JP6514910B2 - Isolated synchronous rectification type DC / DC converter, synchronous rectification controller, power supply using the same, power adapter and electronic equipment - Google Patents

Description

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

  Various home appliances such as TVs and refrigerators operate by receiving commercial AC power from the outside. Laptop computers, electronic devices such as mobile phone terminals and tablet terminals are also operable with commercial AC power, or can be charged with internal batteries in the equipment with commercial AC power. In such home appliances and electronic devices (hereinafter collectively referred to as electronic devices), a power supply device (AC / DC converter) that converts commercial AC voltage into AC / DC (AC / DC) is incorporated. Alternatively, an AC / DC converter may be incorporated in an external power adapter (AC adapter) of the electronic device.

  FIG. 1 is a block diagram showing a basic configuration of an AC / DC converter 100r examined by the present inventor. The AC / DC converter 100r mainly includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC / DC converter 200r.

Commercial alternating voltage V _AC is input to filter 102 through a fuse and an input capacitor (not shown). The filter 102 removes noise of the commercial AC voltage V _AC . The rectifier circuit 104 is a diode bridge circuit that full-wave rectifies the commercial AC voltage V _AC . The output voltage of the rectifier circuit 104 is smoothed by the smoothing capacitor 106 and converted to the DC voltage V _IN .

The isolated DC / DC converter 200r receives the DC voltage V _IN at the input terminal P1 and steps it down to generate an output voltage V _OUT stabilized at the target value, and outputs the output terminal P2 and the ground terminal P3. Supply to a load (not shown) connected between.

  The DC / DC converter 200r includes a primary side controller 202, a photocoupler 204, a shunt regulator 206, an output circuit 210, a synchronous rectification controller 300r, and other circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C1, a switching transistor M1, and a synchronous rectification transistor M2. The topology of the output circuit 210 is that of a general synchronous rectification type flyback converter, so the description thereof is omitted.

By switching the switching transistor M1 connected to the primary winding W1 of the transformer T1, the input voltage V _IN is stepped down to generate the output voltage V _OUT . Then, the primary side controller 202 adjusts the switching duty ratio of the switching transistor M1.

The output voltage V _OUT of the DC / DC converter 200 r is divided by the resistors R 1 and R 2. The cathode (K) terminal of the shunt regulator 206 is connected to the light emitting element (light emitting diode) on the input side of the photocoupler 204, and the anode (A) terminal is grounded. The _divided voltage (voltage detection signal) V _{OUT — S} is input to the reference (REF) terminal of the shunt regulator 206. The shunt regulator 206 includes an error amplifier, amplifies an error between the voltage detection signal V _{OUT — S} and a predetermined reference voltage V _REF (not shown), generates an error current I _ERR according to the error, and Pull in (sink) from a light emitting element (light emitting diode).

In the light receiving element (phototransistor) on the output side of the photocoupler 204, a feedback current I _FB flows according to the error current I _ERR on the secondary side. The feedback current I _FB is smoothed by the resistor and the capacitor, and is input to the feedback (FB) terminal of the primary side controller 202. Primary-side controller 202 adjusts the duty ratio of the switching transistor M1 based on the voltage (feedback _{voltage) V FB} of the FB pin.

  The synchronous rectification controller 300r switches the synchronous rectification transistor M2 in synchronization with the switching of the switching transistor M1. The synchronous rectification controller 300 r includes an internal power supply circuit 302, a pulse generator 304, and a driver 306.

The synchronous rectification controller 300r operates using the output voltage V _OUT of the DC / DC converter 200r as a power supply. Specifically, the synchronous rectification controller 300r is provided with a power supply (VCC) terminal for receiving the output voltage V _OUT or a DC voltage proportional thereto. The internal power supply circuit 302 steps down the voltage of the VCC terminal to generate an internal power supply voltage V _REG stabilized at a predetermined voltage level. The internal power supply voltage V _REG is supplied to the pulse generator 304 and the driver 306.

The pulse generator 304 generates a pulse signal S1 synchronized with the switching of the switching transistor M1. For example, when the switching transistor M1 is turned off, the pulse generator 304 sets the pulse signal S1 to a first state (for example, high level) instructing the synchronous rectification transistor M2 to be turned on. Also, the pulse generator 304 instructs the pulse signal S1 to turn off the synchronous rectification transistor M2 when the secondary current I _S flowing through the secondary winding W2 becomes substantially zero during the on period of the synchronous rectification transistor M2. It is in the state (low level).

  The driver 306 switches the synchronous rectification transistor M2 in response to the pulse signal S1. The above is the overall configuration of the AC / DC converter 100r.

Unexamined-Japanese-Patent No. 2010-074959

  As a result of examining the operation at the time of startup of the DC / DC converter 200r, the present inventors came to recognize the following problem.

FIG. 2 is an operation waveform diagram at the time of startup of the DC / DC converter 200 r. V _{G_P} the gate voltage of the switching transistor _{M1, V D_P} the drain voltage of the switching transistor _{M1, V D_S} the drain voltage of the synchronous rectification transistor _{M2, V GS} is synchronized gate voltage of the rectification transistor _{M2, V OUT} denotes the output voltage.

