Switching power supply device and semiconductor device for power supply control

Abstract

The invention provides a switching power supply device and a semiconductor device for power supply control. The power supply control IC outputs a signal for on / off control of the switching element according to an output detection signal from the secondary side of the transformer, and is provided with: a first external terminal to which a voltage corresponding to the output detection signal is input; a second external terminal to which a voltage proportional to the current flowing through the primary side winding is input; a periodic signal generation circuit that generates a periodic signal for periodically turning on the switching element; a current detection circuit that generates an off timing signal on the basis of the voltages of the first external terminal and the second external terminal; and a correction current circuit for generating a correction current to be output to a second external terminal connected to an external correction resistor, the correction current circuit outputting the correction current in a light load state, and the periodic signal generation circuit generating a periodic signal to the first external terminal by using a predetermined reference value set for the voltage of the first external terminal as a boundary. A periodic signal is generated or output of the periodic signal is stopped.

Description

Technical Field

[0001] The present invention relates to an insulating switching power supply device including a voltage conversion transformer and a power supply control semiconductor device used in the switching power supply device. Background Art

[0002] In a DC power supply device, there is a switch-controlled AC-DC converter (switching power supply device), which controls the current flowing through the primary winding by turning on and off the switching element connected in series with the primary winding of the voltage conversion transformer, thereby controlling the voltage induced in the secondary winding. In addition, in a switch-controlled AC-DC converter, for the control operation of the primary side, a resistor for current detection is provided in series with the switching element on the primary side, and a terminal (current detection terminal) to which the voltage after current-voltage conversion by the resistor is input is provided in the power supply control circuit (IC). And, the peak current of the primary winding is controlled based on the detected current value and the feedback voltage from the secondary side, so as to maintain the output voltage or output current of the secondary side in a fixed manner.

[0003] Furthermore, in an AC-DC converter configured to control the output based on a voltage obtained by current-to-voltage conversion using a resistor for current detection and a feedback voltage from the secondary side, in order to reduce switching losses at light loads and improve power efficiency, control is performed in a PWM manner in medium to heavy load regions, and on the other hand, switching is temporarily stopped in a light load region, thereby performing a sudden operation to output power corresponding to the light load.

[0004] In addition, the applicant has made the following invention and previously filed a patent application: an external terminal for connecting an external resistor is newly provided in a power supply control semiconductor device (power supply control IC) constituting a switching power supply device, and by changing the resistance value of the external resistor connected to the external terminal, a user can easily change the switching frequency control characteristics through external settings in order to cope with noise, or can easily change the reference value of the feedback terminal for stopping and restarting the switch (see patent document 1).

[0005] However, the invention of Patent Document 1 requires the addition of an external terminal (ADJ) for connecting an external resistor, wherein the external resistor is used to change the frequency control characteristics or settings. Therefore, when applied to a semiconductor device for power supply control that does not have a surplus number of terminals, it is necessary to change the package, which leads to a problem of increased costs. In addition, the switching power supply device of Patent Document 1 has the following problems: power efficiency and output ripple are in a trade-off relationship. If the power efficiency is improved, the output ripple increases, and if the output ripple is reduced, the power efficiency decreases.

[0006] In addition, Patent Document 2 discloses an invention related to a switching power supply device, which is configured to allow a correction current to flow through a correction resistor connected to a detection terminal provided in a power supply control semiconductor device, similarly to the invention of the present application. However, the subject of the invention of Patent Document 2 is that in a power supply control semiconductor device having an overcurrent detection circuit and configured to be able to switch output voltages, an overcurrent limit value can be appropriately set according to the output voltage through an external correction resistor, which is different from the subject of the invention of the present application. Due to the difference in subject, the characteristics of the correction current flowing through the correction resistor are also different in the invention of Patent Document 2 and the invention of the present application.

[0007] Patent Document 1: Japanese Patent No. 6424644

[0008] Patent Document 2: Japanese Patent No. 7032648 Summary of the invention

[0009] The present invention is made with a view to the above-mentioned problems, and its object is to provide a switching power supply device and a semiconductor device for power supply control constituting the primary side of the switching power supply device, wherein the switching power supply device does not need to newly set up an external terminal for connecting an external resistor, and in particular, can change the switching frequency control characteristics in a light load region by external settings.

[0010] Another object of the present invention is to provide a switching power supply device and a power supply control semiconductor device, which can easily adjust the relationship between power efficiency and output ripple, which are in a trade-off relationship, by changing the resistance value of an external resistor.

[0011] To achieve the above object, the switching power supply device of the present invention comprises:

[0012] A transformer for voltage conversion, which applies a DC voltage to the primary winding;

[0013] a switching element connected in series with the primary winding;

[0014] a semiconductor device for power supply control, which generates and outputs a control signal for controlling the switching element to be turned on or off; and

[0015] a current-voltage conversion element connected in series with the switching element to convert the current flowing through the primary winding into a voltage,

[0016] The switching element is controlled to be turned on and off so that current intermittently flows through the primary winding, and the current induced in the secondary winding of the transformer is rectified and smoothed to output a DC voltage.

