JP6072881B2 - DC / DC converter, power supply device using the same, and electronic device - Google Patents

Description

  The present invention relates to a DC / DC converter.

  Various home appliances such as TVs and refrigerators, or electronic devices such as laptop computers, mobile phone terminals and PDAs (Personal Digital Assistants) operate by receiving external power, and also from an external power source. The built-in battery can be charged with the power of In home appliances and electronic devices (hereinafter collectively referred to as electronic devices), a power supply device that converts commercial AC voltage into AC / DC (AC / DC) is built-in, or the power supply device is a power source external to the electronic device. Built in the adapter (AC adapter).

The power supply device includes a rectifier circuit (diode bridge circuit) that rectifies an AC voltage, and an insulating DC / DC converter that steps down the rectified voltage and supplies the voltage to a load.
FIG. 1 is a diagram showing a configuration of a DC / DC converter 100r examined by the present inventors. The specific configuration of the DC / DC converter 100r should not be regarded as a general technique well known to those skilled in the art.

In the DC / DC converter 100r, a DC input voltage _VIN from a rectifier circuit (not shown) provided in the preceding stage is input to the input terminal P1. DC / DC converter 100r is supplied to a load (load) which steps down the input voltage _{V IN} is connected to the output terminal _{P OUT.}

The DC / DC converter 100r mainly includes a switching transistor M1, a transformer T1, a first diode D1, a first output capacitor Co1, a control circuit 10r, and a feedback circuit 20r. In the DC / DC converter 100r, the primary side region and the secondary side region of the transformer T1 must be electrically insulated. The feedback circuit 20r includes resistors R1 and R2 that divide the output voltage _VOUT , a shunt regulator 22, and a photocoupler 24.

The shunt regulator 22 is an error amplifier that amplifies an error between the _divided output voltage V _OUT ′ and the reference voltage V _REF corresponding to the target value of the output voltage V _OUT . The photocoupler 24 feeds back a feedback signal corresponding to the error between the output voltage _VOUT and the target voltage to the control circuit 10r. The control circuit 10r controls the ON / OFF duty ratio of the switching transistor M1 using pulse modulation so that the output voltage _VOUT matches the target value.

Control circuit 10r is where is operable at about 10V supply voltage _{V CC,} it is driven with an input voltage _V IN (about 140 V) so the efficiency is deteriorated. On the other hand, since the voltage V _OUT stepped down by the DC / DC converter 100r is generated on the secondary side of the transformer T1, the voltage V _OUT cannot be supplied to the control circuit 10r provided on the primary side.

Therefore, an auxiliary coil L3 is provided on the primary side of the transformer T1. Auxiliary coil L3, a second diode D2 and the second output capacitor Co2 serves as an auxiliary DC / DC converter for generating a supply voltage _{V CC} for the control circuit 10r. In the DC / DC converter 100r, power supply voltage _{V CC} is proportional to the output voltage _{V OUT,} the proportionality coefficient is determined by the winding ratio of the secondary coil L2 and the auxiliary coil L3 of the transformer T1.
V _CC = V _OUT × N _D / N _S
Here, _{N S} is the number of turns in the secondary coil L2, _{N D} is the number of turns of the auxiliary coil L3.

JP-A-9-098571 Japanese Patent Laid-Open No. 2-211055

The present inventors have studied the starting operation of such a DC / DC converter 100r, and have come to recognize the following problems. The starting operation will be described. The input voltage _VIN is supplied to the input terminal P1. With this input voltage _VIN , the capacitor Co2 is charged through the resistor R11, and the power supply voltage _VCC rises. The power supply voltage _{V CC} reaches the threshold voltage _{V UVLO} set inside the control circuit 10r, the control circuit 10r is operable to start the switching of the switching transistor M1 is started. By switching the switching transistor M1, the power supply voltage _VCC is stabilized by an auxiliary DC / DC converter including the auxiliary coil L3, the second diode D2, and the second output capacitor Co2.

Wherein the time from the input voltage _{V IN} is supplied to the control circuit 10r is activated, the time until the power supply voltage _{V CC} reaches the threshold voltage _{V UVLO,} i.e. at a time constant determined by the resistor R11 and the capacitor Co2 Determined. Accordingly, the smaller the resistance value of the resistor R11, the shorter the activation time, and the larger the resistance value, the longer the activation time. On the other hand, after the control circuit 10r is activated, the resistor R11 consumes useless power. The wasteful power consumption by the resistor R11 decreases as the resistance value R11 increases, and increases as the resistance value R11 decreases. That is, in the DC / DC converter 100r of FIG. 1, the startup time and the power consumption are in a trade-off relationship.

  An aspect of the present invention has been made in view of such problems, and one of exemplary purposes thereof is to provide a DC / DC converter that can reduce power consumption and can be started in a short time.

