Integrated circuits and power supply circuits

Abstract

To provide an integrated circuit and a power supply circuit, which can suppress a drop of power supply voltage.SOLUTION: A power supply circuit 10 includes: a transformer 22; a transistor 30 for controlling inductor current; a first capacitor 33; a diode 34 for charging the first capacitor on the basis of voltage of an auxiliary coil L3 of the transformer when the transistor is turned off; a control IC 32 having a first terminal VCC to which voltage of the first capacitor is applied as power supply voltage, a second terminal FB to which feedback voltage corresponding to output voltage is applied, a driving signal output circuit for outputting a driving signal whose switching period of the transistor becomes long on the basis of feedback voltage when load current flowing in a load becomes small, a drive circuit for driving the transistor on the basis of the driving signal and a determination circuit for determining whether power supply voltage drops below a prescribed threshold; and a current output circuit for generating step-up voltage on the basis of voltage of the auxiliary coil and charging the first capacitor on the basis of the step-up voltage when the power supply voltage is lower than first voltage.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an integrated circuit and a power supply circuit. [Background technology]

[0002] The power supply circuit may be provided with an integrated circuit that controls the switching of the power transistor of the power supply circuit using a power supply voltage generated by a voltage from an auxiliary coil of a transformer (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-108517 Summary of the Invention [Problem to be solved by the invention]

[0004] Generally, the voltage of the auxiliary coil is generated by the integrated circuit switching the power transistor, so for example, if the current flowing through the load of the power supply circuit decreases and the switching period of the power transistor becomes longer, the voltage of the auxiliary coil may decrease, causing a drop in the power supply voltage.

[0005] When the power supply voltage drops, a so-called low voltage protection circuit may operate and reset the integrated circuit.

[0006] The present invention has been made in view of the above-mentioned problems in the prior art, and an object of the present invention is to provide an integrated circuit that suppresses a drop in power supply voltage. [Means for solving the problem]

[0007] a first terminal to which the voltage of the first capacitor is applied as a power supply voltage; a second terminal to which a feedback voltage corresponding to the output voltage is applied; a drive signal output circuit that outputs a drive signal based on the feedback voltage to lengthen the switching period of the transistor when the load current flowing through the load becomes small; a drive circuit that drives the transistor based on the drive signal; and a determination circuit that determines whether the power supply voltage has dropped below a first voltage; and the power supply circuit includes: a boost circuit that generates a boost voltage based on the voltage of the auxiliary coil; and a first charging circuit that charges the first capacitor based on the boost voltage when the power supply voltage is lower than the first voltage.

[0008] a first charging circuit for charging the first capacitor; and an integrated circuit for controlling switching of the transistor. The integrated circuit includes a first terminal to which the voltage of the first capacitor is applied as a power supply voltage, a second terminal to which a feedback voltage corresponding to the output voltage is applied, a drive signal output circuit for outputting a drive signal whose switching period is changed by the feedback voltage, a drive circuit for driving the transistor based on the drive signal, and a determination circuit for determining whether the power supply voltage has dropped below a first voltage. When the power supply voltage is lower than the first voltage, the first charging circuit charges the first capacitor based on the boosted voltage.

[0009] a first diode that charges the first capacitor based on the voltage of the auxiliary coil when the transistor is turned off; a determination circuit that determines whether the voltage of the first capacitor has dropped below a first voltage; a boost circuit that generates a boosted voltage based on the voltage of the auxiliary coil; a first charging circuit that charges the first capacitor based on the boosted voltage when the voltage of the first capacitor is lower than the first voltage; and an integrated circuit that controls switching of the transistor, wherein the integrated circuit includes a first terminal to which the voltage of the first capacitor is applied as a power supply voltage; a second terminal to which a feedback voltage corresponding to the output voltage is applied; a drive signal output circuit that outputs a drive signal whose switching period is changed by the feedback voltage; and a drive circuit that drives the transistor based on the drive signal. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an integrated circuit that suppresses a drop in power supply voltage. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating an example of the configuration of an AC-DC converter 10. FIG. [Figure 2] FIG. 2 is a diagram illustrating an example of the configuration of a control IC 32. [Figure 3] FIG. 2 is a diagram showing an example of the configuration of a start-up circuit 53. [Figure 4] 10 is a diagram showing the relationship between the feedback voltage Vfb and the frequency Fsw of the oscillation signal osc_out. FIG. [Figure 5] FIG. 2 is a diagram showing an example of the configuration of a control circuit 57. [Figure 6](A) is a diagram for explaining the operation of generating the drive signal Vq1 in the case of a heavy load, and (B) is a diagram for explaining the operation of generating the drive signal Vq1 in the case of a light load (or when reducing the output voltage Vout). [Figure 7] It is a diagram showing an example of the configuration of the current output circuit 39a. [Figure 8] It is a diagram showing how the power supply voltage Vcc is generated when the feedback voltage Vfb is high. [Figure 9] It is a diagram showing how the power supply voltage Vcc is generated when the feedback voltage Vfb is low. [Figure 10] It is a diagram showing how the power supply voltage Vcc is generated when reducing the output voltage Vout. [Figure 11] It is a diagram showing an example of the configuration of the current output circuit 39b. [Figure 12] It is a diagram showing an example of the configuration of the control circuit 58. [Figure 13] It is a diagram showing an example of the configuration of the current output circuit 39c.

Mode for Carrying Out the Invention

[0012] From the description in this specification and the accompanying drawings, at least the following matters become clear. =====This Embodiment===== FIG. 1 is a diagram showing an example of the configuration of an AC-DC converter 10 according to an embodiment of the present invention. The AC-DC converter 10 is a power supply circuit that generates an output voltage Vout from the AC voltage Vac of a commercial power supply.

[0013] <<Overview of AC-DC Converter 10>> The AC-DC converter 10 includes a full-wave rectifier circuit 20, capacitors 21, 24, 41, a transformer 22, a resistor 23, diodes 25, 27, 28, 40, a control block 26, a constant voltage circuit 42, and a light-emitting diode 43.

[0014] The DC-DC converter 11 is connected to the AC-DC converter 10 and is a load to which power is supplied by the AC-DC converter 10, and to which the output voltage Vout is applied. The current flowing through the DC-DC converter 11 is referred to as the load current Iout.

[0015] Furthermore, the DC-DC converter 11 applies a DC voltage Vdc to an MCU (Micro Controller Unit) 12. Furthermore, the MCU 12 outputs a signal Sig and causes a constant voltage circuit 42 (described later) to switch the level of the DC voltage Vshunt based on the signal Sig. In this case, the AC-DC converter 10 outputs a high output voltage Vout or a low output voltage Vout, as will be described in detail later.

[0016] The full-wave rectifier circuit 20 full-wave rectifies a predetermined AC voltage Vac, which is an input voltage, and applies the resulting voltage Vrec1 to a primary coil L1 of a transformer 22, capacitors 21 and 24, and a resistor 23. The capacitor 21 smoothes the voltage Vrec1. The AC voltage Vac has an effective value of 100 to 240 V and a frequency of 50 to 60 Hz, for example.

[0017] The transformer 22 has a primary coil L1 provided on the input side, a secondary coil L2 magnetically coupled to the primary coil L1, and an auxiliary coil L3 magnetically coupled to the secondary coil. The secondary coil L2 and auxiliary coil L3 are wound so that the voltage generated in the secondary coil L2 and auxiliary coil L3 has the opposite polarity to the voltage generated in the primary coil L1. The primary coil L1 and auxiliary coil L3 are provided on the input side (primary side), and the secondary coil L2 is provided on the output side (secondary side).

