JP2002119053A - Switching regulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching regulator which always maintains its output impedance low, regardless of the conditions of a load and negative feedback. SOLUTION: This switching regulator is provided with a switching circuit 2 for performing switching of DC voltage from a rectifying smoothing circuit 1, a main coil 3 which is connected in series to the primary winding 15 of a transformer 4 and receives the output of the switching circuit 2 as an input, a rectifying circuit 5 and a smoothing circuit 6 for rectifying and smoothing voltage outputted from the secondary winding 16 of the transformer 4 and generating DC voltage, a voltage change detecting circuit 7 which outputs a control signal according to the variation of the DC voltage outputted from the smoothing circuit 6 from a reference voltage, and a frequency-variable oscillating circuit 9 which outputs, as a timing signal for driving the switching circuit 2, an oscillation signal whose oscillation frequency is controlled according to the control signal outputted from the detecting circuit 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a switching regulator.

[0002]

2. Description of the Related Art A DC voltage obtained by rectifying and smoothing an AC voltage from a commercial AC power supply is converted into a high-frequency AC voltage by switching, and a desired DC voltage is converted from the AC voltage using a high-frequency transformer or the like. A switching regulator that generates a stable DC voltage power supply by negatively feeding back the difference between the DC voltage and a predetermined reference voltage is generally called a switching regulator.

A conventional switching regulator for large power generally performs switching of a DC voltage at a constant frequency, and changes a flow angle thereof by negative feedback.
The configuration was such that WM control was performed.

On the other hand, a conventional switching regulator for small power generally stores current energy in a primary winding of a transformer and extracts the energy from a secondary winding when the primary winding is opened. In order to stabilize the output voltage, the energizing time of the primary winding was changed by negative feedback in order to vary the current energy stored in the primary winding of the transformer. .

[0005]

However, in the conventional switching regulator for high power, as shown in FIG. 6, the duty cycle of the PWM control is large at the time of heavy load, so that a sufficient flow angle can be obtained, and there is no negative feedback. Output impedance can be reduced, but at light load
Since the duty cycle of the WM control is small, the flow angle is small, and the output impedance is increased.
As a result, the dead time has increased, and there has been a problem that it is basically difficult to reduce noise such as a zero switch.

Further, the conventional switching regulator for low power requires a time for storing energy in the primary winding of the transformer and a time for releasing the energy through the secondary winding. Since all time cannot be used only for energy transfer, the output impedance in the absence of negative feedback becomes extremely large, which is not suitable for high power use.

The present invention has been conceived under such circumstances, and regardless of the load and negative feedback,
It is an object of the present invention to provide a switching regulator that can always keep the output impedance low.

[0008]

DISCLOSURE OF THE INVENTION In order to solve the above-mentioned problems, the present invention takes the following technical means.

According to a first aspect of the present invention, a first rectifying / smoothing means for rectifying and smoothing an AC voltage to generate a DC voltage, a switching means for switching a DC voltage from the first rectifying / smoothing means, An inductance element connected in series to the primary winding of the transformer and receiving the output of the switching means; and a second element for rectifying and smoothing the voltage output from the secondary winding of the transformer to generate a DC voltage. A rectifying / smoothing means, a voltage change detecting means for outputting a control signal according to a deviation between the DC voltage from the second rectifying / smoothing means and the reference voltage, and an oscillation frequency controlled according to the control signal from the voltage change detecting means And a variable frequency oscillating means for outputting the oscillation signal as a timing signal for driving the switching means. It is.

According to a preferred embodiment, there is provided an auxiliary inductance element connected in parallel to a series circuit of the inductance element and the primary winding of the transformer.

According to another preferred embodiment, the frequency variable oscillating means outputs a first pulse train as a timing signal, and the duty correcting means keeps the duty cycle of the first pulse train from the frequency variable oscillating means at approximately 50%. Having.

