JP2020202677A - Switching power supply - Google Patents

Abstract

To provide a switching power supply that can detect reverse currents of different magnitudes during a predetermined period from the start-up and a period after the predetermined period, and can suppress voltage vibration at the start-up of the switching power supply.SOLUTION: A switching power supply 1 that converts a voltage according to a synchronous rectification method includes a first transistor MP, a second transistor MN that is connected in series with the first transistor and is located on the lower voltage side than the first transistor, a control circuit 10 that controls the on/off of the first transistor and the second transistor, and a detection circuit 30 that controls the second transistor to turn off when the magnitude of a reverse current flowing through the second transistor is greater than or equal to a threshold value. The detection circuit adjust such that a first threshold which is a threshold in a first period after the switching power supply is turned on is smaller than a second threshold which is a threshold in a second period following the first period.SELECTED DRAWING: Figure 2

Description

The present disclosure relates to a switching power supply.

Conventionally, a synchronous rectification type DC-DC converter is known. This DC-DC converter has a first transistor and a second transistor arranged on a lower voltage side than the first transistor. In this DC-DC converter, when the output voltage is not so high (for example, at startup), when the first transistor is turned on and the second transistor is turned off, a current is applied to the smoothing coil on the output side. It flows and is charged to the smoothing capacitor on the output side, and the output voltage rises. Then, at the timing when the first transistor is turned off and the second transistor is turned on, a current flows through the smoothing coil, but the current stops, and a current flows from the smoothing capacitor to the second transistor via the smoothing coil. May start to flow. That is, a discharge from the smoothing capacitor to the second transistor may occur, and a reverse current may occur. In this case, since the voltage of the smoothing coil is lowered, the charging voltage is lowered and the output voltage is lowered. If this is repeated, when the DC-DC converter is started up, the voltage fluctuates (vibrates) up and down, and the voltage conversion efficiency at the time of starting up becomes insufficient.

In FIGS. 1, 10, and 13 of Patent Document 1, a method of detecting the reverse current of the low-side transistor T2 (commutation MOS transistor, corresponding to the above-mentioned second transistor) of synchronous rectification is described. When a comparator is connected between the drain and the source of the transistor T2 and a reverse current flows through the transistor T2, the drain voltage becomes higher than the source voltage of the transistor T2, so that the output of the comparator is inverted and the reverse current is detected. When the reverse current is detected, the transistor T2 is turned off.

In Patent Document 2, the synchronous rectification transistor on the low side is turned off at the time of startup. Therefore, a diode for commutation (flywheel diode) is inserted in order to make a path of the smoothing coil in response to the case where the synchronous rectification transistor is turned off and the main transistor is turned off.

JP-A-2008-125223 Japanese Unexamined Patent Publication No. 11-22874

In Patent Document 1, since a comparator is used for reverse current detection, the circuit scale becomes large. In addition, the reverse threshold voltage of the comparator cannot be set, the magnitude of the reverse current cannot be detected, and the criteria for determining the reverse current are the same at each timing after startup. It may not be appropriate.

In Patent Document 2, when the withstand voltage and current withstand of a power transistor as a diode for commutation is insufficient, the diode for commutation may be destroyed when a large current flows. Moreover, since the diode for commutation is used exclusively at the time of starting, the number of parts increases.

The present disclosure provides a switching power supply capable of suppressing an increase in the number of parts, detecting a reverse current having a different magnitude between a predetermined period and a period after a predetermined period from the start-up, and suppressing voltage vibration at the start-up of the switching power supply. provide.

One aspect of the present disclosure is a switching power supply that converts voltage according to a synchronous rectification method, which is connected in series with a first transistor and the first transistor and is arranged on a lower voltage side than the first transistor. When the magnitude of the reverse current flowing through the second transistor, the control circuit for controlling the on / off of the first transistor and the second transistor, and the reverse current flowing through the second transistor is equal to or larger than the threshold value, the second transistor is used. The detection circuit includes a detection circuit that controls off the transistor of the switching power supply, and the first threshold value, which is the threshold value in the first period after the switching power supply is turned on, follows the first period. It is a switching power supply that is adjusted to be smaller than the second threshold value, which is the threshold value in the second period.

According to the present disclosure, it is possible to cope with a reverse current having a different magnitude between a predetermined period from the start-up and a period after the predetermined period, and it is possible to suppress voltage vibration at the start-up of a switching power supply.

Configuration diagram showing an example of a switching power supply in the embodiment Circuit diagram showing the first configuration example of the reverse current detection circuit of the switching power supply Circuit diagram showing a configuration example of a soft start circuit for a switching power supply Circuit diagram showing a second configuration example of a reverse current detection circuit for a switching power supply The figure which shows the change example of the output voltage of the switching power supply at the time of transition from a soft start period to a normal period in the case where an overcurrent detection circuit does not provide a threshold adjustment circuit. The figure which shows the change example of the inverting input voltage of a comparator at the time of transition from a soft start period to a normal period in the case where an overcurrent detection circuit does not provide a threshold adjustment circuit. The figure which shows the change example of the output voltage at the time of transition from a soft start period to a normal period in the case where an overcurrent detection circuit includes a threshold adjustment circuit. The figure which shows the change example of the inverting input voltage of a comparator at the time of transition from a soft start period to a normal period when an overcurrent detection circuit includes a threshold adjustment circuit.

