JP2006042576A - Dc-dc converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DC-DC converter that detects an overvoltage state, quickly reduces an output voltage for returning to normal control, and prevents the occurrence of undershoot. <P>SOLUTION: The DC-DC converter comprises a control circuit 100 for alternately turning on and off a main switching element 1, a rectifying switching element 2, an inductor 4, a smoothing means 5, and a main switching element 1 for controlling an output voltage and the rectifying switching element 2 by a prescribed on/off time ratio; and an overvoltage protection circuit 50a for detecting the overvoltage state of the output voltage Vout, and connecting a resistor circuit 3 having a prescribed resistance value to the switching element 2 for rectification in parallel when the overvoltage state is detected. Discharging is made via the resistance circuit 3 when an overvoltage is detected, thus gently reducing the output voltage for suppressing the occurrence of undershoot. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a switching DC-DC converter used in various electronic devices, and more particularly to a DC-DC converter having an overvoltage protection circuit.

  In a DC power supply, an overvoltage protection circuit is usually required in order to obtain a stable output voltage. Conventionally, when an output overvoltage state is detected, the operation of the converter is stopped until the converter is released from the overvoltage state. However, in this method, the decrease in the output voltage depends only on the discharge to the load, so that the overvoltage state is maintained at a light load. For this reason, the circuit of Patent Document 1 has been proposed.

  The overvoltage protection circuit of Patent Document 1 will be described with reference to FIG. The power supply circuit shown in FIG. 7 includes a main switching element 1 composed of a P-channel MOSFET and a rectifying switching element 2 composed of an N-channel MOSFET, an inductor 4, a smoothing capacitor 5, and an overvoltage connected in series between a power supply Vin and ground. A protection circuit 50 and a control circuit 102 are included.

  Main switching element 1 and rectifying switching element 2 and inductor 4 constitute a step-down converter, and voltage Vout is output through smoothing capacitor 5. In the overvoltage protection circuit 50, the overvoltage detection comparator 6 compares the output voltage Vout with a predetermined reference voltage Vref1, and outputs a High signal when the output voltage Vout is greater than the reference voltage Vref1. The comparator 7 compares the output voltage Vout with a predetermined reference voltage Vref2, and outputs a High signal when the output voltage Vout is greater than the reference voltage Vref2. The control circuit 102 outputs a drive signal to the main switching element 1 and a drive signal to the rectifying switching element 2 based on the output of the comparator 7. The OR circuit 13 of the protection circuit 50 inputs the output of the overvoltage detection comparator 6 and the drive signal output from the control circuit 102 and outputs the drive signal to the rectifying switching element 2.

  Since the output signal of the overvoltage detection comparator 6 is in a low state when the output voltage Vout is equal to or lower than the reference voltage Vref1, the operation of the rectifying switching element 2 follows only the signal output of the control circuit 102. That is, the main switching element 1 and the rectifying switching element 2 are turned on and off alternately. At this time, the pulse width of the drive signal to the main switching element 1 is modulated so that the output voltage Vout is substantially equal to the second reference voltage Vref2.

On the other hand, when the output voltage Vout rises to the reference voltage Vref1 or more due to a sudden change in the output current or the like, the overvoltage detection comparator 6 outputs a High signal to the rectifying switching element 2 via the OR circuit 13, and the rectifying switching element 2 Turn on the. As a result, the output terminal is short-circuited to the ground via the inductor 4, and the output voltage Vout is forcibly discharged and lowered. In this way, the output overvoltage state is released.
JP-A-11-187651

  FIG. 8 is an operation waveform diagram of each part when the output current of the conventional DC-DC converter suddenly decreases. FIG. 8A shows the output current Iout and the current IL of the inductor 4, FIG. 8B shows the output voltage Vout, and FIG. Indicates the output V6 of the overvoltage detection comparator 6, and (d) indicates the drive signal Vg2 of the rectifying switching element 2.

  In the conventional overvoltage protection circuit, as shown in the operation waveform diagram of FIG. 8, the rectifying switching element 2 is forcibly turned on during the period of detecting the overvoltage state, so that the smoothing capacitor 5 rapidly turns on. The current is discharged. For this reason, when the overvoltage state is released, the discharge current from the smoothing capacitor 5 becomes excessive, which may cause undershoot.

  An object of the present invention is to provide a DC-DC converter that detects an overvoltage state, quickly reduces the output voltage Vout to return to normal control, and prevents the occurrence of undershoot.

