JP3345866B2 - Switching power supply - Google Patents

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 having an output voltage detecting circuit provided with a shunt regulator as a feedback loop for stabilizing a DC output voltage.

[0002]

2. Description of the Related Art Generally, in a switching power supply of this type, various types of output voltage detection circuits are provided on a secondary side of a transformer as a feedback loop for stabilizing a DC output voltage. 62-185490
If a high-precision shunt regulator is used for the output voltage detection circuit, the reference voltage can be created internally by simply supplying the DC output voltage directly to the shunt regulator, There is an advantage that an auxiliary power supply circuit for obtaining a reference voltage is not required and the circuit configuration can be simplified.

FIG. 4 shows an example of an isolated converter in which a shunt regulator is built in an output voltage detecting circuit. In FIG. 4, reference numeral 1 denotes a transformer for insulating a primary side from a secondary side, and 2 denotes a transformer. 1 is an NPN transistor which is a switching element connected to the primary winding 1A. By switching this transistor 2, the DC input voltage Vi from the input terminals 3 and 3A is intermittently applied to the primary winding 1A of the transformer 1. Is applied to A rectifying and smoothing circuit 8 including a rectifying diode 4, a flywheel diode 5, a choke coil 6, and a smoothing capacitor 7 is connected to the secondary winding 1B of the transformer 1. Secondary winding 1B of transformer 1 with switching of transistor 2
Is rectified and smoothed through a rectifying and smoothing circuit 8 and output as a DC output voltage Vo between the output terminals 9 and 9A.

On the other hand, an output voltage detection circuit 11 having a shunt regulator 10 as a feedback loop for stabilizing the DC output voltage Vo output from the secondary winding 1B of the transformer 1 to the output terminals 9, 9A, Output voltage detection circuit 11
And a pulse width control circuit 12 for controlling the pulse conduction width of the drive signal supplied to the base of the transistor 2 based on the detection result from. The output voltage detection circuit 11 includes a series circuit including voltage dividing resistors R1 and R2 between output terminals 9 and 9A, and a series circuit including a first resistor R4, a light emitting diode 13A forming a photocoupler 13, and a shunt regulator 10. Are connected in parallel with each other, and the connection point of the resistors R1 and R2 is connected to the reference of the shunt regulator 10,
A parallel circuit comprising a capacitor C1 for preventing overshoot and a resistor R5 is connected between the cathode of the shunt regulator 10 and the reference. In this case, the equivalent circuit of the output voltage detection circuit 11 is as shown in FIG.
The shunt regulator 10 is replaced with a circuit configuration including a reference power supply 14 for outputting a reference voltage VREF and an operational amplifier 15.

In the output voltage detection circuit 11, the DC output voltage V
o is divided by the resistors R1 and R2 and applied to the reference of the shunt regulator 10, and the current value flowing into the cathode of the shunt regulator 10 is changed according to the difference between the applied voltage and the reference voltage VREF of the shunt regulator 10. Change. Thereby, the light emitting diode 13A
And the output current of the phototransistor 13B of the photocoupler 13 connected to the pulse width control circuit 12 changes in accordance with the change in the light emission amount. When the DC output voltage Vo increases, the output current of the phototransistor 13B decreases, and the pulse width control circuit 12 narrows the pulse conduction width of the transistor 2. On the other hand, when the DC output voltage Vo decreases, the pulse width of the phototransistor 13B increases. The output current increases, and the pulse width control circuit 12 widens the pulse conduction width of the transistor 2 to keep the DC output voltage Vo constant.

[0006]

In the output voltage detecting circuit 11 of the prior art, if the voltage value of the DC output voltage Vo line connected to the plus side output terminal 9 changes by ΔVo, the AC impedance of the light emitting diode 13A is changed. Since the component can be ignored, the variation ΔIF of the current flowing through the light emitting diode 13A is expressed by the following equation.

[0007]

(Equation 1)

In the above equation, s = jω. By further rearranging this equation and obtaining the transfer function of the output voltage detection circuit 11, the following equation is obtained.

