JP2012244652A - Switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost, high-efficiency switching power supply device whose auxiliary power supply voltage is stable even over a wide input voltage range.SOLUTION: The switching power supply device includes: an input power supply 12; a transformer T1 including a primary winding TN1 to which a switching element TR1 is connected in series and a secondary winding TN2 to which an output smoothing circuit 20 comprising synchronous rectifying elements TR21, TR22, an output choke coil Lo, and an output capacitor Co is connected; an auxiliary power supply circuit 22 which charges a capacitor Csub through a diode Dsub; and a control circuit 14 which controls the power supply device. The auxiliary power supply circuit 22 is connected to a tertiary winding TN3 of the transformer T1 through an active clamp circuit 24, and comprises a clamp capacitor C2 and a clamp element TR2 for switching, and the clamp element TR2 is turned on or off alternating with the main switching element TR1.

Description

The present invention relates to a switching power supply device for converting a DC voltage into a desired voltage and supplying it to an electronic device, and more particularly to a switching power supply device that seeks high efficiency.

  The switching power supply device converts the DC power into AC power by turning on / off the switching element, supplies the AC power to the primary side of the transformer, and rectifies and smoothes the AC power transmitted to the secondary side of the transformer to thereby generate a predetermined DC It is a power supply device that obtains electric power. A typical example is a forward converter type switching power supply device that transfers energy from the primary side to the secondary side of the transformer when the switching element is in an on state, and a control circuit that turns the switching element on and off. Is supplied with voltage from an auxiliary power source.

  As an auxiliary power supply circuit for operating a control circuit inside a switching power supply, a circuit in which a tertiary winding is provided in a transformer and a diode and a capacitor are connected is known (see, for example, Patent Document 1).

  The auxiliary power circuit of this system has a wide input voltage range and a switching power supply device with a high switching frequency cannot make the power supply highly efficient.

  For this reason, a circuit configuration in which the diode Dsub1, the diode Dsub2, the choke coil Lsub, and the capacitor Csub are connected to the tertiary winding TN3 is proposed as an auxiliary power supply circuit that achieves high efficiency of the power supply (for example, Patent Document 2) reference).

  In the auxiliary power supply circuit of this system, the auxiliary power supply voltage decreases when the output current of the switching power supply device decreases. That is, when the output current of the switching power supply device becomes small, the duty is controlled to be narrowed, and as a result, the auxiliary power supply voltage is lowered. As a method of improving the drop in the auxiliary power supply voltage, there is a method of synchronously rectifying the output rectifying and smoothing circuit.

  Synchronous rectification of the output smoothing of the secondary winding enables the output choke coil to operate in the continuous current mode regardless of the output current of the switching power supply device, and the reduction of the auxiliary power supply voltage is improved. However, in this auxiliary power circuit, since the auxiliary power choke coil is used in the continuous current mode, the auxiliary power choke coil needs to be a choke coil having a large inductance. Further, the number of turns of the tertiary winding TN3 provided in the transformer increases.

  As a means for realizing a high-efficiency switching power supply device, there is a forward converter having an output rectification smoothing circuit using synchronous rectification in which an active clamp circuit is applied to the primary winding side (see, for example, Patent Document 3).

A forward converter that uses an active clamp circuit on the primary winding side of the transformer can keep the voltage applied to the main switching element low, enabling the use of elements with low conduction resistance, and improving the efficiency of the switching power supply. It can be improved.

JP-A-5-316722 JP 2004-64862 A JP 2002-78328 A

  However, in order to obtain a high-efficiency switching power supply device in which the auxiliary power supply voltage is stable over a wide input voltage range, the conventionally proposed circuits have the problems described below.

  FIG. 4 is a circuit diagram in which a circuit in which a tertiary winding is provided in a transformer and a diode and a capacitor are connected as an auxiliary power supply circuit is applied to a forward converter type switching power supply 50. A main switching element TR1 using a MOS-FET is connected to the primary winding TN1 wound around the isolation transformer T1 from the input power supply 52, and performs a switching operation. A secondary winding TN2 is wound around the isolation transformer T1, and an output rectifying / smoothing circuit 58 including diodes D21 and D22, an output choke coil Lo, and an output capacitor Co is connected to the secondary winding TN2. Further, as the auxiliary power supply circuit 60, a tertiary winding TN3 wound around the insulating transformer T1 is provided, the diode Dsub1 and the capacitor Csub are connected, and the auxiliary power supply voltage Vsub is supplied to the control circuit 54.

