ISOLATED DC/DC CONVERTER, PRIMARY SIDE CONTROLLER, POWER ADAPTER, AND ELECTRONIC DEVICE

Abstract
An isolated DC/DC converter includes a transformer having a first winding and a secondary winding, a switching transistor connected to the primary winding of the transformer, a rectifier element connected to the secondary winding of the transformer, a photocoupler, a feedback circuit configured to drive a light emitting element on an input side of the photocoupler by a forward current corresponding to an error between an output voltage of the DC/DC converter and a target voltage of the DC/DC converter, a conversion circuit configured to convert a collector current flowing in a light receiving element on an output side of the photocoupler into a feedback voltage having a negative correlation with the collector current, a pulse signal generator configured to generate a pulse signal corresponding to the feedback voltage, and a driver configured to drive the switching transistor depending on the pulse signal.
Description
TECHNICAL FIELD

The present disclosure relates to a DC/DC converter.


BACKGROUND

A flyback-type DC/DC converter is used for various power supply circuits such as an AC/DC converter. FIG. 1 is a block diagram illustrating a basic configuration of an AC/DC converter 100R reviewed by the present inventors. The AC/DC converter 100R mainly includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC/DC converter 201.


A commercial AC voltage VAC is input to the filter 102 via a fuse and an input capacitor (not shown). The filter 102 removes noise of the commercial AC voltage VAC. The rectifier circuit 104 is a diode bridge circuit that full-wave rectifies the commercial AC voltage VAC. An output voltage of the rectifier circuit 104 is converted into a DC voltage VIN, which is smoothed by the smoothing capacitor 106.


The isolated DC/DC converter 201 receives the DC voltage VIN at an input terminal P1, steps down the same, and supplies an output voltage VOUT, which is stabilized to a target value, to a load (not shown) connected to an output terminal P2. The DC/DC converter 201 includes a primary side controller 202, a photocoupler 204, a shunt regulator 206, an output circuit 210, and other circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C1, and a switching transistor M1. The topology of the output circuit 210 is that of a typical flyback converter.


As the switching transistor M1 connected to a primary winding W1 of the transformer T1 is switched, the input voltage VIN is stepped down and the output voltage VOUT is generated. Then, the primary side controller 202 stabilizes the output voltage VOUT to the target value by adjusting a switching duty ratio of the switching transistor M1.


The output voltage VOUT of the DC/DC converter 201 is divided by resistors R11 and R12. The shunt regulator 206 amplifies an error between the divided voltage (voltage detection signal) VS and a predetermined reference voltage VREF (not shown) which is internally set, and draws a forward current IF corresponding to the error from a light emitting element (light emitting diode) of an input side of the photocoupler 204 (sink).


A collector current IC corresponding to the forward current IF flows through a light receiving element (phototransistor) of an output side of the photocoupler 204. The collector current IC (feedback current IFB) is input to a feedback (FB) terminal of the primary side controller 202. A feedback voltage VFB having a negative correlation with the collector current IC is generated at the FB terminal. The primary side controller 202 switches the switching transistor M1 with a duty ratio corresponding to the feedback voltage VFB.


An output current IOUT of the DC/DC converter 201 varies depending on a state of the load. When the output current IOUT decreases while the switching transistor M1 is switched at a certain duty ratio, the output voltage VOUT rises. Then, since the forward current IF and the collector current IC increase and the feedback voltage VFB decreases, the duty ratio of the switching transistor M1 decreases and the current supply to the output capacitor C1 decreases to suppress the rise of the output voltage VOUT.


On the contrary, when the output current IOUT increases at a state that the duty ratio of the switching transistor M1 is constant, the output voltage VOUT decreases. Since the forward current IF and the collector current IC decrease and the feedback voltage VFB rises, the duty ratio of the switching transistor M1 increases and the current supply to the output capacitor C1 increases, so that the decrease of the output voltage VOUT is suppressed.



FIG. 2 illustrates a relationship between the output current IOUT and the collector current IC. Here, for simplicity of explanation, it is assumed that the conversion efficiency (gain) of the photocoupler 204 is 100% (IC/IF≈1).


As described above, the collector current IC decreases in a light load state where the output current IOUT is small, and increases in a heavy load state where the output current IOUT is large. An operating point of the circuit should be determined in consideration of the stability of the circuit. For example, as indicated by the solid line, if the collector current IC=0.5 mA is optimum at a rated current IOUT=IRATE, the collector current IC rises to about 1 mA in the light load state (IOUT≈0 mA). Assuming that IC=IF=1 mA and VOUT=24 V, the power consumption on the secondary side is 24 mW.


Due to the recent demand for energy savings, the reduction in power consumption of the light load or no-load state (also referred to as a standby state) is required, specifically a standby power of 100 mW or less is required in the entire AC/DC converter 100R. If the power consumption of 24 mW occurs on the secondary side of the DC/DC converter 201, it becomes difficult to suppress the AC/DC converter 100R as a whole to 100 mW or less.


It is assumed that the operating point of the circuit is determined such that the collector current IC in the light load state (IOUT≈0 mA) is, for example, 0.5 mA, as indicated by the alternate long and short dash line in FIG. 2. In this case, if the conversion efficiency of the photocoupler 204 is maintained at 100%, since the IC=IF=0.5 mA, the power consumption on the secondary side can be reduced to 12 mW but the proportion of the power consumption still exceeds 10% of the allowable power consumption of 100 mW.


When the output current IOUT increases up to the rated current IRATE according to the alternate long and short dash line, the collector current IC decreases up to, for example, 0.25 mA. Although the conversion efficiency of the photocoupler 204 has temperature dependency, the temperature dependency increases as the operating current decreases. Therefore, if the operating point is determined as indicated by the alternate long and short dash line, the gain of the feedback loop greatly fluctuates due to the temperature fluctuation, and the stability of the system degrades.


From the above, it is actually difficult to set the operating point such that the collector current IC decreases as indicated by the alternate long and short dash line.


