JP3251240B2 - DC / DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC / DC converter that converts a DC voltage into a rectangular wave by a switching element, rectifies and smoothes the output, and outputs a predetermined DC voltage. The improvement of the soft start circuit.

[0002]

2. Description of the Related Art In a DC / DC converter used as an internal power supply of an electronic device, a change in an output voltage is detected and a predetermined delay time is provided to stabilize an output voltage, and a switching element is feedback-controlled. A control circuit is used.

On the other hand, a DC / DC converter is required to output a stable output voltage in a short period of time from the start of operation, but the overshoot voltage generated at the start of the DC / DC converter due to the delay time of the control circuit is increased. It becomes a problem.

In order to prevent the overshoot voltage, a soft start circuit having a large time constant, which operates only at the time of starting the DC / DC converter, is provided separately from the feedback loop to limit the pulse width at the time of starting. It is necessary to compensate for the delay in the response speed of the system.

On the other hand, in order to output the rated voltage within a certain period of time after starting and to suppress the occurrence of overshoot voltage, the speed of the feedback loop needs to be increased, and the stability of the circuit operation is sacrificed. .

FIG. 3 shows a configuration of a conventional primary control type forward converter (hereinafter, DC / DC converter) 20 using a soft start circuit utilizing a reference voltage and a dead time control function built in a pulse width control IC. FIG.

FIG. 4 is a diagram showing operation waveforms at the time of startup in FIG. 3, (A) shows a voltage waveform of an input power supply voltage,
(B) shows the internal operation of the pulse width control IC, (C) shows the output waveform of the pulse width control IC, and (D) shows an example of overshoot of the output power supply voltage.

Next, the operation will be described with reference to FIG. 3 and FIG.

When a switch (not shown) is turned on, the power supply voltage VIN of the DC input power supply 17 is changed to the DC / DC converter 2.
0, the input voltage gradually increases as shown in FIG. 4A, and reaches the rated voltage of the DC input power supply via the operation start voltage a of the pulse width control IC 11 and the converter operation start voltage b.

The pulse width control IC 11 starts operating when the input voltage VIN reaches the operation start point a, outputs a predetermined reference voltage to the REF terminal, and outputs the predetermined reference voltage to the input terminal (+) of the differential amplifier IC1 of the soft start circuit 21. Applied. On the other hand, the input voltage VIN is divided into a predetermined voltage by the resistors R1 and R2 and applied to the input terminal (-) of the differential amplifier IC1.

At this time, the input voltage VIN is controlled by the pulse width control I.
After the operation start voltage a of C11 elapses and reaches the converter operation start voltage b, the reference voltage of the REF terminal of the pulse width control IC11 applied to the input terminal (+) of the differential amplifier IC1 is applied to the input terminal (-). By setting R1 and R2 to be higher than the applied voltage,
The output of the differential amplifier IC1 becomes H level.

At the same time, when a predetermined reference voltage is output to the REF terminal after the operation start voltage a of the pulse width control IC 11 has passed, the potential of the DT terminal is changed via the capacitor C2 to the REF terminal as shown in FIG. Is set to the reference voltage. Thus, the maximum pulse width of the pulse width control IC is limited at the time of startup, thereby preventing the DC / DC converter from being broken at the start of operation. Thereafter, the potential of DT gradually discharges due to the time constant of C2 and R4, and stabilizes at the maximum pulse width limited potential in a steady state.

When the input voltage VIN reaches the converter operation start voltage b, the voltage at the input terminal (-) of the differential amplifier IC1 becomes higher than the voltage at the input terminal (+) and the differential amplifier IC1
Is inverted to L level, the pulse width control IC 11 is activated, and a triangular wave is output from the OSC.

As shown in FIG. 4B, when the potential of the DT terminal discharges and decreases to the triangular wave amplitude level, extraction of the pulse width is started, and a rectangular wave pulse having the extracted pulse width is output from the OUT terminal. The output terminal 18-
Output voltage VOUT corresponding to the pulse width of the rectangular wave pulse is output to 1, 18-2.

