JPH04156271A - Current resonance converter - Google Patents

Abstract

PURPOSE:To reduce switching noise by generating current resonance between a leakage inductance produced between the primary and secondary windings of a choke coil inserted into the primary side circuit of a transformer and a capacitor connected to the reverse blocking diode of a coil reset circuit. CONSTITUTION:When turning on/off operations are made to an FET 3, an AC voltage is generated on the secondary side of a transformer 2. This AC voltage is converted into a pulsating flow of DC by means of a rectifier circuit 11 and then smoothed to a smooth DC voltage by means of a smoothing circuit 12. A load 31 is connected to the DC voltage. When a direct current is switched to another direct current by means of the FET 3, a current resonance circuit is formed of the leakage inductance 15 of a primary-side choke coil 10 and resonance capacitor 14. As a result, the electric current on the primary side of the transformer 2 becomes a sine wave from which a high-frequency component is eliminated. Therefore, the alternating current induced on the secondary side of the transformer 2 also becomes a sine wave and, since the high-frequency component is eliminated, the DC voltage outputted from the transformer 2 becomes less in noise.

Description

[Detailed Description of the Invention] [Summary] In communication equipment and other devices, an input DC voltage is converted into a desired DC voltage and supplied using a switching current generated through a resonant circuit formed in a switching circuit. Regarding current resonant converters, for the purpose of reducing switching noise and switching loss, a switching circuit including a choke coil for applying DC power, a transformer, a switching means, etc. connected in series,
A rectifier circuit provided on the secondary side of the transformer, a smoothing circuit connected to the output stage of the rectifier circuit, and energy supplied from the secondary winding of the choke coil to the output stage of the rectifier circuit via a reverse blocking diode. A resonant capacitor is connected in parallel to the reverse blocking diode, and resonance is caused between the resonant capacitor and the leakage inductance of the choke coil.

[Industrial Application Fields] The present invention is used in communication equipment and other devices to convert an input DC voltage into a desired DC voltage and supply it using a switching current generated through a resonant circuit formed in a switching circuit. The present invention relates to a current resonant converter, and more particularly to a forward converter using a current resonant converter.

Switching power supplies have higher efficiency than directly controlled power supplies (series regulators) and generate less heat from active elements, so they have recently come to be frequently used as 5V power supplies for computer equipment. However, since it is a switching method, it inevitably involves switching noise, and reducing noise has become an issue.

F Prior Art] A forward converter turns a DC power supply on and off using a switching circuit, transmits the on/off information to the secondary side using a transformer, and rectifies and smoothes the induced AC on the secondary side to obtain a DC voltage. It is.

FIG. 7 is a circuit diagram showing an example of the configuration of a conventional forward converter. In the figure, a voltage applied from a DC power supply 8 is applied to a primary winding of a transformer 2 and an FET 3 connected in series via a choke coil 1, and is converted into an AC voltage by the switching operation of the FET 3. On the secondary side of transformer 2, the AC voltage transmitted from the primary side is
It is rectified and smoothed by a diode 4 and a capacitor 5 connected in series, and is output as a predetermined DC voltage. A load 9 is connected to this DC voltage.

A diode 6 connected in series to the other terminal of the choke coil 1 is turned on by the reverse voltage induced in the choke coil 1 when the FET 3 changes from the on state to the off state, and the excitation accumulated in the choke coil 1 is turned on. It is for discharging energy. Furthermore, the diode 7 connected in series with the other terminal of the primary winding of the transformer 2 in the reverse direction is turned on by the reverse voltage induced in the transformer 2 when the FET 3 changes from the on state to the off state. The excitation energy accumulated in the transformer 2 is discharged.

In this way, the excitation energy accumulated in the choke coil 1 and the transformer 2 is discharged every switching period, thereby stabilizing the operation.

Note that the FET 3 performs a switching operation in accordance with a predetermined switching control signal given by a pulse width control method, and stabilizes the secondary side DC voltage to a predetermined value.

[Problem to be solved by the invention] In the conventional circuit as described above, as shown in FIG.
The waveform of the voltage between the terminals of T3 (hereinafter referred to as a switching voltage waveform) Vos and the waveform of the current flowing through the FET 3 (hereinafter referred to as a switching current waveform) IDS suddenly change at the time of rising and falling. Therefore, the AC voltage generated by switching contains many high frequency components.

This high frequency component is superimposed on the secondary side DC voltage as switching noise.

In addition, as shown in the figure, the switching current waveform and the switching voltage waveform rise and fall at approximately the same time and change at the same time, so there is a problem that power loss occurs in FET 3 in the overlapping region. .

The present invention has been made in view of such problems, and an object of the present invention is to provide a current resonant converter that can reduce switching noise and switching loss.

