JP2018102113A - Isolation synchronous rectification type dc/dc converter, power adapter and electronic apparatus, control method of dc/dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase reliability of a DC/DC converter.SOLUTION: A secondary side controller 400 drives a light emitting element of a photo coupler 204 so that a detection voltage Vcorresponding to an output voltage Vgenerated in an output capacitor C1 approaches a reference voltage V. A primary side controller 202 controls a switching transistor M1 according to a feedback signal V. A protection circuit 420 is activated upon detection of an abnormal state, and drives the light emitting element of the photocoupler 204. An auxiliary power supply circuit 210 includes a power supply capacitor C2 provided separately from the output capacitor C1 and supplies a power supply voltage Vto the protection circuit 420 and an anode of the light emitting element of the photo coupler 204.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an isolated synchronous rectification type DC / DC converter.

  Flyback DC / DC converters are used in various power supply circuits including AC / DC converters. FIG. 1A is a circuit diagram of a diode rectification type flyback converter 200R, and FIG. 1B is a circuit diagram of a synchronous rectification type flyback converter 200S.

The flyback converter 200R in FIG. 1A receives an input voltage _VIN at its input terminal P1, generates a DC output voltage _VOUT stabilized at a predetermined target voltage, and outputs an output terminal P2 and a ground terminal P3. To a load (not shown) connected between the two. A switching transistor M1 is connected to the primary winding W1 of the transformer T1, and a diode D1 is connected to the secondary winding W2. The output capacitor C1 is connected to the output terminal P2.

A feedback circuit (also referred to as a shunt regulator circuit) 206 drives a light emitting element of the photocoupler 204 with a current I _ERR corresponding to an error between the output voltage _VOUT and the target voltage _{VOUT (REF)} . A feedback current _IFB corresponding to the error flows through the light receiving element of the photocoupler 204. The primary controller (Primary Controller) 202 feedback (FB) pin feedback signal _{V FB} is generated in response to the feedback current _{I FB.} The primary controller 202 generates a pulse signal having a duty ratio (or frequency) corresponding to the feedback signal _VFB , and drives the switching transistor M1.

In a flyback converter diode rectification type in FIG. 1 (a), in the diode D1, the power loss of the Vf × _{I OUT} occurs. Vf is a forward voltage, and _IOUT is a load current. If Vf = 0.5V and I _OUT = 10A, the power loss is 5W. Therefore, in many applications, a heat sink or a heat sink for cooling the diode D1 is required.

  A flyback converter 200S in FIG. 1B includes a synchronous rectification transistor M2 and a synchronous rectification controller (also referred to as a synchronous rectification IC) 300S instead of the diode D1 in FIG. The synchronous rectification controller 300S switches the synchronous rectification transistor M2 in synchronization with the switching of the primary side switching transistor M1.

In the synchronous rectification type flyback converter, the loss of the synchronous rectification transistor M2 is R _ON × I _OUT ² . R _ON is the on-resistance of the synchronous rectification transistor M2, and when R _ON = 5 mΩ and I _OUT = 10 A, the loss is 0.5 W, which is greatly reduced compared to the diode rectification type. Therefore, theoretically, in the synchronous rectification type, a heat sink and a heat sink are unnecessary or can be simplified.

JP 2009-159721 A

  As a result of studying the synchronous rectification converter of FIG. 1B, the present inventors have recognized the following problems.

The flyback converter 200S is provided with a protection circuit such as an overvoltage protection (OVP) circuit 390 in order to improve reliability. For example, the OVP circuit 390 is built in the feedback circuit 206 and supplies a current I _OVP to the light emitting element of the photocoupler 204 in an overvoltage state.

FIG. 2 is an operation waveform diagram of the flyback converter 200S of FIG. Prior to time t _{0, the} state is normal, and the output voltage V _OUT is stabilized at the target value V _{OUT (REF)} . At time t ₀ , some abnormality occurs, and the output voltage V _OUT deviates from the target value V _{OUT (REF)} and starts to rise.

When the output voltage _{V OUT} at time _{t 1} exceeds the overvoltage threshold _{V OVP,} OVP circuit 390, a state for supplying a current _{I OVP} to the light emitting element of the photocoupler 204 is fixed (latched) in that state. As a result, the feedback current I _FB increases, the feedback signal V _FB decreases, and the switching of the switching transistor M1 is stopped.

When the switching of the switching transistor M1 is stopped, charging of the output capacitor C1 is stopped, so that the output voltage _VOUT decreases with time. The feedback circuit 206 receives the output voltage _VOUT as the power supply voltage _VCC . Therefore, when the output voltage _VOUT decreases, the OVP circuit 390 becomes inoperable and cannot maintain the current I _OVP . For example, the feedback circuit 206 incorporates an unillustrated UVLO (Under Voltage Lock Out) circuit, and is configured to reset the OVP state of the feedback circuit 206 when V _CC <V _UVLO .

The decrease in the output voltage _{V OUT} That power supply voltage _{V CC,} the current _{I OVP} (and current _{I ERR)} becomes zero to time _{t 2, the} feedback current _{I FB} becomes zero, the feedback signal _{V FB} rises, the switching transistor M1 Switching resumes. By restarting switching, the output voltage _VOUT begins to rise again.

If the cause of the overvoltage remains, the output voltage _VOUT reaches the overvoltage threshold V _OVP again. The flyback converter 200S repeats operation and stop alternately in a time division manner (referred to as intermittent mode).

  There is a case where the heat generation of the circuit elements constituting the flyback converter 200S, specifically, the synchronous rectification transistor M2 and the switching transistor M1 may be a problem. In the intermittent mode of FIG. 2, heat is generated during the operation period to increase the temperature, and the temperature is relaxed during the stop period. Therefore, when the stop period is short, the temperature of the circuit element increases steadily.

  Although overvoltage protection has been described here, the same problem can occur with other protection circuits.

  The present invention has been made in view of such a problem, and one of exemplary purposes of an embodiment thereof is to provide a DC / DC converter that suppresses heat generation.

1. One embodiment of the present invention relates to an isolated synchronous rectification type DC / DC converter. The DC / DC converter includes a transformer having a primary winding and a secondary winding, a switching transistor connected to the primary winding of the transformer, an output capacitor charged by a current flowing through the secondary winding of the transformer, and light emission A photocoupler including an element and a light receiving element, and a feedback circuit for driving the light emitting element of the photocoupler, and a light receiving element of the photocoupler so that a detection voltage corresponding to an output voltage generated in the output capacitor approaches a reference voltage; A primary side controller that controls the switching transistor according to a feedback signal based on the state of the light receiving element, a protection circuit that is activated when an abnormal state is detected and drives the light emitting element, and a power supply capacitor provided separately from the output capacitor The power supply voltage generated in the power supply capacitor And a auxiliary power supply circuit for supplying to the anode of the light-emitting element of the coupler.

According to this aspect, since the power supply voltage of the protection circuit is maintained even after the output voltage _VOUT has decreased, the state where the light emitting element is driven can be maintained for a long time. Thereby, when operating in the intermittent mode, the heat relaxation time can be lengthened and heat generation can be suppressed.

  The protection circuit includes an abnormality detection circuit that maintains an assertion state of the abnormality detection signal until reset when it detects an abnormal state, and a transistor that is turned on in the assertion state of the abnormality detection signal. A voltage may be supplied.

The abnormality detection signal may be negated when the power supply voltage falls below the release threshold.
The abnormality detection signal may be negated after a predetermined time elapses after the assertion.

  The protection circuit may be an overvoltage protection circuit.

  The auxiliary power supply circuit may further include a charging path from the output capacitor to the power supply capacitor.

  The charging path may include a rectifying element that allows a current from the output line of the DC / DC converter to the power supply capacitor and blocks a reverse current.

  The charging path may include a diode provided such that the anode is on the output line side of the DC / DC converter and the cathode is on the power supply capacitor side.

2.1 Another aspect of the present invention relates to a secondary controller used in an isolated synchronous rectification type DC / DC converter. The secondary-side controller includes a control output pin to be connected to the light emitting element of the photocoupler, a power supply pin to receive a power supply voltage, a control input pin to receive a detection voltage corresponding to the output voltage of the DC / DC converter, a detection voltage, A feedback circuit that amplifies an error of the reference voltage and supplies a current corresponding to the error to the light emitting element of the photocoupler, and a power supply path for supplying power from the control output pin to the power supply pin.