  When the main switching element TR1 is on, a positive voltage is applied to the dot marking side of the primary winding TN1 of the transformer T1, and a positive voltage is generated on the dot marking side of the tertiary winding TN3. The capacitor Csub is charged via the diode Dsub1 by the voltage generated in the tertiary winding TN3. At this time, the voltage of the capacitor Csub becomes the auxiliary power supply voltage Vsub. If the forward voltage drop of the diode Dsub1 is negligible from the input voltage Vin, the number of turns N1 of the primary winding TN1, and the number of turns N3 of the tertiary winding TN3, the auxiliary power supply voltage Vsub is given by the following equation.

本補助電源回路６０は、式（１）から分かるように、入力電圧Ｖｉｎが変化すると補助電源電圧Ｖｓｕｂが比例して変化する。例えば、入力電圧が２倍の範囲で変動する場合、補助電源の電圧も２倍の変動を持つことになる。

As can be seen from the equation (1), the auxiliary power supply circuit 60 changes the auxiliary power supply voltage Vsub in proportion to the change in the input voltage Vin. For example, when the input voltage fluctuates within a range of twice, the voltage of the auxiliary power supply also has a fluctuation of twice.

  In order to make the switching power supply device 50 low loss, it is necessary to supply a voltage sufficient to saturate the on-resistance of the main switching element TR1, so that the gate of the main switching element TR1 is sufficient even when the input voltage Vin is the lowest. Therefore, the turn ratio between the number of turns N1 and the number of turns N3 is set so that a correct voltage can be applied. However, in the switching power supply device 50 of FIG. 4, since the auxiliary power supply voltage Vsub increases as the input voltage Vin increases, the voltage supplied to the control circuit 54 also increases, and therefore, the high voltage is applied to the gate of the main switching element TR1. Will be applied.

  Even if a voltage higher than the saturation voltage is applied to the gate of the main switching element TR1, the on-resistance does not change, and the main switching element TR1 charges and discharges the gate charge when it is turned on / off. If this voltage is applied, the loss increases. This loss becomes more prominent as the gate voltage is higher and the switching frequency is higher. Here, in order to reduce the charge / discharge loss of the gate of the main switching element TR1, a series regulator circuit or the like for stabilizing the voltage can be inserted between the auxiliary power supply circuit 60 and the control circuit 54. In this case, when the input voltage Vin increases, the loss of the series regulator circuit increases.

  Therefore, the auxiliary power supply circuit 60 of this system cannot make the power supply highly efficient in a switching power supply device having a wide input voltage range and a high switching frequency.

  For this reason, a circuit configuration in which a diode Dsub1, a diode Dsub2, a choke coil Lsub, and a capacitor Csub are connected to the tertiary winding TN3 is proposed as an auxiliary power supply circuit that achieves high efficiency of the power supply.

  FIG. 5 shows a circuit diagram in which this auxiliary power supply circuit is applied to a forward converter type switching power supply device. 4, the auxiliary power supply circuit 74 has a diode Dsub2 and an auxiliary power choke coil Lsub added thereto. The inductance of the auxiliary power choke coil Lsub is set so that the critical current of the auxiliary power choke coil Lsub is less than or equal to the output current Isub of the auxiliary power supply. As a result, the auxiliary power choke coil Lsub is always in a state where a current exceeding the critical current flows, and the auxiliary power choke coil Lsub is in a state of operating in the current continuous mode.

  When the main switching element TR1 is on, a positive voltage is applied to the dot marking side of the primary winding TN1 of the transformer T1, and a positive voltage is also generated on the dot marking side of the tertiary winding TN3. The voltage generated in the tertiary winding TN3 causes a current to flow through the capacitor Csub via the diode Dsub1 and the auxiliary power choke coil Lsub, thereby exciting the auxiliary power choke coil Lsub and charging the capacitor Csub.

  When the main switching element TR1 is off, a reset voltage is generated in the transformer T1, thereby generating a positive voltage on the side of the tertiary winding not marked with a dot. At the same time, an induced voltage is generated in the auxiliary power choke coil Lsub, and a positive voltage is generated on the non-dotted side of the auxiliary power choke coil Lsub. As a result, a reverse bias is applied to the diode Dsub1, and a forward bias is applied to the diode Dsub2. Here, a current path is formed from the auxiliary power choke coil Lsub through the diode Csub2 via the capacitor Csub and back to the auxiliary power choke coil Lsub, and the auxiliary power choke coil Lsub is demagnetized and the capacitor Csub is charged.

  The auxiliary power supply voltage Vsub is obtained by the following equation from the duty duty at which the main switching element TR1 is turned on, assuming that the voltage drop of the diodes Dsub1 and Dsub2 can be ignored.

