JP3560100B2 - Resonant DC-DC converter - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a resonant DC-DC converter that converts a DC input into AC by switching with a switch element, converts this AC output into DC, and outputs the DC.
[0002]
[Prior art]
FIG. 2 is a circuit diagram showing a configuration of a conventional resonance type DC-DC converter. In FIG. 1, a ripple removing capacitor 2 is connected between an input terminal 1 to which a DC input voltage Vin is input and the ground, and a capacitor between the input terminal 1 and an output terminal 3 from which an output voltage Vout is output. Has a choke coil 4 and a rectifying diode 5 connected in series.
[0003]
A switch element 6 is connected between the connection point of the choke coil 4 and the rectifying diode 5 and the ground, and an output smoothing capacitor 7 is connected between the output stage of the rectifying diode 5 and the ground. .
[0004]
The differential amplifier 8 detects a variation of the DC output voltage, and the output voltage divided by the voltage divider 9 is input to one input terminal of the differential amplifier 8. The reference voltage 10 is input to the other input terminal of the switch.
[0005]
The differential voltage detected by the differential amplifier 8 is supplied to a pulse width modulation circuit (PWM) 11, and a pulse signal that has been pulse width modulated in accordance with the differential voltage of the differential amplifier 8 is supplied to the switch element 6. By switching the switch element 6 with this pulse signal, the DC input is chopped and converted into AC. The AC output is converted into DC by the diode 5 and smoothed by the capacitor 7 to output the output voltage Vout.
[0006]
The PWM control of the DC-DC converter configured as described above is a pulse width modulation method performed by controlling the on / off time ratio (time ratio) of the switching element 6 while fixing the switching cycle.
[0007]
In this control circuit system, the output voltage divided by the voltage divider 9 of the feedback system is compared with the reference voltage 10 by the differential amplifier 8, and the difference voltage controls the pulse width modulation circuit 11, and when the output voltage is low, When the output voltage is high, the pulse width of the ON time is narrowed to stabilize the output voltage of the power supply.
[0008]
[Problems to be solved by the invention]
However, in the case of the duty ratio control, the switching frequency is fixed, so it is necessary to add an overcurrent protection circuit that prevents the output voltage from rising at a small load current, and an overcurrent protection circuit that prevents an overload or output short circuit. This is disadvantageous in terms of cost and mounting.
[0009]
Further, in the current resonance type DC-DC converter, the resonance frequency is fixed in accordance with the on-time, so that the stabilization of the output voltage is frequency control for stabilizing the output voltage by making the off-time variable.
[0010]
The present invention provides a highly reliable and safe current resonance type which can easily realize overcurrent protection and prevention of a decrease in switching frequency at low cost and easily by making the off time of a switch element variable. It is an object to provide a DC-DC converter.
[0011]
[Means for Solving the Problems]
The present invention relates to a resonance type DC-DC converter having a switch element for chopping a DC voltage at a predetermined frequency and converting the converted AC to an AC, and a rectifying and smoothing unit for converting the converted AC to a DC and smoothing to generate a DC output voltage. a DC converter, an error detection circuit for detecting an error voltage by comparing a pulse generating circuit for outputting a pulse signal for switching a predetermined frequency to said switching element and said DC output voltage and a reference voltage, the switching element A time constant variable circuit for controlling an off time of the pulse generation circuit, the time constant variable circuit comprising a time constant of the pulse generation circuit according to an error voltage between the DC output voltage detected by the error detection circuit and a reference voltage. by also in the range for controlling the off time of the switching element by changing the setting resistor component.
