JP6271175B2 - AC / DC converter and its control circuit, power adapter and electronic device - Google Patents

Description

  The present invention relates to an AC / DC 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 1r examined by the present inventors. The AC / DC converter 1r mainly includes a fuse 2, an input capacitor Ci, a filter 3, a first rectifier circuit 4, a smoothing capacitor Cs, and a DC / DC converter 6r.

Commercial AC voltage V _AC is input to the filter 3 through a fuse 2 and the input capacitor Ci. Filter 3 removes commercial AC voltage _{V AC} noise. The first rectifier circuit 4, a diode bridge circuit for full-wave rectifying the commercial AC voltage V _AC. The output voltage of the first rectifier circuit 4 is smoothed by the smoothing capacitor Cs and converted to a direct-current voltage _VDC .

The insulated DC / DC converter 6r receives a DC voltage V _DC at the input terminal P1, steps down the voltage, and connects the output voltage _VOUT stabilized at a target value to the output terminal P2 (not shown). ).

The DC / DC converter 6r includes a control circuit 10r, an output circuit 7, and a feedback circuit 8. The output circuit 7 includes a transformer T1, a first diode D1, and a first output capacitor Co1. The feedback circuit 8 generates a feedback voltage V _FB corresponding to the output voltage _VOUT and supplies it to the feedback terminal (FB terminal) of the control circuit 10r.

  The control circuit 10r includes a switching transistor M1, a drive circuit 20, a starter circuit 30, a clamp circuit 40, and an AC voltage detection circuit 50.

The drain (DRAIN) terminal of the control circuit 10r is connected to the primary winding W1. A source (SOURCE) terminal of the control circuit 10r is connected to an external detection resistor Rs. The switching transistor M1 is provided between the DRAIN terminal and the SOURCE terminal. The drive circuit 20 switches the switching transistor M1 based on the feedback voltage _VFB .

When the switching transistor M1 is switched, the input voltage V _DC is stepped down, and the output voltage _VOUT is generated. Then, the control circuit 10r adjusts the switching duty ratio of the switching transistor M1, thereby stabilizing the output voltage _VOUT at the target value and controlling the coil current Ip flowing through the primary winding W1 of the transformer T1.

The detection resistor _RS is provided in series with the primary winding W1 of the transformer T1 and the switching transistor M1. Sense resistor is _{R S,} a voltage drop proportional to the current Ip flowing through the primary winding W1 and the switching transistor M1 (the detection _{voltage) V CS} is generated. Control circuit 10r, based on the detection voltage _{V CS,} and controls the current Ip flowing through the primary winding W1.

A second diode D2 and a second output capacitor Co2 are connected to the auxiliary winding W3 of the transformer T1. In response to switching of the switching transistor M1, a DC voltage _VCC is generated in the second output capacitor Co2. The DC voltage _{V CC} is supplied to the power supply of the control circuit 10r (VCC) terminal.

Internal circuit of the control circuit 10r including the driving circuit 20 operates by receiving power supply voltage _{V CC} generated in the VCC terminal. Before the DC / DC converter 6r is activated, the power supply voltage _VCC is zero, so that the control circuit 10r cannot be activated. Therefore, the control circuit 10r includes a starter circuit 30 provided between the DRAIN terminal and the VCC terminal. The starter circuit 30 is connected to the VCC terminal by the input voltage V _DC supplied via the primary winding W1 before starting the switching of the switching transistor M1 of the DC / DC converter 6r when the DC / DC converter 6r is started. The second output capacitor Co2 connected to is charged. Thus, before the switching of the switching transistor M1 is started, it is possible to increase the power supply voltage V _CC to control circuit 10r is operable voltage level.

By the charging by the starter circuit 30, the voltage _{V CC} of the VCC pin can be quite high. Clamp circuit 40 is supplied to the internal circuit clamps the voltage _{V CC} of the VCC terminal to a predetermined upper limit voltage (e.g., 12V) or less, including a drive circuit 20.

