JP3568870B2 - converter - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a converter, and more particularly to a control circuit of a DC-DC converter driven by a DC input power supply and supplying an extremely low output voltage to a load.
[0002]
[Prior art]
The output voltage control of such a converter generally employs a pulse frequency modulation (PFM) control method using the principle of resonance characteristics of a transformer (transformer). FIG. 7 shows a frequency characteristic of an output voltage (Vout) ratio (Vout / Vin) to an input voltage (Vin) when the output load of the transformer is used as a parameter. As shown in FIG. 7, even if the output load fluctuates from a light load (L1) to a heavy load (L2), the output voltage is stabilized by adjusting the drive frequency of the transformer from f2 to f1. . However, in the case of the frequency modulation control method, when the resonance frequency deviates from the resonance frequency point, the power conversion efficiency of the transformer tends to decrease, so that the controllability is limited in a wide range of input voltage and load fluctuations. On the other hand, in a converter that can be used worldwide, such as an AC / DC adapter for a notebook PC (personal computer), for example, the input voltage is 90 Vac to 264 Vac, and the load is 0 to 100% even if the output voltage fluctuates. Stable control is required.
[0003]
In order to meet such a demand, for example, as disclosed in Japanese Patent Laid-Open Publication No. Hei 4-210773, "Method of Controlling Converter Using Electric-to-Mechanical Transformer" (hereinafter referred to as a first conventional technique), There has been proposed a method of stably controlling an output by a combination of control and a duty ratio (or pulse width) modulation (PWM) control. The prior art disclosed in such a patent publication is configured as shown in FIG. That is, the primary drive circuit 1 by high frequency switching, the piezoelectric transformer 2 driven by the primary drive circuit 1, the output rectifier circuit 3 connected to the secondary side of the transformer 2, and the output rectifier circuit 3 The output voltage of the converter is controlled by providing a variable frequency oscillator 5 for varying the switching frequency of the primary side drive circuit 1 and a voltage / time ratio conversion circuit 7 via a detection amplifier circuit 4 for feeding back the output. The primary side drive circuit 1 is supplied with operating power from a DC power supply 6.
[0004]
Another converter is disclosed in Japanese Unexamined Patent Publication No. 9-51675, entitled "Wide Input Piezoelectric Transformer Converter" (hereinafter referred to as "second prior art"). As shown in FIG. 9, a control method has been proposed in which the duty ratio is adjusted according to the change in the input voltage Vin of the input power supply 11 and the frequency is changed according to the change in the load 44. That is, a switching circuit 20 for generating an AC voltage by switching a DC input voltage Vin by two switches S1 and S2 that are turned ON / OFF alternately, a filter circuit 27 for smoothing the AC voltage, and a filter circuit 27 A piezoelectric transformer 30 that converts a voltage in accordance with the AC output voltage of the piezoelectric transformer 30, a rectifying and smoothing circuit 40 that rectifies and smoothes the AC output voltage of the piezoelectric transformer 30, and a detection circuit 50 that detects the DC output voltage of the rectifying and smoothing circuit 40. A frequency modulation circuit 90 that modulates a switching frequency with an output voltage of the detection circuit 50, and a drive circuit 21 that drives the switches S1 and S2 of the switching circuit 20 in synchronization with an AC signal output from the frequency modulation circuit 90. Modulates the duty ratio of the rectangular wave output from the drive circuit 21 by the DC input voltage Vin. By providing a ratio modulation circuit 70 performs a output voltage control of the converter.
[0005]
[Problems to be solved by the invention]
However, in the control method of the first prior art described above, specific proposals are given as to which control method of the frequency modulation control method and the time-ratio modulation control method is prioritized or how interference between controls is avoided. Are not disclosed at all.
[0006]
On the other hand, in the above-mentioned second prior art, the fluctuation of the input voltage Vin is absorbed by the PWM means, and the fluctuation of the output voltage is controlled by frequency modulation. Therefore, two detection circuits are required to detect the input voltage and the output voltage, and the circuit configuration is complicated. Further, when the fluctuation range of the input voltage Vin is very wide, the power loss generated in the detection circuit at a high input voltage becomes large, which hinders downsizing. Further, since the frequency fluctuates according to the fluctuation of the load, the fundamental wave component of the noise generated from the converter fluctuates. Therefore, it is difficult to design a noise filter for reducing or removing this noise.
[0007]
[Object of the invention]
Accordingly, an object of the present invention is to provide a converter that stably controls an output voltage in response to a wide range of input voltage and load fluctuation.