[Means for Solving the Problems] FIG. 1 is a block diagram of the principle of the present invention. Components that are the same as those in FIG. 7 are designated by the same reference numerals. In the figure, 8 is a DC power source to be applied. This DC power supply 8 has a choke coil 10. A switching circuit in which a transformer 2 and a switching means 3 are connected in series is connected. 1
1 is a rectifier circuit provided on the secondary side of the transformer 2, and 12 is a smoothing circuit connected to the output stage of the rectifier circuit 11.

A coil reset circuit 20 supplies energy from the secondary winding of the choke coil 10 to the output stage of the rectifier circuit 11 via the diode 13.

13 is a reverse blocking diode whose one end is connected to the secondary winding of the choke coil 10; 14 is the diode 13;
is a resonant capacitor connected in parallel with . Choke coil 10. The diode 13 and the capacitor 14 constitute a coil reset circuit 20.

[Function] The choke coil 10 inserted in the primary side series circuit has 1
Smooth the next-side DC current. At the same time, when the switching means 3 is turned off, the excitation energy accumulated in the primary side circuit is circulated to the secondary side via the coil reset circuit 20. Further, a current resonance circuit is formed by the leakage inductance 15 of the choke coil 10 and the capacitor 13 connected in parallel to the reverse blocking diode 13, and the waveform of the primary current is made into a sine wave.

FIG. 2 is a diagram showing switching voltage and primary side current waveforms. (a) shows the switching voltage waveform, and (b) shows the primary side current waveform. The primary side current waveform when switching is on is different from the waveform shown in FIG. 8, and has a sine wave shape. Therefore, the alternating current transmitted to the secondary side does not contain high frequency components, and the switching noise included in the direct current output can be reduced. Further, in the switching means 3, since the voltage is 0 when it is on, the power loss is 0 regardless of the primary current. Also, when it is off, the current on the 1 o'clock side is 0, so
Similarly, the loss becomes 0.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 3 is a circuit diagram showing one embodiment of the present invention.

Components that are the same as those in FIG. 1 are designated by the same reference numerals.

In the figure, 8 is a DC power supply supplied to the switching circuit, 2 is a transformer, 10 is a choke coil, and 3 is an FET as the switching means 3.

The DC power supply 8 includes a choke coil 10. A series circuit of transformer 2 and FET 3 is connected. 7 is transformer 2
This diode is connected in reverse direction in series to the other terminal of the primary winding of the transformer 2 via the third winding, and when FET3 changes from the on state to the off state, It is turned on by voltage and discharges the excitation energy stored in the transformer 2.

30 is a drive circuit that generates pulses to drive the switching FET 3. As the drive circuit 30,
For example, a PW that monitors the secondary side DC voltage of the transformer 2 and drives the FET 3 so that the voltage reaches a certain reference value.
An M-type drive circuit is used.

The coil reset circuit 20 is a circuit for causing a primary current to flow when the FET 3 is off.

Without this circuit, an abnormal surge voltage would occur across the primary choke coil when FET3 is off.
This allows the primary current to circulate outward.

The coil reset circuit 20 includes a choke coil secondary winding, a reverse blocking diode 13, and a resonance capacitor 14 connected in parallel to the diode 13. This coil reset circuit 20 includes the rectifier circuit 11 of the secondary side circuit.
and the connection point of the smoothing circuit 12. The reverse blocking diode 13 prevents excessive current from flowing into the secondary winding of the choke coil 10 when the coil reset circuit 20 is off. For example, without the diode 13, the secondary winding of the choke coil 10 would be directly connected to the DC voltage.

Now, assuming that the DC voltage is 5V and the coil resistance is 0.1Ω, approximately 5
A current of 0A flows through the coil and burns it out. Therefore, when the coil reset circuit 20 is off, a reverse blocking diode 13 is provided to prevent the secondary DC voltage from flowing to the coil. Coil reset circuit 20
When the diode 13 is on, a freewheeling current flows through the diode 13 to the secondary circuit.

The rectifier circuit 11 includes a rectifier diode lla and a capacitor 1.
1b and converts alternating current into direct current (pulsating current). Further, the smoothing circuit 12 following the smoothing circuit 12 is composed of a choke coil 12a and a smoothing capacitor 12b, and converts the pulsating current into a flat direct current. 31 is a load resistor connected to the DC output. The operation of the circuit configured as described above will be explained as follows.

When the FET 3 is turned on, power is supplied from the DC power supply 8 and a primary current flows. As a result, 2 of transformer 2
A voltage is induced on the next side. On the other hand, when the FET 3 is turned off, excitation energy flows to the secondary side of the choke coil 10 via the coil reset circuit 20. This current flows through the reverse blocking diode 13 to the secondary DC voltage generation circuit, supplying a small amount of power. Further, the diode 7 is turned on by a reverse voltage induced in the transformer 2 when the FET 3 changes from the on state to the off state, and discharges the excitation energy accumulated in the transformer 2.