As shown in FIG. 4C, the pulse width of the triangular wave is detected corresponding to the potential of the DT terminal. Therefore, at first, a thin pulse corresponding to the tip of the triangular wave is detected, and DT
As the potential of the terminal decreases, a pulse having a wider pulse width is detected. The potential of the DT terminal further decreases, and the pulse width of the detected pulse expands to the maximum pulse width set to prevent DC / DC converter breakdown.

During this time, the pulse width control IC outputs the output voltage VO
The control of the pulse width according to the UT is not performed. Therefore, FIG.
As shown in (D), the output voltage VOUT gradually increases in accordance with the pulse width, and eventually a voltage (overshoot) higher than the rated voltage is output.

On the other hand, the capacitor C1 is charged by the reference voltage of the REF terminal via PC1-1 and R5, and the potential of the FB terminal gradually rises, and when the potential of the FB terminal becomes the same potential (control potential) as the potential of the DT terminal, the pulse width control IC. Starts control of the pulse width.

Error amplification circuit 16 of voltage output detection circuit 16
-1 detects a difference between the output voltage VOUT and a preset reference voltage, and according to the difference, the light emitting unit PC of the photocoupler.
Control the current flowing through 1-1. R of pulse width control IC
The light receiving portion PC1-2 of the photocoupler connected to the EF terminal changes its internal impedance in response to the current flowing through the light emitting portion PC1-1.

The potential of the FB terminal is set by the internal impedance of PC1-2 connected to the REF terminal and the voltage dividing ratio of R6. When the output voltage VOUT is higher than the rated voltage, the current flowing through the light emitting portion PC1-1 of the photocoupler of the output voltage detection circuit 16 increases, the internal impedance of the light receiving portion PC1-2 of the photocoupler of the soft start circuit decreases, and the REF terminal , The current flowing to PC1-2 and R6 increases, the voltage generated across R6 increases, and FB
The potential of the terminal is set high.

When the potential of the FB terminal rises, the pulse width of the rectangular wave pulse output to the OUT terminal becomes narrow,
14 becomes short, and the output voltage VOUT decreases. When the output voltage drops below the rated voltage, PC1
-1 decreases, the internal impedance of PC1-2 increases, and the potential of the FB terminal decreases.
The pulse width of the rectangular wave pulse output to the terminal increases, and the output voltage VOUT increases.

As described above, by lowering the potential of the DT terminal by discharging the capacitor C2 inserted between the DT terminal and the REF terminal, the pulse width control at the start of the DC / DC converter is performed and the output is controlled. The photocoupler is controlled by the difference between the voltage VOUT and the rated voltage to set the control potential of the FB terminal. When the control potential of the FB terminal reaches the same potential as the potential of the DT terminal, the pulse width control starts and the output voltage VOUT is rated. Control so that it becomes voltage.

However, R5 and C1 are connected to the FB terminal for stabilizing the feedback loop, and the control potential (the control potential of the FB terminal is equal to the potential of the DT terminal).
Until the voltage of the DT terminal is reached. During the charging period of the C1, the pulse width of the rectangular wave pulse output to the OUT terminal is widened to the maximum by the potential of the DT terminal and is uncontrolled. Rises and an overshoot voltage is generated.

[0023]

The conventional DC / DC converter is set at the time of starting the pulse width control IC, and has a DT.
The pulse width of the triangular wave is detected based on the potential of the DT terminal, which is reduced by the discharge of the capacitor C2 inserted between the terminal and the REF terminal, and a pulse is output to the OUT terminal to control the FET to generate an output voltage. The potential of the FB terminal is controlled by controlling the photocoupler based on the difference between the output voltage and the rated voltage.
When the capacitor C1 for stabilizing the feedback loop connected to the terminal is charged and the control potential of the FB terminal reaches the same potential as the potential of the DT terminal, the pulse width control according to the output voltage is started and a rectangle is output to the OUT terminal. A wave pulse is output to control the FET 14 so that the output voltage becomes the rated voltage.

