JPH0530145B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a DC-DC conversion circuit that switches DC power and supplies it to the primary winding of a transformer, rectifies and smoothes the output of the secondary winding of the transformer, and obtains a DC output. In particular, it relates to a circuit that separates input and output with high impedance and generates little common-mode switching noise.

<Background> This type of DC-DC conversion circuit is used, for example, as a power source for a digital line termination device installed inside a subscriber's premises in a digital subscriber line transmission system. That is, as shown in FIG. 1, the digital line termination device 11 installed inside the subscriber's premises is generally configured to operate by distant power supply from the station, and the balanced type connected to the digital line termination device 11 is A digital signal and a feeding direct current are superimposed on the subscriber line 12 of the cable. Although not shown in the figure, the other end of the subscriber line 12 is led into the office and connected to a digital line termination device inside the office. Digital signals on subscriber line 12 are transmitted to equipment 11
The signal is inputted to the pulse transmission circuit 15 via the connection point 13 and the transformer 14. The pulse transmission circuit 15 includes an equalization amplifier circuit, a pulse transmission circuit, and the like. The supply direct current on the subscriber line 12 is connected to the connection point 1.
3. Via power separation filters 16a and 16b
The voltage is input to the primary sides 18a and 18b of the DC-DC conversion circuit 17, and a DC voltage of 30 V, for example, is applied between the primary sides 18a and 18b. The DC-DC conversion circuit 17 performs DC-DC conversion, and generates, for example, a DC voltage of 5 V between the secondary sides 19a and 19b of the DC-DC conversion circuit 17. The main part or the whole of the digital line termination device 11 is operated by the outputs of the secondary sides 19a and 19b of the DC-DC conversion circuit 17. A DC blocking capacitor 21 inserted between the connection point 13 and the connection point between the filters 16a and 16b and the transformer 14 is provided to prevent the supply DC current from flowing through the transformer 14. The power separation filters 16a and 16b are constructed of, for example, coils so as to have low impedance for direct current and high impedance for alternating current. This is to avoid short-circuiting the digital signals since the AC impedance between the primary sides 18a and 18b of the DC-DC conversion circuit 17 is low.

Now, the digital line termination device 1 with the above configuration.
When a conventionally configured DC-DC converter circuit is applied to the DC-DC converter circuit 17, which is the power source of No. 1, the following drawbacks occur.

(a) Between the primary side and the secondary side of the DC-DC conversion circuit 17, that is, the primary side 18a and the secondary side 19a
between the primary side 18b and the secondary side 19b
During this period, switching noise _v1 , so-called common mode switching noise, occurs due to switching of the DC-DC conversion circuit 17. This common mode switching noise is converted into differential mode noise according to the amount of unbalanced attenuation determined by the subscriber line 12, the digital line termination device 11, etc., and is transmitted between the secondary sides 22a and 22b of the transformer 14. This can cause code errors in digital signals. For this reason, it is necessary to sufficiently reduce the generation of common-mode switching noise, but conventional
A DC-DC conversion circuit could not satisfy this requirement.

(b) Various types of vertical noise such as impulsive noise from the analog telephone line are induced on the subscriber line 12 and cause code errors in the digital signal, so the unbalanced attenuation of the digital line termination device 11 is It needs to be high enough. In order to eliminate the drawback described in the above item (a), as shown in FIG.
Some devices suppress the common-mode switching noise by connecting 3 between the primary side 18a and the secondary side 19a (or between the primary side 18b and the secondary side 19b). However, with such a configuration and the second part of the DC-DC conversion circuit 17.
When connecting the next side 19a or 19b to earth with low impedance, power isolation filter 1
A resonance point is formed in the vertical circuit from the coil for 6a (or 16b) and the external capacitor 23, and at this resonance point, the digital line termination circuit 11
The unbalanced attenuation of the signal is extremely degraded, resulting in code errors. Therefore, it is necessary to set the resonance point to a frequency sufficiently higher than the pulse transmission band, and for this purpose, the external capacitor 23 is removed and the primary sides 18a, 18b and secondary side 19 of the DC-DC conversion circuit 17 are
a and 19b must be separated by a high impedance comparable to the stray capacitance of the transformer 14. However, in this case, the conventional circuit configuration suffers from the drawbacks described in item (a) above.