  According to this aspect, even if an open abnormality occurs in the power supply pin, since the voltage of the control output pin is supplied to the power supply line of the secondary controller via the power feeding path, the secondary controller Operation can be maintained as a power supply voltage. Thereby, reliability can be improved.

  The power supply path may include a rectifying element. The power supply path may include a diode provided with an orientation in which the anode is on the control output pin side and the cathode is on the power supply pin side.

The secondary controller may further include a protection circuit that drives the light emitting element when an abnormal state is detected. The power supply voltage of the power supply pin may be supplied to at least a part of the protection circuit.
Thereby, when an open abnormality occurs in the power supply pin, it is possible to prevent the protection function by the protection circuit from being lost.

The protection circuit may be configured to activate upon detecting an overvoltage condition and drive the light emitting element. The power supply voltage supplied to the power supply pin may decrease later than the output voltage when the DC / DC converter is stopped.
When the overvoltage state occurs, the light emitting element is driven, the primary side switching is stopped, and the output voltage is lowered. When the output voltage decreases, the driving of the light emitting element by the protection circuit is released. Then, the primary side switching resumes. When the overvoltage state continues, the DC / DC converter shifts to an intermittent operation mode.
Here, since the power supply voltage of the power supply pin is maintained even after the output voltage is lowered, the state where the light emitting element is driven can be maintained for a long time. That is, the stop period of the DC / DC converter can be lengthened. Thereby, when operating in the intermittent mode, the heat relaxation time can be lengthened and heat generation can be suppressed.

  When the overvoltage state is detected, the protection circuit is connected to the overvoltage detection comparator that asserts the abnormality detection signal, the latch circuit that latches the abnormality detection signal, and the light emitting element, and is a protection transistor that is turned on according to the output of the latch circuit And may be included.

  The feedback circuit may include an error amplifier that amplifies an error between the detection voltage and the reference voltage, and a pass transistor that is connected to the control output pin and is driven according to an output signal of the error amplifier.

  The synchronous rectification controller that drives the synchronous rectification transistor on the secondary side of the DC / DC converter may be housed in the same package.

  Another aspect of the present invention relates to a DC / DC converter. The DC / DC converter includes any one of the secondary-side controllers described above.

2.2 Another aspect of the present invention also relates to an isolated synchronous rectification type DC / DC converter. The DC / DC converter includes a transformer having a primary winding and a secondary winding, a switching transistor connected to the primary winding of the transformer, an output capacitor charged by a current flowing in the secondary winding of the transformer, Connected to a photocoupler including a light emitting element and a light receiving element, a feedback circuit for driving the light emitting element of the photocoupler so that a detection voltage corresponding to the output voltage of the DC / DC converter approaches a reference voltage, and the light receiving element of the photocoupler A primary side controller that controls the switching transistor in response to a feedback signal based on the state of the light receiving element, a synchronous rectifying controller that drives the synchronous rectifying transistor, and a protection circuit that is activated when an abnormal state is detected and that drives the light emitting element. A power supply capacitor provided separately from the output capacitor Look, a power supply voltage generated in the power source capacitor, and at least part of the auxiliary power supply circuit for supplying to the power supply line of the protection circuit, and a rectifying element provided between the cathode and the power supply line of the light emitting element of the photocoupler.

  According to this aspect, even if the power supply path from the auxiliary power supply circuit to the power supply line is interrupted, voltage is supplied to the power supply line from the cathode of the light emitting element of the photocoupler via the rectifier element. It can maintain operation. Thereby, reliability can be improved.

The power supply voltage may drop later than the output voltage when the DC / DC converter is stopped.
Thereby, the length of the intermittent operation stop period can be extended.

A power supply voltage from an auxiliary power supply circuit may be supplied to the anode of the light emitting element of the photocoupler.
As a result, during intermittent operation in an abnormal state, the output voltage during the stop period can be lowered to 0 V or in the vicinity thereof.

  The rectifying element may be a diode.

  The protection circuit may include an abnormality detection circuit that maintains assertion of the abnormality detection signal until reset when it detects an abnormal state, and a transistor that is turned on in the assertion state of the abnormality detection signal. A power supply voltage may be supplied to the abnormality detection circuit.

  The abnormality detection signal may be negated when the power supply voltage falls below the release threshold.

  The abnormality detection signal may be negated after a predetermined time elapses after the assertion.

  The protection circuit may be an overvoltage protection circuit. The abnormality detection circuit may include a hysteresis comparator.

  The auxiliary power supply circuit may further include a charging path from the output capacitor to the power supply capacitor. The charging path may include a rectifying element that allows a current from the output line of the DC / DC converter to the power supply capacitor and blocks a reverse current.

  The charging path may include a diode provided such that the anode is on the output line side of the DC / DC converter and the cathode is on the power supply capacitor side.

  The synchronous rectification controller, the feedback circuit, and the protection circuit may be housed in one package.

The feedback circuit and the protection circuit may be integrated on the same chip.
“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, and some resistors are used to adjust circuit constants. And a capacitor may be provided outside the semiconductor substrate. By integrating the circuit on one chip, the circuit area can be reduced and the characteristics of the circuit elements can be kept uniform.

  The synchronous rectification controller, the feedback circuit, and the protection circuit may be integrated on the same chip.

  Another embodiment of the present invention relates to an electronic device. Electronic equipment includes a load, a diode rectifier circuit that full-wave rectifies the commercial AC voltage, a smoothing capacitor that generates a DC input voltage by smoothing the output voltage of the diode rectifier circuit, and steps down the DC input voltage and supplies it to the load Any of the above-described DC / DC converters may be provided.

  Another aspect of the present invention relates to a power adapter. The power adapter includes a diode rectifier circuit that full-wave rectifies a commercial AC voltage, a smoothing capacitor that generates a DC input voltage by smoothing an output voltage of the diode rectifier circuit, and supplies the load by stepping down the DC input voltage. Any one of the DC / DC converters may be provided.

  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.

  Furthermore, the description of this item (means for solving the problem) does not explain all the essential features, and thus the sub-combinations of these features described can also be the present invention.

  According to an aspect of the present invention, the reliability of a DC / DC converter can be increased.

1A and 1B are circuit diagrams of a flyback converter. It is an operation | movement waveform diagram of the DC / DC converter of FIG.1 (b). 1 is a circuit diagram of an insulation type DC / DC converter according to a first embodiment. FIG. FIG. 4 is an operation waveform diagram of the DC / DC converter of FIG. 3. It is an operation | movement waveform diagram of the DC / DC converter which concerns on a comparison technique. FIG. 4 is a circuit diagram illustrating a first configuration example of the DC / DC converter of FIG. 3. FIG. 7 is an operation waveform diagram of the DC / DC converter of FIG. 6. FIGS. 8A and 8B are circuit diagrams of modified examples of the secondary side controller of FIG. FIG. 4 is a circuit diagram illustrating a second configuration example of the DC / DC converter of FIG. 3. It is a circuit diagram of the insulation type DC / DC converter which concerns on 2nd Embodiment. FIG. 11 is a circuit diagram illustrating a first configuration example of the DC / DC converter of FIG. 10. It is an operation | movement waveform diagram of the DC / DC converter which concerns on a 1st comparison technique. FIG. 12 is an operation waveform diagram of the DC / DC converter of FIG. 11. 14A and 14B are circuit diagrams of modified examples of the secondary-side controller of FIG. It is a circuit diagram which shows the 2nd structural example of the DC / DC converter of FIG. It is a circuit diagram of an AC / DC converter provided with a DC / DC converter. It is a figure which shows an AC adapter provided with an AC / DC converter. 18A and 18B 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 other members that do not affect the state or inhibit the function 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, This includes cases where the connection is indirectly made through other members that do not affect the connection state or inhibit the function.

  The vertical and horizontal axes of the waveform diagrams and time charts referred to in this specification are enlarged or reduced as appropriate for easy understanding, and each waveform shown is also simplified for easy understanding. Or exaggerated or emphasized.

FIG. 3 is a circuit diagram of the insulation type DC / DC converter 200 according to the first embodiment. The DC / DC converter 200 is a flyback converter. The DC / DC converter 200 receives an input voltage _VIN at an input terminal P1, generates a DC output voltage _VOUT stabilized at a predetermined target voltage, and outputs an output terminal P2 and a ground terminal. Supply to a load (not shown) connected between P3.

The transformer T1 has a primary winding W1 and a secondary winding W2. One end of the primary winding W1 is connected to the input terminal P1 and receives a DC input voltage _VIN . The drain of the switching transistor M1 is connected to the other end of the primary winding W1 of the transformer T1. A sense resistor for current detection may be inserted between the source of the switching transistor M1 and the ground line.

