JP2599288Y2 - Switching power supply - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a MOS
The present invention relates to a switching power supply that uses an FET and synchronizes the ON / OFF operation of the MOS FET with the switching operation of a switching element.

[0002]

2. Description of the Related Art A diode generally used as a rectifier has a power loss due to the presence of a forward voltage drop. As one means for reducing the power loss in the rectifying element, it is conceivable to use a transistor (here, a MOS FET) having a low power loss in an ON state as the rectifying element. However, when a MOS FET is used as a rectifying element, it is necessary to consider the existence of a parasitic diode of the MOS FET. That is, the reason is based on the following three items. When the parasitic diode conducts, the loss increases due to the forward voltage drop. When a reverse voltage is applied after the parasitic diode conducts, a surge current is generated due to the reverse recovery characteristic of the diode. When the parasitic diode conducts, carriers are present in the channel-drain junction region, and the turn-on / off dV / dt characteristics of the MOS FET deteriorate. Since the above-mentioned phenomenon is not preferable in driving the switching power supply, a means for preventing the parasitic diode of the MOS FET from conducting is required.

FIG. 2 shows an example of a circuit of a conventional switching power supply in which a MOS FET is used as a rectifying element and its parasitic diode is not conducted. In FIG. 2, M1 is a MOS FET as a rectifying element for main rectification.
And M2 is a MOS as a rectifying element for rectification during commutation.
FET. T2 is a converter transformer, which has a plurality of windings in addition to the primary winding N1 and the secondary winding N2, N3 is a driving winding of the commutating rectification MOS FET M2, and N4 is a bias. Winding of the capacitor C2 for charging,
N5 is a driving winding of the rectifying MOS FET M1. The operation of the circuit shown in FIG. 2 will be described below with reference to FIG. 3 showing the voltage waveform at each point of the circuit.

When the switching transistor Q1 is in the ON state, the DC transformer E supplies the converter transformer T
Current flows through the second primary winding N1, and a voltage is induced in each of the windings N2 to N5. At this time, the voltage V ₂ induced in the driving winding N5, rectifying MOS FET M1 is turned on is positively biased. Rectification MOS FET M
1 induced voltage V _t in the secondary winding N2 by the ON state is to supply DC power to the load. Also here, the bias capacitor C2 is in the charging state, the voltage V induced in the drive winding N3 and the terminal voltage V _C
Assuming that the relationship with ₁ is V ₁ > V _C , the commutation-time rectification MOS FET M2 is reverse-biased by the voltage V ₁ and is turned off.

Next, when the switching transistor Q1 shifts to the off state, a voltage in a direction opposite to that of the current is generated in each of the windings N2 to N5 by the inductance of the windings. Since the polarity of the voltage of each winding is inverted, the rectifying MOS FET M1 is reverse-biased and turned off. At this time, the voltage generated in the charging winding N4 becomes a forward voltage of the backflow preventing diode D1 to charge the bias capacitor C2, and the commutating rectifier M1.
OSFET M2 is due to the voltage terminal voltage V _C of the voltage and the bias capacitor C2 generated in the drive winding N3 is superimposed is positively biased to an ON state. When the commutation-time rectification MOS FET M2 is turned on, the commutation-time rectification MOS FET F is connected between both ends of the choke coil L1.
A closed circuit is formed via the main current path of ETM2, and DC power is supplied to the load by the flyback voltage generated in the choke coil L1.

When the switching transistor Q1 is in the off state, the converter transformer T2 is in the magnetic flux reset state, and the voltage generated in each of the windings N2 to N5 becomes zero, the commutating rectification MOS FET M2
It will continue to maintain the ON state by the terminal voltage V _C of the bias capacitor C2. By repeating the above operation, the switching power supply shown in FIG. 2 is driven, and either one of the rectifying MOS FET M1 and the commutating rectifying MOS FET M2 is always turned on, and the parasitic current existing in the MOS FET is always turned on. The diode does not conduct.