According to another preferred embodiment, the frequency variable oscillating means outputs a first pulse train as a timing signal, and based on the first pulse train from the frequency variable oscillating means, relates to the frequency of the first pulse train. And a switching control unit that generates a pair of second pulse trains in which the overlapping periods of the off periods are always constant, and drives the switching unit by the second pulse train.

According to another preferred embodiment, the switching control means includes a delay circuit for delaying the first pulse train by a predetermined time corresponding to the overlap time of the OFF period and outputting the delayed pulse train as a delay pulse train, and a delay pulse train from the delay circuit. And the first
An AND circuit that outputs the logical product of the pulse train as one of the second pulse trains, and a NOR circuit that outputs, as the other of the second pulse trains, a NOT signal of the logical sum of the delayed pulse train and the first pulse train from the delay circuit.

According to the present invention, by connecting an inductance element to the primary winding of the transformer in series and changing the switching frequency, the voltage applied to the primary winding is changed to change the output voltage. Is controlled, the flow angle is always constant, so that the output impedance can always be kept low irrespective of the load or the negative feedback situation.

[0015] Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be specifically described below with reference to the drawings.

FIG. 1 is a circuit block diagram of a switching regulator according to the present invention. This switching regulator includes a rectifying and smoothing circuit 1, a switching circuit 2, a main coil 3, a transformer 4, a rectifying circuit 5, a smoothing circuit 6, a voltage change detection circuit 7, an isolator 8, a variable frequency oscillation circuit 9, a switching control circuit 10, Voltage power supply circuit 11,
It has an auxiliary coil 12, an input terminal 13, an output terminal 14, and a resistor R1. The switching circuit 2 includes four MOS field-effect transistors FET1 to FET4.
It has. The transformer 4 includes a primary winding 15, a secondary winding 16, and a primary auxiliary winding 17.

The rectifying and smoothing circuit 1 rectifies and smoothes an AC voltage from a commercial AC power supply input to an input terminal 13,
The obtained DC voltage is supplied to the switching circuit 2.

The switching circuit 2 is controlled by a switching control circuit 10 to switch the DC voltage from the rectifying / smoothing circuit 1 and to apply the voltage obtained by the switching to the main coil 3 and the primary winding 15 of the transformer 4. To the series circuit.

The main coil 3 is a primary winding 1 of a transformer 4.
5, the voltage from the switching circuit 2 is divided. That is, the main coil 3 is provided to vary the voltage applied to the primary winding 15 according to the frequency of the voltage from the switching circuit 2.

The transformer 4 is a high-frequency transformer,
The voltage applied to the secondary winding 15 is boosted to increase the voltage of the secondary winding 1.
6 is output.

The rectifier circuit 5 includes the secondary winding 1 of the transformer 4.
6 is rectified and supplied to the smoothing circuit 6.

The smoothing circuit 6 smoothes the voltage from the rectifier circuit 5 and supplies the obtained DC voltage to the output terminal 14 and the voltage change detection circuit 7.

The voltage change detection circuit 7 compares the DC voltage from the smoothing circuit 6 with a predetermined reference voltage, and supplies a control signal corresponding to the difference to the variable frequency oscillation circuit 9 via the isolator 8.

The isolator 8 transmits a control signal from the voltage change detection circuit 7 to the variable frequency oscillation circuit 9.

The variable frequency oscillating circuit 9 generates a first pulse train having a frequency corresponding to the control signal from the voltage change detecting circuit 7, and supplies the first pulse train to the switching control circuit 10 as a timing signal. This variable frequency oscillation circuit 9 has a built-in duty correction circuit, and the duty cycle of the first pulse train is maintained at approximately 50%.

The switching control circuit 10 generates a two-phase second pulse based on the first pulse train from the variable frequency oscillator 9.
A pulse train is generated, and one second pulse train is supplied to the gates of the field effect transistors FET1 and FET2 of the switching circuit 2, and the other second pulse train is supplied to the gates of the field effect transistors FET3 and FET4 of the switching circuit 2.

The constant voltage power supply circuit 11 generates a DC voltage having a predetermined voltage value based on the voltage from the primary side auxiliary winding 17 of the transformer 4, and outputs the DC voltage to the frequency variable oscillation circuit 9 and the switching control circuit 10. Power supply.