  The synchronous rectification transistor M2 and the secondary winding W2 of the transformer T1 are provided in series between the output terminal P2 and the ground terminal P3. The output capacitor C1 is connected between the output terminal P2 and the ground terminal P3.

The primary controller 202 is connected to the light receiving element of the photocoupler 204. A feedback signal V _FB corresponding to the feedback current I _FB flowing through the light receiving element of the photocoupler 204 appears at the feedback (FB) terminal of the primary controller 202.

The primary-side controller 202 generates a pulse signal having a duty ratio (or frequency) corresponding to the feedback signal V _FB and outputs it from the output (OUT) terminal to drive the switching transistor M1. The configuration and control method of the primary controller 202 are not particularly limited. For example, the primary controller 202 may be a current mode modulator.

The synchronous rectification controller 300 controls the synchronous rectification transistor M2. For example, the synchronous rectification controller 300 generates a control pulse based on the drain-source voltage _VDS2 of the synchronous rectification transistor M2, and supplies a gate pulse corresponding to the control pulse to the gate of the synchronous rectification transistor M2. The configuration and operation of the synchronous rectification controller 300 are not particularly limited, and a known technique may be used.

The secondary-side controller 400 includes a control input (SH_IN) pin, a control output (SH_OUT) pin, a power supply (VCC) pin, and a ground (GND) pin, and is accommodated in one package. A detection voltage V _OUTS corresponding to the output voltage V _OUT is input to the SH_IN pin. For example, the detection voltage V _OUTS is a voltage obtained by dividing the output voltage _VOUT by the resistors R ₁₁ and R ₁₂ . The SH_OUT pin is connected to the light emitting element of the photocoupler 204. The GND pin is connected to the ground terminal P3 (ground line).

An auxiliary power supply circuit 210 is provided on the secondary side of the DC / DC converter 200. Auxiliary power supply circuit 210 includes a power supply capacitor C2 that is provided separately from the output capacitor C1, the power supply voltage _{V CC} generated in the power supply capacitor C2 is supplied to the VCC pin of the secondary controller 400. The auxiliary power supply circuit 210 includes a charging path 212 provided between the auxiliary power supply circuit 210 and the power supply capacitor C2. The charging path 212 may include a rectifying element that allows a current from the output line 208 of the DC / DC converter 200 toward the power supply capacitor C2 and blocks a reverse current. The rectifying element may include a diode D2 provided such that the anode is on the output line 208 side of the DC / DC converter 200 and the cathode is on the power supply capacitor C2 side. Alternatively, the rectifying element may be a switch (transistor).

During operation of the DC / DC converter 200, the power supply capacitor C2 is charged to the output voltage _{V OUT} is substantially the same potential, VCC pin Therefore, the output voltage _{V OUT} substantially at the same potential supply voltage _{V CC} is Supplied.

The anode of the light emitting element of the photocoupler 204 is connected to the power supply capacitor C2 through the resistor R _3. That is, the light emitting element, the power supply voltage V _CC from the auxiliary power supply circuit 210 is supplied.

When the switching operation of the DC / DC converter 200 stops, the output capacitor C1 is discharged by the load current and decreases with time. On the other hand, the power supply capacitor C2 is discharged by the forward current of the photocoupler 204 and the current flowing through the VCC pin of the secondary controller 400. Capacity of the power supply capacitor C2 in the stop state of the DC / DC converter 200, the power supply voltage _{V CC} is determined to decrease slower than the output voltage _{V OUT.}

The secondary-side controller 400 includes a power supply line 402, a feedback circuit 410, and a protection circuit 420, which are integrated on a single semiconductor substrate. The power supply line 402 is connected to the VCC pin. Feedback circuit 410 and the protection circuit 420 operates by receiving power supply voltage _{V CC} of the power supply line 402.

The feedback circuit 410 is a so-called shunt regulator, and the light emitting element of the photocoupler 204 is arranged so that the detection voltage V _OUTS approaches the reference voltage V _REF , in other words, the output voltage _VOUT approaches the target value V _{OUT (REF).} Drive. Feedback circuit specifically 410, the current _{I PC} corresponding to the error of the detection voltage _{V OUTS} and the reference voltage _{V REF} for driving the light emitting element of the photocoupler 204.

For example, the feedback circuit 410 includes an error amplifier 412 and a pass transistor 414. The error amplifier 412 amplifies an error between the detection voltage V _OUTS and the reference voltage V _REF . Pass transistor 414 is, for example, a P-channel MOSFET, and its source is connected to the SH_OUT pin and its gate is connected to the output of error amplifier 412. The pass transistor 414, the output signal of the error amplifier 412, that is, the current _{I PC} corresponding to the error of the detection voltage _{V OUTS} and the reference voltage _{V REF} flows. The pass transistor 414 may be an N channel. Further, it may be a PNP type or PNP type bipolar transistor.

  The power supply of the error amplifier 412 may be taken from the SH_OUT pin. In this case, the protection circuit 420 is supplied with power from the power supply line 402.

When the protection circuit 420 detects an abnormal state, the protection circuit 420 supplies current to the light emitting element of the photocoupler 204. When an abnormality occurs, the forward current flowing through the light emitting element of the photocoupler 204 increases, the feedback current I _FB increases, the feedback voltage V _FB decreases, the switching of the switching transistor M1 is stopped, and the circuit is protected.

Although not particularly limited, in this embodiment, the abnormal state is an overvoltage state, and the protection circuit 420 is an OVP circuit that supplies the current I _OVP to the photocoupler 204 when the overvoltage state is detected.

The protection circuit 420 includes an abnormality detection circuit 421 and a protection transistor 426. When detecting an abnormal state (here, an overvoltage state), the abnormality detection circuit 421 maintains the asserted state of the abnormality detection signal S _OVP ′ until it is reset. The protection transistor 426 is turned on when the abnormality detection signal S _OVP ′ is asserted.

  The above is the configuration of the DC / DC converter 200. Next, the operation will be described.

FIG. 4 is an operation waveform diagram of the DC / DC converter 200 of FIG. Prior to time t _{0, the} state is normal, and the output voltage V _OUT is stabilized at the target value V _{OUT (REF)} . At time t ₀ , some abnormality occurs, and the output voltage V _OUT deviates from the target value V _{OUT (REF)} and starts to rise. As the output voltage _VOUT rises, the power supply voltage _VCC rises.

When the output voltage V _OUT exceeds the overvoltage threshold V _OVP at time t ₁ , the protection circuit 420 enters a state where the current I _OVP is supplied to the light emitting element of the photocoupler 204 and is fixed (latched) in that state. . As a result, the feedback current I _FB increases, the feedback signal V _FB decreases, and the switching of the switching transistor M1 is stopped.

When the switching of the switching transistor M1 stops, the charging of the output capacitor C1 stops, so that the output voltage _VOUT decreases with time and the power supply voltage _VCC supplied to the secondary controller 400 also decreases. Output voltage _{V OUT} to the time _{t 2} is reduced to 0V. On the other hand, the power supply voltage _{V CC} decreases slower than the output voltage _{V OUT.}

When the voltage V _CC ′ of the VCC pin of the secondary controller 400 drops to the release threshold value V _UVLO at time t ₃ , the protection state of the protection circuit 420 is released, and I _OVP becomes zero. As a result, the feedback current I _FB also becomes zero, the feedback signal V _FB rises, and the switching of the switching transistor M1 is restarted. By restarting switching, the output voltage _VOUT rises again, and the power supply voltage _VCC also rises.

If the cause of the overvoltage remains, the output voltage _VOUT rises again exceeding the target voltage _{VOUT (REF)} . When the output voltage _VOUT exceeds the overvoltage threshold value V _OVP , the protection state is entered again. The DC / DC converter 200 repeats operation and stop alternately in a time division manner while the cause of the overvoltage continues.

  The above is the operation of the DC / DC converter 200. According to this DC / DC converter 200, the stop period in the intermittent mode can be made longer than that of the flyback converter 200S of FIG. As described above, the circuit element generates heat during the operation period and the temperature rises, and the temperature is relaxed during the stop period. However, the temperature rise of the circuit element can be suppressed by lengthening the stop period.

Another advantage of the DC / DC converter 200 becomes clear by comparison with comparative techniques. In the comparison technique, the VCC pin of the secondary controller 400 is connected to the output line 208, and the output voltage _VOUT is used as the power supply voltage.

  The operation of the comparison technique will be described. FIG. 5 is an operation waveform diagram of the DC / DC converter according to the comparative technique.

