JP2009268246A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve startability in a switching power supply formed of a self-excited compound resonance series converter of a voltage feedback type using an induction voltage of an auxiliary winding for driving the switching device, and capable of greatly simplifying a circuit with reduced loss, low noise and cost reduction. <P>SOLUTION: There is provided a startup circuit 11 including: a control power supply Vcc; a relaxation oscillator circuit 12 biased by the control power supply; an oscillation detecting circuit 13 for determining whether the induction voltage of the auxiliary winding L12 reaches a threshold level Vth that can turn on a corresponding power MOSFETQ2; and a logic circuit 14 that applies the oscillation signal of the relaxation oscillator circuit 12 to the power MOSFETQ2 as a startup signal when the voltage does not reach the level Vth, and blocks the oscillation signal when the voltage reaches the level Vth. Accordingly, it is possible to easily set proper startup conditions by setting the relaxation oscillator circuit 12, thus achieving high startability. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a switching power supply device that outputs a DC stabilized voltage / current that is insulated from an input power supply side, and more particularly to a switching power supply that achieves a reduction in switching element loss, a reduction in size and cost, and a stable startability. Relates to the device.

  Conventionally, a composite resonance type series converter circuit is known as a small-sized and high-efficiency switching power supply as described above, and many known techniques such as Patent Document 1 are shown. FIG. 7 shows a main circuit configuration according to the prior art, and FIG. 8 shows waveforms of main parts.

  In this switching power supply device, a series circuit of switching elements Q1 and Q2 (hereinafter referred to as power MOSFET) is connected between both terminals of the DC input power supply E, and a capacitor C0 is connected. A series resonant circuit of an inductor L, a primary winding L11 of the output transformer T, and a resonant capacitor C1 is formed between the connection point of the power MOSFETs Q1 and Q2 and one end of the DC input power supply E, and the power A capacitor C2 is connected in parallel with either MOSFET Q1 or Q2 (FIG. 7 shows an example in which one end of DC input power supply E is connected to the low voltage side and capacitor C2 is connected in parallel to power MOSFET Q2.) Also, diodes D1 and D2 are connected in antiparallel to power MOSFETs Q1 and Q2, respectively (in many cases, they are also used as body diodes for power MOSFETs Q1 and Q2).

  Further, an intermediate tap is provided in the output winding of the output transformer T and divided into two (L21, L22), and a full-wave rectifier circuit is formed by diodes D3 and D4 that rectify their outputs, and between the intermediate taps. A smoothing capacitor C3 and a DC load Load are connected. The power MOSFETs Q1 and Q2 are alternately turned ON / OFF at a preset frequency in consideration of the complex resonance condition by the control unit 1 shown as a block. Therefore, the control unit 1 has a high-frequency oscillation function, a function of alternately driving the two power MOSFETs Q1 and Q2, and a function of setting a dead time period for turning off the two power MOSFETs Q1 and Q2. A feedforward, feedback control function and output variable function for controlling voltage, current, and power are provided.

  Referring to FIG. 8, Vg1 and Vg2 indicate drive signals for power MOSFETs Q1 and Q2 set in advance by control unit 1. A dead time period in which both are turned ON / OFF alternately and both are turned OFF is set. VQ1, IQ1 and VQ2, IQ2 indicate the drain-source voltage and drain current of the power MOSFETs Q1, Q2. When the drive signal Vg1 is High, the drain current IQ1 flows through the power MOSFET Q1, and when it is Low, the voltage VQ1 substantially equal to the DC input power source E is applied (the same applies to the power MOSFET Q2). In the dead time period, the drain-source voltages VQ1 and VQ2 have rising and falling waveforms with an arbitrary slope due to the effect of the capacitor C2, the inductor L, and the excitation inductance of the output transformer T. Further, the drain currents IQ1 and IQ2 have a series resonance current waveform set approximately by the inductor L and the resonance capacitor C1, and these combined currents are generated between the inductor L, the primary winding L11 of the output transformer T, and the resonance capacitor C1. This is the current of the series resonant circuit. VC1 represents the voltage waveform of the resonant capacitor C1, and has a waveform delayed in phase from the current of the series resonant circuit. ID3 and ID4 indicate current waveforms of the output rectifier diodes D3 and D4. The relationship between the drive frequency of the power MOSFETs Q1 and Q2 and the series resonance frequency of the inductor L and the resonance capacitor C1 is expressed as “resonance frequency> drive. By satisfying the condition of “frequency”, it is possible to set the current of one of the diodes D3 and D4 to flow after the current of one of the diodes D3 and D4 has finished flowing. Not transmitted. That is, during the period in which no current flows through the diodes D3 and D4, the secondary side of the output transformer T is considered to be unloaded, and the primary side exciting inductance L of the transformer T is inserted in series into the primary side series resonance circuit. As a result of the switching of the series resonance condition, inflection points are also found in the waveforms of the drain currents IQ1 and IQ2.

