JPH0993942A - Power supply circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a speaker current which flows through the secondary side of a power supply transformer from leaking into the primary side. SOLUTION: In a power supply circuit having a power supply transformer 2, choke coils L3 and L4 are connected in series to the primary coil L1 of the power supply transformer. The choke coils L3 and L4 are composed of independent windings which are not magnetically coupled with the power supply transformer 2 and, further, the inductance of the choking coils is selected to be sufficiently large against the signal of a voice band.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a power supply circuit,
In particular, it is suitable for use in a low frequency amplifier circuit that amplifies an audio signal.

[0002]

2. Description of the Related Art FIG. 5 shows an example of a conventional noise filter circuit provided in a power supply circuit or the like. FIG noise filter circuit 30 shown by a broken line in (a), the coil L _21, L ₂₂
And the capacitors C ₂₁ and C ₂₂ and are provided between the commercial power supply 31 and the primary winding coil (not shown) of the power transformer 32.

When the noise filter circuit 30 is thus inserted between the commercial power supply 31 and the power transformer 32, harmonic noise of 10 KHz or more is cut off from an AC voltage of 100 V, 50 Hz supplied from the commercial power supply 31, for example. It is designed to do.

That is, such a noise filter circuit 30 has a power supply transformer 3 as shown by an arrow in FIG.
When viewed from the 2 side, the coils L ₂₁ , L ₂₂ and the capacitor C ₂₂ which constitute the noise filter circuit 30 act as a low pass filter, and are generated in the circuit block connected to the secondary side of the power transformer. The high frequency noise generated is blocked so that it does not flow out to the commercial power supply 31.

On the other hand, when the noise filter circuit 30 is viewed from the commercial power source 31 side as shown by the arrow in FIG.
The coils L ₂₁ , L ₂₂ and the capacitor C ₂₁ that constitute the noise filter circuit 30 act as a low-pass filter, so that, for example, the harmonic noise superimposed on the commercial power supply 31 is blocked and does not flow into the power supply transformer 32. I have to.

FIG. 6 shows an example of the case where the low frequency amplifier circuit is driven by the power supply circuit provided with the noise filter circuit 30 as described above. In this figure,
The noise filter circuit 30 is connected between the commercial power supply 31 and the power transformer 32.
The harmonic noise of 10 KHz or more superimposed on 1 is cut off so as not to flow into the power transformer 32, and is generated in the secondary side circuit block of the power transformer 32.
The high frequency noise of KHz or more is cut off so as not to flow out to the commercial power supply 31 side.

[0007] Power transformers 32 primary winding coil L ₃₁ indicated by a broken line, the secondary winding coil L _32a, is composed of L _{32 b,} from the commercial power source 31 into the primary side winding coil L ₃₁ For example, the commercial AC voltage of 100 V for 50 cycles applied is boosted / decreased to output an alternating voltage from the secondary side coils L _32a and L _32b .

The rectifying / smoothing circuit 33 shown by the chain line is
Rectifier diodes D ₂₁ , D ₂₂ and smoothing capacitors C ₂₃ , C
₂₄ , and the diode D ₂₁ and the capacitor C ₂₃ rectify and smooth the positive alternating voltage generated in the secondary winding coil L _32a of the power transformer 32 to output a predetermined DC voltage. The diode D ₂₂ and the capacitor C ₂₄ rectify and smooth the negative alternating voltage generated in the secondary winding coil L _32b of the power transformer 32 to output a predetermined DC voltage.

The low frequency amplifier 34 amplifies an audio signal input from a pre-stage circuit block (not shown),
The speaker current Is corresponding to the audio signal is output, and the speaker current Is flows through the smoothing capacitor C ₂₃ (C ₂₄ ) to the speaker 35, but the smoothing capacitor C ₂₃ (C ₂₄ ). When the speaker current Is cannot be sufficiently supplied from the device, a part of the current is supplied to the secondary winding coil L _32a (L _32b ) of the power transformer 32 and the diode D as a speaker current Is ′.
It will flow through ₂₁ (D ₂₂ ).

[0010]

By the way, in the low frequency amplifier circuit as shown in FIG. 6, when the speaker current Is cannot be sufficiently supplied from the smoothing capacitor C ₂₃ (C ₂₄ ), the power transformer 32 Secondary winding coil L _32a (L
_{Since the} speaker current Is ′ flows through _32b ), the power transformer 32 is used as an output transformer, and the current I _R corresponding to the speaker current Is ′ leaks to the primary winding coil L ₃₁ . Note that the power required for the low-frequency amplifier 34 is actually supplied from the primary side circuit (commercial power supply 31) of the power transformer 32 via the rectifier circuit 33, and the amount of this power supplied to the audio current Is. It just changes in proportion.

