JP5184937B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply, and more particularly to a switching power supply that can be used for a step-down chopping switching power supply including a step-down chopping converter, thereby enabling the power supply to be reduced in size and output.

  Conventionally, a step-down chopper circuit having a circuit configuration as shown in FIG. 7 has been used as a power source for low-power devices and other devices of this type. In this circuit, the output voltage of an AC power source, for example, a commercial frequency power source is full-wave rectified by a full-wave rectifier 10 to obtain a voltage smoothed by a smoothing capacitor C1.

This voltage is supplied to the load through a step-down chopper circuit including a switching element Q1, a transformer 20, a flywheel diode D1, and a smoothing capacitor C5 connected to the circuit. Further, the ratio of the ON cycle to the control cycle within one control cycle of the switching element Q1 is controlled by an output voltage detection circuit for making the output voltage constant and a control circuit having control means for controlling the driving of the switching element Q1. Control (for example, refer to Patent Document 1).
JP-A-5-219743

  By the way, the switching power supply has a current critical type switching power supply that shifts the switching element to the OFF state once and then the ON state, and the control without returning the transformer current to zero every switching cycle of the switching element. There is a continuous current type switching power supply.

  However, in the case of the current critical switching power supply described above, since the switching current waveform is a triangular wave, the peak current increases as the load increases. Considering the case of a heavy load, the current capacity of the switching element and the transformer Therefore, there is a problem that the power supply itself becomes large.

  Therefore, the present invention has been made in view of the above-described problems, and an object of the present invention is to provide a small-sized and high-output switching power supply by suppressing the peak current even in the case of a heavy load.

  In order to solve the above-described problems, the present invention proposes the following matters.

(1) The present invention provides a rectifier circuit for rectifying commercial power (for example, equivalent to the rectifier circuit 10 in FIG. 1), and a transformer (for example, FIG. 1 is a switching power supply including a step-down chopping converter (e.g., equivalent to the switching transistor Q1 and the control circuit 30 in FIG. 1) that supplies the switching element (e.g., equivalent to the switching transistor Q1 in FIG. 1). A control circuit (e.g., corresponding to the control circuit 30 in FIG. 1) that drives the switching element in a current critical mode when the load is light, and the current continuity when the load is heavy. the switching element is driven in the mode, the transformer control coil (e.g., corresponding to a control winding Nc of FIG. 1) The control circuit includes trigger control means (e.g., corresponding to the Z / C terminal in the control circuit 30 of FIG. 1) for detecting the voltage of the control coil and turning on the switching element; A timer circuit (for example, the timer circuit 60 in FIG. 1) supplies a signal for turning on the switching element to the trigger control means when the voltage is charged / discharged in the capacitor and the charging voltage exceeds a predetermined threshold. proposes a switching power supply, characterized in that it comprises a corresponding) to.

  According to the present invention, the control circuit for controlling the driving of the switching element is provided. The control circuit drives the switching element in the current critical mode when the load is light, and drives the switching element in the continuous current mode when the load is heavy. Therefore, since the peak current that becomes a problem at the time of heavy load can be suppressed, the switching element and the transformer can be downsized, or the current capacity of the power supply can be increased by using the current switching element and transformer.

Further, according to the present invention, the control circuit detects the voltage of the control coil provided with a trigger control means for turning on the switching element, a timer circuit, the voltage of the control coil to charge discharge electric in the capacitor, charging When the voltage exceeds a predetermined threshold value, a signal for turning on the switching element is supplied to the trigger control means. That is, when the load is light, the switching element is driven by the trigger control means in a current critical mode in which the switching element is turned on. On the other hand, when the load is heavy, the frequency of the voltage of the control coil decreases, so the voltage amplitude due to charging / discharging of C4 in the timer circuit increases. By monitoring this voltage value, the heavy load state is detected. The switching element is driven in the current continuous mode by forcibly turning on the switching element under the control of the timer circuit.

