JP2015095928A - Switching converter and control circuit thereof, current detection method, ac/dc converter, power supply adaptor, and electronic apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress variation and fluctuation of effective threshold voltage.SOLUTION: A control circuit 200 is used in a DC/DC converter 100. The DC/DC converter 100 has at least a coil Land a switching transistor M1. A current detection circuit 300 compares a current Iflowing through the coil Lduring ON period of the switching transistor M1 with a predetermined threshold current I. A threshold voltage generating section 304 generates a threshold voltage V(t) increasing with time during On period of the switching transistor M1. A comparator 302 compares a detection voltage V, corresponding to the coil Lflowing through the switching transistor M1, with the threshold voltage V(t).

Description

  The present invention relates to a switching converter.

  Various home appliances such as TVs and refrigerators operate by receiving commercial AC power from the outside. Electronic devices such as laptop computers, mobile phone terminals, and tablet PCs can also be operated with commercial AC power, or a battery built into the device can be charged with commercial AC power. Such home appliances and electronic devices (hereinafter collectively referred to as electronic devices) have built-in power supply devices (inverters) for converting AC / DC (AC / DC) commercial AC voltage, or the inverters are external to the electronic devices. Built in the power adapter (AC adapter).

  FIG. 1 is a block diagram of an AC / DC converter 400r investigated by the present inventors. The AC / DC converter 400r mainly includes a rectifier circuit 402, a smoothing capacitor 404, and a DC / DC converter (switching converter) 100r.

Rectifier circuit 402, a diode bridge circuit for full-wave rectifying the commercial AC voltage V _AC. The output voltage of the rectifier circuit 402 is smoothed by the smoothing capacitor 404 and converted into a direct-current voltage _VDC .

The direct current voltage V _DC is supplied to the input line 104 of the insulation type DC / DC converter 100r in the subsequent stage. The DC / DC converter 100r steps down the direct-current voltage V _DC to generate an output voltage _VOUT stabilized at a target value, and supplies the output voltage _VOUT to a load (not shown) connected to the output line 106.

The DC / DC converter 100r includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a switching transistor M1, a detection resistor R _CS , a transformer T1, a rectifier diode D1, an output capacitor C1, and a feedback circuit 108. The feedback circuit 108 generates a feedback voltage V _FB corresponding to the output voltage _VOUT and supplies the feedback voltage V _FB to the feedback terminal (FB terminal) of the control circuit 200r.

The switching transistor M1 and the detection resistor _{R CS} is connected to the primary coil _{L P} of the transformer T1. Rectifier diode D1 and the output capacitor C1 is connected to the secondary coil _{L S} of the transformer T1.

The output terminal OUT of the control circuit 200r is connected to the gate of the switching transistor M1. The control circuit 200r generates a pulse signal S _OUT whose duty ratio is adjusted so that the output voltage V _OUT approaches a predetermined target value, and switches the switching transistor M1.

Control circuit 200r is in the on period of the switching transistor M1, detectably configured to current (as the coil current _{I P)} flowing through the primary coil _{L P} and the switching transistor M1. Specifically, the current detection (CS) terminals of the control circuit 200r is connected to the sense resistor _{R CS,} the CS terminal, the detection voltage _{V CS} that is proportional to the coil current _{I P} is input. Current detecting circuit 300r by comparing the detection voltage _{V CS} with a predetermined threshold voltage _{V TH,} the coil current _{I P} of the threshold current _{I TH (= V TH / R} CS) for comparing.
The above is the configuration of the AC / DC converter 400r.

FIG. 2 is an operation waveform diagram of the DC / DC converter 100r. When the pulse signal _SOUT is at a high level, the switching transistor M1 is turned on. When the switching transistor M1 is turned on, the coil current I _P increases with time, accordingly also increases the detection voltage V _CS. When the pulse signal _SOUT becomes low level, the switching transistor M1 is turned off. During the OFF period of the switching transistor M1, a current I _S flows through the secondary coil L _S and is supplied to the output capacitor C1. By repeating the switching of the switching transistor M1, the output voltage _VOUT is stabilized at a desired level.

  As a result of studying the AC / DC converter 400r of FIG. 1, the present inventor has come to recognize the following problems.

