JPH05316726A - Switching power source - Google Patents

Abstract

PURPOSE:To realize a zero-voltage switch without necessity of providing a reverse blocking diode at a secondary side. CONSTITUTION:A rectifier 40 and a smoothing circuit 42 are connected to a secondary winding 10 of a transformer 2 having a primary winding 4 connected to a main switch 8. An equivalent parasitic diode is not included as a secondary switch 44 of the rectifier 40 to be turned ON, OFF by a secondary side control circuit 32. For example, a zero-voltage switch is realized without using a reverse blocking diode by forming it of a switch element made of a bipolar transistor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a switching power supply capable of realizing a zero voltage switch.

[0002]

2. Description of the Related Art Generally, a device shown in FIG. 7 is known as a switching power supply device for supplying a stabilized DC power supply to various electronic devices. That is, FIG. 7 shows a device of the forward converter system, which has input terminals 6A and 6B.
A DC voltage E1 is applied to the main switch section 8 by a control circuit 9 to turn on / off the switch section at a predetermined time ratio, so that an alternating current flows through the primary winding 4 of the transformer 2. Transformer 2
The AC voltage generated in the secondary winding 10 is rectified by a rectifier circuit including two diodes 12 and 14, and
The DC voltage is applied to the load 20 after being smoothed by the smoothing circuit including the choke coil 16 and the capacitor 18.

An auxiliary winding 22 is provided on the primary side of the transformer 2 for clamping the voltage applied to the main switch section 8 to prevent an excessive voltage from being applied, and the auxiliary winding 22 is provided. A diode 24 is connected to the winding 22. In such a switching power supply device,
A DC voltage E1 of, for example, 100 V is applied to the input terminals 6A and 6B, and the main switch unit 8 causes 100 to several hundred KH
By turning on / off at the frequency of z, pulse width modulation control can be performed, and a DC voltage of 5 V, for example, can be obtained at the secondary side output terminals 26A and 26B.

By the way, in this type of power supply device,
When the main switch unit 8 is turned on, there is a certain voltage drop, so that not only power loss occurs, but also when turning on, for a short time, when voltage and current overlap, power loss occurs. was there. That is,
The secondary side choke coil 16 while the main switch unit 8 is on
Energy is stored in the choke coil 16 when it is turned off, the energy in the choke coil 16 is released via the flywheel diode 14 and the output voltage is smoothed. At this time, however, part of the current from the choke coil 16 is secondary. The current also flows through the winding 10 and the diode 12 becomes conductive. When the diode 12 conducts in this way, both ends of the primary winding 4 are 0V.
The DC voltage E1 fixed to the input terminals 6A and 6B is directly applied to the main switch portion 8 and when it is turned on in this state, power loss occurs as described above.

Therefore, in order to eliminate the above-mentioned inconvenience,
A device that enables a so-called zero voltage switch has been proposed. As a device capable of this zero voltage switch, for example, one disclosed in Japanese Patent Laid-Open No. 168853/1990 is known, and this device is shown in FIG. Figure 7
The same parts as those in the inside are designated by the same reference numerals. A MOSFET (field effect transistor) 28 is provided in place of the secondary side diode 12 shown in FIG. 7, the main switch section 8 is turned on / off at a predetermined duty ratio by the primary side control circuit 30, and the MOSFET 28 is Secondary side control circuit 3
Due to 2, the main switch unit 8 is turned on in synchronization with or after the turn-on of the main switch unit 8 and is turned off earlier than the main switch unit 8 is turned off. When the main switch unit 8 is turned off, each winding voltage of the transformer 2 is inverted and a flyback voltage is generated. When the flyback voltage generated in the auxiliary winding 22 reaches the DC voltage E1, the diode 24 becomes conductive and clamps the flyback voltage. Diode 24
When the current flowing in the transformer 2 becomes zero, the flyback voltage drops and each winding voltage of the transformer 2 oscillates due to its exciting inductance and capacitance such as parasitic capacitance. In the case of the circuit configuration of FIG. 7, the diode 12 conducts and each winding voltage is fixed at 0V, but in the case of the circuit of FIG.
Since 28 remains off, each winding voltage is inverted again, so that the applied voltage to the main switch unit 8 is the DC voltage E.
It is lower than 1. If the number of turns of the auxiliary winding is smaller than that of the primary winding 4, the voltage applied to the main switch unit 8 becomes zero, and a zero voltage switch can be realized.