  The DC-DC converter according to the present invention is a DC-DC converter that converts an input DC voltage into a predetermined output voltage. A DC-DC converter includes a main switching element and a rectifying switching element arranged in series between a DC power source and a ground, an inductor having one end connected to a connection point between the main switching element and the rectifying switching element, and an inductor Smoothing means connected to the other end of the output, a control circuit for alternately turning on and off the main switching element and the rectifying switching element at a predetermined on / off time ratio in order to control the output voltage, and an overvoltage state of the output voltage And an overvoltage protection circuit for connecting a resistance circuit having a predetermined resistance value in parallel with the rectifying switching element when an overvoltage state is detected.

  The overvoltage protection circuit may include a comparison circuit that compares the output voltage with a predetermined value and outputs a signal indicating the comparison result. At this time, the resistance circuit is turned on / off according to the output signal from the comparison circuit, and gives a predetermined resistance value when turned on. During normal operation in which the output voltage is smaller than a predetermined value, the rectifying switching element and the resistance circuit may be turned on / off simultaneously. The resistance circuit may include a MOSFET having the same channel as the rectifying switching element.

  While the overvoltage state of the output voltage is detected, both the main switching element and the rectifying switching element may be turned off.

  Alternatively, the main switching element is turned off while the overvoltage state of the output voltage is detected, and the rectifying switching element is turned off after being turned on for a certain period from the start of detection of the overvoltage state. May be.

  The predetermined resistance value in the overvoltage protection circuit is preferably larger than the on-resistance value of the rectifying switching element.

  According to the present invention, undershoot can be prevented when an overvoltage is detected and the output voltage Vout is quickly reduced to return to normal control.

  Hereinafter, embodiments of a DC-DC converter according to the present invention will be described with reference to the accompanying drawings.

(First embodiment)
FIG. 1A is a circuit configuration diagram of a DC-DC converter according to the first embodiment of the present invention. The DC-DC converter of the present embodiment is a step-down converter that steps down an input voltage Vin to a predetermined voltage and outputs it as an output voltage Vout. The DC-DC converter includes a main switching element 1 composed of a P-channel MOSFET, a switching element (hereinafter referred to as “rectifying switching element”) 2 composed of an N-channel MOSFET, an inductor 4, a smoothing capacitor 5, A control circuit 100, a protection circuit 50a, and a comparator 7 are included. The output voltage Vout of the DC-DC converter is taken out from the connection point between the inductor 4 and the smoothing capacitor 5.

  The main switching element 1 and the rectifying switching element 2 are connected in series, the source terminal of the main switching element 1 is connected to the input voltage Vin, and the source terminal of the rectifying switching element 2 is grounded. A drive signal from the control circuit 100 is input to the gate terminals of the main switching element 1 and the rectifying switching element 2.

  One end of the inductor 4 is connected to the connection point of the main switching element 1 and the rectifying switching element 2 (that is, the drain terminals of both switching elements 1 and 2). The smoothing capacitor 5 is connected to the other end of the inductor 4 and the opposite side is grounded.

  The output voltage Vout is input to the non-inverting input terminal of the comparator 7, and the reference voltage Vref2 output from the reference voltage source 9 is input to the inverting input terminal. The output of the comparator 7 is input to the control circuit 100. The control circuit 100 includes a first drive signal Vg1 which is a drive signal for the main switching element 1 and a second drive signal Vg2 which is a drive signal for the rectifying switching element 2 based on the output of the protection circuit 50a and the output of the comparator 7. Is output.

  The protection circuit 50a detects an overvoltage state based on the output voltage Vout, and performs a protection operation when the overvoltage state is detected. Therefore, the protection circuit 50 a includes the resistance circuit 3, the overvoltage detection comparator 6, and the OR circuit 11.

  The output voltage Vout is input to the non-inverting input terminal of the overvoltage detection comparator 6, and the reference voltage Vref <b> 1 serving as a determination criterion for the overvoltage state output from the reference voltage source 8 is input to the inverting input terminal. The output of the overvoltage detection comparator 6 is input to the control circuit 100 and the OR circuit 11. The OR circuit 11 inputs the output signal of the overvoltage detection comparator 6 and the second drive signal Vg2 from the control circuit 100, performs a logical OR operation, and outputs the result to the resistor circuit 3 as the control signal Vg3. .

