JP2003092876A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high efficiency stabilized switching power supply by feeding a boost half bridge system (BHB system) switching power supply from an AC power supply thereby suppressing harmonics. SOLUTION: In a boost half bridge system (BHB system) switching power supply, a bridge diode is used for AC input, time ratio of a first switching element is set at 50% or less, and a first choke coil on the input side is operated in discontinuous mode thus suppressing harmonic currents from an input power supply. Since variation of voltage being applied to a second choke coil on the putput side can be suppressed by varying the number of turns of two secondary windings of a transformer, inductance can be reduced and a high efficiency power supply can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device composed of one converter and having a high efficiency in which a harmonic component of an input current is reduced by an AC input.

[0002]

2. Description of the Related Art The most common method for reducing the harmonic component of the input current of a switching power supply is to use an active filter circuit composed of a boost chopper circuit or the like at the input stage of the switching power supply. However, since the active filter circuit itself is a single converter, it has a configuration in which two converters are connected in series as a whole, which generally causes an increase in the number of parts and a decrease in efficiency.

On the other hand, in a synchronous rectification type switching power supply using a MOSFET for an output rectification circuit, a problem when trying to achieve high efficiency is that the higher the voltage between the source and drain of the MOSFET, the higher the on-resistance. It is large and the power loss is large, so that the efficiency of the power source is lowered. Further, the MOSFET forming the synchronous rectification circuit cannot be driven over the entire off period of the primary side switch, and switching loss occurs during this period.

The present applicant has previously proposed a switching power supply device that solves this problem. (JP-A-11-26226
3) This is a boost half bridge method (BHB method)
This is called a switching power supply device, and FIG. 4 shows an example of this circuit. In FIG. 4, Vin is an input AC power supply, Bd is a bridge rectifier circuit, 2a and 2b are input terminals, L1 is a choke coil, Q1 and Q2 are switch elements (MOSFETs),
C1 and C2 are capacitors, T is an output transformer, N1 is its primary winding Na, Nb is its secondary winding (with an intermediate terminal)
Q3 and Q4 are synchronous rectification MOSFETs, L2 and C3 are choke coils and capacitors respectively constituting an output filter, 16a and 16b are output terminals, 17 is a load, and 18 is a control circuit.

This circuit operation is performed by the primary side switching element Q1,
Q2 is alternately turned on and off (when one switch element is turned on, the other switch is turned off) to give a gate signal to the synchronous rectification MOSFETs, Q3 and Q4 via the output transformer, and load via the choke coil L2. Power 17
In addition, the voltage at the output terminal is detected,
The constant voltage control of the output voltage Vout is performed by changing the on / off ratio of the secondary switch elements Q1 and Q2.

By the way, the above circuit sends power from the input power source Vin to the series circuit of the capacitors C1 and C2 as a boost chopper circuit, and at the same time supplies power to the load from the series circuit of the capacitors C1 and C2 by the operation of the half bridge circuit. . This conventional circuit is a synchronous rectification MOSFET, Q
Since 3 and Q4 can be driven at all times except for a short off period, a synchronous rectification MOSFET having a low withstand voltage and a small on resistance can be used, and an output filter can be made small, thereby providing a highly efficient switching power supply. You can

Further, since the conventional circuit has a choke coil on the input side, if a bridge rectifier circuit is used in front of the input terminal, the input current can be easily operated by operating this choke coil in the discontinuous current mode. Although it is possible to reduce the harmonic component of, the duty ratio of the lower switch element Q1 is 50% to operate this choke in discontinuous mode.
It is necessary to operate with the following. For this reason, the voltage applied to the output choke coil L2 becomes large, so that a choke coil having a large inductance is required as the output choke coil L2, which becomes a factor of reducing the power supply efficiency.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and its purpose is to apply a boost half bridge type (BHB type) power supply device to an output choke coil. It is an object of the present invention to provide a highly efficient power supply device in which the applied voltage is reduced to reduce the inductance and the harmonic components of the input current are reduced.