[0012]
According to the present invention, at the time of an overcurrent such as an overload or an output short circuit, the time constant variable circuit determines the time constant of the pulse generation circuit so that the off time of the switch element is not extremely short, and the overcurrent flows to the load. To prevent Further, at the time of a small load current, the time constant variable circuit determines the time constant of the pulse generation circuit so that the off time of the switch element does not become extremely long, thereby preventing the switching frequency from lowering.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
According to the first aspect of the present invention, there is provided a switch element for chopping a DC voltage at a predetermined frequency and converting it to AC, and a rectifier for converting the converted AC to DC and smoothing the converted AC to generate a DC output voltage. A resonance type DC-DC converter having a smoothing unit, comprising: a pulse generation circuit that outputs a switching pulse signal having a predetermined frequency to the switch element; and detecting an error voltage by comparing the DC output voltage with a reference voltage. An error detection circuit, and a time constant variable circuit that controls an off time of the switch element by changing a time constant setting resistance component of the pulse generation circuit according to an error voltage detected by the error detection circuit. It is provided.
[0014]
The invention according to claim 2, wherein the time constant variable circuit generates a current corresponding to an error voltage from the error detection circuit, a first resistor connected in series to an emitter of the transistor, and the transistor , And a third resistor connected in parallel to a series circuit of the transistor and the first resistor.
[0015]
Therefore, at the time of an overcurrent such as an overload or an output short circuit, the transistor constituting the time constant variable circuit is turned on, and the time constant of the off time of the pulse generation circuit is determined by the combined resistance of the first to third resistors. It is determined. This limits the off time from becoming extremely short and prevents an overcurrent from flowing to the load.
[0016]
Further, at the time of a small load current, the transistor forming the time constant variable circuit is turned off, so that the time constant of the off time of the pulse generation circuit is determined by the combined resistance of the second resistance and the third resistance. This limits the off time from becoming extremely long and prevents the switching frequency from lowering.
[0017]
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0018]
FIG. 1 is a circuit diagram showing a configuration of a current resonance type DC-DC converter according to an embodiment of the present invention. The same components as those shown in FIG. In FIG. 1, a ripple removing capacitor 2 is connected between an input terminal 1 to which a DC input voltage Vin is input and the ground, and furthermore, between the input terminal 1 and an output terminal 3 for outputting an output voltage Vout. , A choke coil 4 and a rectifying diode 5 are connected in series.
[0019]
A resonance choke coil 12 and a switching element 6 are connected between the connection point of the choke coil 4 and the rectifier diode 5 and the ground, and an output smoothing is provided between the output stage of the rectifier diode 5 and the ground. The capacitor 7 is connected. Further, a resonance capacitor 13 is connected in parallel with the rectifier diode 5, and the resonance choke coil 12 and the resonance capacitor 13 form a current resonance circuit.
[0020]
Further, the pulse width control circuit 14 for controlling the switch element 6 compares the reference voltage 15b with a voltage obtained by dividing the output voltage Vout by the voltage divider 15a and has a differential amplifier 15c for detecting an error voltage between the two. It has a detection circuit 15.
[0021]
Further, the pulse width control circuit 14 includes a PNP transistor 16 for generating a current corresponding to the error voltage output from the error detection circuit 15, a resistor 17 connected in series between the emitter terminal of the transistor 16 and the Vcc power supply, and a An IC configured to supply a pulse signal to the switching element 6, a resistor 18 for preventing a decrease in switching frequency connected in parallel between the collector terminal and the Vcc power supply, a resistor 19 for preventing an overcurrent connected in series to the collector terminal of the transistor 16, and the switch element 6. It has a pulse generation circuit 20 composed of a dual monostable multivibrator.
[0022]
Further, a capacitor 21 and a resistor 22, which are connected between the pulse generating circuit 20 and the Vcc power supply to set the ON time of the pulse generating circuit 20, are connected between the pulse generating circuit 20 and the collector terminal of the transistor 16, and A capacitor 23 for setting the off time is provided. The transistor 16 and the resistors 17 to 19 constitute a time constant variable circuit 24 for setting the time constant of the off time of the pulse generation circuit 20.
