Power supply control device and flyback converter

Abstract

To provide a power supply control device that can improve the performance of controlling an electric current when comparing an electric current flowing in the primary coil of a transformer with a threshold level.SOLUTION: A power supply control device comprises: a comparator that compares a current sense signal generated by the I / V conversion of a primary current flowing in a primary coil with a threshold voltage; a switching control unit that turns off a switching element based on the comparison result of the current sense signal and the threshold voltage obtained by the comparator; an external terminal that can be connected to the connection node of an external resistor which is connected in series between one end of an auxiliary coil and an applied end of the ground potential; a current detection unit that detects a terminal current flowing in the external terminal; and a threshold voltage correction unit that corrects the threshold voltage based on the current detection signal from the current detection unit.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a power supply controller for a flyback converter. [Background technology]

[0002] Conventionally, a flyback converter is known as a switching power supply circuit applied to an isolated DC / DC converter or an isolated AC / DC converter (for example, Patent Document 1). A flyback converter chops a DC input voltage with a switching transistor and transmits energy to the secondary side via a transformer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2003-209971 A Summary of the Invention [Problem to be solved by the invention]

[0004] In a flyback converter, the primary current flowing through the primary winding of a transformer may be compared with a threshold value, for example, to detect an overcurrent by comparing the primary current with an overcurrent threshold value.

[0005] An object of the present invention is to provide a power supply control device that improves the current control performance when comparing the current flowing through a primary winding of a transformer with a threshold value. [Means for solving the problem]

[0006] One aspect of the present invention is A switching element; a transformer having a primary winding, a secondary winding, and an auxiliary winding; A rectifying element; A smoothing capacitor; having An input voltage application terminal is connected to one end of the primary winding, The other end of the primary winding is connected to a current input terminal of the switching element, A power supply control device used in a flyback converter, the rectifying element and the smoothing capacitor being provided on a rear stage side of the secondary winding, a comparison unit that compares a current sense signal generated by I / V converting a primary side current flowing through the primary winding with a threshold voltage; a switching control unit that turns off the switching element based on a comparison result by the comparison unit between the current sense signal and the threshold voltage; an external terminal connectable to a connection node of an external resistor connected in series between one end of the auxiliary winding and an application end of a ground potential; a current detection unit that detects a terminal current flowing through the external terminal; a threshold voltage correction unit that corrects the threshold voltage based on a current detection signal from the current detection unit; The power supply control device has the above configuration (first configuration).

[0007] In the first configuration, the current detection section may be a current detection resistor (second configuration).

[0008] In the second configuration, the threshold voltage correction section may include an inverting amplifier that inverts and amplifies the current detection signal (third configuration).

[0009] In any one of the first to third configurations, the threshold voltage correction section may have a sample hold section that samples a voltage based on the current detection signal (fourth configuration).

[0010] In addition, in any one of the first to fourth configurations, a voltage adjustment resistor is connected between the input terminal of the comparison unit and an application terminal of a reference voltage, The threshold voltage correction unit generates an output current based on the current detection signal, The output current may flow through the voltage adjustment resistor and be drawn into the threshold voltage correction section (fifth configuration).

[0011] In the fifth configuration, the threshold voltage correction unit includes a constant current generation circuit that receives a voltage based on the current detection signal and generates a constant current, The output current may be generated based on the constant current (sixth configuration).

[0012] In the sixth configuration, the threshold voltage correction unit a first current mirror having an input terminal connected to an output terminal of the constant current generating circuit and configured with a PMOS transistor; a second current mirror having an input terminal connected to the output terminal of the first current mirror and configured with an NMOS transistor; (seventh configuration).

[0013] In any one of the first to fourth configurations, one input terminal of the comparison unit is connected to a terminal to which a reference voltage is applied, the other input terminal of the comparison unit is connected to one end of a resistor including a sense resistor having one end connected to the current output terminal of the switching element, The threshold voltage correction unit generates an output current based on the current detection signal, The output current may be discharged toward a node to which the other input terminal of the comparator and the one terminal of the resistor are connected (eighth configuration).

[0014] In the eighth configuration, a resistor may be inserted between the other input terminal of the comparing section and the one end of the sense resistor (ninth configuration).

[0015] Furthermore, in any of the first to ninth configurations, a configuration may be made in which a switch is disposed in a path through which the terminal current flows and is on / off controlled based on a signal related to driving the switching element (tenth configuration).

[0016] In any one of the first to tenth configurations, the threshold voltage may be an OCP (overcurrent protection) threshold voltage (eleventh configuration).

[0017] In any one of the first to fourth configurations, a first comparator that compares a feedback voltage based on an output voltage of the flyback converter with a burst threshold voltage; a second comparator serving as the comparison unit for comparing the feedback voltage with the current sense signal; having the switching control unit switches between stopping and restarting the switching of the switching element in response to an output of the first comparator; The threshold voltage may be the burst threshold voltage (twelfth configuration).

