JP4419189B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply apparatus including a rectifying and smoothing circuit that rectifies and smoothes a voltage induced in a secondary winding of a transformer to obtain an output voltage.

  An example of a conventional forward type switching power supply device is shown in FIG. In the figure, 1 and 2 are input terminals to which an input voltage Vin is applied, 3 is a transformer that insulates the primary side from the secondary side, and a primary winding 3a of the transformer 3 and an NPN transistor 4 as a main switch element. Is connected between the input terminals 1 and 2. The base of the transistor 4 is supplied with a pulse drive signal from a control circuit (not shown), whereby the transistor 4 is switched and the input voltage Vin is intermittently applied to the primary winding 3a.

  On the other hand, on the secondary side of the transformer 3, the voltage induced in the secondary winding 3b of the transformer 3 due to switching of the transistor 4 is rectified and smoothed to obtain a DC output voltage Vout between the output terminals 15 and 16. A smoothing circuit 21 is connected. In this rectifying / smoothing circuit 21, the rectifying diode 5 which is a rectifying element is turned on when the transistor 4 is turned on to store energy in the choke coil 7, and the commutating diode 6 which is a commutating element is turned on when the transistor 4 is turned off. The energy stored in the choke coil 7 until then is sent to the output side. More specifically, the dot side of the secondary winding 3 b is connected to the anode of the rectifying diode 5, and the cathode of the rectifying diode 5 is connected to one end of the choke coil 7 that is an inductance and the cathode of the commutation diode 6. Is done. The non-dot side of the secondary winding 3 b is connected to the anode of the commutation diode 6, the other end of the output smoothing capacitor 14 that is a capacitive element, and the output terminal 16. The other end of the choke coil 7 is connected to one end of the output smoothing capacitor 14 and the output terminal 15.

  Next, the operation of the above configuration will be described. The input voltage Vin is intermittently applied to the primary winding 3a of the transformer 3 as the transistor 4 performs a switching operation by a pulse drive signal from the control circuit. When the transistor 4 is turned on in this series of operations, the input voltage Vin applied to the primary winding 3a induces a voltage having a positive polarity on the dot side of the secondary winding 3b, and the rectifying diode 5 becomes conductive. The output voltage Vout smoothed by the output smoothing capacitor 14 while storing energy in the choke coil 7 is output between the output terminals 15 and 16. On the other hand, when the transistor 4 is turned off, a positive voltage is generated at the non-dot side terminal of the secondary winding 3b. Then, this time, the rectifying diode 5 is reverse-biased and becomes non-conductive, the commutation diode 6 becomes conductive, and the energy previously stored in the choke coil 7 is released, while continuing between the output terminals 15 and 16. The output voltage Vout smoothed by the output smoothing capacitor 14 is output.

  In the rectifying / smoothing circuit 21 shown in FIG. 5, when the rectifying diode 5 is turned on, a voltage difference between the induced voltage of the secondary winding 3b and the output voltage Vout is applied across the choke coil 7, and the commutating diode 6 is When conducting, a voltage across the choke coil 7 is output as the output voltage Vout, which constitutes a so-called step-down circuit. FIG. 6 shows a voltage waveform between both ends of the commutation diode 6. If the voltage generated between both ends of the commutation diode 6 when the transistor 4 is on is Vp, the voltage is rectified and smoothed by the rectifying and smoothing circuit. Is substantially equal to Vp × D, which is obtained by multiplying the voltage Vp by the duty ratio (ratio of on-time to one cycle) D of the NPN transistor 4.

  Accordingly, the maximum voltage value that can be taken out as the output voltage Vout from the rectifying and smoothing circuit 21 is determined by the turns ratio of the primary winding 3a and the secondary winding 3b of the transformer 3. Therefore, unless the turns ratio is increased, the output voltage Vout As a result, a higher voltage cannot be extracted. For this reason, particularly when it is desired to increase the output voltage Vout, the transformer 3 becomes larger and heavier, making it difficult to increase the cost and reduce the size.