When the start signal START is asserted (high level) at time t0 and the start of the DC / DC converter 200r is instructed, the primary side controller 202 starts switching the gate voltage V _{G — P} of the switching transistor M1. In accordance with the switching of the switching transistor M1, its drain voltage V _{D — P} is also switched.

The voltage V _{W1 of the} primary winding W1 is V _IN −V _{D — P.} When the windings of the primary winding W1 and the secondary winding W2 are N _P and N _S , the voltage V _{W2 of the} secondary winding _W2 is −V _W1 × N _S / N _P. Therefore, in accordance with the switching of the primary side drain voltage _{VD_P} , the secondary side drain voltage _{VD_S} is also switched.

Here, immediately after startup, the output voltage V _OUT is substantially zero, so that the voltage at the VCC terminal of the synchronous rectification controller 300r is also zero, and the internal power supply voltage V _REG generated by the internal power supply circuit 302 is also zero. Here, if the internal power supply voltage V _REG is zero or less than the specified voltage, the drivability of the driver 306 is insufficient, and the potential of the OUT terminal can not be fixed at 0 V (ground potential). Immediately after start-up, it can be said that the potential of the OUT terminal is indeterminate.

There can not be neglected parasitic capacitance (drain capacitance) C _DS between the gate and drain of the synchronous rectification transistor M2. When the switching transistor M1 is turned on and the drain voltage V _{D _S} on the secondary side rises, the gate voltage V _GS on the secondary side, which should be fixed at low level (ground voltage) by the parasitic capacitance C _DS , jumps up. As a result, when the gate voltage V _GS exceeds the threshold voltage V _{GS (TH)} of the synchronous rectification transistor M2, the synchronous rectification transistor M2 is turned on unintentionally.

When the synchronous rectification transistor M2 is turned on in a section where the drain voltage V _{D _S} on the secondary side is at high level, drain current I _M2 = V _{D _S} / R _ON flows in the synchronous rectification transistor M2. R _ON2 is the on resistance of the synchronous rectification transistor M2. V _{D_S} = _{50 V,} if R ON2 = 10 m [Omega, the drain current _{I M2} is a large current to the extent that reduces the reliability of the synchronous rectification transistor M2 and the transformer T1.

  The above problems should not be regarded as the common technical recognition of those skilled in the art, but the present inventors have uniquely recognized.

  The present invention has been made in view of such problems, and one of the exemplary objects of an aspect thereof is that the synchronous rectification transistor is erroneously turned on during the on period of the switching transistor when the DC / DC converter is activated. To provide a synchronous rectification controller that can prevent the

  One embodiment of the present invention relates to a synchronous rectification controller which is disposed on the secondary side of an isolated synchronous rectification type DC / DC converter and controls a synchronous rectification transistor. The synchronous rectification controller operates with a power supply terminal receiving a DC voltage generated on the secondary side of the DC / DC converter and a DC voltage as a power supply, and generates a pulse signal based on the voltage across the synchronous rectification transistor. , A driver operating with a DC voltage as a power supply, switching a synchronous rectification transistor according to a pulse signal, an auxiliary power supply circuit generating an auxiliary power supply voltage based on a drain voltage of the synchronous rectification transistor, and an auxiliary power supply voltage. And an off-fixing circuit that operates to lock the synchronous rectification transistor off.

  In this aspect, immediately after startup, an auxiliary power supply voltage is generated based on the drain voltage of the synchronous rectification transistor even if a sufficiently high DC voltage is not generated on the secondary side of the DC / DC converter, and the auxiliary power supply voltage is received. The gate voltage of the synchronous rectification transistor is fixed at a low level by an off-fixing circuit that operates as described above. Thus, it is possible to prevent the gate voltage from jumping up as the drain voltage of the synchronous rectification transistor fluctuates, and to prevent the synchronous rectification transistor from being turned on erroneously.

The auxiliary power supply circuit may include a peak hold circuit. The peak hold circuit may include a capacitor, and a diode provided between the drain and the capacitor of the synchronous rectification transistor, with the cathode directed to the capacitor side.
The auxiliary power supply circuit may include a filter circuit. The filter circuit may include a capacitor and a resistor.

  The off-fixing circuit may include an off transistor provided between the gate and source of the synchronous rectification transistor.

  An auxiliary power supply voltage may be input to the control terminal of the turning-off transistor.

The auxiliary power supply circuit may include a diode provided between the control terminal of the off transistor and the drain of the synchronous rectification transistor in a direction in which the cathode is on the side of the control terminal of the off transistor.
According to this configuration, the gate capacitances of the diode and the off transistor form a peak hold circuit. Therefore, when the drain voltage of the synchronous rectification transistor jumps up, the gate voltage of the off transistor is maintained at a high voltage level. Can be fixed on.

The off-fixing circuit may release the off-fixing of the synchronous rectification transistor when the DC voltage exceeds a predetermined voltage.
When a sufficient DC voltage is supplied to the power supply terminal of the synchronous rectification controller, the pulse generator and driver become operable, so the off state of the synchronous rectification transistor is released and the synchronous rectification transistor is switched according to the pulse signal. Thus, the DC / DC converter can be operated in the synchronous rectification mode.