SUMMARY

Some embodiments of the present disclosure provide a DC-DC converter capable of reducing power consumption at a light load.


According to one embodiment of the present disclosure, there is provided an isolated DC/DC converter. The isolated DC/DC converter includes a transformer having a first winding and a secondary winding, a switching transistor connected to the primary winding of the transformer, a rectifier element connected to the secondary winding of the transformer, a photocoupler, a feedback circuit configured to drive a light emitting element on an input side of the photocoupler by a forward current corresponding to an error between an output voltage of the DC/DC converter and a target voltage of the DC/DC converter, a conversion circuit configured to convert a collector current flowing in a light receiving element on an output side of the photocoupler into a feedback voltage having a negative correlation with the collector current, a pulse signal generator configured to generate a pulse signal corresponding to the feedback voltage, and a driver configured to drive the switching transistor depending on the pulse signal. The conversion circuit is configured to change conversion characteristics from the collector current to the feedback voltage based on an output current of the DC/DC converter.


In a light load state, the conversion characteristics can be determined such that a forward current on the input side of the photocoupler becomes small, and in a heavy load state, the power consumption can be reduced. In addition, in the heavy load state, the conversion characteristics can be determined such that the forward current on the input side of the photocoupler is increased, and the stability of the circuit can be improved in the heavy load state.


The DC/DC converter further includes a current sense resistor installed in series with the switching transistor. The conversion characteristics can be controlled based on a voltage drop of the current sense resistor. The output current can also be detected based on the voltage drop of the current sense resistor.


The DC/DC converter further includes a load monitoring circuit including a first comparator configured to compare the voltage drop of the current sense resistor with a threshold value, the load monitoring circuit being configured to control the conversion characteristics depending on an output of the first comparator.


The load monitoring circuit further includes a timer circuit whose output is changed for a certain period of time using an output of the first comparator as a trigger. The load monitoring circuit is configured to control the conversion characteristics depending on the output of the timer circuit.


The timer circuit includes a capacitor connected to the output of the first comparator, a resistor or a current source configured to charge the capacitor, and a second comparator configured to compare a voltage of the capacitor with a determination reference value. The conversion characteristics are controlled depending on an output of the second comparator.


An emitter of the light receiving element of the photocoupler is grounded. The conversion circuit includes a variable impedance circuit installed between a collector of the light receiving element of the photocoupler and a reference voltage line. The conversion circuit is configured such that impedance of the variable impedance circuit is variable depending on the output current.


When the collector current is IC, the voltage of the reference voltage line is VREF, and the impedance of the variable impedance circuit is Z, the feedback voltage VFB is expressed by Eq. (1).






V
FB
=V
REF
−Z×I
C  Eq. (1)


Therefore, the conversion characteristics may be set based on the impedance Z.


The variable impedance circuit includes a first resistor installed between the collector of the light receiving element of the photocoupler and the reference voltage line, and a second resistor and a switch installed in series between both ends of the first resistor. The variable impedance circuit is configured such that ON/OFF of the switch is switchable depending on the output current.


The conversion circuit includes a feedback terminal at which the feedback voltage is generated, a resistor installed between the feedback terminal and the reference voltage line, a first transistor whose collector/drain is connected to the feedback terminal, emitter/source is grounded, and base/gate is connected to the emitter of the light receiving element of the photocoupler, and a variable impedance circuit installed between the base/gate of the first transistor and the ground.


It is assumed that the current flowing in the first transistor Q31 is IFB, and the resistance value of the resistor R31 is R. At this time, the feedback voltage VFB is expressed by Eq. (2).






V
FB
=V
REF
−R×I
FB  Eq. (2)


When the impedance of the variable impedance circuit 260 is Z, the base voltage (or gate voltage) VBE of the first transistor Q31 is expressed by Eq. (3).






V
BE
≈I
C
×Z  Eq. (3)


When the mutual conductance of the first transistor Q31 is written as gm, Eq. (4) is obtained.






I
FB
=V
BE
×gm  Eq. (4)


Eq. (5) is obtained from Eqs. (2) to (4).






V
FB
=V
REF
−R×I
C
×Z×gm  Eq. (5)


In this manner, when the impedance Z of the variable impedance circuit is changed, the slope of the conversion characteristics from the collector current IC to the feedback voltage VFB can be changed.


Assuming that the collector current is IC, the correction current is ICMP, and the current amplification factor of the first transistor is β, the feedback current IFB is expressed by Eq. (6).






I
FB=β×(IC−ICMP)  Eq. (6)


Therefore, the conversion characteristics from the collector current IC to the feedback voltage VFB is expressed by Eq. (7).






V
FB
=V
REF
−R×β×(IC−ICMP)  Eq. (7)


That is, the conversion characteristics can be controlled depending on the correction current ICMP.


The transformer may further include an auxiliary winding installed on the primary side. The DC/DC converter may further include an auxiliary power supply circuit configured to rectify and smooth the current flowing in the auxiliary winding to generate a power source voltage, and a current sense resistor installed in series with the switching transistor.


The conversion characteristics may be discretely changed based on a comparison result between the output current and at least one threshold value. The conversion characteristics may be continuously changed depending on the output current.


According to another embodiment of the present disclosure, there is provided a primary side controller for use in an isolated DC/DC converter. The DC/DC converter includes a transformer having a first winding and a secondary winding, a switching transistor connected to the primary winding of the transformer, a rectifier element connected to the secondary winding of the transformer, a photocoupler, and a feedback circuit configured to drive a light emitting element on an input side of the photocoupler by a forward current corresponding to an error between an output voltage of the DC/DC converter and a target voltage of the DC/DC converter. The primary side controller includes a conversion circuit configured to convert a collector current flowing in a light receiving element on an output side of the photocoupler into a feedback voltage having a negative correlation with the collector current, a pulse signal generator configured to generate a pulse signal corresponding to the feedback voltage, and a driver configured to drive the switching transistor depending on the pulse signal. The conversion circuit is configured to change conversion characteristics from the collector current to the feedback voltage based on an output current of the DC/DC converter.