( 2 ) According to the present invention, in the switching power supply of ( 1 ), the capacitor of the timer circuit (for example, equivalent to the capacitor C4 in FIG. 1) is generated during the ON period of the switching element by the voltage signal from the control coil. Negative charging is performed by a negative voltage level proportional to the input voltage, and positive charging is performed by a positive voltage level proportional to an output voltage generated in the control coil during the OFF period of the switching element. There has been proposed a switching power supply characterized by calculating an off period, measuring one period, and controlling an oscillation period.

According to the present invention, the capacitor of the timer circuit is negatively charged by the negative voltage level proportional to the input voltage generated during the ON period of the switching element by the voltage signal from the control coil, and is controlled during the OFF period of the switching element. The positive voltage level proportional to the output voltage generated in the coil is positively charged, the control coil voltage is calculated between the on period and the off period, one period is counted, and the oscillation period is controlled. Therefore, by controlling the frequency of the switching current to be substantially constant, the current waveform of the switching current can be changed to a trapezoidal current waveform, so that the peak value and the effective value are reduced, and the switching element, transformer, The output smoothing circuit can be reduced in size. In addition, an overpower reduction function can be obtained for a region where power is increased.

( 3 ) In the switching power supply of ( 1 ), the present invention has an exclusive configuration of a signal output from the timer circuit and an on-trigger signal detected from the voltage of the control coil, and is output from the timer circuit. A switching power supply is proposed in which driving of the switching element is controlled by an earlier signal of the signal and the on-trigger signal.

  According to the present invention, the signal output from the timer circuit and the on-trigger signal detected from the voltage of the control coil are exclusively configured, and the signal output from the timer circuit or the on-trigger signal is the earlier signal. , Controlling the driving of the switching element. Therefore, this makes it possible to drive the switching element in the continuous current mode by forcibly turning on the switching element under heavy load.

( 4 ) The present invention provides an output voltage detection circuit (for example, equivalent to the output voltage detection circuit 50 of FIG. 1) for detecting an output voltage in the switching power supply of any one of (1) to ( 3 ), and the output voltage A switching power supply comprising an on period control unit (e.g., corresponding to the F / B terminal in the control circuit 30 of FIG. 1) that controls the on period of the switching element in accordance with the detection voltage of the detection circuit. is suggesting.

  According to the present invention, since the peak current at the time of heavy load can be suppressed, there is an effect that the switching element, the transformer, and the smoothing circuit can be reduced in size. Further, even when the current switching element, transformer, and smoothing circuit are used, there is an effect that the capacity of the power source can be increased.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the constituent elements in the present embodiment can be appropriately replaced with existing constituent elements and the like, and various variations including combinations with other existing constituent elements are possible. Therefore, the description of the present embodiment does not limit the contents of the invention described in the claims.

<Configuration of switching power supply>
As shown in FIG. 1, the switching power supply according to the present embodiment mainly includes a rectifier circuit 10, a transformer 20 including a choke coil Np and a control winding Nc, a control circuit 30, an output smoothing circuit 40, and an output. The voltage detection circuit 50, the timer circuit 60, a switching transistor Q1, a flywheel diode D1, and a photocoupler PH1 are included.

  The rectifier circuit 10 supplies the transformer 20 with a pulsating flow obtained by full-wave rectifying the alternating current of the commercial power supply. The transformer 20 supplies electric power to the load when the switching transistor Q1 is ON, and simultaneously stores electromagnetic energy corresponding to the input / output voltage difference in the choke coil Np. When the switching transistor Q1 is OFF, The electromagnetic energy accumulated in the choke coil Np is supplied to the load. The control winding Nc supplies a signal approximately proportional to the voltage applied between the Np terminals to the Z / C terminal of the control circuit 30. This signal becomes a trigger signal for turning on the switching transistor Q1 in the control circuit 30. The flywheel diode D1 plays a role of releasing energy accumulated in the choke coil Np.