[0017] The power supply control semiconductor device comprises:

[0018] a first external terminal to which a voltage or current corresponding to an output detection signal from a secondary side of the transformer is input;

[0019] A second external terminal, to which the voltage converted by the current-voltage conversion element is input;

[0020] a periodic signal generating circuit for generating a periodic signal for providing a timing for periodically turning on the switching element;

[0021] a current detection circuit that generates a signal for providing a timing for turning off the switching element based on the voltage or current of the first external terminal and the voltage of the second external terminal; and

[0022] a correction current circuit that generates a correction current to be output to the second external terminal,

[0023] A correction resistor is provided between the second external terminal and the current-voltage conversion element.

[0024] According to the above structure, since there is a correction current circuit that outputs a correction current to an external terminal (current detection terminal CS) to which a voltage proportional to the current flowing through the switching element is input, the voltage of the external terminal (CS) is shifted to a higher side by a voltage corresponding to the product of the current value of the output correction current and the resistance value of the external resistor (correction resistor) connected to the current detection terminal, thereby the change of the disconnection timing signal output from the current detection circuit becomes earlier, and the switching cycle changes. Therefore, it is not necessary to provide an external terminal (ADJ) connected to an external resistor as in the invention of Patent Document 1. Therefore, the switching frequency control characteristics can be changed by external settings (resistance value of the external correction resistor) without increasing the number of external terminals.

[0025] Furthermore, by changing the resistance value of the external resistor connected to the external terminal, the power efficiency and output ripple, which are in a trade-off relationship, can be easily adjusted, thereby allowing the designer of the power supply device to freely determine which characteristic of the switching power supply device to prioritize.

[0026] According to the present invention, in an isolated switching power supply device that has a transformer for voltage conversion and controls the output by turning on and off the current flowing through the primary winding, and a current control semiconductor device constituting the primary side of the isolated switching power supply device, it is not necessary to newly provide an external terminal to which an external resistor is connected, and in particular, the switching frequency control characteristics in a light load region can be changed by external settings. In addition, there is an effect that by changing the resistance value of the external resistor connected to the external terminal to which a voltage proportional to the current flowing through the switching element is input, the power efficiency and the output ripple that are in a trade-off relationship can be easily adjusted. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a circuit configuration diagram showing one embodiment of a DC-DC converter as a DC power supply device that is effective as a switching power supply semiconductor device to which the present invention is applied.

[0028] Figure 2 It means in Figure 1 A functional block diagram of an embodiment of a semiconductor device for a switching power supply of the present invention provided on the primary side of a transformer in a DC-DC converter.

[0029] Figure 3 It is a circuit configuration diagram showing a specific example of a light load determination circuit constituting the switching power supply semiconductor device of the embodiment.

[0030] Figure 4 It is a circuit configuration diagram showing a specific example of a cycle timer circuit constituting a semiconductor device for a switching power supply according to an embodiment.

[0031] Figure 5 It is a circuit configuration diagram showing a specific example of a cutoff circuit (current detection circuit) constituting the semiconductor device for a switching power supply of the embodiment.

[0032] Figure 6 1 is a circuit configuration diagram showing a configuration example of a correction current circuit constituting a semiconductor device for a switching power supply according to an embodiment.

[0033] Figure 7 (A) is a graph showing the relationship between the feedback voltage and the period of the periodic signal in the power supply control semiconductor device of the embodiment, and (B) is a graph showing the relationship between the feedback voltage, the switching frequency, and the correction current.

[0034] Figure 8 This is a graph showing the relationship between the output current and the average switching frequency in the semiconductor device for power supply control according to the embodiment.

[0035] Fig. 9 This is a timing chart of PWM operation at medium to heavy loads when a small resistance element is used as a correction resistor in the power supply control semiconductor device of the embodiment.

[0036] Fig.10 This is a timing chart of PWM operation under medium to heavy loads when a large resistance element is used as the correction resistor.

[0037] Fig.11 This is a timing chart during sudden operation (light load) when a small resistance element is used as a correction resistor in the power supply control semiconductor device of the embodiment.

[0038] Fig.12 This is a timing chart of sudden operation (light load) when a large resistance element is used as the correction resistor.

[0039] Fig.13 This is a graph showing the relationship between the output current and the magnitude of the output ripple when the resistance value of the correction resistor is large and when the resistance value is small in the semiconductor device for power supply control according to the embodiment.

[0040] Fig.14 This is a graph showing the relationship between the output current and the efficiency when the resistance value of the correction resistor is large and when the resistance value is small in the semiconductor device for power supply control according to the embodiment. DETAILED DESCRIPTION

[0041] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

[0042] Figure 1 This is a circuit configuration diagram showing one embodiment of an isolated DC-DC converter as a DC power supply device to which the power supply control semiconductor device of the present invention is applied.