The conversion characteristics can be determined such that the forward current on the input side of the photocoupler becomes small in a light load state, and the forward current on the input side of the photocoupler become large in a heavy load state. Thus, the power consumption can be reduced in the light load state and the stability of the circuit can be improved in the heavy load state.


The DC/DC converter further includes a current sense resistor installed in series with the switching transistor. The conversion characteristics may be controlled based on a voltage drop of the current sense resistor.


The primary side controller further includes a load monitoring circuit including a first comparator configured to compare the voltage drop of the current sense resistor with a threshold value, the load monitoring circuit being configured to control the conversion characteristics depending on an output of the first comparator.


The load monitoring circuit further includes a timer circuit whose output is changed for a certain period of time using an output of the first comparator as a trigger and the conversion characteristics are controlled depending on an output of the timer circuit.


The timer circuit includes a capacitor connected to the output of the first comparator, a resistor or a current source configured to charge the capacitor, and a second comparator configured to compare a voltage of the capacitor with a determination reference value, wherein the conversion characteristics are controlled depending on an output of the second comparator.


An emitter of the light receiving element of the photocoupler is grounded. The conversion circuit includes a variable impedance circuit installed between a collector of the light receiving element of the photocoupler and a reference voltage line, and the conversion circuit is configured such that impedance of the variable impedance circuit is variable depending on the output current.


The variable impedance circuit includes a first resistor installed between the collector of the light receiving element of the photocoupler and the reference voltage line, and a second resistor and a switch installed in series between both ends of the first resistor. The variable impedance circuit is configured such that ON/OFF of the switch is switchable depending on the output current.


The primary side controller is integrated on a single semiconductor substrate. The “integration” includes a case of forming all of the constituent elements of the circuit on the semiconductor substrate, or a case of integrating the main constituent elements of the circuit. A part of resistors, capacitors or the like may be installed outside the semiconductor substrate for adjusting a circuit constant. As the circuit is integrated in a single chip, a circuit area can be reduced and the characteristics of the circuit elements can be kept uniform.


According to another embodiment of the present disclosure, there is provided an isolated DC/DC converter. The DC/DC converter includes the aforementioned primary side controller.


According to another embodiment of the present disclosure, there is provided an electronic device. The electronic device includes a load, a diode rectifier circuit configured to full-wave rectify a commercial AC voltage, a smoothing capacitor configured to smooth an output voltage of the diode rectifier circuit to generate a DC input voltage, and the aforementioned DC/DC converter configured to step down the DC input voltage for supply to the load.


According to another embodiment of the present disclosure, there is provided a power adaptor. The power adaptor includes a diode rectifier circuit configured to full-wave rectify a commercial AC voltage, a smoothing capacitor configured to smooth an output voltage of the diode rectifier circuit to generate a DC input voltage, and the aforementioned DC/DC converter configured to step down the DC input voltage for supply to a load.





BRIEF DESCRIPTION OF DRAWINGS


FIG. 1 is a block diagram illustrating a basic configuration of an AC/DC converter reviewed by the present inventor.



FIG. 2 illustrates a relationship between an output current IOUT and a collector current IC.



FIG. 3 is a circuit diagram of an isolated DC/DC converter according to an embodiment of the present disclosure.



FIGS. 4A to 4C are diagrams illustrating examples of conversion characteristics of a conversion circuit.



FIG. 5 is a circuit diagram of a first configuration example of the DC/DC converter.



FIGS. 6A to 6C are circuit diagrams illustrating specific configuration examples of the conversion circuit.



FIGS. 7A and 7B are circuit diagrams illustrating configuration examples of a load monitoring circuit.



FIGS. 8A and 8B are circuit diagrams illustrating more specific configuration examples of the conversion circuit.



FIGS. 9A and 9B are operational waveform diagrams of a conversion circuit 220A in FIG. 8A.



FIG. 10 is a circuit diagram of a second configuration example of the DC/DC converter.



FIGS. 11A to 11D are circuit diagrams illustrating specific configuration examples of the conversion circuit.



FIG. 12 is a diagram illustrating an AC adapter having an AC/DC converter.



FIGS. 13A and 13B are diagrams illustrating electronic devices having an AC/DC converter.





DETAILED DESCRIPTION

Embodiments of the present disclosure will be now described in detail with reference to the drawings. Like or equivalent components, members, and processes illustrated in each drawing are given like reference numerals and a repeated description thereof will be properly omitted. Further, the embodiments are presented by way of example only, and are not intended to limit the present disclosure, and any feature or combination thereof described in the embodiments may not necessarily be essential to the present disclosure.


In the present disclosure, “a state where a member A is connected to a member B” includes a case where the member A and the member B are physically directly connected or even a case where the member A and the member B are indirectly connected through any other member that does not affect an electrical connection state between the members A and B or does not impair functions and effects achieved by combinations of the members A and B.


Similarly, “a state where a member C is installed between a member A and a member B” includes a case where the member A and the member C or the member B and the member C are indirectly connected through any other member that does not affect an electrical connection state between the members A and C or the members B and C or does not impair function and effects achieved by combinations of the members A and C or the members B and C, in addition to a case where the member A and the member C or the member B and the member C are directly connected.



FIG. 3 is a circuit diagram of an isolated DC/DC converter 200 according to an embodiment of the present disclosure.


The DC/DC converter 200 receives a DC voltage VIN at an input terminal P1, steps down the same, and supplies an output voltage VOUT, which is stabilized to a target value, to a load (not shown) connected to an output terminal P2.


The DC/DC converter 200 includes a primary side controller 202, a photocoupler 204, a shunt regulator 206, an output circuit 210, and other circuit components. The output circuit 210 includes a transformer T1, a rectifier element D1, an output capacitor C1, and a switching transistor M1. The topology of the output circuit 210 is equal to that of a general flyback converter, and therefore, a description thereof will be omitted.


The switching transistor M1 is connected to a primary winding W1 of the transformer T1. As this switching transistor M1 is switched, the input voltage VIN is stepped down and an output voltage VOUT is generated. Then, the primary side controller 202 stabilizes the output voltage VOUT to a target value by adjusting a duty ratio of the switching of the switching transistor M1.


