 2G, the feedback voltage FB1 fluctuates according to the output fluctuation as shown in FIG. Along with this, the output Err of the error amplifier 54 also fluctuates (see FIG. 2C), and the ON-DUTY of the signal input to each gate terminal of the first switching element 1 and the second switching element 2 is adjusted. PWM control is performed.

  In the present embodiment, a function for preventing an overcurrent from flowing through the second switching element 2 is provided. Therefore, the instruction unit 6 instructs the control unit 5 to open the second switching element 2 when the current flowing through the second switching element 2 reaches a preset value. As described above, the open / close state of the second switching element 2 is controlled in accordance with the output AND of the AND gate 58, and a drain current flows when the second switching element 2 is closed. This drain current increases or decreases according to the load connected to the output terminal 35. For this reason, when this load is known, the current flowing through the second switching element 2 can be predicted. Therefore, the instruction unit 6 prevents the current from flowing through the second switching element 2 when the current flowing through the second switching element 2 exceeds such a predictable current value and reaches a preset value. The controller 5 is instructed to open the second switching element 2.

  In the present embodiment, the instruction unit 6 includes a second switching element 2 and a third switching element 61 that forms a current mirror circuit, and a resistor 62 connected in series between the third switching element 61 and the ground terminal. And is configured. The third switching element 61 is configured using an n-type MOS-FET in the same manner as the second switching element 2. The drain terminal of the third switching element 61 is connected to the drain terminal of the second switching element 2, and the gate terminal of the third switching element 61 is connected to the gate terminal of the second switching element 2. Therefore, the same signal (PWM signal) is input to the second switching element 2 and the third switching element 61.

  A resistor 62 is provided between the source terminal of the third switching element 61 and the ground terminal. Thereby, the voltage between the terminals of the resistor 62 changes according to the current (drain current) flowing through the third switching element 61. FIG. 3A shows a feedback voltage FB3 corresponding to the voltage between the terminals of the resistor 62. FIG. When an overcurrent starts to flow through the second switching element 2 at time t3, the voltage between the terminals of the resistor 62 increases. When the voltage between the terminals of the resistor 62 becomes a preset voltage, the control unit 5 is instructed to open the second switching element 2.

  In this embodiment, the voltage across the resistor 62 is input to the High-selector 63 as the feedback voltage FB3, and the output Hi-Sel of the High-selector 63 (details will be described later) is applied to the inverting terminal of the comparator 64. Have been entered. On the other hand, the reference voltage Vref from the reference voltage generation unit 51 is input to the non-inverting terminal of the comparator 64. Therefore, as shown at time t4 in FIG. 3B, when the feedback voltage FB3 (output Hi-sel) becomes larger than the reference voltage Vref, the comparator 64 as shown in FIG. 3C. The output Comp2 becomes Low.

  As a result, as shown in FIG. 3D, the output AND of the AND gate 58 is also Low, and the signal input to the gate terminal of the second switching element 2 is also continuously Low (FIG. 3E). reference). Accordingly, the second switching element 2 is opened. Thereby, it is possible to prevent an overcurrent from flowing through the second switching element 2. In the present embodiment, such a High-selector 63 and a comparator 64 also constitute the instruction unit 6.

  A current flows through the resistor 62 when the third switching element 61 is closed. Such a current is a loss in a steady state, which is not preferable from the viewpoint of the efficiency of the switching power supply 100. Therefore, in order to reduce the current flowing through the resistor 62, the size of the third switching element 61 that constitutes the current mirror circuit with the second switching element 2 is set to be several thousand times smaller than the size of the second switching element 2. It is preferable to make it about 1/100.

  In addition, a predetermined ripple voltage is superimposed on the output voltage Vout in the switching power supply 100. Therefore, it is preferable to set the resistance value of the resistor 62 in consideration of this ripple component. Further, even when an unexpected current flows to the second switching element 2, the comparator 64 does not exceed the rated current of the second switching element 2 so that the second switching element 2 is not damaged. It is preferable that the resistance value of the resistor 62 is set so that the above operates.

  Specifically, it is preferable to set the comparator 64 to operate when the output voltage Vout exceeds the abnormal voltage Vover expressed by the following equation (1).

  Here, Vout is an intended output voltage, and Vin is an input voltage. F is the oscillation frequency of the triangular wave, and L is the L value of the coil 3. C is the capacitance of the capacitor 4, and Re is the ESR (equivalent series resistance) of the capacitor 4.