[0008]
[Means for Solving the Problems]
A converter according to the present invention includes a primary-side drive circuit including a switch, a transformer driven by the primary-side drive circuit, and an output rectifier circuit connected to a secondary side of the transformer, and performs frequency modulation on a switch of the primary-side drive circuit. A control method for controlling the frequency modulation method or the time ratio modulation method in accordance with a detection signal from a detection circuit connected to a load, and a control method determination circuit for controlling the frequency conversion method or the time ratio modulation method. A control method switching circuit that switches the primary-side drive circuit to a frequency modulation circuit or a time-ratio modulation circuit in response to a determination signal from the controller, and a priority operation of the time-ratio modulation circuit, and limits the operation of the time-ratio modulation circuit based on the detection signal. When the frequency modulation circuit is operated, the control method determination circuit has a hysteresis operation characteristic. Configured to prevent control interference when switching to a ratio modulation mode when the changeover or frequency modulation scheme to.
[0009]
According to a preferred embodiment of the converter of the present invention, the frequency modulation circuit includes an error amplifier that compares a detection signal of the detection circuit with a reference voltage, and a triangular wave generator that receives an output of the error amplifier and generates a triangular wave of a variable frequency. You. Further, the control system switching circuit clamps the output of the error amplifier of the frequency modulation circuit by the output of the control system determination circuit. The duty ratio limiting circuit is constituted by a voltage divider connected in series between the output of the control method determining circuit and the power supply. Further, the time ratio modulation circuit includes a comparator that compares a triangular wave from the frequency modulation circuit with a detection signal from the detection circuit or an output voltage from the time ratio limitation circuit.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the configuration and operation of a preferred embodiment of a converter according to the present invention will be described in detail with reference to the accompanying drawings.
[0011]
First, FIG. 1 is a block diagram showing a basic configuration of a converter according to the present invention. The converter includes a DC power supply 10, a primary side drive circuit 20, a piezoelectric transformer 30, an output rectifier circuit 40, a detection circuit 50, a control method determination circuit 60, a duty ratio limiting circuit 70, a control method return circuit 80, a frequency modulation circuit 90 And a duty ratio modulation circuit 100. Here, the detection circuit 50, the control method determination circuit 60, the duty ratio limiting circuit 70, the control method switching circuit 80, the frequency modulation circuit 90, and the duty ratio modulation circuit 100 constitute a control circuit 110.
[0012]
FIG. 2 is a specific circuit diagram of a preferred embodiment of the converter having the basic configuration shown in FIG. The primary drive circuit 20 of this converter includes an active clamp circuit 22. The active clamp circuit 22 includes an inductor 23, a first switch (S <b> 1) 24 for periodically connecting the DC input voltage Vin from the DC power supply 11 to the inductor 23, and while the first switch 24 is OFF. It comprises a capacitor 25 for limiting the voltage between both ends of the inductor 23, and a second switch (S2) 26 connected in series with the capacitor 25.
[0013]
The voltage generated at both ends of the inductor 23 is converted into an AC voltage by a filter circuit including the inductor 27 and input to the piezoelectric transformer 30. The piezoelectric transformer 30 transmits only the frequency components near the resonance frequency to the secondary side to which the output rectifier circuit 40 is connected. Therefore, the output of the piezoelectric transformer 30 becomes a sine wave voltage, and is converted into a direct current by the rectifying and smoothing circuit 40 including the bridge rectifier diode 41, the filter coil 42, and the filter capacitor 43. The DC output voltage Vout is supplied to the load 44 and input to the detection circuit 50, and detects the output voltage of the load 44. In accordance with the detection signal from the detection circuit 50, the control method determination circuit 60 determines whether to perform drive frequency modulation or time ratio modulation of the primary side drive circuit 20. A signal for switching between the frequency control and the duty ratio control is transmitted to the control method switching circuit 80 based on the determination result. A signal is also sent to the duty ratio limiting circuit 70 that limits the duty ratio at the same time as switching to the frequency modulation control, and the duty ratio is limited. In the figure, reference numeral 21 denotes a switching transistor drive circuit, and 31 to 35 denote capacitors, coils, and resistors constituting the piezoelectric transformer 30.
[0014]
Then, the control circuit 110 of the converter according to the present invention operates the primary side drive circuit 20 at a preset frequency. The duty ratio modulation circuit 100 operates preferentially, and the output voltage is switched from the control by the duty ratio modulation circuit 100 to the control by the frequency modulation circuit 90 in accordance with the detection signal detected by the detection circuit. When the control method is switched, the duty ratio is forcibly fixed by the duty ratio limiting circuit 70. Further, by providing the control method determination circuit 60 with a hysteresis characteristic, it is possible to prevent interference between controls when switching from the time-ratio modulation method to the frequency modulation method.