The output voltage VOUT of the DC/DC converter 200 is divided by resistors R11 and R12. The feedback circuit 206, which is, for example, a shunt regulator, amplifies an error between the divided voltage (voltage detection signal) VS and a predetermined reference voltage VREF (not shown) which is internally set, and drives a light emitting element (light emitting diode) of an input side of the photocoupler 204 by a forward current IF corresponding to the error. A collector current IC corresponding to the forward current IF flows through a light receiving element (phototransistor) of an output side of the photocoupler 204.


The primary side controller 202 includes a conversion circuit 220, a pulse signal generator 222, and a driver 224. As will be described later, a portion or all of the primary side controller 202 may be integrated on a single semiconductor substrate. The conversion circuit 220 converts the collector current IC flowing through the light receiving element of the output side of the photocoupler 204 into a feedback voltage VFB, which has a negative correlation with the collector current IC. The conversion circuit 220 may be recognized as a current/voltage conversion circuit or may be recognized as a transimpedance circuit.


The pulse signal generator 222 generates a pulse signal SPWM corresponding to the feedback voltage VFB. For example, the pulse signal generator 222 is a pulse width modulator, and the pulse signal SPWM has a duty ratio corresponding to the feedback voltage VFB. The configuration of the pulse signal generator 222 is not specifically limited to the above, and can be configured by adopting a voltage mode, an average current mode, a peak current mode, a pseudo resonance mode, or the like. Alternatively, the pulse signal generator 222 may be another modulator such as a pulse frequency modulator. The driver 224 generates a gate pulse SOUT corresponding to the pulse signal SPWM to drive the switching transistor M1.


The transformer T1 has an auxiliary winding W3. An auxiliary power supply circuit 212 includes a diode D3 and a capacitor C3, and rectifies and smooths a current flowing through the auxiliary winding W3 to generate a power supply voltage VCC. The power supply voltage VCC is supplied to the circuit block of the primary side.


A current sense resistor RCS is inserted between the switching transistor M1 and a ground. A voltage drop (current detection signal VCS), which is proportional to a current IP flowing through the primary winding W1, is generated in the current sense resistor RCS during an ON period of the switching transistor M1. The current detection signal VCS is used for overcurrent protection in the primary side controller 202. Alternatively, the pulse signal generator 222 in the peak current mode or the average current mode generates the pulse signal SPWM based on the current detection signal VCS.


The conversion circuit 220 is configured such that the conversion characteristics from the collector current IC to the feedback voltage VFB are changed based on the output current IOUT of the DC/DC converter 200.



FIGS. 4A to 4C are diagrams illustrating examples of conversion characteristics of the conversion circuit 220. The solid line (i) indicates the characteristics when the output current IOUT is a rated current IRATE (heavy load state), and the alternate long and short dash line (ii) indicates the characteristic when the output current IOUT is substantially zero (light load state). The characteristics may vary continuously or discretely between the solid line (i) and the alternate long and short dash line (ii), depending on the output current our.


The control characteristics (V-I characteristics) of FIG. 4A shows that the slope varies depending on the load current low. The control characteristic of FIG. 4B shows that the Y section (offset amount) varies depending on the load current IOUT. The control characteristic of FIG. 4C shows that both the slope and the Y section (offset amount) vary depending on the load current IOUT. The variation of the control characteristics in the conversion circuit 220 is not limited to FIGS. 4A to 4C, and other control characteristics may be adopted.


The configuration of the DC/DC converter 200 has been described above. Next, an operation thereof will be described.


Here, it will be described according to the control characteristics of FIG. 4A. In the rated output state (heavy load state), the conversion characteristics of (i) are selected. In the rated output state, the DC/DC converter 200 operates in a current continuous mode, and the duty ratio D of the switching does not depend on the output voltage VOUT and becomes a value D1 determined by the input voltage VIN, the output voltage VOUT and the winding ratio of the transformer T1. An operating point at this time is indicated by the broken line OP1. The feedback voltage VFB becomes a voltage V1 corresponding to the duty ratio D1. The collector current IC of the photocoupler 204 at this time becomes I1.


In the light load state, the conversion characteristics of (ii) are selected. In the light load state, the DC/DC converter 200 operates in a discontinuous mode, and the duty ratio D of the switching depends on the output current IOUT. Therefore, the feedback voltage VFB varies along the alternate long and short dash line, depending on the output current IOUT. An operating point of the lightest load state (standby state) where the output current IOUT is minimum (substantially zero) is indicated by the broken line OP2. The duty ratio D at this time becomes a small value D2, and the feedback voltage VFB becomes a value V2 corresponding to the small duty ratio D2. The collector current IC of the photocoupler 204 at this time becomes I2.


In the DC/DC converter 200 of FIG. 2, it is assumed that the operating point is defined such that I1=1 mA and I2=0.02 mA. When the efficiency of the photocoupler 204 is 100%, the forward current IF of the photocoupler 204 falls to 0.02 mA in the standby state. When the VOUT=24 V, the standby power consumption becomes 0.48 mW, which can be reduced to 1/50 compared with the conventional 24 mW.


In the rated load state (heavy load state), since the operating current of the photocoupler 204 is sufficiently large as 1 mA, it can be used in a region where the temperature dependency of the conversion efficiency is small. Thus, it is possible to reduce the temperature dependency of the gain of the feedback loop and to improve the stability of the system.


The operating currents I1 and I2 may be optimized based on the characteristics of the photocoupler 204 to be used, and the above numerical values are merely examples. The same effects can be obtained even when the control characteristics of FIGS. 4B and 4C are used.


The present disclosure is recognized by the block diagram or the circuit diagram of FIG. 3 or intended to cover various devices and circuits derived from the above description, but is not limited to the specific configuration. Hereinafter, more specific configuration examples or embodiment examples will be described to help understand and clarify the essence of the present disclosure and a circuit operation thereof, rather than to narrow the scope of the present disclosure.