In order to avoid such a problem, for example, in Patent Document 1, in a ringing choke type switching power supply device, when the main switch element is turned on, the second capacitor is charged with a voltage induced in the secondary winding of the transformer. On the other hand, when the main switch element is turned off, the first capacitor is charged by the magnetic energy stored in the transformer so far, and the first and second capacitors are connected in series on the output side of the transformer. There has been proposed a circuit in which a higher output voltage can be extracted from between both ends of the circuit.
Japanese Patent Laid-Open No. 6-189539

  In the circuit configuration shown in the cited document 1, a higher output voltage can be obtained from the rectifying / smoothing circuit with a simple configuration using the first and second capacitors. However, in the forward type switching power supply device including the rectifying diode 5 and the commutation diode 6 as a rectifying / smoothing circuit, a capacitor cannot be incorporated as it is, and the number of turns of the primary winding 3a and the secondary winding 3b is not limited. It is impossible to increase or decrease the output voltage with a simple configuration without changing the ratio.

  In view of the above problems, an object of the present invention is to provide a switching power supply device that can extract an arbitrary output voltage with a simple configuration without changing the turns ratio of the transformer.

Switching power supply according to the present invention includes a rectifier smoothing circuit for obtaining an output voltage of the voltage induced in the transformer secondary winding with the switching of the main switching element rectifying and smoothing, this rectifying and smoothing circuit, the main switching device A switching power supply configured such that the rectifier element is turned on when the main switch element is turned on to store energy in the inductance, and the commutation element is turned on when the main switch element is turned off to send the energy stored in the inductance to the output side In the device, there is an ON period in which the series circuit of the secondary winding and the inductance is short-circuited at least while the rectifying element is in conduction, and the switch that sends the energy of the inductance to the output side in the OFF period Current flow from the output side to the switching element during the ON period of the switching element The buck-boost chopper circuit that includes a unidirectional conductive element to prevent provided, and the off timing of the switching element during the conduction period of the rectifier element, the output voltage higher than the induced voltage of the secondary winding A PWM control circuit for determining the off-timing of the switch element so that the off-timing of the switch element is set during the conduction period of the commutator and the output voltage is made lower than the induced voltage of the secondary winding. ing.

  In this case, when the switch element constituting the step-up / step-down chopper circuit is turned on during the conduction period of the rectifying element, a voltage induced in the secondary winding is applied across the inductance, and energy is quickly accumulated in the inductance. After that, if the switch element is off during the conduction period of the rectifying element, the inductance flyback voltage is superimposed on the induced voltage of the secondary winding, and an output voltage higher than the induced voltage of the secondary winding is taken out. Can do. Also, if the switching element is turned off during the commutation period, only the flyback voltage of the inductance is applied to the output side, and an output voltage lower than the induced voltage of the secondary winding can be extracted. it can.

  According to the above configuration, any output can be obtained by appropriately setting the ON / OFF timing of the switch elements constituting the buck-boost chopper circuit without changing the turns ratio of the primary winding and secondary winding of the transformer. The voltage can be taken out.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to FIGS. In addition, about the thing which a structure overlaps with FIG. 5 of a prior art example, the same code | symbol is attached | subjected and the description is abbreviate | omitted as much as possible.

  In FIG. 1 showing the entire configuration of the circuit, in this embodiment, the induced voltage of the secondary winding 3b is stepped up or stepped down between the choke coil 7 constituting the rectifying and smoothing circuit 21 of the transformer 3 and the output smoothing capacitor 14. A step-up / step-down chopper circuit 31 is provided. This step-up / down chopper circuit 31 is a switching element in which a drain (one end) is connected to the other end of the choke coil 7 and a source (the other end) is connected to a connection point between the secondary winding 3 b and the commutation diode 6. The MOSFET 32 includes a diode 33 which is a backflow prevention element for preventing the charge from the output smoothing capacitor 14 from flowing into the choke coil 7 and the MOSFET 32, and a resistor 34 connected to the gate of the MOSFET 32. The choke coil 7 is caused to function as a boosting or step-down inductance by changing the on / off timing of the MOSFET 32 in accordance with the pulse control signal given to. Reference numeral 36 denotes a load connected between the output terminals 15 and 16.

  On the other hand, in order to determine the on / off timing of the MOSFET 32, a PWM control circuit 41 for supplying a pulse control signal having an arbitrary pulse width to the gate of the MOSFET 32 is provided. The step-up PWM control circuit 41 shown in FIG. 1 is for taking out the output voltage Vout higher than the induced voltage of the secondary winding 3b. The step-down PWM control circuit 71 will be described later.