Hereinafter, an embodiment in which the switching power supply according to the present disclosure is specifically disclosed (hereinafter, referred to as “the present embodiment”) will be described in detail with reference to the drawings as appropriate. However, more detailed explanation than necessary may be omitted. For example, detailed explanations of already well-known matters and duplicate explanations for substantially the same configuration may be omitted. This is to avoid unnecessary redundancy of the following description and to facilitate the understanding of those skilled in the art. It should be noted that the accompanying drawings and the following description are provided for those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims.

FIG. 1 is a configuration diagram showing an example of a switching power supply according to the embodiment of the present disclosure. FIG. 2 is a circuit diagram showing a first configuration example of a reverse current detection circuit for a switching power supply.

The switching power supply 1 is a step-down switching power supply in which the coil L1 and the capacitor C _OUT are externally connected to the power supply IC5. In the step-down switching power supply 1, the coil L1 and the capacitor C _OUT are connected in series with the terminal LX. A resistor R _ESR is connected between the coil L1 and the capacitor C _OUT . A load 3 is connected to the coil L1. Resistors R _B1 and R _B2 are connected in series to the coil L1 in parallel with the load 3. The connection point between the resistor R _B1 and the resistor R _B2 is connected to the PWM circuit 10 via the feedback terminal FB of the power supply IC 5.

In the switching power supply 1, the input voltage _{V IN} from the DC power supply 2 is a power supply terminal _V P of power IC5 via a power line _LN, are supplied to the _{V PW.} A bypass capacitor C _IN is connected to the power line LN in parallel with the power supply IC 5. The power supply terminal Vp is connected to a regulator, and the regulator generates a power supply voltage Vreg.

The switching power supply 1 converts a voltage according to a synchronous rectification method. The power supply IC 5 of the switching power supply 1 includes, for example, a transistor MP, a transistor MN, a transistor M _SNS , a resistor R _SNS , a PWM (Pulse Width Modulation) circuit 10, a logic circuit 11, and a Pch driver 12. The Nch driver 13, the soft start circuit 20, and the reverse current detection circuit 30 are provided. The power supply IC 5 is an example of an integrated circuit. The transistor M _SNS and the resistor R _SNS may be included in the reverse current detection circuit 30.

The transistor MP is, for example, a P-channel MOSFET (NMR) and is arranged on the high voltage side (high side side). The transistor MN is, for example, an N-channel MOSFET (NMOS), is connected in series with the transistor MP, and is arranged on the lower voltage side (low side side) than the transistor MP.

The PWM circuit 10 acquires a duty ratio for converting a voltage, generates a pulse signal based on the duty ratio, and outputs the pulse signal to the logic circuit 11. The larger the duty ratio of the pulse signal generated by the PWM circuit 10, the larger the output voltage V _OUT of the switching power supply 1, and the smaller the duty ratio, the smaller the output voltage V _OUT .

The logic circuit 11 performs a level shift operation, a protection operation, a stop operation, and the like. In the level shift operation, the logic circuit 11 converts the pulse signal output from the PWM circuit 10 into a level signal for driving the transistor MP and the transistor MN. In the protection operation, the logic circuit 11 protects the device by suppressing heating of the device (for example, transistor MP and transistor MN). In the stop operation, the logic circuit 11 stops the Pch driver 12 and the Nch driver 13 when the voltage input to the logic circuit 11 is low.

The Pch driver 12 is connected to the gate of the transistor MP and outputs a voltage for driving the transistor MP according to a logic signal output from the logic circuit 11. Further, the Nch driver 13 is connected to the gate of the transistor MN and the gate of the transistor M _SNS , and outputs a voltage for driving the transistor MP and the transistor M _SNS according to the logic signal output from the logic circuit 11.

When the transistor MP is turned on by the Pch driver 12 (transistor MN is turned off), the current from the DC power supply 2 flows through the transistor MP, the coil L1, and the capacitor C _OUT , and the capacitor C _OUT is charged. When the transistor MN is turned on by the Nch driver 13 (transistor MP is turned off), the energy stored in the coil L1 causes a current to flow from the transistor MN to the coil L1 and the capacitor C _OUT to charge the capacitor C _OUT. To.

The soft start circuit 20 is controlled so that the output voltage V _OUT is gradually increased by the control of the PWM circuit 10 after the power of the switching power supply 1 is turned on. According to the output of the soft start circuit 20, it is divided into a soft start period and a normal period that continues after the soft start period ends. In the normal period, steady PWM control is performed. The soft start circuit 20 limits the duty ratio for converting the voltage in the PWM circuit 10 during the soft start period. The soft start circuit 20 is connected to the PWM circuit 10 at one end and the reverse current detection circuit 30 at the other end.

The reverse current detection circuit 30 is a circuit that detects a reverse current (reverse current) flowing through the transistor MN. The reverse direction is a direction in which the transistor MN flows from the output side (for example, the capacitor C _OUT or the coil L1 side). The reverse current detection circuit 30 includes, for example, resistors R1, R2, R3, R4, R5, R6, a comparator 31, transistors M1, M2, M3, M4, and a constant current source 32.