[0018] The resistor 23, the capacitor 24, and the diode 25 constitute a snubber circuit. The snubber circuit suppresses a surge voltage generated by the leakage inductance of the primary coil L1 when the power transistor 30 (described later) is turned off, and prevents the power transistor 30 from being destroyed. The snubber circuit is connected in parallel with the primary coil L1. The anode of the diode 25 is connected to the high-potential side of the power transistor 30 (described later), and the cathode is connected to the resistor 23. The capacitor 24 is connected in parallel with the resistor 23.

[0019] The control block 26 controls the voltage generated in the secondary coil L2 on the secondary side of the transformer 22 by controlling the inductor current IL1 flowing through the primary coil L1 on the primary side of the transformer 22. As a result, an output voltage Vout is generated on the secondary side of the transformer 22.

[0020] The diodes 27 and 28 full-wave rectify the AC voltage Vac to generate a rectified voltage Vrec2, which is applied to a terminal VH of a control IC 32 (described later) included in the control block 26.

[0021] The diode 40 rectifies the inductor current IL2 from the secondary coil L2 of the transformer 22 and supplies the rectified current to the capacitor 41. The capacitor 41 is charged by the current from the diode 40, and an output voltage Vout is generated across the capacitor 41.

[0022] The constant voltage circuit 42 is a circuit that generates a constant DC voltage and is configured using, for example, a shunt regulator. If the DC-DC converter 11 is, for example, a power supply circuit used in a printer (not shown), the constant voltage circuit 42 outputs a high DC voltage Vshunt based on a signal Sig from the MCU 12 that indicates that the printer is operating. On the other hand, the constant voltage circuit 42 outputs a low DC voltage Vshunt based on a signal Sig from the MCU 12 that indicates that the printer is in a standby state.

[0023] The light-emitting diode 43 is an element that emits light with an intensity corresponding to the difference between the output voltage Vout and the voltage Vshunt from the constant voltage circuit 42, and forms a photocoupler together with the phototransistor 38 described later. In this embodiment, as the level of the output voltage Vout increases, the intensity of the light from the light-emitting diode 43 increases.

[0024] <<Overview of Control Block 26>> The control block 26 is a circuit block for controlling the AC-DC converter 10. The control block 26 includes a power transistor 30, resistors 31 and 36, a control IC 32, capacitors 33, 35 and 37, a diode 34, a phototransistor 38, and a current output circuit 39a.

[0025] The power transistor 30 is an NMOS transistor for controlling the power supplied to the DC-DC converter 11, and controls the inductor current IL1 flowing through the primary coil. In this embodiment, the power transistor 30 is a MOS (Metal Oxide Semiconductor) transistor, but is not limited to this. The power transistor 30 may be, for example, a bipolar transistor, etc., as long as it is a transistor that can control power.

[0026] The resistor 31 is a resistor for detecting an inductor current IL1 (i.e., a current flowing through the power transistor 30) that flows through the primary coil L1 when the power transistor 30 is on. One end of the resistor 31 is connected to the source electrode of the power transistor 30, and the other end is grounded.

[0027] The control IC 32 is an integrated circuit that switches the power transistor 30 to generate the output voltage Vout. Specifically, the control IC 32 switches the power transistor 30 based on the inductor current IL1 and the feedback voltage Vfb.

[0028] The control IC 32 is provided with terminals CS, FB, OUT, VCC, VH, and A, which will be described in detail later. The gate electrode of the power transistor 30 is connected to the terminal OUT, and the power transistor 30 is switched by a drive voltage Vg. The actual control IC 32 is also provided with other terminals, but these are omitted for convenience of explanation.

[0029] The capacitor 33 is provided between the terminal VCC and one end of the auxiliary coil L3. The anode of the diode 34 is connected to the auxiliary coil L3, and the cathode is connected to the terminal VCC. The diode 34 charges the capacitor 33 based on the voltage Va at the other end of the auxiliary coil L3. One end of the auxiliary coil L3 is grounded.

[0030] Furthermore, the voltage Va generated in the auxiliary coil L3 is applied to the capacitor 33 via the diode 34. Note that the terminal VCC is connected to the capacitor 33 to which a voltage based on the voltage Va of the auxiliary coil L3 is applied when the power transistor 30 is off, and this voltage becomes the power supply voltage Vcc.

[0031] That is, the voltage of the capacitor 33 is applied to the terminal VCC as the power supply voltage Vcc. The capacitor 33 corresponds to a "first capacitor," the diode 34 corresponds to a "first diode," and the terminal VCC corresponds to a "first terminal."

[0032] Capacitor 35 is provided between terminal CS and ground, and the voltage across resistor 31 generated by the flow of inductor current IL1 is applied via resistor 36. Capacitor 35 and resistor 36 form a low-pass filter, stabilizing voltage Vcs at terminal CS.

[0033] The capacitor 37 is provided between the terminal FB and the ground to stabilize the voltage Vfb at the terminal FB. The voltage Vfb is a feedback voltage that corresponds to the output voltage Vout and is applied to the terminal FB.

[0034] As will be described in detail later, the control IC 32 turns on the power transistor 30 at a frequency corresponding to the voltage Vfb. If the voltage Vcs exceeds the voltage Vfb while the power transistor 30 is on, the control IC 32 turns off the power transistor 30.

[0035] The phototransistor 38 is provided between the terminal FB and ground and receives light from the light-emitting diode 43. When the intensity of the light emitted by the light-emitting diode 43 increases, the phototransistor 38 causes a larger sink current Ia to flow through the terminal FB. As a result, the feedback voltage Vfb decreases, as will be described in detail later. The terminal FB corresponds to the "second terminal."

[0036] When the power supply voltage Vcc drops, the current output circuit 39a operates to charge the capacitor 33. Details of the current output circuit 39a will be described later.

[0037] <<Configuration of Control IC32>> 2 is a diagram showing an example of the configuration of the control IC 32. The control IC 32 switches the power transistor 30 to generate the output voltage Vout. Specifically, the control IC 32 switches the power transistor 30 based on the voltage Vcs corresponding to the inductor current IL1 and the feedback voltage Vfb.

[0038] The control IC 32 includes a low voltage protection circuit (UVLO) 50, a startup element 51, resistors 52 and 54, a startup circuit 53, a drive signal output circuit 55, a drive circuit 56, and a control circuit 57.

[0039] ==Undervoltage protection circuit (UVLO) 50== The low voltage protection circuit 50 outputs a signal rst based on the power supply voltage Vcc. Specifically, when the level of the power supply voltage Vcc reaches a predetermined level Voff, the low voltage protection circuit 50 outputs a high level (hereinafter referred to as "H" level) signal rst that stops the switching of the power transistor 30.

[0040] On the other hand, when the level of the power supply voltage Vcc reaches a predetermined level Von, which is higher than the predetermined level Voff, during operation of the startup circuit 53 (described later), the low voltage protection circuit 50 outputs a low level (hereinafter referred to as "L" level) signal rst that permits switching of the power transistor 30.

[0041] ==Starting element 51, resistor 52== The startup element 51 is an element that generates a predetermined voltage from a current based on the voltage Vh (i.e., the rectified voltage Vrec2) applied to the terminal VH. The resistor 52 is an element that limits the current when the startup circuit 53 charges the capacitor 33 in FIG. 1 via the terminal VCC. When a predetermined voltage is applied to one end of the resistor 52, the resistor 52 generates a voltage Vsup at the other end. The terminal VH corresponds to the "third terminal."