[0043] The insulated DC-DC converter of this embodiment has: a pair of voltage input terminals 11 for inputting a DC voltage; a transformer 12 for voltage conversion having a primary winding Np, a secondary winding Ns, and an auxiliary winding Nb; a switching transistor SW1 connected in series with the primary winding Np of the transformer 12; and a switching power supply semiconductor device (hereinafter referred to as a power supply control IC) 13 for turning on and off the switching transistor SW1. When constituting a switching power supply device (AC-DC converter), a diode bridge circuit and a smoothing capacitor for rectifying an AC voltage from an AC power supply are connected to the front stage of the input terminal 11.

[0044] In the present embodiment, the switching transistor SW1 is formed of an N-channel MOSFET (insulated gate field effect transistor) as a discrete component. The power supply control IC 13 is provided with an output terminal GATE of a gate drive signal for driving the gate of the transistor SW1.

[0045] In addition, in the DC-DC converter of the present embodiment, a rectifying and smoothing circuit is provided on the primary side of the transformer 12. The rectifying and smoothing circuit is composed of a rectifying diode D1 connected in series with the auxiliary winding Nb, and a smoothing capacitor C1 connected between the cathode terminal of the diode D1 and the grounding point GND. The voltage rectified and smoothed by the rectifying and smoothing circuit is applied to the power supply voltage terminal VDD of the power supply control IC13.

[0046] The power supply control IC 13 is provided with an external terminal FB to which a phototransistor (light receiving element) PT is connected. The phototransistor PT constitutes a photocoupler for transmitting an output detection signal on the secondary side to the primary side as a feedback voltage VFB.

[0047] In addition, the power supply control IC 13 is provided with an external terminal CS as a current detection terminal, and a voltage Vcs after current-voltage conversion by a current detection resistor Rs connected between the source terminal of the switching transistor SW1 and the ground point GND is input to the external terminal CS. In addition, a correction resistor Rc described in detail later is connected between the external terminal CS and a connection node between the switching element SW1 and the current detection resistor Rs.

[0048] On the other hand, a rectifying diode D2 connected in series with the secondary winding Ns and a smoothing capacitor C2 connected between the cathode terminal of the diode D2 and the other terminal of the secondary winding Ns are provided on the secondary side of the transformer 12. The AC voltage induced in the secondary winding Ns by intermittently passing a current through the primary winding Np is rectified and smoothed, thereby generating and outputting a DC voltage Vout. The primary winding Np and the secondary winding Ns of the transformer 12 have opposite polarities, and are a flyback converter.

[0049] Moreover, a constant voltage control circuit (parallel regulator) 14 and a light emitting diode (light emitting element) PD are provided on the secondary side of the transformer 12, wherein the constant voltage control circuit 14 constitutes an output voltage detection circuit for detecting the output voltage Vout, and the light emitting diode PD constitutes a photocoupler for transmitting an output voltage detection signal corresponding to the detection voltage of the constant voltage control circuit 14 to the primary side.

[0050] In the light emitting diode PD, a current corresponding to the detection voltage flows through the constant voltage control circuit 14, and is transmitted to the primary side as a light signal having an intensity corresponding to the detection voltage. As a result, a current corresponding to the light intensity flows through the phototransistor PT, and is pulled up by the pull-up resistor ( Figure 2 Rp) etc. is converted into feedback voltage VFB and input.

[0051] The constant voltage control circuit 14 is configured to include a bipolar transistor TR1 connected in series with a light emitting diode PD, resistors R3 and R4 for dividing the output voltage Vout on the secondary side, an error amplifier AMP0 for comparing the divided voltage with a reference voltage Vref0 and outputting a voltage corresponding to the potential difference, and a phase compensation circuit 14a, and the output voltage of the error amplifier AMP0 is applied to the base terminal of the transistor TR1, so that a current corresponding to the output voltage Vout flows. In this embodiment, the higher the output voltage Vout on the secondary side, the more current flows through the light emitting diode PD, thereby the more current flows through the phototransistor PT, and the lower the voltage VFB of the external terminal FB of the power supply control IC13.

[0052] In addition, in the present embodiment, the secondary side circuit uses the rectifying diode D2, but a synchronous rectification method in which the rectifying diode D2 is replaced with a switch may be adopted.

[0053] Next, use Figure 2 to Figure 6 right Figure 1 The functional block structure of an embodiment of IC13 for primary-side power control in a power supply device and the function of each block are described.

[0054] The power supply control IC 13 of this embodiment is configured to be able to perform actions based on PWM control (hereinafter referred to as PWM actions) and burst actions according to the weight of the load. Figure 2 As shown, the power supply control IC13 of this embodiment has: a light load determination circuit 31, which is used to determine the weight of the load; a cycle timer circuit 32, which generates a cycle signal PS that provides a switching cycle; and a cut-off circuit 33 composed of a current detection circuit, which takes the voltage VFB of the above-mentioned external terminal FB and the voltage Vcs of the current detection terminal CS as inputs to generate a timing signal for cutting off the switch element SW1.

[0055] Among these functional circuits, the period timer circuit 32 is configured to generate a period signal PS having a period corresponding to the voltage VFB of the external terminal FB to which the feedback signal from the secondary side is input.

