JPH08331844A - Switching power supply - Google Patents

Abstract

PURPOSE: To improve stability and transient response characteristic with stable oscillation as AC factors through a feed-back signal from an primary side of an auxiliary power supply, by superimposing a divided voltage of an output voltage in the auxiliary power supply circuit on an error voltage signal as a feed back signal from a secondary output voltage through a coupling capacitor. CONSTITUTION: An output voltage signal divided by dividing resistors R1 and R2 from an auxiliary power supply circuit 60 is fed through a coupling capacitor Cac and superimposed as an AC factor on an error voltage signal fed from a photocoupler 40 to a dividing circuit 50. In a switching power supply with an stabilized output voltage, a DC factor is stabilized by a signal from a secondary circuit, and a decrease in a phase margin in the vicinity of a gain cross frequency caused by parasitic characteristics in the secondary circuit can be compensated. Then, stability in output voltage can be increased with good response characteristic in transient characteristics.

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 device used in a DC-DC converter, and more particularly to improvement of controllability in the case of achieving both quick response and stability.

[0002]

2. Description of the Related Art A switching power supply device feed-forward-controls an input voltage to allow a wide range of the input voltage, as disclosed in, for example, Japanese Patent Application Laid-Open No. 4-109867 proposed by the present applicant. The technology is known. Further, Japanese Patent Application Laid-Open No. 60-113662 proposes to improve the controllability against a transient state such as overload or sudden change in load.

FIG. 4 is a circuit diagram of a conventional secondary side output voltage feedback type switching power supply. In the figure, the transformer is provided with a primary winding n ₁ , a secondary winding n ₂ and an auxiliary winding ns. The input voltage source 10 is for rectifying and smoothing an alternating current from a commercial alternating current power source to generate a direct current voltage Vin, and is connected to the primary winding n ₁ and turned on / off by a switching element Q such as a transistor. There is. Then, a switching voltage signal is induced in the secondary winding n ₂ , so that the output voltage Vout converted into a direct current by the rectifying and smoothing circuit of the diode D1 and the output capacitor C1 is supplied to the load 20. Here, since an on / off system or a format called a flyback converter is adopted, energy is stored in the transformer T during the period when the switching element Q is on, and the on period during the period when the switching element Q is off. The magnetic flux of the transformer T that has increased inside is reduced.

The output voltage is stabilized by the following structure. The error amplifier 30 compares the output voltage Vout with the reference voltage and outputs an error signal having a duty ratio proportional to this error voltage. The photocoupler 40 insulates the primary side and the secondary side of the switching power supply, and converts it into an on / off signal corresponding to the error signal output from the error amplifier 30 here. The drive circuit 50 is a photocoupler 40.
The on / off signal sent from the device is input, and the switching control signal in the direction in which the output voltage Vout and the reference voltage match is applied to the switching element Q. The auxiliary power supply circuit 60 generates an operating voltage of the drive circuit 50, and converts the switching signal induced in the auxiliary winding ns into a direct current by the rectifying / smoothing circuit of the diode D2 and the capacitor C2,
Generates auxiliary power supply voltage.

[0005]

However, according to the conventional secondary side output voltage feedback type switching power supply, the phase delay increases in the region of several tens kHz of the frequency characteristic, and the following problems occur. Oscillation easily occurs due to the decrease in the phase margin. Therefore, the gain of the control system cannot be increased and the response to the load fluctuation becomes slow. Therefore, the stability is easily affected by the temperature change of the parasitic resistance of the output capacitor. Therefore, it is difficult to use a high-performance product having a small parasitic resistance for the output capacitor, and it is difficult to downsize the output filter. Because of, it takes time and effort to design the control system. This phase delay is mainly caused by the photocoupler 40 for insulation.
Is caused by the secondary side parasitic component such as the phase delay of 1) and the parasitic resistance of the output capacitor C1.