First Configuration Example


FIG. 5 is a circuit diagram of a first configuration example (200A) of the DC/DC converter 200. A primary side controller integrated circuit (IC) (hereafter, simply referred to as a primary side controller) 300A is a functional IC in which the primary side controller 202 of FIG. 3 is integrated on a single semiconductor substrate.


The primary side controller 300A has a feedback (FB) pin, a current detection (CS) pin, an output (OUT) pin, and a power supply (VCC) pin. The primary side controller 300A further includes a conversion circuit 220A, a pulse signal generator 222, and a driver 224. The CS pin of the primary side controller 300A is connected to a current sense resistor RCS and receives a current detection signal VCS based on a voltage drop of the current sense resistor RCS. The OUT pin is connected to a gate of the switching transistor M1, and a power supply voltage VCC generated by an auxiliary power source circuit 212 is supplied to the VCC pin.


An emitter of the light receiving element of the photodetector 204 is grounded. A collector of the light receiving element of the photodetector 204 is connected to the FB pin. The conversion circuit 220A includes a variable impedance circuit 240 and a load monitoring circuit 250. The conversion circuit 220A is installed between the FB pin and a reference voltage line 230, and it is configured such that its impedance Z varies depending on the output current IOUT of the DC/DC converter 200A. A reference voltage VREF generated by a reference voltage source (not shown) is supplied to the reference voltage line 230.


When the impedance of the variable impedance circuit 240 is Z, the feedback voltage VFB of the FB pin (terminal) is expressed by Eq. (1).






V
FB
=V
REF
−Z×I
C  Eq. (1)


Therefore, the conversion characteristics of the conversion circuit 220 can be controlled by changing the impedance Z.


The load monitoring circuit 250 monitors the output current IOUT, and controls the impedance Z of the variable impedance circuit 240 based on the output current IOUT. Here, a peak value (or an average value) of a current IP flowing in the switching transistor M1 depends on the output current IOUT, where a large amount of the current IP flows in the switching transistor M1 in the heavy load state in which the output current IOUT is large and a small amount of the current IP flows in the switching transistor M1 in the light load state in which the output current IOUT is small. Therefore, the conversion circuit 220A monitors the output current IOUT of the DC/DC converter 200A based on the current detection signal VCS, and changes the impedance Z depending on the current detection signal VCS.



FIGS. 6A to 6C are circuit diagrams illustrating specific configuration examples of the conversion circuit 220A.


Referring to FIG. 6A, the variable impedance circuit 240 includes a first resistor R21 installed between the collector (i.e., FB pin) of the light receiving element of the photocoupler 204 and the reference voltage line 230, and a second resistor R22 and a switch SW21 installed in series between both ends of the first resistor R21. The ON/OFF state of the switch SW21 may be switched depending on the output current IOUT. The switch SW21 may be configured by a bipolar transistor or a field effect transistor (FET). The impedance Z of the variable impedance circuit 240 becomes a first value R21 when the switch SW21 is turned off, and becomes a second value R21//R22 when the switch SW21 is turned on. The symbol “//” represents a combined impedance of parallel resistors. According to the variable impedance circuit 240 of FIG. 6A which may be recognized as a two-stage variable resistor, the variable impedance circuit 240 of FIG. 6A can realize the control characteristic of FIG. 4A.


For example, the load monitoring circuit 250 compares the current detection signal VCS at the CS pin with a threshold voltage VTH, and turns on the switch SW21 when VCS>VTH, namely in the heavy load state, and turns off the switch SW21 when VCS<VTH, namely in the light load state.


In FIG. 6A, it is possible to configure three or more stages of variable resistors by increasing the number of resistors and switches of the variable impedance circuit 240. In this case, the number of threshold voltages VTH in the load monitoring circuit 250 may be increased and the resistance values of the variable impedance circuit 240 may be switched to three or more stages.


Referring to FIG. 6B, the variable impedance circuit 240 includes a first resistor R21 installed between the collector (i.e., FB pin) of the light receiving element of the photocoupler 204 and the reference voltage line 230, and a current source 242 and a switch SW22 installed in series between both ends of the first resistor R21. The ON/OFF state of the switch SW22 may be switched depending on the output current IOUT. When the switch SW22 is turned on, the current source 242 is connected to the FB pin so that the impedance Z of the variable impedance circuit 240 increases. According to the variable impedance circuit 240 of FIG. 6B, it is possible to realize the control characteristics of FIG. 4B. The switch SW22 may be built in the current source 242.


Referring to FIG. 6C, the variable impedance circuit 240 includes a first resistor R21 installed between the collector (i.e., FB pin) of the light receiving element of the photocoupler 204 and the reference voltage line 230, and a variable current source 244 installed in series between both ends of the first resistor R21. The variable current source 244 generates a correction current ICMP corresponding to the output current IOUT. For example, the variable current source 244 may generate a current ICMP corresponding to the current detection signal VCS at the CS pin.


According to the variable impedance circuit 240 of FIG. 6C, it is possible to realize the control characteristics of FIG. 4B and also to continuously change the V-I characteristics between (i) and (ii) depending on the output current IOUT.


In summary, the variable impedance circuit 240 may be configured by discrete or continuous variable resistance, variable current sources, a combination thereof, or the like.



FIGS. 7A and 7B are circuit diagrams illustrating configuration examples of the load monitoring circuit 250. The load monitoring circuit 250 of FIG. 7A includes a first comparator 252 and a timer circuit 253. The first comparator 252 compares a voltage drop (current detection signal) VCS of the current sense resistor RCS with a threshold value VTH. The load monitoring circuit 250 controls the conversion characteristics depending on an output of the first comparator 252.


In the timer circuit 253, its output is changed for a certain period of time using the output of the first comparator 252 as a trigger. The impedance of the variable impedance circuit 240, i.e., the conversion characteristic of the conversion circuit 220A, is controlled depending on the output of the timer circuit 253.