  The control circuit 30 controls the ON timing and the ON time width of the switching transistor Q1 by signals input to the Z / C terminal and the F / B terminal. Specifically, the control winding Nc is connected to the Z / C terminal via the ON timing adjustment resistor R2. When a trigger signal transitioning from “Hi” to “Low” is input to the Z / C terminal, an ON signal is output from the DR terminal to the gate of the switching transistor Q1, and the switching transistor Q1 is turned ON. The F / B terminal is connected to a phototransistor that forms part of the photocoupler PH1, and receives light emitted from a light emitting diode provided in the output voltage detection circuit 50, and the ON time of the switching transistor Q1. Control the width. The output smoothing circuit 40 smoothes the voltage waveform supplied from the choke coil Np to obtain an output voltage.

  The output voltage detection circuit 50 compares the divided voltage values of the resistors R4 and R5 for detecting the output voltage with the reference voltage, and turns on the light emitting diode that forms part of the photocoupler PH1 according to the comparison result. . The light emitted from the light emitting diode is received by the phototransistor connected to the F / B terminal of the control circuit 30, and controls the ON time width of the switching transistor Q1.

  The timer circuit 60 is connected to one end of the control winding Nc of the transformer 20 and an ON trigger timing adjustment resistor R2, and is connected to the base terminal of the switch Q2 via the timer setting resistor R3 and the constant voltage diode D2. ing. A clock capacitor C4 is connected in parallel between the base and emitter terminal of the switch Q2, and the collector terminal of the switch Q2 is connected to the Z / C terminal of the control circuit 30. Further, one end of the time measuring capacitor C3 is connected to the control circuit IC-GND together with the other end of the control winding Nc. The control circuit IC-GND is connected to the cathode side of D1.

  The switching transistor Q1 is connected in series with a current detection resistor R1 and in parallel with a damper capacitor C2.

<Operation of switching power supply>
The operation of the switching power supply according to this embodiment will be described with reference to FIGS.
FIG. 2 shows an operation waveform diagram of the switching power supply according to the present embodiment.
Since the switching power supply according to the present embodiment uses current critical control as a basic circuit, electromagnetic energy is accumulated in the choke coil Np of the transformer 20 when the switching transistor Q1 is turned on. When the time set by the F / B terminal is reached and the switching transistor Q1 is turned OFF, the electromagnetic energy stored in the choke coil Np of the transformer 20 is supplied to the output through the output smoothing circuit 40. When all of the electromagnetic energy stored in the choke coil Np of the transformer 20 is supplied, the conduction of the flywheel diode D1 is lost, and the voltage generated in the control winding Nc of the transformer 20 is reversed from positive to negative. As a result, the Z / C terminal of the control circuit 30 receives a trigger, an ON signal is output from the DR terminal of the control circuit 20 to the switching transistor Q1, and the switching transistor Q1 is turned ON. That is, as indicated by the solid line in FIG. 6, when the electromagnetic energy output from the choke coil Np of the transformer 20 is small, both the ON period and the OFF period are short (the oscillation frequency is high), and when the output electromagnetic energy is large, the period is Operates so as to be long (the oscillation frequency is low).

<Timer circuit operation>
Simultaneously with the series of switching operations, the timer circuit 60 operates as follows.
That is, when the switching transistor Q1 is turned on, a negative voltage proportional to the input voltage is generated in the control winding Nc of the transformer 20, and this voltage is limited by the setting of the timer setting resistor R3 and the constant voltage diode D2. The timer capacitor C4 is negatively charged by the current.

  On the other hand, when the switching transistor Q1 is turned OFF, a positive voltage proportional to the output voltage is generated in the control winding Nc of the transformer 20, and this voltage is generated by the current limited by the time setting resistor R3 and the constant voltage diode D2. The clock capacitor C4 is charged positively, and a triangular wave voltage as shown in the voltage waveform between the clock capacitor C4 terminals in FIG. 2, for example, is generated between the clock capacitor C4 terminals.