[0018] Another aspect of the present invention is a flyback converter having a power supply control device having any of the above configurations, the switching element, the transformer, the rectifier element, the smoothing capacitor, and the external resistor. [Brief description of the drawings]

[0019] [Figure 1] FIG. 11 is a diagram showing an example of waveforms for explaining a first problem. [Diagram 2] FIG. 11 is a diagram showing an example of waveforms for explaining a second problem. [Diagram 3] FIG. 1 illustrates a configuration of a flyback converter according to an exemplary embodiment of the present invention. [Figure 4] FIG. 4 is a diagram illustrating an example of the configuration of a threshold voltage correction unit. [Diagram 5] 4 is a timing chart showing an example of waveforms of various signals in a flyback converter. [Figure 6] FIG. 2 is a diagram showing a configuration of a power supply control device according to a first modified example. [Figure 7] FIG. 13 is a diagram illustrating a configuration of a threshold voltage correction unit according to a first modified example. [Figure 8] FIG. 11 is a diagram showing a configuration of a flyback converter according to a second modified example. [Figure 9] 11A and 11B are diagrams illustrating example waveforms of a feedback voltage and a burst threshold voltage. [Figure 10] FIG. 13 is a diagram showing a configuration of a flyback converter according to another modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Exemplary embodiments of the present invention will now be described with reference to the drawings.

[0021] <1. About the assignment> Before describing the embodiments of the present invention, a problem that the present inventors have independently found will be described with reference to the drawings.

[0022] A flyback converter is provided with a transformer. The transformer has a primary winding and a secondary winding. An input voltage VH, which is a DC voltage, is applied to one end of the primary winding. The other end of the primary winding is connected to a current inflow end of a switching element (e.g., the drain of an NMOS transistor).

[0023] The primary current Ip, which is the current flowing through the primary winding, is I / V converted (current-voltage converted) to produce a current sense signal Vcs. For example, as an OCP (overcurrent protection) function, the current sense signal Vcs is compared with an OCP threshold voltage Vth_ocp. When the switching element is turned on, the primary current Ip rises. Then, in response to the rise in the primary current Ip, the current sense signal Vcs rises, and when it exceeds the OCP threshold voltage Vth_ocp, the switching element is turned off.

[0024] Here, the left side of Fig. 1 shows an example of the waveform of the current sense signal Vcs when the primary side current Ip rises. On the left side of Fig. 1, the solid line shows the waveform of the current sense signal Vcs when the input voltage VH is high, and the dashed line shows the waveform of the current sense signal Vcs when the input voltage VH is low. The higher the input voltage VH, the greater the rising slope of the current sense signal Vcs.

[0025] As shown on the left side of Figure 1, regardless of the input voltage VH, a delay time Dt occurs from the time when the current sense signal Vcs rises and exceeds the OCP threshold voltage Vth_ocp until the switching element is turned off. As a result, if the OCP threshold voltage Vth_ocp is fixed and does not depend on the input voltage VH, as shown on the left side of Figure 1, the current sense signal Vcs at the time when the switching element is turned off, i.e., the peak of the primary current Ip, will differ greatly depending on the input voltage VH. If the peak of the primary current Ip becomes large, it will be necessary to design the primary winding to have more turns so that magnetic saturation does not occur even with a large primary current Ip. This leads to an increase in the size and cost of the transformer.

[0026] Such a change in the input voltage VH occurs, for example, when a flyback converter is applied to an isolated AC / DC converter and the AC voltage changes (for example, AC 100V, 200V, etc.). In this case, the AC voltage is rectified and smoothed to generate the input voltage VH.

[0027] Here, the right side of Fig. 1 shows an example of the waveform of the current sense signal Vcs when the OCP threshold voltage Vth_ocp is changed according to the input voltage VH. In this case, the OCP threshold voltage Vth_ocp1 for the higher input voltage VH is set lower than the OCP threshold voltage Vth_ocp2 for the lower input voltage VH. As a result, as shown on the right side of Fig. 1, at the timing when the switching element is turned off after the delay time Dt has elapsed after the current sense signal Vcs exceeds the OCP threshold voltages Vth_ocp1 and Vth_ocp2, respectively, the difference in the peak of the current sense signal Vcs, i.e., the primary side current Ip, can be suppressed.

[0028] However, when the OCP threshold voltage is changed according to the input voltage VH as described above, as shown in the left side of FIG. 2 (similar to the right side of FIG. 1), when the inductance of the primary winding is a certain value, the difference in peaks of the primary current Ip can be suppressed. However, when the inductance changes depending on the primary winding used, for example, as shown in the right side of FIG. 2, the case results. The right side of FIG. 2 is an example in which the inductance of the primary winding is lower than that of the left side of FIG. 2. In this case, as shown in the right side of FIG. 2, the rising slope of the current sense signal Vcs changes (the slope becomes steeper in the example on the right side of FIG. 2), and the difference in peaks of the primary current Ip at the timing when the switching element is turned off increases.