  In such a complex resonance type series converter, ZVS (zero voltage switching), that is, a condition setting such that a current starts to flow after the applied voltage of the switching elements Q1 and Q2 is reduced is possible, and switching loss is extremely small. Since the recovery loss of the secondary side rectifier diodes D3 and D4 can be avoided, high frequency can be achieved with high efficiency. Further, since the voltage / current waveform at the time of switching is stable and the ringing of the secondary side rectifier diodes D3 and D4 can be suppressed, the noise is also excellent.

  The above-described prior art is a so-called separately-excited switching power supply that drives the two switching elements Q1 and Q2 by setting the frequency in advance with an oscillator while having such various features. A level shifter to the switching element Q1 on the potential side is necessary, and from the viewpoint of frequency followability and loss, there is a technical problem with high frequency when miniaturizing the transformer T (the current level shifter has a limit of about 500 kHz) Further, due to cost problems, self-excited studies as shown in Patent Documents 2 to 4, for example, have been made.

  FIG. 9 is a diagram showing an example of a voltage feedback self-excited separately excited composite resonant series converter using the power MOSFETs Q1 and Q2. It should be noted that the transformer T1 is provided with feedback windings L13 and L12, and the feedback windings L13 and L12 provide positive feedback to the gate terminals of the power MOSFETs Q1 and Q2 through the gate resistors R1 and R2, respectively. This means that the self-oscillation operation is continued by being connected in a predetermined direction.

An activation circuit IGN based on relaxation oscillation is added. The starter circuit IGN forms a series circuit of a resistor R3 and a capacitor C5 between both terminals of the DC input power supply E, and connects these connection points to the gate terminal of the power MOSFET Q2 via the Sidac Q3. As a result, when the potential of the capacitor C5 reaches the breakdown voltage, the Sidac Q3 is turned on, and the charge of the capacitor C5 is discharged through the Sidak Q3, the gate resistor R2, and the feedback winding L12. When the generated voltage drop becomes equal to or higher than the threshold voltage Vth of the gate terminal of the power MOSFET Q2, the power MOSFET Q2 is turned ON for a moment, and the charge of the resonance capacitor C1 charged in advance via the resistor R4 is changed to the primary winding of the transformer T1. Discharge occurs through L11 and the choke coil (inductor) L. As a result of the oscillation of the discharge current of the resonant capacitor C1, a feedback voltage equal to or higher than the gate threshold voltage Vth of the power MOSFET Q1 is obtained in the feedback winding L13, and oscillation is started by a positive feedback action. In this starting circuit IGN, in order to charge the resonant capacitor C1 at the time of starting as described above, a resistor R4 that bypasses the high-side power MOSFET Q1 and a diode D5 that draws current from the resistor R3 are provided. There are several conditions for reliably performing such a starting operation, and satisfying these conditions is a necessary condition.
Japanese Patent No. 2734296 Japanese Patent No. 3371595 JP 2002-262568 A JP 2006-129548 A

FIG. 10 shows the waveform of each part at the time of startup. FIG. 10A shows the potential Vc1 of the resonance capacitor C1 constituting the composite resonance circuit. When the power is turned on, the DC capacitor is charged up to the DC input power supply voltage E with the time constant of the resistor R4 and the resonance capacitor C1. . FIG. 10B shows the potential Vc5 of the capacitor C5 constituting the starter circuit IGN. When the power is turned on, the resistor R3 and the capacitor C5 are charged with a time constant. During the charging, the potential Vc5 is reaches the breakdown voltage _{V BO} of SIDAC Q3, the charge of the capacitor C5 and the SIDAC Q3 is turned ON to discharge the potential Vc5 of the capacitor C5 drops rapidly. When the discharge current value in this case becomes equal to or less than the holding current (valley current) of the Sidac Q3, the Sidac Q3 is turned off, charging from the resistor R3 starts again from that point, and this operation is repeated until the converter is started (said Dac The voltage that turns OFF when Q3 is equal to or lower than the holding current is represented by Voff).

