JP2009232595A - Switching power supply circuit and its power factor improvement circuit - Google Patents

Abstract

A switching power supply circuit having a simple circuit configuration and capable of exhibiting a high power factor is provided.
A capacitor (36) and a series circuit of a transformer (38) and a switching element (40) connected in parallel with the capacitor (36) are provided. By switching the switching element (40), the primary winding of the transformer (36) is intermittently connected. A switching power supply circuit that rectifies the voltage generated on the secondary winding side of the transformer 38 and obtains an output voltage by passing a current to the choke coil 32 connected to one end of a capacitor 36 via a diode 34. And a switching element Q3 for switching the connection between the connection point between the diode 34 and the choke coil 32 and the other end of the capacitor 36.
[Selection] Figure 1

Description

  The present invention relates to a switching power supply circuit and a power factor improving circuit thereof.

  In a switching power supply that supplies a DC voltage to an electric device such as a fluorescent lamp, power factor improvement is required in order to improve efficiency and prevent influence on other electric devices. In order to improve the power factor of the switching power supply, various power factor improvement circuits (PFC: Power Function Controller) have been considered.

  As shown in FIG. 8, the conventional boost chopper type power supply 100 multiplies the operational amplifier 10 that compares the output voltage Vout with the reference voltage Vref, the attenuator 14 that attenuates the output of the rectifier circuit 12, and the outputs of the operational amplifier 10 and the attenuator 14. A multiplier 16, a comparator 18 that changes a state according to a difference between a voltage value Vs obtained by converting a current flowing between the drain and source of the switching element Q into a voltage by a resistor Rs and an output value of the multiplier 16, and a choke coil L A comparator 20 that changes the state according to a voltage value Vc obtained by converting a change in the flowing current into a voltage, a flip-flop 22 that receives the output of the comparator 18 at a reset terminal, and receives the output of the comparator 20 at a set terminal; Controls the gate voltage of switching element Q in response to the output of flop 22 Configured to include a drive circuit 24 that.

  The output voltage Vout is a substantially constant DC voltage in a steady state, and the output from the attenuator 14 receives the output waveform of the rectifier circuit 12 and becomes a full-wave rectified waveform. Therefore, the output of the multiplier 16 also becomes a full-wave rectified waveform. The output of the multiplier 16 is used as a reference voltage for the comparator 18. On the other hand, the voltage Vs increases when the switching element Q is on, and the voltage Vs becomes 0 when the switching element Q is off. When this voltage Vs reaches the reference voltage from the multiplier 16, a reset signal is input to the flip-flop 22, and the switching element Q is turned off. When the switching element Q is turned off, the current flowing through the choke coil L changes, and the voltage Vc corresponding to the change is input to the comparator 20. When the non-inverting terminal (+) of the comparator 20 has a higher voltage than the inverting terminal (−), a set signal is input to the flip-flop 22 and the switching element Q is turned on again. At this time, the multiplier 16 sets the reference voltage of the comparator 18 to a full-wave rectified waveform corresponding to the output waveform of the rectifier circuit 12, thereby widening the conduction angle and improving the power factor of the boost chopper type power supply 100. A circuit configuration in which the input of the comparator 20 is replaced with an oscillation circuit (OSC) is also known.

  Patent Document 1 discloses a step-up type including an abnormal oscillation suppression circuit that reduces the pulse width for controlling the gate of the switching element and forcibly turns off the switching element when the output voltage of the rectifier circuit is extremely smaller than the maximum value. A switching power supply is disclosed.

Japanese Patent Laid-Open No. 10-164828

  However, the conventional power factor improvement circuit (PFC) requires a multiplier, a current detection circuit for a choke coil, a comparator, and the like, and there is a problem that the number of parts increases.

  Accordingly, an object of the present invention is to provide a switching power supply circuit that has a simple circuit configuration and can exhibit a high power factor, and a power factor improving circuit thereof.