  The PWM control circuit 41 includes a pulse shaping circuit 42 that generates an H (high) level rectangular wave a in synchronization with a voltage induced in the secondary winding 3b when the transistor 4 is turned on, When the rectangular wave a rises, the voltage level decreases, and when this rectangular wave falls, the voltage level rises, and an inverted sawtooth wave generating circuit 43 that generates an inverted sawtooth waveform c and a voltage dividing resistor 44 between the output terminals 15 and 16. , 45 are connected in series, and output voltage detection circuit 46 that outputs detection voltage d obtained by dividing output voltage Vout from the connection point of voltage dividing resistors 44 and 45, and detection voltage d from output voltage detection circuit 46 Is compared with the inverted sawtooth wave c from the inverted sawtooth wave generation circuit 43, and a comparator 47 for generating a rectangular wave b of H level or L (low) level according to the comparison result, and from the pulse shaping circuit 42 Square wave a is at H level Only when the rectangular wave b from the comparator 47 is at the L level, the subtractor 48 that outputs the rectangular wave (ab) at the H level, and the rectangular wave (ab) from the subtractor 48 as the switch element. And a buffer 49 as drive means for supplying a pulse control signal (gate drive pulse) to the gate of the MOSFET 32. The circuit configurations other than the step-up / down chopper circuit 31 and the PWM control circuit 41 are all the same as those shown in FIG.

  Next, the operation of the above configuration will be described with reference to the waveform diagram of FIG. In FIG. 2, the upper stage is the secondary tap voltage of the transformer 3 generated on the dot side of the secondary winding 3b. Hereinafter, the rectangular wave a from the pulse shaping circuit 42 and the inverted sawtooth wave generating circuit 43 are shown. Inverted sawtooth wave c and detection voltage d from output voltage detection circuit 46, rectangular wave b from comparator 47, rectangular wave (ab) from subtractor 48, and drain-source voltage of FET 32 Each is shown.

  The input voltage Vin is intermittently applied to the primary winding 3a of the transformer 3 with the switching of the transistor 4 that is the main switch element. In this series of operations, when the transistor 4 is in the ON state, a voltage Vtap having a positive polarity on the dot side of the secondary winding 3b is induced, and the rectifying diode 5 is forward-biased and conducted, while the commutating diode 6 Is reverse biased and becomes non-conductive. Here, in synchronization with the generation of the induced voltage Vtap in the secondary winding 3b, the pulse wave forming circuit 42 of the PWM control circuit 41 generates an H level rectangular wave a slightly earlier than the induced voltage Vtap rises. appear. Here, an H-level rectangular wave a may be generated from the pulse shaping circuit 42 simultaneously with the rise of the induced voltage Vtap. In response to the switching of the rectangular wave a from the pulse shaping circuit 42 to the H level, the inverted sawtooth wave generating circuit 43 outputs an inverted sawtooth wave c whose voltage level is decreased to the comparator 47. The comparator 47 compares the inverted sawtooth wave c and the detected voltage d from the output voltage detection circuit 46. Immediately after the induced voltage Vtap is generated, the voltage level of the inverted sawtooth wave c is higher than the detected voltage d. Outputs an L level rectangular wave b, and the subtractor 48 outputs an H level rectangular wave (ab).

  Thus, in the period in which the induced voltage Vtap is generated in the secondary winding 3b, the H level pulse control signal is supplied to the gate of the MOSFET 32 via the buffer 49 until the voltage level of the inverted sawtooth wave c becomes lower than the detection voltage d. Is applied, and the MOSFET 32 is turned on. Therefore, a closed loop composed of the secondary winding 3b, the choke coil 7, the MOSFET 32, and the secondary winding 3b is formed, and the current flowing through the choke coil 7 increases to store more energy.