  Referring again to FIG. 10A, the capacitance value of the resonance capacitor C1 is normally set to be smaller as the operating frequency of the converter is higher, and sufficiently smaller than the capacitance value of the starting capacitor C5. Accordingly, when the start pulse is applied to the power MOSFET Q2 when the Sidac Q3 is turned on, the discharge is almost completed. When the start is not started, charging via the resistor R4 and discharging for each start pulse are repeated. FIG. 10C shows the gate voltage Vg2 of the power MOSFET Q2. Based on such operations, the conditions for smooth start-up will be clarified.

First, the charging voltage Vc5 of the starting capacitor C5 is as follows.
Vc5 = E * (1-e- ^{(t / C5 * R3)} ) (1)
Then, the charging voltage Vc5 gives starting pulse to the gate of the power MOSFETQ2 to ON is SIDAC Q3 When break reaches down voltage _{V BO} of SIDAC Q3. Therefore, assuming that the operating resistance of the Sidac Q3 is RQ3, the time t1 from the power-on and the starting pulse voltage Vg2 are given by
t1 = −C5 × R3 × ln (1-V _BO / E) (2)
Vg2 = V _BO × R2 / (R2 + RQ3) (3)

If the off-voltage of the Sidac Q3 is V _OFF , the discharge voltage Vc5 ′ of the start-up capacitor C5 and the start pulse width t2 when the Sidak Q3 is _{turned on} can be calculated as follows.
Vc5 ′ = V _BO × e ^{−t / (C5 × (R2 + RQ3))} (4)
t2 = −C5 × (R2 + RQ3) × ln (V _OFF / V _BO ) (5)

Furthermore, the recharging voltage Vc5 ″ of the starting capacitor C5 after the Sidak Q3 is turned off is given by the following equation, and the time t3 until the breakdown voltage V _BO of the Sidak Q3 is reached can be calculated.
Vc5 ″ = V _OFF + (E−V _OFF ) × (1-e− ^{(t / C5 × R3)} ) (6)
t3 = −C5 × R3 × ln (1- (V _BO −V _OFF ) / (E−V _OFF )) (7)

On the other hand, the charging voltage Vc1 and the charging time constant τ of the resonance capacitor C1 are given by the following equations.
Vc1 = E * (1-e- ^{(t / C1 * R4)} ) (8)
τ = C1 × R4 (9)

From the above, the following conditions are necessary for smooth start-up.
(1) Charging time constant of resonant capacitor C1 τ <t3 <t1
(2) Start pulse voltage Vg2> Vth
(3) Start pulse width t2 ≦ π × √ (L × C1)
In this way, by setting the width t2 of the start pulse to ½ or less of the series resonance period of the inductance L and the resonance capacitor C1, an oscillating current suitable for start-up can be obtained. In order to satisfy these conditions, it is necessary to select the component constants that make up the start-up circuit IGN and the resonance circuit. However, characteristics variations and power supply voltage fluctuations of main parts including temperature effects of main parts such as Sidac Q3 When considered, it is not always easy.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a voltage feedback type self-excited composite resonance series converter that can easily set an appropriate start-up condition with a simple configuration, and realizes greatly simplified circuit and cost reduction compared to other excitation types. It is to provide a switching power supply device that can be used.

  In the switching power supply device of the present invention, a series circuit composed of first and second switching elements is connected between both terminals of the DC input power supply, and the connection point between the first and second switching elements and the DC input power supply A series resonant circuit composed of an inductor, a resonant capacitor, and a primary winding of an output transformer is connected between one terminal and the secondary side of the output transformer obtained by switching of the first and second switching elements. The induced current is rectified and smoothed by a diode and a smoothing capacitor and output, and the voltage induced in the first and second auxiliary windings of the output transformer is applied to the control terminals of the first and second switching elements. Switching power supply comprising a voltage feedback self-excited composite resonant series converter that continues switching at The induced voltage of at least one of the control power supply, the oscillation circuit energized by the control power supply, and the first and second auxiliary windings is set to a predetermined level at which the corresponding switching element can be switched. An oscillation detection circuit for determining whether or not the oscillation detection circuit has reached, and in response to the determination result of the oscillation detection circuit, the oscillation signal of the oscillation circuit is used as an activation signal for the corresponding switching element during a period when the predetermined level is not reached. And a starting circuit configured to include a logic circuit that blocks the oscillation signal of the oscillation circuit from being applied to a corresponding switching element when the predetermined level is reached.