ここで、スイッチング電源装置の出力電圧Ｖｏは、出力チョークコイルＬｏが電流連続モードで動作しているとして、ダイオードＤ２１、Ｄ２２の電圧降下が無視できるものとすると、メインスイッチング素子ＴＲ１がオンとなるデューティｄｕｔｙから次式で得られる。

Here, assuming that the output choke coil Lo is operating in the current continuous mode and the voltage drop of the diodes D21 and D22 can be ignored, the output voltage Vo of the switching power supply device is a duty at which the main switching element TR1 is turned on. It is obtained from the duty by the following equation.

また、三次巻線ＴＮ３に設けた補助電源回路７４の構成は、二次巻線ＴＮ２に設けた出力整流平滑回路７２と同じ構成であることから、補助電源電圧Ｖｓｕｂと出力電圧Ｖｏの関係は、次式で表すことができる。

Further, since the configuration of the auxiliary power supply circuit 74 provided in the tertiary winding TN3 is the same as the output rectification smoothing circuit 72 provided in the secondary winding TN2, the relationship between the auxiliary power supply voltage Vsub and the output voltage Vo is It can be expressed by the following formula.

式（４）から、補助電源回路７４の補助電源電圧Ｖｓｕｂは、出力電圧Ｖｏと、二次巻線ＴＮ２と三次巻線ＴＮ３の巻数比で決定されるため、入力電圧Ｖｉｎが上昇しても補助電源電圧Ｖｓｕｂが上昇することが無い。従って、補助電源電圧Ｖｓｕｂが上昇することによる制御回路５４の消費電力の増大からスイッチング電源装置の効率が低下する現象は生じない。このため、図４で示した補助電源回路６０の欠点が解決された回路構成となっている。

From the equation (4), the auxiliary power supply voltage Vsub of the auxiliary power supply circuit 74 is determined by the output voltage Vo and the turn ratio of the secondary winding TN2 and the tertiary winding TN3. The power supply voltage Vsub does not increase. Therefore, a phenomenon in which the efficiency of the switching power supply device is not reduced due to an increase in power consumption of the control circuit 54 due to an increase in the auxiliary power supply voltage Vsub does not occur. Therefore, the circuit configuration has solved the disadvantages of the auxiliary power supply circuit 60 shown in FIG.

  Up to this point, the output choke coil Lo of the output rectifying and smoothing circuit 72 and the auxiliary power supply choke coil Lsub of the auxiliary power supply circuit 74 have been described as operating in the continuous current mode. When the output choke coil Lo of the output rectifying / smoothing circuit 72 enters the current discontinuous mode, the control circuit 54 performs control to reduce the duty duty of the main switching element TR1.

  The output voltage Vo when the output choke coil Lo is in the current discontinuous mode is obtained by the following equation, where Ton is the on time of the main switching element TR1 and Toff is the off time.

補助電源チョークコイルＬｓｕｂが電流連続モードで動作しており、式（２）が成立しているので、出力チョークコイルＬｏのインダクタンスをＬｏとすれば、式（２）と式（５）から、補助電源電圧Ｖｓｕｂは次式となる。

Since the auxiliary power choke coil Lsub operates in the current continuous mode and the equation (2) is established, if the inductance of the output choke coil Lo is Lo, the auxiliary choke coil Lsub is subtracted from the equations (2) and (5). The power supply voltage Vsub is expressed by the following equation.

式（６）より、出力電流Ｉｏが小さくなると補助電源電圧Ｖｓｕｂが低下することが分かる。つまり、スイッチング電源装置７０の出力電流Ｉｏが小さくなると、デューティｄｕｔｙが狭くなる制御が行われ、その結果、補助電源電圧Ｖｓｕｂが低下することになる。この補助電源電圧Ｖｓｕｂの低下を改善する方法として、出力整流平滑回路７２を同期整流化する方法がある。

From equation (6), it can be seen that the auxiliary power supply voltage Vsub decreases as the output current Io decreases. That is, when the output current Io of the switching power supply device 70 is reduced, the duty duty is controlled to be narrowed, and as a result, the auxiliary power supply voltage Vsub is lowered. As a method of improving the decrease in the auxiliary power supply voltage Vsub, there is a method of synchronously rectifying the output rectifying / smoothing circuit 72.

  FIG. 6 shows a switching power supply circuit 80 to which the circuit configuration of the output rectification smoothing circuit 84 obtained by synchronously rectifying the output rectification smoothing circuit 72 of the switching power supply device 70 of FIG. 5 is applied. In the switching power supply device 80 of FIG. 6, the diode D21 in the switching power supply device 70 is a synchronous rectifier TR21 that is an N-channel MOS-FET, and the diode D22 in the switching power supply device 70 is a synchronous rectifier that is an N-channel MOS-FET. It replaces as rectifier element TR22.