The off-fixing circuit may further include a release circuit that turns off the off transistor when the DC voltage exceeds a predetermined voltage.
By turning off the off transistor, the gate voltage of the synchronous rectification transistor can be released.

  The release circuit may include a release transistor which is provided between the gate and the source of the off transistor, and which has a control terminal to which a release signal is asserted which is asserted when the DC voltage exceeds a predetermined voltage.

  The synchronous rectification controller according to an aspect may further include an under voltage lockout circuit that asserts a release signal when the voltage corresponding to the DC voltage exceeds a predetermined threshold voltage.

  The off-fixing circuit may fix the synchronous rectification transistor off by forcibly turning on the low side transistor of the output stage of the driver.

  Another aspect of the present invention is also a synchronous rectification controller. The synchronous rectification controller operates with a power supply terminal receiving a DC voltage generated on the secondary side of the DC / DC converter and a DC voltage as a power supply, and generates a pulse signal based on the voltage across the synchronous rectification transistor. And a driver operating with a DC voltage as a power supply and switching the synchronous rectification transistor according to the pulse signal, an off transistor provided between the gate and source of the synchronous rectification transistor, a control terminal of the off transistor and synchronous rectification And a diode provided between the drain terminals of the transistors in a direction in which the cathode is on the control terminal side of the off-transistor.

  The gate capacitance of the off transistor and the diode form a peak hold circuit. Therefore, when the drain voltage of the synchronous rectification transistor jumps up in response to the turning off of the switching transistor, the gate voltage of the off transistor becomes high level, the off transistor is turned on, and the gate-source voltage of the synchronous rectification transistor is substantially It is possible to lock the synchronous rectification transistor off as it becomes zero.

  The synchronous rectification controller may further comprise a capacitor connected to the control terminal of the off transistor.

  The synchronous rectification controller may further comprise a resistor provided between the control terminal and the drain terminal of the off transistor.

  The synchronous rectification controller may further include a release transistor provided between the control terminal of the off transistor and the ground line and turned on when the DC voltage exceeds a predetermined voltage.

  The synchronous rectification controller further includes a low voltage lockout circuit that compares a voltage corresponding to the DC voltage with a predetermined threshold voltage and asserts a release signal when the DC voltage exceeds the threshold, and the release transistor , May be turned on when the release signal is asserted.

  The synchronous rectification controller may further include an internal power supply circuit that generates an internal power supply voltage based on the voltage of the power supply terminal, and the pulse generator and the driver may operate using the internal power supply voltage as a power supply.

The synchronous rectification controller may be integrated on one semiconductor substrate.
"Integrated integration" includes the case where all of the circuit components are formed on a semiconductor substrate, and the case where the main components of the circuit are integrally integrated. A resistor, a capacitor or the like 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.

  Another aspect of the present invention relates to an isolation 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, a synchronous rectification transistor connected to the secondary winding of the transformer, and a photo A coupler, a primary side controller connected to the output side of the photocoupler and switching a switching transistor according to a feedback signal from the photocoupler, any of the above-mentioned synchronous rectification controllers controlling a synchronous rectification transistor, and a photocoupler And a shunt regulator connected to the input side of the DC / DC converter to generate an error current according to the output voltage of the DC / DC converter.

  The DC / DC converter may be a flyback type or a forward type.

  Another aspect of the present invention relates to a power supply (AC / DC converter). The power supply device includes: a filter for filtering commercial AC voltage; a diode rectification circuit for full-wave rectification of the output voltage of the filter; a smoothing capacitor for smoothing the output voltage of the diode rectification circuit to generate a DC input voltage; And step up and supply the load with the DC / DC converter described above.

  Another aspect of the present invention relates to an electronic device. The electronic device includes a load, a filter for filtering a commercial AC voltage, a diode rectification circuit for full-wave rectifying the output voltage of the filter, and a smoothing capacitor for smoothing the output voltage of the diode rectification circuit to generate a DC input voltage; And d) reducing the DC input voltage and supplying the load to the DC / DC converter.

  Another aspect of the present invention relates to an AC adapter. The AC adapter includes a filter for filtering a commercial AC voltage, a diode rectification circuit for full wave rectification of the output voltage of the filter, a smoothing capacitor for smoothing the output voltage of the diode rectification circuit to generate a DC input voltage, and a DC input voltage And step up to generate a DC output voltage.

  It is to be noted that any combination of the above-described constituent elements, or one in which the constituent elements and expressions of the present invention are mutually replaced among methods, apparatuses, systems, etc. is also effective as an aspect of the present invention.

  According to an aspect of the present invention, it is possible to prevent the synchronous rectification transistor from being accidentally turned on during the on period of the switching transistor when the DC / DC converter is activated.