  The resistance circuit 3 has a control terminal 3a, receives a signal Vg3 from the OR circuit 11 through the control terminal 3a, and performs connection / disconnection operation of the resistance element in parallel with the rectifying switching element 2 according to the signal Vg3. In the present embodiment, the resistance circuit 3 is composed of an N-channel MOSFET as shown in FIG. 1B, and its on-resistance is used as a resistance element connected in parallel to the rectifying switching element 2. The drain terminal of the N-channel MOSFET is connected to the connection point between the main switching element 1 and the rectifying switching element 2, and the source terminal is grounded. The N-channel MOSFET is turned on when the gate potential becomes High, and the on-resistance at that time is larger than that of the rectifying switching element 2. For example, when the on-resistance of the rectifying switching element 2 is 1Ω, the on-resistance of the resistor circuit 3 is about 100Ω. The resistor circuit 3 may be configured as a part of the rectifying switching element 2 only in that the gate is different. Even if configured in this way, there is no influence on the normal operation.

  The operation of the DC-DC converter of the present embodiment configured as described above will be described. First, an operation during normal operation in which the output voltage Vout is equal to or lower than the reference voltage Vref1 will be described.

  During normal operation, the output signal V6 of the overvoltage detection comparator 6 is low. Therefore, the control circuit 100 generates and outputs the drive signal Vg1 and the drive signal Vg2 to the main switching element 1 and the rectifying switching element 2 based only on the output signal of the comparator 7. That is, the main switching element 1 and the rectifying switching element 2 are alternately turned on and off. At this time, the pulse width of the drive signal to the main switching element 1 is modulated so that the output voltage Vout is substantially equal to the second reference voltage Vref2. Further, since the drive signal Vg <b> 2 to the rectifying switching element 2 is input to the control terminal 3 a of the resistance circuit 3 through the OR circuit 11, the resistance circuit 3 performs an on / off operation simultaneously with the rectifying switching element 2. As described above, the on-resistance of the resistance circuit 3 is set larger than the on-resistance of the rectifying switching element 2. The resistance circuit 3 may be configured as a part of the rectifying switching element 2 only in the gate but having no influence on normal operation.

  Next, the operation when an overvoltage state is detected will be described. When the output voltage Vout increases due to a sudden change in the output current Iout and becomes equal to or higher than the reference voltage Vref1, it is detected as an overvoltage state, and the overvoltage detection comparator 6 causes the output voltage V6 to become High, and the OR circuit 11 outputs a high signal. When this signal is input, the control circuit 100 controls the operations of the main switching element 1 and the rectifying switching element 2 to be turned off. When the output V6 of the overvoltage detection comparator 6 is input to the control terminal 3a of the resistance circuit 3 via the OR circuit 11, the resistance circuit 3 is turned on, and the resistance element (ON resistance) is connected in parallel to the rectifying switching element 2. ) Are connected to form a discharge path. As a result, a discharge current flows from the smoothing capacitor 5 through the inductor 4 and the resistance circuit 3 to reduce the output voltage Vout. At this time, since the resistance value of the resistance element (ON resistance) of the resistance circuit 3 is larger than the resistance value of the rectifying switching element 2, the discharge current flows gently, and the sudden decrease in the output voltage Vout can be prevented. , Undershoot can be prevented.

  FIG. 2 is a diagram showing operation waveforms of each part of the DC-DC converter of the present embodiment. This figure shows waveforms from the time of normal operation until an overvoltage state is detected and the output voltage Vout is reduced to return to normal operation.

  2, (a) is the output current Iout and the current IL of the inductor 4, (b) is the output voltage Vout, (c) is the output voltage V6 of the overvoltage detection comparator 6, and (d) is for the main switching element 1. The drive signals Vg1 and (e) indicate the drive signal Vg2 for the rectifying switching element 2, and (f) indicates the control signal Vg3 for the resistor circuit 3, respectively.