At time t ₀ , some abnormality occurs, and the output voltage V _OUT deviates from the target value V _{OUT (REF)} and starts to rise. As the output voltage _VOUT rises, the power supply voltage _VCC rises.

When the output voltage V _OUT exceeds the overvoltage threshold V _OVP at time t ₁ , the protection circuit 420 enters a state where the current I _OVP is supplied to the light emitting element of the photocoupler 204 and is fixed (latched) in that state. . As a result, the feedback current I _FB increases, the feedback signal V _FB decreases, and the switching of the switching transistor M1 is stopped.

When the switching of the switching transistor M1 stops, the charging of the output capacitor C1 stops, so that the output voltage _VOUT decreases with time and the power supply voltage _VCC supplied to the secondary controller 400 also decreases. The power supply voltage _{V CC} is reduced slower than the output voltage _{V OUT.}

When the output voltage V _OUT , that is, the cathode voltage of the light emitting element of the photocoupler 204 is decreased to a certain voltage level at time t ₂ , the protection current I _OVP is decreased and the luminance of the light emitting element is decreased. As a result, the feedback current I _FB decreases, the feedback voltage V _FB increases, and the switching transistor M1 switches. When the output voltage _VOUT rises slightly due to the switching of the switching transistor M1, the protection current I _OVP slightly increases and the switching of the switching transistor M1 is stopped or the switching duty ratio is reduced.

As described above, in the comparative technique, in the stop period of the overvoltage state, the output voltage _VOUT is not completely reduced to 0V, and is in an equilibrium state at a certain voltage level, and the switching of the switching transistor M1 is not completely stopped. The above is the operation of the comparison technique.

According to the DC / DC converter 200 according to the embodiment, since the light emission of the light emitting element of the photocoupler 204 is maintained even when the output voltage _VOUT decreases, the switching transistor M1 is stopped as shown in FIG. The state can be maintained.

  It is understood as one aspect of the present invention, the block diagram and circuit diagram of FIG. 3, or extends to various devices and circuits derived from the above description, and is not limited to a specific configuration. In the following, more specific configuration examples and modifications will be described in order to help understand the essence and circuit operation of the invention and clarify them, not to narrow the scope of the present invention.

(First configuration example)
FIG. 6 is a circuit diagram showing a first configuration example (200A) of the DC / DC converter.

The protection circuit 420A is an overvoltage protection (OVP) circuit. The overvoltage protection (OVP) pin of the secondary side controller 400A is the power supply voltage _{V CC} is supplied through a resistor _{R 31.} At the OVP pin, a voltage V _CC ′ obtained by dividing the power supply voltage _VCC by an external resistor R ₃₁ and a built-in resistor R ₃₂ is generated. Incidentally resistor R ₃₂ may be an external component.

As described above, during the switching operation of the DC / DC converter 200A, since V _CC ≈V _OUT , the voltage V _CC ′ at the VCC pin is a voltage corresponding to the output voltage V _OUT . When the voltage V _CC ′ of the OVP pin exceeds a predetermined overvoltage threshold value V _OVP , the protection circuit 420A generates a current I _OVP and drives the photocoupler 204.

The protection circuit 420A is configured to maintain the driving state of the photocoupler 204 until it is reset when the voltage V _CC ′ of the OVP pin exceeds a predetermined overvoltage threshold V _OVP .

The protection circuit 420A includes an abnormality detection circuit 421 and a protection transistor 426. When detecting an abnormal state (here, an overvoltage state), the abnormality detection circuit 421 maintains the asserted state of the abnormality detection signal S _OVP until it is reset. The protection transistor 426 is turned on when the abnormality detection signal S _OVP is asserted.

Abnormality detection circuit 421 includes an overvoltage detection comparator 422 and a latch circuit 424. The overvoltage detection comparator 422 compares the voltage V _CC ′ of the OVP pin with the overvoltage threshold value V _OVP and, when detecting an overvoltage state (V _CC ′> V _OVP ), asserts the comparison signal S _OVP ′ (for example, high level) To do. The latch circuit 424 latches the comparison signal S _OVP '. The latch circuit 424 may include a flip-flop. The protection transistor 426 is connected to the light emitting element of the photocoupler 204 via the SH_OUT pin, and is turned on in accordance with the output S _OVP of the latch circuit 424.

The secondary-side controller 400A may include a UVLO (Under Voltage Lock Out) circuit 430. The UVLO circuit 430 is a reset circuit that asserts the release signal S _RESET (for example, low level) and resets the latch circuit 424 when V _CC ′ <V _UVLO . When the OVP state of the protection circuit 420A is released by the UVLO circuit 430, the protection transistor 426 is turned off and the current I _OVP is stopped.

In the secondary controller 400A, the power supply voltage _VCC is supplied to the feedback circuit 410 and the protection circuit 420A through the power supply line 402.

  The power supply path 404 is configured to supply power in one direction from the SH_OUT pin to the VCC pin. For example, the power supply path 404 may include a rectifying element. The rectifying element includes a diode provided such that the anode is on the SH_OUT pin side and the cathode is on the VCC pin (power supply line 402) side.

  The above is the configuration of the DC / DC converter 200A. Next, the operation will be described. FIG. 7 is an operation waveform diagram of the DC / DC converter 200A of FIG.

Prior to time t _{0, the} state is normal, and the output voltage V _OUT is stabilized at the target value V _{OUT (REF)} . At time t ₀ , some abnormality occurs, and the output voltage V _OUT deviates from the target value V _{OUT (REF)} and starts to rise.

As the output voltage _VOUT increases, the power supply voltage _VCC and the voltage V _CC ′ of the OVP pin increase. When the voltage V _CC ′ exceeds the overvoltage threshold V _OVP at time t ₁ , the protection circuit 420 enters a state in which the current I _OVP is supplied to the light emitting element of the photocoupler 204 and is fixed (latched) in that state. . As a result, the feedback current I _FB increases, the feedback signal V _FB decreases, and the switching of the switching transistor M1 is stopped.

When the switching of the switching transistor M1 is stopped, the output for the charging of the capacitor C1 is stopped, the output voltage V _OUT will decrease both time, will also decrease supply voltage V _CC is supplied to the secondary side controller 400A. Output voltage _{V OUT} to the time _{t 2} is reduced to 0V. On the other hand, the power supply voltage _{V CC} decreases slower than the output voltage _{V OUT.}

Time _{t 3} to the voltage of the OVP pin _{V CC} 'is released protection state of the protection circuit 420A drops to UVLO voltage _{V UVLO, I OVP} is zero. As a result, the feedback current I _FB also becomes zero, the feedback signal V _FB rises, and the switching of the switching transistor M1 is restarted. By restarting switching, the output voltage _VOUT rises again, and the power supply voltage _VCC also rises.

If the cause of the overvoltage remains, the output voltage _VOUT rises again exceeding the target voltage _{VOUT (REF)} . Then, when the voltage V _CC ′ of the OVP pin exceeds the overvoltage threshold V _OVP , the protection state is entered again. The DC / DC converter 200A repeats operation and stop alternately in a time division manner while the cause of the overvoltage continues. The above is the operation of the DC / DC converter 200A.

  According to the DC / DC converter 200A of FIG. 6, by increasing the length of the stop period, abnormal heat generation of the synchronous rectification transistor M2 can be prevented, and reliability can be improved.

  Further, according to the DC / DC converter 200A, even when an open abnormality occurs in the VCC pin, a voltage is supplied to the power supply line 402 from the SH_OUT pin via the power feeding path 404. Accordingly, the feedback circuit 410 and the protection circuit 420A can continue to operate, and reliability can be improved.

(Modification)
FIGS. 8A and 8B are circuit diagrams of modified examples (400B and 400C) of the secondary-side controller of FIG. In the secondary side controller 400B of FIG. 8A, the protection circuit 420B includes an auto reset circuit 428. Auto-reset circuit 428 also operates in response to voltage _VCC on power supply line 402.

  The auto-reset circuit 428 includes a timer circuit, and resets the latch circuit 424 after a predetermined time has elapsed since the protection circuit 420B is in the OVP state. According to this modification, the stop period can be set according to the time measured by the auto reset circuit 428.

In the secondary-side controller 400C of FIG. 8B, the protection circuit 420C includes a hysteresis comparator 423 and a protection transistor 426. The hysteresis comparator 423 compares the voltage V _CC ′ of the OVP pin with a threshold voltage that changes in two values of V _OVP and V _UVLO , and generates an abnormality detection signal S _OVP according to the comparison result. According to this configuration, the operation of FIG. 5 can be realized.