It is a block diagram which shows the basic composition of the AC / DC converter which this inventor examined. It is an operation | movement wave form diagram at the time of starting of a DC / DC converter. It is a circuit diagram of a DC / DC converter provided with a synchronous rectification controller concerning an embodiment. FIG. 5 is an operation waveform diagram of the DC / DC converter of FIG. 3; It is a circuit diagram showing an example of composition of a synchronous rectification controller. It is an operation | movement wave form diagram of the synchronous rectification controller of FIG. It is a figure showing an AC adapter provided with an AC / DC converter. FIGS. 8A and 8B are diagrams showing an electronic device provided with an AC / DC converter. It is a circuit diagram of the DC / DC converter concerning a 1st modification. FIGS. 10A and 10B are circuit diagrams of a synchronous rectification controller according to a third modification.

  Hereinafter, the present invention will be described 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 duplicating descriptions will be omitted as appropriate. In addition, the embodiments do not limit the invention and are merely examples, and all the features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In the present specification, "a state in which the member A is connected to the member B" means that the members A and B are physically directly connected, or the members A and B are electrically connected. It also includes the case of being indirectly connected via other members that do not affect the state.
Similarly, "a state in which the member C is provided between the member A and the member B" means that the member A and the member C, or the member B and the member C are directly connected, or the electric member It also includes the case of indirect connection via other members that do not affect the connection state.

  FIG. 3 is a circuit diagram of a DC / DC converter 200 including the synchronous rectification controller 300 according to the embodiment. This DC / DC converter 200 can be used for an AC / DC converter as well as the DC / DC converter 200r of FIG. The basic configuration of the DC / DC converter 200 is the same as that of the DC / DC converter 200 r of FIG. 1.

  The synchronous rectification controller 300 has a power supply (VCC) terminal, a switching output (OUT) terminal, a drain voltage (VD) terminal, and a ground (GND) terminal, and is a functional IC (Integrated Circuit) integrated on one semiconductor substrate. It is. The synchronous rectification controller 300 may be housed in the same package as the synchronous rectification transistor M2, and may constitute an integral single module.

The VCC terminal of the synchronous rectification controller 300 is supplied with a DC voltage (output voltage V _{OUT in this case} ) generated on the secondary side of the DC / DC converter 200. The synchronous rectification controller 300 operates with the voltage at the VCC terminal as the main power supply. The VD terminal is connected to the drain of the synchronous rectification transistor M2. The OUT terminal is connected to the gate of the synchronous rectification transistor M2. The GND terminal is connected to the potential that the synchronous rectification controller 300 should make a reference, and in the topology of FIG. 3, the reference potential on the secondary side, that is, the ground voltage V _GND is supplied.

  The synchronous rectification controller 300 includes an internal power supply circuit 302, a pulse generator 304, a driver 306, an auxiliary power supply circuit 310, and an off-fixing circuit 320.

An internal power supply circuit 302 lowers the DC voltage _{V OUT} of the VCC terminal, and generates an internal power supply voltage _{V REG} stabilized to a predetermined voltage level. The internal power supply voltage V _REG is supplied to the power supply terminals of the pulse generator 304 and the driver 306, so that it can be said that the pulse generator 304 and the driver 306 operate using the DC voltage V _OUT of the VCC terminal as a power supply.

The pulse generator 304 generates a pulse signal S1 based on the drain voltage V _{D _S} of the synchronous rectification transistor M2. Synchronous controller 300, the GND terminal is connected to the source and the common ground line of the synchronous rectification transistor M2, since the operating reference to the source voltage _{V GND,} the drain voltage _{V D_S} the VD terminal, the synchronous rectification transistor M2 It is nothing but the voltage (drain-source voltage V _DS ) between

The configuration and operation of the pulse generator 304 are not particularly limited, and known or future available techniques may be used. For example, when the pulse generator 304 detects that the switching transistor M1 on the primary side of the DC / DC converter 200 is turned off, the pulse signal S1 turns on the synchronous rectification transistor M2 to an on level (for example, high level). When the current I _S of the secondary winding W2 is detected substantially to become zero, an off-level (e.g., low level) to indicate the synchronous rectification transistor M2 off the pulse signal S1. The driver 306 switches the synchronous rectification transistor M2 in response to the pulse signal S1.

Since the voltage across the secondary winding W2 is -V _IN × N _S / N _P in the on period of the switching transistor M1, the drain voltage V _{D — S of} the synchronous rectification transistor M2 (that is, the voltage V _DS between drain and source) _Becomes V _D — _S = V _OUT + V _IN × N _S / N _P. N _P and N _S are the number of turns of the primary winding W1 and the winding W2.

When the switching transistor M1 is turned off, the secondary current I _S flows from the source to the drain of the synchronous rectification transistor M2, so that the drain-source voltage becomes a negative voltage. In the continuous mode, the switching transistor M1 is turned on, the secondary current I _S becomes zero, and the drain voltage bounces back to V _D = V _OUT + V _IN × N _S / N _P again. In the discontinuous mode, as the secondary current I _S decreases as the energy stored in the transformer T1 decreases in the on state of the synchronous rectification transistor M2, the absolute value of the drain-source voltage V _DS decreases and eventually When the secondary current I _S becomes substantially zero, the drain-source voltage V _DS also becomes substantially zero, and the drain voltage V _{D — S} rings.