[0009]

SUMMARY OF THE INVENTION The invention described in the claims for achieving the above object is achieved by changing the number of turns of two secondary windings of a transformer in a conventional boost half-bridge type switching power supply. By reducing the voltage applied to the output choke coil and using a choke coil with a small inductance, the loss in the choke coil is reduced, and at the same time, a high-efficiency power supply device that reduces the harmonic components of the input current is realized. be able to.

That is, a bridge type diode connected to an AC power source, a transformer having input terminals 2a and 2b for receiving a DC output of the bridge type diode, a primary winding and a secondary winding, and a connection between the input terminals. A choke coil, a first switch element, a series circuit (first), and a series circuit of a primary winding of the transformer and a capacitor (first) connected between terminals of the first switch element (first). 2), a second switch element connected between terminals of the primary winding of the transformer, a second capacitor, a series circuit (third), and a terminal of each of the secondary windings of the transformer. A switching power supply device comprising a series circuit (fourth) of first and second synchronous rectification MOSFETs connected to each other, and a series circuit (fifth) of an output choke coil and an output capacitor. Line Na, and Providing a difference in number of turns of the secondary winding Nb, is characterized in that so as to reduce the voltage applied to the output choke coils, there is provided a high-efficiency switching power supply.

[0011]

DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows an embodiment of the circuit of the present invention. FIG. 2 shows a voltage waveform applied to the output choke coil in the conventional circuit, and FIG. 3 shows a voltage waveform applied to the output choke coil according to the present invention.

In FIG. 1, Vin is an input power source and 2
a and 2b are input terminals. L1 is a first input choke coil, and a bridge diode is inserted between the input terminal and the choke coil. Q1 and Q2 are the first and second switching elements respectively, C1 and C2 are the first and second capacitors respectively, and T, N1 and NaNb are the transformer, its primary winding and the first two, respectively. Next winding part, second
Is a secondary winding portion, Q3 and Q4 are a first synchronous rectification MOSFET and a second synchronous rectification MOSFET, respectively, and L2 and C3 are a second choke coil and a third choke coil which respectively constitute an output filter. 16a, 1 with the condenser of
6b is an output terminal, 17 is a load, and 18 is a control circuit.

A first secondary winding portion Na of the transformer T,
By providing a difference in the number of turns of the second secondary winding portion Nb of the transformer T, the voltage generated at the intermediate terminal of the secondary winding of the transformer with respect to the connection point of the first and second synchronous rectification MOSFETs. According to the setting of the duty ratio of the input side switching element, the period during which the first switching element Q1 is on and the second period
The same voltage is output during the period when the switch element Q2 is turned on.

The boost half bridge type power supply device has input terminals 2a and 2b for receiving a DC voltage, and is connected between a transformer T having at least a primary winding N1 and secondary windings Na and Nb and the input terminal. The first series circuit of the first choke coil L1 and the first switch element Q1 and the primary winding N1 of the transformer T connected between the first switch element Q1 and the first capacitor C1. And a second switch element Q2 for forming a path through which a part of the current of the first choke coil L1 flows while the first switch element Q1 is off. A third series circuit with the second capacitor C2, a rectifier circuit connected to the secondary winding of the transformer T, a filter circuit connected to the rectifier circuit, and a DC connected to the load connected to the filter circuit. Output terminal 16a for outputting a pressure, 16
b and the output voltage of the output terminal, the first and second switch elements Q1 and Q2 are alternately turned on so that the other is off when one is on, and the first switch A bridge diode for inputting an AC voltage is characterized by having a control circuit for controlling the output voltage of the output terminals 16a and 16b to be constant by changing the ratio of the ON and OFF periods of the element Q1. Bd
Was used.