[0056] In addition, the power supply control IC 13 of the present embodiment includes a correction current circuit 34, which causes the correction current Ics to flow to the correction resistor Rc connected to the current detection terminal CS. The correction current Ics output from the correction current circuit 34 flows to the current detection resistor Rs via the correction resistor Rc. As a result, the voltage Vcs of the current detection terminal CS is shifted (increased) to the higher side by an amount corresponding to the magnitude of the correction current Ics and the correction resistor Rc.

[0057] In addition, the power supply control IC13 includes: a latch circuit 35 composed of an RS trigger, etc., which takes the periodic signal PS output from the above-mentioned timer circuit 32 and the output signal TOFF of the cut-off circuit 33 as input; and a drive circuit 36, which generates a gate drive signal (drive pulse) for turning on and off the above-mentioned switching element SW1 according to the gate control signal GCS output from the latch circuit 35 and outputs it from the external terminal GATE.

[0058] The gate control signal GCS is supplied to the cycle timer circuit 32 as a signal for operating the cycle timer circuit 32. The power supply control IC 13 is provided with an internal power supply circuit 37 composed of a voltage regulator such as a series regulator for supplying an operating voltage to each of the functional circuits.

[0059] like Figure 3 As shown, the light load determination circuit 31 among the functional blocks 31 to 36 constituting the power supply control IC 13 includes: a comparator (voltage comparison circuit) CMP1 that compares the voltage VFB of the external terminal FB with a predetermined reference voltage VREF1 or reference voltage VREF2 (VREF1 < VREF2); and switches S1 and S2 that selectively supply the reference voltage VREF1 or reference voltage VREF2 to the non-inverting input terminal of the comparator CMP1. Furthermore, the output of the comparator CMP1 is supplied as an oscillation stop signal OSS to a period timer circuit 32 of a subsequent stage.

[0060] In addition, the switch S1 is turned on and off by the signal obtained by inverting the output of the comparator CMP1 by the inverter INV1, and the switch S2 is turned on and off by the output signal of the comparator CMP1. Thus, a hysteresis is given to the determination process of whether it is a light load, and when the voltage VFB is near the reference voltage VREF1, the output of the comparator CMP1 can be prevented from repeating a high level and a low level in a short time. However, the hysteresis is not necessary.

[0061] like Figure 4 As shown, the cycle timer circuit 32 has: an AND gate G1, which takes as input the gate control signal GCS output from the latch circuit 35 and the signal after the control signal GCS is delayed and inverted by the delay circuit DLY composed of a series inverter; and an OR gate G2, which takes as input the output of the AND gate G1 and the oscillation stop signal OSS output from the above-mentioned light load determination circuit 31.

[0062] In addition, the cycle timer circuit 32 includes: a variable current source VCS1, which flows a current corresponding to the voltage VFB of the external terminal FB; a capacitor C3, which is charged by the current from the current source VCS1; and a discharge switching transistor S3, which is used to discharge the charge of the capacitor C3 and apply the output of the OR gate G2 to the gate terminal of the switching transistor S3.

[0063] Furthermore, the cycle timer circuit 32 includes: a comparator CMP2 that compares the charging voltage of the capacitor C3 with a predetermined reference voltage Vref1; and an AND gate G3 that receives the output of the comparator CMP2 and the oscillation stop signal OSS output from the light load determination circuit 31 as inputs. Furthermore, when the oscillation stop signal OSS is set to a low level, the AND gate G3 is opened by the output of the inverter INV2, and the output of the comparator CMP2 is output as the cycle signal PS. On the other hand, when the oscillation stop signal OSS is set to a high level, the AND gate G3 is closed by the output of the inverter INV2, and the output of the cycle signal PS is cut off. Furthermore, when the oscillation stop signal OSS is set to a high level, the output of the OR gate G2 is fixed to a high level, and the oscillation operation of the cycle timer circuit 32 is stopped.

[0064] Furthermore, when the load is light, the cycle timer circuit 32 is stopped by the oscillation stop signal OSS. Then, when the oscillation stop signal OSS changes and the cycle timer circuit 32 is activated, the switching element SW1 is repeatedly turned on and off until the cycle timer circuit 32 is stopped again. (See Fig.11 , Fig.12 ).

[0065] exist Figure 4 In the cycle timer circuit 32, when the gate control signal GCS changes from a low level to a high level, the output of the AND gate G1 changes to a high level in a predetermined time (delay time of the delay circuit DLY), the switch transistor S3 is turned on, and the charge of the capacitor C3 is discharged. In addition, when the gate control signal GCS changes to a low level and the switch transistor S3 is turned off, the charging of the capacitor C3 starts, and when a predetermined time determined by the capacitance value of the capacitor C3 and the current value of the variable current source VCS1 has passed, the charging voltage of the capacitor C3 reaches Vref1, and the output of the comparator CMP2 changes from a low level to a high level.

[0066] Thus, the comparator CMP2 outputs a period signal PS having a period corresponding to the voltage VFB. Therefore, it can be said that the period timer circuit 32 includes an oscillation circuit with a variable frequency. Figure 7 (A) shows an example of the relationship between the voltage VFB of the external terminal FB and the period Tps of the periodic signal PS.