AC / DC converters 1r are used in various environments, the AC voltage V _AC input thereto, which may or greater than the rated value, or smaller. If the DC / DC converter 6r is operated in a situation where the _AC voltage VAC is not rated, there is a possibility that a problem may occur.

Therefore, the control circuit 10r is equipped with a function for monitoring the _AC voltage VAC. The second rectifier circuit 9 performs full-wave rectification on the _AC voltage VAC. The first rectified voltage V _RECT generated by the second rectifier circuit 9 may exceed several hundred volts and cannot be input to the control circuit 10r as it is. So first rectified voltage _{V RECT} by resistors R11, R12 pressurized is divided, the voltage _{V IN} after the division is input to the control circuit inputs 10r (VIN) terminal.

AC voltage detection circuit 50, based on the second rectified voltage _{V IN} of the VIN terminal, to detect whether the amplitude _{A AC} of the AC voltage _{V AC} is included in a predetermined range. More specifically, the AC voltage detecting circuit 50, the amplitude _{A AC} of the AC voltage _{V AC} is lower than the threshold amplitude _{A BO,} stops DC / DC converter 6r. This is also called a low AC input voltage protection (brown out) function. In addition, the amplitude of the AC voltage _{V AC} is higher than the threshold amplitude _{A BO,} resume the operation of the DC / DC converter 6r (brown-in).

Capacitance components including the capacitor Ci are present between the positive electrode terminal and the negative electrode terminal to which the _AC voltage VAC is input. Charge remains in this capacitive component even after the outlet plug is removed. As a result, the DC voltage _VDC does not immediately become zero. The AC / DC converter 1r used for the power adapter and the electronic device is required to decrease the voltage at the positive terminal and the negative terminal within a predetermined time after the outlet plug is removed. . Therefore, in the AC / DC converter 1r shown in FIG. 1, a discharge resistor Rdis is inserted between the positive electrode terminal and the negative electrode terminal.

  As a result of studying the AC / DC converter 1r shown in FIG. 1, the inventor has recognized the following problems.

In the AC / DC converter 1r shown in FIG. 1, in order to monitor the _AC voltage VAC, high-breakdown-voltage resistors R11 and R12 that are externally attached to the control circuit 10r are required. Such an external resistance element is expensive and requires an extra mounting area.

  The present invention has been made in view of these problems, and one of exemplary purposes of an embodiment thereof is to provide an AC / DC converter capable of reducing an external resistance and a control circuit thereof.

  One embodiment of the present invention relates to a control circuit used in an AC / DC converter that converts an AC voltage into a first DC voltage. The AC / DC converter includes a first rectifier circuit that rectifies and smoothes an AC voltage and converts the AC voltage into a second DC voltage, an insulating DC / DC converter that boosts or reduces the second DC voltage, and a first DC voltage. And a feedback circuit for generating a feedback voltage according to. The control circuit includes an input detection terminal to which a first rectified voltage obtained by rectifying an AC voltage is input, and a third DC voltage generated on the primary side of the DC / DC converter transformer in the operating state of the DC / DC converter. Is a high-voltage depletion type first NMOS whose input is connected to an external capacitor, a feedback terminal that receives a feedback voltage, a drain connected to the input detection terminal, and a gate connected to the source. An AC voltage based on a second rectified voltage generated at a connection node of the first resistor and the second resistor, including a transistor, a first resistor and a second resistor provided in series between the source of the first NMOS transistor and the ground line AC voltage detection circuit for detecting the amplitude of the AC / DC converter, and provided between the input detection terminal and the power supply terminal. In operation, the starter circuit including a path for charging the capacitor connected to the power supply terminal by the first rectified voltage and the power supply terminal is operated, and the switching transistor of the DC / DC converter is driven according to the feedback voltage. And a driving circuit that is integrated on a single semiconductor substrate.