  Next, when the output electromagnetic energy is large, the oscillation period becomes longer, and the clock capacitor C4 is positively charged during the OFF period of the switching transistor Q1, so that the voltage across the clock capacitor C4 terminal is changed to the switch Q2. When the switch Q2 is turned on, for example, a trigger signal is given to the Z / C terminal of the control circuit 30 as shown in the voltage waveform between the capacitor C4 terminals for timekeeping at the time of heavy load in FIG.

  In the present embodiment, the switching transistor Q1 is configured by a MOSFET, but can also be configured by other switching elements such as a bipolar transistor and an IGBT.

  As described above, in the present embodiment, in the operation region where the output energy is low, that is, in the short cycle state where the stored energy of the transformer 20 is small, the energy output by driving the switching transistor Q1 in the current critical mode. Is large, the cycle is widened, and the time set by the timer circuit 60 becomes longer, the trigger input by the timer circuit 60 is prioritized without waiting for the release of electromagnetic energy from the transformer 20, and the switching transistor Q1 is turned on. The switching transistor Q1 is driven in the current continuous mode.

  On the other hand, in the operation in which the trigger input by the timer circuit 60 is prioritized, the control winding Nc of the transformer 20 is negatively proportional to the input voltage during the ON period of the switching transistor Q1 set by the F / B terminal. Is generated. The generated negative voltage becomes a current limited by the setting of the timer setting resistor R3 and the constant voltage diode D2, and negatively charges the timer capacitor C4.

  When the switching transistor Q1 is turned off, a positive voltage proportional to the output voltage is generated in the control winding Nc of the transformer 20. The generated negative voltage becomes a current limited by the clock setting resistor R3 and the constant voltage diode D2, and positively charges the clock capacitor C4. When the voltage across the capacitor C4 for timekeeping reaches the ON voltage of the switch Q2 and the switch Q2 is turned ON, a trigger signal is given to the Z / C terminal of the control circuit 30 and the switching transistor Q1 is turned ON.

  At this time, since the switching transistor Q1 is turned on before the transformer 20 finishes releasing the electromagnetic energy to the output smoothing circuit 40, the current of the switching transistor Q1 when the transformer 20 is turned on is, for example, at the time of heavy load in FIG. As in the switching current waveform of the switching transistor Q1 at Td, the state is positively biased, and electromagnetic energy is accumulated in the transformer 20 from the biased state.

  In the operation in which the trigger input by the timer circuit 60 is prioritized, the switched current waveform is a trapezoidal wave. That is, when the peak current value is the same peak current value as the triangular wave current waveform in the current critical mode, the electromagnetic energy stored in the transformer 20 increases compared to the current critical mode and supplies a large amount of energy to the output smoothing circuit. It will be possible. Therefore, the output energy can be increased in a switching element having the same maximum current / voltage rating and a transformer having the same inductance value / number of turns / core size, except for a factor due to temperature.

However, when the control by the timer circuit is performed, the switching transistor Q1 is forcibly turned on. Therefore, the switching noise and the switching loss are caused by the influence of the steep discharge current of the resonance capacitor C1 and the recovery current of the flywheel diode D1. It will increase.

  Therefore, the switching power supply according to the present embodiment is particularly suitable for a load that requires a relatively small load power in a steady state and requires a large load power with a short pulse, and a current criticality capable of soft switching up to the steady load power. By setting the operation to be continuous operation by the timer circuit at the time of a large load power with a pulse or an overcurrent such as an abnormality, a large power can be obtained at a small size and at a low cost. A switching power supply with less noise can be provided.

  In addition, in this embodiment, since one cycle is measured by calculating the ON period and the OFF period of the control winding voltage, the oscillation period is controlled, so that overpower is reduced in a region where power is increased. Function is also obtained.