A power supply voltage Vreg is applied to one end of the resistor R1. A constant current Ibias1 flows through the resistor R1. A resistor R2 is connected to the other end of the resistor R1. One end of the resistor R2 is connected to the resistor R1 and the drain of the transistor M1, and the other end is connected to the drain of the transistor M3 and the negative electrode input terminal (− terminal) of the comparator 31. A power supply voltage Vreg is applied to one end of the resistor R3, and the other end is connected to the positive electrode input terminal (+ terminal) of the comparator 31 and the drain of the transistor M4. A current corresponding to the reverse current generated in the transistor MN flows through the resistor R3.

The constant current source 32 is a power source that generates a constant current Ibias 1 even if the magnitude of the load fluctuates. A power supply voltage Vreg is applied to one end thereof, and the other end is connected to the drain of the transistor M2.

The transistor M1 is a MOSFET, and the output signal of the soft start circuit 20 is input to the gate, the power supply voltage Vreg is applied to the source, and the connection points of the resistors R1 and R2 are connected to the drain. The transistor M2 is an NMOS, the gate is connected to the constant current source 32, the drain is connected to the constant current source 32, and the source is connected to the resistor R4. The transistor M3 is an NMOS, the gate is connected to the constant current source 32, the drain is connected to the resistor R2 and the negative electrode input terminal of the comparator 31, and the source is connected to the resistor R5. The transistor M4 is an NMOS, the gate is connected to the constant current source 32, the drain is connected to the resistor R3 and the positive electrode input terminal of the comparator 31, and the source is the resistor R6 and the connection point between the transistor M _SNS and the resistor R _SNS. Is connected. The transistors M2, M3, and M4 form a current mirror circuit.

Further, the transistor M _SNS is an NMOS, the gate is connected to the Nch driver 13, the drain is connected to the terminal LX of the power supply IC 5, and the source is connected to the resistor R _SNS and the connection point of the transistor M4 and the resistor R6. The transistor M _SNS forms a current mirror circuit with the transistor MN. Therefore, a current proportional to the transistor size flows between the drain-source of the transistor MN and the drain-source of the transistor M _SNS . Therefore, by detecting the voltage of the resistor R _SNS , the current of the transistor M _SNS can be detected, and as a result, the current of the transistor MN can be detected.

In the comparator 31, the positive electrode input terminal is connected to the other end of the resistor R3 and the drain of the transistor M4, the negative electrode input terminal is connected to the other end of the resistor R2 and the drain of the transistor M3, and the output terminal is connected to the Nch driver 13. To. The voltage of the negative electrode input terminal of the comparator 31 becomes the reference voltage Vref of the comparator 31. The reference voltage Vref is a value corresponding to the threshold value th1 for detecting the reverse current of the transistor MN.

The reverse current detection circuit 30 includes a first series, a second series, and a third series. The first series includes a constant current source 32, a transistor M2, and a resistor R4. The second series includes resistor R1, resistor R2, transistor M3, and resistor R5. The third series includes resistor R3, transistor M4, and resistor R6. Since the transistors M2, M3, and M4 form a current mirror circuit, in the first series, the second series, and the third series, the current values flowing through the respective elements are equal (current value = Ibias1). Or it is adjusted to be proportional.

FIG. 3 is a circuit diagram showing a configuration example of the soft start circuit 20. The soft start circuit 20 includes a comparator 21, a capacitor Css, and a constant current source 22. A power supply voltage Vreg is applied to one end of the constant current source 22, and the other end is connected to the capacitor Css, the PWM circuit 10, and the negative electrode input terminal of the comparator 21. One end of the capacitor Css is connected to the constant current source 22, the PWM circuit 10, and the negative electrode input terminal of the comparator 21, and the other end is grounded. In the comparator 21, the negative electrode input terminal is connected to the constant current source 22, the capacitor Css, and the PWM circuit 10, the reference voltage Vref1 is applied to the positive electrode input terminal, and the output terminal is connected to the reverse current detection circuit 30.

When the soft start period starts, the capacitor Css is charged by the constant current Ibss from the constant current source 22, the electric charge is accumulated, and the voltage of the capacitor Css rises. When the voltage of the capacitor Css does not reach the reference voltage Vref1, the output of the comparator 21 is High, and when the voltage of the capacitor Css reaches the reference voltage Vref1, the output of the comparator 21 becomes Low, and when it becomes Low, the soft start. The period ends. The output signal of the comparator 21 becomes the output signal of the soft start circuit 20.

Further, the output signal of the soft start circuit 20 is input to the gate of the transistor M1 of the reverse current detection circuit 30. At the start of the soft start period, the high signal from the comparator 21 is input to the transistor M1, and the transistor M1 is turned off. On the other hand, when a predetermined time elapses from the start of the soft start period and the normal period is reached, the Low signal from the comparator 21 is input and the transistor M1 is turned on. The High signal as the output of the soft start circuit 20 has a voltage equal to or higher than the threshold voltage of the gate of the transistor M1, and the Low signal has a voltage lower than the threshold voltage of the gate of the transistor M1.

The operation of the reverse current detection circuit 30 will be described.