Focusing on the coil current I _P in the ON period of the switching transistor M1. In the on period of the switching transistor M1, is across the primary coil L _P DC voltage V _DC is applied, the following equation holds.
V _DC == L _P · dI _P / dt (1)
V _CS = R _CS × I _P (2)
When this is transformed, equation (3) is obtained.
_{V CS = R CS / L P} × ∫V DC dt = (R CS / L P × V DC) × t ... (3)
(R _CS / L _P × V _DC ) is a slope [V / s] of the detection voltage V _CS and is hereinafter referred to as α.

Thus the inclination α of the detected voltage _{V CS} at the on period of the switching transistor M1 is dependent on the inductance of the DC voltage _{V DC} and the primary coil _{L P.}

FIG. 3A is an operation waveform diagram of the current detection circuit 300r, and FIG. 3B is a diagram showing an effective threshold voltage. Current comparator of the detection circuit 300r has a response delay tau _D, the output signal _{S CMP is} from when V CS _{= V TH,} the transition is made after a delay time tau _D elapses. The detection voltage _{V CS} at which the output signal _{S CMP} of the comparator is changed, referred to as the effective threshold voltage _{V TH_EFF.} As shown in FIG. 3 (a), as the effective threshold voltage _{V TH_EFF} inclination α is greater of the detection voltage _{V CS,} higher than its ideal value _{V TH,} is given by Equation (4).
V _TH — _EFF = V _TH + α × τ _D (4)

Therefore, when using an output S _CMP of the comparator in such over-current protection, the variation of the variation and the coil L1 of the power flow voltage V _DC, the inclination α is changed, the effective threshold voltage and thus along with it, the threshold current I _TH will fluctuate or vary.

  It should be noted that the above examination should not be recognized as general technical common knowledge of those skilled in the art, but has been independently recognized by the present inventors.

  The present invention has been made in view of these problems, and one of the exemplary purposes of an embodiment thereof is to provide a control circuit for a switching converter that can suppress fluctuations and variations in the effective threshold voltage.

  One embodiment of the present invention relates to a control circuit used in a switching converter. The switching converter includes at least a switching transistor and a coil connected to the switching transistor. The control circuit includes a current detection circuit that compares the current flowing through the coil with a predetermined threshold current during the ON period of the switching transistor. The current detection circuit includes a threshold voltage generation unit that generates a threshold voltage that increases with time during an ON period of the switching transistor, and a comparator that compares a detection voltage corresponding to the current flowing through the switching transistor with the threshold voltage. .

During the response delay tau _D of the comparator, the voltage width detected voltage V _CS exceeds the threshold voltage V _TH (overshoot amount) [Delta] V is, the larger the inclination α is greater of the detection voltage V _CS. In this aspect, as the elapsed time in the ON period becomes longer, the amount of overshoot ΔV can be canceled by increasing the level of the threshold voltage V _TH , and the fluctuation of the effective threshold voltage V _{TH_EFF} And variations can be suppressed.

  The threshold voltage may increase monotonously during the ON period of the switching transistor.

The threshold voltage V _TH (t) is a predetermined time range.
V _TH (t) = V _TH — _EFF × t / (t + τ _D )
Or you may have a waveform which approximated it. τ _D is the response delay of the comparator.

The threshold voltage V _TH may include a constant component and a correction component that increases with time during the ON period of the switching transistor.

  The correction component may converge to a predetermined level over time.

  The threshold voltage generation unit includes a current source for generating a constant current, a capacitor charged by the current source, and a resistor connected to the capacitor, and outputs the voltage of the capacitor as the threshold voltage. The threshold voltage may be changed according to a time constant determined by a capacitor and a resistor.

  The correction component continues to increase with a substantially constant slope during the ON period of the switching transistor.

  The threshold voltage generator includes a reference current generator that generates a predetermined reference current, and a correction current generator that generates a correction current that is synchronized with switching of the switching transistor and that increases with time during the ON period of the switching transistor. And a current-voltage conversion circuit that converts a current obtained by adding the reference current and the correction current into a threshold voltage.

