JP3026709U - Switching power supply - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To prevent a decrease in conversion efficiency when a load is lightened. SOLUTION: When a DC voltage of an output 11 is going to rise, a pulse generation circuit 1 for generating a switching pulse whose frequency drops is provided, and a switching transistor Q1 is switched by using a switching pulse generated by the pulse generation circuit 1. Switch.

Description

[Detailed description of the device]

[0001]

[Technical field to which the device belongs]

 The present invention relates to a switching power supply that outputs a stabilized DC voltage by switching a DC source etc. obtained by rectifying a commercial power supply. Specifically, when the load decreases, the switching frequency is It is related to the switching power supply which decreases.

[0002]

[Prior art]

 The efficiency of the switching power supply that obtains a stabilized DC output by switching the DC source obtained by rectifying the commercial power supply is greatly affected by the switching characteristics of the switching transistor. Therefore, a device with a small collector-emitter voltage in the saturated state and a high switching speed is used. Furthermore, in order to shorten the turn-off time for the switching transistor to change from the ON state to the OFF state, a resistor is connected between the base and emitter to discharge the accumulated charge quickly, or a speed-up capacitor is connected to the base. A method of pulling the base of the switching transistor to a negative potential at the time of turn-off was adopted.

[0003]

[Problems to be solved by the device]

 However, even when the above method is used, the following problems occur when the load becomes extremely light. In other words, the flyback method, in which power is output from the secondary coil when the switching transistor is turned off, uses a self-excited switching circuit.In this circuit, the switching frequency increases as the load becomes lighter. Become . On the other hand, a turn-on loss and a turn-off loss in the switching transistor occur every time switching is performed. For this reason, when the load becomes lighter and the switching frequency becomes higher, the loss increases, which causes a problem that the conversion efficiency decreases.

Further, in the case of the PWM configuration in which the switching frequency is constant and the switching transistor ON ratio changes, as in the case of the separately excited method, when the load is lightened, the period in which the switching transistor is ON is reduced. It becomes extremely short. That is, the switching pulse width becomes shorter. On the other hand, the loss at the turn-on of the switching transistor is the loss at the period t5 in FIG. 3, and the loss at the turn-off is the loss at the period t7. That is, the loss in the periods t5 and t7 is constant regardless of the pulse width. On the other hand, effective power is transmitted to the secondary coil by the switching mainly in the period t6 when the switching transistor is in the saturated state. Therefore, when the switching pulse width becomes short, the periods t5 and t7 do not decrease, but only the period t6 in which the power transmission is effective decreases. In other words, the ratio of the period t6 during which the active power is transmitted is reduced with respect to the periods t5 and t7 where the loss is generated, which causes a problem that the conversion efficiency is reduced.

The present invention was devised to solve the above problems, and an object of the present invention is to prevent a decrease in conversion efficiency even when the load is lightened. To provide a switching power supply. Another object of the present invention is to provide a switching power supply capable of simplifying the circuit connected to the secondary coil. Another object of the present invention is to provide a switching power supply which can stabilize a DC output voltage with high accuracy. Another object of the invention is to provide a switching power supply capable of feeding back an error in a DC output voltage to the primary side with an inexpensive element. Another object of the present invention is to provide a switching power supply that can simplify the configuration of a pulse generation circuit that generates switching pulses. Another object of the invention is to provide a switching power supply which can avoid complication of the circuit even when a protection circuit is added to the switching transistor.

[0006]

[Means for Solving the Problems]

