JPS6321424B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is applicable to switching elements such as N-channel field effect transistors whose drains are connected to a positive power supply, in which the potential of the drive terminal is floating from ground and fluctuates at the switching frequency. This relates to a circuit that drives a .

An example of a conventional switching circuit is an N-channel power supply with the drain connected to the positive power supply.
FIG. 1 shows a switching power amplifier circuit using a MOS type field effect transistor (hereinafter simply referred to as a field effect transistor). In Figure 1,
Q is a field effect transistor, 1 is a positive power supply terminal, 2 is a pulse width modulation (hereinafter referred to as PWM) signal input terminal, and 3 is a power supply terminal 4 that receives power supply, and is connected to a pulse transformer or an optical coupling element. This is a drive circuit in which input and output terminals are insulated by a drive circuit 3, and a PWM signal from the signal input terminal 2 is applied between the source and gate of a field effect transistor Q by a drive circuit 3. Further, C _S is a capacitor for reducing the alternating current impedance of the power supply terminal 1, and R _g1 and R _g2 are resistors for preventing parasitic vibration of the field effect transistor Q, terminating the drive circuit 3, improving the waveform, etc. Further, D is a flywheel diode, a coil L _A and a capacitor C constitute a low-pass filter, and R _L is a load.

In the above configuration, when a signal from the signal input terminal 2 is applied between the source and gate of the field effect transistor Q by the drive circuit 3, the field effect transistor Q is turned on/off according to the PWM signal.
An OFF switching operation will be performed. When the field effect transistor Q is in the ON state, current flows from the power supply terminal 1 through the coil L _A to the load R _L , and when the field effect transistor Q is in the OFF state, the current flows through the coil L through the flywheel diode D. It is. Therefore, a PWM waveform whose amplitude is the voltage at power supply terminal 1 appears on the cathode side of the flywheel diode D, that is, on the source side of the field effect transistor Q, but the low-pass filter formed by the coil L _A and the capacitor C suppresses the PWM waveform. By blocking the repetition frequency, a voltage of the average value of the PWM waveform is output to the load R _L. With the above operation, the switching power amplifier circuit operates as a switching regulator when used as a power supply, and as an amplitude modulator when the load R _L is a modulated power amplifier.

However, in the circuit operation as described above, due to its characteristics, the source potential of the field effect transistor Q oscillates with a PWM waveform with a large amplitude equal to the voltage of the power supply. Sufficient insulation characteristics are required, and furthermore, the capacitance to ground of the output stage of the drive circuit 3 may adversely affect the switching characteristics of the field effect transistor Q, or the large amplitude of the output stage of the drive circuit 3 may cause unnecessary radiation or switching. There were problems such as the generation of noise.

The present invention has been made to eliminate such problems, and focuses on the low-pass filter that constitutes the switching circuit, and by providing a drive winding in the coil of this low-pass filter, the drive circuit can be improved. In contrast, the object of the present invention is to provide a switching circuit which obtains AC-like insulation characteristics and solves the above-mentioned problems. This will be explained below using figures.

FIG. 2a is a circuit diagram showing a first embodiment of the first invention, and FIG. 2b is a circuit diagram showing the first embodiment of the second invention.

In Figure 2 a and b, L _B is W ₁ , W ₂ ,
A coil having three sets of windings W ₃ , of which windings W ₂ and W ₃ are paired wires or coaxial wires with winding W ₃ as the core wire, and are located on the same magnetic core as winding W ₁ or It has a structure in which it is wound so that magnetic flux passes through it and couples it. Moreover, these windings W ₂ and W ₃ have the same number of turns as the winding W ₁ , and the alternating current voltage at the beginning and end of winding W ₁ and the voltage induced in winding W ₂ or winding W _{3 are different} from each other. As for the direction of the winding, the winding W ₁ ,
W ₂ and W ₃ are wound in the same direction. Further, the drain of the field effect transistor Q is connected to the power supply terminal 1, and its source is connected to the cathode of a flywheel diode D whose anode side is grounded, and the winding W ₁ of the coil L _B forming a low-pass filter.
is connected to. R has a low resistance and the other circuit elements are the same as those in FIG.

First, a first embodiment of the first invention will be described based on FIG. 2a.