  The comparator is provided for overcurrent detection, and the control circuit performs a predetermined overcurrent protection process when the output of the comparator indicates that the detected voltage is higher than the threshold voltage.

  The control circuit compares the detection voltage with the feedback terminal that receives the feedback voltage whose value is adjusted so that the output voltage of the switching converter approaches a predetermined target value, and is asserted when the detection voltage becomes higher than the feedback voltage. And an error comparator that generates an off signal, and a logic unit that generates a pulse signal that transitions to an off level when the off signal is asserted.

The control circuit may be integrated on a single semiconductor substrate.
“Integrated integration” includes the case where all of the circuit components are formed on a semiconductor substrate and the case where the main components of the circuit are integrated. A resistor, a capacitor, or the like may be provided outside the semiconductor substrate. By integrating the control circuit on one IC (Integrated Circuit) chip, the circuit area can be reduced and the characteristics of the circuit elements can be kept uniform.

  Another aspect of the present invention relates to a switching converter. The switching converter may include at least an output circuit including a coil, a switching transistor connected to the coil, a detection resistor provided in series with the switching transistor, and any one of the control circuits described above. .

  Another aspect of the present invention relates to an AC / DC converter. The AC / DC converter includes a rectifier circuit that rectifies an AC voltage, a smoothing capacitor that smoothes the output voltage of the rectifier circuit, and any one of the switching converters that receives the voltage of the smoothing capacitor as an input voltage.

  Another embodiment of the present invention relates to an electronic device. The electronic device may include a load and the above-described AC / DC converter that supplies a DC voltage to the load.

  Another aspect of the present invention relates to a power adapter. The power adapter may include the above-described AC / DC converter.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to an aspect of the present invention, fluctuations and variations in the effective threshold voltage can be suppressed.

It is a block diagram of the AC / DC converter which this inventor examined. It is an operation | movement waveform diagram of a DC / DC converter. FIG. 3A is an operation waveform diagram of the current detection circuit, and FIG. 3B is a diagram showing an effective threshold voltage. FIG. 4 is a circuit diagram of an AC / DC converter including the control circuit according to the embodiment. 5A and 5B are operation waveform diagrams of the current detection circuit of FIG. FIG. _6A shows the relationship between the threshold voltage V _TH (t) and the effective threshold voltage V _{TH_EFF} . FIGS. 7A and 7B are waveform diagrams showing examples of the threshold voltage V _TH (t). It is a circuit diagram of a threshold voltage generation unit according to a first configuration example. FIG. 6 is a circuit diagram of a threshold voltage generation unit according to a second configuration example. It is a figure which shows an AC adapter provided with an AC / DC converter. FIGS. 11A and 11B are diagrams illustrating an electronic device including an AC / DC converter.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are physically directly connected, or the member A and the member B are electrically connected. The case where it is indirectly connected through another member that does not affect the state is also included.
Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as an electrical condition. It includes the case of being indirectly connected through another member that does not affect the connection state.

FIG. 4 is a circuit diagram of an AC / DC converter 400 including the control circuit 200 according to the embodiment. AC / DC converter 400 converts the AC voltage _{V AC} to a DC voltage _{V OUT.} Since the basic configuration of the AC / DC converter 400 is the same as that of the AC / DC converter 400r of FIG. 1, only the differences will be described below.

  The AC / DC converter 400 includes a rectifier circuit 402, a smoothing capacitor 404, and the DC / DC converter 100. The rectifier circuit 402 and the smoothing capacitor 404 are as described with reference to FIG.

The feedback circuit 108 generates a feedback voltage V _FB corresponding to the output voltage V _OUT of the DC / DC converter 100. For example, the feedback circuit 108 includes a shunt regulator 110 and a photocoupler 112. The shunt regulator 110 amplifies an error between a voltage obtained by dividing the DC voltage _VOUT and a predetermined target value _VREF , thereby generating a feedback signal S1 whose level is adjusted so that the error becomes zero.

In the photocoupler 112, the light emitting element on the primary side is controlled by the feedback signal S1, and a signal generated in the light receiving element on the secondary side of the photocoupler 112 is input to the FB terminal of the control circuit 200 as the feedback voltage _VFB. .