(Second configuration example)
FIG. 9 is a circuit diagram showing a second configuration example (200D) of the DC / DC converter. In the DC / DC converter 200D, the secondary controller 400D includes a synchronous rectification controller 300D in the same package in addition to the feedback circuit 410 and the protection circuit 420. They may be integrated on the same semiconductor substrate (die, chip), or may be divided and integrated into a plurality of dies.

The SOURCE pin is a ground terminal of the synchronous rectification controller 300D. The GATE pin and the DRAIN pin are connected to the gate and drain of the synchronous rectification transistor M2. To Synchronous controller 300D The power supply voltage _{V CC} is supplied from the power supply line 402. A synchronous rectification controller 300D may be incorporated in the secondary side controllers 400B and 400C according to the modified examples of FIGS. 8A and 8B.

  Hereinafter, modified examples related to the first embodiment will be described.

(First modification)
The protection circuit 420 is not limited to an overvoltage protection circuit. For example, the switching rectification state in which the synchronous rectification transistor M2 cannot be switched may be detected, and the switching state may be activated to activate the light emitting element of the photocoupler 204. For example, the protection circuit 420 may detect an open abnormality of the GATE pin of the secondary controller 400, or may detect an open abnormality of the DRAIN pin of the secondary controller 400.

(Second modification)
In the embodiment, the synchronous rectification type flyback converter is taken as an example, but the present invention can also be applied to a diode rectification type flyback converter. The present invention can also be applied to an LLC converter.

(Third Modification)
The structure of the power supply path 404 is not limited to the diode as shown in FIG. 2, and may be configured by a switch that is turned on when the voltage of the SH_OUT pin is higher than the voltage of the VCC pin and turned off when the voltage is not so. .

(Fourth modification)
The configuration of the auxiliary power supply circuit 210 is not limited to that shown in FIG. For example, an auxiliary power supply circuit 210, constituted by a step-up charge pump which receives the output voltage V _OUT, may provide its output voltage to the VCC pin.

(Second Embodiment)
The problem to be solved by the second embodiment will be described. As a result of examining the DC / DC converter 200S of FIG. 1B, the present inventors have recognized the following problems. The power supply (VCC) pin of the feedback circuit 206 is supplied with the output voltage V _OUT (or the power supply voltage V _CC originating therefrom), and the internal circuit of the feedback circuit 206 operates using the output voltage _VOUT as the power supply voltage. To do.

If the VCC pin is disconnected from the board or the wiring on the board is disconnected (these are called open abnormalities), the output voltage _VOUT is not supplied to the VCC pin, the feedback circuit 206 becomes inoperable, and consequently the output voltage _VOUT Is out of control.

  A similar problem may occur in a diode rectification type flyback converter including an rectifier diode or an LLC converter instead of the synchronous rectification transistor M2 and the synchronous rectification controller 300. According to the DC / DC converter 200 according to the second embodiment described below, this problem is solved.

FIG. 10 is a circuit diagram of an insulation type DC / DC converter 200 according to the second embodiment. The DC / DC converter 200 is a flyback converter. The DC / DC converter 200 receives an input voltage _VIN at an input terminal P1, generates a DC output voltage _VOUT stabilized at a predetermined target voltage, and outputs an output terminal P2 and a ground terminal. Supply to a load (not shown) connected between P3.

The transformer T1 has a primary winding W1 and a secondary winding W2. One end of the primary winding W1 is connected to the input terminal P1 and receives a DC input voltage _VIN . The drain of the switching transistor M1 is connected to the other end of the primary winding W1 of the transformer T1. A sense resistor for current detection may be inserted between the source of the switching transistor M1 and the ground line.

  The synchronous rectification transistor M2 and the secondary winding W2 of the transformer T1 are provided in series between the output terminal P2 and the ground terminal P3. The output capacitor C1 is connected between the output terminal P2 and the ground terminal P3.

Photocoupler 204 includes a light emitting element and a light receiving element. The light emitting element is biased by resistors R ₂₁ and R ₂₂ .

The primary controller 202 is connected to the light receiving element of the photocoupler 204. A feedback signal V _FB corresponding to the feedback current I _FB flowing through the light receiving element of the photocoupler 204 appears at the feedback (FB) terminal of the primary controller 202.

The primary-side controller 202 generates a pulse signal having a duty ratio (or frequency) corresponding to the feedback signal V _FB and outputs it from the output (OUT) terminal to drive the switching transistor M1. The configuration and control method of the primary controller 202 are not particularly limited. For example, the primary controller 202 may be a current mode modulator.

The synchronous rectification controller 300 controls the synchronous rectification transistor M2. For example, the synchronous rectification controller 300 generates a control pulse based on the drain-source voltage _VDS2 of the synchronous rectification transistor M2, and supplies a gate pulse corresponding to the control pulse to the gate of the synchronous rectification transistor M2. The configuration and operation of the synchronous rectification controller 300 are not particularly limited, and a known technique may be used.

Next, the configuration of the secondary controller 400 will be described.
The secondary-side controller 400 includes a control input (SH_IN) pin, a control output (SH_OUT) pin, a power supply (VCC) pin, and a ground (GND) pin, and is accommodated in one package. A detection voltage V _OUTS corresponding to the output voltage V _OUT is input to the SH_IN pin. For example, the detection voltage V _OUTS is a voltage obtained by dividing the output voltage _VOUT by the resistors R ₁₁ and R ₁₂ . The SH_OUT pin is connected to the light emitting element of the photocoupler 204. The GND pin is connected to the ground terminal P3 (ground line). VCC pin, the power supply voltage _{V CC} is supplied. In Figure 10, the output voltage _{V OUT} is used as the power supply voltage _{V CC,} not limited thereto as described later.

The secondary-side controller 400 includes a power supply line 402, a power supply path 404, a feedback circuit 410, and a protection circuit 420, which are integrated on one semiconductor substrate. The power supply line 402 is connected to the VCC pin. Feedback circuit 410 and the protection circuit 420 operates by receiving power supply voltage _{V CC} of the power supply line 402.

  The power supply path 404 is configured to supply power in one direction from the SH_OUT pin to the VCC pin. For example, the power supply path 404 may include a rectifying element. The rectifying element includes a diode provided such that the anode is on the SH_OUT pin side and the cathode is on the VCC pin (power supply line 402) side.

The feedback circuit 410 is a so-called shunt regulator, and the light emitting element of the photocoupler 204 is arranged so that the detection voltage V _OUTS approaches the reference voltage V _REF , in other words, the output voltage _VOUT approaches the target value V _{OUT (REF).} Drive. Feedback circuit specifically 410, the current _{I PC} corresponding to the error of the detection voltage _{V OUTS} and the reference voltage _{V REF} for driving the light emitting element of the photocoupler 204.

For example, the feedback circuit 410 includes an error amplifier 412 and a pass transistor 414. The error amplifier 412 amplifies an error between the detection voltage V _OUTS and the reference voltage V _REF . Pass transistor 414 is, for example, a P-channel MOSFET, and its source is connected to the SH_OUT pin and its gate is connected to the output of error amplifier 412. The pass transistor 414, the output signal of the error amplifier 412, that is, the current _{I PC} corresponding to the error of the detection voltage _{V OUTS} and the reference voltage _{V REF} flows. The pass transistor 414 may be an N channel. Further, it may be a PNP type or PNP type bipolar transistor.

  The power supply of the error amplifier 412 may be taken from the SH_OUT pin. In this case, the protection circuit 420 is supplied with power from the power supply line 402.

When the protection circuit 420 detects an abnormal state, the protection circuit 420 supplies current to the light emitting element of the photocoupler 204. When an abnormality occurs, the forward current flowing through the light emitting element of the photocoupler 204 increases, the feedback current I _FB increases, the feedback voltage V _FB decreases, the switching of the switching transistor M1 is stopped, and the circuit is protected.

Although not particularly limited, in this embodiment, the abnormal state is an overvoltage state, and the protection circuit 420 is an OVP circuit that supplies the current I _OVP to the photocoupler 204 when the overvoltage state is detected.

The above is the configuration of the DC / DC converter 200. Next, the advantages will be described.
Dislodged VCC pin from the printed circuit board, the wiring is disconnected connecting VCC pin and the output line 208, supply of the power supply voltage _{V CC} to the power line 402 is interrupted. Then, an alternative power supply voltage is supplied from the SH_OUT pin to the power supply line 402 via the power supply path 404. Thus, the operation of the circuit blocks (410, 420) that receive power supply from the power supply line 402 can be maintained.