[0006]

By the way, in order to realize the above-mentioned zero voltage switch, the M used on the secondary side is used.
Since the OSFET 28 always includes the parasitic diode 34,
In order to reliably prevent conduction of the parasitic diode 34 when the main switch portion 8 is turned off, the reverse conduction prevention diode 36 must be provided in order to prevent reverse conduction. For this reason, during operation, a voltage drop occurs not only across the MOSFET 28 but also across the reverse conduction blocking diode 36, and a conduction loss of two elements occurs, resulting in an increase in power consumption, especially when a large current is output. Had a problem of high loss. The present invention focuses on the above problems,
It was created to solve this effectively. An object of the present invention is to provide a switching power supply device that does not require a reverse conduction blocking diode on the secondary side and can realize a zero voltage switch.

[0007]

In order to solve the above problems, the present invention provides a transformer having a primary winding connected to a main switch portion which is turned on and off at a predetermined duty ratio, and the transformer. A rectifying circuit including a secondary switch unit connected to the secondary winding of the above, and a smoothing circuit connected to the rectifying circuit,
In a switching device having a secondary side control circuit that controls ON / OFF of the secondary switch unit based on the output of the smoothing circuit, the secondary switch unit switches so as not to include an equivalent parasitic diode. It is formed by an element.

[0008]

Since the present invention is constructed as described above, an AC voltage is induced in the secondary winding by turning on / off the main switch portion provided in the primary winding of the transformer. The AC voltage is rectified by a rectifier circuit including a secondary switch section that is turned on / off under the control of the secondary side control circuit, smoothed by a smoothing circuit, and supplied to the load as a DC voltage. The main switch section performs a zero-voltage switch operation by the action of the secondary switch section, but since the secondary switch section is composed of switch elements so as not to include an equivalent parasitic diode, it is necessary to provide an extra reverse conduction blocking diode. There is no unnecessary power loss.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a switching power supply device according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing an embodiment of a switching power supply device according to the present invention. The same parts as those of the circuit configuration shown in FIG. 8 are designated by the same reference numerals. This switching power supply device is a device capable of performing a zero voltage switch by a forward converter method, and the primary side of the transformer 2 has a primary winding 4 and an auxiliary winding 22, and this primary winding has For example, a main switch unit 8 made of a MOSFET is connected. A DC voltage E of about 100 V is applied to the input terminals 6A and 6B on the primary side, for example.
1 is applied, and the primary side control circuit 30 connected to the main switch portion 8 turns on / off the main switch portion 8 at a predetermined duty ratio so that an alternating current flows through the primary winding. Is composed of. In addition, the auxiliary winding 2 on the primary side
A diode 24 is connected in series to 2 to prevent an excessive voltage from being applied to the main switch section 8 as in the case shown in FIG.

On the other hand, the secondary winding 1 is provided on the secondary side of the transformer 2.
A rectifying circuit 40 for rectifying the AC voltage generated at 0 and a smoothing circuit 42 for smoothing the rectified voltage are sequentially provided. The output terminals 26A and 26B are provided with the load 2
0 is connected. Specifically, the rectifying circuit 40
Has a secondary switch section 44, which is a feature of the present invention. The secondary switch section 44 is composed of a switch element that does not include an equivalent parasitic diode. Here, the equivalent parasitic diode is a concept that refers to a parasitic diode function that is exhibited when viewed as the entire switch unit. Therefore, as will be described later, when two switch elements including a parasitic diode are connected in series in mutually opposite directions to cancel the functions of both parasitic diodes, the entire switch section should not include an equivalent parasitic diode. become.

Specifically, the secondary switch section 44 is
As shown in the figure, it is composed of one NPN-type bipolar transistor 46 as a switching element. The collector of this transistor 46 is connected to the secondary winding 10 and the emitter thereof is the choke coil 1 of the smoothing circuit 42.
Connected to 6. The base of the transistor 46 is connected to the secondary side control circuit 32, and, for example, in the same manner as in the case shown in FIG. 8, the bipolar transistor 4 is synchronized or delayed with the turn-on of the main switch section 8.
6 is turned on and the bipolar transistor 46 is turned off earlier than the main switch portion 8 is turned off. A diode 14 that exhibits a flywheel function during rectification is connected between the emitter side of the bipolar transistor 46 and the other end of the secondary winding 10, and the choke coil is connected when the transistor 46 is off. The energy in 16 is emitted.