The load monitoring circuit 250 of FIG. 7B includes a peak hold circuit 255 and a third comparator 257. The peak hold circuit 255 peak-holds the current detection signal VCS. The third comparator 257 compares a peak value VCS(PEAK) of the current detection signal VCS with a threshold value VTH. The impedance of the variable impedance circuit 240, i.e., the conversion characteristic of the conversion circuit 220A, is controlled depending on an output of the third comparator 257.



FIGS. 8A and 8B are circuit diagrams illustrating specific configuration examples of the conversion circuit 220A.



FIG. 8A corresponds to FIG. 6A. A transistor Q21 is a PNP-type bipolar transistor, and forms the switch SW21 of FIG. 6A together with resistors R23 and R24.


The timer circuit 253 includes a capacitor C20 connected to the output of the first comparator 252, a resistor R27 (or a current source) for charging the capacitor C20, and a second comparator 254 for comparing a voltage VC20 of the capacitor C20 with a determination reference value K×VREF. The conversion characteristics are controlled depending on an output of the second comparator 254.


Resistors R28 and R29 are installed m series between the reference voltage line 230 and the GND pin, and divides the reference voltage VREF to generate the threshold voltage VTH. The first comparator 252 compares the current detection signal VCS with the threshold voltage VTH. The first comparator 252 has an output of an open collector (open drain), and a time constant circuit including the capacitor C20 and the resistor R27 is connected to its output. Resistors R25 and R26 are installed in series between the reference voltage line 230 and the GND pin, and divides the reference voltage VREF to generate the determination reference voltage K×VREF (0<K<1, here K=½). The second comparator 254 has an output of an open collector (open drain), and when the voltage VC20 of the capacitor C20 is lower than the determination reference voltage VREF/2, the second comparator 254 pulls down the potential of the base of the transistor Q21 to a low level and turns on the transistor Q21.


The configuration of the timer circuit 253 is not particularly limited to the above, and a digital timer for counting a clock, a one-shot multivibration circuit, and other known circuits may be used.


An operation of the conversion circuit 220A of FIG. 8A will be described. FIGS. 9A and 9B are operational waveform diagrams of the conversion circuit 220A in FIG. 8A. FIG. 9A illustrates an operation of a light load state, and FIG. 9B illustrates an operation of a heavy load state.


Referring to FIG. 9A, in the light load state, the amplitude of the primary current IP flowing in the switching transistor M1 is small, and the current detection signal VCS is lower than a threshold voltage VTH. Therefore, the first comparator 252 is in high impedance (open), and the voltage VC20 of the capacitor C20 is kept at a level equal to the reference voltage VREF. Since VC20>VREF/2 is formed, the output of the second comparator 254 is high impedance, and the transistor Q20 is kept in the OFF state.


Referring to FIG. 9B, in the heavy load state, the amplitude of the primary current IP flowing in the switching transistor M1 increases, and the current detection signal VCS exceeds the threshold voltage VTH. Therefore, the first comparator 252 periodically becomes a low level, the capacitor C20 is discharged, and its voltage VC20 drops to 0 V. Thus, the output of the second comparator 254 becomes a low level and the transistor Q21 is turned on.


When the primary current IP becomes zero as the switching transistor M1 is turned off, VCS<VTH and the output of the first comparator 252 becomes a high impedance. As a result, the capacitor C20 is charged via the resistor R27 and the voltage VC20 increases according to a time constant τ of C20 and R27. The time constant τ is defined such that VC20<VREF/2 is kept in the heavy load state.


When the switching transistor M1 is turned on in a next cycle, the primary current IP starts to increase again. When VCS>VTH, the output of the first comparator 252 becomes a low level, the capacitor C20 is discharged, and the voltage VC20 becomes 0 V again. In the heavy load state, the transistor Q21 is kept in the ON state by repeating such operation.


As described above, according to the conversion circuit 220A of FIG. 8A, the impedance Z of the variable impedance circuit 240 can be appropriately changed depending on the current detection signal VCS.


Returning to FIG. 8B, the variable impedance circuit 240 of FIG. 8B further includes a current source 242, in addition to the variable impedance circuit 240 of FIG. 8A. The other components are similar to those of FIG. 8A. The transistor Q21 functioning as the switch may also be installed in the current source 242.


Second Configuration Example


FIG. 10 is a circuit diagram of a second configuration example (200B) of the DC/DC converter 200. The pulse signal generator 222, the driver 224 and a portion of the conversion circuit 220 in the primary side controller 202 of FIG. 3 are built in a primary side controller 300B, and a portion of the conversion circuit 220 is externally attached.


A conversion circuit 220B includes a resistor R31, a first transistor Q31, a variable impedance circuit 260, and a load monitoring circuit 270. The resistor R31 is installed between the reference voltage line 230 and the FB pin. That is, the primary side controller 300B has the same configuration as that of the conventional primary side controller.


The first transistor Q31 is an NPN-type bipolar transistor whose collector is connected to the FB pin, emitter is grounded, and base is connected to the emitter of the light receiving element of the photocoupler 204.


The variable impedance circuit 260 is installed between the base of the first transistor Q31 and the ground. The impedance Z of the variable impedance circuit 260 is configured to be variable depending on the output current IOUT.


It is assumed that the current flowing in the first transistor Q31 is IFB, and the resistance value of the resistor R31 is R. At this time, a feedback voltage VFB is expressed by Eq. (2).






V
FB
=V
REF
−R×I
FB  Eq. (2)


When the base current of the first transistor Q31 approximates to zero and the impedance of the variable impedance circuit 260 is Z, a base voltage VBE of the first transistor Q31 is expressed by Eq. (3).






V
BE
≈I
C
×Z  Eq. (3)


When the mutual conductance of the first transistor Q31 is written as gm, Eq. (4) is obtained.






I
FB
=V
BE
×gm  Eq. (4)


Eq. (5) is obtained from Eqs. (2) to (4).






V
FB
=V
REF
−R×I
C
×Z×gm  Eq. (5)


Therefore, when the impedance Z of the variable impedance circuit 260 is changed, the slope of the conversion characteristics from the collector current IC to the feedback voltage VFB can be changed as illustrated in FIG. 4A. The first transistor Q31 may be configured with an N-channel MOSFET.