These points will be explained in more detail below.

FIG. 3 shows the basic configuration of a conventional DC-DC conversion circuit. The DC input voltage _E1 input from the DC power supply 24 through the primary sides 18a and 18b is converted into an alternating voltage (hereinafter referred to as switching voltage) by the repeated on/off operation (hereinafter referred to as switching) of the switch element 25, A switching voltage is applied between both ends 29 and 31 of the primary winding 27 of the transformer 26.
e ₁ occurs. Therefore, the secondary winding 2 of the transformer 26
A switching voltage e ₂ is induced between both ends 32 and 33 of 8. This switching voltage _e2 is half-wave rectified by a diode 34 and smoothed by an output capacitor 35 to obtain a DC output voltage _E2 , which is supplied to a load 36 through the secondary sides 19a and 19b. Note that the switching voltage _e 1 induced between both ends 29 and 31 of the primary winding 27 and the secondary winding 28
The ratio to the switching voltage e ₂ induced between both ends 32 and 33 of is determined by the winding ratio between the primary winding 27 and the secondary winding 28. An input capacitor 37 is connected between both ends of the DC power supply 24, and a switch element 2
5 is a transistor, for example, and this transistor 25 is inserted in series with the primary winding 27, and the drive circuit 3 is connected between the base and emitter of the transistor 25.
8 are connected.

In the conventional DC-DC conversion circuit shown in FIG. 3, between the primary side and the secondary side, that is, the primary side 18a,
There was a drawback that a large switching voltage was generated between the next side 19a. This is called common mode switching noise, and will be explained below using FIG. 4. Fourth
The figure shows the DC-DC conversion circuit shown in Figure 3.
This figure shows the generation mechanism of common-mode switching noise, focusing on the AC component. The symbols in FIG. 4 are the same as in FIG. 3, e ₁ , e ₂ , v ₁ and v ₂ represent the switching voltages between the respective terminals at any given time, and the arrows represent the mutual relationship between the voltage polarities. Switch element 25
The connection point between the diode 34 and the power supply 24 is the primary side 18b, and the connection point between the diode 34 and the output capacitor 35 is the secondary outside 19b. Capacitor 42 between winding ends 29 and 32, condenser 4 between winding ends 31 and 33
3 represents the stray capacitance distributed between the primary winding 27 and the secondary winding 28 using lumped constant circuits. Switch element 25, primary winding 2
7. The closed circuit consisting of the input capacitor 37 is called a closed circuit, the closed circuit consisting of the primary winding 27, the secondary winding 28, and the capacitors 42 and 43 is called a closed circuit, and the secondary winding 28, the diode 34, and the output capacitor 35 are called a closed circuit.
A closed circuit consisting of the following is called a closed circuit.

In FIG. 4, first focus on the closed circuit. Since the input capacitor 37 is short-circuited (sufficiently low impedance) to the switching frequency component, no switching voltage is generated across it. Therefore, from Kirchhoff's voltage law, the switch element 25
A switching voltage having an amplitude equal to the switching voltage e ₁ induced between both ends 29 and 31 of the primary winding 27 is generated at both ends of the primary winding 27 in opposite phases. Next, we will focus on closed circuits. Since the output capacitor 35 is short-circuited for the switching frequency component, no switching voltage is generated across it. Therefore, according to Kirchhoff's voltage law, a switching voltage with an amplitude equal to the switching voltage e ₂ induced between the ends 32 and 33 of the secondary winding 28 is generated at both ends of the diode 34 in opposite phases.