  Note that, when insulation between the primary side and the secondary side of the transformer T1 is not required, the shunt regulator 110 and the FB terminal may be connected by wiring without using the photocoupler 112. Further, the function of the shunt regulator 110, that is, the error amplifier may be incorporated in the control circuit 200.

The detection resistor _RCS is provided in series with the switching transistor M1. In the on period of the switching transistor M1, 1 primary coil _{L P,} the switching transistor M1, the coil current _{I P} flows through a path of the detection resistor _{R CS.} In the present embodiment, one end of the source and the detection resistor R _CS of the switching transistor M1 is grounded, the other end of the detection resistor R _CS is connected to the FB terminal. The detection voltage V _CS ′ generated at the other end of the detection resistor R _CS is a negative voltage.
_{V CS '= -I P × R} CS
As a matter of course, the switching transistor M1 and the detection resistor _RCS may be laid out as shown in FIG. 1 and the detection voltage _VCS may be a positive voltage.

Hereinafter, a specific configuration of the control circuit 200 will be described.
The control circuit 200 includes an inverting amplifier 202, an error comparator 204, a logic unit 206, a driver 208, and a current detection circuit 300, and is integrated on a single semiconductor substrate.

The inverting amplifier 202 inverts the negative detection voltage V _CS ′ to generate a positive detection voltage V _CS . The error comparator 204 compares the detection voltage V _CS with the feedback voltage V _FB, and asserts an off signal S _OFF (for example, high level) when V _CS > V _FB .

The logic unit 206 generates a pulse signal S _PWM that instructs on / off of the switching transistor M1. When the off signal S _OFF is asserted, the logic unit 206 changes the pulse signal S _PWM to an off level (for example, a low level) corresponding to the switching transistor M1 being turned off. In addition, the logic unit 206 transitions the pulse signal _SPWM to an on level (for example, a high level) corresponding to the switching transistor M1 being turned on every predetermined switching period in synchronization with a clock signal having a predetermined period. Thus, the pulse signal S _PWM is pulse width modulated.
The driver 208 generates a gate pulse signal S _OUT corresponding to the pulse signal S _PWM , and switches the switching transistor M1.

Current detecting circuit 300, the ON period of the switching transistor M1, to compare the coil current _{I P} flowing through the primary coil _{L P} and a predetermined threshold current _{I TH.} For example, the current detection circuit 300 is provided for overcurrent protection (OCP: Over Current Protection). When I _P > I _TH , the logic unit 206 turns off the switching transistor M1 and switches the circuit element from the overcurrent state. Protect.

The current detection circuit 300 includes a comparator 302 and a threshold voltage generation unit 304. The threshold voltage generation unit 304 generates a threshold voltage V _TH (t) that changes with time.

The comparator 302, the detection voltage _{V CS} corresponding to the coil current _{I P,} as compared with the threshold voltage _V TH _(t), when the _{V CS> V TH (t)} , asserts a protection signal _{S OCP} (e.g. high Level).

When the protection signal S _OCP is asserted, the logic unit 206 sets the pulse signal S _PWM to a low level and turns off the switching transistor M1.

  The above is the configuration of the AC / DC converter 400. Subsequently, the operation of the current detection circuit 300 will be described.

5A and 5B are operation waveform diagrams of the current detection circuit 300 of FIG. As shown in FIG. 5A, when the pulse signal S _PWM transitions to the on level (high level), the threshold voltage V _TH (t) increases with time. Accordingly, the threshold voltage V _TH (t) becomes higher as the elapsed time from the transition to the _ON period TON becomes longer.

FIG. 5B shows the relationship between the detection voltage V _CS having a different slope α and the threshold voltage V _TH (t). From the detection voltage _{V CS} is crossed the threshold voltage _V TH (t), the value of the detection voltage _{V CS} in the response delay tau _D time after lapse of the comparator 302 becomes the effective threshold voltage _{V TH_EFF.}

The above is the operation of the current detection circuit 300.
During the response delay τ _D of the comparator, the voltage width (overshoot amount) over which the detection voltage V _CS exceeds the threshold voltage V _TH (t) increases as the inclination α of the detection voltage V _CS increases. In the current detection circuit 300 according to the embodiment, the level of the threshold voltage V _TH (t) is increased as the elapsed time of the _ON period TON becomes longer. As a result, the overshoot amount ΔV can be canceled, and fluctuations and variations in the effective threshold voltage V _{TH_EFF} can be suppressed.