A high voltage of several hundred volts can be applied to the input detection terminal. By connecting a high breakdown voltage depletion-type first NMOS transistor to the input detection terminal, the current flowing through the first NMOS transistor, the first resistor, and the second resistor is limited to the vicinity of a predetermined maximum current I _MAX . Thereby, the voltage between both ends of the first resistor and the second resistor, in other words, the source voltage Vs of the first NMOS transistor can be clamped in the vicinity of V _MAX = (R1 + R2) × I _MAX . That is, it is possible to prevent a high voltage exceeding V _MAX from being applied to the first resistor R1 and the second resistor R2. According to this aspect, since the first resistor R1 and the second resistor R2 may have low withstand voltages, they can be formed of elements on the semiconductor substrate, and the external resistance can be reduced.

“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 as one integrated circuit (IC), the circuit area can be reduced and the characteristics of the circuit elements can be kept uniform.

In one embodiment, the control circuit may further include a third resistor inserted between the source of the first NMOS transistor and the first resistor. The gate of the first NMOS transistor is connected to the connection node between the source and the first resistor.
According to this aspect, the temperature characteristic of the first NMOS transistor can be canceled by the third resistor, and the source voltage of the first NMOS transistor can be stabilized.

The control circuit according to an aspect may further include a discharge transistor provided between the power supply terminal and the ground line.
According to this aspect, since the capacity between the positive electrode and the negative electrode can be discharged by turning on the discharge transistor after the outlet plug is removed, an external discharge resistor is not required, and the cost is reduced. Can be lowered.

  The starter circuit may include a high breakdown voltage second NMOS transistor and a gate control circuit that controls the gate of the second NMOS transistor. The second NMOS transistor is a depletion type, and the source and the back gate may be connected in common.

  The AC voltage detection circuit compares the second rectified voltage with a predetermined threshold voltage, generates a comparison signal indicating the comparison result, and corresponds to the amplitude and threshold voltage of the AC voltage based on the comparison signal. And a logic unit that determines the magnitude relationship of the threshold amplitude.

The logic unit may determine that the amplitude of the AC voltage is smaller than the threshold amplitude when the comparison signal maintains a constant level continuously over a predetermined number of cycles of the AC voltage.
According to this aspect, it is possible to detect a voltage abnormality for a short time over several cycles of the AC frequency.

  The AC voltage detection circuit may stop the DC / DC converter when the amplitude of the AC voltage is smaller than a threshold amplitude set lower than the rated amplitude.

  The size of the first NMOS transistor and the resistance values of the first resistor and the second resistor may be determined so that the first NMOS transistor operates in a linear region when the amplitude of the AC voltage is included in the detection range.

  The threshold voltage may have hysteresis.

  Another aspect of the present invention relates to an AC / DC converter that converts an AC voltage into a first DC voltage. The AC / DC converter rectifies and smoothes AC voltage, converts it to a second DC voltage, an isolated DC / DC converter that boosts or reduces the second DC voltage, and rectifies the AC voltage. A transformer having a second rectifier circuit for generating a first rectified voltage, a primary winding provided on the primary side, an auxiliary winding, and a secondary winding provided on the secondary side; A switching transistor connected to the secondary winding; a first diode having an anode connected to the secondary winding; a first output capacitor having one end grounded and the other end connected to the cathode of the first diode; A second diode connected to the auxiliary winding, a second output capacitor having one end grounded and the other end connected to the cathode of the second diode, and a feed corresponding to the first DC voltage generated in the first output capacitor Comprising a feedback circuit for generating a click voltage, and any of the control circuits described above, the. A second DC voltage is input to the input detection terminal of the control circuit, a second DC voltage generated in the second output capacitor is supplied to the power supply terminal, and a feedback voltage is input to the feedback terminal.

  The feedback circuit includes a shunt regulator that generates a feedback signal whose level is adjusted so that an error between a voltage obtained by dividing the first DC voltage and a predetermined target value becomes zero, and a light emitting element on the primary side thereof includes a feedback signal. A signal generated in the light receiving element on the secondary side of the photocoupler may be supplied to the control circuit as a feedback voltage.

  The AC / DC converter may further include a filter that filters the AC voltage. The second rectifier circuit may rectify the AC voltage after passing through the filter.

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

  Another aspect of the present invention relates to a power adapter. The power adapter includes 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, external resistance can be reduced.