[0042] == Overview of Starter Circuit 53 == For example, when the low-voltage protection circuit 50 outputs a high-level signal rst, the startup circuit 53 outputs a current that charges the capacitor 33 in Fig. 1 via the terminal VCC at a voltage Vsup that corresponds to the voltage Vh. On the other hand, when the low-voltage protection circuit 50 outputs a low-level signal rst, the startup circuit 53 stops operating.

[0043] Specifically, when the level of the power supply voltage Vcc falls below a predetermined level Voff (for example, when the control IC 32 is started up), the low voltage protection circuit 50 outputs a high-level signal rst. In this case, the start-up circuit 53 outputs a current based on the high-level signal rst. Furthermore, the start-up circuit 53 stops operating based on the low-level signal rst.

[0044] ==Details of Start Circuit 53== As shown in FIG. 3, the start-up circuit 53 includes an OR element 70, an NMOS transistor 71, a PMOS transistor 72, resistors 73, 75, and 76, a diode 74, and a hysteresis comparator 77.

[0045] The OR element 70 performs a logical OR operation on the signal rst and the output signal dss_o of the hysteresis comparator 77, and based on the operation result, turns on and off the NMOS transistor 71. Furthermore, when the signal rst at the “H” level is input, the OR element 70 turns on the NMOS transistor 71.

[0046] When the NMOS transistor 71 is turned on, it sets the voltage at one end of the resistor 73 to the ground voltage and turns on the PMOS transistor 72 located between the node to which the voltage Vsup is applied and the anode of the diode 74 .

[0047] The voltage Vsup is applied to the other end of the resistor 73. When the PMOS transistor 72 is turned on, the capacitor 33 in Fig. 1 is charged with a current according to the voltage Vsup via the diode 74 and the terminal VCC.

[0048] On the other hand, when the NMOS transistor 71 is turned off, the PMOS transistor 72 is turned off and the capacitor 33 is not charged.

[0049] Resistors 75 and 76 are connected in series between terminal VCC and ground to form a voltage divider circuit. The voltage at the connection point of resistors 75 and 76 varies depending on the power supply voltage Vcc and is applied to the inverting input terminal of a hysteresis comparator 77.

[0050] A reference voltage Vref_dss is applied to the non-inverting input terminal of the hysteresis comparator 77. The hysteresis comparator 77 generates a high threshold Vdssh and a threshold Vdssl that is lower than the threshold Vdssh, based on the reference voltage Vref_dss.

[0051] When the voltage at the connection point of the resistors 75 and 76 becomes lower than the threshold Vdssl, the hysteresis comparator 77 outputs a signal dss_O at an "H" level. On the other hand, when the voltage at the connection point of the resistors 75 and 76 becomes higher than the threshold Vdssh, the hysteresis comparator 77 outputs a signal dss_O at an "L" level.

[0052] With the above configuration, when an “L” level signal rst is input to the start-up circuit 53 and the power supply voltage Vcc drops below a voltage corresponding to the threshold Vdssl, the start-up circuit 53 charges the capacitor 33 so that the voltage obtained by dividing the power supply voltage Vcc is between the threshold Vdssh and the threshold Vdssl.

[0053] On the other hand, for example, when the voltage Va from the auxiliary coil L3 increases and the power supply voltage Vcc becomes higher than the threshold Vdssh, the start-up circuit 53 does not charge the capacitor 33. The threshold Vdssl is higher than a predetermined level Voff, which will be described later. The threshold Vdssl corresponds to a "second voltage," and the start-up circuit 53 corresponds to a "second charging circuit."

[0054] ==Resistor 54== Returning to Figure 2, the resistor 54 will be described. One end of the resistor 54 is applied with voltage Vdd from an internal power supply (not shown), and the other end is connected to the terminal FB. A sink current Ia from the phototransistor 38 in Figure 1 flows through the resistor 54, and a feedback voltage Vfb is generated based on the voltage generated across the resistor 54.

[0055] Specifically, when the intensity of light from the light-emitting diode 43 increases, the phototransistor 38 causes a large sink current Ia to flow to the terminal FB. As a result, the voltage generated across the resistor 54 increases, and the feedback voltage Vfb decreases. That is, the feedback voltage Vfb corresponding to the output voltage Vout is applied to the terminal FB.

[0056] ==Drive signal output circuit 55== The drive signal output circuit 55 outputs a drive signal Vq1 whose switching period is changed by the feedback voltage Vfb. The drive signal output circuit 55 includes an oscillation circuit 60, a comparator 61, and an SR flip-flop 62.

[0057] ===Oscillator Circuit 60=== The oscillation circuit 60 generates a timing for turning on the power transistor 30. Specifically, the oscillation circuit 60 outputs an oscillation signal osc_out based on the feedback voltage Vfb. Furthermore, the frequency Fsw of the oscillation signal osc_out is normally set to a predetermined frequency Fsw_norm (e.g., 100 kHz) as shown in FIG. 4, for example, and is set so that the frequency Fsw decreases as the feedback voltage Vfb decreases.

[0058] ===Comparator 61=== 2 generates the timing to turn off the power transistor 30. Specifically, when the power transistor 30 is on and the voltage Vcs becomes the feedback voltage Vfb, the comparator 61 outputs an “H” level signal Vr to turn off the power transistor 30.

[0059] ===SR Flip-Flop 62=== When the oscillation circuit 60 outputs an oscillation signal osc_out of "H" level that turns on the power transistor 30, the SR flip-flop 62 outputs a signal Vq1 of "H" level that turns on the power transistor 30.

[0060] On the other hand, when the comparator 61 outputs a high-level signal Vr, the SR flip-flop 62 outputs a low-level signal Vq1 that turns off the power transistor 30. In this way, the switching period of the drive signal Vq1 is changed according to the frequency Fsw that changes depending on the feedback voltage Vfb.

[0061] ==Driver Circuit 56== The drive circuit 56 outputs a drive voltage Vg via a terminal OUT based on the drive signal Vq1 to drive the power transistor 30.

[0062] Specifically, when a drive signal Vq1 of "H" level is input, the drive circuit 56 outputs a drive voltage Vg which is the power supply voltage Vcc, thereby turning on the power transistor 30. On the other hand, when a drive signal Vq1 of "L" level is input, the drive circuit 56 outputs a drive voltage Vg which is the ground voltage, thereby turning off the power transistor 30.

[0063] When a signal to stop the switching of the power transistor 30 is input from a protection circuit (not shown) that protects the AC-DC converter 10, the drive circuit 56 maintains the drive voltage Vg at the ground voltage and stops the switching of the power transistor 30.

[0064] == Overview of Control Circuit 57 == The control circuit 57 determines whether the power supply voltage Vcc has dropped below a threshold Vvccl (described later) and outputs a signal Son to operate the current output circuit 39a. Specifically, when the power supply voltage Vcc is lower than the threshold Vvccl according to the reference voltage Vref_vcc, the control circuit 57 outputs an "H" level signal Son to cause the current output circuit 39a to charge the capacitor 33 in order to maintain the power supply voltage Vcc at a high level. The thresholds Vvcch and Vvccl will be described below with reference to FIG. 5.

[0065] On the other hand, when the power supply voltage Vcc is higher than the threshold Vvcch according to the reference voltage Vref_vcc, the control circuit 57 outputs a signal Son of "L" level to the current output circuit 39a to stop charging the capacitor 33.

[0066] ==Details of Control Circuit 57== 5, the control circuit 57 includes resistors 80 and 81, a hysteresis comparator 82, an NMOS transistor 83, and a Zener diode 84. The resistors 80 and 81 are connected in series between a node to which the power supply voltage Vcc is applied and the ground, forming a voltage divider circuit. The power supply voltage Vcc is applied to one end of the resistor 80, and the other end is connected to one end of the resistor 81. The other end of the resistor 81 is grounded.