The auxiliary coil 12 is provided to allow the inertial current to flow through the flywheel diode of the switching circuit 2 to realize a zero voltage switch.

That is, a field effect transistor FET
1, FET2 or field effect transistor FET3
Current energy is stored in the auxiliary coil 12 during the ON period of the FET 4, and the current energy stored in the auxiliary coil 12 is turned off until the moment when the field effect transistors FET1, FET2 or the field effect transistors FET3, FET4 are turned off. Field effect transistors FET3, FET4 or field effect transistor F
By emitting the light through the flywheel diodes of ET1 and FET2, the flywheel diodes are turned on, and the voltages of the field effect transistors FET3 and FET4 or the field effect transistors FET1 and FET2 are reduced to almost zero.

More specifically, the current flowing through the auxiliary coil 12 immediately before the field-effect transistors FET1 and FET2 or the field-effect transistors FET3 and FET4 are turned off is the field-effect transistors FET1 and F1.
ET2 or field effect transistor FET3, FET
4 keeps flowing even when the transistor 4 is turned off.
T3, FET4 or field effect transistor FET
1. The current continues to flow through the parasitic diode or the external diode of the FET2, and the energy is eventually reduced by the resistance, and the current becomes zero. Since this diode is used to pass an inertial current, it is called a flywheel diode. While the current is flowing through the flywheel diode, the voltage across the corresponding field-effect transistor is almost zero, so if the field-effect transistor is turned on at that timing, almost no surge current flows. In addition, switching loss and noise can be suppressed. This method is called a zero voltage switch method.

During the period in which the current energy is stored in the auxiliary coil 12, the main coil 3 and the transformer 4
Current energy is also stored in the primary side leakage inductance, and this current energy is also discharged through the flywheel diode.

The resistor R1 is a starting resistor.

FIG. 2 is a circuit diagram of the variable frequency oscillation circuit 9. The frequency variable oscillation circuit 9 includes inverters 21 to 2
5, photocoupler 26, output terminal 27, resistors R2 to R
7 and capacitors C1 to C3. The photocoupler 26 includes a light emitting diode 28 on the light emitting side and a phototransistor 29 on the light receiving side. Inverter 21
Is a Schmitt trigger type inverter.

The control signal from the voltage change detection circuit 7 is input to the light emitting diode 28 of the photocoupler 26. As a result, the light emission intensity of the light emitting diode 28 changes according to the control signal, and the impedance of the phototransistor 29 changes accordingly, and the output of the rectangular wave oscillation circuit constituted by the circuit preceding the resistor R4. The frequency of the square wave changes. This square wave is corrected to a duty cycle of approximately 50% by a duty correction circuit after the resistor R4, and the first pulse train 3 is output from the output terminal 27.
It is output as 0 and supplied to the switching control circuit 10 as a timing signal.

FIG. 3 is a circuit diagram of the switching control circuit 10. The switching control circuit 10 includes the inverter 3
1, 32, an AND circuit 33, a NOR circuit 34, an input terminal 35, output terminals 36 and 37, a resistor R8, and a capacitor C4.

FIG. 4 is a signal waveform diagram of each part of the switching control circuit 10. In FIG. 4, A denotes a first pulse train 3 input from the frequency variable oscillation circuit 9 to the input terminal 35.
0, B is the voltage waveform at the connection point between the resistor R8 and the capacitor C4, C is the output voltage waveform of the inverter 32,
D is one of the second pulse trains 3 output from the output terminal 36.
The voltage waveform E of 8 is the voltage waveform of the other second pulse train 39 output from the output terminal 37.