  One aspect of the present invention includes a capacitor, and a series circuit of a transformer and a first switching element connected in parallel to the capacitor, and switching the first switching element, thereby switching the transformer from the capacitor. A switching power supply circuit that obtains an output voltage by rectifying a voltage generated on the secondary winding side of the transformer by intermittently passing a current through the primary winding, and is connected to one end of the capacitor via a diode. And a second switching element that switches a connection between a connection point between the diode and the choke coil and the other end of the capacitor.

  Here, it is preferable that the drive signal for controlling the switching of the first switching element is changed according to the output voltage, and the switching of the second switching element is controlled by the drive signal.

  A capacitor, and a series circuit of a transformer and a first switching element connected in parallel with the capacitor, and intermittently passing from the capacitor to the primary winding of the transformer by switching the first switching element. Is a power factor improving circuit of a switching power supply circuit that rectifies the voltage generated on the secondary winding side of the transformer to obtain an output voltage, and is connected to one end of the capacitor via a diode. A choke coil, and a second switching element that switches a connection between a connection point between the diode and the choke coil and the other end of the capacitor, and controls switching of the first switching element. The switching of the second switching element is controlled by a drive signal. It is a power factor improvement circuit.

  Here, it is preferable to control so that the increase / decrease in the on-time of the first switching element and the on-time of the second switching element coincide with each other.

  In addition, it is preferable to have a circuit that controls switching of the second switching element in accordance with a current flowing through the second switching element. For example, a resistor that converts a current flowing through the second switching element into an overcurrent detection voltage, and a circuit that blocks input of the drive signal to the second switching element according to the overcurrent detection voltage. Therefore, it is preferable to stop the switching when an overcurrent flows through the second switching element.

  It is preferable to have a circuit that controls switching of the second switching element in accordance with a voltage input to the choke coil. For example, a resistor that divides a voltage input to the choke coil and converts the divided voltage into an input detection voltage, and a circuit that blocks input of the drive signal to the second switching element according to the input detection voltage. By providing, it is preferable to stop the switching of the second switching element when the voltage input to the choke coil becomes high.

  In addition, the switching power supply circuit in the present invention may be an RCC switching power supply, a quasi-resonant switching method, or a PWM control switching power supply.

  Furthermore, an overcurrent protection circuit that controls the current flowing through the first switching element so as not to exceed a predetermined value may be provided.

  According to the present invention, it is possible to provide a switching power supply circuit that has a simple circuit configuration and can exhibit a high power factor, and a power factor improving circuit thereof. Thereby, an inexpensive and highly efficient switching power supply can be provided.

  As shown in the block diagram of FIG. 1, the switching power supply circuit 200 according to the embodiment of the present invention includes a rectifier circuit 30, a choke coil 32, a diode 34, a capacitor 36, a transformer 38, a switching element 40, a drive circuit 42, and an adjustment circuit. 44, a secondary side diode 46, a secondary side capacitor 48, an output voltage detection circuit 50, and a power factor improvement circuit (PFC circuit) 52.

  The switching power supply circuit 200 in the present embodiment is configured by a combination of a PFC circuit 52 and a general ringing choke converter (RCC) circuit. Specifically, the switching power supply circuit 200 can be configured as shown in the circuit example of FIG.

  The rectifier circuit 30 is configured by combining four diodes, and full-wave rectifies and outputs the AC power input to the switching power supply circuit 200. The rectifier circuit 30 may include an inrush current prevention resistor Rs connected in series between the output terminals T1 and T2. The rectifier circuit 30 may include a smoothing capacitor connected in parallel between the output terminals T1 and T2.

  The output terminal T1 of the rectifier circuit 30 is connected to one end of the choke coil 32. The other end of the choke coil 32 is connected to the anode of the diode 34. The cathode of the diode 34 is connected to one end of the primary winding L1 of the transformer 38. The other end of the primary winding L1 of the transformer 38 is connected to the output terminal T2 via the switching element 40 and a resistor. In this way, a series circuit is formed between the output terminals T1 and T2 of the rectifier circuit 30 via the choke coil 32, the diode 34, the primary winding L1 of the transformer 38, the switching element 40, and the resistor. A capacitor 36 is connected between the cathode of the diode 34 and the output terminal T2 of the rectifier circuit 30.