  By synchronously rectifying the output smoothing of the secondary winding TN2, the output choke coil Lo can operate in the current continuous mode regardless of the output current Io of the switching power supply 80, and the auxiliary power supply voltage Vsub. However, since the auxiliary power choke coil Lsub is used in the current continuous mode in the auxiliary power circuit 86, the auxiliary power choke coil Lsub needs to be a choke coil having a large inductance.

  For example, the switching power supply 80 of FIG. 6 is configured so that the output voltage Vo = 5V is obtained at the input voltage Vin = 48V by using the transformer T1 having the number of turns N1: the number of turns N2 = 8 turns: 2 turns and the switching frequency fsw = 200 kHz. Suppose that the switching power supply is made. As the auxiliary power supply, a voltage for driving the gate of the MOS-FET is necessary. Therefore, when the auxiliary power supply voltage Vsub = about 10 V and the current necessary for the control circuit 54, the auxiliary power supply current Isub = 0.05 A. think of.

  First, the number of turns N3 required for the tertiary winding TN3 can be obtained from the equation (4) by the following equation.

補助電源チョークコイルＬｓｕｂのインダクタンスＬｓｕｂと臨界電流Ｉｃｒの関係は、補助電源チョークコイルの臨界電流Ｉｃｒ、スイッチング周波数ｆｓｗ、補助電源チョークコイルのインダクタンスＬｓｕｂを考慮して次式で表わされる。

The relationship between the inductance Lsub of the auxiliary power choke coil Lsub and the critical current Icr is expressed by the following equation in consideration of the critical current Icr of the auxiliary power choke coil, the switching frequency fsw, and the inductance Lsub of the auxiliary power choke coil.

補助電源チョークコイルの臨界電流Ｉｃｒが、０．０５Ａ以下になると、電流不連続モードとなることから、臨界電流Ｉｃｒ＝０．０５Ａとして、入力電圧Ｖｉｎ＝４８Ｖ、スイッチング周波数ｆｓｗ＝２００ｋＨｚの場合に対してインダクタンスＬｓｕｂを計算すると、最低でも２９２μＨが必要であることが分かる。入力電圧Ｖｉｎが高い場合や、スイッチング周波数ｆｓｗが低い場合は、さらに大きなインダクタンスＬｓｕｂが必要になる。

When the critical current Icr of the auxiliary power choke coil becomes 0.05 A or less, the current discontinuous mode is entered. Therefore, the critical current Icr = 0.05 A is assumed, and the input voltage Vin = 48 V and the switching frequency fsw = 200 kHz. When calculating the inductance Lsub, it is found that 292 μH is required at the minimum. When the input voltage Vin is high or the switching frequency fsw is low, a larger inductance Lsub is required.

  In order to obtain a choke coil having an inductance of 292 uH, it is necessary to wind a large number of windings around a magnetic material such as a ferrite core, resulting in an increase in cost of the choke coil. In addition, since the relationship of “the output current Isub of the auxiliary power supply> the critical current Icr of the choke coil” must be taken into consideration, it is necessary to examine the equation (8) for the usage environment of the switching power supply device, and the design is complicated. It also has the problem of becoming.

  Further, since the auxiliary power supply voltage Vsub is obtained from the expression (4) with respect to the output voltage Vo, when the auxiliary power supply circuit 86 is used in a switching power supply device having a low output voltage Vo, the tertiary winding TN3 provided in the transformer is used. The number of turns increases (4 turns under the above conditions). If the number of turns N3 of the tertiary winding TN3 increases, the size of the transformer T1 increases. If the size of the transformer T1 cannot be changed, the ratio of the primary winding TN1 and the secondary winding TN2 to the transformer T1 decreases by the amount of the tertiary winding TN3, and the primary winding TN1 and the secondary winding TN2 The conduction resistance of the secondary winding TN2 increases, leading to a decrease in efficiency of the switching power supply device.

  Here, the efficiency of the switching power supply device can be improved by applying an active clamp circuit to the primary transformer of the forward converter having the output rectification smoothing circuit using synchronous rectification.

  6 has a synchronous rectifier circuit in the output rectifier circuit, and an active clamp circuit can be used in combination as a forward converter in which the output choke coil operates in the current continuous mode.

  FIG. 7 shows a switching power supply device 90 in which an active clamp circuit 96 is applied to the forward converter of FIG. The active clamp circuit 96 has a clamp element TR12 and a clamp capacitor C12 connected in series and is connected in parallel to the primary winding TN1 of the transformer T1. The switching power supply device 90 performs an operation of alternately turning on / off the clamp element TR12 and the main switching element TR1.