[0067] The voltage at the connection point of the resistors 80 and 81 is applied to the inverting input terminal of a hysteresis comparator 82. The hysteresis comparator 82 also has a non-inverting input terminal to which a reference voltage Vref_vcc is applied.

[0068] The hysteresis comparator 82 generates a high threshold Vvcch based on the reference voltage Vref_vcc and a threshold Vvccl that is lower than the threshold Vvcch. The threshold Vvccl is higher than a predetermined level Voff.

[0069] When the voltage at the connection point of the resistors 80 and 81 becomes lower than the threshold Vvccl, the hysteresis comparator 82 outputs a signal at the "L" level. On the other hand, when the voltage at the connection point of the resistors 80 and 81 becomes higher than the threshold Vvcch, the hysteresis comparator 82 outputs a signal at the "H" level.

[0070] The NMOS transistor 83 has a gate electrode to which a signal from the hysteresis comparator 82 is input, a source electrode to which the NMOS transistor 83 is grounded, and a drain electrode to which the signal Son is output.

[0071] When the hysteresis comparator 82 outputs a high-level signal, the NMOS transistor 83 turns on. In this case, the control circuit 57 outputs a low-level signal Son.

[0072] On the other hand, when the hysteresis comparator 82 outputs a signal of the "L" level, the NMOS transistor 83 is turned off. In this case, the drain electrode of the NMOS transistor 83 is pulled up by a resistor 121 (described later) in the current output circuit 39a, so that the signal Son becomes the "H" level.

[0073] The Zener diode 84 is an element that determines the voltage level of the signal Son when the signal Son is output at "H" level, and determines the voltage when the current output circuit 39a in FIG. 1 charges the capacitor 33 in FIG.

[0074] In other words, the Zener diode 84 is an element that determines the upper limit of the power supply voltage Vcc that occurs across the capacitor 33 when the current output circuit 39a charges the capacitor 33.

[0075] The Zener diode 84 also functions as an element for protecting the NMOS transistor 83 so that the voltage between the drain and source electrodes of the NMOS transistor 83 does not exceed the breakdown voltage of the NMOS transistor 83 .

[0076] The Zener diode 84 is connected in parallel with the NMOS transistor 83 , with its anode grounded and its cathode connected to the drain electrode of the NMOS transistor 83 .

[0077] Also, when NMOS transistor 83 is turned off, Zener diode 84 divertes current from resistor 121 to ground, maintaining the voltage at the drain electrode of NMOS transistor 83 so that NMOS transistor 120 (described below) can be turned on.

[0078] As a result, the control circuit 57 controls the charging of the capacitor 33 by the current output circuit 39a based on the power supply voltage Vcc, thereby preventing the power supply voltage Vcc from falling below the threshold Vvccl. The threshold Vvccl is higher than the threshold Vdssh and the predetermined level Voff.

[0079] The NMOS transistor 83 corresponds to a "first switch" and a "first NMOS transistor," the threshold value Vvccl corresponds to a "first voltage," and the control circuit 57 corresponds to a "determination circuit."

[0080] <<Generation of drive signal Vq1 and change in power supply voltage Vcc due to drive signal Vq1>> 6A and 6B are diagrams illustrating the operation of the drive signal output circuit 55 to generate the drive signal Vq1. First, the operation of the drive signal output circuit 55 to generate the drive signal Vq1 will be described with reference to (A) of FIG.

[0081] In this case, as will be described in detail later, for example, the DC-DC converter 11 in Fig. 1 is in a heavy load state, and the feedback voltage Vfb is high, so the frequency Fsw of the oscillation signal оsc_out becomes the predetermined frequency Fsw_norm shown in Fig. 4.

[0082] Here, "the DC-DC converter 11 is in a heavy load state" refers to, for example, a case where the current value of the load current Iout flowing through the DC-DC converter 11 is greater than a predetermined value (for example, 1 A). On the other hand, "the DC-DC converter 11 is in a light load state" refers to, for example, a case where the current value of the load current Iout flowing through the DC-DC converter 11 is smaller than a predetermined value (for example, 1 A).

[0083] Furthermore, "DC-DC converter 11 is in a no-load state" refers to a case where the current value of the load current Iout flowing through DC-DC converter 11 is extremely small or 0 (zero) A. Furthermore, although it has been described that the current value of the load current Iout for determining whether the DC-DC converter 11 is in a heavy load state or a light load state is 1 A, for example, this current value can be set in various ways.

[0084] The drive signal output circuit 55 generates a drive signal Vq1 to control the ratio of the on period of the power transistor 30 to the switching period determined by the frequency Fsw of the oscillation signal osc_out (that is, to perform PWM control).

[0085] 6A, when the DC-DC converter 11 is in a heavy load state, that is, when the load current Iout increases and the output voltage Vout decreases, the difference between the output voltage Vout and the voltage Vshunt from the constant voltage circuit 42 decreases. As a result, the light from the light-emitting diode 43 weakens, the sink current Ia in FIG. 1 decreases, and the feedback voltage Vfb increases.

[0086] Furthermore, when the signal Sig from the MCU 12 causes the constant voltage circuit 42 to switch the voltage Vshunt from a low voltage Vshunt to a high voltage Vshunt, the difference between the output voltage Vout and the voltage Vshunt from the constant voltage circuit 42 becomes smaller. Therefore, similarly, the light from the light-emitting diode 43 becomes weaker, the sink current Ia decreases, and the feedback voltage Vfb increases.

[0087] 2 outputs an "H" level oscillation signal osc_out, the SR flip-flop 62 outputs an "H" level drive signal Vq1 that turns on the power transistor 30. As the power transistor 30 turns on, the voltage Vcs increases linearly.

[0088] When the power transistor 30 is turned on, the comparator 61 outputs a high-level signal Vr at time t1, at which the voltage Vcs reaches the feedback voltage Vfb. This causes the SR flip-flop 62 to output a low-level drive signal Vq1 that turns off the power transistor 30. As the power transistor 30 turns off, the voltage Vcs also becomes the ground voltage.

[0089] At time t2, when a time corresponding to a switching period according to the predetermined frequency Fsw_norm has elapsed since time t0, the oscillation circuit 60 again outputs the oscillation signal osc_out at the “H” level. Thereafter, the same operation continues.

[0090] In this case, the inductor current IL1 flowing through the primary coil L1 increases, and the inductor current IL2 generated in the secondary coil L2 also increases. The increase in the inductor current IL2 causes the output voltage Vout to rise.

[0091] Since the secondary coil L2 is magnetically coupled to the auxiliary coil L3, the output voltage Vout increases, and the voltage Va of the auxiliary coil L3 also increases. Therefore, in this case, the power supply voltage Vcc is maintained at a high level even if the current output circuit 39a does not operate.

[0092] Next, with reference to (B) of Fig. 6, the operation of the drive signal output circuit 55 to generate the drive signal Vq1 will be described. In this case, as will be described in detail later, for example, the DC-DC converter 11 in Fig. 1 is in a light load state, so the feedback voltage Vfb drops. As a result, the frequency Fsw of the oscillation signal osc_out becomes a predetermined frequency Fsw_light, which is lower than the predetermined frequency Fsw_norm shown in Fig. 4.

[0093] 6(B), when the DC-DC converter 11 is in a light load state, that is, when the load current Iout decreases and the output voltage Vout increases, the difference between the output voltage Vout and the voltage Vshunt from the constant voltage circuit 42 increases. As a result, the light from the light-emitting diode 43 becomes stronger, the sink current Ia in FIG. 1 increases, and the feedback voltage Vfb decreases.