  According to the above configuration, the switching elements are turned on by applying the induced voltages of the first and second auxiliary windings to the control terminals of the first and second switching elements through the gate resistors, respectively. In a switching power supply device composed of a voltage feedback type self-excited composite resonance series converter which is continuously driven by / OFF driving, a starting circuit is provided. The starting circuit includes a control power supply, an oscillation circuit energized by the control power supply, and an induced voltage of at least one of the first and second auxiliary windings. An oscillation detection circuit that determines whether or not the line has reached a predetermined level at which the corresponding switching element can be switched, and a period in which the line does not reach the predetermined level in response to the determination result of the oscillation detection circuit A logic circuit that applies an oscillation signal of the oscillation circuit to the corresponding switching element as a start signal and blocks the oscillation signal of the oscillation circuit from being applied to the corresponding switching element when the predetermined level is reached. Constitute.

  Therefore, at the time of start-up when a sufficient induced voltage cannot be obtained in the first and second auxiliary windings, the oscillation signal of the oscillation circuit is given to the gate of the corresponding switching element as the start-up signal, and it is driven ON. In a voltage feedback self-excited complex resonant series converter that can realize low-noise and greatly simplified circuit and cost reduction compared to other-excited type, appropriate start-up conditions can be easily set by setting the oscillation circuit. Can be set to improve the startability.

  In the switching power supply device of the present invention, the oscillation circuit includes a relaxation oscillation circuit.

  According to the above configuration, by using a relaxation oscillation circuit that generates a sawtooth wave or a rectangular wave by charging or discharging a capacitor or an inductor via a resistor as the oscillation circuit, an appropriate start-up condition can be obtained by setting the resistance. Setting can be performed easily.

  Furthermore, in the switching power supply device of the present invention, the oscillation detection circuit stops the operation of the relaxation oscillation circuit when an induced voltage of at least one of the first and second auxiliary windings reaches the predetermined level. It is characterized by making it.

  According to the above configuration, since the operation of the relaxation oscillation circuit is stopped during steady self-excited oscillation, power saving can be achieved.

  In the switching power supply device of the present invention, the pulse width of the output of the relaxation oscillation circuit is less than or equal to ½ of the resonance period of the series resonance circuit, and the pulse period is charged prior to activation of the resonance capacitor. It is more than a time constant.

  According to the above configuration, when the start pulse is given from the relaxation oscillation circuit to the control terminal of the switching element, the timing of the start pulse is important, and if sufficient charge is not stored in the resonant capacitor, the auxiliary winding by the start pulse is used. Since the other switching element cannot be turned on by the induced voltage of the line, the start can be surely performed by selecting as described above.

  Furthermore, in the switching power supply device of the present invention, when there is no feedback voltage from the first and second auxiliary windings, the corresponding first and second switching from the first and second auxiliary windings. A circuit that reaches the control terminal of the element is made open or has a high impedance, and has a starting auxiliary means for making the circuit conductive or low impedance after a feedback voltage is generated from the first and second auxiliary windings. To do.

  According to the above configuration, the start assist means can suppress the start signal from being supplied to the first and second auxiliary windings at the start, and the consumption of the oscillation circuit that creates the start signal Power can be reduced.

  Preferably, the start assisting means is inserted in series in a circuit from the first and second auxiliary windings to the control terminals of the corresponding first and second switching elements so that the direction is a forward direction. And a switch provided in parallel with the diode and turned on when the diode is reverse-biased.

  As described above, the switching power supply device of the present invention applies the induced voltages of the first and second auxiliary windings to the control terminals of the first and second switching elements through the gate resistors, respectively. In a switching power supply device comprising a voltage feedback type self-excited composite resonance series converter configured to continue switching by ON / OFF driving those switching elements, a starting circuit is provided, and the starting circuit is a control power source, The induced voltage of at least one of the oscillation circuit energized by the control power source and the first and second auxiliary windings can switch the switching element to which the first and second auxiliary windings correspond. An oscillation detection circuit for determining whether or not a predetermined level has been reached, and in response to a determination result of the oscillation detection circuit, the predetermined level Logic that blocks the oscillation signal of the oscillation circuit from being supplied to the corresponding switching element as a start signal during a period of not reaching, and preventing the oscillation signal of the oscillation circuit from being applied to the corresponding switching element when the predetermined level is reached And a circuit.