 In order to solve the above-mentioned problems, the switching power supply according to the invention of claim 1 stabilizes by switching the primary coil of the transformer using a switching transistor and rectifying the voltage generated in the secondary coil. It is applied to a switching power supply that outputs a regulated DC voltage, and is provided with a pulse generation circuit that generates a switching pulse whose frequency decreases when the DC voltage rises, and switching of the switching transistor by the switching pulse. It is configured to do. The switching power supply according to the invention of claim 2 is of a flyback type in which electric power is output from the secondary coil when the switching transistor is in an off state. Further, the switching power supply according to the invention of claim 3 is applied to a switching power supply in which an error of the DC voltage is fed back to the primary side by an element whose input and output are insulated, and the pulse generating circuit is The switching pulse is generated based on the output of the element. Further, in the switching power supply according to the invention of claim 4, the element is a photocoupler. According to a fifth aspect of the present invention, in the switching power supply, a conversion circuit for converting an output current of the photocoupler into an error voltage and a positive input of an output of the conversion circuit are input to the pulse generation circuit via a first resistor. Connected to the output terminal of the comparator, the other terminal connected to the plus input, and one terminal connected to the minus input of the comparator. , A capacitor having the other terminal connected to 0 potential, and a third resistor having one terminal connected to the minus input and the other terminal connected to the output terminal, wherein the error voltage is The voltage range lower than approximately the intermediate value of the input voltage range of the comparator is changed, and the DC voltage is lowered when it is about to rise. According to a sixth aspect of the present invention, there is provided a switching power supply which, when an emitter current of the switching transistor exceeds a predetermined value, reduces a base current of the switching transistor and an inverting transistor which reverses an output of the comparator. Circuit, and the output of the inverting circuit is led to the base of the protection transistor.

[0007]

[Embodiment of device]

 Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an electrical connection according to an embodiment of the present invention. Specifically, it is a flyback type and separately excited type switching power supply. In the figure, a positive input 12 and a zero potential input (primary side ground level) 13 are connected to a DC source (not shown) obtained by rectifying and smoothing a commercial power source such as 120V. ing. Further, one terminal of the primary coil 8 of the transformer 7 is connected to the plus input 12, and the other terminal is connected to the collector of the switching transistor Q1. A snubber circuit including a resistor R 16, capacitors C5 and C6, and a diode D6 is connected in parallel to the primary coil 8.

The secondary coil 10 has one terminal connected to the ground level (secondary ground level) and the other terminal connected to the rectifying diode D7. A capacitor C8 for smoothing the output 11 is connected to the cathode of the diode D7. An error detection circuit for driving the photocoupler 3 with a current corresponding to the error of the DC voltage of the output 11 and a secondary coil for generating a voltage different from that of the output 11 are not shown. The emitter of the switching transistor Q1 is connected to the 0 potential input 13 via a resistor R17 for detecting the emitter current. Further, the emitter of the switching transistor Q1 is led to the base of the protection transistor Q2 via a diode D4 which prevents the reverse flow of current. The protection transistor Q2 has its collector connected to the base of the switching transistor Q1 and its emitter connected to the zero potential input 13. Therefore, the protection transistor Q2, when turned on, bypasses the base current of the switching transistor Q1 to turn off the switching transistor Q1.

One terminal of the auxiliary coil 9 is connected to the 0-potential input 13, and the other terminal is connected to the rectifying diode D 5 for generating the auxiliary current source 15. A smoothing capacitor C7 is connected to the cathode of the diode D5. The auxiliary current source 15 is led to the base of the switching transistor Q1 via a diode D3 for backflow prevention and a resistor R15 for current limiting. The cathode of the diode D3 is connected to a resistor R14 whose one terminal is led to the plus input 12 in order to supply a starting base current to the switching transistor Q1. The Zener diode D2 is an element for stabilizing the voltage of the auxiliary power supply 14 for operating the pulse generation circuit 1 and the inverting circuit 4. Therefore, the cathode of the Zener diode D2 is connected to the plus input 12 via the resistor R7 for ensuring the initial current at the time of start-up, and also connected to the auxiliary current source via the resistor R8 and the diode D1. It is connected to 15. A smoothing capacitor C3 is connected in parallel with the Zener diode D2.

The pulse generation circuit 1 includes six resistors R1 to R6, two capacitors C1 and C2, and a comparator 5, and generates a switching pulse for switching the switching transistor Q1. The generated switching pulse changes its frequency to the lower side as the load connected to the output 11 becomes lighter. Specifically, a conversion circuit 2 for converting the output current of the photocoupler 3 into an error voltage, and a comparator 5 in which the output (error voltage) of the conversion circuit 2 is led to the plus input via the first resistor R1. Is equipped with. In addition, one terminal is connected to the output terminal of the comparator 5, the other terminal is connected to the positive input of the second resistor R2, and one terminal is connected to the negative input of the comparator 5 and the other terminal is 0. It comprises a capacitor C1 connected to the potential input 13 and a third resistor R3 having one terminal connected to the minus input and the other terminal connected to the output terminal. The first resistor R1 and the second resistor R2 give hysteresis to the positive input of the comparator 5. The capacitor C1 and the third resistor R3 determine the oscillation frequency. Further, since the output of the comparator 5 is an open collector, it is pulled up by the resistor R6.