First, the reverse current detection by the reverse current detection circuit 30 is performed, for example, as follows.

The transistor MP is turned off, the transistor MN is turned on, a current flows from the source of the transistor MN to the drain, and a current flows to the coil L1. Then, when the current of the drain from the source of the transistor MN decreases and a reverse current is generated, a current flows from the drain of the transistor MN to the source, and in the transistor M _SNS , a current proportional to the transistor size with respect to the transistor MN is generated. It flows. Then, a current from the transistor M _SNS and a current from the source of the transistor M4 flow through the resistor R _SNS . Therefore, the voltage applied to the resistor R _SNS when the reverse current is detected becomes larger than the voltage applied to the resistor R _SNS when the reverse current is not detected. Since the voltage is equal to the source voltage of the transistor M4, the source voltage of the transistor M4 also becomes large. As a result, the voltage applied to the resistor R3 connected in series with the transistor M4 becomes small, the voltage drop at the resistor R3 becomes small, and the voltage at the positive electrode input terminal of the comparator 31 becomes large.

When the voltage of the positive input terminal of the comparator 31 becomes equal to or higher than the reference voltage Vref, the comparator 31 detects the reverse current flowing through the transistor MN and outputs a High signal (a signal having a voltage equal to or higher than a predetermined voltage). On the other hand, when the voltage of the positive input terminal of the comparator 31 is less than the reference voltage Vref, the comparator 31 detects the reverse current flowing through the transistor MN and outputs a Low signal (a signal having a voltage less than a predetermined voltage).

When the High signal is input from the comparator 31, the Nch driver 13 outputs a predetermined voltage to the gates of the transistors MN and M _SNS . The predetermined voltage is, the transistor _MN, a gate voltage lower than the threshold voltage of _{M SNS,} to the transistor _MN, off _{M SNS.} On the other hand, when the Low signal is input from the comparator 31, the Nch driver 13 does not output a predetermined voltage to the gates of the transistors MN and M _SNS . Therefore, the transistors MN and M _SNS are not turned off.

Further, the adjustment of the threshold value th1, that is, the adjustment of the reference voltage Vref may be performed as follows, for example.

When the magnitude of the reverse current flowing through the transistor MN is equal to or greater than the threshold th1, the reverse current detection circuit 30 controls the transistor MN to be turned off because the voltage of the positive input terminal of the comparator 31 is equal to or greater than the reference voltage Vref.

The voltage (reference voltage Vref) of the negative electrode input terminal of the comparator 31 can be adjusted by the resistors R1 and R2 included in the reverse current detection circuit 30. For example, when the resistance value of the resistors R1 and R2 is larger, the voltage drop at the resistors R1 and R2 is larger, the reference voltage Vref is smaller, the threshold value th1 is smaller, and a small voltage is input to the positive electrode input terminal. However, the reverse current can be detected. That is, the reverse current detection sensitivity is increased. On the other hand, for example, the smaller the resistance value of the resistors R1 and R2, the smaller the voltage drop in the resistors R1 and R2, the larger the reference voltage Vref, the larger the threshold th1, and the voltage of a certain large value is input to the positive electrode input terminal. If so, the reverse current can be detected. That is, the reverse current detection sensitivity is lowered.

The resistors R1 and R2 may be connected to, for example, a switch. In this case, the enable / disable of the resistors R1 and R2 can be changed by adjusting the on / off of the switch from the outside of the reverse current detection circuit 30. As a result, the voltage drop amount due to the resistors R1 and R2 may be adjusted to adjust the voltage of the negative electrode input terminal of the comparator 31, and the threshold th1 may be changed.

Further, as the output signal of the soft start circuit 20, as described above, the high signal is output during the soft start period, so that the transistor M1 is turned off. On the other hand, since the Low signal is input in the normal period, the transistor M1 is turned on. When the transistor M1 is off, the resistor R1 is not short-circuited, and the voltage dropped from the power supply voltage Vreg by the resistors R1 and R2 becomes the reference voltage Vref of the comparator 31. When the transistor M1 is on, the resistor R1 is short-circuited, and the voltage dropped by the resistor R2 without dropping by the resistor R1 from the power supply voltage Vreg becomes the reference voltage Vref of the comparator 31. Therefore, the reference voltage Vref of the comparator 31 becomes smaller in the soft start period than in the normal period, and it becomes easier to detect the reverse current. The reference voltage Vref in the soft start period corresponds to the threshold th11 as the threshold th1 in the soft start period. The reference voltage Vref in the normal period corresponds to the threshold th12 as the threshold th1 in the normal period.

Therefore, the reverse current detection circuit 30 can be adjusted so that the threshold value th11 corresponding to the reference voltage Vref in the soft start period becomes smaller than the threshold value th12 corresponding to the reference voltage Vref in the normal period by turning on / off the transistor M1. Therefore, the reverse current generated in the transistor MN can be detected by using the same reverse current detection circuit 30 in both the soft start period and the normal period.

FIG. 4 is a circuit diagram showing a second configuration example of the reverse current detection circuit 30. FIG. 4 includes a threshold value adjusting circuit 40 in the reverse current detection circuit 30.