The circuit operation shown in FIG. 2a is performed in the same manner as in the case of FIG.
In other words, the signal from the signal input terminal 2 is applied between the source and gate of the field effect transistor Q via two windings W ₂ and W ₃ wound together with the winding W ₁ of the coil L _B of the low-pass filter. As a result, the field effect transistor Q performs an ON/OFF switching operation. Drive circuit 3 at this time
, that is, the potential at point D of winding W ₂ , first, the potential at point A of winding W ₁ is a PWM waveform whose amplitude is the power supply voltage of power supply terminal 1, and the potential at point D of winding W ₁ is
The potential at point B is the average value of the waveform at point A, that is, it has a waveform that is almost _{DC .}
has the same number of turns as winding W ₁ and is magnetically coupled, so the voltage of winding W ₁ , that is, the voltage equal to the AC voltage waveform between points A and B, is the same as the voltage at point C of winding W ₂ ,
It is induced between points D and between points E and F of winding _W3 .
In other words, the winding is started from the large amplitude PWM waveform at point A.
Just as the AC component is absorbed between points A and B of W ₁ and the waveform at point B becomes close to DC with little change, the AC voltage waveform induced in winding W ₂ changes to point C, that is, A.
Since the AC voltage component is absorbed from the same PWM waveform as point D, the voltage at point D is also the same voltage as point B, that is, the same potential as the capacitor C potential.

Also, since the windings W ₂ and W ₃ are closely magnetically coupled, the voltage induced in the winding W ₂ and the voltage induced in the winding W ₃ are equal, so that the voltage induced in the control terminal of the switching element is That is, the voltage between the C point of winding W ₂ and the winding
The voltage between point E of W ₃ is the output of drive circuit 3, that is, the voltage between point D of winding W ₂ and point F of winding W ₃ , and the induced voltage of winding W ₂ or winding W ₃ . It is well known that it is not affected by

Next, consider a case where the output of the drive circuit 3, that is, the potential at point D of the winding _W2 , and the potential of the capacitor C become different due to stray capacitance between the windings of the coil L _B or winding stray inductance. . In order to prevent this potential from being different and maintain the same potential, when point B of winding W ₁ and point D of winding W ₂ are connected, load current flows to winding W ₁ and winding W ₂ , and the winding A potential difference occurs due to the line resistance, and this acts as a negative feedback between the gate and source of the field effect transistor Q, resulting in a disadvantage that it adversely affects its switching characteristics.

The first embodiment of the second invention eliminates this drawback by connecting point B of winding W ₁ and point D of winding W ₂ via a low resistance R, as shown in FIG. 2b. This is what I did. As a result, the load current flowing through the winding _W2 can be ignored, and therefore the negative feedback can be ignored. In an actual circuit, if a low resistance R of about 5Ω to 10Ω is connected, the AC fluctuation at point D will be the same as at point B, and the value of 5Ω to 10Ω is compared to the winding resistance (usually 0.1Ω or less). is sufficiently large, so the load current flowing through the winding _W2 can be ignored.

Next, looking at the potential of capacitor C mentioned above, the potential of capacitor C is a direct current with almost no fluctuation when applied as a switching regulator, and when applied as an amplitude modulator, it is a direct current with almost no fluctuation. The following fluctuations are required, and therefore, the insulation characteristics required between the input and output of the drive circuit 3 are mainly those of direct current.

As explained above, in the first embodiment of the first and second inventions, insulation characteristics at the switching frequency can be obtained by the two windings W ₂ and W ₃ newly added to the winding of the low-pass filter. A switching circuit that prevents large amplitude fluctuations in the output potential of the drive circuit 3, in other words, a switching circuit that switches the field effect transistor Q whose negative terminal is common to the switch control terminal using a positive power supply can be easily constructed.

Therefore, when the above switch circuit is used as a switching regulator, the negative terminal can be commonly grounded, and when used as an amplitude modulator, the negative side of the modulated power amplifier circuit, which is the load, can be grounded. Furthermore, at the output stage of the drive circuit 3,
Since a switching voltage with a large amplitude does not appear compared to the conventional one, the insulation characteristics required between the input and output of the drive circuit 3 are DC-like, and the circuit is simpler than the conventional one. This has the advantage of reducing unnecessary radiation and switching noise from the output stage of the circuit.

Note that the first aspect of the first and second inventions described above
In the embodiment, the coil L is used as a low-pass filter.
Although the case of one stage of capacitor C has been described, it is also possible to use a low pass filter with multiple stages or regardless of the structure of the low pass filter, as shown in FIGS. 3a and 3b as another example.
Even if it is composed of L ₁ , C ₁ , L ₂ , C ₂ , C ₃ , L ₃ , C ₄ , and C ₅ , the circuit network R _L ′ on the load side from L ₁ and C ₁ can be considered as part of the load. For example, it is clear that the same operations, effects, and effects as in the first embodiment of the first and second inventions occur. In FIGS. 3a and 3b, the coil L ₁ of the low-pass filter, which is the main part of the present invention, has a first winding W ₁ as the main winding and a second winding W ₂ as the drive winding. , and a third winding _W3 , but an example is shown in which an outer conductor and a core wire of a coaxial wire are used as the second winding and the third winding.