[0094] Furthermore, when the signal Sig from the MCU 12 causes the constant voltage circuit 42 to switch the voltage Vshunt from a high voltage Vshunt to a low voltage Vshunt, the difference between the output voltage Vout and the voltage Vshunt from the constant voltage circuit 42 increases. Therefore, similarly, the light from the light-emitting diode 43 becomes stronger, the sink current Ia increases, and the feedback voltage Vfb decreases.

[0095] In Fig. 6B when the feedback voltage Vfb is low, the drive signal output circuit 55 generates the drive signal Vq1 in the same manner as in Fig. 6A. However, because the frequency Fsw of the oscillation signal osc_out is lower, the switching period corresponding to the predetermined frequency Fsw_light between time t10 and time t12 is longer than in Fig. 6A. In other words, when the DC-DC converter 11 is in a light load state and the load current Iout is smaller, the switching period becomes longer.

[0096] Furthermore, since the feedback voltage Vfb is reduced, the period during which the power transistor 30 is on is reduced compared to the case of FIG. 6(A).

[0097] In this case, the inductor current IL1 flowing through the primary coil L1 decreases, and the inductor current IL2 generated in the secondary coil L2 also decreases. The decrease in the inductor current IL2 causes the output voltage Vout to decrease.

[0098] As described above, the secondary coil L2 is magnetically coupled to the auxiliary coil L3, so that the output voltage Vout decreases and the voltage Va of the auxiliary coil L3 also decreases. Therefore, in this case, unless the current output circuit 39a operates, the power supply voltage Vcc cannot be maintained at a high level.

[0099] On the other hand, when the switching of the power transistor 30 is stopped, the voltage Va of the auxiliary coil L3 also drops, and therefore the power supply voltage Vcc is not maintained at a high level unless the current output circuit 39a operates. In this case, the output voltage Vout also drops, just as when the DC-DC converter 11 is in a heavy load state.

[0100] Therefore, when switching of the power transistor 30 starts, the AC-DC converter 10 increases the output voltage Vout. As a result, when switching of the power transistor 30 starts, the voltage Va of the auxiliary coil L3 increases and the power supply voltage Vcc is also maintained at a high level.

[0101] <<Configuration of current output circuit 39a>> ==Current output circuit 39a== Fig. 7 is a diagram showing an example of the configuration of the current output circuit 39a. When the power supply voltage Vcc drops, the current output circuit 39a charges the capacitor 33 shown in Fig. 1. As will be described in detail later, when the voltage according to the power supply voltage Vcc is lower than the threshold value Vvccl, the current output circuit 39a charges the capacitor 33.

[0102] On the other hand, if the voltage according to the voltage Vcc is higher than Vvcch, the current output circuit 39a does not charge the capacitor 33. The current output circuit 39a includes a boost circuit 90 and a charging circuit 91, as shown in FIG.

[0103] ===Boost circuit 90=== The boost circuit 90 generates a boosted voltage Vchg based on the voltage Va of the auxiliary coil L3. Specifically, the boost circuit 90 generates the boosted voltage Vchg by adding the voltage Va to the voltage charged in the capacitor 101.

[0104] The boost circuit 90 includes a diode 100 and a capacitor 101. The anode of the diode 100 is connected to one grounded end of the auxiliary coil L3, and the cathode is connected to one end of the capacitor 101. The other end of the capacitor 101 is connected to the other end of the auxiliary coil L3, and a voltage Va is applied to it.

[0105] As a result, when the voltage Va becomes negative, the capacitor 101 is charged by a current through the diode 100. On the other hand, when the voltage Va becomes positive, a boosted voltage Vchg is generated in the capacitor 101. The diode 100 corresponds to a "second diode," and the capacitor 101 corresponds to a "second capacitor."

[0106] ===Charging circuit 91=== 1 based on the boosted voltage Vchg from the boost circuit 90 when the power supply voltage Vcc is lower than the threshold Vvccl. Specifically, when the control circuit 57 outputs a high-level signal Son, the charging circuit 91 charges the capacitor 33 based on the boosted voltage Vchg. On the other hand, when the control circuit 57 outputs a low-level signal Son, the charging circuit 91 does not charge the capacitor 33.

[0107] ====Switch Circuit 110 and Diode 111==== The charging circuit 91 includes a switch circuit 110 and a diode 111. The switch circuit 110 connects one end of the capacitor 101 to the diode 111 based on a signal Son. The switch circuit 110 includes an NMOS transistor 120 and a resistor 121.

[0108] The NMOS transistor 120 is provided between one end of the capacitor 101 and the anode of the diode 111. Specifically, the drain electrode of the NMOS transistor 120 is connected to one end of the capacitor 101, and the source electrode is connected to the anode of the diode 111. In addition, a resistor 121 is provided between the drain electrode and gate electrode of the NMOS transistor 120.

[0109] A signal Son is input to the gate electrode of the NMOS transistor 120. The resistor 121 functions as a current limiting resistor for generating a potential at the gate electrode when the NMOS transistor 120 is turned on.

[0110] With the above configuration, when the power supply voltage Vcc is lower than the threshold Vvccl and the control circuit 57 outputs an “H” level signal Son, the switch circuit 110 applies to the anode of the diode 111 a voltage obtained by subtracting the threshold voltage Vth of the NMOS transistor 120 from the voltage of the “H” level signal Son (hereinafter referred to as “voltage (Son-Vth)”).

[0111] On the other hand, when the power supply voltage Vcc is higher than the threshold Vvcch and the control circuit 57 outputs the signal Son at the “L” level, the switch circuit 110 does not apply the voltage (Son−Vth) to the anode of the diode 111 .

[0112] When the voltage (Son-Vth) is applied to the anode, the diode 111 supplies a current to charge the capacitor 33 based on the boosted voltage Vchg because the cathode is connected to the capacitor 33. On the other hand, when the voltage (Son-Vth) is not applied to the anode, the diode 111 does not supply a current to charge the capacitor 33.

[0113] Even if the power supply voltage Vcc is higher than the voltage (Son-Vth), the diode 111 prevents a current based on the power supply voltage Vcc from flowing toward the boost circuit 90.

[0114] As a result, when the power supply voltage Vcc becomes lower than the threshold Vvccl, the current output circuit 39a charges the capacitor 33 with the boosted voltage Vchg. On the other hand, when the power supply voltage Vcc becomes higher than the threshold Vvcch, the current output circuit 39a does not charge the capacitor 33.

[0115] As a result, the control IC 32 maintains the power supply voltage Vcc high so that the start-up circuit 53 does not charge the capacitor 33 and the low-voltage protection circuit 50 does not reset the control IC 32. The diode 111 corresponds to the "third diode," the NMOS transistor 120 corresponds to the "second switch" and the "second NMOS transistor," and the charging circuit 91 corresponds to the "first charging circuit."

[0116] <<Generating power supply voltage Vcc according to feedback voltage Vfb>> First, generation of the power supply voltage Vcc when the DC-DC converter 11 is in a heavy load state and the feedback voltage Vfb is high will be described with reference to Fig. 8. In this case, the control IC 32 can maintain the power supply voltage Vcc higher than the threshold value Vvccl without using the current output circuit 39a, i.e., even when the switch circuit 110 is turned off.

[0117] 7 will not be taken into consideration unless necessary, and the start-up of the AC-DC converter 10 will be completed. Also, the power supply voltage Vcc will be higher than the threshold value Vvccl.