  Using these properties, the pulse generator 304 may generate the pulse signal S1 based on the drain voltage (voltage between drain and source) of the synchronous rectification transistor M2.

  The driver 306 switches the synchronous rectification transistor M2 in response to the pulse signal S1.

The auxiliary power supply circuit 310 generates an auxiliary power supply voltage V _AUX based on the drain voltage V _D of the synchronous rectification transistor M2 input to the VD terminal. As described later, the auxiliary power supply circuit 310 can be configured by a peak hold circuit, a filter circuit, a combination thereof, or the like.

The off-fixing circuit 320 operates in response to the auxiliary power supply voltage V _AUX and fixes the potential of the OUT terminal of the synchronous rectification transistor M2 to low level, that is, the voltage V _GS between the gate and the source of the synchronous rectification transistor M2 is substantially zero. Fix the synchronous rectification transistor M2 off.

In the off-fixing circuit 320, when the DC voltage V _OUT exceeds a predetermined voltage, in other words, when the internal power supply voltage V _REG exceeds the minimum operating voltage of the pulse generator 304 and the driver 306, the synchronous rectification transistor M2 is turned off. Release the fixation.

The above is the basic configuration of the synchronous rectification controller 300. Subsequently, the operation will be described.
FIG. 4 is an operation waveform diagram of DC / DC converter 200 of FIG. When the start signal START is asserted (high level) at time t0 and the start of the DC / DC converter 200 is instructed, the primary side controller 202 starts switching the gate voltage V _{G — P} of the switching transistor M1. According to the switching of the switching transistor M1, the drain voltage V _{D _ P} on the primary side and the drain voltage V _{D _ S} on the secondary side switch.

When the drain voltage V _{D — S} on the secondary side jumps up, the auxiliary power supply voltage V _AUX generated by the auxiliary power supply circuit 310 rises. As a result, the off-fixing circuit 320 becomes operable to fix the synchronous rectification transistor M2 in the off state. As a result, the turning on of the synchronous rectification transistor M2 as shown in FIG. 2 is prevented.

When the output voltage V _OUT exceeds a threshold voltage V _UVLO at time t2, the off-fixing by the off-fixing circuit 320 is released. The pulse generator 304 and the driver 306 become operable, and the synchronous rectification transistor M2 is switched according to the pulse signal S1 to shift to the synchronous rectification mode.

That is, immediately after startup, the auxiliary power supply voltage V _AUX is generated based on the drain voltage V _{D _S} of the synchronous rectification transistor M 2 even if a sufficiently high DC voltage V _OUT is not generated on the secondary side of the DC / DC converter 200. Ru. The gate voltage V _{G — S} of the synchronous rectification transistor M 2 is fixed at the low level by the off-fixing circuit 320 that operates in response to the auxiliary power supply voltage V _AUX .

The above is the operation of the DC / DC converter 200. According to the DC / DC converter 200, to prevent the gate voltage _{V G_s} with the fluctuation of the drain voltage _{V D_S} of the synchronous rectification transistor M2 jumps up, it can be prevented from being turned on accidentally synchronous rectification transistor M2.

  Subsequently, a specific configuration example of the synchronous rectification controller 300 will be described. The present invention extends to various forms understood from the circuit diagram and the block diagram of FIG. 3 and the above description, and is not limited to the specific configuration described below.

FIG. 5 is a circuit diagram showing a configuration example of the synchronous rectification controller 300. As shown in FIG. The off fixing circuit 320 includes an off transistor M 3 and a release circuit 322. The off transistor M3 is provided between the gate and the source of the synchronous rectification transistor M2. When the off transistor M3 is turned on, the gate-to-source voltage V _{GS of the} synchronous rectification transistor M2 becomes zero and is fixed to be off.

The auxiliary power supply voltage V _AUX from the auxiliary power supply circuit 310 is input to the control terminal (gate) of the turning-off transistor M3. When the auxiliary power supply voltage V _AUX exceeds the threshold voltage V _{GS (TH)} between the gate and the source of the off transistor M 3, the off transistor M 3 is turned on, and the synchronous rectification transistor M 2 is fixed to off.

The release circuit 322 turns off the off transistor M3 when the DC voltage V _OUT exceeds a predetermined voltage, in other words, when the pulse generator 304 and the driver 306 become operable, the off fixing circuit 320 turns off the synchronous rectification transistor M2. Release the fixation.

For example, the release circuit 322 includes a release transistor M4 provided between the gate and the source of the off transistor M3. A release signal UVLO, which is asserted when the DC voltage V _OUT exceeds a predetermined voltage, is input to the control terminal (gate) of the release transistor M4. For example, the release signal UVLO is generated by a UVLO (Under Voltage Lock Out) circuit 330. The release signal UVLO is asserted (for example, high level) when a voltage corresponding to the DC voltage V _OUT exceeds a predetermined threshold voltage V _UVLO . When the release signal UVLO is asserted, the release transistor M4 is turned on, the off transistor M3 is turned off, and the fixed-off state of the synchronous rectification transistor M2 is released.