  In the present embodiment, a function of preventing an overcurrent from flowing through the second switching element 2 even when an overvoltage is applied to the output terminal 35 is provided. In order to realize such a function, the instructing unit 6 controls the control unit 5 when the voltage at the output terminal 35 that outputs the output voltage Vout becomes a preset voltage value higher than the intended output voltage Vout. Is instructed to open the second switching element 2.

  Here, the output voltage Vout of the switching power supply 100 has a predetermined variation due to fluctuations in the input voltage Vin, fluctuations in the load, fluctuations in temperature, and the like even during normal operation. Further, there is a variation due to an initial variation of a setting resistor for setting the output voltage Vout. Due to such variations, the output voltage Vout varies by about ± several percent with respect to the center value. In such a variation, it is not necessary to open the second switching element 2. For this reason, the instruction | indication part 6 instruct | indicates to make the 2nd switching element 2 into an open state with respect to the control part 5, when it has shifted | deviated larger than such +/- several%. Specifically, it is preferable to set the output voltage Vout to about 1.5 times or twice the center value. Alternatively, it can be set to the same voltage value as the input voltage Vin.

  Here, the output voltage Vout of the switching power supply 100 is set by dividing the inter-terminal voltage of the capacitor 4 with a plurality of resistors. In the present embodiment, at least three resistors 71, 72, 73 are provided in series between the output terminal 35 corresponding to the terminals of the capacitor 4 and the ground terminal. These three resistors 71, 72, 73 set the output voltage of the switching power supply 100 to the desired output voltage Vout. In the present embodiment, the potential of the node to which the resistor 72 and the resistor 72 are connected is input to the error amplifier 54 as the feedback voltage FB1.

  The instruction unit 6 instructs the control unit 5 when the terminal voltage of the resistor 73 on the side close to the ground terminal among the at least three resistors 71, 72, 73 becomes a preset voltage. The preset voltage is the voltage of the output terminal 35 that serves as a trigger for opening the second switching element 2 as described above. This voltage can be set according to the resistance ratio of the respective resistance values of the resistor 72 and the resistor 73.

  In this embodiment, the voltage between the resistors 72 and 73 (hereinafter referred to as “feedback voltage FB2”) is input to the High-selector 63, and the output of the High-selector 63 (details will be described later) is output. The signal is input to the inverting terminal of the comparator 64. On the other hand, the reference voltage Vref from the reference voltage generation unit 51 is input to the non-inverting terminal of the comparator 64. Accordingly, the voltage across the resistor 73 increases at time t3 as shown in FIG. 3A, and the output Hi-sel of the High-selector 63, as shown at time t4 in FIG. That is, when the feedback voltage FB2 becomes larger than the reference voltage Vref, the output Comp2 of the comparator 64 becomes Low as shown in FIG. As a result, as shown in FIG. 3D, the output AND of the AND gate 58 also becomes Low, and the signal input to the gate terminal of the second switching element 2 becomes Low (see FIG. 3E). Accordingly, the second switching element 2 is opened. In the present embodiment, such resistors 72, 73 and 74 also constitute the instruction unit 6.

  The switching power supply 100 according to the present embodiment is configured to open the second switching element 2 according to the current flowing through the second switching element 2 or the voltage applied to the output terminal 35. Therefore, when at least one of the voltage between the terminals of the resistor 62 and the voltage between the terminals of the resistor 73 becomes larger than a preset voltage value, the second switching element 2 is opened. . Therefore, the feedback voltage FB3 corresponding to the voltage between the terminals of the resistor 62 and the feedback voltage FB2 corresponding to the voltage between the terminals of the resistor 73 are input to the High-selector 63, respectively. The High-selector 63 outputs the higher one of the input feedback voltage FB2 and feedback voltage FB3 to the inverting terminal of the comparator 64.

  Here, when the switching power supply 100 is operating normally, the output Hi-sel of the High-selector 63 is set to be smaller than the reference voltage Vref. As a result, the output Comp2 of the comparator 64 becomes High, and the AND gate 58 can change the signal level according to the output Comp1 of the comparator 56.