[0029] <2. Overall configuration of flyback converter> An embodiment devised to solve the above problems will be described below. Fig. 3 is a diagram showing the configuration of a flyback converter 28 according to an exemplary embodiment of the present invention. The flyback converter 28 performs DC / DC conversion of an input voltage VH, which is a DC voltage, to an output voltage VOUT, which is also a DC voltage. As mentioned above, the flyback converter 28 is preferably applied to an isolated AC / DC converter in which the input voltage VH often changes. However, the flyback converter 28 may also be applied to an isolated DC / DC converter in which the input voltage VH changes.

[0030] As shown in Fig. 3, the flyback converter 28 includes the power supply control device 1, external resistors 16 and 17, a light receiving element 18, a capacitor 19, a sense resistor 20, a transformer 21, a rectifier diode 22, a smoothing capacitor 23, a feedback circuit 24, a light emitting element 25, a rectifier diode 26, and a smoothing capacitor 27. The power supply control device 1 is a semiconductor device (semiconductor package) including an IC in which the internal components shown in Fig. 3 are integrated into one chip. The external resistors 16 and 17, the light receiving element 18, the capacitor 19, the sense resistor 20, the transformer 21, the rectifier diode 22, the smoothing capacitor 23, the light emitting element 25, the rectifier diode 26, and the smoothing capacitor 27 are discrete elements disposed outside the power supply control device 1.

[0031] The power supply control device 1 includes a PWM (Pulse Width Modulation) control unit 2, a pull-up resistor 3, a voltage divider unit 4, a PWM comparator 5, a flip-flop 6, a driver 7, a switching element 8, an OCP comparator 9, an OVP (overvoltage protection) comparator 10, a short detection comparator 11, a current detection resistor 12, a switch transistor 13, and a threshold voltage correction unit 14. The switching element 8 may be external to the power supply control device.

[0032] The power supply control device 1 also has a VCC terminal, a DRAIN terminal, a SOURCE terminal, an FB terminal, and a ZT terminal, which are external terminals for establishing electrical connections with the outside.

[0033] The transformer 21 has a primary winding 21A, a secondary winding 21B, and an auxiliary winding 21C. One end of the primary winding 21A is connected to an application terminal of the input voltage VH. The other end of the primary winding 21A is connected to the DRAIN terminal.

[0034] One end of the secondary winding 21B is connected to the anode of the rectifier diode 22. The cathode of the rectifier diode 22, together with one end of the smoothing capacitor 23, is connected to the output terminal T1. The other end of the secondary winding 21B, together with the other end of the smoothing capacitor 23, is connected to a ground terminal T2. The ground terminal T2 is connected to an application terminal for a ground potential. An output voltage VOUT is generated at the output terminal T1.

[0035] The feedback circuit 24 and the light-emitting element 25 are disposed on the secondary side. The feedback circuit 24 drives the light-emitting element 25 with a current corresponding to the error between the output voltage VOUT and its target voltage. The light-emitting element 25 and the light-receiving element 18 constitute a photocoupler. The light-receiving element 18 and a capacitor 19 are externally connected to the FB terminal. The FB terminal is also connected to one end of a pull-up resistor 3 inside the power supply control device 1. The other end of the pull-up resistor 3 is connected to an application end of the internal power supply voltage. When the light-receiving element 18 receives the light output from the light-emitting element 25, a current flows through the pull-up resistor 3 and the light-receiving element 18, and an FB terminal voltage corresponding to the current is generated. Therefore, an FB terminal voltage corresponding to the above error is generated.

[0036] The FB terminal voltage is divided by the voltage divider 4 to produce a feedback voltage Vfb. The feedback voltage Vfb is applied to the inverting input terminal (−) of the PWM comparator 5.

[0037] The switching element 8 is composed of an NMOS transistor. The drain (current inflow end) of the switching element 8 is connected to the DRAIN terminal. The source (current outflow end) of the switching element 8 is connected to the SOURCE terminal. The SOURCE terminal is externally connected to one end of a sense resistor 20. The other end of the sense resistor 20 is connected to an application end of a ground potential. A primary current Ip flowing through the primary winding 21A and the switching element 8 is I / V converted by the sense resistor 20 to become a current sense signal Vcs. The current sense signal Vcs is applied to the non-inverting input end (+) of the PWM comparator 5.

[0038] The PWM control unit 2 controls the flip-flop 6 by sending signals to the set terminal (S) and reset terminal (R) of the flip-flop 6. More specifically, the PWM control unit 2 determines the turn-on timing (off period) of the switching element 8 by sending a signal synchronized with a clock signal having a constant frequency output from an oscillator (not shown) to the set terminal. Furthermore, when the PWM comparator 5 detects that the current sense signal Vcs has exceeded the feedback voltage Vfb, the PWM control unit 2 sends a reset signal to the reset terminal and determines the turn-off timing (on period) of the switching element 8.

[0039] The Q output signal output from the Q output terminal of the flip-flop 6 is input to the driver 7. The driver 7 generates a gate signal Vg whose level corresponds to the level of the Q output signal, and applies the generated gate signal Vg to the gate of the switching element 8. More specifically, when the Q output signal is High, the gate signal Vg is High, and when the Q output signal is Low, the gate signal Vg is Low. When the gate signal Vg is High, the switching element 8 is in the ON state, and when the gate signal Vg is Low, the switching element 8 is in the OFF state. In this way, the switching element 8 is switched and controlled (ON / OFF controlled) by PWM control, thereby controlling the output voltage VOUT.