[0118] At time t20, the control IC 32 outputs the drive voltage Vg, which is the power supply voltage Vcc, turning on the power transistor 30. When the power transistor 30 turns on, an inductor current IL1 flows through the primary coil L1.

[0119] Since the primary coil L1 and the auxiliary coil L3 are wound to have opposite polarities, in this case, the voltage Va of the auxiliary coil L3 becomes a negative voltage V1. When the voltage Va becomes the negative voltage V1, the capacitor 101 is charged via the diode 100.

[0120] At this point, the boost voltage Vchg is approximately the ground voltage. Also, since the voltage Va of the auxiliary coil L3 is a negative voltage V1, the capacitor 33 is not charged via the diode 34. Therefore, the power supply voltage Vcc gradually decreases due to the power consumption of the control IC 32.

[0121] At time t21, when the control IC 32 outputs the drive voltage Vg, which is the ground voltage, the power transistor 30 is turned off. When the power transistor 30 is turned off, the inductor current IL1 does not flow through the primary coil L1. In this case, the voltage Va of the auxiliary coil L3 becomes a positive voltage V0.

[0122] When the voltage Va becomes the positive voltage V0, the boosted voltage Vchg becomes the positive voltage V0 minus the negative voltage V1, and the capacitor 33 is charged via the diode 34. Furthermore, because the power supply voltage Vcc is higher than the threshold Vvccl, the switch circuit 110 is turned off. Therefore, the charging circuit 91 does not charge the capacitor 33 with the boosted voltage Vchg.

[0123] After time t22, the same operation is repeated. In the above case, the power transistor 30 is turned on for a long period of time, and the inductor current IL1 flowing through the primary coil L1 is large. In addition, the switching period of the power transistor 30 is also short.

[0124] Therefore, the power supply voltage Vcc can be maintained at a level sufficiently higher than the threshold voltage Vvccl simply by charging the capacitor 33 with the voltage Va. In this case, the start-up circuit 53 does not charge the capacitor 33 via the terminal VCC, so there is no power loss due to the start-up element 51.

[0125] 9, generation of the power supply voltage Vcc when the DC-DC converter 11 is in a light load state and the feedback voltage Vfb is low will be described. In this case, the control IC 32 can maintain the power supply voltage Vcc higher than the threshold value Vdssl by using the current output circuit 39a, i.e., by turning on the switch circuit 110.

[0126] In the following, for the sake of simplicity, the forward voltages of the diodes 100 and 111 in FIG. 7 will not be taken into consideration unless necessary, and it is assumed that the start-up of the AC-DC converter 10 has been completed.

[0127] At time t30, the control IC 32 outputs the drive voltage Vg, which is the power supply voltage Vcc, to turn on the power transistor 30. When the power transistor 30 turns on, an inductor current IL1 flows through the primary coil L1.

[0128] In this case, the voltage Va of the auxiliary coil L3 becomes a negative voltage V3. When the voltage Va becomes the negative voltage V3, the capacitor 101 is charged via the diode 100.

[0129] At this point, the boosted voltage Vchg is equal to the voltage V5 (specifically, the ground voltage minus the forward voltage Vdth of the diode 100). Furthermore, since the voltage Va of the auxiliary coil L3 is a negative voltage V3, the capacitor 33 is not charged via the diode 34. Therefore, the power supply voltage Vcc gradually decreases due to the power consumption of the control IC 32.

[0130] At time t31, when the control IC 32 outputs the drive voltage Vg, which is the ground voltage, the power transistor 30 is turned off. When the power transistor 30 is turned off, the inductor current IL1 does not flow through the primary coil L1. In this case, the voltage Va of the auxiliary coil L3 becomes a positive voltage V2.

[0131] Furthermore, when the voltage Va becomes the positive voltage V2, the boosted voltage Vchg becomes the voltage generated in the capacitor 101 plus the voltage Va (specifically, the positive voltage V2 minus the negative voltage V3). Furthermore, the capacitor 33 is charged via the diode 34.

[0132] At time t32, the voltage Va, although a positive voltage, begins to fluctuate due to the resonance between the primary coil L1 and the parasitic capacitance of the power transistor 30. When the fluctuation begins, the voltage Va drops. The boost voltage Vchg then fluctuates in accordance with the fluctuation of the voltage Va. When the fluctuation of the voltage Va ends, the current based on the voltage Va from the auxiliary coil L3 stops. Thereafter, the power supply voltage Vcc gradually drops due to the power consumption of the control IC 32.

[0133] At time t33 when the power supply voltage Vcc falls below the threshold Vvccl, the control circuit 57 outputs an “H” level signal Son, turning on the NMOS transistor 120 and charging the capacitor 33 with the boosted voltage Vchg. As a result, the power supply voltage Vcc rises and the boosted voltage Vchg falls.

[0134] At time t34 when the power supply voltage Vcc becomes higher than the threshold value Vvcch, the control circuit 57 outputs the signal Son at the "L" level, turning off the NMOS transistor 120 and preventing the capacitor 33 from being charged with the boosted voltage Vchg. Therefore, the power supply voltage Vcc gradually decreases due to the power consumption of the control IC 32, and the boosted voltage Vchg remains constant.

[0135] After time t35, the same operation is repeated. In the above case, the period during which the power transistor 30 is on is short, and the inductor current IL1 flowing through the primary coil L1 is small. Also, the switching period of the power transistor 30 is long.

[0136] Therefore, the voltage Va of the auxiliary coil L3 becomes small, and the power supply voltage Vcc may not be able to be maintained at a state sufficiently higher than the threshold voltage Vvccl by simply charging the capacitor 33 with the voltage Va.

[0137] However, since the capacitor 33 is charged with the boosted voltage Vchg, when the operation from time t30 to time t35 is repeated, the drop in the power supply voltage Vcc is suppressed.

[0138] In this case, due to the operation of the charging circuit 91, the power supply voltage Vcc is lower than the threshold value Vdssl, causing the startup element 51 to operate, and the startup circuit 53 no longer charges the capacitor 33 via the terminal VCC, thereby suppressing power loss due to the startup element 51.

[0139] Finally, referring to Fig. 10, the generation of the power supply voltage Vcc when the AC-DC converter 10 reduces the output voltage Vout will be described. In this case, the feedback voltage Vfb reduces in the same way as when the DC-DC converter 11 is in a light load state. Furthermore, by using the current output circuit 39a, i.e., by turning on the switch circuit 110, the control IC 32 can maintain the power supply voltage Vcc higher than the threshold Vdssl.

[0140] In the following, for the sake of simplicity, the forward voltages of the diodes 100 and 111 in FIG. 7 will not be taken into consideration unless necessary, and it is assumed that the start-up of the AC-DC converter 10 has been completed.

[0141] At time t40, the control IC 32 outputs the drive voltage Vg, which is the power supply voltage Vcc, to turn on the power transistor 30. When the power transistor 30 turns on, an inductor current IL1 flows through the primary coil L1.

[0142] In this case, the voltage Va of the auxiliary coil L3 becomes a negative voltage V3. When the voltage Va becomes the negative voltage V3, the capacitor 101 is charged via the diode 100.

[0143] At this point, the boosted voltage Vchg is equal to the voltage V5 (specifically, the ground voltage minus the forward voltage Vdth of the diode 100). Furthermore, since the voltage Va of the auxiliary coil L3 is a negative voltage V3, the capacitor 33 is not charged via the diode 34. Therefore, the power supply voltage Vcc gradually decreases due to the power consumption of the control IC 32.

