JPH07298610A - Switching power source - Google Patents

Abstract

PURPOSE:To operate a MOSFET of a secondary rectifier without increasing number of components without complicating a structure of a transformer. CONSTITUTION:A smoothing circuit 30 having FETs 25, 26 and a choke coil 30 is connected as a secondary rectifier 27 to a secondary side of a transformer 21. When a FET 22 is ON, the FET 25 is conducted. When the FET 22 is OFF, the FET 26 is conducted. Thus, an output voltage Vo is generated from output terminals +Vout, -Vout. Thus, a drive signal of a sufficient voltage to operate the FETs 25, 26 is supplied to the FETs 25, 26 in synchronization with the switching of the FET 22.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronous rectification type switching power supply device for rectifying a voltage induced on a secondary side of a transformer which constitutes an inverter by a MOS type FET.

[0002]

2. Description of the Related Art Generally, a switching power supply device of this type is designed to obtain a predetermined DC output by switching a DC input voltage applied to a primary winding of a transformer forming an inverter by a switching element. When a diode is used as a secondary rectifier circuit that rectifies the voltage induced in the secondary winding of the transformer, there is a problem in that the forward voltage drop of the diode reduces the efficiency of the power supply device.

FIG. 2 shows an example of a push-pull type converter using a MOS type FET in the secondary rectifying section in order to improve the efficiency of such a power supply device. In the figure, 1 and 1A are input terminals to which a DC input voltage Vin is supplied, 2 is a capacitor connected between the input terminals 1 and 1A, and the input voltage Vi is two switching elements M.
When the OS type FETs 3 and 4 are turned on and off, they are alternately applied to the primary windings 5A and 5B of the transformer 5. On the other hand, on the secondary side of the transformer 5, a pair of Ms connected to one ends of the secondary windings 5C and 5D of the transformer 5 instead of the rectifying diode.
A smoothing circuit 10 including a choke coil 8 and a smoothing capacitor 9 is connected together with the OS type FETs 6 and 7, and one ends of auxiliary windings 5E and 5F forming the transformer 5 have resistors 11 and
Connected to the gates of FETs 6 and 7 via 12. Then, each of these elements constitutes a power conversion circuit 13 for converting the DC input voltage Vi into the DC output voltage Vo.

Reference numeral 14 is a pulse width control circuit which stabilizes the output voltage Vo by controlling the pulse conduction width to each FET 3, 4. This pulse width control circuit 14 has a resistor 15,
The FETs 3 and 4 are alternately turned on and off via 16 and a voltage proportional to the primary windings 5A and 5B of the main transformer 5 is turned on when either one of the FETs 3 and 4 is turned on.
Induce to C and 5D. At this time, the FET 4 is turned on, and if the potential on the dot side of the secondary windings 5C and 5D is higher than that on the non-dot side, a drive signal is given from the auxiliary winding 5E to the gate of the FET 6 and synchronized with the FET 4. Then FE
T6 turns on. Therefore, the output voltage Vo is supplied from the secondary winding 5C to the output terminals 17 and 17A via the FET 6. On the other hand, when the FET 3 is turned on and the potential on the non-dot side of the secondary windings 5C and 5D becomes higher than that on the dot side, a drive signal is supplied from the auxiliary winding 5F to the gate of the FET 7 conversely. , FE in synchronization with FET3
T7 turns on. Then, from the secondary winding 5D to FE
The output voltage Vo is supplied to the output terminals 17 and 17A through T7. When both FETs 3 and 4 are off, the output voltage Vo is supplied from the choke coil 8 to the output terminals 17 and 17A.

[0005]

In the above prior art, when the FETs 6 and 7 are used in place of the rectifying diode, the loss in the secondary rectifying section is reduced as compared with the conventional one.
Although it is possible to improve the efficiency of the power supply device, the auxiliary windings 5E and 5F are further wound around the transformer 5 to
It is necessary to supply a predetermined drive voltage to 6 and 7, resulting in a complicated transformer structure and an increase in cost. In addition, in order to avoid such a situation, the secondary windings 5C, 5
Although a method of directly applying the voltage induced in D to the gates of the FETs 6 and 7 as a driving voltage has been known, the output voltage Vo is 5 V or less, for example, at a low voltage of about 2 V or 3.3 V, A drive voltage sufficient to turn on the FETs 6 and 7 cannot be obtained, and the method can be applied only to a specific output voltage range in which the output voltage Vo is relatively high.