  When the output current Iout suddenly changes from a heavy load to a light load, the output voltage Vout increases as the charging current to the smoothing capacitor 5 becomes excessive. When the output voltage Vout exceeds the reference voltage Vref1, an overvoltage state is detected, and the output V6 of the overvoltage detection comparator 6 becomes High. During this overvoltage state, the drive signal Vg1 for the main switching element 1 is high, the drive signal Vg2 for the rectifying switching element 2 is low, and both the main switching element 1 and the rectifying switching element 2 are in the off state. On the other hand, the control signal Vg3 to the resistance circuit 3 becomes High while the overvoltage state is continuously detected, the resistance circuit 3 is turned on, and the resistance element (on resistance) is connected to the rectifying switching element 2 in parallel. Therefore, when the smoothing capacitor 5 is discharged by the resistor circuit 3, the output voltage Vout in the overvoltage state decreases. Since the resistance value of the resistance element (ON resistance) of the resistance circuit 3 is not smaller than the resistance value of the rectifying switching element 2, the discharge current from the smoothing capacitor 5 by the resistance circuit 3 is not excessive and suppresses the occurrence of undershoot. It returns to normal operation smoothly.

(Second Embodiment)
Another configuration example of the DC-DC converter according to the present invention will be described. FIG. 3 shows a circuit configuration diagram of a DC-DC converter according to the second embodiment of the present invention. This embodiment differs from the first embodiment in the following points. That is, the DC-DC converter of the present embodiment further includes an OR circuit 12 and a one-shot pulse generation circuit 14, and the output V 6 of the overvoltage detection comparator 6 together with the control circuit 100 and the OR circuit 11 is a one-shot pulse generation circuit. 14 is input. The output of the one-shot pulse generation circuit 14 and the second drive signal Vg2 for the rectifying switching element 2 output from the control circuit 100 are input to the OR circuit 12, and the output of the OR circuit 12 is the drive signal to the rectifying switching element 2. Vg20.

  In the DC-DC converter according to the present embodiment, the operation during the normal operation in which the output voltage Vout is equal to or lower than the reference voltage Vref1 is the same as that in the first embodiment, so that the description is omitted. An operation in an overvoltage state in which Vout increases and becomes equal to or higher than the reference voltage Vref1 will be described below.

  In the overvoltage state, the High signal is input from the overvoltage detection comparator 6 to the control circuit 100, the OR circuit 11, and the one-shot pulse generation circuit 14. At this time, the control circuit 100 sets the first drive signal Vg1 to High and the second drive signal Vg20 to Low so that both the main switching element 1 and the rectifying switching element 2 are turned off. However, in the present embodiment, the one-shot pulse generation circuit 14 generates a pulse having a predetermined pulse width in synchronization with the rising timing of the output V6 of the overvoltage detection comparator 6, and rectifies it via the OR circuit 12. Output to the gate terminal of the switching element 2. Therefore, during a certain period after the overvoltage state is detected (a period corresponding to the pulse width of the output pulse of the one-shot pulse generation circuit 14), the main switching element 1 is in the off state and the rectifying switching element 2 is in the on state. On the other hand, the output of the overvoltage detection comparator 6 is input to the control terminal of the resistor circuit 3 via the OR circuit 11, and the resistor circuit 3 is turned on during the overvoltage state.

  As described above, in this embodiment, both the rectifying switching element 2 and the resistance circuit 3 are turned on for a certain period from the detection of the overvoltage, and two discharge paths are formed. After a certain period of time, the rectifying switching element 2 is turned off, only the resistance circuit 3 is turned on, and the discharge path becomes one system. For this reason, a certain period from the detection of the overvoltage rapidly escapes from the overvoltage state via the two discharge paths, and after that, when the certain period elapses, the discharge is alleviated by the discharge path of only the resistance circuit 3, and normal control Realize a gradual return to

  FIG. 4 is an operation waveform diagram of each part of the DC-DC converter of this embodiment. This figure shows a waveform from the time of normal operation until an overvoltage state is detected and the output voltage Vout is lowered to return to normal operation.

  4, (a) is the output current Iout and the current IL of the inductor 4, (b) is the output voltage Vout, (c) is the output voltage V6 of the overvoltage detection comparator 6, and (d) is the one-shot pulse generation circuit. 14 is an output voltage Vp, (e) is a drive signal Vg1 from the control circuit 100 to the main switching element 1, (f) is a drive signal Vg20 from the control circuit 100 to the rectifying switching element 2, and (g) is an OR circuit. Drive signals Vg2 and (h) from 12 to the rectifying switching element 2 indicate waveforms of the gate voltage Vg3 of the resistor circuit 3, respectively.