  Since the forward converter using the active clamp circuit can keep the voltage applied to the main switching element TR1 low, it is possible to use an element with a low withstand voltage as the switching element. An element having a low withstand voltage has a characteristic that the conduction resistance is smaller than that of an element having a high withstand voltage, so that conduction loss can be reduced and the efficiency of the switching power supply can be improved. Further, the synchronous rectifier circuit can reduce the conduction resistance of the secondary side synchronous rectifier element TR21 and the synchronous rectifier element TR22, and the switching power supply can be made more efficient.

  However, even if the switching power supply device 90 shown in FIG. 7 can improve the efficiency of the switching power supply, the auxiliary power supply circuit 96 still needs a large inductance. The usage environment, that is, the input voltage, switching frequency, and auxiliary power circuit current must be taken into consideration, and the problems such as difficulty in design and the increase in the number of turns of the tertiary winding remain as they are. Since the number of parts increases, there is also a problem that the switching power supply device becomes large and expensive.

An object of the present invention is to solve the above-described problems and to provide a high-efficiency switching power supply apparatus in which an auxiliary power supply voltage is stable even over a wide input voltage range.

The switching power supply device according to the present invention includes:
In order to intermittently generate a DC input voltage and generate an intermittent voltage, a DC input power source connected in series and a primary winding of an isolation transformer and a main switching element that performs on / off operation,
An output rectifying / smoothing circuit that rectifies and smoothes the intermittent voltage generated in the secondary winding of the isolation transformer by the on / off operation of the main switching element to generate a DC output voltage and supplies power to the load;
A control circuit for controlling the on / off operation of the main switching element;
In a forward converter type switching power supply device including an auxiliary power supply circuit that is connected to a tertiary winding provided in an isolation transformer and supplies power for a control circuit,
The auxiliary power supply circuit is a switching power supply device that is connected to the tertiary winding via an active clamp circuit.

  In the active clamp circuit, a clamp element that performs on / off operation connected in series and a clamp capacitor are connected in parallel with the tertiary winding, and an auxiliary power supply circuit is connected to both ends of the clamp element.

  The auxiliary power supply circuit includes a diode and a capacitor connected in series to the cathode of the diode, and supplies a voltage generated across the capacitor as an auxiliary power supply voltage to the control circuit.

The output rectifying / smoothing circuit includes a synchronous rectifying element driving circuit, two synchronous rectifying elements that are ON / OFF controlled by the synchronous rectifying element driving circuit and connected in parallel to the secondary winding, and both ends of the synchronous rectifying element. And an output choke coil and an output capacitor connected in series, and supplies a voltage generated across the capacitor to the load.

The switching power supply according to the present invention is connected to an active clamp circuit that regulates not to exceed a certain power supply to the tertiary winding of a transformer used as an auxiliary power supply, and the intermittent voltage generated at both ends of the clamp element is The auxiliary power supply is taken out using a rectifying / smoothing circuit composed of a diode and a capacitor. This makes it possible to obtain an auxiliary power supply voltage with small fluctuations even in a wide input voltage range. In addition, the choke coil is unnecessary from the conventional auxiliary power supply circuit, so that it is not necessary to design the inductance of the choke coil, the number of tertiary windings wound around the transformer can be reduced, and the number of parts of the switching power supply device can be reduced. This is effective in reducing the size and size.

The figure explaining the switching power supply device by this invention. The figure explaining each part voltage of the switching power supply by this invention. The figure which shows the relationship between an input voltage and an auxiliary power supply voltage. The figure explaining the conventional switching power supply device which provided the auxiliary power circuit in the conventional forward converter power supply. The figure explaining the conventional switching power supply which improved the auxiliary power circuit of FIG. The figure explaining the switching power supply device which aimed at the high efficiency using the synchronous rectification element for the output smoothing circuit of the switching power supply device of FIG. The figure explaining the switching power supply device which aimed at the high efficiency using the active clamp circuit for the primary side trans | transformer of the switching power supply device of FIG.

  A switching power supply device according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a circuit diagram illustrating a switching power supply device 10 according to the present invention, which is of an insulating synchronous rectification forward converter type, and an auxiliary power supply circuit 22 is a tertiary that is wound around a transformer T1 via an active clamp circuit 24. It is connected to the winding TN3.