The present invention solves such a problem, and provides a switching power supply device which is not affected by the parasitic characteristics of the output capacitor and the photocoupler and which can obtain controllability that is both stable and responsive. The purpose is to

[0007]

According to the present invention for achieving the above object, a DC input voltage Vin applied to a primary winding n ₁ is turned on / off by a switching element Q and induced in a secondary winding n _2. While rectifying and smoothing the switching current and supplying the output voltage Vout to the load, energy is stored in the transformer T having the primary winding and the secondary winding while the switching element is on, and during the off period. Flyback converter 10 for transmitting energy to the secondary side
An error amplifier 30 that compares the output voltage with a reference voltage to obtain an error voltage, and a means 40 that insulates the error voltage signal output from the error amplifier between the primary side and the secondary side of the converter. , A drive circuit 50 for applying a switching control signal for reducing the error voltage signal sent via the insulating means to the switching element, and a rectifying and smoothing the switching current induced in the auxiliary winding ns to drive the switching current. A switching power supply device having an auxiliary power supply circuit 60 for sending operating power to the circuit has the following configuration.

That is, the means R1 and R2 for detecting the output voltage of the auxiliary power supply circuit and the coupling for superimposing the signal obtained by the auxiliary power supply output voltage detection means on the error voltage signal sent to the drive circuit in an alternating manner. It is characterized in that means C _AC are provided.

[0009]

According to the structure of the present invention, the DC accuracy of the output voltage is obtained by the secondary side output voltage feedback of the error amplifier and the drive circuit. By feeding back the AC component contained in the auxiliary power supply output voltage to the drive circuit by the auxiliary power supply output voltage detection means and the coupling means C _AC , the _AC stability related to oscillation and the like is returned to the feedback signal from the primary side auxiliary power supply circuit. Is gained by. According to the control theory, the stability of oscillation etc. is determined by the phase margin at the gain crossover frequency, but in order to reduce the effect of the parasitic resistance of the secondary side circuit, the characteristics of the primary side circuit are dominant near the gain crossover frequency. I am trying to become.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG.
FIG. 1 is a configuration diagram of a switching power supply device showing an embodiment of the present invention. In FIG. 1, components having the same functions as those in FIG. 4 are designated by the same reference numerals and the description thereof will be omitted. In the figure, the voltage dividing resistors R1 and R2 divide the output voltage of the auxiliary power supply circuit 60, and generate a signal proportional to the output voltage of the auxiliary power supply circuit 60. The coupling capacitor C _ac is a _{voltage dividing} resistor R
The output voltage signals sent from the R1 and R2 are AC-superposed on the error voltage signals sent from the photocoupler 40 to the drive circuit 50 AC. The output capacitor C
1 is a high-performance product with a small parasitic resistance.

The operation of the thus constructed device will be described below. FIG. 2 is a frequency characteristic diagram in an open loop of the switching power supply, where (A) is gain [dB] and (B) is phase [d].
eg] is shown. In the figure, the solid line is the embodiment of the present invention shown in FIG. 1, and the broken line is the characteristic diagram of the secondary side output voltage feedback type in FIG. In the open loop characteristic, it is known that when the phase at the gain crossover frequency is delayed by more than -180 deg, the closed loop control system oscillates. here,
The gain crossover frequency is a frequency at which the gain becomes 0 dB. In the conventional example shown by the broken line, the gain crossover frequency f
s is 6.5 kHz, and the phase −210 deg at this time is
The phase margin Δθs with respect to −180 deg is −30 deg, which is a value indicating oscillation. On the other hand, in the present embodiment shown by the solid line, the gain crossover frequency fp is 8.6 kHz, and it can be seen that the phase margin Δθp with respect to −180 deg at this time is +14 deg, which is in a stable region without oscillation.

The reason why the phase margin is improved in the vicinity of the gain crossover frequency is that the characteristic on the primary side becomes dominant and the phase delay due to the parasitic component on the secondary side is improved. However, in the conventional example shown by the broken line, the gain needs to be reduced by about 10 dB in order to set the phase margin Δθs at the gain crossover frequency fs = 6.5 kHz to +0 deg so that oscillation can be prevented. It is expected that the transient response characteristic of the output voltage Vout with respect to will deteriorate. According to the present embodiment, oscillation does not occur even if the gain is reduced, and the transient response characteristic becomes good.