[0067] Figure 5 FIG. 2 shows a specific circuit configuration example of the cutoff circuit 33 formed by the current detection circuit. Figure 5 As shown, the cutoff circuit 33 includes: an operational amplifier AMP1 that inputs the voltage Vcs of the external terminal CS to the non-inverting input terminal; and a comparator CMP3 that inputs the output of the operational amplifier AMP1 to the non-inverting input terminal and inputs the voltage VFB of the external terminal FB to the inverting input terminal. In addition, resistors R5 and R6 are connected in series between the output terminal of the operational amplifier AMP1 and the ground point, and the voltage divided by the resistors R5 and R6 is input to the inverting input terminal of the operational amplifier AMP1.

[0068] Thus, the operational amplifier AMP1 operates as a non-inverting amplifier circuit having an amplification factor (K1) corresponding to the resistance ratio of the resistors R5 and R6, and outputs a signal amplified to K1×Vcs. Furthermore, if the voltage (K1×Vcs) output from the operational amplifier AMP1 exceeds the voltage VFB of the external terminal FB, the output of the comparator CMP3 changes from a low level to a high level. This output is supplied to the latch circuit 35 as a cutoff signal TOFF, and the switch element SW1 is turned off.

[0069] Figure 6 A specific circuit configuration example of the correction current circuit 34 is shown.

[0070] Figure 6 The correction current circuit 34 shown has an inverting amplifier circuit 41, which has an input resistor R7, an operational amplifier AMP2, and a feedback resistor R8, and amplifies the voltage VFB of the external terminal FB; and a clamp circuit 42, which has a diode D3 and a constant voltage source CVS, and clamps the output of the inverting amplifier circuit 41 input through the resistor R9. In addition, the correction current circuit 34 of this embodiment has a voltage-current conversion circuit 43, which has an operational amplifier AMP3, which inputs the voltage passed through the clamp circuit 42 to the non-inverting input terminal; and MOS transistors M1, M2 and a resistor R10 connected in series between the power supply voltage terminal and the ground point. The clamp circuit 42 can regulate the correction current Ics1 in the light load region where the voltage VFB of the external terminal FB is low.

[0071] Furthermore, the correction current circuit 34 of this embodiment has: a current output circuit 44, which has a MOS transistor M3 connected to the MOS transistor M1 of the voltage-current conversion circuit 43 by current mirror connection and a MOS transistor M4 whose drain is connected to the transistor M3; and a bias circuit 45, which applies a predetermined voltage to the gate terminal of the transistor M4. The bias circuit 45 has a diode-connected MOS transistor M5 and a resistor R11, which are arranged in series between the power supply voltage terminal and the ground point, and the gate terminal of the MOS transistor M5 is connected to the gate terminal of the transistor M4 of the current output circuit 44. Thus, the current output circuit 44 outputs a current corresponding to the potential difference between the voltage applied to the gate terminal of the transistor M3 from the voltage-current conversion circuit 43 and the voltage applied to the gate terminal of the transistor M4 from the bias circuit 45, as the correction current Ics.

[0072] By using the correction current circuit 34 having the above structure, as Figure 7 As shown by the solid line in (B), a correction current Ics having the following characteristics is generated and flows to the correction resistor Rc outside the IC connected to the current detection terminal CS. The characteristic is that the correction current Ics is large in the area where the voltage VFB is low, is small in the area where the voltage VFB is high, and changes relatively rapidly in the middle.

[0073] from Figure 7 As can be seen from the solid line of (B), when the voltage VFB of the external terminal FB is the reference voltage VREF2, a relatively large current (first reference current Ics1) flows to the correction resistor Rc outside the IC. On the other hand, when the voltage VFB of the external terminal FB is the reference voltage VREF3, a relatively small current (second reference current Ics2) flows to the correction resistor Rc outside the IC. That is, Ics1>Ics2.

[0074] In the possession Figure 6 In the power supply control IC 13 of the present embodiment having the correction current circuit 34, in a light load state where the voltage VFB of the external terminal FB is lower than the reference voltage VREF2, the voltage Vcs of the current detection terminal CS increases by (correction current Ics×correction resistance Rc). Therefore, compared with a case where there is no correction current Ics, the output (K1×Vcs) of the operational amplifier AMP1 reaches the voltage VFB of the external terminal FB more quickly, and thus the rising timing of the off signal TOFF output from the current detection circuit 33 is advanced.

[0075] As a result, the energy injected into the transformer by one turn-on of the switching element SW1 becomes smaller, and the average switching frequency becomes higher than when the correction current Ics does not flow. However, the ripple of the output voltage Vout becomes smaller. On the other hand, in the medium to heavy load region where the voltage VFB of the external terminal FB is higher than the reference voltage VREF3, the correction current Ics becomes smaller, so the correction current Ics has little effect on the operation of the IC.

[0076] Hereinafter, the operation of the power supply control IC 13 according to the present embodiment based on the correction current Ics and the correction resistor Rc will be described in detail.