  Thus, according to the secondary side controller 400 which concerns on 2nd Embodiment, reliability can be improved.

  The present invention is understood as the block diagram and circuit diagram of FIG. 10 or extends to various devices and circuits derived from the above description, and is not limited to a specific configuration. In the following, more specific configuration examples and modifications will be described in order to help understand the essence and circuit operation of the invention and clarify them, not to narrow the scope of the present invention.

(First configuration example)
FIG. 11 is a circuit diagram showing a first configuration example (200A) of the DC / DC converter 200. As shown in FIG. An auxiliary power supply circuit 210 is provided on the secondary side of the DC / DC converter 200A of FIG. Auxiliary power supply circuit 210 includes a power supply capacitor C2 that is provided separately from the output capacitor C1, the power supply voltage _{V CC} generated in the power supply capacitor C2 is supplied to the VCC pin of the secondary side controller 400A. The auxiliary power supply circuit 210 includes a charging path 212 provided between the auxiliary power supply circuit 210 and the power supply capacitor C2. The charging path 212 may include a rectifying element that allows a current from the output line 208 of the DC / DC converter 200A toward the power supply capacitor C2 and blocks a reverse current. The rectifying element may include a diode D2 provided such that the anode is on the output line 208 side of the DC / DC converter 200A and the cathode is on the power supply capacitor C2 side. Alternatively, the rectifying element may be a switch (transistor).

During operation of the DC / DC converter 200, the power supply capacitor C2 is charged to the output voltage _{V OUT} is substantially the same potential, VCC pin Therefore, the output voltage _{V OUT} substantially at the same potential supply voltage _{V CC} is Supplied.

In FIG. 11, the anode of the light emitting element of the photocoupler 204 is connected to the power supply capacitor C2 via a resistor. That is, the light emitting element, the power supply voltage V _CC from the auxiliary power supply circuit 210 is supplied.

When the switching operation of the DC / DC converter 200A stops, the output capacitor C1 is discharged by the load current and decreases with time. On the other hand, the power supply capacitor C2 is discharged by the forward current of the photocoupler 204 and the current flowing through the VCC pin of the secondary controller 400A. Capacity of the power supply capacitor C2 in the stop state of the DC / DC converter 200A, the power supply voltage _{V CC} is determined to decrease slower than the output voltage _{V OUT.}

The protection circuit 420A is an overvoltage protection (OVP) circuit. The overvoltage protection (OVP) pin of the secondary side controller 400A is the power supply voltage _{V CC} is supplied through a resistor _{R 31.} At the OVP pin, a voltage V _CC ′ obtained by dividing the power supply voltage _VCC by an external resistor R ₃₁ and a built-in resistor R ₃₂ is generated. Incidentally resistor R ₃₂ may be an external component.

As described above, during the switching operation of the DC / DC converter 200A, since V _CC ≈V _OUT , the voltage V _CC ′ at the VCC pin is a voltage corresponding to the output voltage V _OUT . When the voltage V _CC ′ of the OVP pin exceeds a predetermined overvoltage threshold value V _OVP , the protection circuit 420A generates a current I _OVP and drives the photocoupler 204.

The protection circuit 420A is configured to maintain the driving state of the photocoupler 204 until it is reset when the voltage of the OVP pin exceeds a predetermined overvoltage threshold V _OVP .

The protection circuit 420A includes an abnormality detection circuit 421 and a protection transistor 426. When detecting an abnormal state (here, an overvoltage state), the abnormality detection circuit 421 maintains the asserted state of the abnormality detection signal S _OVP until it is reset. The protection transistor 426 is turned on when the abnormality detection signal S _OVP is asserted.

Abnormality detection circuit 421 includes an overvoltage detection comparator 422 and a latch circuit 424. The overvoltage detection comparator 422 compares the voltage V _CC ′ of the OVP pin with the overvoltage threshold value V _OVP and, when detecting an overvoltage state (V _CC ′> V _OVP ), asserts the comparison signal S _OVP ′ (for example, high level) To do. The latch circuit 424 latches the comparison signal S _OVP '. The latch circuit 424 may include a flip-flop. The protection transistor 426 is connected to the light emitting element of the photocoupler 204 via the SH_OUT pin, and is turned on in accordance with the output S _OVP of the latch circuit 424.

The secondary-side controller 400A may include a UVLO (Under Voltage Lock Out) circuit 430. The UVLO circuit 430 is a reset circuit that asserts the release signal S _RESET (for example, low level) and resets the latch circuit 424 when V _CC ′ <V _UVLO . When the OVP state of the protection circuit 420A is released by the UVLO circuit 430, the protection transistor 426 is turned off and the current I _OVP is stopped.

In the secondary controller 400A, the power supply voltage _VCC is supplied to the feedback circuit 410 and the protection circuit 420A through the power supply line 402.

The above is the configuration of the DC / DC converter 200A.
According to the DC / DC converter 200A, even when an open abnormality occurs in the VCC pin, a voltage is supplied to the power supply line 402 from the SH_OUT pin via the power feeding path 404. Accordingly, the feedback circuit 410 and the protection circuit 420A can continue to operate and can improve reliability.

According to the DC / DC converter 200A, another problem that may occur in the DC / DC converter 200 of FIG. 10 is solved. As shown in FIG. 10, the overvoltage protection of the configuration (first comparison technique) in which the output voltage _VOUT is directly supplied to the VCC pin will be described.

FIG. 12 is an operation waveform diagram of the DC / DC converter 200 according to the first comparative technique. Prior to time t _{0, the} state is normal, and the output voltage V _OUT is stabilized at the target value V _{OUT (REF)} . At time t ₀ , some abnormality occurs, and the output voltage V _OUT deviates from the target value V _{OUT (REF)} and starts to rise.

When the output voltage V _OUT exceeds the overvoltage threshold V _OVP at time t ₁ , the protection circuit 420 enters a state in which the current I _OVP is supplied to the light emitting element of the photocoupler 204 and remains in that state until reset. Fixed (latched). As a result, the feedback current I _FB increases, the feedback signal V _FB decreases, and the switching of the switching transistor M1 is stopped.

When the switching of the switching transistor M1 stops, the charging of the output capacitor C1 stops, so that the output voltage _VOUT decreases with time and the power supply voltage _VCC supplied to the secondary controller 400 also decreases. Therefore, when the output voltage _VOUT decreases, the protection circuit 420 becomes inoperable and cannot maintain the current I _OVP .

The decrease in the output voltage _{V OUT} That power supply voltage _{V CC,} the current _{I OVP} (and current _{I ERR)} becomes zero to time _{t 2, the} feedback current _{I FB} becomes zero, the feedback signal _{V FB} rises, the switching transistor M1 Switching resumes. By restarting switching, the output voltage _VOUT begins to rise again.

If the cause of the overvoltage remains, the output voltage _VOUT reaches the overvoltage threshold V _OVP again. The DC / DC converter 200S repeats operation and stop alternately in time division (referred to as intermittent mode).

  There is a case where heat generation of the circuit elements constituting the DC / DC converter 200, specifically, the synchronous rectification transistor M2 or the switching transistor M1 may be a problem. In the intermittent mode of FIG. 12, heat is generated during the operation period to increase the temperature, and the temperature is relaxed during the stop period. Therefore, when the stop period is short, the temperature of the circuit element increases steadily. The above is a problem that may occur in the first comparative technique.

Next, the overvoltage protection operation of the DC / DC converter 200A of FIG. 11 will be described.
FIG. 13 is an operation waveform diagram of the DC / DC converter 200A of FIG.
Prior to time t _{0, the} state is normal, and the output voltage V _OUT is stabilized at the target value V _{OUT (REF)} . At time t ₀ , some abnormality occurs, and the output voltage V _OUT deviates from the target value V _{OUT (REF)} and starts to rise.

As the output voltage _VOUT increases, the power supply voltage _VCC and the voltage V _CC ′ of the OVP pin increase. When the voltage V _CC ′ exceeds the overvoltage threshold V _OVP at time t ₁ , the protection circuit 420 enters a state in which the current I _OVP is supplied to the light emitting element of the photocoupler 204 and is fixed (latched) in that state. . As a result, the feedback current I _FB increases, the feedback signal V _FB decreases, and the switching of the switching transistor M1 is stopped.