  Therefore, at the time of start-up in which a sufficient induced voltage cannot be obtained in the first and second auxiliary windings, the oscillation signal of the oscillation circuit is given as the start signal to the gate of the corresponding switching element and is driven ON, so that low loss・ In a voltage feedback self-excited complex resonant series converter that can achieve low-noise and greatly simplified circuit and cost reduction compared to other-excited type, an appropriate starting condition can be set by setting the oscillation circuit. It can be set easily and the startability can be improved.

  In addition, as described above, the switching power supply device of the present invention uses a relaxation oscillation circuit that generates a sawtooth wave or a rectangular wave by charging or discharging a capacitor or inductor via a resistor as the oscillation circuit.

  Therefore, it is possible to easily set an appropriate starting condition by setting the resistance.

  Furthermore, as described above, the switching power supply device according to the present invention is configured such that the oscillation detection circuit causes the relaxation oscillation when the induced voltage of at least one of the first and second auxiliary windings reaches the predetermined level. Stop circuit operation.

  Therefore, the operation of the relaxation oscillation circuit is stopped during steady self-excited oscillation, so that power saving can be achieved.

  In the switching power supply device of the present invention, as described above, the pulse width of the output of the relaxation oscillation circuit is set to 1/2 or less of the resonance period of the series resonance circuit, and the pulse period is set prior to the start of the resonance capacitor. More than the time constant to be charged.

  Therefore, reliable activation can be performed.

  Furthermore, as described above, when there is no feedback voltage from the first and second auxiliary windings, the switching power supply device of the present invention corresponds to the first corresponding from the first and second auxiliary windings. And a starting auxiliary means for making the circuit leading to the control terminal of the second switching element open or high impedance, and making the circuit conductive or low impedance after the feedback voltage is generated from the first and second auxiliary windings. Provide.

  Therefore, it is possible to suppress the start signal from being supplied to the first and second auxiliary windings at the time of start-up, and to suppress the power consumption of the oscillation circuit that generates the start signal.

  FIG. 1 is a block diagram of a switching power supply device that is a voltage feedback type self-excited composite resonance series converter according to an embodiment of the present invention. This switching power supply device is intended to reduce the loss of the switching elements Q1 and Q2, and to reduce the size and cost, and to increase the operation frequency. Between both terminals of the DC input power supply E, a power source smoothing capacitor C0 is connected to a series circuit of switching elements Q1 and Q2 (hereinafter referred to as power MOSFETs), and the power MOSFET Q1 is bypassed for startup. A resistor R4 for charging the resonant capacitor C1 is connected. A series resonance circuit of an inductor L, a primary winding L11 of the output transformer T1, and the resonance capacitor C1 is formed between a connection point of the power MOSFETs Q1 and Q2 and one end of the DC input power supply E, and A capacitor C2 is connected in parallel with one of the power MOSFETs Q1 and Q2 (FIG. 1 shows an example in which one end of the DC input power source E is connected to the low voltage side and the capacitor C2 is connected in parallel to the power MOSFET Q2). Further, diodes are connected in antiparallel to the power MOSFETs Q1 and Q2, respectively (in FIG. 1, the body diodes of the power MOSFETs Q1 and Q2 are also used and omitted).

  Further, an intermediate tap is provided in the output winding of the output transformer T1 and divided into two (L21, L22), and a full-wave rectifier circuit is formed by diodes D3 and D4 that rectify their outputs, and between the intermediate taps. A smoothing capacitor C3 and a DC load Load are connected. Further, the output transformer T1 is provided with a first auxiliary winding L12, the opposite polarity side of the primary main winding L11 is connected to the gate of the power MOSFET Q2 via the gate resistor R2, and the auxiliary winding L12 is generated. The power MOSFET Q2 is configured to be able to be self-excited with voltage. Similarly, for the gate drive circuit of the high-voltage side power MOSFET Q1, a second auxiliary winding L13 is provided in the output transformer T1, and the same polarity side as the primary main winding L11 is connected to the power MOSFET Q1 via the gate resistor R1. The power MOSFET Q1 is connected to the gate and configured so that the power MOSFET Q1 can be driven by self-excitation with the voltage generated in the auxiliary winding L13. Thus, the point that the power MOSFETs Q1 and Q2 are alternately turned ON / OFF alternately by the feedback voltages from the two auxiliary windings L13 and L12 to continue the self-excited oscillation is the voltage feedback type self-excited complex resonance series converter. This is the same as the switching power supply device shown in FIG.