[0015]
Next, FIG. 3 is a detailed configuration of the control circuit 110 of the converter shown in FIG. 1 and FIG. 3, the detection circuit 50 includes an error amplifier 51 that compares an output voltage Vout applied to a load 44 with a first reference voltage source (Vref1) 52. The control method determination circuit 60 includes an error amplifier 61 that compares the output of the error amplifier 51 with a third reference voltage source (Vref3) 62. The control system switching circuit 80 includes a first resistor 82 and a transistor 81 for switching to a frequency modulation circuit 90 according to the output voltage of the error amplifier 61. The duty ratio limiting circuit 70 is composed of a voltage divider composed of a second resistor 71 and a third resistor 72 connected in series to determine a voltage to be input to the duty ratio modulation circuit 100 according to the output of the error amplifier 61. You. The time ratio modulation circuit 100 compares the output voltage Vf of the error amplifier 51, the voltage VL, which is lower than the voltage from the time ratio limit circuit 70, with the oscillator voltage Vx from the frequency modulation circuit 90, and outputs the result. It is composed of The frequency modulation circuit 90 includes an error amplifier 93 that compares the output voltage of the error amplifier 51 with a second reference voltage source (Vref2) 94, and a Vf converter (voltage) that varies the frequency according to the output voltage of the error amplifier 93. (Frequency converter) 92 and an oscillator 91. The Vf converter 92 and the oscillator 91 constitute a triangular wave generator that generates a triangular wave having a constant amplitude as described later.
[0016]
Hereinafter, the operation of the embodiment of the converter according to the present invention will be described with reference to the operation waveform diagram shown in FIG. 4A shows the gate voltage of the first switch (S1) 24, that is, VGS1, FIG. 4B shows the gate voltage of the second switch (S2) 26, that is, VGS2, and FIG. 4C shows the first switch (S1). S1) is the drain voltage, that is, VDS1, (d) is the voltage of the inductor 23, that is, VLr, and (e) is the output voltage of the transformer 35, that is, V2. First, it will be described that the converter using the active clamp circuit 22 can control the stabilization of the output voltage VO by adjusting the duty ratio D of the first switch (S1) 24. The voltage VLr between both ends of the inductor 23 becomes a substantially trapezoidal wave as shown in FIG. 4D, and its amplitude can be expressed by Expression (1), where D is the duty ratio of the first switch (S1) 24.
VLr = Vin / 2 (1-D) (1)
[0017]
For this reason, this voltage VLr is approximated to a square wave, and a voltage of only the fundamental wave component obtained by Fourier series expansion is output via the inductor 27 and the piezoelectric transformer 30, and the amplitude of the fundamental wave component is expressed by the following equation (2). Is represented by
| VLr | (1 ^st ) = Vin2sinDπ / (1-D) (2)
Therefore, the voltage applied to the input of the piezoelectric transformer 30 can be varied by adjusting the duty ratio D.
[0018]
Next, the operation of the control circuit unit 110 will be described with reference to the operation waveform diagram of FIG. The drive signal of the first switch (S1) 24 and the second switch (S2) 26 of the primary drive circuit 20 is driven by the drive signal Vd from the switching transistor drive circuit 21. The drive signal Vd includes an output voltage Vf from the detection circuit 50 according to the output voltage, a low voltage VL output by the control method determination circuit 60 and the duty ratio limiting circuit 70 according to the output voltage Vf, and a frequency modulation circuit. The output voltage Vx of the oscillator 91 in the circuit 90 is obtained by comparison. When Vf described above is lower than the third reference voltage Vref3 from the reference voltage source 62 (Vf <Vref3), the output of the error amplifier 61 is H (high level), so VL is also H. The output signal Vo of the error amplifier 101 is controlled by comparing Vf and Vx with a comparator. Therefore, the duty ratio of Vd is adjusted as D1, D2, and D3 according to the voltage of Vf. That is, in the input voltage region where Vf is lower than Vref3, the duty ratio modulation control for adjusting the duty ratio at a fixed frequency is performed.
[0019]
On the other hand, when the voltage of Vf is higher than the voltage of Vref3 (Vf> Vref3), the output of the error amplifier 61 becomes L (low level) as shown in FIG. Then, since the voltage VL obtained by dividing Vcc by the second resistor 71 and the third resistor 72 constituting the duty ratio limiting circuit 70 becomes smaller than Vf, the pulse width of the output signal Vd of the duty ratio modulation circuit 100 becomes ( It is fixed (as long as the frequency of the triangle wave is constant). At this time, since the error amplifier 93 operates, frequency control is performed. Further, since the error amplifier 61 of the control method determination circuit 60 has a hysteresis characteristic, once switching from the time-ratio modulation control method to the frequency modulation control method, Vf needs to be reduced to some extent. Unstable operation at the time of switching does not occur.