Therefore, it is necessary to charge the capacitor C1 until the potential of the FB terminal reaches the control potential of the pulse width (the potential of the FB terminal is equal to the potential of the DT terminal). The pulse width of the rectangular wave pulse output to the terminal (the pulse width of the triangular wave extracted by the potential of the DT) expands to the maximum by the potential of the DT terminal and is uncontrolled, so that the output voltage rises and an overshoot voltage occurs. I do.

Therefore, the overshoot voltage can be prevented by increasing the response speed of the feedback loop and eliminating the non-control period, but the response speed is limited due to the stability of the feedback loop.

An object of the present invention is to provide a DC / DC capable of stabilizing a feedback loop and suppressing an overshoot voltage.
To provide a converter.

[0027]

A DC / DC converter according to the present invention comprises a REF terminal for outputting a reference voltage, a DT terminal for setting a maximum pulse width limiting potential, an FB terminal for inputting a pulse width control potential, and a start signal. CONT end <br/> terminal for inputting, by controlling the pulse width control IC with an OUT terminal for outputting a square wave pulse, the pulse width control IC D C /
A soft start circuit for suppressing an overshoot voltage of the output voltage generated at the start of the DC converter, and a power supply
Pulse width control connected in series with the primary winding of a transformer
ON / OFF in response to the square wave pulse output by the IC
FET and the F connected to the secondary side of the power transformer.
A rectifier circuit for rectifying the rectangular pulse output by the ET;
An output voltage detecting circuit for detecting a change in a rectified output voltage output from the rectifying circuit and controlling a control potential of the FB terminal, and converting an input voltage to a predetermined DC voltage by switching control and outputting the DC / DC. in the converter, the
The soft start circuit includes resistors R1, which divide the input voltage.
R2 and one of the input terminals are connected to the resistors R1 and R2.
And the other input terminal is connected to the pulse width control IC.
And the output terminal is connected to the pulse width
A differential amplifier circuit connected to the CONT terminal of the control IC
And the REF terminal of the pulse width control IC and the FB
Connected to the output voltage detection circuit
A light receiving section of a photocoupler that operates in response to the light emitting section;
The FB terminal of the pulse width control IC and the negative side of the input voltage
, The FB terminal and the input voltage.
A resistor R5 and a capacitor connected in series between
C1, the REF terminal of the pulse width control IC and the
Connected in series with the negative side of the input voltage and the midpoint is
Resistors R3 and R connected to the DT terminal of the pulse width control IC
4 and the cathode side is forwarded to the output of the differential amplifier circuit.
The node side is the midpoint of the connection between the resistor R5 and the capacitor C1.
And a diode D1 connected to
Pulse width control of the potential of the FB terminal when starting the width control IC
Set to a possible control potential, and by the capacitor C1.
The input voltage is used to start the DC / DC converter.
Voltage until the input voltage reaches the DC
Discharge starts when DC / DC converter startup operating voltage is reached
It was started, the FB terminal conductive by discharge electricity of the capacitor C1
Oscillator built in the pulse width control IC is generated based on the pressure
The pulse width of the generated triangular wave is detected.

[0028]

[0029]

[0030]

[0031]

[0032]

[0033]

Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows the DC of the present invention.
3 is a block diagram illustrating a main configuration of a / DC converter. FIG. 2 is a diagram showing voltage waveforms of main parts at the time of startup in the DC / DC converter of FIG.