[0009]

(Equation 2)

The above equation is given by G1 = 1 / sC1 (R1 + R
5) and G2 = 1 + sC1 (R1 + R5)
Is multiplied by a constant of (R5 + R1) / R4R1, the sum of the gain diagrams relating to the elements of each transfer function is obtained, and this is translated by a constant to obtain FIG. The gain-frequency characteristic shown in FIG.
In this case, the transfer function G2 has a zero point (knee frequency) ωz shown in the following equation.

[0011]

(Equation 3)

That is, as is apparent from the gain characteristic diagram shown in FIG. 6, in the conventional output voltage detecting circuit 11, the zero point ωz
Even in the higher frequency region, since it has a certain gain, it becomes impossible to remove high-frequency components inside the output voltage detection circuit 11, and noise generated on the secondary side of the transformer 1 Is transmitted to the primary side via the

In view of the above problems, an object of the present invention is to provide a switching power supply that does not transmit noise generated on the secondary side of a transformer to the primary side.

[0014]

According to the present invention, a DC input voltage is intermittently applied to a primary winding of a transformer by switching of a switching element, and a voltage induced in a secondary winding of the transformer is rectified and smoothed. And output a DC output voltage between output terminals, and as a feedback loop for stabilizing the DC output voltage, connect a series circuit of a first resistor, a photocoupler, and a shunt regulator between the output terminals, In a switching power supply device including an output voltage detection circuit that divides and applies a DC output voltage to the reference of the shunt regulator, a phase compensation circuit that inverts a differential value of the DC output voltage and applies the inverted value to the reference of the shunt regulator. It is provided in the output voltage detection circuit. In this case, a differentiating circuit including a series circuit of a capacitor and a second resistor connected to the phase compensation circuit between the output terminals, and a connection point between the capacitor and the second resistor configuring the differentiating circuit And an inverter connected between the shunt regulator and the reference.

[0015]

According to the above configuration, the phase compensation circuit inverts the differential value of the DC output voltage and applies it to the reference of the shunt regulator constituting the output voltage detection circuit. Thereby, the phase lead element of the output voltage detection circuit is compensated, and the gain decreases linearly as the frequency increases.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. The same parts as those in the above-described conventional example are denoted by the same reference numerals, and detailed description of the common parts is omitted. This embodiment detects the differential value of the DC output voltage Vo,
A phase compensation circuit 21 that inverts this detection signal and applies it to the reference of the shunt regulator 10 is an output voltage detection circuit.
11 is different from the conventional example. The phase compensating circuit 21 includes a differentiating circuit 22 as a phase lead circuit comprising a series circuit of a capacitor C2 and a second resistor R6 connected between the output terminals 9 and 9A, and an input terminal connected to the capacitor C2 and the second resistor R6. An inverter 23 is connected to a connection point with the resistor R6, and has an output terminal connected to the reference of the shunt regulator 10 via the resistor R3. FIG. 2 shows an equivalent circuit in which the output voltage detection circuit 11 and the phase compensation circuit 21 are combined, and the shunt regulator 10 is replaced with a circuit configuration including a reference power supply 14 for outputting a reference voltage VREF and an operational amplifier 15. Can be

Next, the operation of the above configuration will be described. The DC input voltage Vi is intermittently applied to the primary winding 1A of the transformer 1 by the switching of the transistor 2, and the voltage induced in the secondary winding 1B of the transformer 1 is rectified and smoothed by the rectifying and smoothing circuit. , 9A is supplied with a DC output voltage Vo. Further, the output voltage detection circuit 11 divides the DC output voltage Vo by the resistors R1 and R2, applies the divided voltage to the reference of the shunt regulator 10, and the applied voltage and the reference voltage VREF of the shunt regulator 10.
The current value flowing into the cathode of the shunt regulator 10 is changed according to the difference between As a result, the output current of the phototransistor 13B changes according to the change in the light emission amount of the light emitting diode 13A, and the pulse width control circuit 12 controls the pulse conduction width of the transistor 2 so as to keep the DC output voltage Vo constant. .