  One embodiment of the present invention relates to a DC / DC converter. The DC / DC converter has a transformer having a primary coil to which an input voltage is applied at one end, a secondary coil, and an auxiliary coil provided on the primary coil side, a potential at one end fixed, and the other end output. A first output capacitor connected to the terminal, a first diode provided between the other end of the first output capacitor and one end of the secondary coil so that the cathode faces the first output capacitor; A switching transistor provided on the path of the next coil, a second output capacitor having a fixed potential at one end thereof, and a cathode between the other end of the second output capacitor and one end of the auxiliary coil. A second rectifying element provided in a direction on the capacitor side; and a control circuit for controlling on / off of the switching transistor. The control circuit is biased so as to be normally on, provided between the power supply terminal connected to the other end of the second output capacitor, the high voltage terminal to which the input voltage is input, and the high voltage terminal and the power supply terminal. A charging transistor which is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a charging transistor from a high voltage terminal to a power supply terminal in a first state where the voltage of the power supply terminal is lower than a predetermined first threshold voltage And the charging current is reduced to substantially zero in a second state in which the voltage at the power supply terminal is higher than a second threshold voltage determined higher than the first threshold voltage. A current limiting circuit.

  According to this aspect, immediately after the DC / DC converter is started, the second output capacitor is charged by the current flowing from the high voltage terminal to the power supply terminal via the charging transistor. When the current limiting circuit is ignored, the charging current becomes larger as the voltage at the power supply terminal is lower. Therefore, immediately after starting the DC / DC converter, when the voltage at the power supply terminal is low, the second output capacitor can be charged with a large charging current, so that the starting time can be shortened. Further, when the voltage of the power supply terminal becomes higher than the second threshold voltage by the current limiting circuit, the current consumption can be reduced by making the charging current substantially zero. Further, when there is no current limiting circuit, if the power supply terminal is grounded, the current flowing from the high voltage terminal to the power supply terminal becomes very large. In this aspect, the voltage of the power supply terminal is changed to the first threshold voltage by the current limiting circuit. When it is lower, the circuit can be suitably protected by limiting the charging current.

The current limiting circuit may include a bypass switch provided on a charging current path between the high voltage terminal and the power supply terminal, and a first current source that supplies a predetermined current to the power supply terminal. In the first state, the bypass switch is turned off and the first current source is turned on. In the second state, both the bypass switch and the first current source are turned off, and the voltage at the power supply terminal is higher than the first threshold voltage and In the third state lower than the two threshold voltages, at least the bypass switch may be turned on.
According to this aspect, in the first state, the charging current flowing between the high voltage terminal and the power supply terminal can be limited to the current level generated by the first current source. In the second state, the charging current can be substantially zero, and in the third state, the charging current can be reduced as the voltage at the power supply terminal increases.

  The control circuit may further include a first diode provided between the current limiting circuit and the power supply terminal so that the cathode faces the power supply terminal side.

  The first current source is connected to form a current mirror circuit with the first transistor provided on the path of the reference current, and the second transistor provided between the high voltage terminal and the power supply terminal. And a control switch provided in series with the second transistor between the high voltage terminal and the power supply terminal.

  The bypass switch may include a third transistor that is an NPN-type bipolar transistor provided on a path between the high voltage terminal and the power supply terminal, and a bias circuit that controls a base current of the third transistor.

  The bias circuit includes a fourth transistor that is an NPN bipolar transistor provided between the base collector of the third transistor, a second current source provided between the base of the third transistor and the ground terminal, and a fourth transistor. A third current source for supplying current to the base of the first transistor, a first switch provided between the base of the fourth transistor and the ground terminal, and a second switch provided in series between the base of the fourth transistor and the ground terminal. A switch and a second diode.

  The current limiting circuit compares the voltage at the power supply terminal with the first threshold voltage, compares the voltage at the power supply terminal with the second threshold voltage, and a first comparator that generates a first detection signal indicating the comparison result. And a second comparator that generates a second detection signal indicating the comparison result. The state of the current limiting circuit may be controlled based on the first and second detection signals.

  Another aspect of the present invention is a power supply device. This power supply apparatus includes an AC / DC converter that converts a commercial AC voltage into a DC voltage, and the DC / DC converter according to any one of the above aspects that receives the DC voltage and supplies a voltage obtained by stepping down the DC voltage to a load. .

  Yet another embodiment of the present invention is an electronic device. This electronic apparatus includes a microcomputer and the DC / DC converter according to any one of the above-described aspects that supplies the output voltage to the microcomputer.

  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, it is possible to provide a DC / DC converter that reduces power consumption and can be started in a short time.

It is a figure which shows the structure of the DC / DC converter which this inventor examined. It is a circuit diagram which shows the structure of the electronic device which concerns on embodiment. FIG. 3 is a circuit diagram illustrating a configuration example of a control circuit in FIG. 2. FIG. 4 is a circuit diagram showing a more specific configuration of the control circuit of FIG. 3. It is a figure which shows the relationship between the voltage of a power supply terminal, and a charging current. It is a wave form diagram which shows operation | movement of the electronic device which concerns on embodiment. It is a circuit diagram which shows another structural example of a control circuit.