  A junction point between the choke coil 32 and the diode 34 is switching-controlled by a PFC circuit 52 described later, and a voltage applied to the capacitor 36 via the choke coil 32 and the diode 34 can be controlled.

  As shown in FIG. 2, it is also preferable to bypass the output terminal T1 of the rectifier circuit 30 and the capacitor 36 with a diode D1. In this configuration, a voltage is applied to the capacitor 36 via the choke coil 32 and the diode 34, and a voltage is directly applied from the rectifier circuit 30 via the diode D1.

  The transformer 38 forms an electromagnetic coupling between the primary winding L1 and the secondary winding L2, and outputs a change in voltage applied between the terminals of the primary winding L1 from between the terminals of the secondary winding L2. Is converted to a voltage to be output. The transformer 38 forms an electromagnetic coupling between the primary winding L1 and the feedback winding L3, and outputs a change in voltage applied between the terminals of the primary winding L1 from between the terminals of the feedback winding L3. Is converted to a voltage to be output. The output voltage of the feedback winding L3 is converted by the drive circuit 42 into the drive signal VDD (gate voltage) of the switching element 40.

  The anode of the secondary diode 46 is connected to one end of the secondary winding L2 of the transformer 38. The output terminal T3 of the switching power supply circuit 200 is connected to the cathode of the secondary diode 46. The output terminal T4 of the switching power supply circuit 200 is connected to the other end of the secondary winding L2 of the transformer 38. The secondary side capacitor 48 is connected in parallel between the output terminals T3 and T4 of the switching power supply circuit 200.

  In the RCC circuit, the current flowing from the capacitor 36 to the primary winding L1 of the transformer 38 is intermittently switched by the switching element 40, and the voltage generated in the secondary winding L2 of the transformer 38 due to the switching operation is switched to the secondary side. The current is rectified by the diode 46 and the secondary side capacitor 48 and output between the output terminals T3 and T4 of the switching power supply circuit 200.

  The output voltage detection circuit 50 outputs the control voltage FB to the adjustment circuit 44 according to the output voltage Vout between the output terminals T3 and T4. The control voltage FB is used to control the drive signal VDD applied to the gate of the switching element 40 when the input voltage of the switching power supply circuit 200 increases or the output voltage decreases. As shown in FIG. 2, the output voltage detection circuit 50 can be composed of a combination of a Zener diode, a resistor, and a capacitor.

  The switching element 40 includes a transistor. The forward voltage of the transformer 38 is fed back to the drive circuit 42 by the feedback winding L3 of the transformer 38, and the drive signal VDD applied to the gate of the switching element 40 is controlled by the drive circuit 42.

  As shown in FIGS. 1 and 2, the drive circuit 42 can be configured by a parallel circuit of a capacitor CT and a diode DT and a resistor RT. The voltage generated in the feedback winding L3 of the transformer 38 is output via the capacitor CT and the resistor RT. When the voltage across the capacitor CT reaches the forward voltage of the diode DT, the voltage generated in the feedback winding L3 of the transformer 38 is also output through the diode DT. The output of the drive circuit 42 is applied to the gate of the switching element 40 as the drive signal VDD.

  As a result, the switching element 40 is controlled to be in the on state only for the time until the drain current Id of the switching element 40 reaches its peak value, and the primary current L1 and the secondary winding L2 of the transformer 38 are used. The output voltage Vout of the switching power supply circuit 200 is held almost constant.

  The adjustment circuit 44 controls the drive signal VDD applied to the gate of the switching element 40 in accordance with the control voltage FB output from the output voltage detection circuit 50. As shown in FIG. 2, the adjustment circuit 44 includes transistors Q1 and Q2. The drain of the transistor Q1 is connected to the drain of the switching element 40, and the source is connected to the base of the transistor Q2 via a resistor. The drive signal VDD is applied to the gate of the transistor Q1. The collector of the transistor Q2 is connected to the gate of the switching element 40 and the gate of the transistor Q1, and the drive signal VDD is applied. The emitter of the transistor Q2 is connected to the source of the switching element 40. A control voltage FB is applied to the base of the transistor Q2.