  When the output current Iout suddenly changes from a heavy load to a light load, the output voltage Vout increases as the charging current to the smoothing capacitor 5 becomes excessive. When the output voltage Vout exceeds the reference voltage Vref1, an overvoltage state is detected, and the output V6 of the overvoltage detection comparator 6 becomes High. The drive signal Vg1 of the main switching element 1 is forcibly set to Low and turned off. On the other hand, the drive signal Vg2 for the rectifying switching element 2 is forcibly set to High only for a certain period by the output Vp from the one-shot pulse generation circuit 14, so that the rectifying switching element 2 is turned on for a certain period. Further, while the overvoltage state continues to be detected, the control signal Vg3 of the resistance circuit 3 becomes High, and the resistance circuit 3 is turned on.

  Therefore, during a certain period from the start of the overvoltage state detection, the smoothing capacitor 5 is discharged by the rectifying switching element 2 and the resistance circuit 3, so that the output voltage Vout rapidly decreases. After being turned off, the output voltage Vout gradually decreases due to discharge only through the resistance circuit 3. When the output voltage Vout is lower than the first reference voltage Vref1, the on-resistance of the resistor circuit 3 is not as small as that of the rectifying switching element 2, so that the discharge current from the smoothing capacitor 5 by the resistor circuit 3 is not excessive and undershoot. Smoothly returns to normal operation while suppressing the occurrence of noise.

  In the above description, the fixed period in which the rectifying switching element 2 is forcibly turned on after the start of overvoltage detection is set by the output from the one-shot pulse generation circuit 14. The setting method of can also be considered. For example, a reference voltage Vref3 higher than the reference voltage Vref1 is set, and as an overvoltage detection level, a first level in which the output voltage Vout exceeds the reference voltage Vref1 and becomes equal to or lower than the reference voltage Vref3, and a second level exceeding the reference voltage Vref3 These two levels are set so that the resistor circuit 3 is turned on in the first level overvoltage state, and the rectifying switching element 2 is turned on together with the resistor circuit 3 in the second level overvoltage state. May be. Alternatively, the current of the rectifying switching element 2 is detected, and in the overvoltage state, the rectifying switching element 2 is turned on together with the resistance circuit 3 only until the current of the rectifying switching element 2 falls below a predetermined value. It may be. The predetermined value is preferably a value equal to or less than zero, which is the discharge current from the smoothing capacitor 5.

(Third embodiment)
Although the second embodiment has been described using a step-down DC-DC converter, the idea of the present invention can also be applied to a step-up DC-DC converter. In the following, an example of application to a boost converter will be described.

  FIG. 5A is a diagram showing a configuration example when the technical idea of the present invention is applied to a step-up DC-DC converter. The DC-DC converter of this embodiment is a boost converter that boosts an input voltage Vin to a predetermined voltage and outputs the boosted voltage as an output voltage Vout. The DC-DC converter includes a main switching element 21 composed of an N-channel MOSFET, a switching element (rectifying switching element) 22 composed of a P-channel MOSFET, a inductor 20, a smoothing capacitor 24, a control circuit 101, A protection circuit 50c and a comparator 27 are included. The output voltage Vout of the DC-DC converter is taken out from the smoothing capacitor 24.

  Drive signals Vg21 and Vg22 from the control circuit 101 are input to the gate terminals of the main switching element 21 and the rectifying switching element 22.

  The protection circuit 50 c includes a resistance circuit 23, an overvoltage detection comparator 26, NOR circuits 31 and 32, and a one-shot pulse generation circuit 34. In the overvoltage detection comparator 26, the output voltage Vout is input to the non-inverting input terminal, and the reference voltage Vref1 is input to the inverting input terminal. The output of the overvoltage detection comparator 26 is input to the control circuit 101, the NOR circuit 32, and the one-shot pulse generation circuit 34. The output of the NOR circuit 32 is input to the control terminal 23a of the resistance circuit 23, and operates as a resistance element when the potential of the control terminal 23a becomes Low. In the present embodiment, the resistance circuit 23 is composed of a P-channel MOSFET as shown in FIG. 5B and is connected in parallel with the rectifying switching element 22. The resistance element of the resistance circuit 23 uses the on-resistance of the P-channel MOSFET. The resistance value when the resistance circuit 23 is on is set to be larger than the on-resistance value of the rectifying switching element 22. Note that the resistor circuit 23 may be configured as a part of the rectifying switching element 22 only in the gate, and even if configured in this way, there is no influence on normal operation.