The first pulse train 3 input to the input terminal 35
0 is supplied to one input terminal of the AND circuit 33 and the NOR circuit 34, is inverted by the inverter 31, is integrated by the integration circuit including the resistor R8 and the capacitor C4, and is inverted and shaped by the inverter 32. And the AND circuit 33 and the NOR circuit 3
4 is supplied to the other input terminal. Inverters 31 and 3
2. The resistor R8 and the capacitor C4 constitute a delay circuit, and the output of the inverter 32 is delayed from the first pulse train 30 by a delay time determined by the time constant of the resistor R8 and the capacitor C4. I have. Therefore, one second pulse train 38 and the other second pulse train 39
As is clear from FIG. 4, each pulse has an off period, that is, a dead time at each rise and fall of each pulse.

In this embodiment, since the MOS field effect transistors FET1 to FET4 are employed as the switching elements of the switching circuit 2, the dead time is set to about 0.1 to 0.5 μsec. The second
The upper limit of the frequency of the pulse trains 38 and 39 depends on the response speed of the switching elements of the switching circuit 2 and is about 1 to 2 MHz because MOS field effect transistors FET1 to FET4 are employed. The second pulse train 38,
The lower limit of the frequency of 39 is about several tens of kHz higher than the upper limit of the audible frequency.

FIG. 5 is a circuit diagram of the voltage change detection circuit 7. The voltage change detection circuit 7 includes a photocoupler 41, a constant voltage diode 42, transistors 43 and 44, and an input terminal 4.
5, 46, and resistors R9 to R13. The photocoupler 41 includes a light emitting diode 47 on the light emitting side and a phototransistor 48 on the light receiving side. The transistors 43 and 44 constitute a differential amplifier.

When a DC voltage from the smoothing circuit 6 is input to the input terminals 45 and 46, the DC voltage and a reference voltage determined by the constant voltage diode 42 are compared by a differential amplifier composed of transistors 43 and 44. Then, a current corresponding to the deviation flows through the light emitting diode 47 of the photocoupler 41, and the light emission intensity of the light emitting diode 47 is varied. As a result, the phototransistor 48 of the photocoupler 41 supplies a current corresponding to a deviation between the DC voltage from the smoothing circuit 6 and the reference voltage to the frequency variable oscillation circuit 9 via the isolator 8 as a control signal.

Next, the operation of the switching regulator will be described.

The AC voltage from the commercial AC power supply input to the input terminal 13 is rectified and smoothed by the rectification and smoothing circuit 1 and is switched by the switching circuit 2.
It is supplied to a series circuit of the main coil 3 and the primary winding 15 of the transformer 4. Thereby, the secondary winding 16 of the transformer 4
Is rectified by the rectifier circuit 5, smoothed by the smoothing circuit 6, output from the output terminal 14, and supplied to the voltage change detection circuit 7. As a result, the voltage change detection circuit 7 supplies a control signal corresponding to the deviation between the DC voltage from the smoothing circuit 6 and the reference voltage to the frequency variable oscillation circuit 9, and the frequency variable oscillation circuit 9 , The frequency of the first pulse train 30 is changed. Specifically, when the DC voltage from the smoothing circuit 6 is higher than the reference voltage, the frequency of the first pulse train 30 increases accordingly, and conversely, the DC voltage from the smoothing circuit 6 is lower than the reference voltage. In such a case, the frequency of the first pulse train 30 decreases accordingly.

That is, the impedance Z of the main coil 3
1 is Z1 if the inductance of the main coil 3 is L
= JωL. The impedance Z2 viewed from the primary side of the transformer 4 is given by Z2, where N is the turns ratio of the primary winding 15 to the secondary winding 16, and r is the load resistance.
= N ² r. Therefore, when the load becomes light, in other words, when the load resistance r increases, the impedance Z2 expected from the primary side of the transformer 4 increases, and the output voltage from the smoothing circuit 6 becomes higher than the reference voltage. 2 has a higher switching frequency. Here, considering the transient response of the current when a step voltage is applied to the series circuit of the main coil 3 and the primary winding 15 of the transformer 4, the current increases almost linearly during the transient period. Therefore, when the switching frequency of the switching circuit 2 increases, the primary winding 15
The period of increase of the current flowing through is shortened, and the peak value of the current decreases. As a result, the primary winding 15 of the transformer 4
, The output voltage from the smoothing circuit 6 also decreases, and becomes equal to the reference voltage.