  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 to each other. Including the case of being indirectly connected through other members that do not substantially affect the state of connection, or do not impair the functions and effects achieved by the combination thereof.
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 their electric It includes cases where the connection is indirectly made through other members that do not substantially affect the general connection state, or that do not impair the functions and effects achieved by their combination.

FIG. 2 is a circuit diagram illustrating a configuration of the electronic apparatus 1 according to the embodiment.
The electronic device 1 is, for example, a home appliance such as a television, a refrigerator, or an air conditioner, or a computer. The electronic device 1 includes a microcomputer 2, a signal processing circuit 4, a DC / DC converter 100, and a rectifier circuit 102. The electronic device 1 is divided into a primary side and a secondary side that are insulated from each other. A part of the rectifier circuit 102 and the DC / DC converter 100 is arranged on the primary side, and another part of the DC / DC converter 100, the microcomputer 2 and the signal processing circuit 4 are arranged on the secondary side.

The rectifier circuit 102 is, for example, a diode rectifier circuit, receives an _AC voltage VAC such as a commercial AC voltage, full-wave rectifies it, and smoothes it by the capacitor C1 to generate a DC voltage V _DC (= V _IN ). When V _AC = 100V, V _DC = 144V.

The DC / DC converter 100 receives the direct-current input voltage _VIN at its input terminal P1, steps down the voltage, and outputs it from the output terminal P2. A PFC (Power Factor Correction) circuit (not shown) may be provided between the DC / DC converter 100 and the rectifier circuit 102. The output voltage _VOUT from the output terminal P2 is output to the microcomputer 2 and the signal processing circuit 4. The microcomputer 2 controls the entire electronic device 1 in an integrated manner. The signal processing circuit 4 is a block that performs specific signal processing, and examples thereof include an interface circuit that performs communication with an external device, an image processing circuit, an audio processing circuit, and the like. Needless to say, the actual electronic device 1 is provided with a plurality of signal processing circuits 4 according to their functions.

  The above is the overall configuration of the electronic device 1. Next, a DC / DC converter 100 that can be suitably used for such an electronic apparatus 1 will be described.

  The DC / DC converter 100 mainly includes a transformer T1, a first diode D1, a second diode D2, a first output capacitor Co1, a second output capacitor Co2, a switching transistor M1, a control circuit 10, and a feedback circuit 20.

The transformer T1 has a primary coil L1, a secondary coil L2, and an auxiliary coil L3 provided on the primary coil side. The number of turns of the primary coil L1 of the number of turns of _N P, 2 coil L2 and _{N S.} The number of turns of the auxiliary coil L3 and _{N D.}

  The switching transistor M1, the primary coil L1, the secondary coil L2, the first diode D1, and the first output capacitor Co1 form a first converter (main converter). One end of the first output capacitor Co1 is grounded, and its potential is fixed. The first diode D1 is provided between the other end of the first output capacitor Co1 and one end N2 of the secondary coil L2 in such a direction that the cathode is on the first output capacitor Co1 side. The other end of the secondary coil L2 is grounded and the potential is fixed.

An input voltage _VIN is applied to one end of the primary coil L1. The switching transistor M1 is provided on the path of the primary coil L1. A switching signal OUT from the control circuit 10 is input to the gate of the switching transistor M1 through the resistor R10.

  The switching transistor M1, the primary coil L1, the auxiliary coil L3, the second diode D2, and the second output capacitor Co2 form a second converter (auxiliary converter).

  The potential at one end of the second output capacitor Co2 is fixed. The second diode (second rectifier element) D2 is provided between the other end of the second output capacitor Co2 and one end N3 of the auxiliary coil L3. The other end of the auxiliary coil L3 is grounded, and its potential is fixed. The second diode D2 is arranged in such a direction that its cathode is on the second output capacitor Co2 side.

The second output capacitor Co2, the power supply voltage _{V CC} in accordance with the winding ratio _(N D / N _S) is generated.
V _CC = N _D / N _S × V _OUT (1)

The power supply terminal VCC (8th pin) of the control circuit 10 is connected to the second output capacitor Co2, charges the second output capacitor Co2 immediately after startup, and the voltage (power supply) generated in the second output capacitor Co2 after startup. Receives _{VCC (} also called voltage). The control circuit 10 adjusts the duty ratio of the switching signal OUT using pulse width modulation (PWM), pulse frequency modulation (PFM) or the like so that the level of the output voltage _VOUT approaches the target value, and controls the switching transistor M1. Control. A method for generating the switching signal OUT is not particularly limited.