  In the comparator 27, the output voltage Vout is input to the non-inverting input terminal, and the reference voltage Vref2 output from the reference voltage source 29 is input to the inverting input terminal. The output of the comparator 27 is input to the control circuit 101. Based on the outputs of the overvoltage detection comparator 26 and the comparator 27, the control circuit 101 has a first drive signal Vg21 which is a drive signal for the main switching element 21 and a second drive signal which is a drive signal for the rectifying switching element 22. Vg220 is output. The output Vp of the one-shot pulse generation circuit 34 and the second signal Vg220 output from the control circuit 101 are input to the NOR circuit 31, and the output of the NOR circuit 31 becomes the drive signal Vg22 to the rectifying switching element 22. The NOR circuit 32 receives the output signal Vp of the overvoltage detection comparator 26 and the second drive signal Vg 220 from the control circuit 101. The output signal Vg23 of the NOR circuit 32 is input to the control terminal 23a of the resistance circuit 23.

The operation of the DC-DC converter configured as described above will be described.
First, an operation during normal operation in which the output voltage Vout is equal to or lower than the reference voltage Vref1 will be described. In the normal operation, the output signal V26 of the overvoltage detection comparator 26 is Low. Therefore, the control circuit 101 outputs drive signals Vg21 and Vg220 to the main switching element 21 and the rectifying switching element 22 based only on the output signal of the comparator 27. That is, the main switching element 21 and the rectifying switching element 22 are alternately turned on and off. At this time, the pulse width of the drive signal to the main switching element 21 is modulated so that the output voltage Vout is substantially equal to the second reference voltage Vref2. Further, since the drive signal Vg 220 to the rectifying switching element 22 is also input to the control terminal 23 a of the resistance circuit 23 via the NOR circuit 32, the resistance circuit 23 performs an on / off operation simultaneously with the rectifying switching element 22.

Next, the operation in the overvoltage state of the DC-DC converter of this embodiment will be described.
When the output voltage Vout rises to the reference voltage Vref1 or more due to a sudden change in the output current or the like, an overvoltage state is detected, and a high signal is sent from the overvoltage detection comparator 26 to the control circuit 101, the NOR circuit 32, and the one-shot pulse generation circuit 34. Is input. At this time, the control circuit 101 controls the first signal Vg21 to be Low, the second signal Vg220 to be High, and both the switching element 21 and the rectifying switching element 22 to be turned off. However, with respect to the rectifying switching element 22, a pulse having a predetermined pulse width is generated by the one-shot pulse generation circuit 34 and is input to the gate terminal of the rectifying switching element 22 via the NOR circuit 31. Therefore, the main switching element 21 is in the off state and the rectifying switching element 22 is in the on state for a certain period after the start of the overvoltage state detection. On the other hand, since the output of the overvoltage detection comparator 26 is input to the control terminal 23a of the resistance circuit 23 via the NOR circuit 32, the resistance circuit 23 is turned on during detection of the overvoltage state, and the resistance component is rectified. A discharge path is formed by being connected in parallel to the switching element 22.

  Therefore, since a discharge path is formed by the rectifying switching element 22 and the resistance circuit 23 within a certain period from the start of overvoltage detection, it is possible to quickly escape from the overvoltage state. Then, after the elapse of a certain period from the start of overvoltage detection, only the resistance circuit 23 is turned on, and only the resistance circuit 23 having a large resistance value becomes a discharge path, so that a gentle return to normal control is realized.

  FIG. 6 is an operation waveform diagram of each part of the step-up DC-DC converter of this embodiment. This figure shows waveforms from the time of normal operation until an overvoltage state is detected and the output voltage Vout is reduced to return to normal operation.

  6, (a) is the output current Iout and the current IL of the inductor 20, (b) is the output voltage Vout, (c) is the output voltage V26 of the overvoltage detection comparator 26, and (d) is the one-shot pulse generation circuit. The output voltage Vp of 34, (e) shows the drive signal Vg22 of the rectifying switching element 22, (f) shows the drive signal Vg21 of the main switching element 21, and (g) shows the waveform of the control signal Vg23 of the resistance circuit 23, respectively.