It is a block diagram of the AC / DC converter which this inventor examined. It is a circuit diagram which shows the AC / DC converter provided with the control circuit which concerns on embodiment. It is a figure which shows the current-voltage characteristic of a depletion type NMOS transistor. 4A and 4B are operation waveform diagrams of the AC / DC converter of FIG. It is a circuit diagram which shows a part of control circuit which concerns on a 1st modification. It is a figure which shows an AC adapter provided with an AC / DC converter. FIGS. 7A and 7B 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. 2 is a circuit diagram illustrating the AC / DC converter 1 including the control circuit 10 according to the embodiment. AC / DC converter 1 converts an AC voltage _{V AC} to the first DC voltage _{V OUT.} Since the basic configuration of the AC / DC converter 1 is the same as that of the AC / DC converter 1r shown in FIG. 1, only the differences will be described below.

The feedback circuit 8 generates a feedback voltage V _FB corresponding to the first DC voltage _VOUT . For example, the feedback circuit 8 includes a shunt regulator 81 and a photocoupler 82. The shunt regulator 81 generates a feedback signal S1 whose level is adjusted so that an error between a voltage obtained by dividing the first DC voltage _VOUT and a predetermined target value _VREF becomes zero.

In the photocoupler 82, 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 82 is input to the FB terminal of the control circuit 10 as the feedback voltage _VFB. .

Hereinafter, a specific configuration of the control circuit 10 will be described.
Second rectifier circuit 9, the AC voltage _{V AC,} rectifies the AC voltage _{V AC} passed through the filter 3 and more specifically, to generate a first rectified voltage _{V RECT1.} The first rectified voltage V _RECT1 is input to the input detection terminal VIN of the control circuit 10.

An external second output capacitor Co2 is connected to the VCC terminal, and the third DC voltage generated on the primary side of the transformer T1 of the DC / DC converter 6 in the operating state of the DC / DC converter 6 is connected to the VCC terminal. (also referred to as a power supply _{voltage) V CC} is supplied.

  The control circuit 10 includes a switching transistor M1, a drive circuit 20, a starter circuit 30, a clamp circuit 40, a first NMOS transistor MN1, an AC voltage detection circuit 50, a discharge transistor MN3, and a discharge controller 60, and is integrated on a single semiconductor substrate. It becomes.

  The switching transistor M1, the drive circuit 20, and the clamp circuit 40 have already been described. Note that the configurations of the drive circuit 20 and the clamp circuit 40 are not particularly limited, and a known circuit may be used. The drive circuit 20 may include a peak current mode or average current mode modulator.

The first NMOS transistor MN1 is a high breakdown voltage transistor and is configured as a depletion type. “High breakdown voltage” means having a breakdown voltage that can withstand the maximum voltage that can be applied to the input detection terminal VIN. The input detection terminal VIN, the peak voltage of approximately 140V at the AC voltage _{V AC} rated 100V is applied, the temporal variation of the AC voltage _{V AC,} the input detection terminal VIN, the high voltage of about 700V Can be applied. In this case, the first NMOS transistor MN1 is composed of a 700V withstand voltage element. Since the device structure of the high voltage transistor element may be a known technique, the description thereof is omitted.

  The drain of the first NMOS transistor MN1 is connected to the input detection terminal VIN, and its gate is connected to its source. The back gate of the first NMOS transistor MN1 is also connected to the source.

The AC voltage detection circuit 50 includes a first resistor R1 and a second resistor R2 provided in series between the source of the first NMOS transistor MN1 and the ground line. The source voltage Vs of the first NMOS transistor MN1 is divided by the first resistor R1 and the second resistor R2. The AC voltage detection circuit 50 detects the amplitude A _AC of the _AC voltage V _AC based on the second rectified voltage V _RECT2 generated at the connection node N1 of the first resistor R1 and the second resistor R2.