  It should be noted that in the present embodiment, the activation circuit 11 is provided between the gate and the source of the power MOSFET Q2. The startup circuit 11 is generally configured to include an internal power supply Vcc for startup, a relaxation oscillation circuit 12, an oscillation detection circuit 13, a logic gate circuit 14, and an output circuit 15. Then, when the activation pulse is applied from the relaxation oscillation circuit 12 to the gate of the power MOSFET Q2 via the logic gate circuit 14, and the steady state oscillation is detected by the oscillation detection circuit 13, the logic gate circuit 14 blocks the output of the activation pulse. The oscillation is continued with the feedback voltage generated in the auxiliary winding L12. The output circuit 15 is composed of a power amplifier or a buffer circuit, and its output is given to the gate of the power MOSFET Q2 via a diode D11.

  FIG. 2 is a block diagram of an activation circuit 111 which is a specific configuration example of the activation circuit 11. The relaxation oscillation circuit 12 is an astable multivibrator composed of a timer IC 121 realized by an LMC555 manufactured by National Semiconductor, etc., and has a series circuit composed of resistors R11 and R12 connected to the internal power supply Vcc and a resonance capacitor C11. The frequency and ON duty can be set easily and in detail from the voltage at each connection point. The output of the IC 121 is inverted by a logic gate circuit (NOT) 16 and input to a two-input logic gate circuit (AND) 14. The other input of the logic gate circuit 14 is connected to one end of a feedback winding L 12. The inverted signal of the output obtained by integrating the feedback voltage from is integrated by an integrating circuit composed of the resistor R13, the diode D12, and the capacitor C12 of the oscillation detecting circuit 13. A discharging resistor R14 is connected in parallel to the integrating capacitor C12. The resistors R13 and R14, the diode D12, and the capacitor C12 constitute an oscillation detection circuit 131 that is a configuration example of the oscillation detection circuit 13. The output of the logic gate circuit 14 is supplied from the output circuit 15 to the gate of the power MOSFET Q2 via the diode D11.

  FIG. 3 is a diagram showing operation waveforms of the activation circuit 111 configured as described above. (A)-(e) respond | corresponds to each part ae of FIG. 2, respectively. (A) shows the output signal of the timer IC 121, and the inverted signal shown in (b) is given to one input of the logic gate circuit 14. Until startup, the power MOSFETs Q1 and Q2 are not switched. Therefore, there is no feedback voltage from the feedback winding L12 shown in (c), and the output from the oscillation detection circuit 131 shown in (d) is also at a low level. Thus, the other input of the logic gate circuit 14 is at a high level. Therefore, the output from the output circuit 15 shown in (e) is an inverted signal of the output signal of the timer IC 121 shown in (b).

  On the other hand, when self-excited oscillation is started by the start pulse from the timer IC 121, the feedback voltage shown in (c) is obtained from the feedback winding L12, and the capacitor C12 constituting the integrating circuit has (d) Is generated, the output of the oscillation detection circuit 131 is at a high level. As a result, the logic gate circuit 14 is blocked by the other input, and the start pulse disappears at the same time as the start, and thereafter, it switches to self-excited oscillation by the feedback voltage.

  In FIG. 3, the pulse width of the output of the timer IC 121, which is a relaxation oscillation circuit, is ½ or less of the resonance period of the series resonance circuit, and the pulse period is when the resonance capacitor C1 is charged prior to activation. It is chosen more than a constant. Here, when giving a starting pulse to the gate of the power MOSFET Q2, the timing of the starting pulse is important, and if sufficient charge is not stored in the resonant capacitor C1, the other voltage is induced in the auxiliary winding L13 by the starting pulse. Since the power MOSFET Q1 cannot be turned on, by selecting as described above, reliable activation can be performed.