The auxiliary power supply circuit 310 includes a capacitor C11, a diode D11, and a resistor R11.
The capacitor C11 is connected to the control terminal (gate) of the turning-off transistor M3. The diode D11 is provided between the control terminal (gate) and the VD terminal of the turning-off transistor M3 in a direction in which the cathode is on the gate side.

The capacitor C11 and the diode D11 can be grasped as a peak hold circuit. When the drain voltage V _{D — S at the} VD terminal jumps up, the diode D11 turns on in the forward direction, and the capacitor C11 is charged to (V _{D — S} −V _F ). V _F is the forward voltage of the diode D11. The voltage of the capacitor C11 is supplied to the off-fixing circuit 320 as the auxiliary power supply voltage V _AUX .

  When the gate capacitance of the turning-off transistor M3 is large to some extent, the capacitor C11 can be omitted, and a peak hold circuit can be configured by the gate capacitance and the diode D11.

  The resistor R11 is provided in series with the diode D11. The resistor R11 is provided for over current protection and over voltage protection. Alternatively, it can be understood that the resistor R11 and the capacitor C11 (or the gate capacitance of the off transistor M3) constitute a low pass filter.

The above is the configuration example of the synchronous rectification controller 300. Subsequently, the operation will be described.
FIG. 6 is an operation waveform diagram of the synchronous rectification controller 300 of FIG.

When the start signal START is asserted (high level) at time t0, the switching of the switching transistor M1 starts. When the drain voltage V _{D — S} on the secondary side jumps up in response to the _turning on of the switching transistor M 1, the auxiliary power supply voltage V _AUX rises to near V _{D — S −} V _F and is peak-held.

When the auxiliary power supply voltage V _AUX exceeds the threshold voltage V _{GS (TH)} of the off transistor M 3, the off transistor M 3 is turned on, the gate voltage V _{G — S} of the synchronous rectification transistor M 2 is fixed at 0 V, and the synchronous rectification transistor M2 is fixed off.

  As described above, according to the synchronous rectification controller 300 of FIG. 5, the synchronous rectification transistor M2 can be preferably fixed off immediately after the start of the DC / DC converter 200.

Further, when the output voltage V _OUT exceeds the threshold voltage V _UVLO at time t1, the releasing transistor M4 is turned on and the off transistor M3 is turned off, and the off-fixing of the synchronous rectification transistor M2 is released. Then, the off transistor M3 can be switched according to the pulse signal S1, and the DC / DC converter 200 can be shifted to the synchronous rectification mode.

(Use)
Subsequently, applications of the DC / DC converter 200 described in the embodiment will be described.
FIG. 7 is a diagram showing an AC adapter 800 provided with the AC / DC converter 100. As shown in FIG. The AC adapter 800 includes a plug 802, a housing 804, and a connector 806. Plug 802 receives commercial AC voltage V _AC from an outlet (not shown). AC / DC converter 100 is mounted in housing 804. DC output voltage V _OUT generated by AC / DC converter 100 is supplied from connector 806 to electronic device 810. The electronic device 810 is exemplified by a notebook PC, a digital camera, a digital video camera, a mobile phone, a mobile audio player, and the like.

FIGS. 8A and 8B illustrate an electronic device 900 including the AC / DC converter 100. FIG. The electronic device 900 in FIGS. 8A and 8B is a display device, but the type of the electronic device 900 is not particularly limited, and is an device incorporating a power supply device such as an audio device, a refrigerator, a washing machine, a vacuum I hope there is.
Plug 902 receives commercial AC voltage V _AC from an outlet (not shown). AC / DC converter 100 is mounted in housing 804. The DC output voltage V _OUT generated by the AC / DC converter 100 is used for loads such as a microcomputer, a DSP (Digital Signal Processor), a power supply circuit, lighting equipment, an analog circuit, a digital circuit, etc. mounted in the same housing 904. Supplied.

  The present invention has been described above based on the embodiments. It is understood by those skilled in the art that this embodiment is an exemplification, and that various modifications can be made to the combination of each component and each processing process, and such a modification is also within the scope of the present invention. is there. Hereinafter, such modifications will be described.

(First modification)
In the embodiment, the synchronous rectification controller 300 is used on a platform in which the synchronous rectification transistor M2 is disposed on the lower potential side than the secondary winding W2, but the present invention is not limited thereto. The synchronous rectification controller 300 can also be used on a platform in which the synchronous rectification transistor M2 is disposed on the higher potential side than the secondary winding W2, that is, on the output terminal P2 side.

FIG. 9 is a circuit diagram of a DC / DC converter 200c according to a first modification. The auxiliary winding W4, the diode D4 and the capacitor C4 of the transformer T1 form an auxiliary converter, and generate a DC voltage V _CC1 higher than the output voltage V _OUT . The DC voltage V _CC1 is supplied to the VCC terminal. The GND terminal of the synchronous rectification controller 300 is connected to the source of the synchronous rectification transistor M2, and the VD terminal is connected to the drain of the synchronous rectification transistor M2. The configuration of the synchronous rectification controller 300 is the same as that of the embodiment. Also in this modification, the same effect as that of the embodiment can be obtained.