Most of the leakage current I _R flowing through the primary side of the power transformer 32 has a low frequency in the same audio band as the speaker current Is, so that the conventional 10K
The noise filter circuit 30 that blocks noise above Hz has a drawback that it cannot block the outflow to the commercial power supply 31 side.

If the leakage current I _R flows through the primary side of the power transformer 32, especially when the speaker 35 is driven with a large output, the impedance characteristic of the low frequency amplifier circuit 34 with respect to the speaker 35 changes. That is, the speaker current Is fluctuates under the influence of the power supply side, and as a result, the sound quality of the sound output from the speaker 35 deteriorates.

[0013]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and in a power supply circuit having a power transformer, a single winding coil or a plurality of windings in series with a primary winding coil of the power transformer. A coil is provided so that the coil or coils are not magnetically coupled to the power transformer. Further, the inductance of the coil is set to have a sufficiently large value for a voice band signal.

According to the present invention, by connecting a single or a plurality of coils to the primary winding coil of the power transformer,
1 of the power transformer in the frequency band of the audio signal
Since the impedance of the secondary side becomes large, it is possible to prevent the speaker current flowing through the secondary side of the power transformer from flowing out to the primary side of the power transformer and reduce the deterioration of the sound quality.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 1 shows an example of a low frequency amplifier circuit using a power supply circuit according to an embodiment of the present invention. The commercial power supply 1 is a power supply that outputs a commercial AC voltage of 100 V and 50 Hz, for example. The power transformer 2 shown by the broken line is a primary winding coil L ₁ and secondary winding coils L _2a and L ₂ .
_2b , the AC voltage applied from the commercial power source 1 to the primary winding coil L ₁ is stepped up and down to output a predetermined alternating voltage from the secondary winding coils L _2a and L _2b. Has been done.

The winding of the power transformer 2 has an AC voltage of about 90 applied to the primary winding coil L ₁ , which will be described later.
The alternating voltage generated in the V and secondary winding coils L _2a and L _2b is set to about + 20V and -20V, respectively.

For the choke coils L ₃ and L ₄ , for example, iron cores are inserted inside, one end is connected to both ends of the primary winding side coil L ₁ of the power transformer 2, and the other end is a commercial AC power source. 1 is connected to each. Also,
The choke coils L ₃ and L ₄ are independent windings that are not magnetically coupled to the power transformer 2, and the commercial AC power source 1
About 100V of AC voltage output from each
The value of the inductance is set so that the voltage of V is applied.

In order to apply an AC voltage of 5 V to each of the choke coils L ₃ and L ₄ , for example, assuming that the frequency of the commercial power source 1 is 50 Hz, the primary side circuit of the power transformer 2 will see. The value of 5/90 of the inductance may be set to be the value of the inductance of the choke coils L ₃ and L ₄ . That is, the choke coil L
The combined inductance of ₃ and L ₄ is 1 of the power transformer 2.
It may be set so as to have a value of 10/90 of the inductance viewed from the secondary circuit.

Further, as described above, the choke coils L ₃ , L
_When the value of the inductance of ₄ is set, about 10% of the commercial AC voltage of 100 V becomes reactive power in the choke coils L ₃ and L ₄ , but iron loss is the main factor, so the overall power conversion efficiency is not lowered. Absent. This allows the power transformer 2
As a result, an AC voltage of 90 V is applied to the primary winding coil L ₁ of
Will be applied.

The rectifying / smoothing circuit 3 shown by the alternate long and short dash line is composed of rectifying diodes D ₁ and D ₂ and smoothing capacitors C ₁ and C ₂ , and the rectifying diode D ₁ and the smoothing capacitor C ₁ are 2 of the power transformer 2. The positive alternating voltage (+ 20V) generated in the secondary winding coil L _2a is rectified and smoothed and output,
The rectifying diode D ₂ and the smoothing capacitor C ₂ rectify and smooth the negative alternating voltage (−20 V) generated in the secondary winding coil L _2b of the power transformer 2 and output it.

A low-frequency amplifier (power amplifier) 4 shown by a broken line is an audio signal (50 Hz to 20 K) input to an input terminal T from a pre-stage circuit block (not shown).
Hz), and outputs a speaker current corresponding to the audio signal to the speaker 5, which is a complementary PP type amplifier, and includes NPN type transistors Tr1 and T2.
r3, PNP-type transistor Tr2, resistors R ₁ to R _8,
It is composed of capacitors C ₃ and C ₄ .

A resistor R constituting the low frequency amplifier 4
_{1 to} R ₄ and the transistors Tr ₁ and Tr 2 are connected to the resistor R ₁
To R ₄ are used as bias resistors to form an output stage for driving the speaker 5, resistors R _{5 to} R ₈ , and a transistor T.
r3, and the capacitor C ₃ constitute an amplifying stage that drives the transistors Tr1, Tr2 in the input audio signal, these output stages, and the amplifier stage are coupled by a coupling capacitor C _3.