Next, the waveform of the threshold voltage V _TH (t) necessary for making the effective threshold voltage V _{TH_EFF} constant will be described. FIG. _6A shows the relationship between the threshold voltage V _TH (t) and the effective threshold voltage V _{TH_EFF} .

Assume that the transition to the ON period occurs at time t = 0. The time from the transition to the on period until the detection voltage V _CS reaches the threshold voltage V _TH (t) is t ₁ , and the period from the transition to the on period to the change in the output S _{OCP of the} comparator 302 It is referred to as _{T 2.}
t ₂ = t ₁ + τ _D

Equation (5) is satisfied with respect to the slope of the detection voltage _{V CS α.}
α = V _TH (t ₁ ) / t ₁ = V _{TH_EFF} / (t ₁ + τ _D ) (5)

When formula (5) is transformed, formula (6) is obtained.
V _TH — _EFF = V _TH (t ₁ ) / t ₁ × (t ₁ + τ _D ) = const (6)
In order to make V _{TH_EFF} in the equation (6) constant for an arbitrary t ₁ , the threshold voltage V _TH (t) ideally satisfies the equation (7).
V _TH (t) = V _TH — _EFF × t / (t + τ _D ) (7)

FIG. 6B shows a waveform of an ideal threshold voltage V _TH (t) that satisfies Expression (7). In reality, in many applications, the slope α is constrained within a certain range, so it is not necessary to satisfy Equation (7) in all time ranges, and in the time range _TRGN corresponding to the range of the slope α, What is necessary is just to satisfy | fill Formula (7).

It is difficult to generate a waveform that satisfies Equation (7) using limited hardware resources. Therefore, the threshold voltage generation unit 304 may generate the threshold voltage V _TH (t) based on a waveform that approximates Equation (7) in the time range _TRGN .

FIGS. 7A and 7B are waveform diagrams showing examples of the threshold voltage V _TH (t). In FIGS. 7A and 7B, the waveform of Expression (7) is indicated by a broken line (i). The threshold voltage V _TH (t) in FIG. 7A is a linear approximation of the waveform of equation (7), as shown by the solid line (ii). The threshold voltage V _TH (t) in FIG. 7B is obtained by approximating the waveform of Expression (7) with a curve as shown by a solid line (iii).

Each threshold voltage V _TH (t) in FIGS. 7A and 7B can be expressed by Expression (8).
V _TH (t) = V _CNST + V _CMP (t) (8)
V _CNST is a constant component, and V _CMP (t) is a correction component that increases with time. More specifically, the correction component V _CMP (t) increases monotonously with time.

The correction component V _CMP (t) of the waveform (ii) in FIG. 7A satisfies Expression (9).
V _CMP (t) = β × t (9)
β is a predetermined coefficient.

The correction component V _CMP (t) of the waveform (iii) in FIG. 7B increases with time and converges to a predetermined level with the passage of time.

Next, a configuration example of the threshold voltage generation unit 304 will be described.
FIG. 8 is a circuit diagram of the threshold voltage generation unit 304 according to the first configuration example. The threshold voltage generation unit 304 includes a current source 310, a capacitor C2, resistors R1 and R2, and a switch SW1. The current source 310 generates a constant current Ic. For example, the current source 310 includes a constant current source 312 and a current mirror circuit 314.
The capacitor C2 is charged with a constant current Ic. The first resistor R1 is provided in parallel with the capacitor C2. The second resistor R2 and the switch SW1 are provided in parallel with the capacitor C2. Switch SW1 is switched in synchronism with the pulse signal _{S PWM,} off the ON period _{T ON} of the switching transistor M1.

According to the threshold voltage generator 304 of FIG. 8, the threshold voltage _V TH at the ON period _{T ON} of the switching transistor M1 (t) varies according to a CR time constant. Thereby, the threshold voltage V _TH (t) according to the waveform (iii) of FIG. 7B can be generated.