[0144] At time t41, when the control IC 32 outputs the drive voltage Vg, which is the ground voltage, the power transistor 30 is turned off. When the power transistor 30 is turned off, the inductor current IL1 does not flow through the primary coil L1. In this case, the voltage Va of the auxiliary coil L3 becomes a positive voltage V6.

[0145] Note that the positive voltage V6 is lower than the positive voltage V0 when the output voltage Vout is high, depending on the output voltage Vout, because the output voltage Vout decreases. In this case, charging the capacitor 33 via the diode 34 makes it impossible to make the power supply voltage Vcc higher than the threshold Vvccl.

[0146] Furthermore, when the voltage Va becomes the positive voltage V6, the boosted voltage Vchg becomes the voltage generated in the capacitor 101 plus the voltage Va (specifically, the positive voltage V6 minus the negative voltage V3).

[0147] At time t42 when the power supply voltage Vcc reaches the threshold value Vvccl, the control circuit 57 outputs an “H” level signal Son, turning on the NMOS transistor 120 and charging the capacitor 33 with the boosted voltage Vchg. As a result, the power supply voltage Vcc rises and the boosted voltage Vchg falls.

[0148] At time t43 when the power supply voltage Vcc becomes higher than the threshold value Vvcch, the control circuit 57 outputs the signal Son at the "L" level, turning off the NMOS transistor 120 and preventing the capacitor 33 from being charged with the boosted voltage Vchg. Therefore, the power supply voltage Vcc gradually decreases due to the power consumption of the control IC 32, and the boosted voltage Vchg remains constant.

[0149] Thereafter, the voltage Va, although a positive voltage, begins to fluctuate due to the resonance between the primary coil L1 and the parasitic capacitance of the power transistor 30. Once the fluctuation begins, the voltage Va drops. The boost voltage Vchg then fluctuates in accordance with the fluctuation of the voltage Va. Once the fluctuation of the voltage Va ends, the current based on the voltage Va from the auxiliary coil L3 stops. The power supply voltage Vcc then gradually drops due to the power consumption of the control IC 32.

[0150] After this, the operations at times t42 and t43 are repeated until time t44. Then, at time t44 when the power transistor 30 is turned on, the boosted voltage Vchg becomes the negative voltage V5, so even if the NMOS transistor 120 is turned on, the current output circuit 39a cannot charge the capacitor 33, and the power supply voltage Vcc temporarily becomes lower than the threshold value Vvccl.

[0151] At time t45 when the power transistor 30 is turned off, the boosted voltage Vchg becomes higher than the voltage that can charge the capacitor 33, so the current output circuit 39a charges the capacitor 33 until the power supply voltage Vcc reaches the threshold Vvcch. After time t45, the operation from time t41 to t45 is repeated.

[0152] === Variations === In the above-described embodiment, the control circuit 57 is configured to include an NMOS transistor 83 and a Zener diode 84. However, as shown in FIG. 11 , the NMOS transistor 83 and the Zener diode 84 may be moved to a current output circuit 39 b and replaced with an NMOS transistor 122 and a Zener diode 123.

[0153] Specifically, when the power supply voltage Vcc is low, the current output circuit 39b charges the capacitor 33 based on the signal Son1 from the control IC 32. The current output circuit 39b includes a boost circuit 90 and a charging circuit 92. In the modified example, components that are assigned the same reference numerals in Figures 10, 11, and 12 as those in the above-described embodiment are the same as those in the above-described embodiment.

[0154] Similar to the charging circuit 91, when the power supply voltage Vcc is low, the charging circuit 92 charges the capacitor 33 based on the boosted voltage Vchg from the boost circuit 90. The charging circuit 92 includes a diode 111 and a switch circuit 112.

[0155] Similar to the switch circuit 110, the switch circuit 112 provides electrical continuity between one end of the capacitor 101 and the diode 111. The switch circuit 112 includes NMOS transistors 120 and 122, a resistor 121, and a Zener diode 123. The functions of the NMOS transistor 122 and the Zener diode 123 are the same as those of the NMOS transistor 83 and the Zener diode 84.

[0156] In this case, the control circuit 57 is modified to be like the control circuit 58 in Fig. 12. The NMOS transistor 122 corresponds to the "first switch" and the "third NMOS transistor", the Zener diode 123 corresponds to the "first Zener diode", and the signal Son1 corresponds to the "determination result".

[0157] In addition, in the above-described embodiment, the current output circuits 39a and 39b are controlled from the control IC 32, but as shown in FIG. 12, the current output circuit 39c may be configured to operate appropriately without being controlled by the control IC 32.

[0158] 13 is a diagram showing an example of the configuration of a current output circuit 39c, which is a modified example of the current output circuit 39a. The current output circuit 39c charges the capacitor 33 when the power supply voltage Vcc is low without receiving a signal from the control IC 32. The current output circuit 39c includes a boost circuit 90 and a charging circuit 93.

[0159] Similar to the charging circuits 91 and 92, when the power supply voltage Vcc is low, the charging circuit 93 charges the capacitor 33 based on the boosted voltage Vchg from the boost circuit 90. The charging circuit 93 includes a diode 111, a switch circuit 112, a Zener diode 113, and a resistor 114. The operations of the Zener diode 113 and the resistor 114 will be described later.

[0160] 13, when the power supply voltage Vcc is higher than a predetermined level, the voltage at the connection point between the Zener diode 113 and the resistor 114 increases, turning on the NMOS transistor 122. As a result, the current output circuit 39c does not charge the capacitor 33 with the boosted voltage Vchg.

[0161] On the other hand, when the power supply voltage Vcc is lower than a predetermined level, the voltage at the connection point between the Zener diode 113 and the resistor 114 decreases, turning off the NMOS transistor 122, causing the current output circuit 39c to charge the capacitor 33 with the boosted voltage Vchg.

[0162] ===Summary=== The AC-DC converter 10 of this embodiment has been described above. The control IC 32 includes a terminal VCC, a terminal FB, a drive signal output circuit 55, a drive circuit 56, and a control circuit 57. The AC-DC converter 10 also includes a boost circuit 90 and a charging circuit 91. In this case, when the power supply voltage Vcc drops, the control IC 32 controls the charging circuit 91 to charge the capacitor 33 with the boosted voltage Vchg. This allows the control IC 32 to suppress a drop in the power supply voltage Vcc. This makes it possible to provide an integrated circuit that suppresses a drop in the power supply voltage.

[0163] The boost circuit 90 also includes a diode 100 and a capacitor 101. The charging circuit 91 also includes a switch circuit 110 and a diode 111. When the voltage Va of the auxiliary coil L3 is a negative voltage, the boost circuit 90 charges the capacitor 101 via the diode 100. When the voltage Va is a positive voltage, the boost circuit 90 generates a boosted voltage Vchg in the capacitor 101. This makes it possible to generate the boosted voltage Vchg with a simple circuit. When the power supply voltage Vcc is lower than the threshold Vvccl, the charging circuit 91 can charge the capacitor 33 with the boosted voltage Vchg.

[0164] Furthermore, the control circuit 57 includes an NMOS transistor 83, and the switch circuit 110 includes a first switch. This allows the current output circuits 39a and 39b to be controlled as to whether or not to charge the capacitor 33 based on the signals Son and Son1 from the control IC 32.

[0165] Furthermore, the control circuit 57 includes a Zener diode 84, and the switch circuit 110 includes an NMOS transistor 120 and a resistor 121. As a result, even if the boosted voltage Vchg becomes too high, the power supply voltage Vcc becomes a voltage corresponding to the Zener voltage of the Zener diode 84 applied to the gate of the NMOS transistor 120 when the NMOS transistor 120 is turned on.