A feedback signal V _FB corresponding to the output voltage V _OUT is input to the feedback terminal FB (second pin) of the control circuit 10 via the feedback circuit 20 including a photocoupler. The capacitor C3 is provided for the purpose of phase compensation.

For example, the feedback circuit 20 includes a shunt regulator 22, a photocoupler 24, and voltage dividing resistors R1 and R2. The voltage dividing resistors R1 and R2 divide the output voltage _VOUT of the DC / DC converter 100 at a voltage dividing ratio K. The shunt regulator 22 amplifies an error between the _divided output voltage V _OUT ′ (= V _OUT × K) and a predetermined reference voltage (V _REF ), and outputs a current I _FB corresponding to the error. The path of the output current I _FB of the shunt regulator 22, the input side of the light emitting diode of the photocoupler 24 is provided. The photocoupler 24 outputs a feedback signal V _FB corresponding to the error between the output voltage V _OUT ′ and the reference voltage V _REF to the FB terminal of the control circuit 10. The resistors R21 and R22 are provided to appropriately bias the light emitting diode of the photocoupler 24.

The control circuit 10 receives the feedback signal V _FB , generates a switching signal OUT whose duty ratio is adjusted so that the _divided output voltage V _OUT ′ matches the reference voltage V _REF, and drives the switching transistor M1. .
When the voltage dividing ratio of the voltage dividing circuit 26 is K, the output voltage V _OUT is
V _OUT = V _REF / K (2)
It is stabilized to satisfy.

The input voltage _VIN is applied to the high voltage terminal VH of the control circuit 10. As will be described later, the high voltage terminal VH and the power supply terminal _VCC are connected via a charging path inside the control circuit 10. During a period before the second converter normally operates, the power supply terminal VCC of the control circuit 10 is charged via a charging path inside the control circuit 10.

Next, a specific configuration example of the control circuit 10 will be described.
For example, the control circuit 10 determines the switching signal according to the output voltage V _OUT generated in the first output capacitor Co1, the current I _M1 flowing through the switching transistor M1 (primary coil L1), and the voltage V _D generated at one end N3 of the auxiliary coil L3. OUT is generated.

The detection resistor Rs is provided for detecting the current I _M1 flowing through the switching transistor M1. A voltage drop (detection signal) Vs generated in the detection resistor Rs is input to a current detection terminal (CS terminal: third pin) of the control circuit 10. Further, the voltage V _D1 of the tap TP of the auxiliary coil L3 of the control circuit 10 is input to the ZT terminal (first pin) through a low-pass filter including a resistor R4 and a capacitor C4.

  FIG. 3 is a circuit diagram showing a configuration example of the control circuit 10 of FIG. The control circuit 10 includes an off signal generation unit 52, an on signal generation unit 54, a drive unit 56, a charging transistor M2, a current limiting circuit 40, and a diode D3.

The off signal generation unit 52 includes a comparator that compares the detection signal Vs with the feedback signal _VFB, and generates an off signal Soff that defines the timing at which the switching transistor M1 is turned off. The off signal Soff generated by the off signal generation unit 52 is asserted when the current I _M1 flowing through the switching transistor M1 reaches a level corresponding to the feedback signal _VFB .

For example, when the output voltage V _OUT ′ becomes lower than the reference voltage V _REF , the feedback signal V _FB becomes higher, the timing at which the off signal Soff is asserted is delayed, and the ON period Ton of the switching transistor M1 becomes longer, resulting in output. Feedback is applied in the direction in which the voltage _VOUT increases. Conversely, when the output voltage V _OUT ′ becomes higher than the reference voltage V _REF , the feedback signal V _FB becomes lower, the timing at which the off signal Soff is asserted becomes earlier, and the on period Ton of the switching transistor M1 becomes shorter, and as a result The feedback is applied in the direction in which the output voltage _VOUT decreases.

The on signal generation unit 54 generates an on signal Son that is asserted after the off signal Soff is asserted. On signal generation unit 54 of FIG. 3, the potential _{V D} of the end N3 of the auxiliary coil L3, a comparator for comparing a predetermined level Vth. The on signal generation unit 54 asserts the on signal Son when the potential V _D decreases to the predetermined level Vth.

When the switching transistor M1 is turned on, a current I _M1 flows through the primary coil L1, and energy is stored in the transformer T1. Thereafter, when the switching transistor M1 is turned off, the energy stored in the transformer T1 is released. The on signal generator 54 can detect that the energy of the transformer T1 has been completely released by monitoring the voltage V _D generated in the auxiliary coil L3. When detecting the release of energy, the on signal generation unit 54 asserts the on signal Son to turn on the switching transistor M1 again.