In the present embodiment, the AC voltage detection circuit 50 is used for the brownout / brownin function. The AC voltage detection circuit 50 determines the magnitude relationship between the amplitude A _{AC of the AC} voltage V _AC and a predetermined threshold amplitude A _BO based on the second rectified voltage V _RECT2, and _browns when A _AC <A _BO . Out and brown in when A _AC > A _BO . The threshold of the brown-out, if you want to set the effective value of 72V of the AC voltage _{V AC,} is the threshold amplitude _{A BO} for brown-out, is about 72x√2 = 101V.

In order to prevent chattering, the threshold amplitude A _BO preferably has hysteresis. Specifically, brown-in may be performed when the effective value of _AC voltage VAC is lower than 72V and browned out, and the effective value is higher than 80V. In this case, the threshold amplitude A _BO may be changed between two values of about 101V and 113V.

AC voltage detection circuit 50 includes, for example, a comparator 51 and a logic unit 52. The comparator 51 compares the second rectified voltage V _RECT2 with a predetermined threshold voltage V _TH determined corresponding to the threshold amplitude A _BO and generates a comparison signal S2 indicating the comparison result. The logic unit 52 determines the magnitude relationship between the amplitude A _{AC of the AC} voltage VAC and the threshold amplitude A _BO based on the comparison signal S2.

FIG. 3 is a diagram showing current-voltage characteristics of a depletion type NMOS transistor. The horizontal axis represents the drain-source voltage Vds, and the vertical axis represents the drain current Id. The drain current Id is limited near a predetermined maximum current value I _MAX when the drain-source voltage Vds increases.

The control circuit 10, when the amplitude _{A AC} of the AC voltage _{V AC} is contained in the range (effective value 101V~141V of the AC voltage _{V AC)} to cover the nominal amplitude and the threshold amplitude _{A BO,} i.e. the AC voltage _{V AC} of when the amplitude a _AC is included in the detection range, as the 1NMOS transistor MN1 is operated in the linear region, the operating point of the circuit is determined. Specifically, this operating point can be set according to the size of the first NMOS transistor MN1 and the resistance values of the first resistor R1 and the second resistor R2.

When the amplitude A _{AC of the AC} voltage V _AC is smaller than the rated amplitude (141 V), the first NMOS transistor MN1 operates in the linear region. Therefore, the drain current Id depends on the amplitude A _AC of the _AC voltage, and a certain function f ( ) Is used to express the equation (1).
Id = f (A _AC ) (1)

The second rectified voltage V _RECT2 is given by equation (2).
V _RECT2 = Id × R2 (2)

Equation (3) is obtained from equations (1) and (2).
V _RECT2 = f (A _AC ) × R2 (3)
That is, in the linear region, the second rectified voltage V _RECT2 has a one-to-one correspondence with the amplitude A _AC .

The threshold voltage V _TH is set by equation (4).
V _TH = f (A _BO ) × R2 (4)

When the 1NMOS transistor MN1 enters the saturation region, the drain current Id is substantially takes a constant value _{I MAX.} That drain current Id is limited to the vicinity of the maximum current value _{I MAX.} When the amplitude A _{AC of the AC} voltage V _AC exceeds the voltage corresponding to the boundary between the linear region and the saturation region (hereinafter referred to as the clamp voltage V _CL ), the voltage across the first resistor R1 and the second resistor R2, in other words, For example, the source voltage Vs of the first NMOS transistor MN1 is clamped around V _MAX = (R1 + R2) × I _MAX .

Returning to FIG. For example, when the comparison signal S2 maintains a constant level (for example, low level) continuously over a predetermined number of cycles of the _AC voltage VAC, the logic unit 52 determines that A _AC <A _BO and stops the drive circuit 20. .

The starter circuit 30 is provided between the input detection terminal VIN and the VCC terminal, and charges the second output capacitor Co2 connected to the VCC terminal by the first rectified voltage V _RECT1 when the AC / DC converter 1 is started. including. Preferably, the starter circuit 30 includes a second NMOS transistor MN2 that is a high voltage transistor having a withstand voltage of 700 V, and a gate control circuit 32 that controls on / off of the second NMOS transistor MN2. For example, the second NMOS transistor MN2 may be the same depletion type as the first NMOS transistor MN1, and the source and back gate may be connected in common.