Next, we will focus on closed circuits. Normal capacitor 42,
The capacitance values of 43 are both small, and the impedance is sufficiently high over high frequencies including the switching frequency. Now, in a closed circuit, capacitor 4
If the switching voltages generated at both ends of 2 and 43 are v ₁ and v ₂ , respectively, then from Kirchhoff's voltage law,
It satisfies the relationship v ₁ + v ₂ = e ₁ − e ₂ . Also the voltage v ₁
and v ₂ is inversely proportional to the capacitance value of each capacitor 42, 43. The switching voltage v ₁ generated across capacitor 42 is common mode switching noise. The following description will be made with reference to FIG. 5, which is an enlarged view of the closed circuit portion. The symbols in FIG. 5 correspond to those in FIG. 4. The capacitance values of capacitors 42 and 43 are C ₁ ,
Let it be C ₂ . From FIG. 5, the amplitude value v ₁ of the common mode switching noise is v ₁ = C ₂ /C ₁ +C ₂ (e ₂ - e ₁ ) (1), and the switching voltage e ₁ generated across the primary winding 27 is and the switching voltage e ₂ induced across the secondary winding 28.
In a normal DC-DC conversion circuit, e ₁ ≠ e ₂ . Therefore, the primary side 18a, 18b, the secondary side 19
In order to isolate between a and 19b with high impedance and to reduce the occurrence of common mode switching noise, the capacitance value _C2 of the capacitor 43 must be made sufficiently smaller than the capacitance value _C1 of the capacitor 42. . However, the capacitance value C ₂ of the capacitor 43
is determined by the shapes of the primary winding 27, secondary winding 28, and core of the transformer, and the capacitance value _C2 cannot be made very small. On the other hand, the capacitance value C ₁ of the capacitor 42
When increasing (for example, by adding an external capacitor),
The common mode switching noise will be smaller, but 1
The impedance between the next sides 18a, 18b and the secondary sides 19a, 19b is low. From the above, the primary side 18
It has been difficult in the prior art to isolate the secondary sides 19a, 19b from the secondary sides 19a, 19b with high impedance and to reduce common mode switching noise. As an example, in the configuration shown in Figure 3, the DC input voltage E ₁ = 30V, the DC output voltage E ₂ = 5V, and the output voltage 1W.
In a DC-DC conversion circuit of about 100 volts, common-mode switching noise occurs as a ripple component of about 10 Vpp. However, this noise value differs slightly depending on the configuration of the primary and secondary windings.

<Summary of the Invention> The purpose of the present invention is to provide a DC-DC conversion circuit that isolates the primary side and the secondary side with high impedance over a high frequency range including DC and switching frequencies, and that generates little common mode switching noise. Our goal is to provide the following.

According to this invention, an electrostatic shielding layer is arranged between the primary winding and the secondary winding, and this electrostatic shielding layer is connected to the stationary end of the primary winding and inside the secondary winding. From the point of view, the voltage induced against the electrostatic shielding layer on one side of the secondary winding and the voltage induced against the electrostatic shielding layer on the other side of the primary winding have opposite polarity. A canceling means is provided.

<First Embodiment> FIG. 6 shows a first embodiment of the present invention, in which diodes 34a and 34b for half-wave rectification are connected to the secondary winding 28.
are connected in series to both ends of the The other ends of the diodes 34a and 34b are connected to both ends of the output capacitor 35. An electrostatic shielding layer 46 is interposed between the primary winding 27 and the secondary winding 28 . This electrostatic shielding layer 46 is connected to a winding end 29 which is an AC zero potential point (hereinafter referred to as a static end) of the primary winding 27. The primary side 18b is also a stationary end, and the effect is the same even if the electrostatic shielding layer 46 is connected to the primary side 18b. Other symbols are in accordance with Figure 3. This structure has the effect of reducing the switching voltage generated between the primary side 18a and the secondary side 19a, that is, the common mode switching noise, as described below. According to the configuration shown in FIG.
(i) An electrostatic shielding layer 46 is provided between the primary winding 27 and the secondary winding 28, and this is connected to the winding end 29, which is the stationary end of the primary winding 27, so that Switching voltage e ₁ generated across the secondary winding 27
is not generated as a voltage (common mode switching noise) between the primary side 18a and the secondary side 19a. (ii) Diodes 34a and 34b for half-wave rectification are connected to both ends of the secondary winding 28, and the electrostatic shielding layer 4
6 - Switching voltage generated between the winding end 32 (or 33) and switching generated between the winding end 32 (or 33) and the secondary side 19a (or 19b), which are both ends of the diode 34b (or diode 34a) The voltages have opposite polarities and cancel each other out, so that the switching voltage e2 generated at both ends of the secondary winding 28 becomes the switching voltage _e2 on the primary side 18a-
It does not occur as a voltage (common mode switching noise) across the secondary side 19a.