In view of the above problems, therefore, the present invention aims to sufficiently operate the MOS FET in any output voltage range without complicating the structure of the transformer and increasing the number of parts. An object is to provide a possible switching power supply device.

[0007]

According to the present invention, a MOS is provided on a secondary side of an inverter having a switching element and a transformer.
In the switching power supply device, which connects a secondary rectification unit composed of a FET and a smoothing circuit having a choke coil to supply a drive signal to the MOS type FET in synchronization with the switching of the switching element, The auxiliary winding is wound, and the MO is attached to one end of the auxiliary winding.
The gate of the S-type FET is connected.

[0008]

With the above structure, a predetermined voltage is generated in the choke coil connected to the secondary side of the transformer in synchronization with the on / off operation of the switching element. At this time, a sufficient voltage is induced in the auxiliary winding to operate the MOS type FET that constitutes the secondary rectification unit.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. This embodiment is applied to a forward type converter. In the figure, reference numeral 21 is a transformer such as a pulse transformer which insulates the primary side and the secondary side from each other. The primary winding 21A of this transformer 21 and a MOS type which is a switching element. FET22
And a series circuit as an inverter 23 has an input terminal + Vin,
It is connected between −Vin, and the DC input voltage Vi is applied to the primary winding 21A via the input terminals + Vin and −Vin. A drive signal from a pulse width control circuit 24 which is a control circuit is given to the gate of the FET 22 with an appropriate dead time, and the FET 22 constituting the inverter 23 is turned on and off by controlling the pulse width of this drive signal. By doing so, the input voltage Vi applied to the primary winding 21A of the transformer 21 is interrupted, and the predetermined DC output voltage V
o is obtained from the output terminals + Vout and -Vout.

On the secondary side of the transformer 21, M connected between the non-dot side terminal of the secondary winding 21B and the output terminal -Vout.
Secondary rectification for rectifying the voltage induced in the secondary winding 21B by the OS type FET 25 and another MOS type FET 26 connected between the source of the MOS type FET 25 and the dot side terminal of the secondary winding 21B. The part 27 is configured. These FET25,
In order to improve efficiency, 26 preferably has characteristics suitable for a synchronous rectification method, such as low on-resistance. Also, between the drain and source of each FET25,26,
Body diode 2 whose anode is connected to the source side
There are 8 and 29. Reference numeral 30 is a choke coil connected between the secondary winding 21B of the transformer 21 and the output terminal + Vout, and 31 is a smoothing capacitor connected between the output terminals + Vout and −Vout. Thus, the smoothing circuit 32 that smoothes the voltage rectified by the secondary rectification unit 27 is configured. Auxiliary windings 33 and 34 that supply a drive signal to the FETs 25 and 26 of the secondary rectification unit 27 are wound around the choke coil 30 in synchronization with the switching of the FET 22. One of the auxiliary windings 33 has a dot side terminal of FE
It is connected to the gate of T25 and the non-dot side terminal is connected to the output voltage -Vout line. Further, in the other auxiliary winding 34, the non-dot side terminal is connected to the gate of the FET 26, and the dot side terminal is the output voltage -Vout.
Connected to the line. Each auxiliary winding 33, 34 is connected to each FET2
It is necessary to have enough windings to generate enough voltage to turn on 5 and 26.

Next, the operation of the above configuration will be described. On the primary side of the transformer 21, the input signal Vin is intermittently applied to the primary winding 21A of the transformer 21 by repeatedly turning the FET 22 on and off by a drive signal from the pulse width control circuit 24. At this time, when the FET 22 is turned on, a positive voltage proportional to the primary winding 21A is induced on the dot side in the secondary winding 21B of the transformer 21, and a current from the secondary winding 21B to the choke coil 30 is generated. The positive voltage is also applied to the dot side of the choke coil 30 in accordance with the movement of the choke coil 30, and the positive voltage is induced on the dot side of the auxiliary windings 33 and 34 wound around the choke coil 30. A sufficient voltage for turning on the FET 25 is supplied from the auxiliary winding 33 between the gate and the source of the FET 25 constituting the secondary rectifying unit 27, and the source and the drain of the FET 25 become conductive, but the FET 26 Becomes lower than the source potential, and the source and drain of the FET 26 become non-conductive. Therefore, in this case, the current flowing from the secondary winding 21B to the source side of the FET 26 is cut off by the body diode 29 of the FET 26, and the secondary winding 21B → choke coil 30 → output terminal + V.
An output current is supplied through the path of out → output terminal −Vout → FET25 (or body diode 28) → secondary winding 21B.