When a DC power source is input to the input power source Vin, there is no influence of the harmonic current, but when used as an AC power source, it is affected by the harmonic current.
When an AC power supply is used for in, an active filter circuit generally composed of a boost chopper circuit or the like is used. However, in such a circuit configuration, the number of parts is large and the efficiency tends to be lowered in general. Since the boost half-bridge type power supply device has the choke coil L1 disposed on the input side, the AC input is rectified by the bridge diode Bd to obtain DC power, and the first choke coil L1 on the input side is disconnected. By continuously operating, the harmonic components of the input current can be easily removed, and the DC power can be efficiently supplied.

Actually, the first choke coil L1 on the input side
As a means for discontinuously operating the switch, the ON time of the lower first switch element Q1 is made small, and it has a difference from the ON time of the upper second switch element Q2, and the duty ratio is 50% or less. Is required, and as a result, the transformer T
When the secondary windings Na and Nb have the same number of turns, the output side second choke coil L2 has a lower first side as shown in FIG.
A different voltage is generated depending on the on-time of the switch element Q1 of 1 and the on-time of the upper second switch element Q2, and a large ripple current is generated in the second choke coil L2 on the output side. A stable output voltage cannot be obtained unless a large inductance is provided to the second choke coil L2.

FIG. 2 shows an input voltage waveform of the output-side second choke coil L2 indicated by a point R. The primary winding of the transformer T is in a period in which the first switch element Q1 and the second switch element Q2 are on. The voltage between the terminals of N1 is converted by the winding ratio of the primary winding N1 of the transformer T and the first secondary winding Na or the second secondary winding Nb. Since the terminal voltage varies depending on the duty ratio, the voltage of the output-side second choke coil L2 also varies.

That is, assuming that the ON time of the first switch element Q1 is the voltage Vc1 of the first capacitor, the second output side
The voltage Vg of the choke coil L2 can be expressed by the following equation.

[Equation 1] Further, assuming that the ON time of the second switch element Q2 is the voltage Vc2 of the second capacitor, the voltage Vh of the second choke coil L2 on the output side can be expressed by the following equations.

[Equation 2] Therefore, for different Vc1 and Vc2, the number of turns of Na and Nb
By setting the number of turns of Na 'and the number of turns of Nb', the voltage Vg 'of the output-side second choke coil L2 is
Vh 'can be made equal.

In the present invention, the secondary winding N of the transformer
Since the number of turns of each of a and Nb is changed, even if the duty ratio is 50% or less, as shown in FIG.
The voltage applied to the choke coil L2 of the first switch element Q1 is turned on and the second switch element Q2 is turned off and is output, and the voltage applied to the choke coil L2 is turned on and the second switch element Q2 is turned on. Is turned off and output, there is no difference in both output voltages, and the ripple in the current flowing through the output-side second choke coil L2 is reduced,
As the second choke coil on the output side, a coil having a small inductance can be used, and the loss is reduced.

Although the above description has been given of the case where the input voltage is constant, the same can be said when the input voltage varies. That is, when the input voltage changes, the voltage applied to the output choke coil changes, but when the input voltage is the rated voltage, the secondary winding of the transformer is reduced so as to reduce the fluctuation of the voltage applied to the output choke coil. Na'N
By setting the winding b ′, the change in the voltage applied to the output choke coil can be reduced even when the input voltage changes, and the same effect can be obtained.

In the embodiment of FIG. 1, the first and second switches Q
Since 1 and Q2 are in a symmetrical positional relationship, the series circuit of the bridge type diode Bd and the first choke coil L1 is connected not between the drain and source terminals of the first switch element Q1 but the drain source of the second switch element Q2. Even if they are connected between terminals, the roles of Q1 and Q2 are interchanged, but the other circuit operations are the same.

Further, in the circuit of FIG. 1, the first and second synchronous rectification MOSFETs Q3 and Q4 are driven, and the first and second synchronous rectification MOSFETs Q and Q of the circuit of FIG.
3 and Q4 are respectively driven by the drain-source voltage of the other synchronous rectification MOSFET, but the driving method of this gate terminal is not limited to the method shown in FIG. If there is a similar effect.