These will be explained in detail using FIG. 7. FIG. 7 shows the effect of reducing the common mode switching noise in the DC-DC conversion circuit shown in FIG. 6, focusing on the alternating current component. The symbols in FIG. 7 are the same as in FIG. 6, e ₁ , e ₂ , v ₁ and v ₂ indicate the switching voltages between the terminals at any given time, and the arrows indicate the mutual relationship between the voltage polarities.

The capacitor 47 represents the stray capacitance distributed between the secondary winding 28 and the electrostatic shielding layer 46 using a lumped constant circuit, and the capacitor 47 is connected between the terminal 32 of the secondary winding 28 and the static shielding layer 46. It has been done.
The capacitor 51 is connected to the secondary winding 28 and the electrostatic shielding layer 4.
6 is represented by a lumped constant circuit, and the capacitor 51 is connected between the terminal 33 of the secondary winding 28 and the electrostatic shielding layer 46. The capacitors 52 and 53 represent the stray capacitance distributed between the primary winding 27 and the electrostatic shielding layer 46 using a lumped constant circuit, and the capacitors 52 and 53 represent the stray capacitance distributed between the primary winding 27 and the electrostatic shielding layer 46. layer 46. capacitor 37, primary winding 27,
The closed circuit of the switch element 25 is a closed circuit, and the primary winding 27, the capacitors 52 and 53, and the electrostatic shielding layer 4
The closed circuit of 6 is a closed circuit, and the closed circuit is the secondary winding 2.
8, consists of capacitors 47 and 51, and an electrostatic shielding layer 46, and the closed circuit is formed by a secondary winding 28 and a diode 3.
It is composed of capacitors 35 4a and 34b. First, the above item (i) will be explained. In FIG. 7, attention is first paid to the closed circuit. Since the input capacitor 37 is short-circuited (sufficiently low impedance) to the switching frequency component, no switching voltage is generated across it. Therefore, from Kirchhoff's voltage law, the primary winding 27 is connected to both ends of the switch element 25.
A switching voltage with an amplitude equal to the switching voltage e ₁ induced between the ends 29 and 31 of is generated in opposite phase. Next, we will focus on closed circuits. capacitor 5
2 and 53 represent the stray capacitance distributed between the primary winding 27 and the electrostatic shielding layer 46 using lumped constant circuits. Capacitor 52 is connected between winding end 29 and electrostatic shielding layer 46 , and capacitor 53 is connected between winding end 31 and electrostatic shielding layer 46 . Here, since both ends of the capacitor 52 are short-circuited by the electrostatic shielding layer 46, no switching voltage is generated. Therefore, according to Kirchhoff's voltage law in a closed circuit, a switching voltage with an amplitude equal to the switching voltage e ₁ generated between both ends 29 and 31 of the primary winding 27 is generated at opposite ends of the capacitor 53 in opposite phases. From the above, the switching voltage _e1 generated on the primary side is closed only to the primary side by the electrostatic shielding layer 46, and the switching voltage e1 generated on the primary side is
It does not affect the voltage between a.