  On the other hand, when the current flowing through the second switching element 2 reaches a preset current value, or when the potential of the output terminal 35 becomes higher than the preset voltage value, the High-selector 63 The output Hi-sel is set to be larger than the reference voltage Vref. As a result, the output Comp2 of the comparator 64 becomes Low, and the output AND of the AND gate 58 becomes Low regardless of the output Comp1 of the comparator 56. Accordingly, the second switching element 2 is opened. In this way, the switching power supply 100 can prevent the second switching element 2 from being destroyed even if the output voltage Vout is abnormal.

[Other Embodiments]
In the said embodiment, the 1st switching element 1 and the 2nd switching element 2 demonstrated as n-type MOS-FET. However, the scope of application of the present invention is not limited to this. It is also possible to configure using an IGBT.

  In the embodiment described above, it is assumed that at least three resistors 71, 72, 73 for setting the output voltage Vout of the switching power supply 100 are provided. However, the scope of application of the present invention is not limited to this. Of course, it is possible to provide four or more.

  In the above embodiment, the instruction unit 6 including the third switching element 61 and the resistor 62 that instructs the control unit 5 to open the second switching element 2 in accordance with the current flowing through the second switching element 2. And an instruction unit 6 including resistors 71, 72, and 73 that instructs the control unit 5 to open the second switching element 2 in accordance with the voltage value of the output terminal 35. did. However, the scope of application of the present invention is not limited to this. Of course, the above-described instruction unit 6 may include at least one to constitute the switching power supply 100.

  That is, from the third switching element 61 and the resistor 62 instructing the control unit 5 to open the second switching element 2 according to the current flowing through the second switching element 2 as shown in FIG. It is possible to provide only the instruction unit 6 as shown in FIG. 5, and as shown in FIG. 5, the control unit 5 is configured to open the second switching element 2 according to the voltage value of the output terminal 35. It is also possible to provide an instruction unit 6 composed of resistors 71, 72, and 73 for instructing. In such a case, it is preferable that the outputs of the respective instruction units 6 are not input to the High-selector 63 but directly input to the inverting terminal of the comparator 64. Therefore, the High-selector 63 is not necessary. Even in such a configuration, it is naturally possible to open the second switching element 2 in an abnormal state.

  In the above embodiment, the control unit 5 has been described as performing PWM control. However, the scope of application of the present invention is not limited to this. Of course, the control unit 5 may be configured to perform PFM control.

  The present invention relates to a step-down synchronous rectification switching power supply.

1: First switching element 2: Second switching element 3: Coil 4: Capacitor 5: Control unit 6: Instruction unit 31: Power supply line 61: Third switching element 62: Resistor 71: Resistor 72: Resistor 73: Resistor 100: Synchronous rectification type switching power supply Vin: Input voltage Vout: Output voltage

Claims (4)

A first switching element connected in series to a power supply line to which an input voltage is applied;
A coil having one terminal connected to the output end of the first switching element;
A second switching element connected between an output terminal of the first switching element and a ground terminal;
A capacitor connected between the other terminal of the coil and a ground terminal;
A control unit for controlling an open / close state of the first switching element and the second switching element so that the input voltage becomes an intended output voltage;
An instruction unit that instructs the control unit to open the second switching element when a current flowing through the second switching element reaches a preset value;
Synchronous rectification type switching power supply.   The indicating unit includes a third switching element that forms a current mirror circuit with the second switching element, and a resistor connected in series between the third switching element and a ground terminal. The synchronous rectification type switching power supply according to claim 1, wherein the control unit is instructed when a voltage between terminals of the terminal becomes a preset voltage. A first switching element connected in series to a power supply line to which an input voltage is applied;
A coil having one terminal connected to the output end of the first switching element;
A second switching element connected between an output terminal of the first switching element and a ground terminal;
A capacitor connected between the other terminal of the coil and a ground terminal;
A control unit for controlling an open / close state of the first switching element and the second switching element so that the input voltage becomes an intended output voltage;
Instructing the control unit to open the second switching element when the voltage of the output terminal that outputs the output voltage becomes a preset voltage value higher than the intended output voltage. An instruction unit to
Synchronous rectification type switching power supply. At least three resistors for setting the output voltage to the desired output voltage are provided in series between the output terminal and a ground terminal;
4. The synchronization according to claim 3, wherein the instruction unit instructs the control unit when a voltage between terminals of a resistor closer to a ground terminal among the at least three resistors becomes a preset voltage. 5. Rectification switching power supply.

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