Conversely, when the load becomes heavy, in other words, when the load resistance r becomes small, the impedance Z2 viewed from the primary side of the transformer 4 becomes small, and the output voltage from the smoothing circuit 6 becomes lower than the reference voltage. As a result, the switching frequency of the switching circuit 2 decreases. Here, considering the transient response of the current when a step voltage is applied to the series circuit of the main coil 3 and the primary winding 15 of the transformer 4, the current increases almost linearly during the transient period.
Therefore, when the switching frequency of the switching circuit 2 decreases, the period of increase of the current flowing through the primary winding 15 of the transformer 4 increases, and the peak value of the current increases. As a result, the voltage applied to the primary winding 15 of the transformer 4 increases, and the output voltage from the smoothing circuit 6 also increases, and becomes equal to the reference voltage.

Thus, the DC voltage output from the output terminal 14 is feedback-controlled so that it is always equal to the reference voltage.

In order to vary the voltage applied to the primary winding 15 of the transformer 4 by varying the switching frequency of the switching circuit 2, the main coil 3 and the primary winding 15 of the transformer 4 are required. It is necessary to use only the transient period in which the current flowing in the series circuit becomes linearly constant before it becomes a constant steady current. To this end, it is necessary to increase the switching frequency of the switching circuit 2 as the load becomes lighter. However, in the present embodiment, the switching frequency of the switching circuit 2 is increased when the load is reduced, and the switching frequency of the switching circuit 2 is decreased when the load is increased. become.

As described above, the primary winding 15 of the transformer 4
The main coil 3 is inserted in series with the main coil 3, and the switching voltage is divided by the impedance of the main coil 3 and the impedance of the primary winding 15 of the transformer 4. If the output voltage is higher, the switching frequency of the switching circuit 2 is increased to lower the voltage applied to the primary winding 15 of the transformer 4, and if the output voltage output from the output terminal 14 is lower than the reference voltage, Since the voltage applied to the primary winding 15 of the transformer 4 is increased by lowering the switching frequency of the switching circuit 2, the output voltage can be stabilized.

Moreover, regardless of the load and the state of the negative feedback,
Output impedance can always be kept low. That is,
Since the output voltage is controlled by changing the frequency instead of changing the duty cycle of the second pulse trains 38 and 39, the operation is always performed at a large flow angle regardless of a load change or the like. It is possible to obtain a stabilized output voltage which has a small rise and is particularly suitable for high power applications.

Since the duty cycle of the first pulse train 30 is maintained at about 50% by incorporating a duty correction circuit in the frequency variable oscillation circuit 9, no DC component is applied to the transformer 4 and abnormal operation is performed. Can be favorably prevented.

Since the switching control circuit 10 is configured so that the second pulse trains 38 and 39 have a fixed dead time regardless of the frequency, the field effect transistors FET1 and FET4 or the field effect transistors FET3 and FET2 are Generation of flowing through current can be favorably prevented. That is, a field effect transistor FET
1 and field-effect transistor FET4, or field-effect transistor FET3 and field-effect transistor FET2
Are turned on at the same time to prevent a through current from flowing, all the field effect transistors FET1 to FET4
Is turned off, that is, a so-called dead time is required. In a normal PWM controller, this period is 5 to 5 with respect to the switching cycle.
Although it is configured to have a constant ratio of about 10%, the required dead time depends on the response speed of the field effect transistors FET1 to FET4 as switching elements, and does not depend on the switching frequency. Therefore, it is preferable that the dead time is fixed regardless of the frequency of the second pulse trains 38 and 39.

Further, since the auxiliary coil 12 is connected in parallel with the series circuit of the main coil 3 and the primary winding 15 of the transformer 4, an inertial current can flow through the flywheel diode of the switching circuit 2 by the auxiliary coil 12. From that
When the field effect transistors FET1 to FET4 are switched from off to on, the field effect transistors FET1 to FE
A so-called zero voltage switch in which no voltage is applied to T4 can be realized.