[0077] Figure 7 (B) also shows the relationship between the voltage VFB of the external terminal FB and the switching frequency Fsw in the power supply control IC 13 of this embodiment. Figure 7 In (B), the dotted line represents the relationship between the voltage VFB and the switching frequency Fsw.

[0078] The correction resistor Rc is selected in the range of 0 to 5 kΩ, and the switching frequency Fsw (the inverse of the switching period) is set to be small when VFB is low and large when VFB is high.

[0079] The correction voltage VCOMP based on the correction current Ics and the correction resistor Rc is represented by VCOMP=Rc×Ics. Here, if the current flowing through the current detection resistor Rs connected in series with the switch element SW1 is set to Ip, the input VLIM of the current detection circuit 33 when the current peak is turned on is VLIM=Rs×Ip+VCOMP=Rs×Ip+Rc×Ics. Since VREF1=VREF2 is assumed, the reference voltage of the current detection circuit 33 during the sudden change is represented by VREF1 / K1 (K1 is the amplification factor of the current detection circuit 33).

[0080] From the above, it can be seen that during the sudden action, the current Ip flowing through the current detection resistor Rs is Ip=(VLIM-VCOMP) / Rs≈(VREF1 / K1-Rc×Ics) / Rs, and the larger the Rc, the smaller the Ip.

[0081] In addition, Figure 8 The dotted line and solid line in FIG. 1 respectively represent the relationship between the output current Iout and the average switching frequency Fsw when the correction resistor Rc is large and small. Figure 8 It can be seen that in the burst operation region where the output current Iout is small, the larger the correction resistance Rc, the higher the average switching frequency. Therefore, when the correction resistance Rc is large, the average switching frequency becomes high, so the efficiency decreases, but the output ripple can be reduced.

[0082] On the other hand, when the load is heavy (= high VFB) without sudden action, the correction current Ics is not needed. Therefore, if the correction current Ics is reduced as in this embodiment, the loss of the power supply control IC13 is reduced by the amount of the correction current Ics reduction, and the efficiency becomes higher. For example, when the correction current Ics2 is set to 1 / 50 of Ics1, the correction voltage VCOMP during heavy load becomes 1 / 50 of that during sudden action, and the correction resistor dependence of the load-frequency characteristic during heavy load can be suppressed. That is, the correction resistor Rc has little effect on the action during heavy load, and the correction resistor Rc can be used to adjust only the sudden action.

[0083] In addition, the power supply control IC 13 of the present embodiment is provided with a correction current circuit 34, so that in a normal state, the correction current Ics is output to the current detection terminal CS according to the load, but when an open circuit abnormality occurs in which the wiring to the current detection terminal CS is cut off, the correction current Ics does not flow. Therefore, by providing a circuit (for example, a voltage comparison circuit) that detects whether the correction current flows according to the potential of the current detection terminal CS, the open circuit abnormality of the terminal CS is detected. When the abnormality is detected, for example, the latch circuit 35 is reset to make the switch element SW1 in an off state, or the cycle timer circuit 32 is stopped, thereby preventing abnormal heating of the power supply device. In addition, an external terminal for outputting a signal for notifying the abnormality to the outside when the open circuit abnormality of the current detection terminal CS is detected can also be provided.

[0084] In addition, even if the open circuit abnormality detection circuit described above is not provided, for example, an overcurrent protection function is sometimes provided in a power supply control IC constituting a switching power supply. When the voltage Vcs of the current detection terminal CS becomes higher than a predetermined potential, the overcurrent protection function detects an overcurrent state, stops the switching action, or puts the switching element into an open state (for example, Patent Document 2).

[0085] In the power supply control IC with overcurrent protection function as described above, when an open circuit abnormality occurs at the terminal CS, the voltage Vcs of the current detection terminal CS is pulled up to a potential higher than the overcurrent detection level by the correction current circuit 34, and the overcurrent protection function takes effect, and the switching operation can be stopped or the switch element SW1 can be turned off. Therefore, as a modified example of this embodiment, a power supply control IC with overcurrent protection function is considered.

[0086] Furthermore, the overvoltage protection function of the power supply control IC may be utilized to stop the switching operation or turn off the switch element SW1 when an open circuit abnormality occurs at the terminal CS.

[0087] Next, use Figures 9 to 14The switching power supply device ( Figure 1 ) shows the difference in operation and characteristics when a small resistance element is used as the correction resistor Rc and when a large resistance element is used.

[0088] exist Fig. 9 The timing diagram of the PWM operation under medium to heavy loads when a small resistance element is used as the correction resistor Rc is shown in FIG. Fig.10 1 is a timing chart of PWM operation at medium to heavy loads when a large resistance element is used as the correction resistor Rc. In the figure, ID is the waveform of the drain current flowing through the switching element SW1.

[0089] Will Fig. 9 and Fig.10 The waveform of the drain current ID is compared with that of the device with a large resistance value. Fig.10 In contrast, the use of a small resistance element as the correction resistor Rc Fig. 9 The peak of ID (drain current peak) is higher. In addition, when the drain current peak is high, the energy injected into the primary winding of the transformer is large during the one-time switching of the switching element SW1, so the average switching frequency becomes lower, thereby suppressing the so-called switching loss associated with the switching element SW1's switching on and off action, and improving efficiency.