The DC / DC converter 10 shown in FIG.
A CONT terminal for inputting a start signal, a REF (reference) terminal for outputting a reference voltage, and an FB (feedback) for inputting a control voltage corresponding to an error voltage of the output voltage.
Terminal and a set voltage to set the maximum pulse width
A pulse width control IC 11 including a T (dead time control) terminal, a Vcc terminal for inputting power, and an OUT terminal for outputting a rectangular pulse, and a software for suppressing an overshoot voltage of the output voltage VOUT at the time of startup. A start circuit 12, an FET 14 connected in series to a primary winding of a power transformer 13, and turned on / off in response to a rectangular wave pulse output from the pulse width control IC 11;
A rectifier circuit 15 that rectifies a rectangular wave pulse output from the secondary winding of the power transformer 13 and an output voltage detection circuit 16 that detects a variation in the output voltage VOUT output by the rectifier circuit
, A DC input power supply 17, and output terminals 18-1 and 18-2.
It is composed of

The soft start circuit 12 includes resistors R1 and R2 for dividing an input power supply voltage VIN supplied from a DC input power supply 17 via a switch (not shown), and one input terminal is connected between the resistors R1 and R2. Point (divided voltage), the other input terminal is connected to the REF terminal of the pulse width control IC 11, and the output terminal is CONT of the pulse width control IC.
A differential amplifier IC1 connected to the terminal and a pulse width control I
A photodetector PC1-2 of the photocoupler connected between the REF terminal and the FB terminal of C11, and a resistor R connected to the FB terminal of the pulse width control IC 11 and the negative side of the DC input power supply 17
6, a resistor R5 and a capacitor C1 connected in series between the FB terminal and the negative side of the DC input power supply, and a series connection between the REF terminal and the negative side of the DC input power supply 17 and having a middle point. A resistor R connected to the DT terminal of the pulse width control IC 11
3, R4, and a diode D1 that is connected on the cathode side to the output terminal of the differential amplifier IC1 and connected on the forward side to the connection point between the resistor R5 and the capacitor C1.

The rectifier circuit 15 includes rectifiers D2 and D3, an obstruction loop L1, and a capacitor C3. The output voltage detection circuit 16 includes a light emitting section PC1-1 of a photocoupler,
An error amplifier 16-1 controls the operating current of the light emitting section PC1-2 of the photocoupler based on the difference between the output voltage VOUT and a preset reference voltage.

Next, an embodiment of the present invention will be described with reference to FIG. 1 and FIG. 2A and 2B are diagrams showing operation waveforms at the time of startup in FIG. 1, wherein FIG. 2A shows a voltage waveform of an input voltage, FIG. 2B shows an internal operation of a pulse width control IC, and FIG. Shows the output waveform of the IC,
(D) is a diagram showing an example of an output voltage.

When a switch (not shown) is turned on, the power supply voltage VIN of the DC input power supply 17 is changed to the DC / DC converter 1
0 and the input voltage VIN as shown in FIG.
Gradually rises and reaches the rated voltage of the DC input power supply via the operation start voltage a of the pulse width control IC 11 and the converter operation start voltage b.

The pulse width control IC 11 starts operating when the input power supply voltage reaches the operation start voltage a, outputs a predetermined reference voltage to the REF terminal, and is applied to the input terminal (+) of the differential amplifier IC1. On the other hand, the input voltage VIN is divided into a predetermined voltage by the resistors R1 and R2 and applied to the input terminal (-) of the differential amplifier IC1.

At this time, the input voltage VIN changes to the pulse width control I.
After the operation start voltage a of C11 elapses and reaches the converter operation start voltage b, the reference voltage of the REF terminal of the pulse width control IC11 applied to the input terminal (+) of the differential amplifier IC1 is applied to the input terminal (-). By setting R1 and R2 to be higher than the applied voltage,
The output of the differential amplifier IC1 becomes H level.

The H level output of the differential amplifier IC1 is input to the CONT terminal of the pulse width control IC11 to set the pulse width control IC11 in a standby state (standby state) and to charge the capacitor C1 via the diode D1. , Capacitor C1 is charged to substantially the same potential as the output level of differential amplifier IC1. This makes it possible for the FB terminal to have the same function as that of the conventional circuit, which is prevented from being damaged by the DT terminal voltage when the DC / DC converter is activated, by the potential of the FB terminal.