The conversion circuit 2 is composed of two resistors R4 and R5 forming a voltage dividing circuit and a capacitor C2 connected in parallel with the resistor R5 and giving a delay when the error voltage rises. The output error voltage changes in a voltage range lower than a substantially intermediate value of the input voltage range of the comparator 5. Further, when the DC voltage of the output 11 is about to rise, the error voltage falls. The photocoupler 3 is an element whose input and output are insulated from each other, and feeds back an error generated in the direct current voltage of the output 11 to the primary side. The inverting circuit 4 is a block for inverting the output of the comparator 5 (output of the pulse generation circuit 1), and is composed of five resistors R9 to R13, a capacitor C4, and a comparator 6. Specifically, the resistors R9 and R10 divide the voltage of the auxiliary power supply 14 into two. The resistor R11 pulls up the open collector output of the comparator 6, and the resistor R12 sets the base current of the protection transistor Q2 to a predetermined value. The capacitor C4 is for speeding up, and the resistor R13 limits the maximum value of the base current of the protection transistor Q2 (generated by charging the capacitor C4). The operating power supply for the comparators 5 and 6 is supplied by the auxiliary power supply 14.

FIG. 2 is an auxiliary diagram for explaining the operation of the pulse generation circuit 1, and the operation of the embodiment having the above configuration will be described with reference to the figure as needed. The output current of the photocoupler 3 decreases in current value when the DC voltage of the output 11 is about to decrease. Further, when the load becomes lighter and the DC voltage of the output 11 tries to rise, the current value increases. Therefore, the error voltage that is the output of the conversion circuit 2 is approximately the middle of the voltage of the auxiliary power supply 14 when an appropriate load is connected to the output 11 and the voltage of the output 11 does not increase so much. It rises to a voltage close to the value. Further, when the load of the output 11 becomes lighter and the voltage of the output 11 tends to increase more, the error voltage decreases.

A curve 21 in FIG. 2 shows the discharge characteristic of the capacitor C1. A curve 22 shows the charging characteristic of the capacitor C1. Further, the voltages V1 and V2 represent the voltage width of the hysteresis determined by the resistors R1 and R2, and V1 = V2. Therefore, when an appropriate load is connected to the output 11, the voltage of the positive input of the comparator 5 becomes the voltage V3 when the output terminal is at the H level and the voltage V4 when the output terminal is at the L level. Further, when the load of the output 11 becomes light, the voltage of the positive input of the comparator 5 becomes the voltage V5 when the output terminal becomes the H level and becomes the voltage V6 when the output terminal becomes the L level.

From the above, when the load is appropriate, the switching transistor Q1 is turned on in the period t2 and the switching transistor Q1 is turned off in the period t3. When the load becomes lighter, the switching transistor Q1 is turned on in the period t1 and the switching transistor Q1 is turned off in the period t4. Therefore, as is clear from FIG. 2, the switching cycle is t1 + t4> t2 + t3. In other words, when the load becomes lighter, the switching cycle becomes longer and the switching frequency becomes lower. Therefore, when the load on the secondary side becomes light and the amount of active power transmitted from the primary side to the secondary side decreases, the loss due to switching of the switching transistor Q1 decreases corresponding to the degree of decrease. Therefore, even if the load becomes light, the efficiency is prevented from decreasing.

The invention described in claim 1 can be similarly applied to a forward system in which electric power is transmitted to the secondary side when the switching transistor is turned on. Further, in the inventions according to claims 1 and 2, an error detection circuit provided on the primary side without feeding back the error from the secondary side (a circuit for detecting an error by the voltage of the auxiliary current source 15 or the like). It can also be applied to a configuration that stabilizes the output voltage. Further, in the invention according to claim 6, the inverting circuit may have a circuit configuration using one transistor and a small number of passive elements.