When the switching of the switching transistor M1 is stopped, the output for the charging of the capacitor C1 is stopped, the output voltage V _OUT will decrease both time, will also decrease supply voltage V _CC is supplied to the secondary side controller 400A. Output voltage _{V OUT} to the time _{t 2} is reduced to 0V. On the other hand, the power supply voltage _{V CC} decreases slower than the output voltage _{V OUT.}

Time _{t 3} to the voltage of the OVP pin _{V CC} 'is released protection state of the protection circuit 420A drops to UVLO voltage _{V UVLO, I OVP} is zero. As a result, the feedback current I _FB also becomes zero, the feedback signal V _FB rises, and the switching of the switching transistor M1 is restarted. By restarting switching, the output voltage _VOUT rises again, and the power supply voltage _VCC also rises.

If the cause of the overvoltage remains, the output voltage _VOUT rises again exceeding the target voltage _{VOUT (REF)} . Then, when the voltage V _CC ′ of the OVP pin exceeds the overvoltage threshold V _OVP , the protection state is entered again. The DC / DC converter 200A repeats operation and stop alternately in a time division manner while the cause of the overvoltage continues.

  The above is the operation of the DC / DC converter 200A. According to the DC / DC converter 200A, the stop period in the intermittent mode can be made longer than that in the DC / DC converter 200 of FIG. As described above, the circuit element generates heat during the operation period and the temperature rises, and the temperature is relaxed during the stop period. However, the temperature rise of the circuit element can be suppressed by lengthening the stop period.

The length of the stop period can be determined based on the capacitance value of the power supply capacitor C2 so as to be within an appropriate temperature range regardless of the rate of decrease of the output voltage _VOUT .

  On the other hand, also in the first comparative technique, when the discharge current of the capacitor C1 during the stop period is sufficiently small, the stop period of FIG. 12 becomes long. Accordingly, in such a case, the power supply pin of the secondary controller 400 may be supplied from the output line 208.

  The DC / DC converter 200A of FIG. 11 will further explain the following advantages over the DC / DC converter 200 of FIG. In order to clarify this advantage, the operation of a configuration in which the light emitting element of the photocoupler 204 is connected to the output line 208 as shown in FIG.

In the second comparison technique, when the protection circuit 420 drives the photocoupler 204 in an abnormal state (for example, an overvoltage state), the output capacitor C1 is discharged by the forward current and the output current, so that the output voltage _VOUT decreases. When the output voltage _VOUT decreases, the forward current (I _OVP ) of the photocoupler 204 decreases and light emission stops, the primary side operation resumes, the output voltage _VOUT increases, and the photocoupler 204 can emit light. Become. By repeating this operation, in an overvoltage state, if the output voltage _VOUT is higher than 0V, the voltage is balanced at a certain voltage level.

Depending on the application, it may be desired to completely stop the switching on the primary side during the stop period and to reduce the output voltage _VOUT to 0V completely. This is a problem that may occur in the second comparison technique.

According to the DC / DC converter 200A in FIG. 11, the light emitting element of the photocoupler 204, the power supply voltage _{V CC} from the auxiliary power supply circuit 210 is supplied. Supply voltage _{V CC,} in order to decrease slower than the output voltage _{V OUT,} the forward current of the photocoupler 204, after the output voltage _{V OUT} has decreased to 0V, continues to flow to the power supply voltage _{V CC} is sufficiently small. Thereby, in the stop period, the switching of the switching transistor M1 can be stopped completely, and the output voltage _VOUT can be lowered to zero V.

(Modification)
14A and 14B are circuit diagrams of modified examples (400B, 400C) of the secondary-side controller 400A of FIG. In the secondary controller 400B of FIG. 14A, the protection circuit 420B includes an auto reset circuit 428. Auto-reset circuit 428 also operates in response to voltage _VCC on power supply line 402.

  The auto-reset circuit 428 includes a timer circuit, and resets the latch circuit 424 after a predetermined time has elapsed since the protection circuit 420B is in the OVP state. According to this modification, the stop period can be set according to the time measured by the auto reset circuit 428.

In the secondary side controller 400C of FIG. 14B, the protection circuit 420C includes a hysteresis comparator 423 and a protection transistor 426. The hysteresis comparator 423 compares the voltage V _CC ′ of the OVP pin with a threshold voltage that changes in two values of V _OVP and V _UVLO , and generates an abnormality detection signal S _OVP according to the comparison result. According to this configuration, the operation of FIG. 13 can be realized.

(Second configuration example)
FIG. 15 is a circuit diagram showing a second configuration example (200D) of the DC / DC converter. In the DC / DC converter 200D, the secondary controller 400D includes a synchronous rectification controller 300D in the same package in addition to the feedback circuit 410 and the protection circuit 420. They may be integrated on the same semiconductor substrate (die, chip), or may be divided and integrated into a plurality of dies.

The SOURCE pin is a ground terminal of the synchronous rectification controller 300D. The GATE pin and the DRAIN pin are connected to the gate and drain of the synchronous rectification transistor M2. To Synchronous controller 300D The power supply voltage _{V CC} is supplied from the power supply line 402. The synchronous rectification controller 300D may be built in the secondary side controllers 400B and 400C according to the modified examples of FIGS. 14 (a) and 14 (b).

  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)
The protection circuit 420 is not limited to an overvoltage protection circuit. For example, the switching rectification state in which the synchronous rectification transistor M2 cannot be switched may be detected, and the switching state may be activated to activate the light emitting element of the photocoupler 204. For example, the protection circuit 420 may detect an open abnormality of the GATE pin of the secondary controller 400, or may detect an open abnormality of the DRAIN pin of the secondary controller 400.

(Second modification)
In the embodiment, the synchronous rectification type flyback converter is taken as an example, but the present invention can also be applied to a diode rectification type flyback converter. The present invention can also be applied to an LLC converter.

(Third Modification)
The configuration of the power supply path 404 is not limited to the diode as shown in FIG. 10, and may be configured by a switch that is turned on when the voltage of the SH_OUT pin is higher than the voltage of the VCC pin and turned off when the voltage is not so. .

(Fourth modification)
As described in relation to the second comparative technique described above, in an application where no problem occurs even when the output voltage _VOUT is stabilized at a certain voltage level in an abnormal state, the light-emitting element of the photocoupler 204 in FIG. The anode may be connected to the output line 208 via a resistor.

(5th modification)
The power supply voltage supplied to the VCC pin of the secondary controller 400 is not limited to the output voltage _VOUT or the voltage generated by the auxiliary power supply circuit 210 of FIG. For example, an auxiliary power supply circuit 210, constituted by a step-up charge pump which receives the output voltage V _OUT, may provide its output voltage to the VCC pin.

  It should be noted that the arbitrary technical features of the first embodiment and the arbitrary technical features of the second embodiment can be combined within a range in which there are no obstruction factors, and the combination technique is also described in this book. It is included in the scope of the invention.

(Use)
Subsequently, the application of the DC / DC converter 200 described in the first and second embodiments will be described. The DC / DC converter 200 can be used for the AC / DC converter 100. FIG. 16 is a circuit diagram of an AC / DC converter 100 including the DC / DC converter 200.

The AC / DC converter 100 includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC / DC converter 200. The filter 102 removes noise from the _AC voltage VAC. The rectifier circuit 104 is a diode bridge circuit that full-wave rectifies the _AC voltage VAC. The smoothing capacitor 106 smoothes the full-wave rectified voltage and generates a DC voltage _VIN . The DC / DC converter 200 receives the direct current voltage _VIN and generates an output voltage _VOUT .

FIG. 17 is a diagram illustrating an AC adapter 800 including the AC / DC converter 100. The AC adapter 800 includes a plug 802, a housing 804, and a connector 806. Plug 802 is subjected to a commercial AC voltage _{V AC} from the wall outlet (not shown). The AC / DC converter 100 is mounted in the housing 804. The DC output voltage V _OUT generated by the AC / DC converter 100 is supplied from the connector 806 to the electronic device 810. Examples of the electronic device 810 include a laptop computer, a digital camera, a digital video camera, a mobile phone, and a mobile audio player.

  18A and 18B are diagrams showing an electronic device 900 including the AC / DC converter 100. FIG. 18A and 18B is a display device, but the type of the electronic device 900 is not particularly limited, and may be a device incorporating a power supply device such as an audio device, a refrigerator, a washing machine, or a vacuum cleaner. I just need it.

Plug 902 is subjected to a commercial AC voltage _{V AC} from the wall outlet (not shown). The AC / DC converter 100 is mounted in the housing 904. The DC output voltage V _OUT generated by the AC / DC converter 100 is applied 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. Supplied.