The discharge transistor MN3 is provided between the VCC terminal and the ground line. When the outlet plug is pulled out, the discharge controller 60 turns on the discharge transistor MN3. The state in which the outlet plug is pulled out can be detected by the AC voltage detection circuit 50 because the second rectified voltage V _RECT2 becomes substantially zero.

  The above is the configuration of the control circuit 10 and the AC / DC converter 1 including the control circuit 10. Next, the operation will be described.

4A and 4B are operation waveform diagrams of the AC / DC converter 1 of FIG. FIG. 4A shows an operation when the amplitude A _{AC of the AC} voltage V _AC is the rated amplitude 141 V (solid line), and an operation when the AC amplitude A _AC is smaller than the threshold amplitude A _BO (one-dot chain line). It is.

When the AC voltage _{V AC} is near the rated, second 1NMOS transistor MN1, the voltage-current characteristic of FIG. 3, it operates in a linear region. At this time, the second rectified voltage V _RECT2 becomes higher than the threshold value V _TH every cycle of the _AC voltage _VAC , and the comparison signal S2 becomes high level every cycle.

When the amplitude A _{AC of the AC} voltage V _AC becomes smaller than the threshold amplitude A _{BO, the} amplitude of the second rectified voltage V _RECT2 becomes smaller and lower than the threshold voltage V _TH . As a result, the comparison signal S2 maintains a low level. Logic unit 52 determines when it detects that the comparison signal S2 is sustained at a low level for several cycles, as amplitude A _AC of the AC voltage V _AC is smaller than the threshold amplitude A _BO. In this case, the logic unit 52 stops the drive circuit 20.

FIG. 4B shows an operation when the amplitude A _{AC of the AC} voltage V _AC becomes very large. The amplitude of the AC voltage _{V AC} is increased, the range exceeding the clamp voltage _{V CL,} the current-voltage characteristics of Figure 3, the drain current Id is limited at about a predetermined value _{I MAX,} the source voltage Vs of the 1NMOS transistor MN1 Is clamped near a predetermined voltage V _MAX = I _MAX × (R1 + R2).

  The operation of the AC / DC converter 1 has been described above.

In the AC / DC converter 1, the maximum current value I _MAX can be designed according to the transistor size of the first NMOS transistor MN1. Therefore, the voltage V _MAX can be set to an arbitrary value, for example, about 100 V, according to the transistor size of the first NMOS transistor MN1 and the resistors R1 and R2.

  As a result, it is possible to prevent a high voltage of several hundreds V (up to 700V) from being applied to the first resistor R1 and the second resistor R2, and the first resistor R1 and the second resistor R2 on the semiconductor substrate having a 100V breakdown voltage These elements can be used. That is, the external resistors R11 and R22 that are necessary in FIG. 1 can be reduced, and the cost can be reduced.

  Further, when the outlet plug is pulled out, the discharge transistor MN3 is turned on. Thereby, the electric charge of the capacity | capacitance between + electrode and-electrode can be extracted by the path | route of the 2nd rectifier circuit 9, the starter circuit 30, and discharge transistor MN3. Therefore, it is possible to reduce the discharge resistance Rdis required in the AC / DC converter 1r of FIG. 1 and to reduce the cost.

(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)
FIG. 5 is a circuit diagram showing a part of the control circuit 10a according to the first modification. The control circuit 10a further includes a third resistor R3 inserted between the source of the first NMOS transistor MN1 and the first resistor R1. The gate of the first NMOS transistor MN1 is connected to the connection node N2 of the source and the first resistor R1.

The current Id flowing through the first NMOS transistor MN1 depends not only on the gate-source voltage but also on the temperature. According to this modification, the current Id of the 1NMOS transistor MN1 with temperature fluctuation tends to increase, the voltage drop _{V R3} of the third resistor R3 is increased, a bias voltage between the gate and the source of the first 1NMOS transistor MN1 , The current Id is shifted in the direction of decreasing. That is, the temperature characteristic of the first NMOS transistor MN1 can be canceled by the third resistor R3, and the source voltage Vs of the first NMOS transistor MN1 can be stabilized.