FIG. 3a shows a second embodiment of the first invention, in which an electron tube is used as the switching element, and will be briefly described. V is a triode whose plate is connected to the power supply terminal 1 and whose cathode is connected to the diode D whose anode is grounded, and whose cathode is connected to the coils L ₁ , L ₂ , L ₃ and the capacitor.
Each of the input sides of a low-pass filter consisting of C ₁ , C ₂ , C ₃ , C ₄ , and C ₅ is grounded. Also, this triode V
As in the first embodiment, a control voltage from the drive circuit 3 is applied between the grid and the cathode by a coaxial line wound around the first stage coil _L1 of the low-pass filter, thereby performing a switching operation. In other words, the triode V performs ON/OFF switching operation by the control voltage from the signal input terminal 2 according to the PWM signal, and the 3-stage low-pass filter
A voltage proportional to the duration of the switching ON state is output to the load R _L connected to L ₁ , L ₂ , and L ₃ .

In the above embodiments, a field effect transistor or an electron tube is used as the switching element, but other devices such as an SIT or an NPN bipolar transistor may also be used, where one of the control terminals as a switching element is connected to the negative polarity side (cathode) of the switching element. It is clear that the same effect can be obtained when a switching circuit common to the negative side is constructed using elements common to the negative side.

In this embodiment, as in the first embodiment, depending on the structure of L ₁ , the first winding W ₁ and the coaxial wires W ₂ and W ₃
The potential of the output stage D of the drive circuit 3 may not become equal to the potential of _C1 due to the influence of the stray capacitance and stray inductance between the two, and a switching waveform may occur.
The problem can be solved by connecting a low resistance R as shown in FIG. b, as in the case of the first embodiment of the second invention.

As described above, according to the present invention, a switching regulator or a switching amplifier common to the negative side can be constructed using field effect transistors, electron tubes, or the like, and the insulation characteristics required for the drive circuit of the switching element can be achieved. Furthermore, excellent effects such as less unnecessary radiation and less switching noise can be expected, so it can be used in regulators and amplitude modulators with high switching frequencies.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of a conventional switching power amplifier circuit, FIGS. 2 and 3 are circuit diagrams according to the present invention, and FIG. 2a shows a first embodiment of the first invention. The circuit diagram, FIG. 2b is a circuit diagram showing the first embodiment of the second invention, and FIG. 3a is the circuit diagram showing the second embodiment of the first invention.
FIG. 3b is a circuit diagram showing a second embodiment of the second invention. 1 and 4 are power supply terminals, 2 is a signal input terminal, 3 is a drive circuit, Q is a field effect transistor, D is a flywheel diode, L _A and L _B are coils, and R _L is a load.

Claims (1)

[Claims] 1. According to a pulse width modulation (PWM) signal
In the switching circuit, one of the switching elements that turn ON and OFF is connected to a power supply, and the other is connected to the input side of a low-pass filter and a diode with one side grounded, and the switching circuit supplies power to a load from the output side of the low-pass filter. The coil of the filter is constituted by the first winding of the transformer, which is composed of magnetically coupled first, second and third windings of the same number of turns, and from the first winding to the second and third windings. controlling one end of the second and third windings of the switching element such that the voltage induced in the third winding is in the same direction as the AC voltage of the first winding with respect to the switching element side; A switching circuit connected between terminals and applying the pulse width modulation signal from the other end. 2. The switching circuit according to claim 1, wherein a pair wire or a coaxial wire is used as the second and third windings. 3 According to the pulse width modulation (PWM) signal
In the switching circuit, one of the switching elements that turn ON and OFF is connected to a power supply, and the other is connected to the input side of a low-pass filter and a diode with one side grounded, and the switching circuit supplies power to a load from the output side of the low-pass filter. The coil of the filter is constituted by the first winding of the transformer, which is composed of magnetically coupled first, second and third windings of the same number of turns, and from the first winding to the second and third windings. controlling one end of the second and third windings of the switching element so that the voltage induced in the third winding is in the same direction as the voltage induced in the first winding with respect to the switching element side; the terminals, and the other end of the second or third winding and the load side end of the first winding are connected through a low resistance; A switching circuit characterized in that the pulse width modulation signal is applied from the other end. 4. The switching circuit according to claim 3, wherein a pair wire or a coaxial wire is used as the second and third windings.