The drive unit 56 turns on the switching transistor M1 when the on signal Son is asserted, and turns off the switching transistor M1 when the off signal Soff is asserted. The drive unit 56 includes a flip-flop 58, a pre-driver 60, and a driver 62. The flip-flop 58 receives an on signal Son and an off signal Soff at a set terminal and a reset terminal, respectively. The state of the flip-flop 58 changes according to the on signal Son and the off signal Soff. As a result, the duty ratio of the output signal Smod of the flip-flop 58 is modulated so that the output voltage _VOUT matches the target value _VREF . In FIG. 3, the high level of the drive signal Smod and the switching signal OUT is associated with the switching transistor M1 being on, and the low level is associated with the switching transistor M1 being off.

  The pre-driver 60 drives the driver 62 according to the output signal Smod of the flip-flop 58. A dead time is set for the output signals SH and SL of the pre-driver 60 so that the high-side transistor and the low-side transistor of the driver 62 are not turned on simultaneously. A switching signal OUT is output from the driver 62.

The charging transistor M2 is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor), is provided between the high voltage terminal VH and the power supply terminal VCC, and is biased so as to be normally on. Specifically, the gate and back gate of the charging transistor M2 are connected to the ground terminal GND, and the drain of the charging transistor M2 is connected to the high voltage terminal VH. A diode D4 is connected between the gate and source of the charging transistor M2. Ignoring current limiting circuit 40, a current _{I M2} decreases flowing through the charging transistor M2 as the voltage _{V CC} of the power supply terminal VCC is high, the more current _{I M2} low voltage _{V CC} of the power supply terminal VCC increases.

The current limiting circuit 40 controls the charging current I _CHG that flows from the high voltage terminal VH to the power supply terminal VCC via the charging transistor M2. In voltage _{V CC} is the first state φ1 lower than the first threshold voltage _{V TH1} of a predetermined power supply terminal VCC, the current limiting circuit 40 limits the limit current _{I LMT} in the charging current _{I CHG.} For example, the limit current I _LMT is a very small current of about 200 to 300 μA. Also in the second threshold voltage _V higher than _TH2 second state φ3 voltage _{V CC} of the power supply terminal VCC is defined higher than the first threshold voltage _{V TH1,} substantially reduces to zero the charging current _{I CHG} . The second threshold voltage V _TH2 may coincide with the lowest voltage V _UVLO (UVLO: Under Voltage Lock Out) at which the control circuit 10 can operate.

More specifically, the current limiting circuit 40 includes a bypass switch SW1, a first current source CS1, a first comparator CMP1, and a second comparator CMP2. The first comparator CMP1, the voltage _{V CC} of the power supply terminal VCC compared with the first threshold voltage _{V TH1,} and generates a first detection signal DET1 indicating the comparison result. The second comparator CMP2 is a voltage _{V CC} compared with the second threshold voltage _{V TH2,} it generates a second detection signal DET2 indicating the comparison result. The current limiting circuit 40 detects the first to third states based on the detection signals DET1 and DET2. That is, the first state φ1 when DET1 and DET2 are both low, the second state φ2 when both DET1 and DET2 are high, and the third state φ3 when DET1 is low and DET2 is low.

The bypass switch SW1 is provided in series with the charging transistor M2 on the path of the charging current between the high voltage terminal VH and the power supply terminal VCC. The first current source (limit current source) CS1 is configured to be switched on and off, and supplies the limit current I _LMT to the power supply terminal VCC in the on state.

In the first state φ1, the bypass switch SW1 is turned off and the first current source CS1 is turned on. Accordingly, the charging current I _CHG supplied to the second output capacitor Co2 is limited to the limit current I _LMT .
In the second state φ2, both the bypass switch SW1 and the first current source CS1 are turned off. As a result, the charging current I _CHG becomes substantially zero.
In the third state φ3, at least the bypass switch SW1 is turned on. As a result, the current limiting circuit 40 becomes conductive, and the current I _M2 flowing through the charging transistor M2 is supplied to the second output capacitor Co2 as the charging current I _CHG . In the present embodiment, it is assumed that the first current source CS1 is off in the third state φ3.

  The charging transistor M2, the bypass switch SW1, and the diode D3 function as a charging circuit that charges the second output capacitor Co2 instead of the auxiliary converter immediately after the DC / DC converter 100 is started and before the auxiliary converter operates.

FIG. 4 is a circuit diagram showing a more specific configuration of the control circuit 10 of FIG. In FIG. 4, blocks relating to the generation of the switching signal OUT are omitted. The reference bias circuit 48 generates a reference current I _REF . The first transistor M11 is provided on the path of the reference current _IREF . The second transistor M12 is connected to form a current mirror circuit together with the first transistor M11. The control switch SW2 is provided on the path of the second transistor M12. The control switch SW2 is turned on when the second detection signal DET2 is at a high level. In the ON state of the control switch SW2, a limit current I _{LMT that} is proportional to the reference current I _REF flows through the second transistor M12. That is, the reference bias circuit 48, the first transistor M11, the second transistor M12, and the control switch SW2 form a first current source CS1.