[0040] <3. Auxiliary winding> One end of the auxiliary winding 21C is connected to a terminal to which the ground potential is applied. The other end of the auxiliary winding 21C is connected to an anode of a rectifier diode 26. The cathode of the rectifier diode 26 is connected to one end of a smoothing capacitor 27. The other end of the smoothing capacitor 27 is connected to a terminal to which the ground potential is applied. The auxiliary winding voltage VD generated in the auxiliary winding 21C becomes a pulse voltage by switching control of the switching element 8, is rectified by the rectifier diode 26, and is smoothed by the smoothing capacitor 27 to generate a power supply voltage Vcc. The generated power supply voltage Vcc is applied to the VCC terminal and becomes the power supply voltage of the power supply control device 1.

[0041] <4.ZT terminal> The other end of the auxiliary winding 21C is connected to one end of the external resistor 16. The other end of the external resistor 16 is connected to one end of the external resistor 17 at a node N1. The other end of the external resistor 17 is connected to the application end of the ground potential. That is, the other end of the auxiliary winding 21C is connected to the application end of the ground potential via the series connection configuration of the external resistor 16 and the external resistor 17. The node N1 is externally connected to the ZT terminal.

[0042] Inside the power supply control device 1, the ZT terminal is connected to the non-inverting input terminal (+) of the OVP comparator 10 and also to the non-inverting input terminal (+) of the short detection comparator 11.

[0043] A reference voltage Ref10 is applied to the inverting input terminal (-) of the OVP comparator 10. The output of the OVP comparator 10 is input to the PWM control unit 2. A reference voltage Ref11 is applied to the inverting input terminal (-) of the short detection comparator 11. The output of the short detection comparator 11 is input to the PWM control unit 2.

[0044] When the switching element 8 is turned off, a flyback voltage VOR is generated in the primary winding 21A. It is expressed as VOR=(VOUT+VF)×(Np / Ns), where VF is the forward voltage of the rectifier diode 22, Np is the number of turns of the primary winding, and Ns is the number of turns of the secondary winding. At this time, the auxiliary winding voltage VD=VOR×(Nd / Np), where Nd is the number of turns of the auxiliary winding.

[0045] The ZT terminal voltage is the auxiliary winding voltage VD divided by external resistors 16 and 17. As described above, VD contains information about the output voltage VOUT, so an overvoltage of the output voltage VOUT can be detected by comparing the ZT terminal voltage with reference voltage Ref10 by OVP comparator 10. Also, a short circuit between the output terminal T1 and the ground potential can be detected by comparing the ZT terminal voltage with reference voltage Ref11 by short detection comparator 11.

[0046] In this way, the ZT terminal makes it possible to detect an abnormality in the output voltage VOUT on the primary side. Note that the ZT terminal in this embodiment should not be confused with the ZT terminal provided in a so-called quasi-resonant controller type power supply control device. The ZT terminal in a quasi-resonant controller type power supply control device is a terminal for detecting the zero timing of the secondary side current flowing through the secondary winding. However, there is no prohibition on applying the present invention to the ZT terminal in a quasi-resonant controller type power supply control device.

[0047] <5. OCP threshold voltage variable control function> Next, a description will be given of an OCP threshold voltage variable control function provided in the power supply control device 1. The OCP threshold voltage Vocp is applied to the inverting input terminal (-) of the OCP comparator 9. The current sense signal Vcs is applied to the non-inverting input terminal (+) of the OCP comparator 9.

[0048] The components related to the OCP threshold voltage variable control function are external resistors 16 and 17, a current detection resistor 12, a switch transistor 13, a threshold voltage correction unit 14, and a voltage adjustment resistor 15.

[0049] As described above, the ZT terminal connected to the node N1 to which the external resistors 16 and 17 are connected is connected to one end of the current detection resistor 12. The other end of the current detection resistor 12 is connected to the drain of the switch transistor 13 formed of an NMOS transistor. The source of the switch transistor 13 is connected to an application terminal of the ground potential. The gate of the switch transistor 13 is driven by a gate signal Vg. Note that the gate of the switching transistor 13 is not limited to the gate signal Vg, and may be any signal related to the drive of the switching element 8, such as the Q output signal of the flip-flop 6.

[0050] The threshold voltage correction unit 14 corrects the OCP threshold voltage Vocp based on the current detection signal Vi generated in the current detection resistor 12. A specific configuration example of the threshold voltage correction unit 14 is shown in FIG.

[0051] 4, the threshold voltage correction unit 14 has an inverting amplifier 14A, a sample-and-hold unit 141, and a V / I (voltage-current) conversion unit 142. A current detection signal Vi, which is a voltage generated across the current detection resistor 12, is input to the inverting amplifier 14A, and the inverting amplifier 14A inverts and amplifies the input current detection signal Vi. An amplifier output Va after the inverting and amplification is input to the sample-and-hold unit 141.