The threshold value adjustment circuit 40 is a circuit that gradually increases the reference voltage Vref and the threshold value th11 of the comparator 31 during the soft start period. The threshold value adjusting circuit 40 includes an inverter 41, a transistor M5, a transistor M6, a capacitor C1, and a constant current source 42.

The inverter 41 inverts the logical level of the input voltage and outputs it, the input end is connected to the soft start circuit 20, and the output end is connected to the gate of the transistor M5 and the gate of the transistor M6. The transistor M5 is a MOSFET, the gate is connected to the output end of the inverter 41, the power supply voltage Vreg is applied to the source, and the drain is connected to the drain of the transistor M6, the capacitor C1, and the transistor M1. The transistor M6 is an NMOS, the gate is connected to the output end of the inverter 41, the source is connected to the constant current source 42, and the drain is connected to the drain of the transistor M5, the capacitor C1 and the transistor M1. One end of the capacitor C1 is connected to the connection point of the transistor M5 and the transistor M6 and the gate of the transistor M1, and the other end is connected to the constant current source 42 and is grounded. The constant current source 42 supplies a constant current Ibias2, one end of which is connected to the source of the transistor M6, and the other end of which is grounded.

An operation example of the threshold value adjusting circuit 40 will be described.

Since the transistor M5 is a MOSFET, it is turned on when the gate node of the transistors M5 and M6 is below the threshold voltage, and turned off when the voltage is above the threshold voltage. Since the transistor M6 is an NMOS, it is turned off when the gate node of the transistors M5 and M6 is below the threshold voltage, and turned on when the voltage is above the threshold voltage.

The High signal as the output of the soft start circuit 20 becomes a Low signal by the inverter 41, and the gate node of the transistors M5 and M6 of the threshold adjustment circuit 40 becomes a voltage lower than the threshold voltage. The Low signal as the output of the soft start circuit 20 becomes a High signal by the inverter 41, and the gate node of the transistors M5 and M6 becomes a voltage equal to or higher than the threshold voltage.

Therefore, in the soft start period when the power supply of the switching power supply 1 is started, the output signal of the soft start circuit 20 is a high signal, so that the transistor M5 is turned on and the transistor M6 is turned off via the inverter 41. Therefore, the capacitor C1 is momentarily charged and reaches the power supply voltage Vreg.

When the soft start period ends, the output signal of the soft start circuit 20 becomes a Low signal. Therefore, the gate nodes of the transistors M5 and M6 become a voltage equal to or higher than the threshold voltage via the inverter 41, and the transistor M5 is turned off. Transistor M6 is turned on. Then, the electric charge charged in the capacitor C1 is discharged through the transistor M6 which is on. In this case, due to the presence of the constant current source 42 on the source side of the transistor M6, the current of the transistor M6 does not exceed the predetermined current Ibias2, so that the current is not discharged instantaneously but gradually.

Since the transistor M1 of the reverse current detection circuit 30 is a MOSFET, it is turned on when the gate voltage becomes less than the threshold voltage. When the change in the vicinity of the threshold voltage of the gate of the transistor M1 causes a current to gradually flow through the transistor M1, the reference voltage Vref and the threshold value th1 change gradually, not instantaneously. In this way, the threshold adjustment circuit 40 can slowly turn on the transistor M1 when switching from the soft start period to the normal period.

Next, changes in the output voltage V _OUT and the reference voltage Vref (inverting input voltage) of the comparator 31 when shifting from the soft start period to the normal period will be described.

FIG. 5A is a diagram showing an example of a change in the output voltage of the switching power supply when shifting from the soft start period to the normal period when the reverse current detection circuit 30 does not include the threshold value adjustment circuit 40. FIG. 5B is a diagram showing an example of a change in the reference voltage Vref of the comparator 31 when shifting from the soft start period to the normal period when the reverse current detection circuit 30 does not include the threshold adjustment circuit 40.

As shown in FIGS. 5A and 5B, when the reverse current detection circuit 30 does not include the threshold adjustment circuit 40, the transistor M1 changes from on to off during the transition from the soft start period to the normal period, and the comparator 31 The reference voltage Vref of is rapidly changed from the voltage v1 to the voltage v2 at time t1. Therefore, as shown in FIG. 5B, the current corresponding to the change in the reference voltage Vref is drawn from the capacitor C _OUT , and the output voltage V _OUT drops significantly.

FIG. 6A is a diagram showing an example of a change in the output voltage V _OUT when shifting from the soft start period to the normal period when the reverse current detection circuit 30 includes the threshold value adjustment circuit 40. FIG. 6B is a diagram showing an example of a change in the reference voltage Vref of the comparator 31 when shifting from the soft start period to the normal period when the reverse current detection circuit 30 includes the threshold value adjustment circuit 40.

As shown in FIGS. 6A and 6B, when the reverse current detection circuit 30 includes the threshold value adjustment circuit 40, the gate voltage of the transistor M1 is gradually changed when shifting from the soft start period to the normal period. M1 gradually changes from on to off. Therefore, the reference voltage Vref of the comparator 31 gradually changes from the voltage v1 to the voltage v2 from time t1 to t2. As a result, when shifting from the soft start period to the normal period, as shown in FIG. 6A, it is possible to reduce the momentary decrease in the output voltage when the current corresponding to the change in the reference voltage Vref is drawn from the capacitor C _OUT .