  Eventually, when the voltage level of the inverted sawtooth wave c becomes lower than the detection voltage d from the output voltage detection circuit 46 during the period in which the induced voltage Vtap is generated on the dot side of the secondary winding 3b, The rectangular wave (ab) turns to the L level, and the MOSFET 32 is turned off. As a result, a back electromotive force is generated between the both ends of the choke coil 7 due to the energy stored so far, and the flyback of the choke coil 7 is caused between the drain and source of the MOSFET 32 to the induced voltage Vtap of the secondary winding 3b. A boosted voltage is generated by adding the voltage Vflybuck. For this reason, the output voltage Vout generated between the output terminals 15 and 16 becomes substantially the same level as the boosted voltage Vtap + Vflybuck.

  Thereafter, when the transistor 4 as the main switch element is turned off, a voltage having a positive polarity on the non-dot side of the secondary winding 3b is generated, and the rectifying diode 5 is reverse-biased and becomes non-conductive, The diode 6 is forward biased and becomes conductive. At this time, the pulse shaping circuit 42 of the PWM control circuit 41 generates an L-level rectangular wave a slightly later than the induced voltage Vtap rises. Here, an L-level rectangular wave a may be generated from the pulse shaping circuit 42 simultaneously with the fall of the induced voltage Vtap. In response to the switching of the rectangular wave a from the pulse shaping circuit 42 to the L level, the inverted sawtooth wave generating circuit 43 outputs the inverted sawtooth wave c whose voltage level increases in slope to the comparator 47. The comparator 47 compares the inverted sawtooth wave c with the detection voltage d from the output voltage detection circuit 46. The voltage level of the inverted sawtooth wave c is lower than the detection voltage d, and the comparator 47 outputs an H level rectangular wave b. Since the rectangular wave a from the pulse shaping circuit 42 is at the L level, the rectangular wave (ab) from the subtractor 48 remains at the L level, and the MOSFET 32 continues to be in the OFF state. At this time, no voltage is generated between the drain and source of the MOSFET 32.

  Eventually, when the period in which the induced voltage Vtap is generated in the secondary winding 3b approaches and an H level rectangular wave a is generated from the pulse shaping circuit 42, the MOSFET 32 is turned on, and the above-described operation is repeated. While the MOSFET 32 is on, energy transfer from the output smoothing capacitor 14 side to the choke coil 7 and the MOSFET 32 is blocked by the diode 33.

  Up to this point, the boosting operation of the step-up / step-down chopper circuit 31 has been described. By turning on the switching control of the MOSFET 32 slightly later than the timing at which the induced voltage Vtap is generated, energy is stored in the choke coil 7 for a short period of time. After the transistor 4 is turned off and a voltage having a positive polarity on the non-dot side of the secondary winding 3b is generated, if the MOSFET 32 is turned off, the energy stored in the choke coil 7 is released and the secondary winding is released. An output voltage Vout lower than the voltage induced on the line 3b can be obtained.

  FIG. 3 shows a circuit configuration for realizing such a step-down operation. The PWM control circuit 71 here increases the voltage level when the rectangular wave a from the pulse shaping circuit 42 rises instead of the inverted sawtooth wave generation circuit 43 in FIG. 1, and the voltage level rises when this rectangular wave falls. A sawtooth wave generating circuit 73 for generating a descending sawtooth waveform c ′ is provided, and instead of the subtractor 48, the rectangular wave a from the pulse shaping circuit 42 and the rectangular wave b from the comparator 47 are both at the H level. Only in this case, an AND circuit 78 for outputting an H level rectangular wave (a & b) is provided. Other configurations are the same as those shown in FIG.

  The operation of the circuit configuration of FIG. 3 will be described with reference to the waveform diagram of FIG. 4. When the transistor 4 as the main switch element is turned on, a positive induced voltage Vtap is generated on the dot side of the secondary winding 3b. Then, a rectangular wave a of H level is generated from the pulse generating circuit 42, and the sawtooth wave generating circuit 73 has a voltage level in response to the switching of the rectangular wave a from the pulse forming circuit 42 to the H level. The sawtooth wave c ′ that rises in inclination is output to the comparator 47. The comparator 47 compares the sawtooth wave c 'with the detection voltage d from the output voltage detection circuit 46. Immediately after the induced voltage Vtap is generated, the voltage level of the sawtooth wave c' is lower than the detection voltage d. Outputs a rectangular wave b of L level, and the rectangular wave (a & b) from the AND circuit 78 becomes L level. As a result, a differential voltage between the induced voltage Vtap of the secondary winding 3b and the output voltage Vout is applied across the choke coil 7, and energy is stored in the choke coil 7. Here, an H-level rectangular wave a may be generated from the pulse shaping circuit 42 simultaneously with the rise of the induced voltage Vtap.