FIG. 3 is a waveform diagram of the transient response characteristic of the output voltage with respect to load current fluctuations, where (A) represents the output voltage and (B) represents the load current. Load current is 0.75A every 20mS
1.50A flows alternately. On the other hand, the output voltage Vout
Where the rating is 5.00V, 0 for each switching of load current.
Although an output voltage fluctuation with a pulse width of about 0.5 mS appears at 07 V, it is understood that the influence on the load 20 is slight because it is settled immediately.

In the above embodiment, the primary side AC
The signal from the auxiliary power supply circuit 60 is used for returning the component by the coupling capacitor C _ac, but an auxiliary winding may be provided independently. Although the capacitor C _ac is used for the feedback of the primary side AC component, other source follower circuits using FETs, emitter follower circuits using transistors, high-pass filter circuits using OP amplifiers, transformer coupling circuits, etc. An AC coupling circuit may be used.

[0015]

As described above, according to the present invention, in the switching power supply device which uses the signal of the secondary side circuit in order to stabilize the DC component in stabilizing the output voltage, the parasitic characteristic of the secondary side circuit is used. In order to compensate for the decrease in the phase margin near the gain crossover frequency, the primary side AC component appearing in the auxiliary power supply circuit 60 is _fed back by the coupling capacitor C _ac to improve the phase margin, so that the following effects are obtained. is there.

The stability of the output voltage is increased and the transient response characteristic is also improved. Although the parasitic resistance of the output capacitor C1 varies depending on the temperature, the AC component on the primary side becomes dominant in the vicinity of the gain crossover frequency, so that it is difficult to be affected by this temperature variation and the robustness referred to in the field of control is improved. . When a high performance product using a solid electrolyte and a small parasitic resistance is used as in the embodiment, the output filter can be downsized, which contributes to the cost reduction of the switching power supply. Since the detection signal from the primary side ensures stability and the detection signal from the secondary side ensures the DC accuracy of the output voltage, it is possible to separate the design of the primary side and the secondary side, and Control system design becomes easier.

[Brief description of drawings]

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

FIG. 2 is a frequency characteristic diagram in an open loop of a switching power supply.

FIG. 3 is a waveform diagram of a transient response characteristic of an output voltage with respect to a load current fluctuation.

FIG. 4 is a circuit diagram of a conventional secondary side output voltage feedback type switching power supply.

[Explanation of symbols]

10 Flyback Converter 20 Load 30 Error Amplifier 40 Photocoupler 50 Drive Circuit 60 Auxiliary Power Supply Circuit C _ac Coupling Means

Front Page Continuation (72) Inventor Akira Uchida 2-9-32 Nakamachi, Musashino City, Tokyo Yokogawa Electric Co., Ltd.

Claims (1)

[Claims] 1. A DC input voltage (Vin) applied to a primary winding (n ₁ ) is turned on and off by a switching element (Q),
The output current (Vout) is supplied to the load by rectifying and smoothing the switching current induced in the secondary winding (n ₂ ), and while the switching element is on, the primary winding and the secondary winding are turned on. Energy is stored in a transformer (T) having a line, and a flyback converter (10) in which energy is transmitted to a secondary side during an off period, and an error amplifier (which compares the output voltage with a reference voltage to obtain an error voltage ( 30), a means (40) for insulating the error voltage signal output from the error amplifier between the primary side and the secondary side of the converter, and a small error voltage signal sent via the insulating means. Drive circuit (50) for applying a switching control signal in the direction to the switching element, and rectifying and smoothing the switching current induced in the auxiliary winding (ns) to operate this drive circuit A switching power supply device having an auxiliary power supply circuit (60) for transmitting power for use, and means (R1, R for detecting an output voltage of the auxiliary power supply circuit
2) and a coupling means (C _AC ) for superimposing the signal obtained by the auxiliary power supply output voltage detection means on the error voltage signal sent to the drive circuit in an alternating current manner. apparatus.