Next, the reverse current detection process is supplemented using a mathematical formula. An example is shown below, and the reverse current detection process may be performed using another mathematical formula.

The reverse current detection circuit 30 detects the reverse current in which the current of the transistor MN on the low voltage side of the synchronous rectification flows from the coil L1 to the ground side. The same voltage as the drain-source voltage V _{DS_MN} of the transistor MN when the reverse current flows is generated in the resistor R _SNS . Since the source voltage of the transistor M4 is V _{DS_MN} and a current mirror circuit is formed between the transistor M4 and the transistor M2, it may be represented by the following equation (1).

なお、Ｖ_ＧＳ２は、トランジスタＭ２のゲート−ソース間電圧である。Ｖ_ＧＳ４は、トランジスタＭ４のゲート−ソース間電圧である。Ｒ４は、抵抗Ｒ４の抵抗値である。

Note that _VGS2 is the gate-source voltage of the transistor M2. _VGS4 is the gate-source voltage of the transistor M4. R4 is the resistance value of the resistor R4.

The gate-source voltage of the transistor M _SNS may be expressed by the equation (2), and if the transistor sizes of the transistor M2 and the transistor M4 are the same, the equation (1) may be expressed by the equation (3). ..

なお、Ｉ_Ｄはドレイン電流である。μ_０はチャネル表面移動度である。Ｃｏｘは単位面積当たりゲート酸化膜容量である。Ｗはチャネル幅である。Ｌはチャネル長である。ＶｔｈはトランジスタＭ２，Ｍ４のゲートの閾値電圧である。βは、トランスコンダクタンスパラメータである。

Note that _ID is the drain current. μ ₀ is the channel surface mobility. Cox is the gate oxide film capacity per unit area. W is the channel width. L is the channel length. Vth is the threshold voltage of the gate of the transistors M2 and M4. β is a transconductance parameter.

From equation (3), the drain of the transistor MN - the source voltage _{V DS_MN,} the drain current _{I D4} of the transistor M4 may be represented by the formula (4).

Transistor MN of on-resistance _{R ON_N,} when a reverse current from the drain of the transistor MN to the source and _{I D_MN,} drain current _{I D4} of the transistor M4 may be represented by the formula (5).

From equation (5), as the reverse current increases in transistor MN, the drain current _{I D4} of transistor M4 day become smaller. The comparator 31 compares the voltage drop due to the resistors R3 with the voltage drop due to the resistors R1 and R2. The voltage dropped from the power supply voltage Vreg by the resistors R1 and R2 is the product of the constant currents Ibias1 and R4 if the transistor sizes of the transistors M2 and M3 are the same and the values of the resistors R4 and R5 are equal. It can be determined and becomes the reference voltage of the comparator 31. The reference voltage of the comparator 31 corresponds to the threshold th1.

The voltage of the positive electrode input terminal and the voltage of the negative electrode input terminal of the comparator 31 are represented by the following equations (6) and (7).
Positive electrode voltage (V +) = Vreg-I _D4 x R3 ... (6)
Negative electrode voltage (V-) = Vreg-Ibias1 × (R1 + R2) ... (7)
R1 is the resistance value of the resistor R1, R2 is the resistance value of the resistor R2, and R3 is the resistance value of the resistor R3.

When the transistor M1 is off, the voltage drop between the resistors R1 and R2 is larger than when the transistor M1 is on, and a smaller reverse current can be detected. For example, by setting the values of the resistors R1 and R2 so that the drain current _{ID_MN} of the transistor MN is detected at 0A, the reverse current can be detected when the transistor MN starts to flow backward. Therefore, the switching power supply 1 can prevent the current from being drawn from the capacitor C _OUT by turning off the transistor MN when the reverse current is detected.

In this way, the switching power supply 1 activates the function of the reverse current detection circuit 30 for the transistor MN during the soft start period, and monitors the presence or absence of the reverse current for each switching of the transistor MN. The reverse current detection circuit 30 turns off the transistor MN when the voltage corresponding to the reverse current becomes the threshold value th1 or more. Then, the threshold value th1 can be set differently between the soft start period and the normal period.

By turning off the transistor M1 during the soft start period, the switching power supply 1 can suppress the occurrence of current withdrawal from the capacitor C _OUT due to the reverse current of the transistor MN when the output voltage V _OUT is low. In addition, the vibration phenomenon in which the output voltage V _OUT fluctuates can be suppressed and the voltage can be raised smoothly. Then, the switching power supply 1 sets the threshold value th1 for turning on the transistor M1 to detect the reverse current after the end of the soft start period to a threshold value th12 larger than the threshold value th11, and enables the generation of the output voltage V _OUT in the normal period. it can.

As described above, the switching power supply 1 of the present embodiment is a switching power supply that converts the voltage according to the synchronous rectification method, and is connected in series with the transistor MP (an example of the first transistor) and the transistor MP. Transistor MN (an example of a second transistor) arranged on the lower voltage side, PWM circuit 10 (an example of a control circuit) that controls on / off of transistor MP and transistor MN, and the magnitude of reverse current flowing through transistor MN. When the value is equal to or higher than the threshold value th1, a reverse current detection circuit 30 (an example of the detection circuit) that controls the transistor MN to be turned off may be provided. In the reverse current detection circuit 30, the threshold value th11 (an example of the first threshold value) in the soft start period (an example of the first period) after the switching power supply 1 is turned on is a normal period (second example) following the soft start period. It may be adjusted to be smaller than the threshold value th12 (an example of the second threshold value) in (an example of the period).