  Eventually, when the voltage level of the sawtooth wave c ′ becomes higher than the detection voltage d from the output voltage detection circuit 46 during the period in which the induced voltage Vtap is generated on the dot side of the secondary winding 3b, the rectangular shape from the AND circuit 78 The wave (a & b) turns to H level and the MOSFET 32 is turned on. In this case, a closed loop composed of the secondary winding 3b, the choke coil 7, the MOSFET 32, and the secondary winding 3b is formed, and the current flowing through the choke coil 7 increases to store more energy.

  Thereafter, when the transistor 4 is turned off, a voltage having a positive polarity on the non-dot side of the secondary winding 3b is generated, and the rectifying diode 5 is reverse-biased and non-conductive, while the commutation diode 6 is forward-biased. Is conducted. At this time, the pulse shaping circuit 42 of the PWM control circuit 41 generates an L-level rectangular wave a slightly later than the induced voltage Vtap rises. Here, an L-level rectangular wave a may be generated from the pulse shaping circuit 42 simultaneously with the fall of the induced voltage Vtap. In response to the switching of the rectangular wave a from the pulse shaping circuit 42 to the L level, the sawtooth wave generating circuit 73 outputs a sawtooth wave c 'whose voltage level is decreased to the comparator 47. The comparator 47 compares the sawtooth wave c ′ with the detection voltage d from the output voltage detection circuit 46, and continues to output the rectangular wave b at the H level. However, since the rectangular wave a from the pulse shaping circuit 42 is at the L level, the AND circuit The square wave (a & b) from 78 goes low and MOSFET 32 is turned off there. As a result, only the flyback voltage Vflybuck of the choke coil 7 is generated between the drain and source of the MOSFET 32, and the output voltage Vout generated between the output terminals 15 and 16 is almost equal to the flyback voltage Vflybuck lower than the induced voltage Vtap. It becomes the same level.

  Eventually, the voltage level of the sawtooth wave c ′ becomes lower than the detection voltage d from the output voltage detection circuit 46, but the rectangular wave (a & b) from the AND circuit 78 remains at the L level, and the MOSFET 32 is turned off. maintain. Then, the transistor 4 is turned on again, and the above operation is repeated. While the MOSFET 32 is on, energy transfer from the output smoothing capacitor 14 side to the choke coil 7 and the MOSFET 32 is blocked by the diode 33.

  As described above, the step-up / step-down chopper circuit 31 of the present embodiment has the MOSFET 32 provided with a period for turning on the MOSFET 32 and storing energy in the choke coil 7 while the transistor 4 is on, that is, while the rectifier diode 5 is conducting. The output voltage Vout is made higher or lower than the induced voltage of the secondary winding 3b depending on whether the turn-off timing is during the conduction period of the rectifier diode 5 or otherwise during the conduction period of the commutation diode 6. be able to. Therefore, in order to take out a high output voltage Vout as in the prior art, it is possible to incorporate a capacitor connected in series or to change the turns ratio of the primary winding 3a and the secondary winding 3b of the transformer 3 without changing the winding ratio. An arbitrary output voltage Vout can be extracted simply by incorporating the step-up / step-down chopper circuit 31 on the secondary side and appropriately setting the ON / OFF timing of the MOSFET 32.

As described above, this embodiment includes the rectifying / smoothing circuit 21 that rectifies and smoothes the voltage induced in the secondary winding 3b of the transformer 3 in accordance with the switching of the transistor 4 as the main switch element, and obtains the output voltage Vout. In this rectifying / smoothing circuit 21, the rectifying diode 5 which is a rectifying element is turned on when the transistor 4 is turned on to store energy in the choke coil 7 which is an inductance, and the commutation diode 6 is turned on when the transistor 4 is turned off. In the switching power supply device configured to send the energy stored in the choke coil 7 to the output side, the secondary winding 2c and the choke coil 7 are connected in series while at least the rectifying diode 5 is conducting. There is an ON period in which the circuit is short-circuited, and the energy of the choke coil 7 is sent to the output side during the OFF period. A step-up / step-down chopper circuit 31 including a MOSFET 32 as a switch element and a diode 33 as a one-way conduction element that prevents current from flowing from the output side to the MOSFET 32 during the ON period of the MOSFET 32 is provided . The MOSFET 32 is turned off during the conduction period of the commutation diode 6 so that the output voltage Vout becomes higher than the induced voltage of the secondary winding 3b during the conduction period of the rectifying diode 5, and the output voltage Vout is reduced to two. PWM control circuits 41 and 71 that determine the OFF timing of the MOSFET 32 are provided so as to be lower than the induced voltage of the next winding 3b .