  The bypass switch SW1 includes a third transistor Q3 and a bias circuit 44. The third transistor Q3 is an NPN bipolar transistor, and is provided in series with the charging transistor M2 on the charging path between the high voltage terminal VH and the power supply terminal VCC. The bias circuit 44 controls the base current of the third transistor Q3 and controls the conduction state of the third transistor Q3.

  The bias circuit 44 includes a fourth transistor Q4, a second current source CS2, a diode D4, a first switch SW11, and a second switch SW12. The fourth transistor Q4 is an NPN bipolar transistor provided between the base collector of the third transistor Q3. The second current source CS2 is provided between the base of the third transistor Q3 and the ground terminal. The third current source CS3 supplies current to the base of the fourth transistor Q4. The first switch SW11 is provided between the base of the fourth transistor Q4 and the ground terminal, and is turned on when the first detection signal DET1 is at a high level. The second switch SW12 and the diode D4 are provided in series between the base of the fourth transistor Q4 and the ground terminal.

Figure 5 is a graph showing the relationship between the voltage _{V CC} of the power supply terminal VCC and the charging current _{I CHG.} The charging current I _CHG changes as shown by the solid line by the charging transistor M2 and the current limiting circuit 40 described with reference to FIGS.

The above is the configuration of the DC / DC converter 100. Next, the operation will be described. FIG. 6 is a waveform diagram showing the operation of the electronic apparatus 1 according to the embodiment. Prior to time t0, the electronic device 1 is off. When the user turns on (starts up) the power supply at time t0, the input voltage _VIN is supplied to the input terminal P1. Immediately after the start-up, since V _CC <V _TH1 (first state φ1), the charging current I _CHG is limited to the limit current I _LMT , and the power supply voltage _VCC gradually increases. When V _CC > V _{TH1 at} time t1 (third state φ3), the charging current I _CHG increases as shown in FIG. 5, and the rising speed of the power supply voltage _VCC increases. When V _CC > V _TH2 is _{satisfied at} time t2 (second state φ2), the charging current I _CHG is substantially reduced to zero. When V _CC > V _TH2 , the control circuit 10 becomes operable and switching of the switching transistor M1 starts. As a result, not the charging path inside the control circuit 10 but the level of the power supply voltage _VCC is adjusted and stabilized by the second converter.

Thus, according to the DC / DC converter 100 according to the embodiment, after the start of activation, it is possible in a short time increase the power supply voltage _{V CC} to control circuit 10 is operable level _{V UVLO.} Further, after the operation of the control circuit 10, the charging current I _CHG becomes substantially zero, so that an increase in current consumption can be suppressed. That is, it is possible to achieve both shortening of startup time and reduction of power consumption. This is the first advantage.

Furthermore, the DC / DC converter 100 according to the embodiment also has a second advantage described below. In the waveform diagram of FIG. 6, it is assumed that the power supply terminal VCC has a ground fault at time t3. Then, since V _CC <V _TH1 , charging current I _CHG decreases to limit current I _LMT . Therefore, even if the ground fault state lasts for a long time, the heat generation amount can be suppressed, and the reliability of the DC / DC converter 100 can be improved. In other words, the limit current I _LMT is desirably set to a level that does not impair the reliability of the circuit when a ground fault occurs.

The second advantage becomes clear by comparison with the control circuit 10c of FIG. The control circuit 10c in FIG. 7 is obtained by omitting the first current source CS1, the first switch SW11, and the first comparator CMP1 from the control circuit 10 in FIG. 5 indicates the relationship between the power supply voltage _VCC and the charging current I _CHG in the control circuit 10c of FIG. That is, the charging current I _CHG is not limited to the limit current I _LMT in the first state φ1, and the charging current I _CHG is maximized in the ground fault state (V _CC = 0V). In FIG. 6, the waveform of the charging current I _CHG when a ground fault occurs in the control circuit 10c of FIG. 7 is indicated by a one-dot chain line, and a large charging current I _CHG flows in the ground fault state.

  On the other hand, according to the control circuit 10 according to the embodiment, the reliability of the circuit in the ground fault state can be improved.

  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.

  In the embodiment, the case where the shunt regulator (error amplifier) 22 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.

Those skilled in the art will appreciate that there are various types of control circuit 10 and that the configuration is not limited in the present invention.
For example, a timer circuit that measures a predetermined off time Toff may be used as the on signal generation unit 54 in FIG. 3 instead of the comparator. It is also possible to fix the off time Toff by estimating in advance the time required for energy release. In this case, the circuit can be simplified in exchange for the deterioration of energy efficiency.

  In the embodiment, the case where the DC / DC converter 100 is mounted on the electronic device 1 has been described. However, the present invention is not limited thereto, and can be applied to various power supply apparatuses. For example, the DC / DC converter 100 can be applied to an AC adapter that supplies power to an electronic device. Examples of the electronic device in this case include a laptop computer, a desktop computer, a mobile phone terminal, and a CD player, but are not particularly limited.