As a result, the switching power supply 1 can perform reverse current detection using the same reverse current detection circuit 30 during the soft start period and the normal period, so that it is not necessary to provide a reverse current detection circuit dedicated to each period. It is possible to suppress an increase in the number of parts. Further, by changing the threshold value th1 for detecting the reverse current in the reverse current detection circuit 30, it is possible to deal with the reverse current having different magnitudes in the soft start period and the normal period. For example, the switching power supply 1 can detect the reverse current with high accuracy even in the soft start period when the voltage is not sufficiently large, and can suppress the vibration of the output voltage of the switching power supply 1.

Further, the reverse current detection circuit 30 has a resistor R1 (an example of a first resistor) in which a power supply voltage Vreg (an example of a first voltage) is applied to one end and a constant current Ibias1 flows, and one end is in series with the resistor R1. A resistor R2 (an example of a second resistor) connected to the resistor R2 may be provided. The voltage at the other end of the resistor R2 corresponds to the voltage of the threshold value th1, and the resistor R1 may be short-circuited in a normal period.

As a result, in the soft start period, the power supply voltage Vreg is applied to the resistors R1 and R2, so that the voltage drop from the power supply voltage Vreg becomes large, the reference voltage Vref becomes small, and the threshold th1 becomes small. In the normal period, the resistor R1 is short-circuited and the power supply voltage Vreg is applied to the resistor R2, so that the voltage drop from the power supply voltage Vreg becomes small, the reference voltage Vref becomes large, and the threshold th1 becomes large. Therefore, in the switching power supply 1, the voltage of the threshold value th1 or more is more likely to be detected in the soft start period than in the normal period, and the detection sensitivity of reverse current detection can be increased.

Further, the reverse current detection circuit 30 may include a transistor M1 (an example of a third transistor) in which a power supply voltage Vreg is applied to the source and a resistor R2 is connected to the drain. In the soft start period, the transistor M1 may be turned off, and in the normal period, the transistor M1 may be turned on.

As a result, the switching power supply 1 can select a resistor to which the power supply voltage Vreg is applied by switching the transistor M3, and can change the threshold value th1.

Further, in the reverse current detection circuit 30, a resistor R3 (an example of a third resistor) in which a power supply voltage Vreg is applied to one end and a current flows according to the magnitude of the reverse current, and the other end of the resistor R2 is a negative electrode input terminal. 31 may be provided, and the other end of the resistor R3 is connected to the positive electrode input terminal.

As a result, the switching power supply 1 has a threshold th1 for detecting the reverse current by changing the voltage (reference voltage Vref) applied to the negative electrode input terminal of the comparator 31 between the soft start period and the normal period. The voltage can be changed. Further, the switching power supply 1 can suitably detect the reverse current in the soft start period and the normal period by applying a voltage corresponding to the current corresponding to the magnitude of the reverse current to the positive electrode input terminal of the comparator 21. ..

Further, the switching power supply 1 includes a transistor M _SNS (an example of a fourth transistor) forming a current mirror circuit with a transistor MN, and a resistor R _SNS (fourth) in which one end is connected to the transistor M _SNS and the other end is grounded. An example of resistance) and may be provided. The reverse current detection circuit 30 comprises a transistor M2 (an example of a fifth transistor) in which a drain is connected to a constant current source 32 (an example of a first constant current source) that generates a constant current Ibias 1, and a transistor M2. A current mirror circuit may be formed between the transistors, and a transistor M4 (an example of a sixth transistor) in which the drain is connected to the resistor R3 and the source side is connected to the resistor R _SNS may be provided.

When a reverse current is generated while the transistor MN is on, a current flows from the drain of the transistor MN to the source, and the same current as that of the transistor MN flows in the transistor M _SNS . The resistor _{R SNS,} flows a current from the transistor _{M SNS} and the transistor M4, the voltage across the resistor _{R SNS} is increased. Since the voltage is equal to the source voltage of the transistor M4, the source voltage of the transistor M4 also increases. As a result, the voltage applied to the resistor R3 connected in series with the transistor M4 becomes small, and the voltage drop at the resistor R3 becomes small, so that the positive electrode voltage (voltage of the positive electrode input terminal) of the comparator 31 becomes high. In this way, the reverse current detection circuit 30 can detect the reverse current by the comparator 31.

Further, the switching power supply 1 may include a threshold value adjusting circuit 40 that gradually increases the threshold value th1.

As a result, the switching power supply 1 can suppress a sudden change in the output voltage due to the momentary change of the threshold value th1 as compared with the case where the threshold value th1 is changed momentarily by gradually changing the threshold value th1.