At the same time, when a predetermined reference voltage is output to the REF terminal after the operation start voltage a of the pulse width control IC 11 has passed, the potential of the DT terminal becomes the resistance R as shown in FIG.
3 is set to the reference voltage of the REF terminal. Since the reference voltage of the REF terminal is constant, the maximum pulse width of the pulse width control IC is determined by the DT terminal voltage, thereby preventing the converter from being destroyed when the operation of the pulse width control IC starts.

When the input voltage VIN reaches the converter operation start voltage b, the voltage at the input terminal (-) of the differential amplifier IC1 becomes higher than the voltage at the input terminal (+) and the differential amplifier IC1
Is inverted to the L level, the pulse width control IC 11 is activated, a triangular wave is output from the OSC, and at the same time, the capacitor C1 starts discharging through R5 and is stabilized at the control potential.

As shown in FIG. 2B, when the potential of the FB discharges and decreases to the amplitude level of the triangular wave, the detection of the pulse width is started, and a rectangular wave pulse having the detected pulse width is output from the OUT terminal to output a rectifier circuit. 15 and output terminals 18-1, 1
8-2 shows the output voltage VO according to the pulse width of the rectangular wave pulse.
The UT is output.

As shown in FIG. 4C, the pulse width of the triangular wave is detected corresponding to the potential of FB. Therefore, at first, a thin pulse corresponding to the tip of the triangular wave is detected, and as the potential of FB decreases, a pulse having a wider pulse width is detected. As shown in FIG. 2D, the output voltage VOUT changes according to the pulse width. The voltage gradually increases, and the rated voltage is output.

Error amplification circuit 16 of voltage output detection circuit 16
-1 detects a difference between the output voltage VOUT and a preset reference voltage, and according to the difference, the light emitting unit PC of the photocoupler.
Control the current flowing through 1-1. R of pulse width control IC
The light receiving portion PC1-2 of the photocoupler connected to the EF terminal changes its internal impedance in response to the current flowing through the light emitting portion PC1-1.

The potential of the FB terminal is set by the internal impedance of PC1-2 connected to the REF terminal and the voltage dividing ratio of R6. When the output voltage VOUT is higher than the rated voltage, the current flowing through the light emitting portion PC1-1 of the photocoupler of the output voltage detection circuit 16 increases, the internal impedance of the light receiving portion PC1-2 of the photocoupler of the soft start circuit decreases, and the REF terminal The current flowing from PC2 to PC1-2 and R6 increases, the voltage generated across R6 increases, and FB
The potential of the terminal is set high.

When the potential of the FB terminal rises, the pulse width of the rectangular wave pulse output to the OUT terminal becomes narrow,
14 becomes short, and the output voltage VOUT decreases. When the output voltage VOUT falls below the rated voltage, the current flowing through the PC1-1 decreases, the internal impedance of the PC1-2 increases, and the potential of the FB terminal decreases. The pulse width increases and the output voltage VOUT increases.

As described above, at the start of operation of the pulse width control IC before the start of operation of the converter, the capacitor C1 is charged by the output voltage of the differential amplifier IC1, and when the DC / DC converter is started, the FB terminal has the same function as the DT terminal. , And in the pulse width control standby state, and the input voltage VI
When N reaches the starting voltage of the converter, the capacitor C
By gradually lowering the potential of the FB terminal and gradually increasing the pulse width by the discharge of 1, it is possible to prevent occurrence of an overshoot voltage of the output voltage at the time of starting the converter.

[0050]

As described above, according to the present invention, the input voltage VIN is supplied from the DC input power supply and the pulse width control IC 11
, The DT terminal is set to the maximum pulse width control potential for preventing the converter from being damaged, that is, the same potential as the REF terminal voltage, and the output voltage of the differential amplifier IC1 is set to the capacitor C1 via the diode D1. And the potential of the FB terminal is set to a state where pulse width control can be started. When the input voltage VIN reaches the operation start voltage of the converter, the Yuzu force of the differential amplifier IC1 is inverted, and the capacitor C1 starts discharging. The pulse width of the triangular wave is detected based on the discharge potential, and the FET 14 responds to the detected pulse width.
Is configured to generate an output voltage VOUT by performing a switching operation. Therefore, without delaying the response speed of the feedback loop for stable operation of the converter, the pulse width for the soft start is limited in the feedback loop, and The transition from the potential required for starting to the control potential can be stably and smoothly continued.