[0020]
Next, FIG. 6 shows the input fluctuation characteristics of the duty ratio D and the frequency f with respect to the parameters in which the load current Io is maximized and minimized. That is, FIG. 6A shows the relationship between the input voltage (horizontal axis) and the duty ratio D (vertical axis), and FIG. 6B shows the relationship between the input voltage (horizontal axis) and the frequency f. The control operation when the input voltage is gradually increased will be described. When the load current is minimum, the duty ratio D is adjusted at a fixed frequency in the range of the input voltage (a to b). On the other hand, in the high input voltage region where the input voltage is equal to or higher than b, the frequency is adjusted at a fixed duty ratio D, thereby performing the stabilization control of the output voltage Vout.
[0021]
The configuration and operation of the preferred embodiment of the converter according to the present invention have been described above in detail. However, such embodiments are merely examples of the present invention and do not limit the present invention in any way. It will be readily apparent to those skilled in the art that various modifications can be made according to the specific application without departing from the spirit of the present invention.
[0022]
【The invention's effect】
As is apparent from the above description, according to the converter of the present invention, a common (or single) detection circuit for detecting the output voltage supplied to the load is used to cover a wide range of input voltage fluctuations and load fluctuations. By switching the switch of the primary-side drive circuit to the duty ratio modulation method or the frequency modulation method, a remarkable practical effect that the output voltage can be easily and reliably maintained constant can be obtained.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a basic principle of a converter according to the present invention.
FIG. 2 is a circuit diagram of a preferred embodiment of a converter according to the present invention, constructed according to the principle of FIG. 1;
FIG. 3 is a block diagram showing a specific configuration of a control circuit of the converter shown in FIGS. 1 and 2.
4 is an operation waveform diagram of a main part of the converter shown in FIG.
FIG. 5 is an operation waveform diagram for explaining an operation of the control circuit shown in FIG. 3;
6 is a graph showing a relationship between an input voltage versus time ratio D and a frequency f of the control circuit shown in FIG. 3;
FIG. 7 is a frequency characteristic diagram of an input voltage versus an output voltage of a piezoelectric transformer in a conventional converter.
FIG. 8 is a block diagram showing a basic configuration of a conventional converter.
FIG. 9 is a specific circuit diagram of the conventional converter shown in FIG.
[Explanation of symbols]
Reference Signs List 10 DC power supply 11 Input voltage 20 Primary drive circuit (switching circuit)
21. Switching transistor drive circuit (drive circuit)
24, 26 switch 30 piezoelectric transformer 40 output rectifier circuit (rectifier smoothing circuit)
Reference Signs List 50 detection circuit 51, 61, 93 error amplifier 52, 62, 94 reference voltage 60 control method determination circuit 70 time ratio limiting circuit 71, 72 voltage dividing resistor 80 control method switching circuit 90 frequency modulation circuit 91 oscillator 92 Vf converter 100 Time ratio modulation circuit 110 Control circuit

Claims (5)

A primary side drive circuit including a switch, a transformer driven by the primary side drive circuit, and an output rectifier circuit connected to a secondary side of the transformer, wherein the switch of the primary side drive circuit is a frequency modulation type or a time ratio. In a converter controlled by a modulation method,
A control method determination circuit that selects a frequency modulation method or a time ratio modulation method according to a detection signal from a detection circuit connected to a load, and the primary side drive circuit according to a determination signal from the control method determination circuit A control circuit switching circuit for switching to a modulation circuit or a time ratio modulation circuit , wherein the time ratio modulation circuit is operated preferentially, and the frequency modulation circuit is operated when the operation of the time ratio modulation circuit is limited based on the detection signal. With
The control method determination circuit has a hysteresis operation characteristic, and prevents control interference when switching from the time ratio modulation method to the frequency modulation method control or when switching from the frequency modulation method to the time ratio modulation method. A converter characterized by doing. 2. The frequency modulation circuit according to claim 1, further comprising: an error amplifier that compares the detection signal with a reference voltage; and a triangular wave generator that receives an output of the error amplifier and generates a triangular wave of a variable frequency. The converter described. The converter according to claim 2, wherein the control method switching circuit clamps an output of the error amplifier of the frequency modulation circuit by an output of the control method determination circuit. 4. The converter according to claim 1, wherein the duty ratio limiting circuit includes a voltage divider connected in series between an output of the control method determination circuit and a power supply. 3. The time ratio modulation circuit according to claim 2, wherein the comparator is configured to compare the triangular wave from the frequency modulation circuit with the detection signal from the detection circuit or an output voltage of the time ratio limitation circuit. A converter according to any one of claims 1 to 4.

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