The threshold adjustment circuit 40 includes a transistor M5 (an example of a seventh transistor), a transistor M6 (an example of an eighth transistor), a capacitor C1, and a constant current source 42 (an example of a second constant current source). And may be provided. A power supply voltage Vreg may be applied to the source of the transistor M5. The drain of the transistor M6 may be connected to the drain of the transistor M5. One end of the capacitor C1 may be connected to the drain of the transistor M5, one end may be connected to the drain of the transistor M6, and the other end may be grounded. One end of the constant current source 42 may be connected to the source of the transistor M6, and the other end may be grounded.

As a result, the electric charge charged in the capacitor C1 is discharged through the transistor M6 which is on. In this case, due to the presence of the constant current source 42 connected to the source of the transistor M6, the current of the transistor M6 does not exceed the predetermined current Ibias2, so that the capacitor C1 is discharged gradually rather than instantaneously. For example, the transistor M1 of the reverse current detection circuit 30 is turned on when the gate voltage becomes equal to or lower than the threshold voltage, and a current gradually flows through the transistor M1 when the value near the threshold changes. Therefore, the threshold value adjusting circuit 40 can gradually change the threshold value th1 for detecting the reverse current of the reverse current detection circuit 30 rather than instantaneously.

Further, the threshold adjustment circuit 40 may input a signal from the soft start circuit 20 (an example of the limiting circuit) that limits the duty ratio for converting the voltage in the PWM circuit 10 during the soft start period.

As a result, the threshold value adjusting circuit 40 can change the threshold value th1 based on the output signal of the soft start circuit 20.

Although the embodiment of the switchg power supply according to the present disclosure has been described above with reference to the drawings, the present disclosure is not limited to such an example. It is clear that a person skilled in the art can come up with various modifications, modifications, substitutions, additions, deletions, and equality examples within the scope of the claims. It is understood that it naturally belongs to the technical scope of the present disclosure.

In the above embodiment, it has been illustrated that each transistor is generated by MOSFET or NMOS, but the present invention is not limited to this. If the function of the embodiment can be realized, the MPa may be an NMOS, the NMOS may be a MPa, or may be composed of other transistors.

The present disclosure is useful for a switching power supply or the like that can detect a reverse current having a different magnitude between a predetermined period and a period after the predetermined period from the start-up and can suppress voltage vibration at the start-up of the switching power supply.

1 Switching power supply 2 DC power supply 3 Load 10 PWM circuit 20 Soft start circuit 30 Reverse current detection circuit 31 Comparator 32 Constant current source 40 Threshold adjustment circuit 41 Inverter 42 Constant current source C1, Css, C _OUT capacitor M1, M2, M3 M4, M5, M6, MP, MN, M _SNS transistor R1, R2, R3, R4, R5, R6, R _SNS resistor

Claims (8)

A switching power supply that converts voltage according to the synchronous rectification method.
The first transistor and
A second transistor connected in series with the first transistor and arranged on a lower voltage side than the first transistor,
A control circuit that controls the on / off of the first transistor and the second transistor,
When the magnitude of the reverse current flowing through the second transistor is equal to or greater than the threshold value, the detection circuit that controls the second transistor to be turned off and the detection circuit.
With
In the detection circuit, the first threshold value, which is the threshold value in the first period after the switching power supply is turned on, is larger than the second threshold value, which is the threshold value in the second period following the first period. Adjust to be smaller,
Switching power supply. The detection circuit
A first resistor to which a first voltage is applied to one end and a constant current flows,
A second resistor whose end is connected in series with the first resistor,
With
The voltage at the other end of the second resistor corresponds to the threshold voltage.
The first resistor is short-circuited during the second period.
The switching power supply according to claim 1. The detection circuit
A third transistor is provided in which the first voltage is applied to the source and the second resistor is connected to the drain.
In the first period, the third transistor is turned off and
In the second period, the third transistor is turned on.
The switching power supply according to claim 2. The detection circuit
A third resistor to which the first voltage is applied at one end and a current flows according to the magnitude of the reverse current, and
The switching power supply according to claim 3, further comprising a comparator in which the other end of the second resistor is connected to the negative electrode input terminal and the other end of the third resistor is connected to the positive electrode input terminal. The second transistor and the fourth transistor forming the current mirror circuit,
It further comprises a fourth resistor, one end connected to the fourth transistor and the other end grounded.
The detection circuit
A fifth transistor having a drain connected to the first constant current source that generates a constant current, and a fifth transistor.
A current mirror circuit is formed with the fifth transistor, and a sixth transistor having a drain connected to the third resistor and a source connected to the fourth resistor is provided.
The switching power supply according to claim 4. A threshold adjustment circuit for gradually increasing the first threshold value is further provided.
The switching power supply according to any one of claims 1 to 5. The threshold adjustment circuit
It comprises a seventh transistor, an eighth transistor, a capacitor, and a second constant current source.
In the seventh transistor, a first voltage is applied to the source.
The drain of the eighth transistor is connected to the drain of the seventh transistor.
One end of the capacitor is connected to the drain of the seventh transistor, one end is connected to the drain of the eighth transistor, and the other end is grounded.
One end of the second constant current source is connected to the source of the eighth transistor, and the other end is grounded.
The switching power supply according to claim 6. The threshold adjustment circuit inputs a signal from a limiting circuit that limits the duty ratio for converting a voltage in the control circuit during the first period.
The switching power supply according to claim 6 or 7.

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