On the other hand, the output voltage detecting circuit 11 is provided with a phase compensating circuit 21 for canceling the zero point ωz existing in the output voltage detecting circuit 11. When the voltage value of the DC output voltage Vo changes by ΔVo, the phase compensation circuit 21
From the connection point between the capacitor C2 and the second resistor R6, the detection signal indicating the voltage change amount ΔVo is output as a differential waveform. The inverter 23 inverts the detection signal from the differentiating circuit 22 and applies the inverted signal to the reference of the shunt regulator 10 via the resistor R3.

Here, a gain-frequency characteristic when the phase compensation circuit 21 is provided in the output voltage detection circuit 11 is obtained. First, the open-loop transfer function Δ of the phase compensation circuit 21 is obtained in the same procedure as when the open-loop transfer function of the output voltage detection circuit 11 is obtained.
When IF / ΔVo is obtained, when the amplification of the inverter 23 is −A, the following equation is obtained by multiplying Equation 2 by the transfer function of the differentiating circuit 22 and the amplification of the inverter.

[0020]

(Equation 4)

In the above equation, if C1R5 = C2R6, the following equation is obtained.

[0022]

(Equation 5)

Accordingly, the total open-loop transfer function of the output voltage detection circuit 11 having the phase compensation circuit 21 is given by the following equation, based on the principle of superposition.

[0024]

(Equation 6)

Here, the amplification A of the inverter 23 is calculated by the following equation (7).
, The open-loop transfer function of Equation 6 is as shown in Equation 8.

[0026]

(Equation 7)

[0027]

(Equation 8)

In the above equation (8), the term of the transfer function G2 which is the phase lead element having the zero point ωz shown in the above equation (2) is eliminated by the transfer function of the differentiating circuit 22 and the amplification -A of the inverter 23. Therefore, as shown in FIG.
In the gain-frequency characteristic of (1), as the frequency ω increases, the gain diagram linearly decreases at a rate of −20 dB / dec. Therefore, in the case of the present embodiment, since the zero point ωz of the output voltage detection circuit 11 disappears, noise generated on the secondary side of the transformer 1 is cut off in a high frequency region, and transmission to the primary side is prevented. . In addition, the output voltage detection circuit 11 has an open-loop characteristic so that a high-frequency region is easily attenuated, and noise can be reduced in the circuit.

As described above, according to the above-described embodiment, the DC input voltage Vi is intermittently applied to the primary winding 1A of the transformer 1 by switching the transistor 2. The voltage induced in the secondary winding 1B is rectified and smoothed to output a DC output voltage Vo between the output terminals 9 and 9A, and the output terminals 9, 9A are used as a feedback loop for stabilizing the DC output voltage Vo. A switching power supply device including an output voltage detection circuit 11 for connecting a series circuit of a first resistor R4, a photocoupler 13 and a shunt regulator 10 therebetween, and dividing and applying a DC output voltage Vo to a reference of the shunt regulator 10 , A phase compensation circuit 21 for inverting the differential value of the DC output voltage Vo and applying the inverted value to the reference of the shunt regulator 10 is provided in the output voltage detection circuit 11. Those were. Therefore, in this case, the provision of the phase compensation circuit 21 cancels out the phase lead element of the output voltage detection circuit 11 having the zero point ωz, and the gain of the output voltage detection circuit 11 decreases linearly as the frequency ω increases. As a result, the noise generated on the secondary side of the transformer 1 can be cut off in the high-frequency region, thereby preventing transmission to the primary side.

In the present embodiment, the phase compensating circuit 21 is a differential circuit 22 composed of a series circuit of a capacitor C2 connected between the output terminals 9 and 9A and a second resistor R6. And a capacitor C constituting the differentiating circuit 22
Connection point between the second resistor R6 and the second resistor R6 and the shunt regulator
This is constituted by an inverter 23 connected between the reference and the reference 10. Therefore, since the phase compensation circuit 21 can be composed of only three components including the capacitor C2, the second resistor R6, and the inverter 23, the output voltage detection circuit 11 Of the transformer 1 can be easily prevented from transmitting the noise to the primary side of the transformer 1. In this case, the value of the capacitor C2 and the value of the second resistor R6 and the amplification degree of the inverter 23-
By appropriately selecting A, it is possible to cancel the zero point ωz of the output voltage detection circuit 11 without changing the existing power supply device at all.