  With such a configuration, after starting, steady oscillation is continued by the feedback voltage from the feedback winding L12 without being affected by the starting circuit (timer IC 121), and when the oscillation stops, the starting operation is promptly performed. It is possible to enter. With such a configuration, the period and width of the activation pulse can be easily set by the CR constant of the relaxation oscillation circuit (the timer IC 121), and appropriate activation conditions can be easily set. Further, the voltage of the start pulse can be selected by the internal power supply Vcc.

  FIG. 4 is a block diagram of a startup circuit 112 which is another specific configuration example of the startup circuit 11. The activation circuit 112 is similar to the activation circuit 111 described above, and corresponding portions are denoted by the same reference numerals and description thereof is omitted. It should be noted that in this activation circuit 112, the output of the oscillation detection circuit 131 composed of the integration circuit is inverted by the logic gate circuit (NOT) 17 and input to the Reset terminal of the timer IC 121. It is to stop the relaxation oscillation. The waveform of each part in this starting circuit 112 is shown in FIG. (A)-(e) respond | corresponds to each part ae of FIG. 4, respectively. (D ′) is the output of the logic gate 17, and when the oscillation detection circuit 131 detects the feedback voltage from the feedback winding L12, the output from the logic gate 17 becomes an active low level, and the timer The IC 121 is reset. Thus, when the self-excited oscillation is started, the relaxation oscillation of the timer IC 121 is stopped and the inverted pulse from the logic gate 16 is not output, as indicated by a virtual line.

  By configuring in this way, after starting up, steady oscillation is continued by the feedback voltage from the feedback winding L12 without being influenced by the starting circuit 112, and when the oscillation stops, the starting operation can be started promptly. It becomes possible and power saving can be achieved.

  The startup circuit 11 (111, 112) described above uses the internal power supply Vcc. For this reason, it is desired to reduce the power of the start pulse, and FIG. 6 shows a specific method. As described above, as shown in FIG. 6A, the current due to the start pulse from the start circuit 11 flows from the gate resistor R2 through the feedback winding L12, and the gate resistor R2 is shown in FIG. 6A. Voltage is generated in the polarity. This voltage is supplied as a substantial activation pulse to the gate of the power MOSFET Q2. However, since the gate resistor R2 is set to several tens of Ω or less in consideration of the switching delay caused by the integration effect with the gate capacitance of the power MOSFET Q2, the capacitance required for the power source of the starting pulse increases. Therefore, it is desired to reduce the capacity as described above.

  A method for solving such a problem is shown in FIGS. FIG. 6 (b) shows a principle method. By switching the switch SW inserted in series from the gate resistor R2 to the gate of the power MOSFET Q2, the circuit on the feedback winding L12 side is cut off at the time of start-up. This means that the startup circuit 11 side is shut off after startup.

  Such activation assisting means is not limited to one that completely opens the shut-off side, such as the switch SW, and may be one that has a high impedance. For example, in FIG. 6C, a diode D21 is inserted in series between the gate resistor R2 and the gate of the power MOSFET Q2 so that the feedback voltage is in the forward direction, and the start-up from the start-up circuit 11 is performed by the diode D21. Prevents the wraparound of the pulse to the feedback winding L12 side (high impedance). After startup, turn on the switch SW that bypasses the diode D21 (conduction), and release the charge remaining on the gate during reverse bias and turn it off. Is maintained.

  FIG. 6D shows an example in which the diode D21 and the switch SW in FIG. FIG. 6E shows a case in which a resistor R21 is connected in parallel with the diode D21 to ensure the reverse bias and reduce the power consumption of the start pulse.

  In FIG. 6F, a Zener diode ZD is used instead of the diode D21. With this configuration, by setting the Zener voltage of the Zener diode ZD to be equal to or higher than the threshold voltage Vth of the power MOSFET Q2, it is possible to secure a certain degree of reverse bias and reduce the power consumption of the start pulse. It is.

  FIG. 6G shows that the reverse bias is prevented by the diode D21 inserted in series from the gate resistor R2 to the gate of the power MOSFET Q2 with the switch SW provided between the gate and the source. It is configured by a concrete circuit as shown in (h). That is, the period in which the reverse bias is necessary is detected by the synchronizing circuit 20, and during that period, the MOSFET Q22 is turned on via the diode D22, and the charge remaining on the gate is discharged to keep it OFF.