Next, when the FET 22 is turned on, the back electromotive force generated in the primary winding of the transformer 21 causes
A positive voltage is induced on the non-dot side of the secondary winding 21B. In addition, the choke coil 30 generates a positive voltage on the non-dot side in order to maintain the continuity of the current, and the auxiliary winding
Positive voltage is induced on the non-dot side of 33 and 34. At this time, between the gate and source of FET26, this FET26
A voltage sufficient to turn on the FET is supplied from the auxiliary winding 34, and the source and drain of the FET26 become conductive, but the gate potential of the FET25 becomes lower than the source potential, and the source and drain of the FET25 are turned off. It becomes a state.
Therefore, the current that tries to flow from the secondary winding 21B to the source side of the FET 25 is the body diode of the FET 25.
The energy of the choke coil 30 that is cut off by the 28 and stored during the ON period of the FET 22 is changed to the choke coil 30 →
Output terminal + Vout → Output terminal −Vout → FET26
(Or body diode 29) → It is supplied as an output current through the path of choke coil 30. Thus, FET22
Output current is alternately supplied from the secondary winding 21 and the choke coil 30 every time the ON and OFF operations of are repeated, and the output terminals + Vout and -Vou are supplied via the smoothing capacitor 31.
A predetermined output voltage Vo is generated during t.

As described above, according to the above embodiment, in the switching power supply device using the pair of FETs 25 and 26 as the secondary rectifying unit 27 for rectifying the voltage induced in the secondary winding 21B of the transformer 21, the smoothing is performed. By winding the auxiliary windings 33 and 34 around the choke coil 30 forming the circuit 32 and connecting the gates of the FETs 25 and 26 to one ends of the auxiliary windings 33 and 34,
A drive signal having a voltage sufficient to operate each FET 25, 26 can be supplied from the auxiliary winding 33, 34 to the FET 25, 26. That is, the structure of the transformer 21 can be obtained by simply winding the auxiliary windings 33 and 34 around the choke coil 30 that constitutes the existing forward type converter and connecting one end of the auxiliary windings 33 and 34 to the gates of the FETs 25 and 26. The FE that configures the secondary rectification unit 27 for any output voltage range without increasing the complexity and increasing the number of parts.
It becomes possible to operate T25 and 26 sufficiently in synchronization with the switching of the FET22.

Although the forward type converter has been described in detail in the present embodiment, it is applicable to various converters having the choke coil 30 as at least the smoothing circuit 32, that is, center tap type, half bridge type, full bridge type converters and the like. , The same action and effect are exhibited. Further, as the switching element, a transistor or the like can be used instead of the MOS type FET.

The present invention is not limited to the above embodiment, but various modifications can be made within the scope of the present invention.

[0016]

According to the present invention, a secondary rectifying section composed of a MOS type FET and a smoothing circuit having a choke coil are connected to the secondary side of an inverter having a switching element and a transformer to switch the switching element. In a switching power supply device that supplies a drive signal to the MOS type FET in synchronization with, an auxiliary winding is wound around the choke coil, and the gate of the MOS type FET is connected to one end of the auxiliary winding. It is possible to provide a switching power supply device capable of sufficiently operating a MOS FET in any output voltage range without complicating the structure of the transformer and increasing the number of components.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram showing a conventional example.

[Explanation of symbols]

 21 transformer 22 MOS type FET (switching element) 23 inverter 25, 26 MOS type FET 27 secondary rectification section 30 choke coil 32 smoothing circuit 33, 34 auxiliary winding

Claims (1)

[Claims] 1. A secondary rectification unit composed of a MOS FET and a smoothing circuit having a choke coil are connected to the secondary side of an inverter having a switching element and a transformer, and synchronized with switching of the switching element. The MO
A switching power supply device for supplying a drive signal to an S-type FET, wherein an auxiliary winding is wound around the choke coil, and the gate of the MOS-type FET is connected to one end of the auxiliary winding.