  For example, when the PWM control operation is performed with a high power output and the input voltage is high, a negative high voltage proportional to the input voltage is generated in the control winding Nc of the transformer 20 during the ON period of the switching transistor Q1. . Then, this voltage is negatively charged by the current-limiting capacitor R4 by the current limited by the time-setting resistor R3 and the constant voltage diode D2.

  In the control winding Nc of the transformer 20, a negative high voltage proportional to the input voltage is generated, so that a negative voltage generated at the clock capacitor C4 terminal increases. However, the control winding Nc of the transformer 20 is OFF during the OFF period. Since the positive voltage proportional to the output voltage generated at the same time does not change, the time until the switch Q2 reaches the ON voltage becomes longer.

  Therefore, the time counted by the timer circuit 60 becomes longer, the transformer 20 releases more electromagnetic energy to the output smoothing circuit 40, and the bias current due to the electromagnetic energy when the switching transistor Q1 is turned on becomes smaller. As a result, the current waveform of the switching transistor Q1 is changed from a trapezoidal wave to a triangular wave, so that when the input voltage is high, the voltage generated at R1 tends to increase. Therefore, the overcurrent detection circuit for the OPC terminal of the control circuit 30 The output power limitation due to becomes easier to operate.

  In the case of overcurrent protection by detecting the source current of a general PWM control switching transistor Q1, the output power value at which the overcurrent protection works increases as the input voltage increases. An increase in the power value of overcurrent protection can be reduced.

  Further, during the ON period of the switching transistor Q1, a negative voltage proportional to the input voltage is generated in the control winding Nc of the transformer 20. This voltage is negatively charged to the time-measurement capacitor C4 by a current limited by the setting of the time-setting resistor R3 and the constant voltage diode D2.

  On the other hand, during the OFF period of the switching transistor Q1, a positive voltage proportional to the output voltage is generated in the control winding Nc of the transformer 20. This voltage is positively charged to the time measuring capacitor C4 by the current limited by the time setting resistor R3 and the constant voltage diode D2.

  When the voltage across the capacitor C4 for timekeeping reaches the ON voltage of the switch Q2 and the switch Q2 is turned ON, a trigger signal is given to the Z / C terminal of the control circuit 30 and the switching transistor Q1 is turned ON, so that the current waveform is trapezoidal. The output energy can be increased with waves.

  In the event of an abnormality due to output overload, if the output is reduced by the current limit circuit, the control winding output generates a voltage proportional to the output voltage during the OFF period, so the control winding voltage is also proportional to the output. At the same time, the timing capacitor C4 is slowly charged. Therefore, when the OFF time is widened, the time until the switch Q2 is turned on is prolonged, and when the OFF period is widened, the oscillation period is prolonged and the output energy is lowered.

  Then, the output decreases due to further overload, and finally, the switch Q2 cannot always be turned on due to the decrease in the control winding output during the OFF period, that is, the current continuous mode is entered, and the output energy is limited. Safety during output overload is maintained.

<Output characteristics>
FIG. 4 shows an example of output characteristics of the switching power supply according to the present embodiment.
In the figure, the solid line portion shows the output characteristics in the conventional circuit, and the broken line portion shows the output characteristics when the circuit of the present invention is used. According to this, when the circuit of the present invention is used, larger power is obtained than when the conventional circuit is used, and as the output voltage decreases due to overcurrent, the output power also decreases. It shows that overcurrent characteristics are obtained when a circuit is used.

FIG. 5 shows current waveforms at each point (“A”, “B”, “C”, “D” in FIG. 4) of the output characteristics shown in FIG.
According to this figure, at the time of a light load such as “A” in FIG. 4, since the switching transistor Q1 is driven in the current critical mode as in the prior art, the switching current waveform is a triangular wave with a small peak value. Yes. “B”, “C”, and “D” in FIG. 4 are in a heavy load state, the timer circuit 60 is activated, and the switching transistor Q1 is driven in the current continuous mode. The frequency is controlled to be substantially constant (see FIG. 6), and the ON period of “B” in FIG. 4 is maintained substantially constant, and becomes “C” and “D” in FIG. It is trapezoidal. That is, in “C” and “D” in FIG. 4, the area of the current waveform increases while the ON period is maintained approximately constant, so that the electromagnetic energy accumulated in the transformer 20 increases.