(Modification 2)
In the embodiment, the case where the threshold amplitude is set to the burnout threshold in the AC voltage detection circuit 50 has been described, but the present invention is not limited to this. For example, the threshold amplitude may be set to a voltage level for overvoltage detection higher than the rated amplitude 141V.

The AC voltage detection circuit 50 may compare the amplitude A _AC with a plurality of threshold values.

(Modification 3)
In the embodiment, the case where the shunt regulator (error amplifier) 212 is provided on the secondary side of the transformer T1 has been described. However, this error amplifier may be provided on the primary side, and further incorporated in the control circuit 10. May be.

(Modification 4)
The DC / DC converter 6 is not limited to the step-down type, and may be a step-up type or a step-up / step-down type.

(Use)
Finally, the use of the AC / DC converter 1 will be described. The AC / DC converter 1 is suitably used for an AC adapter or a power supply block of an electronic device.

FIG. 6 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 receives commercial AC voltage VAC from an 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. 7A and 7B are diagrams illustrating an electronic device 900 including an AC / DC converter. 7A and 7B is a display device, but 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.
The commercial AC voltage VAC is received from the plug 902 and an 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.

P1 ... input terminal, P2 ... output terminal, Co1 ... first output capacitor, Co2 ... second output capacitor, D1 ... first diode, D2 ... second diode, T1 ... transformer, W1 ... primary winding, W2 ... 2 Next winding, W3 ... auxiliary winding, M1 ... switching transistor, RS ... detection resistor, 1 ... AC / DC converter, 2 ... fuse, Ci ... input capacitor, 3 ... filter, 4 ... first rectifier circuit, Cs ... smoothing Capacitor 6 ... DC / DC converter 7 ... Output circuit 8 ... Feedback circuit 9 ... Second rectifier circuit 10 ... Control circuit 20 ... Drive circuit 30 ... Starter circuit 32 ... Gate control circuit 40 ... Clamp Circuit: 50 ... AC voltage detection circuit, 51 ... Comparator, 52 ... Logic unit, 60 ... Discharge control unit, 81 ... Shunt regulator, 82 ... Photocoupler, V C ... power supply terminal, VIN ... input detection terminal, VAC ... alternating voltage, VOUT ... first DC voltage, VDC ... second DC voltage, VCC ... power supply voltage, VRECT1 ... first rectified voltage, VRECT2 ... second rectified voltage, R1 ... first resistor, R2 ... second resistor, R3 ... third resistor, MN1 ... first NMOS transistor, MN2 ... second NMOS transistor, MN3 ... discharge transistor, 800 ... AC adapter, 802 ... plug, 804 ... housing, 806 ... Connector, 810, 900 ... electronic device, 902 ... plug, 904 ... housing.

Claims (13)