  The input power supply 12 is a DC power supply, and is connected to the primary winding TN1 of the isolation transformer T1, and the main switching element TR1 is further connected in series from the primary winding TN1. The main switching element TR1 is controlled by the control circuit 14, and intermittently turns on / off the DC voltage and converts it into an intermittent voltage. An output rectifying / smoothing circuit 20 is connected to the secondary winding TN2 of the insulating transformer T1 to supply power to the load 16. In the output rectifying and smoothing circuit 20, a synchronous rectifying element TR21 and a synchronous rectifying element TR22 connected in series are connected in parallel with the insulating transformer T1. The synchronous rectifying element TR21 and the synchronous rectifying element TR22 are controlled by the synchronous rectifying element driving circuit 18. An output capacitor Co and an output choke coil Lo connected in series are connected in parallel to both ends of the synchronous rectifier element TR22, and a voltage generated at both ends of the output capacitor Co is supplied to the load 16 as an output voltage.

  The active clamp circuit 24 has a configuration in which a clamp capacitor C2 and a clamp element TR2 connected in series are connected in parallel with the tertiary winding TN3. The clamp element TR2 is controlled by the control circuit 14 to be turned on / off alternately with the main switching element TR1.

  The auxiliary power supply circuit 22 is a rectifying / smoothing circuit including a diode Dsub and a capacitor Csub. The capacitor Csub is charged with a current flowing from the anode to the cathode of the diode Dsub. Connected to both ends. The voltage generated in the capacitor Csub is supplied to the control circuit 14 as the auxiliary power supply voltage Vsub.

  The main switching element TR1, the synchronous rectifying element TR21, the synchronous rectifying element TR22, and the clamp element TR2 use N-channel MOS-FETs, but other elements may be used as long as they operate similarly. .

  In order to specifically describe the operation of the switching power supply 10 of the present invention, an example in which the output voltage Vo = 5V and the auxiliary power supply voltage Vsub for driving the MOS-FET in the range of the input voltage Vin = 30V to 60V and about 10V is obtained. Will be described. The number of turns N1 of the primary winding TN1 of the insulating transformer T1 is 8 turns, and the number of turns N2 of the secondary winding TN2 is 2 turns. In the present invention, since the choke coil is not used in the auxiliary power supply circuit, it is not necessary to consider the relationship between the critical current, the output current of the auxiliary power supply, and the switching frequency.

  In the switching power supply device 10 of the present invention, the clamp element TR2 is turned off when the main switching element TR1 is on, and the clamp element TR2 is turned on when the main switching element TR1 is off. In actual control, by setting a dead time between the turn-off of the clamp element TR2 and the turn-on of the main switching element TR1, control is performed so that the drain-source voltage of the main switching element TR1 is turned on, and The switching loss accompanying the switching operation of the main switching element TR1 is reduced.

  In the present invention, in order to operate the output choke coil Lo in the current continuous mode regardless of the output current Io of the switching power supply device 10, a synchronous rectification forward converter system is used. In the synchronous rectifier driving circuit 18, when the main switching element TR1 is on, the synchronous rectifying element TR21 is on and the synchronous rectifying element TR22 is off. When the main switching element TR1 is off, the synchronous rectifying element TR21 is off and the synchronous rectifying element TR22 is off. The synchronous rectifying element TR21 and the synchronous rectifying element TR22 are controlled in synchronization with the main switching element TR1 so that is turned on.

  FIG. 2 (A) shows the operation waveforms of each part of the switching power supply device 10 according to the present invention when the input voltage Vin is 30V, and FIG. 2 (B) shows the present invention when the input voltage Vin is 60V. The operation | movement waveform of each part of the switching power supply device 10 by is shown. In FIG. 2, the gate-source voltage Vgs of the main switching element TR1 is (a), the drain-source voltage Vds is (b), and the gate-source voltage of the clamp element TR2 in the active clamp circuit 24. The voltage VGS is shown in (c), and the drain-source voltage VDS is shown in (d). In order to keep the input voltage constant, when the input voltage is low, that is, when the input voltage Vin is 30 V, the gate-source voltage of the main switching element TR1 is shown in FIG. When the duty time is increased by extending the ON time Ton of Vgs and the input voltage is high, that is, when the input voltage Vin is 60 V, the gate-source voltage of the main switching element TR1 as shown in FIG. (A) Control is performed to shorten the duty time by shortening the Vgs on-time Ton.

  When the main switching element TR1 is on, the input voltage Vin is applied to the primary winding of the isolation transformer T1. The isolation transformer T1 of the active clamp type forward converter operates to demagnetize the amount excited when the main switching element TR1 is turned on when the main switching element TR1 is turned off. Are working to be the same. As shown by (b) Vds indicating the drain-source voltage of the main switching element TR1 in FIGS. 2A and 2B, in order to make excitation and demagnetization the same, (input voltage Vin × main switching) The voltage Vr is generated in the insulating transformer T1 so that the ON time Ton of the element TR1 is equal to (the voltage Vr generated in the transformer when the main switching element TR1 is OFF × the OFF time Toff of the main switching element TR1). For this reason, when the input voltage Vin is low, the voltage Vr generated in the insulating transformer T1 increases, and when the input voltage Vin is high, the voltage Vr generated in the insulating transformer T1 decreases.