(2nd modification)
If the voltage level of the output voltage V _OUT is close to the operating voltage of the pulse generator 304 and the driver 306, the internal power supply circuit 302 of the synchronous rectification controller 300 may be omitted.

(Third modification)
FIGS. 10A and 10B are circuit diagrams of a synchronous rectification controller 300a according to a third modification. The driver 306 includes an output stage 307 including a high side transistor M21 and a low side transistor M22, and a predriver 308 driving the high side transistor M21 and the low side transistor M22 according to the pulse signal S1. In this modification, the off-fixing circuit 320a fixes the synchronous rectification transistor M2 off by forcibly turning on the low-side transistor M22 of the output stage 307 of the driver 306.

FIG. 10B shows a configuration example of the off fixing circuit 320a. The off-fixing circuit 320a is provided between the control terminal of the low side transistor M22 and the output line 312 of the auxiliary power supply circuit 310, and includes a transistor M31 that is turned on when the auxiliary power supply voltage V _AUX rises. For example, the transistor M31 is a P-channel MOSFET, and the ground voltage may be input to its gate. The configuration of the off fixing circuit 320a is not limited to this.

(4th modification)
Although the flyback converter has been described in the embodiment, the present invention is also applicable to a forward converter. In this case, a plurality of synchronous rectification transistors are disposed on the secondary side of the transformer T1. The synchronous rectification controller may be configured to switch the plurality of synchronous rectification transistors. The converter may also be pseudo-resonant.

(5th modification)
At least one of the switching transistor and the synchronous rectification transistor may be a bipolar transistor or an IGBT.

  While the present invention has been described using specific terms based on the embodiments, the embodiments merely show the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement can be made without departing from the concept 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 S1 pulse signal M3 ... transistor for OFF, M4 ... transistor for cancellation, 100 ... AC / DC converter, 102 ... filter, 104 ... rectification circuit, 106 ... smoothing capacitor, 200 ... DC / DC converter, 202 ... primary side controller, 204 ... photocoupler 206: Shunt regulator 210: output circuit 300: synchronous rectification controller 302: internal power supply circuit 304: pulse generator 306: driver 310: auxiliary power supply circuit 320: off fixed circuit 322: release circuit 800 AC adapter 802 plug 804 housing 806 Kuta, 810,900 ... electronic devices, 902 ... plug, 904 ... housing.

Claims (23)