  In this case, when the MOSFET 32 constituting the step-up / step-down chopper circuit 31 is turned on during the conduction period of the rectifying diode 5, the voltage induced in the secondary winding 3 b is applied across the choke coil 7. Accumulate energy promptly. Thereafter, if the MOSFET 32 is turned off during the conduction period of the rectifying diode 5, the flyback voltage of the choke coil 7 is superimposed on the induced voltage of the secondary winding 3b and is higher than the induced voltage of the secondary winding 3b. The output voltage Vout can be taken out. Further, if the MOSFET 32 is turned off during the conduction period of the commutation diode 6, only the flyback voltage of the choke coil 7 is applied to the output side, and the output voltage Vout lower than the induced voltage of the secondary winding 3b. Can be taken out. Therefore, it is possible to extract an arbitrary output voltage Vout by appropriately setting the on / off timing of the MOSFET 32 without changing the turns ratio of the primary winding 3a and the secondary winding 3b of the transformer 3. .

  In addition, this invention is not limited to the said Example, The following various deformation | transformation are possible. For example, a MOS type FET or the like can be used as the main switch element in addition to the transistor 4. Similarly, for example, a transistor may be used as the MOSFET 32 as the switch element. Further, a rectification / commutation switch element such as a MOSFET may be used as the rectification / commutation element of the rectification / smoothing circuit 21, and these rectification / commutation switch elements may be driven in synchronization with the switching of the main switch element. . Thereby, the loss of the rectifying / smoothing circuit 21 can be reduced and the efficiency can be improved. Furthermore, the circuit configuration of the rectifying and smoothing circuit 21 may be a center tap type. Further, the rectifying diode 5 may be connected not to the dot side terminal side of the secondary winding 3b but to the non-dot side terminal side.

It is a circuit diagram which shows an example of the forward type switching power supply device in a present Example. FIG. 4 is a waveform diagram showing voltages at various parts during the boosting operation. It is a circuit diagram of the forward type switching power supply device which shows another modification same as the above. FIG. 4 is a waveform diagram showing voltages at various parts during the step-down operation. It is a circuit diagram which shows an example of the conventional forward type switching power supply device. It is a wave form diagram which shows the voltage between the both ends of the diode for commutation same as the above.

3 Transformer 4 Transistor (Main switch element)
7 Choke coil (inductance)
21 Rectifier smoothing circuit
31 Buck-boost chopper circuit
32 MOSFET (switch element)
33 Diode (unidirectional conducting element)
41, 71 PWM control circuit

Claims (1)

A rectifying / smoothing circuit that obtains an output voltage by rectifying and smoothing the voltage induced in the secondary winding of the transformer with the switching of the main switch element,
This rectifying and smoothing circuit conducts the rectifying element when the main switch element is turned on and stores energy in the inductance, and when the main switch element is turned off, the commutation element conducts and outputs the energy stored in the inductance. In the switching power supply device configured to be sent to the side,
There is an on period in which a series circuit of the secondary winding and the inductance is short-circuited at least while the rectifying element is conducting, and a switching element that sends energy of the inductance to the output side in the off period; A step-up / step-down chopper circuit provided with a one-way conduction element that prevents a current from flowing from the output side to the switch element during an ON period of the switch element ;
The switch element is turned off during the conduction period of the rectifying element, the output voltage is made higher than the induced voltage of the secondary winding, and the switch element is turned off during the conduction period of the commutation element. A switching power supply device comprising: a PWM control circuit that determines an off timing of the switch element so that the output voltage is lower than an induced voltage of the secondary winding .

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