JPH066970A - Switching power supply - Google Patents

Abstract

PURPOSE:To realize a switching power supply in which switching noise due to stray capacitance between primary and secondary windings of transformer can be suppressed. CONSTITUTION:DC current flowing through the primary winding 3 of a transformer 2 is switched by means of a switching element 1 being turned ON/OFF at high frequency thus generating high frequency voltage. The high frequency voltage is transmitted, while being converted into a predetermined magnitude, to the secondary winding 7 of the transformer 2 comprising a core 4 and electrostatic shields 5, 6. Diodes 8, 9 are connected with the opposite ends of the secondary winding 7 and intermediate terminals A, B, C thereof and full-wave rectified DC current is smoothed through a coil 10 and a capacitor 11 to be fed to a load. The diodes 8, 9 are connected, at the cathode sides thereof, with meter casing 12 having same potential as a primary common wire through a capacitor 17. This circuitry suppresses switching noise effectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply, and more particularly to reduction of switching noise.

[0002]

2. Description of the Related Art A switching power supply has a primary side (input side).
The DC input voltage is switched at a high frequency to be converted into an AC, and then transmitted to the secondary side (output side) via a transformer for rectification and smoothing. Such a switching power supply is widely used as a power supply for various electronic devices because the power supply input and the power supply output are galvanically isolated and the entire power supply can be downsized.

By the way, in the switching power supply configured as described above, the size of the transformer is reduced as the size of the transformer is reduced.
The stray capacitance between the secondary winding and the secondary winding increases, and the switching frequency becomes higher, so that the voltage applied to the primary winding has a greater effect on the common potential on the secondary side. As a result, the potential waveform associated with the electrostatic coupling of the switching signal that drives the switch element 1 appears between the primary side and the secondary side common as switching noise.

This switching noise appears as an inter-terminal noise voltage between the input terminal of the measurement signal and the case of the measuring instrument when such a switching power supply is used as the power source of the measuring instrument. The noise voltage between the terminals becomes an input noise of the measuring instrument and becomes an external noise signal for the device to be measured connected to the input terminal, which is not preferable. Therefore, in order to reduce such a noise voltage between terminals, various measures have been conventionally taken.

FIG. 5 is a circuit diagram showing an example of a main part when a conventional power source such as this is used in a measuring instrument. In the figure, 1 is a switch element, and a primary winding 3 of a transformer 2
Are connected in series. Although a DC input voltage is applied to the series circuit of the switch element 1 and the primary winding 3 of the transformer 2, it is not shown. Reference numeral 4 is the core of the transformer 2.
5 and 6 are 2 between the primary winding 3 and the secondary winding 7 of the transformer 2.
It is a heavy electrostatic shield. One electrostatic shield 5 is connected to the common on the primary side, and the other electrostatic shield 6 is connected to the common on the secondary side. A rectifying / smoothing circuit composed of diodes 8, 9 and a coil 10 and a capacitor 11 is connected to both ends A and C of the secondary winding 7, and the midpoint B of the secondary winding 7 is secondary with the electrostatic shield 6. Is connected to the common side. 12 is the case of the measuring instrument, 13 and 14
Is the input terminal of the measuring instrument.

[0006]

However, the structure in which the electrostatic shields 5 and 6 are provided and connected to the commons as described above is effective in reducing the influence of the stray capacitance in the transformer for the general commercial power supply. However, in the case of a switching power supply in which the switching frequency is several thousand times the commercial frequency, the effect of electrostatic coupling due to stray capacitance is very large, so a sufficient shielding effect cannot be obtained.

Therefore, a capacitor 15 is connected between the secondary side common and the primary side common, and a common mode choke 16 is connected to the input terminals 13 and 14. However, a sufficient switching noise reduction effect cannot be obtained. Further, when the capacitor 15 is connected, AC insulation is not performed, and when the common mode choke 16 is connected, the performance of the measuring device is deteriorated.

The present invention solves such a problem, and an object thereof is to realize a switching power supply capable of reducing switching noise caused by a stray capacitance between the primary and secondary windings of a transformer. To do.

[0009]

In order to solve such a problem, the present invention switches a primary side DC input voltage at a high frequency to convert it into an alternating current, and then transfers it to a secondary side via a transformer. In a switching power supply configured to transmit and rectify and smooth, a capacitor is connected between a secondary side rectification output terminal and a primary side common to reduce switching noise transmitted to the secondary side. To do.

[0010]

The switching signal component electrostatically coupled by the stray capacitance between the primary winding and the secondary winding of the transformer flows out from the output terminal on the secondary side to the common on the primary side through the capacitor. This reduces the switching noise output to the secondary side.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of an embodiment of the present invention, and the portions common to FIG. 5 are designated by the same reference numerals. 1 is different from FIG. 4 in that a capacitor 17 is connected between the cathodes of the diodes 8 and 9 on the secondary side and the common (case 12 of the measuring instrument in this embodiment) on the primary side.

In such a structure, the switching noise output to the cathodes of the diodes 8 and 9 on the secondary side flows out to the common on the primary side via the capacitor 17.
This reduces the switching noise output to the secondary side. FIG. 2 is a circuit diagram of an essential part of another embodiment of the present invention, which is an example in which the primary side is configured as a push-pull type, and the same parts as in FIG. In the configuration of FIG. 2, DC power is applied to the center tap of the primary winding 3, and push-pull driven switch elements 1a and 1b are connected to both ends of the primary winding 3. A capacitor 17 is connected between the cathodes of the diodes 8 and 9 on the secondary side and the common on the primary side. In such a configuration, electric power is supplied to the load on the secondary side with one of the switch elements 1a and 1b turned on.

FIG. 3 is a waveform diagram of each part of FIG. (A)
Shows an example of common mode noise that appears at the rectified output terminal on the secondary side and the common on the secondary side when the capacitor 17 is not connected. (B) shows the input of the primary side which is the source of this noise. An example is shown. In this example, the voltage on the primary side is 10 V, the stray capacitance between the primary winding 3 and the secondary winding 7 is 10 pF, and the common mode noise on the secondary side is 20 mV.
It has become. In order to reduce such common mode noise to, for example, 1/100, the stray capacitance (10 pF) of 100 is set as the capacitance of the capacitor 15 in the conventional configuration of FIG.
About twice as many as 1000 pF is required.

However, focusing on the rectified output terminal on the secondary side, a full-wave rectified voltage as shown in (C) is output.
This voltage waveform is equal to the input waveform on the primary side which is a noise source. Therefore, in the present invention, a part of the waveform of the rectified output terminal on the secondary side is made to flow to the common on the primary side via the capacitor 17. That is, by adjusting the value of the capacitor 17, the common mode noise transmitted from the primary side to the secondary side and the voltage transmitted from the secondary side to the common on the primary side can be made equal, resulting in Common mode noise can be eliminated. The capacitance C of the capacitor 17 is approximately 8.3 pF from C = 10 × 10/12 when the voltage at the secondary side rectification output terminal is 12V.

In each of the above embodiments, one end of the capacitor 17 is directly connected to the rectified output terminal on the secondary side, but as shown in FIG. 4, a resistor voltage divider circuit composed of resistors R ₁ and R ₂ is used. You may connect via. In this case, the capacitance C ′ of the capacitor 17 is a value obtained by multiplying the capacitance C of the capacitor 17 in FIG. 2 by the resistance division ratio 1 / {R ₂ / (R ₁ + R ₂ )}. For example, if R ₁ = ₂ kΩ and R ₂ = 10 kΩ, then C ′ = 8.3.
From x1.2 to 10 pF. Such a configuration is effective when the value of the capacitor 17 cannot be arbitrarily selected.

It is also possible to change the correction of noise by connecting a separate capacitor in parallel with the resistor R ₁ or R ₂ so that the voltage waveform at the voltage dividing point is different from the waveform at the rectification output terminal. Further, in the above-described embodiment, the waveform is described as a sine wave, but the same effect can be obtained for a waveform such as a rectangular wave.

[0017]

As described above, according to the present invention,
It is possible to realize a switching power supply that can reduce switching noise caused by stray capacitance between the primary and secondary windings of a transformer, and is suitable as a power supply for various devices that require high sensitivity.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram of each part of FIG.

FIG. 3 is a circuit diagram of a main part of another embodiment of the present invention.

FIG. 4 is a circuit diagram of a main part of another embodiment of the present invention.

FIG. 5 is a circuit diagram showing an example of a main part when a conventional switching power supply is used for a measuring instrument.

[Explanation of symbols]

1 switch element 2 transformer 3 primary winding 4 core 5,6 electrostatic shield 7 secondary winding 8,9 diode 10 coil 11,17 capacitor 12 measuring instrument case R ₁ , R ₂ resistance (voltage divider circuit)

Claims (1)

[Claims] 1. A switching power supply configured to switch a primary side DC input voltage at a high frequency to convert into an alternating current, and then transmit it to a secondary side through a transformer for rectification and smoothing. A switching power supply characterized by connecting a capacitor between the rectification output terminal and the primary side common to reduce switching noise transmitted to the secondary side.