Next, the above item (ii) will be explained. In FIG. 7, attention is paid to the closed circuit. Since the output capacitor 35 is short-circuited for the switching frequency component, no switching voltage is generated across the output capacitor 35. Further, a switching voltage e ₂ is induced between both ends 32 and 33 of the secondary winding 28. Here, from Kirchhoff's voltage law in a closed circuit, the diode 34a,
Both ends 32 and 33 of the secondary winding 28 are connected to both ends of 34b.
A switching voltage having an opposite phase to the switching voltage e ₂ induced between the two is generated with the amplitude divided into two. Considering the variation in diode characteristics,
Let the amplitudes of the switching voltages generated across the diodes 34a and 34b be e ₂ /2+Δ and e ₂ /2−Δ, respectively.

Next, we will focus on closed circuits. Capacitor 47, 51
is a lumped constant circuit representing the stray capacitance distributed between the electrostatic shielding layer 46 and the secondary winding 28, and the capacitor 47 is connected between the electrostatic shielding layer 47 and the winding end 32. , capacitor 51 is connected between electrostatic shielding layer 46 and winding end 33. Normally, capacitance values of both capacitors 47 and 51 are small, and the impedance is sufficiently high over a high frequency range including the switching frequency. Now, when the switching voltages generated across the capacitors 47 and 51 are respectively v ₁ and v ₂ , the following relationship is satisfied from Kirchhoff's voltage law in a closed circuit: v ₁ +v ₂ =e ₂ . The following description will be made with reference to FIG. 8, which is an enlarged view of the closed circuit portion. The symbols in FIG. 8 correspond to those in FIG. Let the capacitance values of the capacitors 47 and 51 be C ₃ and C ₄ , respectively. Solving the cycle equation for the cycle in Figure 8 yields v ₁ = C ₄ /C ₃ + C ₄ e ₂ (2). Now, let's go back to FIG. 7 and explain.

The common mode switching voltage is on the primary side 18a-
_This is the switching voltage that occurs between the secondary side 19a, and if this _is written as v ₃ , _then from FIG. Substituting , v ₃ = C ₄ − C ₃ /C ₃ +C ₄・e ₂ /2+Δ (4). In equation (4), the value of Δ is sufficiently small, and the common mode switching noise is reduced as the difference between the capacitance value C ₃ of the capacitor 47 and the capacitance value C ₄ of the capacitor 51 becomes smaller. Now, in FIG. 6, the technique of equalizing the stray capacitance between the electrostatic shielding layer 46 and the winding end 32 and the stray capacitance between the electrostatic shielding layer 46 and the winding end 33 is relatively easy; The physical position of winding end 32 with respect to layer 46 and the physical position of winding end 33 with respect to electrostatic shielding layer 46 may be symmetrical. For example, the secondary winding 28 may be configured to have one layer of winding around the electrostatic shielding layer 46. That is, in FIG. 8, the capacitance value C ₃ of the capacitor 47 and the capacitance value C ₄ of the capacitor 51 are
Techniques for making these approximately equal are known. From the above, by connecting the half-wave rectifying diodes 34a and 34b to both ends of the secondary winding 2, the switching voltage e ₂ induced across the secondary winding 28 is
It has been explained that the influence on the voltage between the primary side 18a and the secondary side 19a can be reduced.

As described above, with the configuration shown in FIG. 6, the primary side 18a, 1
It is possible to provide a DC-DC conversion circuit that isolates the secondary sides 19a and 19b with high impedance and generates less common mode switching noise.

In FIG. 6, when the secondary side is a DC-DC converter circuit with multiple outputs and multiple secondary windings, the primary winding and multiple secondary windings are sequentially formed on the same axis,
By providing an electrostatic shielding layer between the secondary windings and connecting this electrostatic shielding layer to the electrostatic shielding layer 46 between the primary winding and the secondary winding, common mode This is advantageous in reducing the occurrence of switching noise.