[0052] The sample-and-hold unit 141 has a switch 14B and a capacitor 14C. When the switch 14B is in an on state, a sampling operation is performed in which the amplifier output Va is directly output as a sampling output Vsp. When the switch 14B is in an off state, a holding operation is performed in which the sampling output Vsp immediately before the switch 14B is switched from the on state to the off state is held by the capacitor 14C. The sampling output Vsp is input to the V / I conversion unit 142.

[0053] The V / I conversion unit 142 includes a constant current generating circuit 1421, a first current mirror 14G, and a second current mirror 14H. The constant current generating circuit 1421 includes an error amplifier 14D, an NMOS transistor 14E, and a resistor 14F.

[0054] The sampling output Vsp is input to the non-inverting input terminal (+) of the error amplifier 14D. The output terminal of the error amplifier 14D is connected to the gate of the NMOS transistor 14E. The source of the NMOS transistor 14E is connected to one terminal of a resistor 14F at a node N14. The other terminal of the resistor 14F is connected to an application terminal of a ground potential. The node N14 is connected to the inverting input terminal (-) of the error amplifier 14D. As a result, the voltage of the node N14 is controlled to be the sampling output Vsp, and a constant current I1 is generated by the voltage of the node N14 and the resistor 14F.

[0055] The first current mirror 14G has a PMOS transistor PM1 on the input side and a PMOS transistor PM2 on the output side. The drain of the PMOS transistor PM1 is connected to the drain of the NMOS transistor 14E. The gate and drain of the PMOS transistor PM1 are shorted. The sources of the PMOS transistors PM1 and PM2 are connected to an application terminal of the internal power supply voltage. The gates of the PMOS transistors PM1 and PM2 are connected to each other.

[0056] The second current mirror 14H has an NMOS transistor NM1 on the input side and an NMOS transistor NM2 on the output side. The drain of the NMOS transistor NM1 is connected to the drain of the PMOS transistor PM2. The gate and drain of the NMOS transistor NM1 are shorted. The sources of the NMOS transistors NM1 and NM2 are connected to a terminal to which a ground potential is applied. The gates of the NMOS transistors NM1 and NM2 are connected to each other.

[0057] As a result, the constant current I1 generated by the constant current generating circuit I1 is mirrored by the first current mirror 14G and the second current mirror 14H to become the output current I2. In this manner, the V / I conversion unit 142 performs V / I conversion on the sampling output Vsp to the output current I2.

[0058] 3, one end of the voltage adjustment resistor 15 and the inverting input terminal (-) of the OCP comparator 9 are connected at a node N2. The drain of the NMOS transistor NM2 in the second current mirror 14H is connected to the node N2. The other end of the voltage adjustment resistor 15 is connected to the application terminal of the reference voltage Ref9. When the output current I2 generated by the threshold voltage correction unit 14 flows through the voltage adjustment resistor 15, a voltage that is dropped from the reference voltage Ref9 by the voltage adjustment resistor 15 is generated at the node N2 as the OCP threshold voltage Vocp. In this way, the threshold voltage correction unit 14 corrects the OCP threshold voltage Vocp by generating the output current I2 based on the current detection signal Vi generated in the current detection resistor 12.

[0059] Here, Fig. 5 is a timing chart showing example waveforms of various signals in the flyback converter 28. In Fig. 5, from the top, the waveforms of the gate signal Vg, the DRAIN terminal voltage, the auxiliary winding voltage VD, the current detection signal Vi, the sampling output Vsp, the OCP threshold voltage Vocp, and the current sense signal Vcs are shown.

[0060] When the gate signal Vg rises to High at timing t1 shown in Fig. 5, the switching element 8 is turned on. As a result, the DRAIN terminal voltage falls to 0 V, and the current sense signal Vcs starts to rise. At this time, the auxiliary winding voltage VD falls to -VH × (Nd / Np). In addition, the gate signal Vg turns on the switch transistor 13, so that a terminal current Izt flows from the application end of the ground potential to the application end of VD via the switch transistor 13, the ZT terminal, and the external resistor 16, as shown in Fig. 3.

[0061] At this time, a current detection signal Vi corresponding to the terminal current Izt is generated by the current detection resistor 12, and the terminal current Izt is detected. Here, Vi=-ΔV, as shown in Fig. 5. Since VD contains information about the input voltage VH, ΔV is proportional to the input voltage VH.

[0062] The negative voltage Vi is input to the inverting amplifier 14A (FIG. 4) and inverted and amplified. If the amplification factor is N, then the amplifier output Va=Vi×(-N)=-ΔV×(-N). When the gate signal Vg is High, the switch 14B in the sample-and-hold unit 141 is switched from the OFF state to the ON state, the amplifier output Va is sampled, and the sampling output Vsp is generated. Also, when the gate signal Vg is High, the switch 14B is switched from the ON state to the OFF state, and the sampling output Vsp is held.

[0063] The sampling output Vsp is converted into an output current I2 by the V / I conversion unit 142, and flows through the voltage adjustment resistor 15, thereby correcting the OCP threshold voltage Vocp.