A control circuit used in an AC / DC converter that converts an AC voltage into a first DC voltage,
The AC / DC converter
A first rectifier circuit that rectifies and smoothes the AC voltage and converts it to a second DC voltage;
An insulated DC / DC converter that boosts or steps down the second DC voltage;
A feedback circuit for generating a feedback voltage according to the first DC voltage;
With
The control circuit includes:
An input detection terminal to which a first rectified voltage that rectifies the AC voltage and is not smoothed is input;
An external capacitor is connected, and in the operating state of the DC / DC converter, a power supply terminal to which a third DC voltage generated on the primary side of the transformer of the DC / DC converter is supplied;
A discharge transistor provided between the power supply terminal and a ground line;
A discharge controller for controlling the discharge transistor;
A feedback terminal for receiving the feedback voltage;
A depletion-type high breakdown voltage first NMOS transistor having a drain connected to the input detection terminal and a gate connected to the source;
The AC includes a first resistor and a second resistor provided in series between a source of the first NMOS transistor and a ground line, and the AC is based on a second rectified voltage generated at a connection node of the first resistor and the second resistor. An AC voltage detection circuit for detecting the amplitude of the voltage;
A starter circuit including a path that is provided between the input detection terminal and the power supply terminal and charges a capacitor connected to the power supply terminal by the first rectified voltage when the AC / DC converter is activated;
A drive circuit that operates in response to the voltage of the power supply terminal and drives the switching transistor of the DC / DC converter according to the feedback voltage;
Integrated into a single semiconductor substrate,
The AC voltage detection circuit includes a logic unit that determines a magnitude relationship between the amplitude of the AC voltage and a threshold amplitude corresponding to a predetermined threshold voltage,
The logic unit stops the DC / DC converter when determining that the amplitude of the AC voltage is smaller than the threshold amplitude, and controls the discharge control unit to turn on the discharge transistor to thereby turn on the power supply terminal. A control circuit for discharging the capacitor connected to the capacitor. A third resistor inserted between the source of the first NMOS transistor and the first resistor;
The control circuit according to claim 1, wherein a gate of the first NMOS transistor is connected to a connection node between the third resistor and the first resistor.   The control circuit according to claim 1, wherein the starter circuit includes a high-breakdown-voltage second NMOS transistor.   4. The control circuit according to claim 3, wherein the second NMOS transistor is a depletion type, and a source and a back gate are connected in common. The AC voltage detection circuit is:
A comparator that compares the second rectified voltage with the threshold voltage and generates a comparison signal indicating a comparison result;
The logic unit that determines a magnitude relationship between the amplitude of the AC voltage and the threshold amplitude corresponding to the threshold voltage based on the comparison signal;
The control circuit according to claim 1, further comprising:   The logic unit determines that the amplitude of the AC voltage is smaller than the threshold amplitude when the comparison signal maintains a constant level continuously over a predetermined number of cycles of the AC voltage. Item 6. The control circuit according to Item 5.   The size of the first NMOS transistor and the resistance values of the first resistor and the second resistor are determined such that the first NMOS transistor operates in a linear region when the amplitude of the AC voltage is included in a detection range. The control circuit according to claim 1, wherein the control circuit is characterized in that:   8. The control circuit according to claim 1, wherein the threshold voltage has a hysteresis. An AC / DC converter for converting an AC voltage into a first DC voltage,
A first rectifier circuit that rectifies and smoothes the AC voltage and converts it to a second DC voltage;
An insulated DC / DC converter that boosts or steps down the second DC voltage;
A second rectifier circuit that rectifies the alternating voltage and generates a first rectified voltage;
A transformer having a primary winding provided on the primary side, an auxiliary winding, and a secondary winding provided on the secondary side;
A switching transistor connected to the primary winding of the transformer;
A first diode having an anode connected to the secondary winding;
A first output capacitor having one end grounded and the other end connected to the cathode of the first diode;
A second diode having an anode connected to the auxiliary winding;
A second output capacitor having one end grounded and the other end connected to the cathode of the second diode;
A feedback circuit for generating a feedback voltage according to the first DC voltage generated in the first output capacitor;
The second DC voltage is input to the input detection terminal, the second DC voltage generated in the second output capacitor is supplied to the power supply terminal, and the feedback voltage is input to the feedback terminal. A control circuit according to any one of claims 1 to 8,
An AC / DC converter comprising: The feedback circuit includes:
A shunt regulator that generates a feedback signal whose level is adjusted such that an error between a voltage obtained by dividing the first DC voltage and a predetermined target value is zero;
A photocoupler whose light-emitting element on the primary side is controlled by the feedback signal;
The AC / DC converter according to claim 9, wherein a signal generated in a light receiving element on the secondary side of the photocoupler is supplied to the control circuit as the feedback voltage. A filter for filtering the AC voltage;
The AC / DC converter according to claim 9 or 10, wherein the second rectifier circuit rectifies the AC voltage after passing through the filter. Load,
The AC / DC converter according to any one of claims 9 to 11, wherein the first DC voltage is supplied to the load.
An electronic device comprising:   A power adapter comprising the AC / DC converter according to any one of claims 9 to 11.

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