[0016]

[Effect of device]

 In the switching power supply according to the first aspect of the present invention, when the output DC voltage rises, the frequency of the switching pulse generated by the pulse generation circuit decreases. For this reason, when the load becomes light and the active power transmitted decreases, the switching frequency becomes low and the switching loss decreases. Therefore, even when the load becomes light, it is possible to prevent the conversion efficiency from decreasing. The switching power supply according to the second aspect of the invention is a flyback system in which electric power is output from the secondary coil when the switching transistor is in the off state. Therefore, a predetermined DC output can be obtained only by connecting the diode and the capacitor to the secondary coil, which makes it possible to simplify the circuit connected to the secondary coil. Also, in the switching power supply according to the third aspect of the present invention, since the error is fed back to the primary side by the element whose input and output are insulated, it is possible to stabilize the output DC voltage with high accuracy. Has become. Further, in the switching power supply according to the invention as set forth in claim 4, since the element for feeding back the error is the photocoupler, the error of the DC voltage of the output can be fed back to the primary side by the inexpensive element. ing. According to a fifth aspect of the present invention, in the switching power supply, the pulse generation circuit is provided with a conversion circuit for converting the output current of the photocoupler into an error voltage, and one comparator and a small number of passive elements are provided to handle the error voltage. Since the generated switching pulse is generated, it is possible to simplify the configuration of the pulse generation circuit. According to a sixth aspect of the present invention, there is provided a switching power supply including an inverting circuit that inverts an output of a comparator that constitutes a pulse generating circuit, and a base of a protection transistor that protects the output of the inverting circuit from an overcurrent. Therefore, even when a protection circuit is added to the switching transistor, it is possible to avoid complication of the circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an electrical connection of an embodiment of the present invention.

FIG. 2 is an auxiliary diagram for explaining the operation of a pulse generation circuit.

FIG. 3 is an auxiliary diagram for explaining a relationship between a switching pulse waveform and a loss.

[Explanation of symbols]

 1 pulse generation circuit 2 conversion circuit 3 photocoupler 4 inverting circuit 5 comparator 7 transformer 8 primary coil 10 secondary coil C1 capacitor Q1 switching transistor Q2 protection transistor R1 first resistance R2 second resistance R3 third resistance

Claims (6)

[Scope of utility model registration request] 1. A switching power supply that outputs a stabilized DC voltage by switching a primary coil of a transformer using a switching transistor and rectifying a voltage generated in a secondary coil, wherein the DC voltage rises. When trying to do so, a switching power supply comprising a pulse generation circuit that generates a switching pulse of which the frequency drops, and switching of the switching transistor by the switching pulse. 2. The switching power supply according to claim 1, wherein the switching power supply is of a flyback type in which electric power is output from the secondary coil when the switching transistor is in an off state. 3. A switching power supply in which an error of the DC voltage is fed back to a primary side by an element having an input and an output insulated from each other, wherein the pulse generation circuit outputs the switching pulse based on an output of the element. The switching power supply according to claim 1, wherein the switching power supply is generated. 4. The switching power supply according to claim 3, wherein the element is a photocoupler. 5. The pulse generation circuit includes a conversion circuit that converts an output current of the photocoupler into an error voltage, a comparator in which an output of the conversion circuit is led to a positive input through a first resistor, and Is connected to the output terminal of the comparator,
A second resistor having the other terminal connected to the plus input, a capacitor having one terminal connected to the minus input of the comparator and the other terminal connected to 0 potential, and one terminal having the minus input. And a third resistor having the other terminal connected to the output terminal, the error voltage changing in a voltage range lower than a substantially intermediate value of an input voltage range of the comparator, and the DC voltage The switching power supply according to claim 4, wherein the switching power supply is lowered when it is going to rise. 6. A protection transistor that reduces the base current of the switching transistor when the emitter current of the switching transistor exceeds a predetermined value, and an inverting circuit that inverts the output of the comparator. The switching power supply according to claim 5, wherein the switching power supply is led to the base of the protection transistor.