  FIG. 6 (i) shows an example in which a parallel circuit of a capacitor C21 and a diode D23 is used as a starting pulse wraparound suppression means. For example, a Zener diode is used instead of the diode D23, or a discharge resistor R22 is provided. A device can be appropriately devised. With the configuration as shown in FIG. 6, it is possible to reduce the power supply capacity of the start pulse.

1 is a block diagram of a switching power supply device that is a voltage feedback type self-excited composite resonance series converter according to an embodiment of the present invention. FIG. FIG. 3 is a block diagram of a specific configuration example of a startup circuit in the switching power supply device. FIG. 3 is an operation waveform diagram of the activation circuit shown in FIG. 2. It is a block diagram of the concrete other structural example of the starting circuit in the said switching power supply device. FIG. 5 is an operation waveform diagram of the activation circuit shown in FIG. 4. It is a figure which shows the example of the gate drive circuit which implement | achieves the power saving of the said starting circuit. It is a block diagram which shows the basic composition of a separately excited composite resonance series converter. FIG. 8 is an operation waveform diagram of FIG. 7. It is a block diagram which shows the example of the conventional voltage feedback type self-excitation compound resonance series converter. FIG. 10 is an operation waveform diagram of FIG. 9.

Explanation of symbols

11; 111, 112 Start-up circuit 12 Relaxation oscillation circuit 121 IC for timer
13, 131 Oscillation detection circuits 14, 16, 17 Logic gate circuit 15 Output circuit 20 Synchronization circuit C0 Capacitor (for power supply smoothing)
C1 resonant capacitor C2 capacitor (for dead time)
C3 Smoothing capacitor C11 Capacitors D1 to D4; D11, D12; D21, D22, D23 Diode E DC input power supply L Inductor L11 Primary winding L12, L13 Auxiliary winding Load DC load Q1, Q2 Power MOSFET
Q21, Q22 MOSFET
R1, R2 Gate resistance R4 Resistance (for starting)
R11 to R14; R21 resistor R22 discharge resistor T1 output transformer SW switch Vcc internal power supply ZD Zener diode

Claims (6)

A series circuit composed of first and second switching elements is connected between both terminals of the DC input power supply, and between the connection point of the first and second switching elements and one terminal of the DC input power supply, A series resonant circuit comprising an inductor, a resonant capacitor and a primary winding of an output transformer is connected, and a secondary induced current of the output transformer obtained by switching of the first and second switching elements is generated by a diode and a smoothing capacitor. Rectification / smoothing is output, and the voltage induced in the first and second auxiliary windings of the output transformer is applied to the control terminals of the first and second switching elements to continue switching. In a switching power supply comprising a voltage feedback type self-excited composite resonant series converter,
Control power,
An oscillation circuit energized by the control power supply;
An oscillation detection circuit that determines whether an induced voltage of at least one of the first and second auxiliary windings has reached a predetermined level at which a corresponding switching element can be switched;
In response to the determination result of the oscillation detection circuit, the oscillation signal of the oscillation circuit is supplied to the corresponding switching element as a start signal during a period when the predetermined level is not reached, and when the predetermined level is reached, the oscillation circuit A switching power supply comprising: a startup circuit configured to include a logic circuit that blocks an oscillation signal from being applied to a corresponding switching element.   2. The switching power supply device according to claim 1, wherein the oscillation circuit comprises a relaxation oscillation circuit.   3. The oscillation detecting circuit according to claim 2, wherein when the induced voltage of at least one of the first and second auxiliary windings reaches the predetermined level, the oscillation detecting circuit stops the operation of the relaxation oscillation circuit. Switching power supply.   The pulse width of the output of the relaxation oscillation circuit is ½ or less of the resonance period of the series resonance circuit, and the pulse period is more than a time constant for charging the resonance capacitor prior to starting. The switching power supply device according to claim 2 or 3.   When there is no feedback voltage from the first and second auxiliary windings, the circuit from the first and second auxiliary windings to the control terminals of the corresponding first and second switching elements is opened or high. 5. The apparatus according to claim 1, further comprising start assisting means for setting impedance and making the circuit conductive or low impedance after the feedback voltage from the first and second auxiliary windings is generated. The switching power supply device described in 1.   The start-up assisting means includes a diode inserted in series with a circuit extending from the first and second auxiliary windings to the control terminals of the corresponding first and second switching elements so that the direction is a forward direction. 6. The switching power supply device according to claim 5, further comprising a switch provided in parallel with the diode and turned on when the diode is reverse-biased.

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