Further, regarding the connection location of the second switch element Q2, the second capacitor C2 and the third series circuit,
In the embodiment of FIG. 1, it is connected between the terminals of the primary winding N1 of the transformer T, but even if it is connected between the terminals of the first switch element Q1, the equivalent circuit is the same and the same effect can be obtained.

The same effect can be obtained when the series circuit of the second switch element Q2 and the second capacitor C2 is transferred to the secondary winding of the transformer T or separated to the tertiary winding of the transformer.

[0025]

As is apparent from the above description, in the circuit of the present invention, the bridge diode is used for the AC input, the duty ratio of the first switching element is set to 50% or less, and the first switching element on the input side is used. The harmonic current from the input power source is reduced by operating the choke coil in the discontinuous mode, and the voltage fluctuation of the second choke coil on the output side is changed by changing the number of turns of each of the two secondary windings of the transformer. Since it is possible to reduce the output voltage, a stable output voltage can be obtained with a small inductance as the second choke coil on the output side, and a highly efficient power supply device can be manufactured.

[0026]

[Brief description of drawings]

FIG. 1 is an example of a power supply device showing an embodiment of the present invention.

FIG. 2 is a diagram showing a voltage characteristic applied to a second choke coil on the output side according to a conventional method.

FIG. 3 is a diagram showing a voltage characteristic applied to a second choke coil on the output side according to the method of the present invention.

FIG. 4 is a diagram showing a conventional DC voltage input power supply device.

[Explanation of symbols]

2a, 2b ... Input terminals 16a, 16b ... Output terminal 17 ... Load 18 ... Control circuit C1 ... First capacitor C2 ... Second capacitor C3 ... Output filter third capacitor Bd ... Bridge diode L1 ... First choke Coil L2 ... Second choke coil Q1 of output filter ... First switch element Q2 ... Second switch element Q3, Q4 ... Synchronous rectification MOSFET T ... Transformer N1 ... Primary winding Na of transformer T ... First of transformer T 1 secondary winding Nb ... 2nd secondary winding Na 'of transformer T ... 1st secondary winding Nb' of transformer T according to the present invention 2nd secondary winding of transformer T according to the present invention Vin ... Input power source V (R) R point voltage Vg ... R point voltage generated by C1 voltage Vh ... R point voltage Vg 'generated by C2 voltage ... R point generated by C1 voltage according to the present invention Voltage Vh '... book The voltage at the point R generated by the voltage of C2 according to the invention ... T1 of Q1 ... T2 of Q2 ... Ts of Q2 ...

Claims (5)

[Claims] 1. A transformer having a bridge type diode connected to an AC power source, an input terminal for receiving a DC output of the bridge type diode, a primary winding and a secondary winding, and a choke connected between the input terminals. A series circuit (first) of a coil and a first switch element, a primary winding of the transformer connected between terminals of the first switch element, and a capacitor (first).
1) and a series circuit (second) of the transformer, a series circuit (third) of a second switch element and a capacitor (second) connected between terminals of the primary winding of the transformer, First and second synchronous rectification M connected between each one terminal of the secondary winding
OSFET series circuit (fourth), intermediate terminal of secondary winding of the transformer, and the first and second synchronous rectification MOSFETs
In a power supply device including an output choke coil and an output capacitor connected between the second secondary winding and the connection point of the second secondary winding, the number of turns of the first secondary winding and the second secondary winding of the secondary winding is different. A switching power supply device, wherein the switching power supply device is provided to reduce fluctuations in voltage applied to the output choke coil. 2. The switching power supply device according to claim 1, wherein the transformer is provided with a tertiary winding. 3. The switching power supply device according to claim 1, wherein the third series circuit is connected between terminals of the first switch element. 4. The switching power supply device according to claim 1, wherein the third series circuit is connected between the secondary windings of the transformer. 5. The switching power supply device according to claim 2, wherein the third series circuit is connected between the tertiary windings of the transformer.