<Second Embodiment> The above is a so-called current transmission type, that is, in FIG. 6, when the primary side switch element 25 is off,
Diodes 34a and 34b for half-wave rectification on the secondary side
This invention was applied to a DC-DC conversion circuit of the type in which the current is conductive, but it is also applicable to a DC-DC conversion circuit of the so-called voltage transmission type, in which the half-wave rectifier diode on the secondary side conducts when the switch element on the primary side is on. -The present invention can also be applied to a DC conversion circuit. The voltage transmission type DC-DC conversion circuit of the present invention corresponding to FIG. 6 is shown in FIG. 9.
As shown in the figure. In these cases, the rectifying diodes 34a, 34b have opposite polarities to those in the case of FIG. 6, and the other configurations are the same.

<Effects> As explained above, the present invention provides a DC switching system that isolates the primary side and secondary side with high impedance over a high frequency range including DC and switching frequencies, and generates less common mode switching noise.
-Since we can provide DC conversion circuits, we can provide
It has an advantage in application as a power receiving power source for a digital line termination device installed inside a subscriber's premises that operates by supplying power from a remote station. Specifically, DC
- Switching noise generated by the DC conversion circuit is less likely to enter the pulse transmission system circuit, and high impedance separation between the digital line termination device and the subscriber line is possible. The effect of this is that a high unbalanced attenuation of the digital line termination device can be obtained in the pulse transmission band, and code errors in the digital signal can be suppressed as much as possible against large vertical noise induced on the subscriber line.

A numerical example is shown next. In the configuration shown in FIGS. 6 and 9, the input voltage E ₁ is approximately 26 V, the input current is approximately 24 mA, the output voltage E ₂ is 5 V ± 3.5%, the output voltage is approximately 500 mW, and the primary winding 27 turns. The number of lines is about 80 times,
The number of turns of the secondary winding 28 is about 16, the electrostatic shielding layer 46 is copper foil, the switch element 25 is a MOS-FET,
In the DC-DC conversion circuit, the rectifying diodes 34a and 34b are shot key barrier diodes, the transformer secondary winding 28 is wound in one layer around the electrostatic shielding layer 46, and the switching frequency is approximately 70 KHz.
Common-mode switching noise is a lip component of approximately
The power conversion efficiency was 0.5Vpp and approximately 80%. In addition,
The common mode switching noise was the same whether the switch element drive circuit 38 was of the separately excited type or the self-excited type.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing the configuration of a digital line termination device installed inside a subscriber's premises, respectively. Figure 3 is a connection diagram showing the basic configuration of a conventional current transmission type DC-DC conversion circuit. Figure 4 is the DC of Figure 3.
- An explanatory diagram of the common-mode switching noise generation mechanism in a DC conversion circuit, Figure 5 is an enlarged view of the closed circuit part of Figure 4, and Figure 6 shows the current transmission type of this invention.
A connection diagram showing an example applied to a DC-DC conversion circuit, Fig. 7 is an explanatory diagram of the common mode switching noise reduction effect in the DC-DC conversion circuit of Fig. 6, and Fig. 8 is a closed circuit diagram of Fig. 7. Enlarged view of part, No. 9
The figure is a connection diagram showing another embodiment in which the present invention is applied to a voltage transmission type DC-DC conversion circuit. 24...DC power supply, 25...Switch element, 2
6...Transformer, 27...Primary winding, 28...2
Next winding, 34a, 34b... Diode for half-wave rectification, 35... Output capacitor, 36... Load,
37... Input capacitor, 38... Switch element drive circuit, 46... Electrostatic shielding layer.

Claims (1)

[Claims] 1. In a DC-DC conversion circuit that switches and supplies DC power to the primary winding of a transformer and rectifies and smoothes the output of the secondary winding of the transformer to obtain a DC output, the primary winding and the secondary An electrostatic shielding layer is provided between the winding and the static end of the primary winding, and first and second half-wave rectifiers for the rectification are provided at both ends of the secondary winding, respectively. the first diode
A DC-DC conversion circuit characterized in that the anode of the diode and the cathode of the second diode are connected in series with a secondary winding interposed therebetween.