[0064] As a result, the OCP threshold voltage Vocp becomes variable according to the input voltage VH. When the switching element 8 is in the on state, the OCP comparator 9 compares the rising current sense signal Vcs with the OCP threshold voltage Vocp, and outputs a High signal to the PWM control unit 2 when Vcs exceeds Vocp. At this time, the PWM control unit 2 resets the flip-flop 6 and turns off the switching element 8. There is a delay from when the OCP comparator 9 detects that Vcs has exceeded Vocp until the switching element 8 is turned off. However, since the OCP threshold voltage Vocp becomes variable according to the input voltage VH, as described in the above-mentioned problem, it is possible to suppress the difference in the peak of Vcs, i.e., the peak of the primary side current Ip, at the timing when the switching element 8 is turned off due to the change in the input voltage VH.

[0065] Furthermore, in this embodiment, the resistance values ​​of the external resistors 16 and 17 can be adjusted to change the total resistance value without changing the voltage division ratio, depending on the inductance of the primary winding 21A used. For example, the resistance values ​​of the external resistors 16 and 17 can be changed from 100 kΩ to 10 kΩ. By adjusting the resistance value of the external resistor 16, the terminal current Izt can be adjusted.

[0066] This makes it possible to correct the OCP threshold voltage Vocp according to the inductance of the primary winding 21A without affecting the overvoltage detection by the OVP comparator 10 using the ZT terminal and the short circuit detection by the short circuit detection comparator 11. Therefore, as described above in the problem section, it is possible to suppress an increase in the difference in the peak of Vcs, i.e., the peak of the primary current Ip, at the timing when the switching element 8 is turned off due to a change in the inductance of the primary winding 21A. In other words, it is possible to improve the state on the right side of FIG. 2 described above.

[0067] <6. First modified example> FIG. 6 is a diagram showing the configuration of a power supply control device 1x according to a first modification. The difference in configuration between the power supply control device 1x shown in FIG. 6 and the embodiment described above (FIG. 3) will be described. The configuration of a threshold voltage correction unit 14x included in the power supply control device 1x is shown in FIG. 7. As shown in FIG. 7, the threshold voltage correction unit 14x does not have a current mirror using NMOS transistors in the rear stage as a difference in configuration from the threshold voltage correction unit 14 (FIG. 4) according to the embodiment described above. As shown in FIG. 6, an insertion resistor R is inserted between the non-inverting input terminal (+) of the OCP comparator 9 and the SOURCE terminal (i.e., one end of the sense resistor 20). The drain of the PMOS transistor PM2 in the current mirror 14G (FIG. 7) is connected to a node Nx to which the non-inverting input terminal (+) of the OCP comparator 9 and one end of the insertion resistor R (the end opposite to the SOURCE terminal) are connected. That is, one end of the resistor including the sense resistor 20 and the insertion resistor R is connected to the non-inverting input terminal (+) of the OCP comparator 9. Moreover, a reference voltage Ref9 is applied to the inverting input terminal (−) of the OCP comparator 9.

[0068] With this configuration, the output current I2 generated and output by the current mirror 14G as shown in FIG. 7 flows into the node Nx (FIG. 6). The output current I2 flows from the node Nx through the insertion resistor R and the sense resistor 20. Meanwhile, the primary side current Ip flows through the sense resistor 20. That is, while the above-mentioned embodiment (FIG. 3) has a sink type configuration that draws in the output current I2, this embodiment has a source type configuration that discharges the output current I2. The voltage Vr generated at the node Nx by the insertion resistor R and the sense resistor 20 is compared with the reference voltage Ref9 by the OCP comparator 9.

[0069] Even with the configuration of this embodiment, it is substantially possible to correct the OCP threshold voltage, which is compared with the current sense signal Vcs obtained by I / V conversion of the primary side current Ip, based on the current detection signal Vi.

[0070] In particular, since the resistance value of the sense resistor 20 is often very small, a current of, for example, several hundred mA must be passed in order to pass a current through the sense resistor 20 and generate a voltage. Therefore, in the configuration of Figure 6, an insert resistor R with a relatively large resistance is provided so that a voltage can be generated even with a small current, thereby reducing power consumption. Note that the insert resistor R is not essential.

[0071] <7. Second modified example> 8 is a diagram showing the configuration of a flyback converter 28' according to a second modified example. The configuration of a power supply control device 1' included in the flyback converter 28' differs from that of the above-described embodiment in that a burst comparator 29 is included.

[0072] A feedback voltage Vfb is applied to the inverting input terminal (-) of the burst comparator 29. A non-inverting input terminal (+) of the burst comparator 29 is connected to one terminal of a voltage adjustment resistor 30 at a node N3. The other terminal of the voltage adjustment resistor 30 is connected to an application terminal of a reference voltage Ref29.