[0166] The switch circuit 112 includes a first switch and a second switch, which allows the control circuit 58 to be configured with a simple circuit.

[0167] The switch circuit 112 also includes NMOS transistors 120 and 122, a resistor 121, and a Zener diode 123. As a result, even if the boost voltage Vchg becomes too high, the power supply voltage Vcc becomes a voltage corresponding to the Zener voltage of the Zener diode 123 applied to the gate of the NMOS transistor 120 when the NMOS transistor 120 is turned on.

[0168] The control IC 32 also includes a terminal VH and a startup circuit 53. The threshold value Vdssl of the hysteresis comparator 77 of the startup circuit 53 is lower than the threshold value Vvccl of the hysteresis comparator 82 of the control circuits 57 and 58. As a result, even if the power supply voltage Vcc drops, the current output circuits 39a and 39b operate, reducing the amount of current from the startup element 51 that charges the capacitor 33. This reduces the power loss caused by the startup element 51.

[0169] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. Furthermore, the present invention may be modified or improved without departing from the spirit thereof, and the present invention includes equivalents thereof. [Explanation of symbols]

[0170] 10 AC-DC converter 11 DC-DC converter 20 Full wave rectifier circuit 21, 24, 33, 35, 37, 41, 101 Capacitors 22 Transformer 23, 31, 36, 52, 54, 73, 75, 76, 80, 81, 114, 121 Resistance 25, 34, 27, 28, 40, 74, 100, 111 diodes 26 Control Blocks 30 Power transistor 38 Phototransistor 39a,39b,39c Current output circuit 42 Constant voltage circuit 43 Light-emitting diode 51 Starting element 53 Starter circuit 50 Low voltage protection circuit 55 Drive signal output circuit 56 Drive circuit 57,58 Control circuit 60 Oscillator Circuit 61 Comparator 62 SR Flip-Flop 70 OR element 71, 83, 120, 122 NMOS transistors 72 PMOS transistors 77,82 Hysteresis Comparator 84,113,123 Zener diode 90 Boost circuit 91,92,93 Charging circuit 110,112 Switch circuit

Claims

1. an integrated circuit for controlling switching of the transistor of a power supply circuit that generates an output voltage from an input voltage and applies the output voltage to a load, the power supply circuit including: a transformer including a primary coil, a secondary coil, and an auxiliary coil; a transistor that controls an inductor current flowing through the primary coil; a first capacitor; and a first diode that rectifies a voltage of the auxiliary coil and charges the first capacitor when the transistor is turned off; a first terminal to which the voltage of the first capacitor is applied as a power supply voltage; a second terminal to which a feedback voltage corresponding to the output voltage is applied; a drive signal output circuit that outputs a drive signal based on the feedback voltage such that a switching period of the transistor is lengthened when a load current flowing through the load becomes smaller; a drive circuit that drives the transistor based on the drive signal; a determination circuit that determines whether the power supply voltage has dropped below a first voltage; a terminal for outputting a determination result of the determination circuit; Including, The power supply circuit includes: a boost circuit that generates a boosted voltage based on the voltage of the auxiliary coil; a first charging circuit connected to the terminal; Including, The first charging circuit When the determination result indicates that the power supply voltage is lower than the first voltage, charging the first capacitor with the boosted voltage. Integrated circuit.

2. 10. The integrated circuit of claim 1, one end of the first capacitor is connected to one end of the auxiliary coil; the first diode charges the first capacitor based on a voltage at the other end of the auxiliary coil; The boost circuit comprises: a second capacitor having one end connected to the other end of the auxiliary coil and generating the boosted voltage; a second diode that charges the second capacitor based on a voltage at one end of the auxiliary coil when the transistor is on; Including, The first charging circuit a third diode having a cathode connected to the first capacitor; a switch circuit that, when the power supply voltage is lower than the first voltage, conducts between the other end of the second capacitor and the anode of the third diode; An integrated circuit comprising:

3. 3. An integrated circuit according to claim 2, the determination circuit includes a first switch that is turned on and off based on whether the power supply voltage is lower than the first voltage; The switch circuit a second switch provided between the other end of the second capacitor and the anode of the third diode, the second switch being turned on and off based on a state of the first switch; Integrated circuit.

4. 4. An integrated circuit according to claim 3, the determination circuit includes a Zener diode connected in parallel to the first switch; the first switch is a first NMOS transistor; the second switch is a second NMOS transistor having a drain connected to a gate of the first NMOS transistor; The switch circuit a resistor connecting the gate of the second NMOS transistor and the other end of the second capacitor; Integrated circuit.

5. 3. An integrated circuit according to claim 2, The switch circuit a first switch that is turned on and off based on the determination result of the determination circuit; a second switch provided between the other end of the second capacitor and the anode of the third diode, the second switch being turned on and off based on the state of the first switch; An integrated circuit comprising:

6. 6. An integrated circuit according to claim 5, the first switch is a third NMOS transistor; the second switch is a second NMOS transistor having a drain connected to a gate of the third NMOS transistor; The switch circuit a first Zener diode connected in parallel to the third NMOS transistor; a resistor connecting the gate of the second NMOS transistor and the other end of the second capacitor; Integrated circuit.

7. An integrated circuit according to any one of claims 1 to 6, a third terminal to which a voltage corresponding to the input voltage is applied; a second charging circuit configured to charge the first capacitor based on a voltage at the third terminal when the power supply voltage drops below a second voltage lower than the first voltage; Integrated circuit.

8. A power supply circuit that generates an output voltage from an input voltage and applies the output voltage to a load, a transformer including a primary coil, a secondary coil, and an auxiliary coil; a transistor for controlling an inductor current flowing through the primary coil; A first capacitor; a first diode that rectifies the voltage of the auxiliary coil and charges the first capacitor when the transistor is turned off; a boost circuit that generates a boosted voltage based on the voltage of the auxiliary coil; a first charging circuit for charging the first capacitor; an integrated circuit that controls the switching of the transistor; The integrated circuit comprises: a first terminal to which the voltage of the first capacitor is applied as a power supply voltage; a second terminal to which a feedback voltage corresponding to the output voltage is applied; a drive signal output circuit that outputs a drive signal whose switching period is changed by the feedback voltage; a drive circuit that drives the transistor based on the drive signal; a determination circuit that determines whether the power supply voltage has dropped below a first voltage; a terminal for outputting a determination result of the determination circuit; Including, The first charging circuit When the determination result indicates that the power supply voltage is lower than the first voltage, the first capacitor is charged with the boosted voltage. power circuit.

9. A power supply circuit that generates an output voltage from an input voltage and applies the output voltage to a load, a transformer including a primary coil, a secondary coil, and an auxiliary coil; a transistor for controlling an inductor current flowing through the primary coil; A first capacitor; a first diode that rectifies the voltage of the auxiliary coil and charges the first capacitor when the transistor is turned off; a determination circuit that determines whether the voltage of the first capacitor has dropped below a first voltage; a boost circuit that generates a boosted voltage based on the voltage of the auxiliary coil; a first charging circuit that charges the first capacitor with the boosted voltage when the determination result of the determination circuit indicates that the voltage of the first capacitor is lower than a first voltage; an integrated circuit that controls the switching of the transistor; The integrated circuit comprises: a first terminal to which the voltage of the first capacitor is applied as a power supply voltage; a second terminal to which a feedback voltage corresponding to the output voltage is applied; a drive signal output circuit that outputs a drive signal whose switching period is changed by the feedback voltage; a drive circuit that drives the transistor based on the drive signal; Including, power circuit.

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