Now, the operation of the low frequency amplifier circuit using such a power supply circuit will be described. When the audio signal of the positive half cycle as shown in the input terminal T in FIG. 1 for example is input, the audio transistor Tr3 is turned on, the coupling capacitor C ₃ which is input in a direction as shown in FIG. 1 The current I ₁ corresponding to the signal flows. When the current I ₁ flows through the coupling capacitor C ₃ , the transistors Tr 1 and T which are output stages in the class B amplification type are used.
Of r2, the transistor Tr1 is cut off, the transistor Tr2 is turned on, the speaker current I ₂ according to the audio signal is supplied to the speaker 5, and the speaker 5 outputs the audio.

The speaker current I ₂ flowing through the speaker 5 is
Most of the current in the high frequency range flows as the current I ₃ through the smoothing capacitor C _2, and in the case of a large output, a part of the current in the low frequency range is generated by the diode D.
₂ through the secondary winding coil L _2b of the power transformer. Further, when the current I ₃ flowing through the smoothing capacitor C _2, although the charge of the smoothing capacitor C ₂ is discharged,
The current for compensating for this discharge is supplied from the secondary winding coil L _2b of the power transformer 2. Therefore, the current I ₄ flowing through the secondary winding coil L _2b becomes a part of the low frequency band flowing through the speaker 5 and a current (charging current) that supplements the discharge of the smoothing capacitor C ₂ .

Further, the current I ₄ flowing through the secondary side winding coil L _2b of the power transformer 2, rectifier the diode flowing through the D _2, the period of the rectifying diode D ₂ is conducting, secondary winding The alternating current generated in the wire coil L _2b is superimposed and flows.

When a current I ₄ other than the rectified current flows through the secondary winding coil L _2b of the power transformer 2 by driving the speaker 5 in this manner, this current is conventionally supplied to the primary side of the power transformer 2. The current corresponding to I ₄ leaks, and the leakage current I ₅ corresponding to the audio signal flows to the primary winding coil L _1.
Will occur.

Therefore, in this embodiment, the power transformer 2 is used.
By connecting the choke coils L ₃ and L ₄ in series to both ends of the primary winding coil L ₁ , the primary impedance of the power transformer 2 in the frequency band of the audio signal becomes high impedance, The leakage current I ₅ leaking to the primary side of the transformer 2 is reduced.

Here, for example, when the frequency of the current I ₄ flowing corresponding to the speaker current I ₂ is 50 Hz, the choke coils L ₃ and L connected to the primary side of the power transformer 2 are used.
The leakage current I ₅ associated with the audio signal in _4, assuming that the voltage E ₅₀ is applied, the frequency of the current I ₅ is 500 Hz, the voltage E ₅₀₀ applied to the choke coil L _3, L ₄ is When the frequency becomes 10 times and the frequency of the current I ₅ becomes 5 KHz, the voltage E _5K applied to the choke coils L ₃ and L ₄ becomes 100 times. That is, the sum of the currents flowing through the choke coils L ₃ and L ₄ is 500 Hz.
Then, it becomes 1/10 at 50 Hz and 1/100 at 50 kHz at 5 KHz.

That is, if the leakage current I ₅ leaking to the primary side of the power transformer 2 is constant, the choke coil L ₃ ,
L ₄ has a larger impedance as the frequency of the leakage current I ₅ becomes higher, and has an effect of blocking the leakage current I ₅ flowing out to the primary side of the power transformer 2 for the main frequency components 500 Hz to ₅ KHz of the audio signal. growing.

By thus providing the choke coils L ₃ and L ₄ on the primary side of the power transformer, the impedance of the primary side of the power transformer 2 is low when viewed from the low frequency amplifier 4 side in the main audio frequency band. Since the impedance is as high as 10 to 100 times that of the conventional one, the leakage current I ₅ leaking from the secondary side of the power transformer 2 to the primary side can be reduced.

Further, when the primary side of the power transformer 2 is made to have a high impedance when viewed from the low frequency amplifier 4 side in this way, the leakage current I ₅ corresponding to the main frequency band of the audio signal is generated by the low frequency amplifier 4. Since it is not related to the load characteristic, the output characteristic of the speaker is not affected by the impedance characteristic of the primary side of the power transformer 2, and the sound quality output from the speaker 5 can be improved.

Further, even when noise having a frequency close to the voice band is superimposed on the commercial AC power supply 1, if the choke coils L ₃ and L ₄ having a small distribution capacitance are adopted, the primary side of the power transformer 2 has a high impedance. Therefore,
This signal can be blocked from leaking to the secondary side, and noise can be prevented from flowing into the secondary side of the power transformer 2, that is, the audio circuit side.

Further, there is an advantage that it is possible to prevent a pulsed current (for example, a speaker or a counter electromotive force of an inductance) generated on the secondary side of the power transformer 2 from leaking to the primary side of the power transformer 2. .