[0073] A burst threshold voltage Bst is generated at node N3. As shown in FIG. 9, when the load becomes light, the feedback voltage Vfb decreases. When the feedback voltage Vfb falls below the burst threshold voltage Bst (timing t21 in FIG. 9), the output signal of the burst comparator 29 becomes High. This causes the PWM control unit 2 to stop switching of the switching element 8 (maintain it in the off state). Then, the feedback voltage Vfb increases due to the stopping of switching, and when the feedback voltage Vfb exceeds the burst threshold voltage Bst (timing t22), the output signal of the burst comparator 29 becomes Low. This causes the PWM control unit 2 to start switching of the switching element 8 (recovery from the stop of switching). Such a burst function can reduce switching loss during light load.

[0074] In the above burst operation, when returning from a switching stop, the switching element 8 is turned on and the current sense signal Vcs (primary side current Ip) rises, and when the PWM comparator 5 detects that it has exceeded the feedback voltage Vfb, the PWM control unit 2 turns off the switching element 8. The feedback voltage Vfb at the time of returning from a switching stop, which is equal to the burst threshold voltage Bst, is compared with the current sense signal Vcs as a threshold voltage, and there is a delay time from when it is detected that Vcs has exceeded Bst until the switching element 8 is turned off. Therefore, the same problem as the OCP described above occurs.

[0075] 8, the output current I2 generated by the threshold voltage correction unit 14 flows through the voltage adjustment resistor 30, and the voltage dropped from the reference voltage Ref29 by the voltage adjustment resistor 30 is generated as the burst threshold voltage Bst. In this case, the drain of the NMOS transistor NM2 (FIG. 4) of the second current mirror 14H in the threshold voltage correction unit 14 may be connected to the node N3.

[0076] This allows the burst threshold voltage Bst to be corrected according to changes in the input voltage VH and the inductance of the primary winding 21A, thereby suppressing the difference that occurs in the peak of the primary side current Ip when returning from the stop of switching in the burst operation.

[0077] The correction using the threshold voltage correction unit may be applied to both the OCP threshold voltage and the burst threshold voltage.

[0078] <8.Other>

[0079] Although the embodiment of the present invention has been described above, the embodiment can be modified in various ways within the scope of the spirit of the present invention.

[0080] For example, the secondary side rectifier element is not limited to the rectifier diode 22 shown in Figures 3, 6, and 8 described above, but a rectifier diode having a cathode connected to the other end of the secondary winding 21B and an anode connected to the ground terminal may be used.

[0081] Alternatively, a synchronous rectifier transistor 31 may be used as a rectifier element on the secondary side, as in a flyback converter 280 shown in Fig. 10. In this case, a synchronous rectifier controller 32 is provided on the secondary side together with the synchronous rectifier transistor 31. The synchronous rectifier controller 32 switches the synchronous rectifier transistor 31 in synchronization with the switching of a switching element on the primary side (not shown in Fig. 10, but similar to the switching element 8 in Figs. 3, 6, and 8). [Industrial Applicability]

[0082] The present invention can be used, for example, in an isolated AC / DC converter or an isolated DC / DC converter. [Explanation of symbols]

[0083] 1, 1x, 1' Power Control Unit 2 PWM control section 3 Pull-up resistor 4. Voltage divider 5 PWM Comparator 6. Flip-Flop 7 Drivers 8 Switching elements 9 OCP Comparator 10 OVP Comparator 11 Short circuit detection comparator 12 Current sensing resistor 13 Switch Transistor 14, 14x threshold voltage correction unit 14A Inverting Amplifier 14B Switch 14C Capacitor 14D Error Amplifier 14E NMOS transistor 14F Resistor 14G First current mirror 14H Second current mirror 15 Voltage adjustment resistor 16,17 External Resistor 18 Photodetector 19 Capacitor 20 Sense resistor 21. Trans 21A Primary Winding 21B Secondary winding 21C Auxiliary Winding 22 Rectifier diode 23 Smoothing capacitor 24 Feedback Circuit 25 Light emitting element 26 Rectifier Diode 27 Smoothing capacitor 28, 28' Flyback Converter 29 Burst Comparator 30 Voltage adjustment resistor 31 Synchronous rectifier transistor 32 Synchronous Rectification Controller 141 Sample and hold section 142 V / I conversion section 280 Flyback Converter 1421 Constant current generation circuit NM1, NM2 NMOS transistors PM1, PM2 PMOS transistors T1 output terminal T2 Ground terminal R Insertion resistance

Claims

1. A switching element; a transformer having a primary winding, a secondary winding, and an auxiliary winding; A rectifying element; A smoothing capacitor; having An input voltage application terminal is connected to one end of the primary winding, The other end of the primary winding is connected to a current input terminal of the switching element, A power supply control device used in a flyback converter, the rectifying element and the smoothing capacitor being provided on a rear stage side of the secondary winding, a comparison unit that compares a current sense signal generated by I / V converting a primary side current flowing through the primary winding with a threshold voltage; a switching control unit that turns off the switching element based on a comparison result by the comparison unit between the current sense signal and the threshold voltage; an external terminal connectable to a connection node of an external resistor connected in series between one end of the auxiliary winding and an application end of a ground potential; a current detection unit that detects a terminal current flowing through the external terminal; a threshold voltage correction unit that corrects the threshold voltage based on a current detection signal from the current detection unit; having The power supply control device, wherein the current detection unit is a current detection resistor.