[0023]
Next, the operation of the present embodiment will be described. First, the time constant of the pulse generation circuit 20, that is, the on-time determined by the capacitor 21 and the resistor 22 corresponds to the resonance frequency of the resonance current generated by the choke coil 12 and the capacitor 13.
[0024]
On the other hand, in the error detection circuit 15, an error voltage between the output voltage divided by the voltage divider 15a and the reference voltage 15b is amplified by the differential amplifier 15c. When this error voltage is added to the base of the transistor 16, the error Since a current according to the voltage flows to the collector of the transistor 16, this is equivalent to an apparent change in the parallel resistance of the resistor 17 and the resistance 18 and the combined resistance of the resistance 19 connected in series to this parallel circuit. The time constant of the off time of the pulse generation circuit 20 is determined by the combined resistance and the capacitor 23.
[0025]
That is, at the time of an overcurrent such as an overload or an output short circuit, the transistor 16 is completely turned on. Therefore, the time constant of the off time of the pulse generation circuit 20 is determined by the combined resistance of the resistor 17 and the resistor 18 and the resistor 19 and the capacitor. 23. This limits an extremely short off-time and prevents an overcurrent from flowing to the load.
[0026]
At the time of a small load current, the transistor 16 is completely turned off, so that the time constant of the off time of the pulse generation circuit 20 is determined by the combined resistance of the resistors 14 and 19 and the capacitor 23. This limits the extremely long off-time and prevents the switching frequency from dropping to the audible band.
[0027]
As described above, according to the present embodiment, it is possible to limit the shortening of the off-time due to the combined resistance of the resistors 17 to 19 at the time of an overcurrent, and the off-time due to the combined resistance of the resistors 18 and 19 at the time of a small load current. Limiting the shortening can prevent the switching frequency from lowering.
[0028]
【The invention's effect】
According to the present invention, by Gosuru system by changing depending OFF time of the switching element to the error voltage to a constant setting resistor component detected by the error detection circuit when the pulse generation circuit, the overcurrent prevention and Switching Frequency The reduction can be prevented with a simple circuit configuration, which is advantageous in terms of cost reduction and mounting, and also provides a safe and reliable resonant DC-DC converter.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a configuration of a resonance type DC-DC converter according to an embodiment of the present invention. FIG. 2 is a circuit diagram showing a configuration of a conventional resonance type DC-DC converter.
DESCRIPTION OF SYMBOLS 1 Input terminal 2 Ripple removal capacitor 3 Output terminal 4 Choke coil 5 Rectifier diode 6 Switch element 7 Smoothing capacitor 12 Resonance choke coil 13 Resonance capacitor 14 Pulse width control circuit 15 Error detection circuit 15a Voltage divider 15b Reference voltage 15c Difference Dynamic amplifier 16 Transistor 17 Resistance 18 Resistance 19 Resistance 20 Pulse generation circuit 21 Capacitor 22 Resistance 23 Capacitor 24 Time constant variable circuit

Claims (2)

A resonant DC-DC converter having a switch element for chopping a DC voltage at a predetermined frequency and converting it to AC, and a rectifying and smoothing unit for converting the converted AC to DC and smoothing to generate a DC output voltage. So,
A pulse generation circuit that outputs a switching pulse signal having a predetermined frequency to the switch element; an error detection circuit that compares the DC output voltage with a reference voltage to detect an error voltage; and controls an off time of the switch element. A time constant variable circuit, wherein the time constant variable circuit changes a time constant setting resistance component of the pulse generation circuit according to an error voltage between the DC output voltage detected by the error detection circuit and a reference voltage. The resonant DC-DC converter controls the off time of the switch element by causing the switch element to turn off . The time constant variable circuit includes a transistor that generates a current corresponding to an error voltage from the error detection circuit, a first resistor connected in series to an emitter of the transistor, and a first resistor connected in series to a collector of the transistor. 2. The resonant DC-DC converter according to claim 1, comprising a second resistor and a third resistor connected in parallel to a series circuit of the transistor and the first resistor. 3.

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