  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, M1 ... switching transistor, M2 ... synchronous rectification transistor, C1 ... output capacitor, T1 ... transformer, W1 ... primary winding, W2 ... secondary winding, 100 ... AC / DC converter, DESCRIPTION OF SYMBOLS 102 ... Filter, 104 ... Rectifier circuit, 106 ... Smoothing capacitor, 200 ... DC / DC converter, 202 ... Primary side controller, 204 ... Photocoupler, 210 ... Auxiliary power supply circuit, D2 ... Diode, C2 ... Power supply capacitor, 400 ... Two Secondary controller 402 ... Power supply line 404 ... Power feeding path 410 ... Feedback circuit 412 ... Error amplifier 414 ... Pass transistor 420 ... Protection circuit 421 ... Abnormality detection circuit 422 ... Overvoltage detection comparator 423 ... Hysteresis comparator 424... Latch circuit 42 ... Protection transistor, 428 ... auto reset circuit, 430 ... UVLO circuit, 300 ... synchronous rectification controller, 800 ... AC adapter, 802 ... plug, 804 ... housing, 806 ... connector, 810,900 ... electronic device, 902 ... plug, 904: A housing.

Claims (32)

An insulated synchronous rectification type DC / DC converter,
A transformer having a primary winding and a secondary winding;
A switching transistor connected to the primary winding of the transformer;
An output capacitor charged by a current flowing through the secondary winding of the transformer;
A photocoupler including a light emitting element and a light receiving element;
A feedback circuit that drives the light emitting element of the photocoupler so that a detection voltage corresponding to an output voltage generated in the output capacitor approaches a reference voltage;
A primary controller that is connected to the light receiving element of the photocoupler and controls the switching transistor according to a feedback signal based on a state of the light receiving element;
A protection circuit that activates upon detection of an abnormal state and drives the light emitting element;
An auxiliary power supply circuit including a power supply capacitor provided separately from the output capacitor, and supplying a power supply voltage generated in the power supply capacitor to the anode of the light emitting element of the protection circuit and the photocoupler;
A DC / DC converter comprising: The protection circuit is
When an abnormal state is detected, an abnormality detection circuit that maintains the asserted state of the abnormality detection signal until it is reset,
A transistor that is turned on in the asserted state of the abnormality detection signal;
The DC / DC converter according to claim 1, wherein the power supply voltage is supplied to the abnormality detection circuit.   The DC / DC converter according to claim 2, wherein the abnormality detection signal is negated when the power supply voltage falls below a release threshold.   3. The DC / DC converter according to claim 2, wherein the abnormality detection signal is negated after a predetermined time elapses after the assertion.   The DC / DC converter according to any one of claims 1 to 4, wherein the protection circuit is an overvoltage protection circuit.   6. The DC / DC converter according to claim 1, wherein the auxiliary power circuit further includes a charging path from the output capacitor to the power capacitor.   The DC / DC converter according to claim 6, wherein the charging path includes a rectifying element that allows a current from the output line of the DC / DC converter to the power supply capacitor and blocks a reverse current. .   8. The DC / DC converter according to claim 6, wherein the charging path includes a diode provided in such a direction that an anode is on an output line side of the DC / DC converter and a cathode is on the power supply capacitor side. . The feedback circuit is built in the secondary controller,
In addition to the feedback circuit, the secondary controller
A control output pin to be connected to the light emitting element of the photocoupler;
A power supply pin to receive the power supply voltage; and
A control input pin to receive a detection voltage according to the output voltage of the DC / DC converter;
A power supply path for supplying power from the control output pin to the power supply pin;
With
9. The feedback circuit according to claim 1, wherein the feedback circuit amplifies an error between the detection voltage and a reference voltage and supplies a current corresponding to the error to the light emitting element of the photocoupler. DC / DC converter.   The DC / DC converter according to claim 9, wherein the power supply path includes a rectifying element.   11. The DC / DC converter according to claim 9, wherein the power supply path includes a diode provided in an orientation in which an anode is on the control output pin side and a cathode is on the power supply pin side. It is activated when an abnormal state is detected, and further comprises a protection circuit that drives the light emitting element,
12. The DC / DC converter according to claim 9, wherein the power supply voltage of the power supply pin is supplied to at least a part of the protection circuit. When the overvoltage state is detected, the protection circuit is configured to maintain a state of driving the light emitting element until reset.
13. The DC / DC converter according to claim 12, wherein the power supply voltage supplied to the power supply pin decreases later than the output voltage when the DC / DC converter is stopped. The protection circuit is
When detecting the overvoltage state, an overvoltage detection comparator that asserts an abnormality detection signal;
A latch circuit for latching the abnormality detection signal;
A protection transistor connected to the light emitting element and turned on in accordance with an output of the latch circuit;
The DC / DC converter according to claim 13, comprising: The protection circuit is
A hysteresis comparator that asserts an abnormality detection signal when the monitored voltage exceeds the upper threshold, and negates the abnormality detection signal when the monitored voltage falls below the lower threshold;
A protection transistor connected to the light emitting element and turned on in response to the abnormality detection signal;
The DC / DC converter according to claim 13, comprising: The feedback circuit includes:
An error amplifier for amplifying an error between the detection voltage and the reference voltage;
A pass transistor connected to the control output pin and driven in accordance with an output signal of the error amplifier;
The DC / DC converter according to claim 9, comprising:   17. The DC according to claim 9, wherein a synchronous rectification controller for driving a secondary side synchronous rectification transistor of the DC / DC converter is housed in the same package as the secondary side controller. / DC converter.   The DC / DC converter according to any one of claims 1 to 17, further comprising a rectifying element provided between a cathode of the light emitting element of the photocoupler and a power supply line of the protection circuit.   19. The DC / DC converter according to claim 18, wherein the power supply voltage decreases later than the output voltage when the DC / DC converter is stopped.   The DC / DC converter according to claim 19, wherein the power supply voltage from the auxiliary power supply circuit is supplied to an anode of the light emitting element of the photocoupler.   21. The DC / DC converter according to claim 18, wherein the rectifying element is a diode. The protection circuit is
When an abnormal state is detected, an abnormality detection circuit that maintains the asserted state of the abnormality detection signal until it is reset,
A transistor that is turned on in the asserted state of the abnormality detection signal;
The DC / DC converter according to any one of claims 18 to 21, wherein the power supply voltage is supplied to the abnormality detection circuit.   The DC / DC converter according to claim 22, wherein the abnormality detection signal is negated when the power supply voltage falls below a release threshold.   The DC / DC converter according to claim 22, wherein the abnormality detection signal is negated after a predetermined time elapses after the assertion.   25. The DC / DC converter according to claim 18, wherein the protection circuit is an overvoltage protection circuit.   26. The DC / DC converter according to claim 18, wherein the auxiliary power circuit further includes a charging path from the output capacitor to the power capacitor.   27. The DC / DC converter according to claim 26, wherein the charging path includes a rectifying element that allows a current from the output line of the DC / DC converter to the power supply capacitor and blocks a reverse current. .   28. The DC / DC converter according to claim 26 or 27, wherein the charging path includes a diode provided in such a direction that an anode is on an output line side of the DC / DC converter and a cathode is on the power supply capacitor side. . Load,
A diode rectifier circuit for full-wave rectification of commercial AC voltage;
A smoothing capacitor that smoothes the output voltage of the diode rectifier circuit and generates a DC input voltage;
The DC / DC converter according to any one of claims 1 to 28, wherein the DC input voltage is stepped down and supplied to a load.
An electronic device comprising: A diode rectifier circuit for full-wave rectification of commercial AC voltage;
A smoothing capacitor that smoothes the output voltage of the diode rectifier circuit and generates a DC input voltage;
The DC / DC converter according to any one of claims 1 to 28, wherein the DC input voltage is stepped down and supplied to a load.
A power adapter comprising: A method for controlling an isolated synchronous rectification type DC / DC converter, comprising:
Controlling a switching transistor connected to the primary winding of the transformer;
Rectifying the current flowing in the secondary winding of the transformer to charge the output capacitor and generating an output voltage;
Generating a power supply voltage by a power supply capacitor provided separately from the output capacitor;
Supplying the power supply voltage to a secondary-side controller that drives a light-emitting element of a photocoupler and a cathode of the light-emitting element;
Detecting an abnormal state, maintaining a state of driving the light emitting element;
A method comprising the steps of:   32. The method of claim 31, further comprising a step of supplying power to the secondary controller from a cathode of the light emitting element of the photocoupler when power supply to the secondary controller is cut off. The method described in 1.

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