JP2781978B2 - Switching power supply - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulation type switching power supply using a power MOS transistor as a switching element. More specifically, a switching power supply that provides a capacitor charging / discharging circuit in parallel with the MOS transistor to slow down the voltage rise when the MOS transistor is turned off, reduce high-frequency noise, and reduce switching loss. It is.

[Related Art] Switching power supplies have been increasingly used at higher frequencies due to strong demands for miniaturization from the side of electronic devices using the switching power supplies. This is because when the switching frequency is increased, the size of the main transformer for performing voltage conversion, the capacitor of the rectifying and smoothing circuit, and the like can be reduced.

As the switching element, a bipolar transistor has been used, but in recent years, a power MOS transistor capable of high-speed switching has been used.

FIG. 3 shows a circuit configuration of a main part of a conventional switching power supply. This is an insulated switching power supply in which the input and output are DC-insulated by the main transformer. Provided power MOS transistor Q which is a switching element in series with the primary winding N ₁ of the main transformer T, thereby switching the DC input. And performs voltage conversion in the main transformer T, to obtain a DC output from the rectifying and smoothing circuit 10 connected to the secondary winding N _2. By controlling the on / off time of the MOS transistor Q by a drive signal applied to the gate thereof, the output is stabilized at a constant voltage.

The series circuit of the resistor R and the capacitor C connected in parallel to the MOS transistor Q is for surge absorption. The circuit of the windings N ₄ and the diode D are reset circuit of the main transformer T.

[Problems to be Solved by the Invention] As the switching frequency increases, the noise generated from the switching power supply increases. Since noise interferes with various electronic devices, noise voltage limits are officially regulated around the world.

Therefore, even if the frequency is increased to reduce the size of the switching power supply, it is necessary to strengthen the noise filter and the absorber to prevent noise. In the prior art, a considerable cost and volume are required for the noise filter, and when the absorber is strengthened, heat generation is increased and efficiency is reduced. As described above, it is considered that the increase in frequency and the reduction in noise are contradictory problems, and this has been a major bottleneck in downsizing the power supply.

SUMMARY OF THE INVENTION An object of the present invention is to provide a switching power supply which can solve the above-mentioned drawbacks of the prior art, can reduce noise even at higher frequencies, and can reduce switching loss.

Means for Solving the Problems The present invention provides a secondary winding of a main transformer by connecting a MOS transistor as a switching element in series with the primary winding of a main transformer and controlling the on / off time thereof. Is an insulated switching power supply that extracts a DC output from a rectifying and smoothing circuit connected to the power supply. In order to achieve the above object, according to the present invention, a charging circuit having a first diode and a capacitor connected in series is provided in parallel with the MOS transistor, and the charging circuit is provided in the main transformer substantially in the same direction as the primary winding. A third winding having a number of turns is provided such that the winding start side is connected to the winding start side of the primary winding, and the third winding is provided with a connection point between the first diode and the capacitor and a winding end side of the third winding. A second diode is provided therebetween to form a capacitor discharge circuit.

Here, an inductance is provided in series with the second diode, and a small resistor is inserted into each of the charge and discharge circuits in order to eliminate the imbalance of the electromotive force generated in the primary winding and the third winding of the main transformer. Is preferred.

[Operation] When the MOS transistor is turned off, the drain-source voltage V _DS rises while charging the capacitor, so that the gradient becomes gentle and high-frequency noise decreases,
In addition, unnecessary vibration and the like in the rising waveform are extremely small, so that switching loss is drastically reduced.

Discharging of the capacitor is performed separately when the back electromotive force of the main transformer ends and when the MOS transistor is turned on.

Although it is inevitable that the inductance of each winding varies to some extent due to leakage, etc., the first
Diode prevents resonance due to the inductance of the third winding and the capacitor even if the inductance of the third winding is smaller than the inductance of the first winding, thereby reducing loss and stabilizing circuit operation. Perform the function of bringing.

Embodiment FIG. 1 is a circuit diagram showing a main part of an embodiment of a switching power supply according to the present invention. The basic configuration of the switching unit may be the same as that of the prior art shown in FIG.
That is, to connect the MOS transistor Q which is a switching element in series with the primary winding N ₁ of the main transformer T, connected to the secondary winding N ₂ of the main transformer T by controlling the on-off time This is an insulating switching power supply that extracts a DC output from the rectifying and smoothing circuit 10.

That the present invention differs significantly and the prior art, provided with the charging circuit of the capacitor C ₁ in parallel to the MOS transistor Q, in that the formation of the discharge circuit of the capacitor C ₁ is provided with another coil to the main transformer T is there. That is the portion surrounded by the broken line in FIG.

That, Q. providing a charging circuit and a first diode D ₁ and capacitor C ₁ formed by series connected in parallel with the MOS transistor Q Also provided as the third winding N ₃ the winding start side of substantially the same number of turns in the same direction as the primary winding N ₁ in the main transformer T is connected to the winding start side of the primary winding N _1, the first the diode D ₁ and the connection point a and the third and the capacitor C ₁ of the windings
The second diode D ₂ is provided to form a discharge circuit of the capacitor C ₁ between the winding end of N _3. The first diode D ₁ has its anode to the drain side of the MOS transistor Q, the cathode is provided so as to face the capacitor C ₁ side. The second diode D ₂ has its anode capacitor C ₁ side, the cathode is provided so as to face the third winding N ₃ side.