From another point of view, the variable impedance circuit may also be recognized as a variable current source. Assuming that the current amplification factor of the first transistor Q31 is β and the current flowing in the variable impedance circuit 260 is ICMP, Eq. (6) is obtained.






I
FB=β×(IC−ICMP)  Eq. (6)


By substituting Eq. (6) into Eq. (2), Eq. (7) is obtained.






V
FB
=V
REF
−R×β×(IC−ICMP)  Eq. (7)


By modifying Eq. (7), Eq. (7′) is obtained.






V
FB
=V
REF
+R×β×I
CMP−(R×β×IC)  Eq. (7′)


VREF+R×β×ICMP corresponds to the Y section of FIG. 4B. When the impedance of the variable impedance circuit 260 is changed, the current ICMP is changed, so that the offset of the control characteristics may be changed.


It can be said that the conversion circuit 220B of FIG. 10 shows one of the characteristics of FIGS. 4A, 4B, and 4C depending on the operating point of the conversion circuit 220.


The load monitoring circuit 270 controls the impedance Z of the variable impedance circuit 260 depending on the output current IOUT. The load monitoring circuit 270 may detect the output current IOUT based on the current detection signal VCS.



FIGS. 11A to 11D are circuit diagrams illustrating specific configuration examples of the conversion circuit 220B.


Referring to FIG. 11A, the variable impedance circuit 260 includes a resistor R32 and a variable current source 262. The resistor R32 may be omitted. The variable current source 262 generates a current ICMP corresponding to (having a positive correlation with) the current detection signal VCS. This correction current ICMP is sunk from the base of the first transistor Q31.


Referring to FIG. 11B, the variable impedance circuit 260 includes resistors R32 and R33, and a second transistor Q32. The second transistor Q32 may be recognized to correspond to the variable current source 262 of FIG. 11A, or the second transistor Q32 may be recognized to correspond to a switch SW31 of FIG. 11C.


The load monitoring circuit 270 determines whether it is in the heavy load state or the light load state based on the current detection signal VCS, and controls the impedance of the variable impedance circuit 260 based on the determination result. For example, the load monitoring circuit 270 may be configured similarly to the load monitoring circuit 250 of FIG. 8A. In this case, a power source voltage VCC or a reference voltage VREF generated by the primary side controller 300A may be supplied to a signal line corresponding to the reference voltage line 230 of FIG. 8A.


Referring to FIG. 11C, the variable impedance circuit 260 includes resistors R32 and R34, and the switch SW31. The load monitoring circuit 270 compares the current detection signal VCS with a threshold value VTH, and turns on the switch SW31 in the heavy load state where VCS>VTH, and turns off the switch SW31 in the light load state where VCS<VTH. The load monitoring circuit 270 of FIG. 11C may be configured similarly to the load monitoring circuit 250 of FIG. 8A.


Referring to FIG. 11D, the variable impedance circuit 260 includes a resistor R32, a current source 264, and a switch SW32. The load monitoring circuit 270 is similar to the load monitoring circuit 270 of FIG. 11C. When the switch SW32 is turned on, the current source 264 becomes active and generates a correction current ICMP.


The configuration example of the variable impedance circuit 260 has been described above.


In the second embodiment, a conventional controller may be used as the primary side controller 300B, and the same effects as those of the first embodiment can be obtained by adding the peripheral circuits. Furthermore, the conversion circuit 220B of FIG. 10 may be integrated in the primary side controller 300B.


(Applications)

Next, applications of the DC/DC converter 200 will be described.



FIG. 12 is a diagram illustrating an AC adapter 800 having an AC/DC converter 100. The AC adaptor 800 includes a plug 802, a housing 804, and a connector 806. The plug 802 receives a commercial AC voltage VAC from a socket (not shown). The AC/DC converter 100 is mounted inside the housing 804. A DC output voltage VOUT generated by the AC/DC converter 100 is supplied from the connector 806 to an electronic device 810. Examples of the electronic device 810 include a laptop computer, a digital camera, a digital video camera, a mobile phone, a portable audio player, and the like.



FIGS. 13A and 13B are diagrams illustrating electronic devices 900 having an AC/DC converter 100. The electronic devices 900 of FIGS. 13A and 13B are display devices, but the types of the electronic devices 900 are not particularly limited thereto and the devices may be devices incorporating a power source device such as an audio device, a refrigerator, a washing machine, a vacuum cleaner, or the like.


The plug 902 receives a commercial AC voltage VAC from a socket (not shown). The AC/DC converter 100 is mounted inside a housing 904. A DC output voltage VOUT generated by the AC/DC converter 100 is supplied to a load such as a microcomputer, a digital signal processor (DSP), a power supply circuit, a lighting device, an analog circuit, a digital circuit, or the like, which is mounted inside the same housing 904.


The present disclosure has been described above based on the embodiment. It should be understood by those skilled in the art that the embodiment is merely illustrative and may be differently modified by any combination of the components or processes, and the modifications are also within the scope of the present disclosure. Hereinafter, these modifications will be described.


First Modification

A synchronous rectification transistor and its control circuit may be installed on the secondary side of the DC/DC converter 200, instead of the rectifier element D1. The rectifier element D1 may be installed on a high potential side of the secondary winding W2 or may be provided on a low potential side thereof.


Second Modification

In some embodiments, there has been described a case in which the comparator compares the current detection signal VCS on the primary side with the threshold value to determine whether it is a light load or a heavy load, but the present disclosure is not limited thereto. For example, the current detection signal VCS is converted into a digital value by an A/D converter, and the conversion characteristics (e.g., the impedance and the current of the variable impedance circuits 240 and 260) of the conversion circuit 220 may be controlled based on the digital value.