2 to 4 show modified examples of the power supply circuit of this embodiment as described above. The power supply circuit shown in FIG. 2 shows a modification in which one end of the primary side circuit of the power supply transformer 2 is grounded. The same parts are designated by the same reference numerals and the description thereof will be omitted. Power transformer 2
If one end of the primary side circuit of is grounded, choke coil L
₃ and L ₄ , it is possible to limit the in-phase noise current of the audio signal that leaks through the electrostatic capacity of the power transformer 2.

The power supply circuit shown in FIG. 3 shows a modification in which a full-wave rectifier circuit is used as the rectifier circuit. In the power supply circuit shown in this figure, the alternating voltage generated in the secondary winding coil L ₂ of the power transformer 2 is applied to the diode D ₁₀
The full-wave rectifying circuit composed of D ₁₃ to D ₁₃ rectifies it, the smoothing capacitor C ₁₀ smoothes it, and supplies it to the amplifier circuit.

In this case, a part of the current flowing through the speaker (not shown) amplified and output by the amplifier circuit 4 is taken as the current I ₄ , and the secondary winding coil L of the power transformer 2 is supplied.
₂ flows in both directions, and accordingly, the leakage current I _{5 in} both directions is also generated on the primary side of the power transformer 2.
The leakage current I ₅ that flows when using such a full-wave rectifier circuit can also be cut off by the choke coils L ₃ and L ₄ .

The power supply circuit of FIG. 4 shows a modification in which a conventional noise filter circuit 30 and a capacitor C ₁₁ are attached to the primary side of the power supply transformer 2 in addition to the choke coils L ₃ and L ₄ . It is a thing. Thus choke coil L
_A noise filter circuit 30 is provided between ₃ , L ₄ and the AC power supply 1.
Are connected in series and the capacitor C ₁₁ is connected in parallel, the noise signal in the audio band and the above-mentioned leakage current I ₅ can be cut off.

Although the case where the choke coils L ₃ and L ₄ are connected to both ends of the power transformer 2 has been described in the present embodiment, the present invention is not limited to this, and the choke coil is provided only on one side of the power transformer 2. You may do it. In this case, the inductance of the choke coil may be set so that a voltage of about 10% of the commercial AC voltage is divided and applied. Further, although the case where the low frequency amplifier circuit is applied as the circuit connected to the secondary side of the power transformer has been described in the present embodiment, the present invention is not limited to this.

[0039]

As described above, according to the power supply circuit of the present invention, a single unit in series with the primary winding coil of the power transformer,
Or, by connecting a plurality of coils, it is possible to increase the primary impedance of the power transformer in the frequency band of the audio signal.
It is possible to reduce the current in the low frequency band flowing from the secondary side to the primary side, and to reduce the current from the primary side of the power transformer by 2
It is possible to reduce noise in a low frequency band that flows into the next side.

When a low frequency amplifier circuit or the like is connected to the secondary side of the power transformer to amplify an audio signal, the audio current output from this low frequency amplifier circuit is the impedance of the primary side of the power transformer. It is not affected by the characteristics and the sound quality can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a low frequency amplifier circuit using a power supply circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a modified example of the power supply circuit of the present embodiment.

FIG. 3 is a diagram showing a modification of the power supply circuit of the present embodiment.

FIG. 4 is a diagram showing a modified example of the power supply circuit of the present embodiment.

FIG. 5 is a diagram showing an example of a conventional noise filter circuit provided in a conventional power supply circuit or the like.

FIG. 6 is a diagram showing an example of a low-frequency amplifier circuit using a conventional power supply circuit.

[Explanation of symbols]

1,31 Commercial AC power supply 2,32 Power supply transformer 3,33 Rectification / smoothing circuit 4,34 Low frequency amplifier 5 Speaker 30 Noise filter circuit L ₁ Primary winding coil L _2a , L _2b Secondary winding coil L ₃ , L ₄ choke coils D _1, D ₂ rectifier diode C ₁ -C ₂ smoothing capacitor R ₁ to R ₈ resistor Tr1, Tr3 NPN type transistor Tr2 PNP-type transistor

Claims (3)

[Claims] 1. A power supply circuit having a power supply transformer, wherein a single or a plurality of coils are provided in series with a primary winding coil of the power supply transformer so that the single or a plurality of coils are not magnetically coupled to the power supply transformer. A power supply circuit characterized by being configured. 2. The power supply circuit according to claim 1, wherein the inductance of the coil is set to have a sufficiently large value for a voice band signal. 3. The secondary winding coil of the power transformer is provided with a rectifying / smoothing means, and the output of the rectifying / smoothing means is connected to drive a low frequency amplifier for amplifying an audio signal. The power supply circuit according to claim 1, wherein: