JPS629697Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a circuit for preventing whining noise of a power transformer.

In recent years, the performance of audio amplifiers, tuners, receivers, etc. has improved, and there is a desire to reduce their noise levels as much as possible, and the whining noise of power transformers, which was not very noticeable in the past, has become a problem.

As is well known, a power transformer passes alternating current through the primary winding to generate alternating magnetic flux in the iron core, which induces voltage in the secondary winding.As the magnetic flux changes, a magnetostrictive phenomenon occurs in the iron core. definitely induces. However, this magnetostrictive phenomenon has previously been reduced to a level that does not pose a problem in practical use by using measures such as using the iron core of the power transformer within a range that does not cause magnetic saturation and making the iron core structure difficult to generate whining noise. There is.

By the way, a power transformer originally has the function of converting a balanced AC voltage, but when applied to an unbalanced AC voltage that contains a DC component in the power supply, the iron core becomes DC magnetized, so the magnetization effect may be abnormal. will come. In particular, in a toroidal power transformer in which the iron core has no air gap, significant unbalanced magnetization occurs because the iron core has steep saturation characteristics.

FIG. 1 shows the magnetization curve a and the excitation current b of the iron core when the power transformer is used with a perfectly balanced AC voltage. As shown in the figure, when the primary winding of a power transformer is connected to a power source that does not contain a DC component, the value of the excitation current flowing through the primary winding must be The waveform is also small, the waveform is symmetrical, the magnetostrictive phenomenon of the iron core is small, and the whine noise generated is also small.

FIG. 2 shows the magnetization curve c and excitation current d of the iron core when a DC component is superimposed on the power source to which the power transformer is connected. In this case, since the iron core is magnetized with direct current, the magnetization curve becomes asymmetric, and 1
The exciting current d flowing in the next winding also has a peak waveform in one direction, and the magnetic flux density of the iron core reaches the saturation region in the direction of DC magnetization, so the magnetostriction phenomenon of the iron core increases, causing a whining noise. will also be significantly larger.

In general, commercial power sources are considered to be completely balanced AC power sources that do not include DC components, but recently, as various devices are connected, the waveform of the power source has become distorted, and the integral value of the + side voltage and - side voltage has become distorted. are often not necessarily equal.

When a power transformer is connected to such a power source, only the primary winding resistance of the power transformer limits the difference between the voltages on the + side and the - side, that is, the DC current flowing due to the DC electromotive force.

However, in general, the primary winding resistance is often designed to be small in order to improve the efficiency of the power transformer, and for this reason, the amount of DC current flowing through the primary winding is quite large, and therefore the DC current that the iron core receives is Magnetization also cannot be ignored.

To explain this with a concrete example, Fig. 4 shows the whirring noise when the power transformer 3 (see Fig. 3) of a 500 VA audio amplifier is used with a perfectly balanced AC voltage, and the noise generated when switch 4 is turned on in the circuit shown in Fig. 3. This is a graph showing a comparison of the whirring noise when a half-wave rectifier circuit ² , which may be turned on and normally connected, is connected in parallel to the terminal end of a power cord 1 (approximately 2 m long) with a wire diameter of 2 mm 2 .

As is clear from this graph, switch 4
When the half-wave rectifier circuit is connected in parallel with the power supply turned on, there will be a difference between the + and - sides of the power supply, and the noise level of the power transformer will be approximately
It becomes 15dB larger.

This invention was developed with a focus on these conventional problems, and its purpose is to create a system that can effectively reduce the whining noise of the iron core even when applied to unbalanced AC voltages. The object of the present invention is to provide a prevention circuit.

In order to achieve the above object, this invention provides a pair of diode strings each consisting of one or more diodes connected in series with the same polarity, connects both diode strings in antiparallel to each other, and connects both ends of the diode strings through a resistor. The circuit is characterized in that the connected circuit is connected in series with the primary winding of a power transformer.

A preferred embodiment of this invention will be described in detail below with reference to the accompanying drawings.

When a power transformer is connected to a power source with an unbalanced voltage waveform, the magnetostrictive effect of the iron core can be reduced by reducing the DC current flowing due to the DC voltage contained in the power source.

Therefore, it is possible to insert a relatively large DC resistance in series with the primary winding of the power transformer, but if this is done, the voltage drop due to the DC resistance will become large when the load current flows, making it impractical.

Therefore, if the exciting current of the core is made to flow through a DC resistor and the load current is made to flow through a separate circuit with less voltage drop, it is possible to realize a power transformer with less whining noise.

FIG. 5 shows an example of the circuit according to the present invention which realizes such a principle. This circuit has a diode array 9, 10 formed by connecting two diodes 5, 6 or 7, 8 in series with the same polarity. The primary winding 13 of the power transformer 12 is connected to a circuit in which a pair of diode arrays 9 and 10 are connected in antiparallel to each other, and both ends of the diode arrays are connected through a resistor 11.
It is connected in series with. In addition, in the figure, 14 is a power supply, and 15 is a load.

Also, the value of the resistor 11 is the value of the primary winding 1 in this example.
In addition, in this example, the voltage waveform of the power supply 14 is such that the integral value on the + side is several times that of the resistor 3, as shown by curve h in Figure 6. The unbalanced AC voltage is slightly larger than the integral value.

Next, the operation of the circuit of the present invention will be explained. Diodes 5 to 7 have non-linear impedances,
If the voltage across each resistor is less than approximately 0.7V per resistor, the resistance value becomes infinite, so as long as the voltage across the resistor 11 does not exceed approximately 1.4V in Figure 5, all current will flow through the resistor 11. By appropriately selecting the value of the excitation current i (see Fig. 6) and the resistance value of the transformer, it becomes possible to flow the excitation current i through the resistor 11, reducing DC magnetization and The whining noise can be significantly reduced.

As shown in Figure 6, the excitation current has a peak value on the + side as shown by curve i, the peak of this current flows approximately 90 degrees behind the phase of voltage h, and its value is smaller than load current j. This value is several tenths smaller.

Next, as shown in curve j in Figure 6, the load current flows in the same phase as the voltage waveform and in a different phase from the excitation current shown by curve i, and its value is equal to the excitation current i.
Several times larger than that.

Therefore, assuming that the load current j flows through the resistor 11, the voltage across the resistor 11 will be a very large value, but in the circuit of the present invention, the diode array 9,
10 are connected in antiparallel, so the resistor 11
Most of the current such that the voltage across the terminal becomes 1.4 V or more flows through the diode arrays 9 and 10, and therefore most of the primary current due to the load flows through the diode circuit. Therefore, even if this circuit is inserted into the primary side of the power transformer 12, the voltage drop on the primary side will not exceed 1.4V even at maximum load.

Next, the graph of FIG. 7 compares the roaring noise level when the present invention is applied to the primary winding 13 of the power transformer 12 and when it is not applied. As is clear from the graph, there is a significant difference in the whine noise between the case without the whine prevention circuit and the circuit of the present invention.

Note that in the above embodiment, two diodes were connected in series to form a diode string;
Of course, a pair of diodes or three or more diodes may be used, and a pair of diodes connected in anti-parallel may be further connected in series, and a resistor may be connected in parallel for each anti-parallel circuit. Of course, the same effect can be obtained even if

As is clear from the above description of the embodiments, this invention provides a pair of diode strings each consisting of one or more diodes connected in series with the same polarity, connects both diode strings in antiparallel to each other, and connects both ends with a resistor. Since the circuit is connected in series with the primary winding of the power transformer, even when an unbalanced AC voltage is used as the power source, the whirring noise generated from the power transformer can be avoided. It can be significantly reduced.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the magnetization curve and excitation current when a balanced AC power source is connected to a normal power transformer, and Figure 2 is a graph showing the relationship between the magnetization curve and excitation current when a balanced AC power source is connected to a power transformer. A similar graph, Figure 3, is a circuit diagram showing a circuit used in an experiment in which a balanced AC power source and an unbalanced AC power source are switched and connected to a power transformer to compare the roaring noise level, and Figure 4 is a circuit diagram of the circuit used in Figure 3. Graphs obtained from experiments in circuits,
Fig. 5 is a circuit diagram of the circuit of the present invention, Fig. 6 is a waveform diagram showing the signal status of each part in Fig. 5, and Fig. 7 is a comparison of the whirring sound level when using and not using the circuit of the present invention. This is a graph shown as follows. 5, 6, 7, 8...diode, 9, 10...
Diode string, 11...Resistor, 12...Power transformer, 13...Primary winding, 14...Power supply, 15...
...Load circuit.

Claims (1)

[Scope of utility model registration request] A circuit consisting of a pair of diode strings each consisting of one or more diodes connected in series with the same polarity, both diode strings connected in anti-parallel to each other, and both ends connected via a resistor is connected to the primary circuit of the power transformer. A power transformer noise prevention device characterized by being connected in series with a winding.