Third Modification

In the embodiment, the output current IOUT is monitored based on the current detection signal VCS on the primary side, but the present disclosure is not limited thereto. A circuit for monitoring the output current IOUT is installed on the secondary side of the DC/DC converter 200, a detection signal indicating the monitoring result may be transmitted to the primary side using a photocoupler different from the photocoupler 204 so that the conversion characteristics of the conversion circuit 220 may be changed based on the detection signal.


According to the present disclosure in some embodiments, it is possible to reduce the power consumption at a light load.


While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures.

Claims
  • 1. A primary side controller for use in an isolated DC/DC converter, comprising: a feedback terminal;a conversion circuit configured to: convert a current flowing at the feedback terminal into a feedback voltage having a negative correlation with the current; andchange conversion characteristics from the current to the feedback voltage based on an output current of the isolated DC/DC converter;a pulse signal generator configured to generate a pulse signal corresponding to the feedback voltage; anda driver configured to generate a driving signal based on the pulse signal.
  • 2. The primary side controller of claim 1, wherein the conversion circuit includes a variable impedance circuit installed between the feedback terminal and a reference voltage line, and wherein the conversion circuit is configured such that impedance of the variable impedance circuit is variable based on the output current.
  • 3. The primary side controller of claim 2, wherein the variable impedance circuit includes: a first resistor installed between the feedback terminal and the reference voltage line; anda second resistor and a switch which are installed in series between both ends of the first resistor, andwherein the variable impedance circuit is configured such that ON/OFF of the switch is switchable based on the output current.
  • 4. The primary side controller of claim 2, wherein the variable impedance circuit includes: a resistor installed between the feedback terminal and the reference voltage line; anda current source and a switch which are installed in series between both ends of the resistor, andwherein the variable impedance circuit is configured such that ON/OFF of the switch is switchable based on the output current.
  • 5. The primary side controller of claim 2, wherein the variable impedance circuit includes: a resistor installed between the feedback terminal and the reference voltage line; anda variable current source installed between both ends of the resistor and configured to generate a correction current corresponding to the output current.
  • 6. The primary side controller of claim 2, further comprising a current detection terminal, wherein the conversion characteristics are controlled based on a voltage drop at the current detection terminal.
  • 7. The primary side controller of claim 6, wherein the conversion circuit further includes a load monitoring circuit configured to monitor the output current and control the impedance of the variable impedance circuit based on the output current.
  • 8. The primary side controller of claim 7, wherein the load monitoring circuit includes: a comparator configured to compare the voltage drop at the current detection terminal with a threshold value; anda timer circuit whose output is changed for a certain period of time using an output of the comparator as a trigger, andwherein the conversion characteristics are controlled based on the output of the timer circuit.
  • 9. The primary side controller of claim 7, wherein the load monitoring circuit includes: a peak hold circuit configured to peak-hold a current detection signal at the current detection terminal; anda comparator configured to compare a peak value of the current detection signal with a threshold value to control the impedance of the variable impedance circuit, andwherein the conversion characteristics are controlled based on an output of the comparator.
  • 10. The primary side controller of claim 6, wherein the current detection terminal is connected to a current sense resistor installed in series with a switching transistor of the isolated DC/DC converter, and wherein the conversion characteristics are controlled based on the voltage drop of the current sense resistor.
  • 11. The primary side controller of claim 1, wherein the conversion characteristics are discretely changed based on a comparison result between the output current and at least one threshold value.
  • 12. The primary side controller of claim 1, wherein the conversion characteristics are continuously changed based on the output current.
  • 13. The primary side controller of claim 1, wherein the primary side controller is integrated on a single semiconductor substrate.
  • 14. A primary side controller for use in an isolated DC/DC converter, comprising: a feedback terminal;a current detection terminal;a conversion circuit configured to: convert a current flowing at the feedback terminal into a feedback voltage having a negative correlation with the current; andchange conversion characteristics from the current to the feedback voltage based on a voltage drop at the current detection terminal;a pulse signal generator configured to generate a pulse signal corresponding to the feedback voltage; anda driver configured to generate a driving signal based on the pulse signal.
  • 15. The primary side controller of claim 14, wherein the conversion circuit includes a variable impedance circuit installed between the feedback terminal and a reference voltage line, and wherein the conversion circuit is configured such that impedance of the variable impedance circuit is variable based on the voltage drop at the current detection terminal.
  • 16. The primary side controller of claim 15, wherein the variable impedance circuit includes: a first resistor installed between the feedback terminal and the reference voltage line; anda second resistor and a switch which are installed in series between both ends of the first resistor,wherein the variable impedance circuit is configured such that the switch is capable of being switched based on the voltage drop at the current detection terminal.
  • 17. The primary side controller of claim 15, wherein the variable impedance circuit includes: a resistor installed between the feedback terminal and the reference voltage line; anda current source and a switch which are installed in series between both ends of the resistor, andwherein the variable impedance circuit is configured such that the switch is capable of being switched based on the voltage drop at the current detection terminal.
  • 18. The primary side controller of claim 15, wherein the variable impedance circuit includes: a resistor installed between the feedback terminal and the reference voltage line; anda variable current source installed between both ends of the resistor and configured to generate a correction current corresponding to the voltage drop at the current detection terminal.
  • 19. The primary side controller of claim 15, wherein the conversion circuit further includes a load monitoring circuit configured to monitor the voltage drop at the current detection terminal and to control the impedance of the variable impedance circuit based on the voltage drop at the current detection terminal.
  • 20. The primary side controller of claim 14, wherein the primary side controller is integrated on a single semiconductor substrate.
Priority Claims (1)
Number Date Country Kind
2016-236447 Dec 2016 JP national
CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation application of U.S. patent application Ser. No. 15/830,461, filed on Dec. 4, 2017, the entire contents of which are incorporated herein by reference and priority to which is hereby claimed. Priority under 35 U.S.C. § 119(a) and 35 U.S.C. § 365(b) is hereby claimed from Japanese Application No. 2016-236447, filed Dec. 6, 2016, the disclosures of which are both also incorporated herein by reference.

Continuations (1)
Number Date Country
Parent 15830461 Dec 2017 US
Child 16296775 US