Wherein the second diode D ₂ in series with the inductance L
And it is preferable to insert resistors R ₁ and R ₂ into the charging circuit and the discharging circuit of the capacitor C ₁ , respectively. Note that winding N
The circuit of ₄ and the diode D is a reset circuit of the main transformer T.

By applying a drive signal to the gate of the MOS transistor Q, the MOS transistor Q performs switching induces voltage in the secondary winding N ₂ of the main transformer T, load it in the direct current rectifying and smoothing circuit 10 (Not shown). The output voltage is stabilized at a constant voltage by controlling the pulse width of the drive signal to the gate of the MOS transistor Q.

When MOS transistor Q is turned off, the drain of the conventional MOS transistor in the switching power supply - the source voltage V _DS but rises sharply, the present invention rises while charging the capacitor C ₁ through the first diode D _1. For this reason, the rising gradient is gentle and smooth, high frequency noise is reduced, and switching loss is drastically reduced. While being charged in the capacitor C _1, the discharge is blocked by the third work of winding N _3. That is, the third
The winding N ₃ is for the same counter-electromotive force in the same direction as the primary winding N ₁ occurs.

Discharge of the electric charge stored in the capacitor C ₁ is the main transformer T
When the generation of the back electromotive force is finished and the MOS transistor Q
Is performed twice when the player turns on. When the generation of the counter electromotive force of the main transformer T has been completed, the inductance L, the second diode D _2, the discharge is performed a third through the winding N ₃ to the input side.

Here, the inductance L functions to block a steep current. Therefore, it is preferable to arrange them, but it is not necessary in principle. The resistances R ₁ and R ₂ are for preventing the current from circulating due to the imbalance of the voltage generated between the primary winding N ₁ and the third winding N ₃ when the MOS transistor Q is turned on. . If the primary winding N ₁ and the third winding N ₃ is sufficient that the same number of turns, the resistors R _1, R ₂ is theoretically not necessary, preferably actually provided.

FIGS. 2 and 4 show examples in which the voltage and current waveforms when the MOS transistor is turned off are observed with an oscilloscope. FIG. 2 shows the case of the circuit configuration of the present invention, and FIG. 4 shows the case of the prior art shown in FIG. In the scale, the voltage on the vertical axis is 50 V / div, the current is 1 A / div, and the time on the horizontal axis is 50 nS / div. As is clear from a comparison between FIG. 2 and FIG. 4, in the present invention, the rise of the voltage waveform is gradual and uniform, and almost no oscillation is observed in the rise of the current waveform. On the other hand, in the conventional circuit, the rise of the voltage waveform is steep and oscillation occurs while the current waveform also shows oscillation. When the switching loss at the time of turning off the MOS transistor is calculated, it is about 2.73 W in the circuit of the present invention, which is much improved compared to about 6.82 W in the conventional circuit.

[Effect of the Invention] As described above, in the present invention, a capacitor charging circuit is formed in parallel with a MOS transistor, and a separate winding is provided in a main transformer, thereby forming a capacitor discharging circuit.
When the MOS transistor is turned off, the rise of the voltage is gradual and uniform, so that high-frequency noise can be reduced and switching loss can be significantly reduced. As a result, the size and performance of the switching power supply can be reduced without increasing the size of the noise filter and the absorber circuit.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a main part of one embodiment of a switching power supply according to the present invention, and FIG. 2 is a voltage / current waveform diagram at the time of turn-off. FIG. 3 is a circuit diagram showing an example of the prior art, and FIG. 4 is a voltage / current waveform diagram when the device is turned off. T: Main transformer, N ₁ ... Primary winding, N ₂ ... Secondary winding,
N ₃ ...... third winding, Q ...... MOS transistors, C ₁ ...... capacitor, D ₁ ...... first diode, D ₂ ...... second diode, L ...... inductance.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-89052 (JP, A) JP-A-61-210871 (JP, A) JP-A-61-157264 (JP, A) JP-A-2- 106165 (JP, A) JP-A-2-174557 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H02M 3/00-3/44

Claims (2)

(57) [Claims] 1. A rectifying / smoothing circuit connected to a secondary winding of a main transformer by connecting a MOS transistor as a switching element in series with a primary winding of a main transformer and controlling an on / off time thereof. In an insulated switching power supply for extracting output, a charging circuit having a first diode and a capacitor connected in series is provided in parallel with the MOS transistor, and a third transformer having substantially the same number of turns in the main transformer in the same direction as the primary winding. Is provided such that the winding start side is connected to the winding start side of the primary winding, and the second winding is provided between the connection point between the first diode and the capacitor and the winding end side of the third winding. A switching power supply characterized in that a discharge circuit of a capacitor is formed by providing the diode of (1). 2. An inductor and a second diode are provided in series between a connection point between a first diode and a capacitor and a winding end of a third winding, and a capacitor is provided together with the third winding. 2. The switching power supply according to claim 1, wherein the discharge circuit is formed.