  At this time, the gate-source voltage VGS of the clamp element TR2 is on / off controlled in the opposite phase to the main switching element TR1, as shown in (c) VGS of FIGS. 2 (A) and 2 (B). As shown in (d) VDS, the drain-source voltage VDS of the clamp element TR2 is a voltage having the same phase as (b) Vds indicating the gate-source voltage of the clamp element TR2.

  At this time, the drain-source voltage VDS of the clamp element TR2 can be obtained as follows.

  The switching power supply device 10 of the present invention is a synchronous rectification forward converter in which the output choke coil Lo operates in the current continuous mode, and the duty duty of the main switching element TR1 is the following expression obtained by modifying Expression (3).

メインスイッチング素子ＴＲ１のオン時間Ｔｏｎとメインスイッチング素子ＴＲ１のオフ時間Ｔｏｆｆの比は、デューティｄｕｔｙと（１−ｄｕｔｙ）の比に等しいため、次式が成立する。

Since the ratio of the on-time Ton of the main switching element TR1 and the off-time Toff of the main switching element TR1 is equal to the ratio of the duty duty and (1-duty), the following equation is established.

メインイッチング素子ＴＲ１がオンの時、絶縁トランスＴ１の一次巻線ＴＮ１に印加された入力電圧Ｖｉｎが三次巻線ＴＮ３に伝えられる。絶縁トランスＴ１の三次巻線ＴＮ３は、図１においてドット印を付した側にプラスの電圧が発生する。このとき、三次巻線ＴＮ３に発生する電圧ＶＮ３は次式となる。

When the main switching element TR1 is on, the input voltage Vin applied to the primary winding TN1 of the isolation transformer T1 is transmitted to the tertiary winding TN3. The tertiary winding TN3 of the insulating transformer T1 generates a positive voltage on the side marked with a dot in FIG. At this time, the voltage VN3 generated in the tertiary winding TN3 is expressed by the following equation.

従って、三次巻線ＴＮ３に発生する電圧ＶＮ３は、入力電圧Ｖｉｎに比例し、入力電圧Ｖｉｎが低いと三次巻線ＴＮ３に発生する電圧ＶＮ３が低くなり、入力電圧Ｖｉｎが高いと三次巻線ＴＮ３に発生する電圧ＶＮ３が高くなる。

Therefore, the voltage VN3 generated in the tertiary winding TN3 is proportional to the input voltage Vin. When the input voltage Vin is low, the voltage VN3 generated in the tertiary winding TN3 is low, and when the input voltage Vin is high, the voltage VN3 is applied to the tertiary winding TN3. The generated voltage VN3 increases.

  When the main switching element TR1 is off, the voltage Vr is generated in the primary winding TN1, and at the same time, a voltage proportional to the voltage Vr is generated in the tertiary winding TN3. The tertiary winding TN3 generates a positive voltage on the side without the dot mark in FIG. Since the voltage generated in the tertiary winding TN3 is stored in the voltage VC2 of the clamp capacitor C2, if the forward voltage drop of the diode Dsub can be ignored, the voltage VC2 of the clamp capacitor C2 is expressed by the following equation.

メインスイッチング素子ＴＲ１がオンの時、クランプ素子ＴＲ２のドレインには、クランプコンデンサＣ２の電圧ＶＣ２に、三次巻線ＴＮ３に発生する電圧ＶＮ３を加えた値が印加される。従って、クランプ素子ＴＲ２のドレインに印加される電圧は、メインスイッチング素子ＴＲ１のドレインに印加される電圧をトランスの巻数比で換算した値と等しく、クランプ素子ＴＲ２に印加される電圧ＶＤＳは次式で得られる。

When the main switching element TR1 is on, a value obtained by adding the voltage VN3 generated in the tertiary winding TN3 to the voltage VC2 of the clamp capacitor C2 is applied to the drain of the clamp element TR2. Therefore, the voltage applied to the drain of the clamp element TR2 is equal to the value obtained by converting the voltage applied to the drain of the main switching element TR1 by the turns ratio of the transformer, and the voltage VDS applied to the clamp element TR2 is can get.