  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, D3 ... third diode, T1 ... transformer, L1 ... primary Coil, L2 ... secondary coil, L3 ... auxiliary coil, M1 ... switching transistor, 100 ... DC / DC converter, 10 ... control circuit, 40 ... current limiting circuit, M2 ... charging transistor, SW1 ... bypass switch, SW2 ... control Switch, CS1 ... first current source, CS2 ... second current source, CS3 ... third current source, 44 ... bias circuit, SW11 ... first switch, SW12 ... second switch, CMP1 ... first comparator, CMP2 ... second Comparator 48 ... reference bias circuit Q3 ... third transistor Q4 ... fourth transistor 20 ... feed Click circuit, 22 ... shunt regulator, R1 ... first resistor, R2 ... second resistor, 1 ... electronic device, 2 ... microcomputer 4 ... signal processing circuit, 102 ... rectifier circuit.

Claims (15)

A transformer having a primary coil to which an input voltage is applied at one end thereof, a secondary coil, and an auxiliary coil provided on the primary coil side;
A first output capacitor having a potential fixed at one end and the other end connected to an output terminal;
A first diode provided between the other end of the first output capacitor and one end of the secondary coil in a direction in which the cathode is on the first output capacitor side;
A switching transistor connected to the other end of the primary coil;
A second output capacitor having a fixed potential at one end thereof;
A second rectifying element provided between the other end of the second output capacitor and one end of the auxiliary coil in a direction in which the cathode is on the second output capacitor side;
A control circuit for controlling on and off of the switching transistor;
With
The control circuit includes:
A power supply terminal connected to the other end of the second output capacitor;
A high voltage terminal connected to the one end side of the primary coil and to which the input voltage is input;
A charging transistor which is provided between the high voltage terminal and the power supply terminal and which is biased to be normally on and which is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor);
A first terminal connected to a source of the charging transistor; a second terminal connected to the power supply terminal; a bypass switch provided between the first terminal and the second terminal; (i) the power supply In a first state where the terminal voltage is lower than a predetermined first threshold voltage, the bypass switch is off, and the charging current flowing from the high voltage terminal to the power supply terminal via the charging transistor is limited. And (ii) in a second state in which the voltage at the power supply terminal is higher than a second threshold voltage set higher than the first threshold voltage, the bypass switch is off, and the charging current is substantially reduced. (Iii) in a third state where the voltage at the power supply terminal is higher than the first threshold voltage and lower than the second threshold voltage, the current control in which the bypass switch is on. And the circuit,
Bei to give a,
A DC / DC converter characterized in that a resistor is not included on a path from the one end of the primary coil to the high voltage terminal, the bypass switch and the power supply terminal . The current limiting circuit is:
A first current source for supplying a predetermined current to the power supply terminal;
Further including
In the first state, the bypass switch is turned off, the first current source is turned on,
In the second state, both the bypass switch and the first current source are turned off,
2. The DC / DC switch according to claim 1, wherein at least the bypass switch is turned on in a third state in which the voltage of the power supply terminal is higher than the first threshold voltage and lower than the second threshold voltage. DC converter. A transformer having a primary coil to which an input voltage is applied at one end thereof, a secondary coil, and an auxiliary coil provided on the primary coil side;
A first output capacitor having a potential fixed at one end and the other end connected to an output terminal;
A first diode provided between the other end of the first output capacitor and one end of the secondary coil in a direction in which the cathode is on the first output capacitor side;
A switching transistor connected to the other end of the primary coil;
A second output capacitor having a fixed potential at one end thereof;
A second rectifying element provided between the other end of the second output capacitor and one end of the auxiliary coil in a direction in which the cathode is on the second output capacitor side;
A control circuit for controlling on and off of the switching transistor;
With
The control circuit includes:
A power supply terminal connected to the other end of the second output capacitor;
A high voltage terminal connected to the one end side of the primary coil and to which the input voltage is input;
A charging transistor which is provided between the high voltage terminal and the power supply terminal and which is biased to be normally on and which is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor);
(I) limiting a charging current flowing from the high voltage terminal to the power supply terminal via the charging transistor in a first state in which the voltage of the power supply terminal is lower than a predetermined first threshold voltage; ii) reducing the charging current to substantially zero in a second state in which the voltage at the power supply terminal is higher than a second threshold voltage set higher than the first threshold voltage; and (iii) the power supply A current limiting circuit in which, in a third state in which a voltage at a terminal is higher than the first threshold voltage and lower than the second threshold voltage, the charging current larger than the charging current limited in the first state flows; ,
Bei to give a,
A DC / DC converter characterized in that a resistor is not included on a path from the one end of the primary coil through the high voltage terminal to the power supply terminal . The current limiting circuit is:
A bypass switch provided on a path of the charging current between the high voltage terminal and the power supply terminal;
A first current source for supplying a predetermined current to the power supply terminal;
Including