In this embodiment, the phase compensating circuit 21 is configured to cancel out the phase lead element of the output voltage detecting circuit 11. However, instead of the phase compensating circuit 21, the first resistor R4
A Zener diode is provided at a connection point between the light emitting diode 13 and the light emitting diode 13, and the DC output voltage Vo is
May be absorbed to remove the noise component. Further, the present invention is not limited to the forward type converter shown in the embodiment, but can be applied to a multi-fed type converter such as a flyback type or another push-pull type. In this case, the switching element is not limited to the transistor 2, and various switching elements such as a MOS FET may be used. Further, an integration circuit including a resistor and a capacitor is inserted and connected between the phototransistor 13B of the photocoupler 13 and the pulse width control circuit 12, so that the intrusion of noise into the primary side of the transformer 1 is further suppressed. Good.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the present invention.

[0033]

According to the first aspect of the present invention, a switching power supply device intermittently applies a DC input voltage to a primary winding of a transformer by switching a switching element, and rectifies and smoothes a voltage induced in a secondary winding of the transformer. To output a DC output voltage between output terminals, and as a feedback loop for stabilizing the DC output voltage, connect a series circuit of a first resistor, a photocoupler, and a shunt regulator between the output terminals. A switching power supply device having an output voltage detection circuit for dividing and applying a DC output voltage to a reference of the shunt regulator, wherein a phase compensation circuit for inverting a differential value of the DC output voltage and applying the derivative to the reference of the shunt regulator Is provided in the output voltage detection circuit, and noise generated on the secondary side of the transformer is It is possible not to transmit.

In the switching power supply according to the present invention, the phase compensating circuit includes a differentiating circuit including a series circuit of a capacitor and a second resistor connected between the output terminals, and the differential circuit forming the differentiating circuit. Capacitor and the second
, And an inverter connected between the reference point of the shunt regulator and the reference of the shunt regulator. It is possible to prevent noise generated on the side from being transmitted to the primary side.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of a switching power supply device showing one embodiment of the present invention.

FIG. 2 is a circuit configuration diagram showing an equivalent circuit of a main part of the same.

FIG. 3 is a graph showing a gain-frequency characteristic according to the first embodiment;

FIG. 4 is a circuit configuration diagram of a switching power supply device in a conventional example.

FIG. 5 is a circuit configuration diagram showing an equivalent circuit of a main part of the above.

FIG. 6 is a graph showing gain-frequency characteristics according to the first embodiment;

[Explanation of symbols]

 Reference Signs List 1 transformer 2 transistor (switching element) 9, 9A output terminal 10 shunt regulator 11 output voltage detection circuit 13 photocoupler 21 phase compensation circuit 22 differentiator 23 inverter C2 capacitor R4 first resistor R6 second resistor

Claims (2)

(57) [Claims] A DC input voltage is intermittently applied to a primary winding of a transformer by switching of a switching element.
A DC output voltage is output between output terminals by rectifying and smoothing the voltage induced in the secondary winding of the transformer, and a first loop is provided between the output terminals as a feedback loop for stabilizing the DC output voltage. A switching power supply device comprising an output voltage detection circuit for connecting a series circuit of a resistor, a photocoupler, and a shunt regulator, and dividing and applying a DC output voltage to a reference of the shunt regulator, wherein a differential value of the DC output voltage is provided. A switching power supply device, wherein a phase compensation circuit for inverting the voltage and applying the inverted voltage to the reference of the shunt regulator is provided in the output voltage detection circuit. 2. A differential circuit comprising a series circuit of a capacitor and a second resistor connected to the phase compensation circuit between the output terminals, and a differential circuit comprising the capacitor and the second resistor constituting the differential circuit. 2. The switching power supply according to claim 1, wherein the switching power supply comprises an inverter connected between a connection point and a reference of the shunt regulator.