FIG. 9 is a circuit diagram of the threshold voltage generation unit 304 according to the second configuration example.
The threshold voltage generation unit 304 includes a reference current generation unit 320, a correction current generation unit 330, and a current / voltage conversion circuit 340.

Reference current generation unit 320 generates a reference current I _CNST having a predetermined current amount. The correction current generation unit 330 generates a correction current I _CMP (t) that is synchronized with the switching of the switching transistor M1 and increases with time in the _ON period TON of the switching transistor M1. The configurations of the reference current generation unit 320 and the correction current generation unit 330 are not particularly limited.

The current-voltage conversion circuit 340 converts a current obtained by adding the reference current I _CNST and the correction current I _CMP (t) into a threshold voltage V _TH (t). For example, the current-voltage conversion circuit 340 includes a resistor R3 provided on a current path obtained by adding the reference current I _CNST and the correction current I _CMP (t), and the voltage drop of the resistor R3 is reduced to the threshold voltage V _TH ( t).

The threshold voltage generation unit 304 of FIG. 9 can generate the threshold voltage V _TH (t) according to the waveform (ii) of FIG. The waveform (ii) in FIG. 7A has a smaller error from the ideal waveform (i) than the waveform (iii) in FIG. Therefore, according to the threshold voltage generation unit 304 of FIG. 9, fluctuations and variations in the effective threshold voltage V _{TH_EFF} can be further suppressed as compared with the threshold voltage generation unit 304 of FIG. 8.

(Modification)
The present invention has been described based on the embodiments. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are within the scope of the present invention. is there. Hereinafter, such modifications will be described.

(First modification)
In the embodiment, the current detection circuit 300, a case has been described to be used for overcurrent protection present invention is not limited thereto, it is available in a variety of signal processing based on the coil current I _P.

(Second modification)
In the embodiment, the control circuit of the flyback DC / DC converter 100 has been described, but the type of the switching converter is not particularly limited. For example, the DC / DC converter 100 may be a forward type, a buck converter or a boost converter. The topology of the output circuit 102 may be changed according to the type of the switching converter.

(Use)
Finally, the use of the AC / DC converter 400 will be described. The AC / DC converter 400 is suitably used for an AC adapter or a power supply block of an electronic device.

FIG. 10 is a diagram illustrating an AC adapter 800 including an AC / DC converter. The AC adapter 800 includes a plug 802, a housing 804, and a connector 806. Plug 802 is subjected to a commercial AC voltage _{V AC} from the wall outlet (not shown). The AC / DC converter is mounted in the housing 804. The DC output voltage V _OUT generated by the AC / DC converter is supplied from the connector 806 to the electronic device 810. Examples of the electronic device 810 include a notebook PC, a digital camera, a digital video camera, a mobile phone, and a mobile audio player.

FIGS. 11A and 11B are diagrams illustrating an electronic device 900 including an AC / DC converter. Although the electronic device 900 in FIGS. 11A and 11B is a display device, the type of the electronic device 900 is not particularly limited, and is a device including a power supply device such as an audio device, a refrigerator, a washing machine, or a vacuum cleaner. I just need it.
Plug 902, receives a commercial AC voltage _{V AC} from the wall outlet (not shown). The AC / DC converter is mounted in the housing 804. The DC output voltage VOUT generated by the AC / DC converter is supplied to a load such as a microcomputer, a DSP (Digital Signal Processor), a power supply circuit, a lighting device, an analog circuit, or a digital circuit mounted in the same housing 904. The

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement are permitted without departing from the spirit of the present invention.

400 ... AC / DC converter 402 ... rectifier circuit 404 ... smoothing capacitor 100 ... DC / DC converter 102 ... output circuit 104 ... input line 106 ... output line 108 ... feedback circuit 110 ... shunt regulator 112 ... photo coupler, 200 ... control circuit, 202 ... inverting amplifier, 204 ... error comparator, 206 ... logic unit, 208 ... driver, M1 ... switching transistors, R _{CS ...} detection resistor, T1 ... transformer, _{L P} ... 1 primary coil, L _S ... secondary coil, C1 ... output capacitor, D1 ... rectifier diode, 300 ... current detection circuit, 302 ... comparator, 304 ... threshold voltage generator, 310 ... current source, 312 ... constant current source, 314 ... current Mirror circuit, R1... First resistor, R2... Second resistor, SW1 Switch, C2 ... Capacitor, 320 ... Reference current generator, 330 ... Correction current generator, 340 ... Current-voltage conversion circuit, 800 ... AC adapter, 802 ... Plug, 804 ... Housing, 806 ... Connector, 810,900 ... Electronic Equipment, 902... Plug, 904.