[0090] In addition, Fig.11 The timing diagram of the sudden action at light load when using a small resistance element as the correction resistor Rc is shown in FIG. Fig.12 FIG. 2 shows a timing diagram of sudden operation at light load when a large resistance element is used as the correction resistor Rc. Fig.11 and Fig.12 By comparing the waveform of the output voltage Vout with that of the device with a small resistance value, Fig.11 In contrast, the use of a large resistance element as the correction resistor Rc Fig.12 The switching stop period (the period of the high level of the oscillation stop signal OSS) becomes shorter, and ΔVout indicating the size of the output ripple becomes smaller.

[0091] exist Fig.13 In the figure, the relationship between the output current Iout and the output ripple ΔVout when the resistance value of the correction resistor Rc is small and large is shown for the switching power supply using the power supply control IC 13 of the present embodiment. Fig.14 2 shows the relationship between the output current Iout and the power efficiency when the resistance value of the correction resistor Rc is large and small.

[0092] exist Fig.13and Fig.14 In FIG. 1 , the solid line represents the characteristic when the resistance value of the correction resistor Rc is small, and the dotted line represents the characteristic when the resistance value of the correction resistor Rc is large. Fig.13 It can be seen that in the area where the output current Iout is small, the larger the correction resistance Rc is, the smaller the output ripple is. Fig.14 It can be seen that in a region where the output current Iout is small, the smaller the correction resistance Rc is, the higher the power efficiency is.

[0093] According to the above, when designing a switching power supply device that prioritizes suppressing the ripple of the output voltage over power efficiency, a component with a large resistance value is used as the correction resistor Rc connected to the current detection terminal CS. In addition, on the contrary, it can be seen that when designing a switching power supply device that prioritizes power efficiency over suppressing the ripple of the output voltage, a component with a small resistance value can be used as the correction resistor Rc. Therefore, according to the power supply control IC of the above embodiment, the relationship between power efficiency and output ripple, which are in a trade-off relationship, can be easily adjusted by the external correction resistor Rc without changing the IC used.

[0094] Next, the difference between the phenomenon produced when the adjustment resistor (Rt) connected to the external terminal (ADJ) is switched in the power control IC described in the above-mentioned patent document 1 and the phenomenon produced when the adjustment resistor (Rc) connected to the external terminal (CS) is switched in the power control IC of the present embodiment is described.

[0095] The difference between the two is shown in table 1. In Table 1, "conventional technology" refers to the power supply control IC of Patent Document 1. In Table 1, the phenomenon surrounded by a bold frame refers to an undesirable phenomenon.

[0096] [Table 1]

[0097]

[0098] As shown in Table 1, in the case where the resistor (Rt) is changed from a high resistance value to a low resistance value in the power supply control IC of the prior art, the reference voltage VREF1 changes from a high potential to a low voltage. In contrast, in the case where the resistor (Rc) is changed from a high resistance value to a low resistance value in the power supply control IC of the present embodiment, the reference voltage VREF1 does not change but remains fixed.

[0099] In addition, in the power supply control IC of the prior art, when the resistance (Rt) of the element is changed from a high resistance value to a low resistance value, the on-current peak changes from high to low, the average switching frequency changes from low to high, the efficiency during sudden action changes from high to low, and the output ripple changes from large to small.

[0100] In contrast, in the case of changing the resistor (Rc) from a high resistance value to a low resistance value in the power supply control IC of the present embodiment, the on-current peak changes from low to high, the average switching frequency changes from high to low, the efficiency during sudden action changes from low to high, and the output ripple changes from small to large, which is the opposite phenomenon. In addition, the unpreferable phenomenon surrounded by the thick frame is also different in the prior art and the present embodiment. Therefore, the invention of Patent Document 1 is significantly different from the invention of the present application, and it can be said that it is an invention produced by different technical ideas.

[0101] The invention made by the present inventors has been specifically described above based on the embodiments, but the present invention is not limited to the above embodiments. For example, in the above embodiments, the correction current circuit 34 is configured so that when the voltage VFB of the external terminal FB is higher than the reference voltage VREF3, the correction current circuit 34 outputs a current Ics2 that is smaller than the correction current Ics1, but the correction current circuit 34 may be configured so that when VFB>VREF3, instead of Ics2 flowing, the output current is made zero (Ics2=0).

[0102] In addition, in the above-mentioned embodiment, in the cutoff circuit (current detection circuit) 33, the voltage after the voltage Vcs of the current detection terminal CS is amplified is compared with the voltage VFB of the feedback terminal FB to generate the cutoff signal TOFF, but the voltage Vcs of the terminal CS can also be compared with the voltage after the voltage VFB of the feedback terminal FB is divided to generate the cutoff signal TOFF.

[0103] Furthermore, in the above-described embodiment, the period timer circuit 32 is configured to generate the period signal PS having a variable frequency, but the frequency of the period signal may be fixed.