Then, the choke coil 1 of the smoothing circuit 42
A smoothing capacitor 18 is provided on the output side of 6,
The action of the capacitor 18 and the choke coil 16 is configured to smooth the pulsating flow from the rectifier circuit 40. The output of the smoothing circuit 42 is connected to the secondary side control circuit 32 as well as the load 20, and the control circuit 32 can control the ON period of the bipolar transistor 46 based on this output. Is configured.

In the case shown in FIG. 1, the secondary switch section 44
Although the single bipolar transistor 46 which does not include the parasitic diode is used in the above, the present invention is not limited to this, and any single switch element which does not include the parasitic diode may be used. Instead of the bipolar transistor, for example, as shown in FIG. IGBT (insulated gateb)
As shown in FIG. 3 and FIG. 3, the SIT (static induction) is used.
Alternatively, the transformer 50 may be used. The IGBT 48 has a structure in which a MOSFET and a bipolar transistor are combined and does not include a parasitic diode. Further, since it is a voltage-driven element, the driving power is small, and high-speed switching operation, high breakdown voltage and high current density are possible. Similarly, the SIT 50 does not include a parasitic diode and functions as a switch element. For example, by controlling the voltage between the gate and the source, the height of the potential barrier in the channel portion sandwiched between the gates is changed to control the drain current.

Next, the operation of this embodiment configured as described above will be described. First, the input terminal 6A on the primary side,
The DC voltage E1 applied to 6B is applied to the primary side control circuit 30.
Is converted into an AC voltage by the main switch unit 8 which is turned on and off at a predetermined duty ratio by the control of the above, and is applied to the primary winding 4 of the transformer 2. As a result, an AC voltage is output from the secondary winding 10, and this AC voltage is rectified by the rectifier circuit 40 including the secondary switch unit 44 and the diode 14, and the smoothing circuit 42 including the choke coil 16 and the smoothing capacitor 18. Is smoothed into a direct current, and is supplied to the load 20 from the output terminals 26A and 26B. And
The second control drive circuit 12 controls the ON period of the bipolar transistor 46, which is the secondary switch section 44, based on the output values at the output terminals 26A and 26B, and is synchronized or delayed with the turn-on of the main switch section 8. The transistor 46 is turned on and the transistor 46 is turned off earlier than the main switch section 8 is turned off.

When the secondary switch section 44 is turned off, the current flowing through it becomes zero, the power supply between the primary-secondary windings 4 and 10 of the transformer 2 is cut off, and the current flowing through the main switch section 8 is cut off. Is only the exciting current of the primary winding 4.
At this time, the voltage across the secondary side choke coil 16 is inverted and a flywheel current flows through the diode 14.
Then, when the main switch unit 8 is turned off, a flyback voltage is generated in each winding of the transformer, and when the flyback voltage generated in the auxiliary winding 22 reaches the DC voltage E1, the diode 24 becomes conductive and the flyback voltage is clamped. To do. Then, when the current flowing through the diode 24 becomes zero, the flyback voltage decreases, and each winding voltage of the transformer 2 oscillates due to its exciting inductance and capacitance such as the parasitic capacitance of the main switch unit 8. At this time, the secondary switch unit 4
Since 4 remains in the off state, the voltage of each winding is inverted again, so that the voltage applied to the main switch unit 8 becomes lower than the DC voltage E1 between the input terminals 6A and 6B, and the zero voltage switch operates. Be seen. Such an operation is substantially the same as the operation of the device shown in FIG.

Here, in this embodiment, a single bipolar transistor 46 containing no parasitic diode is used as the secondary switch section 44 forming a part of the rectifier circuit 40 on the secondary side. It is not necessary to provide the reverse conduction blocking diode 36 (see FIG. 8) required to realize the zero voltage switch in FIG. 8, and therefore, the power accompanying the voltage drop (about 0.5 V) generated in this part is not necessary. It is possible to prevent loss. In particular, when applied to a switching power supply in which a large current has been promoted, the power consumption is greatly reduced and high efficiency can be achieved. Such an effect is similarly exhibited when the switching element that does not include the parasitic diode as shown in FIGS. 2 and 3 is adopted in the secondary switch section 44.