In the above embodiment, the switching circuit 2 is of a full bridge type, but may be of a half bridge type depending on the application.

In the above embodiment, the photocouplers 26 and 41 having the light emitting diodes 28 and 47 and the phototransistors 29 and 48 are employed.
A photo-MOS-FET, CdS, or the like may be used as the light-receiving element, or a normal lamp or the like may be used as the light-emitting element.

In the above embodiment, the main coil 3 is used as the inductance element, but a saturable reactor may be used instead of the coil.

In the above embodiment, the auxiliary coil 12 is connected in parallel to the series circuit of the main coil 3 and the primary winding 15 of the transformer 4, but the auxiliary coil 12 is not necessarily provided.

In the above-described embodiment, the duty variable correction circuit is built in the frequency variable oscillation circuit 9.
It is not always necessary to provide this duty correction circuit.

Further, in the above embodiment, the switching control circuit 10 is designed to generate a fixed dead time.
Is configured, but it is not always necessary to configure in this way.

Of course, the specific circuit configuration of the above-described switching calculator is merely an example, and
It is not limited in this way.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram of a switching regulator according to the present invention.

FIG. 2 is a circuit diagram of the variable frequency oscillation circuit shown in FIG.

FIG. 3 is a circuit diagram of the switching control circuit shown in FIG. 1;

4 is a signal waveform diagram of each part of the switching control circuit shown in FIG.

FIG. 5 is a circuit diagram of the voltage change detection circuit shown in FIG.

FIG. 6 is an explanatory diagram of a PWM method employed in a conventional high-power switching regulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rectifier smoothing circuit 2 Switching circuit 3 Main coil 4 Transformer 5 Rectifier circuit 6 Smoothing circuit 7 Voltage change detection circuit 8 Isolator 9 Frequency variable oscillation circuit 10 Switching control circuit 11 Constant voltage power supply circuit 12 Auxiliary coil 15 Primary winding 16 2 Secondary winding 17 Primary auxiliary winding

Claims (5)

[Claims] A first rectifying / smoothing means for rectifying and smoothing an AC voltage to generate a DC voltage; a switching means for switching a DC voltage from the first rectifying / smoothing means; and a primary winding of a transformer. An inductance element connected in series and receiving an output of the switching means, a second rectifying and smoothing means for rectifying and smoothing a voltage output from a secondary winding of the transformer to generate a DC voltage; Voltage change detecting means for outputting a control signal corresponding to a deviation between the DC voltage from the second rectifying and smoothing means and the reference voltage; an oscillation frequency controlled according to a control signal from the voltage change detecting means; And a variable frequency oscillating means for outputting a signal as a timing signal for driving the switching means. 2. The switching regulator according to claim 1, further comprising an auxiliary inductance element connected in parallel to a series circuit of the inductance element and a primary winding of the transformer. 3. The frequency variable oscillating unit outputs a first pulse train as a timing signal, and has a duty correction unit that keeps a duty cycle of the first pulse train from the frequency variable oscillating unit at approximately 50%. Or the switching regulator according to 2. 4. The variable frequency oscillating means outputs a first pulse train as a timing signal. Based on the first pulse train from the variable frequency oscillating means, the first and second off-periods are independent of the frequency of the first pulse train. The switching regulator according to any one of claims 1 to 3, further comprising switching control means for generating a pair of second pulse trains each having a constant overlapping time and driving the switching means by the second pulse train. 5. The switching control means includes: a delay circuit that delays the first pulse train by a predetermined time corresponding to an overlap time of the off period and outputs the delayed pulse train as a delay pulse train; and a delay pulse train from the delay circuit and the first pulse train. AN that outputs a logical product of the pulse train and one of the second pulse trains
5. The switching regulator according to claim 4, further comprising: a D circuit; and a NOR circuit that outputs, as the other of the second pulse train, a NOT signal of the logical sum of the delay pulse train from the delay circuit and the first pulse train.