Claims (16)

A control circuit used in a switching converter,
The switching converter includes at least a switching transistor and a coil connected to the switching transistor,
The control circuit includes a current detection circuit that compares a current flowing through the coil during an ON period of the switching transistor with a predetermined threshold current.
The current detection circuit includes:
A threshold voltage generator that generates a threshold voltage that increases with time in the ON period of the switching transistor;
A comparator that compares a detection voltage corresponding to a current flowing through the switching transistor with the threshold voltage;
A control circuit comprising:   The control circuit according to claim 1, wherein the threshold voltage monotonously increases during an ON period of the switching transistor. When the response delay of the comparator is τ _D , the threshold voltage V _TH (t) is
V _TH (t) = V _TH — _EFF × t / (t + τ _D )
The control circuit according to claim 1, wherein the control circuit has a waveform approximated thereto. 4. The control circuit according to claim 1, wherein the threshold voltage V _TH includes a constant component and a correction component that increases with time during an ON period of the switching transistor.   The control circuit according to claim 4, wherein the correction component converges to a predetermined level over time.   The control circuit according to claim 4, wherein the correction component increases with a substantially constant slope during an ON period of the switching transistor. The threshold voltage generator is
A current source that generates a constant current;
A capacitor charged by the current source;
A resistor connected to the capacitor;
And is configured to output the voltage of the capacitor as the threshold voltage,
5. The control circuit according to claim 1, wherein the threshold voltage changes according to a time constant determined by the capacitor and the resistor. The threshold voltage generator is
A reference current generator for generating the predetermined reference current;
A correction current generator that is synchronized with the switching of the switching transistor and generates a correction current that increases with time in an ON period of the switching transistor;
A current-voltage conversion circuit that converts a current obtained by adding the reference current and the correction current into the threshold voltage;
The control circuit according to claim 1, further comprising: The comparator is provided for overcurrent detection,
9. The control circuit according to claim 1, wherein the control circuit performs a predetermined overcurrent protection process when the output of the comparator indicates that the detected voltage is higher than the threshold voltage. Control circuit. The control circuit includes:
A feedback terminal for receiving a feedback voltage whose value is adjusted so that the output voltage of the switching converter approaches a predetermined target value;
An error comparator that compares the detected voltage with the feedback voltage and generates an off signal that is asserted when the detected voltage is higher than the feedback voltage;
A logic unit that generates a pulse signal that transitions to an off level triggered by the off signal;
The control circuit according to claim 1, further comprising:   11. The control circuit according to claim 1, wherein the control circuit is integrated on a single semiconductor substrate. An output circuit including at least a coil, a switching transistor connected to the coil, and a detection resistor provided in series with the switching transistor;
A control circuit according to any one of claims 1 to 11,
A switching converter comprising: A rectifier circuit for rectifying an alternating voltage;
A smoothing capacitor for smoothing the output voltage of the rectifier circuit;
A switching converter that receives the voltage of the smoothing capacitor as an input voltage;
With
The switching converter is
An output circuit including at least a coil, a switching transistor connected to the coil, and a detection resistor provided in series with the switching transistor;
A control circuit according to any one of claims 1 to 12,
An AC / DC converter comprising: Load,
The AC / DC converter according to claim 13, which supplies a DC voltage to the load.
An electronic device comprising:   A power adapter comprising the AC / DC converter according to claim 13. A current detection method in a switching converter,
The switching converter includes at least a switching transistor and a coil connected to the switching transistor,
The current detection method includes:
Generating a detection voltage according to a current flowing through the switching transistor;
Generating a threshold voltage that increases with time during an on period of the switching transistor;
Comparing the detected voltage with the threshold voltage;
A current detection method comprising:

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