Therefore, it is possible to suppress the generation of the overshoot voltage of the output voltage at the time of starting the converter.

Since a large time constant is not required for the soft start circuit, the rise time of the converter can be shortened, and the rated voltage can be output in a short time at the time of starting.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a main configuration of a DC / DC converter of the present invention.

FIG. 2 is a diagram showing voltage waveforms at main parts when the DC / DC converter of the present invention is started.

FIG. 3 is a block diagram showing a main configuration of a conventional DC / DC converter.

FIG. 4 is a diagram showing voltage waveforms at main parts when a conventional DC / DC converter is started.

[Explanation of symbols]

 Reference Signs List 10 DC / DC converter 11 Pulse width control IC 12 Soft start circuit 13 Transformer 14 FET 15 Rectifier circuit 16 Output voltage detection circuit 16-1 Error amplification circuit 17 DC input power supply 18-1 and 18-2 Output terminal 20 DC / DC converter 21 Soft start circuit

──────────────────────────────────────────────────続 き Continued on the front page (56) References JP-A 6-52391 (JP, U) JP-A 6-66292 (JP, U) (58) Fields surveyed (Int. Cl. ⁷ , DB name) H02M 3/28

Claims (1)

(57) [Claims] 1. A REF terminal for outputting a reference voltage, a DT terminal for setting a maximum pulse width limiting potential, an FB terminal for inputting a pulse width control potential, and a CON for inputting a start signal.
T terminal, and the pulse width control IC with an OUT terminal for outputting a rectangular wave pulse, D to control the pulse width control IC
A soft start circuit for suppressing an overshoot voltage of an output voltage generated at the time of starting the C / DC converter;
The pulse width connected in series with the primary winding of the power transformer
ON, OFF in response to the square wave pulse output by the control IC
FET connected to the secondary side of the power transformer
Rectifier circuit for rectifying the square wave pulse output from the FET
And an output voltage detection circuit for detecting a change in a rectified output voltage output by the rectifier circuit and controlling a control potential of the FB terminal, and converts an input voltage into a predetermined DC voltage by switching control and outputs the DC voltage. / DC converter, the soft start circuit includes a resistor R for dividing an input voltage.
1 and R2 and one input terminal is connected to the resistors R1 and R2.
Connected to the middle point and the other input terminal controls the pulse width.
Connected to the REF terminal of the IC and the output terminal
Differential amplifier connected to the CONT terminal of the width control IC
A circuit; the REF terminal of the pulse width control IC;
Connected between the FB terminal and the output voltage detection circuit.
A light-receiving part of a photocoupler that operates in response to the light-emitting part
Between the FB terminal of the pulse width control IC and the input voltage.
A resistor R6 connected to the negative side, the FB terminal and the input
A resistor R5 connected in series with the negative side of the voltage and a capacitor
And the REF terminal of the pulse width control IC.
Connected in series with the negative side of the input voltage and
A resistor R connected to the DT terminal of the pulse width control IC
3, R4 and the cathode side to the output of the differential amplifier circuit,
The forward side connects the resistor R5 and the capacitor C1.
And a diode D1 connected to the middle point, and when the pulse width control IC is activated, the potential of the FB terminal is pulsed.
Set the control potential to control the pulse width, and
The input voltage is increased by the DC / DC converter
Until the start operating voltage of the input voltage is reached.
Has reached the starting operating voltage of the DC / DC converter.
Discharge starts and the capacitor C1 discharges
Built-in pulse width control IC based on FB terminal voltage
Especially that the oscillator to detect the pulse width of the triangular wave that occurs
DC / DC converter.