2. 2. The power supply control device according to claim 1, wherein the threshold voltage correction unit includes an inverting amplifier that inverts and amplifies the current detection signal.

3. A switching element; a transformer having a primary winding, a secondary winding, and an auxiliary winding; A rectifying element; A smoothing capacitor; having An input voltage application terminal is connected to one end of the primary winding, The other end of the primary winding is connected to a current input terminal of the switching element, A power supply control device used in a flyback converter, the rectifying element and the smoothing capacitor being provided on a rear stage side of the secondary winding, a comparison unit that compares a current sense signal generated by I / V converting a primary side current flowing through the primary winding with a threshold voltage; a switching control unit that turns off the switching element based on a comparison result by the comparison unit between the current sense signal and the threshold voltage; an external terminal connectable to a connection node of an external resistor connected in series between one end of the auxiliary winding and an application end of a ground potential; a current detection unit that detects a terminal current flowing through the external terminal; a threshold voltage correction unit that corrects the threshold voltage based on a current detection signal from the current detection unit; having The threshold voltage correction unit includes a sample-and-hold unit that samples a voltage based on the current detection signal.

4. A switching element; a transformer having a primary winding, a secondary winding, and an auxiliary winding; A rectifying element; A smoothing capacitor; having An input voltage application terminal is connected to one end of the primary winding, The other end of the primary winding is connected to a current input terminal of the switching element, A power supply control device used in a flyback converter, the rectifying element and the smoothing capacitor being provided on a rear stage side of the secondary winding, a comparison unit that compares a current sense signal generated by I / V converting a primary side current flowing through the primary winding with a threshold voltage; a switching control unit that turns off the switching element based on a comparison result by the comparison unit between the current sense signal and the threshold voltage; an external terminal connectable to a connection node of an external resistor connected in series between one end of the auxiliary winding and an application end of a ground potential; a current detection unit that detects a terminal current flowing through the external terminal; a threshold voltage correction unit that corrects the threshold voltage based on a current detection signal from the current detection unit; having a voltage adjusting resistor connected between the input terminal of the comparison unit and a terminal to which a reference voltage is applied; The threshold voltage correction unit generates an output current based on the current detection signal, The output current flows through the voltage adjustment resistor and is absorbed into the threshold voltage correction unit. Power control device.

5. the threshold voltage correction unit has a constant current generation circuit that receives a voltage based on the current detection signal and generates a constant current; The power supply control device according to claim 4 , wherein the output current is generated based on the constant current.

6. The threshold voltage correction unit is a first current mirror having an input terminal connected to an output terminal of the constant current generating circuit and configured with a PMOS transistor; a second current mirror having an input terminal connected to the output terminal of the first current mirror and configured with an NMOS transistor; The power supply control device according to claim 5 , further comprising:

7. A switching element; a transformer having a primary winding, a secondary winding, and an auxiliary winding; A rectifying element; A smoothing capacitor; having An input voltage application terminal is connected to one end of the primary winding, The other end of the primary winding is connected to a current input terminal of the switching element, A power supply control device used in a flyback converter, the rectifying element and the smoothing capacitor being provided on a rear stage side of the secondary winding, a comparison unit that compares a current sense signal generated by I / V converting a primary side current flowing through the primary winding with a threshold voltage; a switching control unit that turns off the switching element based on a comparison result by the comparison unit between the current sense signal and the threshold voltage; an external terminal connectable to a connection node of an external resistor connected in series between one end of the auxiliary winding and an application end of a ground potential; a current detection unit that detects a terminal current flowing through the external terminal; a threshold voltage correction unit that corrects the threshold voltage based on a current detection signal from the current detection unit; having One input terminal of the comparison unit is connected to a terminal to which a reference voltage is applied, the other input terminal of the comparison unit is connected to one end of a resistor including a sense resistor having one end connected to the current output terminal of the switching element, The threshold voltage correction unit generates an output current based on the current detection signal, The output current is discharged toward a node to which the other input terminal of the comparison unit and the one end of the resistor are connected.

8. 8. The power supply control device according to claim 7, further comprising an insertion resistor inserted between the other input terminal of the comparison section and the one end of the sense resistor.

9. 9. The power supply control device according to claim 1, further comprising a switch that is disposed in a path through which the terminal current flows and that is on / off controlled based on a signal related to driving of the switching element.

10. The power supply control device according to claim 1 , wherein the threshold voltage is an OCP (overcurrent protection) threshold voltage.

11. a first comparator for comparing a feedback voltage based on an output voltage of the flyback converter with a burst threshold voltage; a second comparator serving as the comparison unit for comparing the feedback voltage with the current sense signal; having the switching control unit switches between stopping and restarting the switching of the switching element in response to an output of the first comparator; The power supply control device according to claim 1 , wherein the threshold voltage is the burst threshold voltage.

12. 12. A flyback converter comprising: the power supply control device according to claim 1; the switching element; the transformer; the rectifier element; the smoothing capacitor; and the external resistor.

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