<Oscillation control in continuous current operation>
The present embodiment also has a feature that it can be controlled to a stable oscillation state in the continuous current operation that is operated by the timer circuit 60.

FIG. 3 is an explanatory diagram of the operation. When the load changes from a state where the frequency is fixed, that is, a state where the sum of the ON period and the OFF period (T _on + T _off ) is fixed, the output current There also vary, with constant oN period _{T on} of the switching transistor Q1, almost no relation to the oN period _{T on} the control by the feedback would respond.

When the output voltage Vo decreases due to the increase in the output current, a state in which more residual energy is held in the transformer 20, that is, the current of the flywheel diode D1 indicates that the current at the point Da illustrated in FIG. Since the ON trigger set as a fixed period is entered, the switching transistor Q1 is turned ON, and the current Qa at the time of rising in the ON period of the next switching transistor Q1 increases. Thereby, in this period, the energy stored in the transformer 20 increases, the power supplied to the output smoothing circuit 40 increases, and the output voltage Vo is kept constant.

However, in this operation, when the supplied power becomes excessive, the output voltage rises to the point Vc shown in FIG. At this time, since the OFF period Toff is constant, the slope of the emission current becomes steep due to the increase of the output voltage Vo, and the electromagnetic energy stored in the transformer 20 is likely to be released. As shown by the current of the flywheel diode D1, when the ON trigger set as a fixed cycle is entered in the state of the point Dc, the switching transistor Q1 is turned ON, and the current at the rise is the next ON cycle of the switching transistor Q1. The energy stored in the transformer 20 is reduced by decreasing the Qc point. As a result, the power supplied to the output smoothing circuit 40 is reduced and the output voltage Vo is lowered. When the output voltage Vo is excessively lowered as shown by the Ve point in FIG. 3, the above operation is repeated. Become. Furthermore, if the operation does not converge, unstable oscillation continues.

  Further, the repeated cycle is a frequency region where it is difficult to stabilize the output by feedback. When the non-converging state is continued, an increase in output voltage ripple or an oscillation cycle in the audible region occurs. In some cases, oscillation sound from the transformer 20 is generated.

  However, in the switching power supply according to the present embodiment, the frequency is controlled for each oscillation period by a signal from the control winding Nc rather than being completely fixed, so that correction is performed for each oscillation period separately from the feedback circuit. A stable oscillation can be realized without requiring a special correction circuit.

  That is, in the switching power supply according to the present embodiment, the voltage of the output capacitor C5 decreases due to the increase of the output current, and at the same time, the positive voltage proportional to the output voltage generated in the control winding Nc during the OFF period of the switching transistor Q1 decreases. As a result, charging of the timing capacitor C3 is moderated and the OFF period is extended. In this way, since the OFF period is extended, a large amount of energy is released by the output circuit, so that the electromagnetic energy stored in the transformer 20 is reduced, and the current increase at the next rise of the switching transistor Q1 is increased. The electromagnetic energy stored in the transformer 20 during the ON period can be reduced and the supply to a steep output can be suppressed.

  Further, even when the voltage of the output capacitor C5 rises, the positive voltage proportional to the output voltage generated in the control winding Nc during the OFF period of the switching transistor Q1 rises, thereby increasing the charging current of the timing capacitor C4. In addition, the OFF period is shortened. Due to the shortening of the OFF period, the release of electromagnetic energy to the output circuit is suppressed, a steep decrease in the residual energy of the transformer 20 is suppressed, and the residual energy when the switching transistor Q1 is turned on does not decrease transiently. It is controlled by. With the above operation, a transient response to a load change is suppressed, and a stable operation can be provided.