  When the output current Iout suddenly changes from a heavy load to a light load, the output voltage Vout rises because the charging current to the smoothing capacitor 24 becomes excessive. When the output voltage Vout exceeds the reference voltage Vref1, an overvoltage state is detected, and the output V26 of the overvoltage detection comparator 26 becomes High. The gate voltage Vg21 of the switching element 21 is forcibly set to Low and turned off. At the same time, the gate voltage Vg23 of the resistance element 23 becomes High while the overvoltage state is continuously detected, and the resistance element 23 is turned on. At this time, the drive signal Vg22 for the rectifying switching element 22 becomes Low for a certain period from the start of detection of the overvoltage state by the output Vp from the one-shot pulse generation circuit 34. As a result, the resistance circuit 23 is turned on for a certain period from the start of detection of the overvoltage state. Therefore, the smoothing capacitor 24 is discharged by the two current paths of the rectifying switching element 22 and the resistance circuit 23, so that the output voltage Vout rapidly decreases. After the rectification switching element 22 is turned off after a predetermined period has elapsed from the start of detection of the overvoltage state, the output voltage Vout gradually decreases only by the resistance element 23. When the output voltage Vout is lower than the first reference voltage Vref1, the on-resistance of the resistance element 23 is not as small as that of the rectifying switching element 22. Therefore, the discharge current from the smoothing capacitor 24 by the resistance element 23 is not excessive and undershoot. Smoothly returns to normal operation while suppressing the occurrence of

  The resistor circuits 3 and 23 that form the discharge path having a predetermined resistance value when turned on, which are described in the above embodiments, are configured by N-channel or P-channel MOSFETs. However, as long as similar functions can be realized. Other circuit configurations may be used.

  The overvoltage protection circuit of the present invention can detect an overvoltage state of an output voltage of various electronic circuits such as a DC-DC converter, and can quickly return to the normal control state from the overvoltage state. -Useful for DC converters.

1 is a configuration diagram of a DC-DC converter according to Embodiment 1 of the present invention. Operation Waveform Diagram of Each Part of DC-DC Converter of Embodiment 1 Configuration diagram of DC-DC converter in Embodiment 2 of the present invention Operation Waveform Diagram of Each Part of DC-DC Converter of Embodiment 2 Configuration diagram of DC-DC converter in Embodiment 3 of the present invention Operation Waveform Diagram of Each Part of DC-DC Converter of Embodiment 3 Circuit diagram of conventional DC-DC converter Operation waveform diagram of each part of conventional DC-DC converter

Explanation of symbols

1,22 P-channel MOSFET
2,21 N-channel MOSFET
3,23 Resistor circuit 4,20 Inductor 5,24 Smoothing capacitor 6,26 Overvoltage detection comparator 7,27 Comparator 8,28 Reference voltage source Vref1
9,29 Reference voltage source Vref2
11-13 OR circuit 14, 34 One-shot pulse generation circuit 31, 32 NOR circuit 50a-50c Overvoltage protection circuit 100-102 Control circuit

Claims (7)

A DC-DC converter that converts an input DC voltage into a predetermined output voltage,
A main switching element and a rectifying switching element arranged in series between the DC power source and the ground;
An inductor having one end connected to a connection point between the main switching element and the rectifying switching element;
Smoothing means connected to the other end of the inductor;
A control circuit for alternately turning on and off the main switching element and the rectifying switching element at a predetermined on / off time ratio in order to control the output voltage;
An overvoltage protection circuit for detecting an overvoltage state of the output voltage and connecting a resistance circuit having a predetermined resistance value in parallel with the rectifying switching element when the overvoltage state is detected; DC converter.   The overvoltage protection circuit includes a comparison circuit that compares the output voltage with a predetermined value and outputs a signal indicating the comparison result, and the resistance circuit is turned on / off in response to an output signal from the comparison circuit. 2. The DC-DC converter according to claim 1, wherein a predetermined resistance value is given when the DC-DC converter is operated.   3. The DC-DC converter according to claim 2, wherein the rectifying switching element and the resistance circuit are simultaneously turned on and off during a normal operation in which the output voltage is smaller than a predetermined value.   3. The DC-DC converter according to claim 2, wherein the resistance circuit includes a MOSFET having the same channel as that of the rectifying switching element.   The DC-DC converter according to claim 1, wherein both the main switching element and the rectifying switching element are turned off while an overvoltage state of the output voltage is detected.   While the overvoltage state of the output voltage is detected, the main switching element is turned off, and the rectifying switching element is turned off after being turned on for a certain period from the start of detection of the overvoltage state. The DC-DC converter according to claim 1, wherein: 2. The DC-DC converter according to claim 1, wherein the predetermined resistance value is larger than an on-resistance value of the rectifying switching element.

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