[0007]

When a MOS FET is used as a rectifying element of a switching power supply, an MO
It was mentioned earlier that it is necessary to keep the parasitic diodes present in the SFET from conducting. In the circuit shown in FIG. 2, the converter transformer T2 has a primary winding N to prevent the parasitic diode from conducting.
In addition to the primary and secondary windings N2, a large number of windings such as a driving winding N3, a charging winding N4, and a driving winding N5 must be provided. Providing a large number of windings on a single transformer in this way is not sufficient for insulation between the windings, especially for the insulation between the primary winding and the secondary and other windings, and also due to the heat generated by the windings. This is not an advantage in terms of measures for heat dissipation of the transformer and miniaturization of the device. Therefore, the present invention aims to obtain a switching power supply that uses a MOS FET as a rectifying element, and the MOS FET is driven in synchronization with the switching element and minimizes an increase in the number of windings provided in a converter transformer. And

[0008]

According to the present invention, a choke coil having a secondary winding and a converter transformer having a tertiary winding are used, and the secondary winding of the choke coil is charged. The tertiary winding of the converter transformer is used as a drive winding, and a bias capacitor and a drive winding are connected in series between the gate and source of a MOSFET for rectification during commutation. A series circuit of a charging winding and a diode for preventing backflow is provided so as to be connected in parallel, and the forward direction of the diode for preventing backflow at this time is a terminal of a bias capacitor generated by being charged by the charging winding. MOS F for rectification when inter-voltage is commutated
A switching power supply characterized in that ET is set in a direction of positively biasing.

[0009]

FIG. 1 shows a circuit of an embodiment of a switching power supply according to the present invention in which an increase in the number of windings provided in a converter transformer is minimized. In FIG. 1, the same parts as those in FIG. 2 are denoted by the same reference numerals. In FIG.
A primary winding N1 of a converter transformer T1 having a primary winding, a secondary winding and a tertiary winding as a driving winding between both ends of a DC power supply E, and a main current path of a switching transistor Q1 formed by an N-channel MOSFET are connected in series. Connect to The output terminal of the PWM control circuit, not shown in FIG. 1, is connected to the gate of the switching transistor Q1. One end of the secondary winding N2 of the converter transformer T1 is connected to a primary winding L1 of a choke coil CL having a primary winding and a secondary winding as a charging winding and a smoothing capacitor C1.
, And connected to the source of a rectifying MOSFET M1 as a rectifying element composed of an N-channel type MOS FET,
The drain of the rectifying MOS FET M1 is connected to the other end of the secondary winding N2 of the converter transformer T1. At this time,
One end of the secondary winding N2 of the converter transformer T1 on the primary winding L1 side of the choke coil CL has the same polarity as the one end of the primary winding N1 of the converter transformer T1 connected to the high potential side of the DC power supply E. I do.

The gate of the rectifying MOS FET M1 is
It is connected to a connection point between the secondary winding N2 of the converter transformer T1 and the primary winding L1 of the choke coil CL. A commutation-time rectifying MOS FET M2 as a rectifying element composed of an N-channel type MOS FET so as to be parallel to the primary winding L1 of the choke coil CL and the smoothing capacitor C1.
Are connected with the smoothing capacitor C1 side as a source. The gate of the commutation-time rectification MOS FET M2 is connected to one end of the tertiary winding N3 of the converter transformer T1 via the bias capacitor C2, and the other end of the tertiary winding N3 is connected to the commutation-time rectification MOS FET. Connect to the source of FET M2. At this time, one end of the tertiary winding N3 of the converter transformer T1 on the side connected to the source of the commutation-time rectification MOS FET M2 is connected to one end of the converter transformer T1.
It has the same polarity as one end of the next winding N1 connected to the high potential side of the DC power supply E.