  With this configuration, when the control voltage FB applied to the base of the transistor Q2 increases, the transistor Q2 draws current from the switching element 40 and the gate of the transistor Q1, and the drive signal VDD applied to the gate of the switching element 40 is Reduce. Since the control voltage FB changes with changes in the input voltage and output voltage of the switching power supply circuit 200, the switching operation of the switching element 40 can be controlled in accordance with the input voltage and output voltage of the switching power supply circuit 200. As a result, the output voltage Vout of the switching power supply circuit 200 is controlled to be constant.

  The PFC circuit 52 includes a switching element Q3 that opens and closes a connection point between the choke coil 32 and the diode 34 and the output terminal T2 of the rectifier circuit 30 by switching. The switching operation of the switching element Q3 is controlled according to the drive signal VDD.

  That is, the drive signal VDD is input to the set terminal of the flip-flop circuit FF, and when the drive signal VDD becomes higher than a predetermined threshold value, the flip-flop circuit FF is set and a voltage is applied from the output terminal Q to the gate of the switching element Q3. Is applied, and the switching element Q3 is turned on. Thereby, the connection point between the choke coil 32 and the diode 34 and the output terminal T2 of the rectifier circuit 30 are connected via the resistor R02.

  On the other hand, the reset terminal of the flip-flop circuit FF has a drive signal VDD, an input detection voltage V1 obtained by resistance-dividing the output voltage between the output terminals T1 and T2 of the rectifier circuit 30 with resistors R1L and R2L, and An AND element A is provided that receives the logical product of the overcurrent detection voltage V2 obtained by converting the current flowing through the switching element Q3 into a voltage by the resistor R02. That is, when the drive signal VDD is higher than a predetermined value, the input detection voltage V1 is higher than a predetermined value, and the overcurrent detection voltage V2 is lower than a predetermined value, the flip-flop circuit FF is reset.

  FIG. 3 shows changes in the gate signals of the switching element 40 and the switching element Q3 when the input detection voltage V1 is higher than a predetermined value and the overcurrent detection voltage V2 is lower than the predetermined value, that is, when the reset signal is low (L). FIG. Both the switching element 40 and the switching element Q3 are subjected to switching control in accordance with the change of the drive signal VDD.

  FIG. 4 shows the switching element 40 and the switching element when the input detection voltage V1 becomes lower than a predetermined value or the overcurrent detection voltage V2 becomes higher than a predetermined value, that is, when the reset signal becomes high (H). It is the figure which showed the change of the gate signal of Q3. The switching element 40 is switching-controlled in accordance with the change of the drive signal VDD, but is abnormal when the input detection voltage V1 becomes lower than a predetermined value or when the overcurrent detection voltage V2 becomes higher than a predetermined value. It is turned off regardless of the drive signal VDD as an operating state.

  In this way, the charging voltage applied to the capacitor 36 via the diode 34 is adjusted by switching the current flowing through the choke coil 32 with the switching element Q3.

  That is, when the AC power supply voltage Vin input from the input terminal Tin is low or the output load power is large, the drive signal VDD is set so that the ON time of the switching element 40 is increased by the output voltage detection circuit 50 and the adjustment circuit 44. Be controlled. When the on-time of the switching element 40 increases, the on-time (Ton) of the switching element Q3 also increases, and the off-time (Toff) decreases.

  Since the voltage Vc applied from the choke coil 32 to the capacitor 36 via the diode 34 is expressed as Vc = (Ton + Toff) / Toff × (voltage of the output terminal T1 of the rectifier circuit 30), the OFF time of the switching element Q3 ( When Toff) decreases, the voltage Vc increases.