[0104] In the above embodiment, the switching transistor SW1 for intermittently passing current through the primary winding of the transformer is provided as a separate element from the power supply control IC 13 . However, the switching transistor SW1 may be incorporated into the power supply control IC 13 to form a single semiconductor integrated circuit.

[0105] Furthermore, in the above-mentioned embodiment, the case where the present invention is applied to a power supply control IC constituting a flyback AC-DC converter is described, but the present invention can also be applied to a power supply control IC constituting a forward type or quasi-resonant type AC-DC converter.

[0106] Description of Reference Numerals

[0107] 12…transformer, 13…semiconductor device for switching power supply (IC for power supply control), 14…constant voltage control circuit, 31…light load determination circuit, 32…cycle timer circuit (cycle signal generation circuit), 33…cutoff circuit (current detection circuit), 34…correction current circuit, 35…latch circuit (control signal generation circuit), 36…drive circuit, 37…internal power supply circuit, 41…inverting amplifier circuit, 42…clamp circuit, 43…voltage-current conversion circuit, 44…current output circuit, 45…bias circuit, SW1…switching element, Rs…current detection resistor (current-voltage conversion element).

Claims

1. A switching power supply device, comprising: A transformer for voltage conversion, which applies a DC voltage to the primary winding; a switching element connected in series with the primary winding; a semiconductor device for power supply control, which generates and outputs a control signal for controlling the switching element to be turned on or off; and a current-voltage conversion element connected in series with the switching element and converting the current flowing through the primary winding into a voltage, The switching element is controlled to be turned on and off so that current intermittently flows through the primary winding, and the current induced in the secondary winding of the transformer is rectified and smoothed to output a DC voltage. It is characterized in that The power supply control semiconductor device comprises: a first external terminal to which a voltage or current corresponding to an output detection signal from a secondary side of the transformer is input; A second external terminal, to which the voltage converted by the current-voltage conversion element is input; a periodic signal generating circuit for generating a periodic signal for providing a timing for periodically turning on the switching element; a current detection circuit that generates a signal for providing a timing for turning off the switching element based on the voltage or current of the first external terminal and the voltage of the second external terminal; as well as a correction current circuit that generates a correction current to be output to the second external terminal, A correction resistor is provided between the second external terminal and the current-voltage conversion element.

2. The switching power supply device according to claim 1, characterized in that: The correction current circuit outputs the correction current at least in a light load state. The periodic signal generating circuit is configured to generate the periodic signal or stop outputting the periodic signal based on a predetermined reference value set in a light load region, depending on whether the voltage or current of the first external terminal is higher or lower than the reference value.

3. The switching power supply device according to claim 2, characterized in that: The correction current circuit is configured to output a correction current having a first current value in a light load region and a second current value smaller than the first current value in a heavy load region based on the voltage of the first external terminal.

4. The switching power supply device according to claim 3, characterized in that: The periodic signal generating circuit includes an oscillating circuit with a variable frequency, and generates a periodic signal with a long period in a light load region and generates a periodic signal with a short period in a heavy load region according to the voltage of the first external terminal.

5. The switching power supply device according to any one of claims 2 to 4, characterized in that: The switching power supply device includes a light load determination circuit, the light load determination circuit determining whether the first external terminal is in a light load state based on the voltage of the first external terminal. The light load determination circuit determines that the state is a light load state when the voltage of the first external terminal is lower than a first voltage value set corresponding to the reference value, and determines that the state is not a light load state when the voltage of the first external terminal is higher than a second voltage value higher than the first voltage value. The periodic signal generating circuit is configured to generate and output the periodic signal when the light load determination circuit determines that the state is not a light load state, and to stop outputting the periodic signal when the light load determination circuit determines that the state is a light load state.

6. A semiconductor device for power supply control constituting a switching power supply device, the switching power supply device controlling the on / off of a switching element connected in series with a primary winding of a voltage conversion transformer, causing a current to flow intermittently through the primary winding, rectifying and smoothing a current induced in a secondary winding of the transformer to output a DC voltage, It is characterized in that The power supply control semiconductor device comprises: a first external terminal to which a voltage or current corresponding to an output detection signal from a secondary side of the transformer is input; a second external terminal to which a voltage corresponding to a current flowing through the switching element is input; a periodic signal generating circuit for generating a periodic signal for providing a timing for periodically turning on the switching element; a current detection circuit that generates a signal for providing a timing for turning off the switching element based on the voltage or current of the first external terminal and the voltage of the second external terminal; A control signal generating circuit, which generates a control signal to turn on and off the switch element; a light load determination circuit, which determines whether it is a light load state according to the voltage or current of the first external terminal; as well as a correction current circuit that generates a correction current to be output to the second external terminal, The periodic signal generating circuit generates and outputs the periodic signal when the light load determination circuit determines that the state is not a light load state, and stops outputting the periodic signal when the light load determination circuit determines that the state is a light load state. The correction current circuit outputs a correction current having a first current value in a light load region, and outputs a correction current having a second current value smaller than the first current value in a heavy load region, based on the voltage or current of the first external terminal.

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