A synchronous rectification controller disposed on the secondary side of an isolated synchronous rectification type DC / DC converter and controlling a synchronous rectification transistor, comprising:
A power supply terminal for receiving a DC voltage generated on the secondary side of the DC / DC converter;
A pulse generator that operates using the DC voltage as a power supply and generates a pulse signal based on a voltage across the synchronous rectification transistor;
A driver that operates using the DC voltage as a power supply and switches the synchronous rectification transistor according to the pulse signal;
An auxiliary power supply circuit that generates an auxiliary power supply voltage based on a drain voltage of the synchronous rectification transistor;
An off-fixing circuit which operates in response to the auxiliary power supply voltage and fixes the synchronous rectification transistor off;
Bei to give a,
The synchronous rectification controller , wherein the off fixing circuit releases the fixing of the synchronous rectification transistor off when the DC voltage exceeds a predetermined voltage .   The synchronous rectification controller according to claim 1, wherein the auxiliary power supply circuit includes a peak hold circuit. The peak hold circuit
Capacitors,
A diode provided between the drain of the synchronous rectification transistor and the capacitor, the cathode being directed to the capacitor side;
The synchronous rectification controller according to claim 2, comprising:   The synchronous rectification controller according to claim 1, wherein the auxiliary power supply circuit includes a filter circuit.   The synchronous rectification controller according to any one of claims 1 to 4, wherein the off-fixing circuit includes an off transistor provided between the gate and the source of the synchronous rectification transistor.   The synchronous rectification controller according to claim 5, wherein the auxiliary power supply voltage is input to a control terminal of the off transistor.   The auxiliary power supply circuit includes a diode provided between the control terminal of the off transistor and the drain of the synchronous rectification transistor in a direction in which the cathode is on the control terminal side of the off transistor. Synchronous rectification controller according to 5. A synchronous rectification controller disposed on the secondary side of an isolated synchronous rectification type DC / DC converter and controlling a synchronous rectification transistor, comprising:
A power supply terminal for receiving a DC voltage generated on the secondary side of the DC / DC converter;
A pulse generator that operates using the DC voltage as a power supply and generates a pulse signal based on a voltage across the synchronous rectification transistor;
A driver that operates using the DC voltage as a power supply and switches the synchronous rectification transistor according to the pulse signal;
An auxiliary power supply circuit that generates an auxiliary power supply voltage based on a drain voltage of the synchronous rectification transistor;
An off-fixing circuit which operates in response to the auxiliary power supply voltage and fixes the synchronous rectification transistor off;
Equipped with
The off fixed circuit is
An off transistor provided between the gate and the source of the synchronous rectification transistor;
When the DC voltage exceeds the predetermined voltage, Synchronous controller characterized in that it comprises a and a release circuit for turning off the off-transistor. The release circuit is provided between the gate and the source of the off transistor, and includes a release transistor to which a release signal that is asserted when the DC voltage exceeds a predetermined voltage is input to the control terminal. The synchronous rectification controller according to claim 8 .   10. The synchronous rectification controller according to claim 9, further comprising a low voltage lockout circuit that asserts the release signal when a voltage corresponding to the DC voltage exceeds a predetermined threshold voltage. The synchronous rectification controller according to any one of claims 8 to 10 , wherein the auxiliary power supply voltage is input to a control terminal of the turning-off transistor. The auxiliary power supply circuit includes a diode provided between the control terminal of the off transistor and the drain of the synchronous rectification transistor in a direction in which the cathode is on the control terminal side of the off transistor. The synchronous rectification controller according to any one of 8 to 10 . A synchronous rectification controller disposed on the secondary side of an isolated synchronous rectification type DC / DC converter and controlling a synchronous rectification transistor, comprising:
A power supply terminal for receiving a DC voltage generated on the secondary side of the DC / DC converter;
A pulse generator that operates using the DC voltage as a power supply and generates a pulse signal based on a voltage across the synchronous rectification transistor;
A driver that operates using the DC voltage as a power supply and switches the synchronous rectification transistor according to the pulse signal;
An auxiliary power supply circuit that generates an auxiliary power supply voltage based on a drain voltage of the synchronous rectification transistor;
An off-fixing circuit which operates in response to the auxiliary power supply voltage and fixes the synchronous rectification transistor off;
Equipped with
The off-fixing circuit, by turning force the low-side transistor of the output stage of the driver, synchronous rectifier controller characterized in that fixed to turn off the synchronous rectification transistor. A synchronous rectification controller disposed on the secondary side of an isolated synchronous rectification type DC / DC converter and controlling a synchronous rectification transistor, comprising:
A power supply terminal for receiving a DC voltage generated on the secondary side of the DC / DC converter;
A pulse generator that operates using the DC voltage as a power supply and generates a pulse signal based on a voltage across the synchronous rectification transistor;
A driver that operates using the DC voltage as a power supply and switches the synchronous rectification transistor according to the pulse signal;
An off transistor provided between the gate and the source of the synchronous rectification transistor;
A diode provided between the control terminal of the off transistor and the drain terminal of the synchronous rectification transistor in a direction in which the cathode is on the control terminal side of the off transistor;
A releasing transistor provided between the control terminal of the turning off transistor and the ground line and turned on when the DC voltage exceeds a predetermined voltage;
A synchronous rectification controller comprising: 15. The synchronous rectification controller according to claim 14 , further comprising a capacitor connected to the control terminal of the off transistor. 16. The synchronous rectification controller according to claim 14, further comprising a resistor provided between the control terminal and the drain terminal of the off transistor. The voltage release circuit further includes a low voltage lockout circuit that compares a voltage corresponding to the DC voltage with a predetermined threshold voltage and asserts a release signal when the DC voltage exceeds the threshold, the release transistor comprising: The synchronous rectification controller according to any one of claims 14 to 16, wherein the synchronous rectification controller is turned on when the release signal is asserted.   18. An internal power supply circuit for generating an internal power supply voltage based on a voltage of the power supply terminal, the pulse generator and the driver operate using the internal power supply voltage as a power supply. Synchronous rectification controller described in.   The synchronous rectification controller according to any one of claims 1 to 18, wherein the synchronous rectification controller is integrated on a single semiconductor substrate. Isolated synchronous rectification DC / DC converter,
A transformer having a primary winding and a secondary winding;
A switching transistor connected to the primary winding of the transformer;
A synchronous rectification transistor connected to the secondary winding of the transformer;
Photo coupler,
A primary side controller connected to the output side of the photocoupler and switching the switching transistor according to a feedback signal from the photocoupler;
The synchronous rectification controller according to any one of claims 1 to 19, which controls the synchronous rectification transistor;
A shunt regulator connected to the input side of the photocoupler and generating an error current according to the output voltage of the DC / DC converter;
A DC / DC converter characterized by comprising: A filter that filters commercial AC voltage,
A diode rectification circuit that full-wave rectifies an output voltage of the filter;
A smoothing capacitor that smoothes the output voltage of the diode rectifier circuit to generate a DC input voltage;
21. The DC / DC converter according to claim 20, wherein the DC input voltage is stepped down and supplied to a load;
A power supply device comprising: Load,
A filter that filters commercial AC voltage,
A diode rectification circuit that full-wave rectifies an output voltage of the filter;
A smoothing capacitor that smoothes the output voltage of the diode rectifier circuit to generate a DC input voltage;
21. The DC / DC converter according to claim 20, wherein the DC input voltage is stepped down and supplied to a load;
An electronic device comprising: A filter that filters commercial AC voltage,
A diode rectification circuit that full-wave rectifies an output voltage of the filter;
A smoothing capacitor that smoothes the output voltage of the diode rectifier circuit to generate a DC input voltage;
21. The DC / DC converter according to claim 20, wherein the DC input voltage is stepped down and supplied to a load;
A power adapter comprising:

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