In the above embodiment, the secondary switch section 44
However, the present invention is not limited to this, and the secondary switch unit 44 is composed of two MOSFETs 52 and 54 including a parasitic diode as a single unit as shown in FIG. However, the MOSFETs 52 and 54 may be connected in series so that the parasitic diodes 52A and 54A included therein face in opposite directions. This makes it possible to eliminate the equivalent parasitic diode in the secondary switch unit 44 as a whole. The gates of the two MOSFETs 52 and 54 are connected to the secondary side control circuit 32 via resistors 56 and 58.
Connect to. Further, the secondary switch unit 44 may be configured by connecting a saturable reactor 60 having a switching function not including a parasitic diode and a diode 62 in series. When the saturable reactor 60 is used, the turn-on operation of this switch function is controlled by the secondary side control circuit 32, and the turn-off operation is synchronized with the turn-off of the primary side main switch unit 8 (see FIG. 1). Therefore, the control becomes very easy. In addition, the loss at the time of turning on is only the copper loss of the winding, and the PN of the diode etc.
Less than power loss at the junction.

FIG. 6 shows the relationship between the current and the voltage drop in the secondary switch section shown in FIGS. 4 and 8. Figure 6
In FIG. 8, a straight line A shows the relationship between the current and the voltage drop in the secondary switch section formed by connecting a Schottky diode and a MOSFET in series as shown in FIG. 8, and a straight line B shows two MOSFETs as shown in FIG. Consists of 2
The relationship between the current and the voltage drop in the next switch section is shown.
Further, the straight lines C and D show the relationship between the current and the voltage drop of one Schottky diode and one MOSFET, respectively. The graph of FIG. 6 shows the case where the temperature of the MOSFET is not so high. As shown in the figure, the voltage drop of the straight line B showing the characteristic of the power supply device of the present embodiment is considerably lower than the voltage drop of the straight line A showing the characteristic of the secondary switch part of the conventional power supply device, and the secondary switch of the present embodiment. The power consumption in the section can be reduced significantly. For example, when the current value 30A used is taken as an example, the straight line A and the straight line B have a difference in voltage drop of 0.23 V. Therefore, in the present embodiment, the on-time ratio of the secondary switch unit is 0. It has been found that a power consumption of 33 can save about 2.3 W.

[0019]

As described above, according to the switching power supply device of the present invention, the following excellent operational effects can be exhibited. It is possible to realize a zero-voltage switch that can perform pulse width modulation control at a predetermined switching frequency, and since the equivalent parasitic diode has been eliminated, it is not necessary to provide an extra element, and power consumption can be drastically reduced. Higher frequency and higher efficiency can be achieved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply device according to the present invention.

FIG. 2 is a circuit diagram showing a modified example of the switching power supply device shown in FIG.

FIG. 3 is a circuit diagram showing another modification of the switching power supply device shown in FIG.

FIG. 4 is a circuit diagram showing another embodiment of the switching power supply device of the present invention.

FIG. 5 is a circuit diagram showing still another embodiment of the switching power supply device of the present invention.

FIG. 6 is a graph showing a relationship between a current and a voltage drop of a secondary switch unit for comparing the device of the present invention with a conventional device.

FIG. 7 is a circuit diagram showing an example of a conventional switching power supply device.

FIG. 8 is a circuit diagram showing another example of a conventional switching power supply device.

[Explanation of symbols]

 2 transformer 4 primary winding 8 main switch section 10 secondary winding 30 primary side control circuit 32 secondary side control circuit 40 rectifier circuit 42 smoothing circuit 44 secondary switch section 46 bipolar transistor 48 IGBT 50 SIT 52, 54 MOSFET 52A, 54A Parasitic diode 60 Saturable reactor

Claims (3)

[Claims] 1. A rectifier circuit including a transformer having a primary winding connected to a main switch portion which is turned on and off at a predetermined duty ratio, and a secondary switch portion connected to a secondary winding of the transformer. And a smoothing circuit connected to the rectifier circuit, and a secondary-side control circuit that controls ON / OFF of the secondary switch unit based on the output of the smoothing circuit. Is a switching power supply device characterized by being formed by a switching element so as not to include an equivalent parasitic diode. 2. The switching power supply device according to claim 1, wherein the secondary switch unit is formed of a switch element that does not include a parasitic diode. 3. The switching power supply device according to claim 1, wherein the secondary switch unit is formed by connecting two switch elements including a parasitic diode in series in mutually opposite directions.