In this series of on/off operations of the FET 3, an alternating current voltage is generated on the secondary side of the transformer 2. This alternating current voltage is converted into a pulsating direct current by a rectifier circuit 11, and then smoothed by a smoothing circuit 12 to become a flat direct current voltage. A load 31 is connected to this DC voltage.

By the way, when the DC voltage is switched by the FET 3, a current resonance circuit is formed by the leakage inductance 15 of the primary choke coil 10 and the resonance capacitor 14. As a result, the primary current has a sinusoidal waveform as shown in FIG. 2(b), with high frequency components removed. As a result, the AC waveform induced on the secondary side of the transformer 2 also becomes sinusoidal, and as a result of removing high frequency components, the DC voltage obtained as an output has less noise. Also, the relationship between the primary side voltage waveform and current waveform is
Since the relationship shown in the figure is established, power loss due to switching of FET 3 is also eliminated.

FIG. 4 is a diagram showing operating waveforms of each part of the circuit of FIG. 3.

(a) shows the drain-source voltage VD5 of FET3, (
b) is the primary current ID, (C) is the current waveform flowing from the DC power supply 8, (d) is the drive voltage waveform V66 applied to the gate of FET3, and (e) is the waveform induced in the secondary winding of the transformer 2. (f) is the current waveform I of the secondary circuit. , (g) is a circulating current waveform ILO supplied from the coil reset circuit 20 to the secondary circuit. Both show actual operating waveforms. As is clear from (a) and (b), the current waveform is sinusoidal with respect to the voltage waveform.

FIG. 5 is a diagram showing voltage waveforms and current waveforms when the values of each component of the circuit of FIG. 3 are determined as shown in FIG. 6.

The number of turns of the transformer 2 is 10t on the primary side and 3t on the secondary side. The choke coil 10 has a primary side of 1.5t,
The secondary side is 4t.

The choke coil 10 has a primary inductance of 38.2
μH, and the secondary side is 2.77 μH. The primary side choke coil makes it possible to smooth the primary side current, and the inductance of the choke coil 12a of the secondary side smoothing circuit 12 is 9.9μH1, and the capacitance of the capacitor 12b is 22pF, so components with small values are used. is now possible. Note that when the circuit of the present invention is used in a power supply circuit whose primary side voltage is relatively low, it is possible to achieve even lower noise.

In the above description, an example is given in which FETs are used as switching means, but the present invention is not limited to FETs, and ordinary transistors may also be used.

[Effects of the Invention] As explained above in detail, according to the present invention, the leakage inductance between the primary winding and the secondary winding of the choke fill inserted in series in the primary side circuit of the transformer and the coil reset circuit can be reduced. By causing current resonance between the reverse blocking diode and the capacitor connected in parallel, the current waveform flowing through the primary circuit can be made sinusoidal, and the transformer 2
It is possible to prevent high-frequency component noise (switching noise) from being added to the DC voltage generated by the next-side circuit.

Further, according to the present invention, the relationship between the primary side voltage and current can be maintained constant as shown in FIG. 2, and power loss accompanying switching can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the principle of the present invention, Fig. 2 is a diagram showing the operating waveforms of the circuit shown in Fig. 1, Fig. 3 is a circuit diagram showing an embodiment of the present invention, and Fig. 4 is a diagram showing the circuit shown in Fig. 3. FIG. 5 is a diagram showing operating waveforms of each part; FIG. 6 is a diagram showing a specific configuration example of the circuit shown in FIG. 3; FIG. 7 is a diagram showing an example configuration of a conventional forward converter; FIG. 8 is a diagram showing operating waveforms of a conventional forward converter. In Fig. 1, 2 is a transformer, 3 is a switching means, 8 is a DC power supply, 10 is a choke coil, 11 is a rectifier circuit, 12 is a smoothing circuit, 13 is a reverse blocking diode, 14 is a resonant capacitor, 15 is a leakage inductance, 20 is a coil reset circuit.

Claims (1)

[Claims] A choke coil (10) to which a DC power supply (8) is applied;
a switching circuit in which a transformer (2) and a switching means (3) are connected in series; a rectifier circuit (11) provided on the secondary side of the transformer (2); and a switching circuit connected to the output stage of the rectifier circuit (11). smoothing circuit (1
2), and a coil reset circuit (
20), a resonant capacitor (14) is connected in parallel to the reverse blocking diode (13), and resonance occurs between the resonant capacitor (14) and the leakage inductance (15) of the choke coil (10). A current resonant converter characterized in that it is configured to cause.