  When the voltage Vc rises, the output voltage detection circuit 50 and the adjustment circuit 44 decrease the on-time of the switching element 40 to keep the output voltage constant. When the ON time of the switching element 40 decreases, the ON time (Ton) of the switching element Q3 also decreases and the OFF time (Toff) increases. As a result, the voltage Vc decreases.

  By repeating such switching control, the voltage Vc can be stabilized in synchronization with the switching control of the switching element 40.

  FIG. 5 is a diagram showing the improvement of the power factor in the present embodiment. The solid line in FIG. 5 is an input current waveform in a circuit in which the PFC circuit 52 is provided, and the broken line is an input current waveform in a circuit in which the PFC circuit 52 is not provided. As is apparent from the figure, the current conduction angle is increased in the circuit provided with the PFC circuit 52, and the power factor is improved.

  FIG. 6 is a diagram showing the time change of the output voltage in the present embodiment. The solid line in FIG. 6 represents the time change of the output voltage in the circuit provided with the PFC circuit 52, and the broken line represents the time change of the output voltage in the circuit not provided with the PFC circuit 52. As is clear from the figure, the circuit provided with the PFC circuit 52 was able to obtain a stable output voltage as in the case where the PFC circuit 52 was not provided.

  FIG. 7 is a diagram showing a time change of the terminal voltage of the capacitor 36 in the present embodiment. The solid line in FIG. 7 represents the time change of the terminal voltage Vc in the circuit provided with the PFC circuit 52, and the broken line represents the time change of the terminal voltage Vc in the circuit not provided with the PFC circuit 52. In the circuit provided with the PFC circuit 52, the terminal voltage Vc increased by about 1.4 times compared to the circuit without the PFC circuit 52.

  According to the switching power supply circuit 200 in the present embodiment, since the on-duty of the switching element 40 is usually 50% or less, the output voltage of the PFC unit can be kept within a voltage within twice the input voltage without controlling the output voltage of the PFC unit. Can do.

  When the load power is increased, the on-duty of the switching element 40 is increased, and the boosting effect of the voltage Vc is increased. By this boosting effect, the increasing rate of the current flowing through the switching element 40 is reduced, and the on-resistance loss can be reduced.

  Further, when the load is lightened, the boosting effect of the voltage Vc is reduced. The loss at light load is easily affected by the switching loss, and the loss in switching at the switching element 40 can be reduced by decreasing the voltage Vc. Therefore, the loss of the switching element 40 is reduced as the load power decreases.

  Further, when the input voltage becomes low, the on-duty of the switching element 40 becomes long. In a normal switching power supply, when the input voltage is low, a large current flows through the switching element 40, increasing the on-resistance loss and causing the efficiency to deteriorate. In the switching power supply circuit 200 according to the present embodiment, when the input voltage decreases, the on-duty increases, and accordingly, the on-duty of the switching element Q3 in the PFC unit also increases, and the output voltage of the PFC unit also increases. Therefore, the on-resistance loss can be reduced.

  In addition, since the switching element 40 and the switching element Q3 are configured to be synchronized, the switching element 40 and the switching element Q3 operate at the same frequency. Therefore, noise processing becomes easy.

  In addition, the voltage range input to the converter becomes narrow, and it is possible to apply the specifications of the transformer winding with improved efficiency in this voltage range.

  Further, since it is not necessary to detect the output voltage of the PFC unit, an ATT, a multiplier, a voltage control amplifier, an oscillator, and the like in the conventional PFC circuit are unnecessary, and the circuit configuration is simplified and can be configured at low cost. Therefore, by applying to a switching power supply with relatively small output power, there is a great merit in terms of manufacturing cost. For example, it is considered to be particularly suitable for a switching power supply having an output power in the range of 75 W to 150 W.

  Note that the switching power supply circuit 200 according to the present embodiment is provided with an overcurrent protection circuit, and the above-described operations and effects become more effective. Further, as disclosed in Patent Document 1, when the output voltage of the rectifier circuit is extremely smaller than the maximum value, the pulse width for controlling the gate of the switching element is reduced, and the abnormal oscillation is suppressed by forcibly turning off the switching element. By including the circuit, the operation of the switching power supply circuit 200 is further stabilized.