In the first state, the bypass switch is turned off, the first current source is turned on,
In the second state, both the bypass switch and the first current source are turned off,
4. The DC / according to claim 3, wherein at least the bypass switch is turned on in a third state in which the voltage of the power supply terminal is higher than the first threshold voltage and lower than the second threshold voltage. DC converter.   5. The DC / DC converter according to claim 1, wherein, in the third state, a larger charging current is generated as a voltage of the power supply terminal is lower. The control circuit includes:
6. The DC according to claim 1, further comprising a third diode provided between the output of the current limiting circuit and the power supply terminal in a direction in which a cathode is on the power supply terminal side. / DC converter. The first current source is
A first transistor provided on a reference current path;
A second transistor connected to form a current mirror circuit with the first transistor and provided between the high voltage terminal and the power supply terminal;
A control switch provided in series with the second transistor between the high voltage terminal and the power supply terminal;
The DC / DC converter according to claim 2 or 4, characterized by comprising: The bypass switch is
A third transistor which is an NPN bipolar transistor provided on the path of the high voltage terminal and the power supply terminal;
A bias circuit for controlling a base current of the third transistor;
The DC / DC converter according to claim 2 or 4, characterized by comprising: The bias circuit includes:
A fourth transistor, which is an NPN-type bipolar transistor, provided between the base collector of the third transistor;
A second current source provided between a base of the third transistor and a ground terminal;
A third current source for supplying current to the base of the fourth transistor;
A first switch provided between a base of the fourth transistor and a ground terminal;
A second switch and a fourth diode provided in series between a base of the fourth transistor and a ground terminal;
The DC / DC converter according to claim 8, comprising: The current limiting circuit is:
A first comparator for comparing a voltage of the power supply terminal with the first threshold voltage and generating a first detection signal indicating a comparison result;
A second comparator for comparing the voltage of the power supply terminal with the second threshold voltage and generating a second detection signal indicating a comparison result;
Further including
The first switch is turned on when the first detection signal indicates that the voltage of the power supply terminal is lower than the first threshold voltage;
10. The DC / DC according to claim 9, wherein the second switch is turned on when the second detection signal indicates that the voltage of the power supply terminal is higher than the second threshold voltage. converter.   The DC / DC converter according to claim 10, wherein the bypass switch is turned on when the second detection signal indicates that the voltage of the power supply terminal is lower than the second threshold voltage. . The current limiting circuit is:
A first comparator for comparing a voltage of the power supply terminal with the first threshold voltage and generating a first detection signal indicating a comparison result;
A second comparator for comparing the voltage of the power supply terminal with the second threshold voltage and generating a second detection signal indicating a comparison result;
The DC / DC converter according to claim 1, further comprising: a state of the current limiting circuit controlled based on the first and second detection signals. A transformer having a primary coil to which an input voltage is applied at one end thereof, a secondary coil, and an auxiliary coil provided on the primary coil side;
A first output capacitor having a potential fixed at one end and the other end connected to an output terminal;
A first diode provided between the other end of the first output capacitor and one end of the secondary coil in a direction in which the cathode is on the first output capacitor side;
A switching transistor connected to the other end of the primary coil;
A second output capacitor having a fixed potential at one end thereof;
A second rectifying element provided between the other end of the second output capacitor and one end of the auxiliary coil in a direction in which the cathode is on the second output capacitor side;
A control circuit for controlling on and off of the switching transistor;
With
The control circuit includes:
A power supply terminal connected to the other end of the second output capacitor;
A high voltage terminal connected to the one end side of the primary coil and to which the input voltage is input;
A charging circuit that is provided between the high voltage terminal and the power supply terminal and supplies a charging current to the second output capacitor connected to the power supply terminal at the time of startup, and (i) a voltage of the power supply terminal Is higher than a predetermined first threshold voltage and lower than a predetermined second threshold voltage, a larger charging current is generated as the voltage at the power supply terminal is lower, and (ii) the voltage at the power supply terminal is When higher than the second threshold voltage, the charging current is reduced to substantially zero; and (iii) when the voltage at the power supply terminal is lower than the first threshold voltage, the charging current is set to a predetermined limit. A charging circuit that limits current, and
Bei to give a,
A DC / DC converter characterized in that a resistor is not included on a path from the one end of the primary coil through the high voltage terminal to the power supply terminal . An AC / DC converter that converts commercial AC voltage to DC voltage;
The DC / DC converter according to any one of claims 1 to 13, which receives the direct current voltage and supplies a voltage obtained by stepping down the direct current voltage to a load.
A power supply apparatus comprising: A microcomputer,
The DC / DC converter according to any one of claims 1 to 13, wherein the output voltage is supplied to the microcomputer;
An electronic device comprising:

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