本スイッチング電源装置１０は、メインスイッチング素子ＴＲ１がオフの時にトランスに発生する電圧Ｖｒは、入力電圧Ｖｉｎが低いと高くなり、入力電圧Ｖｉｎが高いと低くなる特性を持つため、入力電圧Ｖｉｎが大きく変化しても、入力電圧Ｖｉｎとメインスイッチング素子ＴＲ１がオフの時にトランスに発生する電圧Ｖｒを足した電圧は大きく変動しない。従って、クランプ素子ＴＲ２のドレインに印加される電圧も大きく変化しない。

In this switching power supply device 10, the voltage Vr generated in the transformer when the main switching element TR 1 is off has a characteristic that it increases when the input voltage Vin is low and decreases when the input voltage Vin is high. Even if it changes, the voltage obtained by adding the input voltage Vin and the voltage Vr generated in the transformer when the main switching element TR1 is OFF does not vary greatly. Therefore, the voltage applied to the drain of the clamp element TR2 does not change greatly.

  The auxiliary power supply circuit 22 is composed of a drain-source voltage VDS of the clamp element TR2 by a rectifier circuit including a diode Dsub and a capacitor Csub, and the rectified voltage is used as an auxiliary power supply for the control circuit. The voltage Vsub is the voltage VDS applied to the clamp element TR2. For this reason, in order to obtain about 10 V as the voltage of the auxiliary power supply, the number of turns N3 of the tertiary winding TN3 may be set to 1 turn from the equation (13).

  FIG. 3 shows the voltage Vr generated in the transformer and the auxiliary power supply voltage Vsub when the main switching element TR1 is off with respect to the input voltage Vin of the switching power supply apparatus of the present invention. As can be seen from FIG. 3, when the input voltage Vin increases, the voltage Vr generated in the transformer decreases when the main switching element TR1 is turned off. On the other hand, even if the input voltage Vin changes greatly, it can be seen that the auxiliary power supply voltage Vsub hardly changes and is about 10V.

As mentioned above, although embodiment of this invention was described, this invention contains the appropriate deformation | transformation which does not impair the objective and advantage, Furthermore, it does not receive the restriction | limiting by said embodiment.

DESCRIPTION OF SYMBOLS 10 Switching power supply device 12 Input power supply 14 Control circuit 16 Load 18 Synchronous rectification element drive circuit 20 Output rectification smoothing circuit 22 Auxiliary power supply circuit 24 Active clamp circuit T1 Insulation transformer TN1 Primary winding N1 Number of primary windings TN2 Insulation transformer Secondary winding N2 Number of turns of secondary winding of isolation transformer TN3 Tertiary winding TR1 Main switching element TR21 Synchronous rectifying element TR22 Synchronous rectifying element Vin Input voltage Vo Output voltage Vsub Auxiliary power supply voltage TR2 Clamping element Io Output current Lo Output choke Coil Vsub Auxiliary power supply voltage Isub Auxiliary power supply current Vds Drain-source voltage of main switching element Vgs Gate-source voltage of main switching element VDS Drain-source voltage of clamp element The gate-source voltage of the GS clamping element Dsub diode Csub capacitor C2 clamp capacitor

Claims (4)

In order to intermittently generate a DC input voltage and generate an intermittent voltage, a DC input power source connected in series and a primary winding of an isolation transformer and a main switching element that performs on / off operation,
An output rectifying / smoothing circuit that rectifies and smoothes the intermittent voltage generated in the secondary winding of the isolation transformer by the on / off operation of the main switching element to generate a DC output voltage and supplies power to the load;
A control circuit for controlling the on / off operation of the main switching element;
In a forward converter type switching power supply device including an auxiliary power supply circuit that is connected to a tertiary winding provided in the isolation transformer and supplies power for the control circuit,
The auxiliary power supply circuit is connected to the tertiary winding via an active clamp circuit.
In claim 1,
The active clamp circuit is:
A clamp element that performs on / off operation connected in series and a clamp capacitor are connected in parallel with the tertiary winding,
The auxiliary power circuit is connected to both ends of the clamp element;
A switching power supply device.
In claim 2,
The auxiliary power circuit is composed of a diode and a capacitor connected in series to the cathode of the diode,
A switching power supply device characterized in that a voltage generated across the capacitor is supplied to the control circuit as an auxiliary power supply voltage.
In claim 1,
The output rectifying / smoothing circuit includes: a synchronous rectifying element driving circuit; two synchronous rectifying elements that are ON / OFF controlled by the synchronous rectifying element driving circuit and connected in parallel to the secondary winding; and the synchronous rectifying element A switching power supply comprising: an output choke coil and an output capacitor connected in series in parallel at one end of the capacitor, and supplying a voltage generated at both ends of the capacitor to a load.

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