  The PFC circuit 52 in the present embodiment synchronizes the switching of the transistor Q3 with the switching of the switching element 40 in not only the RCC switching power supply circuit but also other switching power supply circuits such as a quasi-resonant method and a PWM control method. The same can be applied.

It is a figure which shows the structure of the switching power supply circuit in embodiment of this invention. It is a figure which shows the circuit example of the switching power supply circuit in embodiment of this invention. It is a figure explaining control of a switching element in an embodiment of the invention. It is a figure explaining control of a switching element in an embodiment of the invention. It is a figure which shows the time change of the input current of the switching power supply circuit in embodiment of this invention. It is a figure which shows the time change of the output voltage of the switching power supply circuit in embodiment of this invention. It is a figure which shows the time change of the terminal voltage of the capacitor | condenser of the switching power supply circuit in the modification of implementation of this invention. It is a figure which shows the structure of the conventional switching power supply circuit.

Explanation of symbols

  10 operational amplifiers, 12 rectifier circuits, 14 attenuators, 16 multipliers, 18 comparators, 20 comparators, 22 flip-flops, 24 drive circuits, 30 rectifier circuits, 32 choke coils, 34 diodes, 36 capacitors, 38 transformers, 40 switching elements, 42 drive circuit, 44 adjustment circuit, 46 secondary side diode, 48 secondary side capacitor, 50 output voltage detection circuit, 52 power factor improvement circuit, 100, 200 switching power supply.

Claims (10)

A capacitor, and a series circuit of a transformer and a first switching element connected in parallel with the capacitor,
Switching that obtains an output voltage by switching the first switching element to rectify the voltage generated on the secondary winding side of the transformer by intermittently passing a current from the capacitor to the primary winding of the transformer. A power circuit,
A choke coil connected to one end of the capacitor via a diode;
A second switching element for switching a connection between a connection point between the diode and the choke coil and the other end of the capacitor;
A switching power supply circuit comprising: The switching power supply circuit according to claim 1,
A switching power supply circuit, wherein a drive signal for controlling switching of the first switching element is changed according to the output voltage, and switching of the second switching element is controlled by the drive signal. The switching power supply circuit according to claim 1 or 2,
A switching power supply circuit, wherein the increase / decrease in the on-time of the first switching element and the on-time of the second switching element are matched. A switching power supply circuit according to any one of claims 1 to 3,
A switching power supply circuit comprising a circuit for controlling switching of the second switching element in accordance with a current flowing through the second switching element. A switching power supply circuit according to any one of claims 1 to 4,
A switching power supply circuit comprising a circuit for controlling switching of the second switching element in accordance with a voltage input to the choke coil. A switching power supply circuit according to any one of claims 1 to 5,
A switching power supply circuit characterized by being an RCC switching power supply. A switching power supply circuit according to any one of claims 1 to 5,
A switching power supply circuit characterized by being a quasi-resonant switching power supply. A switching power supply circuit according to any one of claims 1 to 5,
A switching power supply circuit characterized by being a PWM control switching power supply. A switching power supply circuit according to any one of claims 1 to 8,
A switching power supply circuit comprising: an overcurrent protection circuit that controls the current flowing through the first switching element so as not to exceed a predetermined value. A capacitor, and a series circuit of a transformer and a first switching element connected in parallel with the capacitor, and intermittently current flows from the capacitor to the primary winding of the transformer by switching the first switching element. A switching power supply circuit that rectifies the voltage generated on the secondary winding side of the transformer to obtain an output voltage,
A choke coil connected to one end of the capacitor via a diode;
A second switching element that switches a connection between a connection point between the diode and the choke coil and the other end of the capacitor;
A power factor improvement circuit, wherein switching of the second switching element is controlled by a drive signal for controlling switching of the first switching element.

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