  As described above, according to the present embodiment, when the load is light, the switching element is driven in the current critical mode, and when the load is heavy, the switching element is driven in the current continuous mode. Since the peak current can be suppressed, the switching element and the transformer can be downsized, or the current switching element and the transformer can be used to increase the current capacity of the power source. In the continuous current mode, the switching current frequency can be made to be a trapezoidal current waveform by controlling the frequency of the switching current to be substantially constant. The transformer and the output smoothing circuit can be reduced in size. In addition, by performing detection using the control winding, it is possible to obtain control stability and a “f” -shaped overcurrent protection characteristic for the overcurrent region.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to embodiment mentioned above, A various deformation | transformation and application are possible within the range which does not deviate from the summary of this invention.

It is the circuit diagram which showed one Example of the switching power supply which concerns on this invention. It is an operation | movement waveform of the Example in the illustration circuit of FIG. It is operation | movement explanatory drawing which concerns on this invention. FIG. 2 is an output characteristic diagram of the embodiment in the illustrated circuit of FIG. 1. It is a figure which shows the current waveform at the time of light load and heavy load. It is the figure which showed the relationship between the oscillation frequency and output electric power in a current critical mode and a continuous current mode. It is the circuit diagram which showed the prior art example of the switching power supply which concerns on this invention.

Explanation of symbols

Np choke coil Nc control winding Q1 switching transistor Q2 switch D1 in timer circuit 60 flywheel diode D2 constant voltage diode C1 in timer circuit 60 damper capacitor C2 smoothing capacitor C3 timing adjustment capacitor C4 for ON trigger timing timer 60 Timing capacitor C5 Smoothing capacitor R1 in output smoothing circuit 40 Switching transistor Q1 Current detection resistor R2 Timing adjustment resistor R3 for ON trigger Timing setting resistors R4 and R5 in timer circuit 60 Output voltage setting resistor PH1 For feedback Photocoupler GND Ground Vo Output voltage 10 Rectifier circuit 20 Transformer 30 Control circuit 40 of switching transistor Q1 Output smoothing circuit 50 Output voltage detection circuit 60 Timer circuit (oscillation cycle control circuit) )
ON trigger input terminal F / B in the Z / C control circuit 30 ON width control input terminal DR in the control circuit 30 Gate drive output OCP of the switching transistor Q1 in the control circuit 30 Overcurrent detection terminal in the control circuit 30

Claims (4)

A switching power supply comprising a rectifier circuit for rectifying a commercial power supply, and a step-down chopping converter for driving the switching element to step-down chopping and supplying the output of the rectifier circuit to a transformer,
A control circuit for controlling the driving of the switching element;
The control circuit drives the switching element in a current critical mode at light load, and drives the switching element in a current continuous mode at heavy load ,
The transformer includes a control coil;
The control circuit includes trigger control means for detecting the voltage of the control coil and turning on the switching element;
A timer circuit that charges and discharges a voltage of the control coil to a capacitor and supplies a signal for turning on the switching element to the trigger control means when the charging voltage exceeds a predetermined threshold value ; A switching power supply.   The capacitor of the timer circuit is negatively charged by a negative voltage level proportional to the input voltage generated during the ON period of the switching element by a voltage signal from the control coil, and is supplied to the control coil during the OFF period of the switching element. 2. The positive charge level proportional to the generated output voltage is positively charged, the ON period and OFF period of the control coil voltage are calculated, one period is counted, and the oscillation period is controlled. Switching power supply described in   The signal output by the timer circuit and the on-trigger signal detected from the voltage of the control coil are configured exclusively, and the earlier signal of the signal output by the timer circuit and the on-trigger signal, The switching power supply according to claim 1, wherein driving of the switching element is controlled.   An output voltage detection circuit for detecting the output voltage;
  4. The switching power supply according to claim 1, further comprising an on period control unit configured to control an on period of the switching element in accordance with a detection voltage of the output voltage detection circuit. .

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