A series circuit of a secondary winding L2 of the choke coil CL and a diode D1 for backflow prevention is connected to a diode D1 so as to be in parallel with the bias capacitor C2.
Is connected to the gate side of the rectifying MOS FET M2 during commutation of the bias capacitor C2. At this time, one end of the secondary winding L2 of the choke coil CL on the side of the bias capacitor C2 and one end of the primary winding L1 of the choke coil CL on the side of the secondary winding N2 of the converter transformer T1 have the same polarity. To do. Note that both ends of the smoothing capacitor C1 serve as output terminals of the switching power supply, and a load _RL is connected in parallel to the smoothing capacitor C1. With the circuit configuration described above, the secondary winding N2 of the converter transformer T1 according to the present invention fulfills the functions of the secondary winding N2 and the driving winding N5 of the converter transformer T2 in FIG. The tertiary winding N3 of T1 is connected to the converter transformer T2 in FIG.
Performs the function of the driving winding N3, and the choke coil CL
Is connected to the converter transformer T in FIG.
2 performs the function of the charging winding N4. As a result, an increase in the number of turns of the converter transformer can be suppressed without the conduction of the parasitic diode existing in the MOS FET.

In the circuit of the present invention shown in FIG.
The secondary winding L2 of the choke coil CL, which functions as a charging winding, charges the bias capacitor C2 when the switching transistor Q1 shifts to the off state and a voltage corresponding to the flyback voltage is generated. Polarity. When the switching transistor Q1 is in the ON state, the polarity of the choke coil CL is changed to the opposite polarity.
The bias capacitor C2 may be charged by an electromotive voltage induced in the next winding L2. Also, in the circuit shown in FIG.
The embodiment has been described using the N-channel type for ET.
A P-channel type MOS FET can also be used. In this case, the circuit may be configured so that the polarity of each winding is reversed. Further, in order to reduce spike noise when the MOSFET is turned off, a Schottky barrier diode having a smaller forward voltage drop than a parasitic diode of the MOSFET is provided in parallel with the MOSFET as a rectifier. In some cases.

[0013]

As described above, according to the present invention, in a switching power supply using a MOS FET as a rectifying element, a MOS FET is driven without conducting a parasitic diode existing in the MOS FET. The secondary winding provided is a charging winding. As a result, the number of windings of the converter transformer can be reduced, so that the load on the insulation measures between the input side and the output side of the converter transformer is reduced, and the insulation of the windings and measures for heat dissipation are facilitated. In addition, since the converter transformer can be downsized in addition to these, it also contributes to downsizing of the power supply device and cost reduction.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of a conventional switching power supply.

FIG. 3 is a voltage waveform at each point of the circuit shown in FIG. 2;

[Explanation of symbols]

 E DC power supply T1 Converter transformer with tertiary winding CL Choke coil with secondary winding M1 MOS FET for main rectification M2 MOS FET for rectification during commutation C2 Bias capacitor

Claims (2)

(57) [Scope of request for utility model registration] 1. A switching element connected to a primary winding of a converter transformer performs a switching operation, so that a stabilized DC output is supplied to a load through a rectifying and smoothing circuit connected to a secondary winding of the converter transformer. In the switching power supply to be supplied, the rectifying / smoothing circuit includes a rectifying element, a rectifying element for commutation, a choke coil, and a smoothing capacitor, and uses a MOSFET as the rectifying element and the rectifying element for commutating. Has a secondary winding, the converter transformer has a tertiary winding, a secondary winding of the choke coil is a charging winding, and a tertiary winding of the converter transformer is a driving winding. A bias capacitor and the drive winding are connected in series between the gate and source of the commutating MOS FET during commutation, and the bias capacitor is connected to the A series circuit of the charging winding and a diode for preventing backflow is provided so as to be connected in parallel, and the forward direction of the diode for preventing backflow at this time is generated by being charged by the charging winding. When the voltage between the terminals of the bias capacitor is connected in such a direction as to positively bias the rectifying MOS FET at the time of commutation, and when the switching element is on, the rectifying MOS FET is turned on. When the switching MOSFET is turned off and the switching element is turned off, the rectifying MOS FET is turned off.
A synchronous rectification type switching power supply in which the S FET is turned off and the commutating MOS FET is turned on. 2. The switching power supply according to claim 1,
A switching power supply, wherein a gate drive voltage of the rectifying MOS FET is obtained from a voltage generated in a secondary winding of a converter transformer.