JP2001197730A - Dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC-DC converter which can use a small size inductive element and assures high operation efficiency. SOLUTION: A control circuit 3A is a PWM control system comprises an oscillator 8, an error amplifier 6 and a comparator 5. The control circuit 3A further comprises a frequency compensation circuit 9. The frequency compensation circuit 9 and oscillator 8 are structured to operate to lower the oscillation frequency of the reference waveform signal when the input voltage is comparatively higher, or to raise the oscillation frequency of the reference waveform signal when the input voltage is comparatively lower. As a result, loss generated in the switching element can be reduced by lowering the switching frequency when the input voltage is high and loss generated in the inductive element is also reduced by raising the switching frequency when the input voltage is low.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC system capable of miniaturizing components and increasing efficiency.
The present invention relates to a technique for improving a converter.

[0002]

2. Description of the Related Art FIG. 2 shows a circuit diagram of a conventional general DC-DC converter. The circuit shown in FIG.
A choke coil L1 as an inductive element and a diode D1 as a rectifier are connected in series between an input terminal 1 and an output terminal 2 of the C converter. A capacitor C1 as a smoothing element is connected between the output terminal 2 and the ground. The connection point between the choke coil L1 and the diode D1 connects the collector of the transistor Q1 as a switching element, and the emitter of the transistor Q1 is connected to the ground. The base of the transistor Q1 is
A drive signal for causing an on / off operation to appear on the PW
It is connected to the output terminal OUT of the control circuit 3B of the M control system. An output detection terminal FB of the control circuit 3B for detecting the output voltage of the DC-DC converter is connected to the output terminal 2 of the DC-DC converter. The control to operate the circuit 3B, and connect the power terminal V _CC of the control circuit 3B to the input terminal 1 has a circuit configuration of connecting the ground terminal GND to the ground.

[0003] In the DC-DC converter having the above-described configuration, a case is considered where the input voltage _VIN supplied to the input terminal 1 changes within a predetermined range. When the input voltage _VIN is high, the flyback voltage generated in the choke coil L1 may be small in maintaining the output voltage _VO at a desired value. Therefore, as shown by the line a in FIG. 3, the ON period of the transistor Q1 is relatively short, and the peak value of the current flowing through the choke coil L1 is low. However, when the input voltage _VIN is low, a large flyback voltage must be generated in the choke coil L1 in order to maintain the output voltage _VO at a desired value. Then, the line b in FIG.
As shown in the figure, the on-period of the transistor Q1 is longer than when the input voltage _VIN is high, and the choke coil L
1, a current having a high peak value flows.

At this time, a loss corresponding to the peak value of the current occurs in the choke coil L1. Choke coil L1
Most of the loss that occurs is copper loss, and the smaller the inductive element used as the choke coil L1 is, the larger the copper loss tends to be. This is because, when comparing two large and small inductive elements having the same inductance value, generally, a small one uses a wire having a smaller cross-sectional area than a large one to form a winding. For this reason, the reason is that, in a small inductive element, the electric resistance of the winding increases and the copper loss increases. To reduce the copper loss generated in the choke coil L1 may be lower the peak value of the current I _L flowing through the choke coil L1. Current flowing through the choke coil L1 _{I L}
The means for lowering the peak value of the transistor Q1
Is to increase the switching frequency.

[0005] A high switching frequency, than when the switching frequency is low, distribution number of the current per unit of time current I _L flowing through the choke coil L1 increases. However, if the input voltage V _IN supplied to the input terminal 1 of the DC-DC converter, the output voltage V _O appearing at the output terminal 2, and the output current I _O flowing out of the output terminal 2 to the outside remain unchanged. , even going to change the switching frequency, the average value per unit time of the current I _L flowing through the choke coil L1 is almost unchanged. Even if the number of circulations per unit time increases by increasing the switching frequency, if the average value per unit time does not change,
The peak value of the current I _L flowing per one time is lowered. This is as shown in the current waveform of FIG. FIG.
The middle line c shows the case where the switching frequency is low, and the line d shows the case where the switching frequency is high.

As described above, by increasing the switching frequency, the peak value of the current flowing through the choke coil L1 decreases, and the copper loss generated in the choke coil L1 can be reduced. However, the switching frequency cannot be blindly increased. This is because the higher the switching frequency, the greater the switching loss that occurs in the transistor Q1. The switching loss is an extremely short time when the state of the transistor Q1 transitions from on to off or from off to on. However, a state occurs in which both the collector-emitter voltage and the collector current are not zero. Since the switching loss is substantially proportional to the number of transitions between the on and off states of the transistor Q1, the switching loss increases as the switching frequency increases. In particular, when the switching frequency and the input voltage _VIN are high, the switching loss occurring in the transistor Q1 must be considered in improving the efficiency of the DC-DC converter over the copper loss occurring in the choke coil L1. Loss.

[0007]

Since the switching loss of the transistor Q1 increases, the switching frequency cannot be increased unnecessarily. However, when the switching frequency is reduced, the peak value of the current increases, and when the input voltage is low, the copper loss particularly generated in the choke coil L1 increases. For this reason, conventionally, a large-sized inductive element used as the choke coil L1 has to be used to prevent the copper loss from increasing even when the switching frequency is low. Therefore, an object of the present invention is to provide a DC-DC converter that can use a small inductive element and can increase the efficiency.

[0008]

A DC-DC converter according to the present invention for solving the above-mentioned problems comprises a frequency correction circuit for outputting a correction signal corresponding to a voltage value of an input voltage, and an on / off operation performed by a switching element. An oscillator for outputting a reference signal as a reference for an operation switching frequency and changing an oscillation frequency of the reference signal according to the correction signal;
It is characterized by including a comparator for generating a drive signal for operating a switching element with an on / off period according to the output voltage of the DC converter and a switching frequency according to a reference signal. Here, the oscillator lowers the oscillation frequency of the reference signal when the input voltage is relatively high, and lowers the oscillation frequency of the reference signal when the input voltage is relatively low, according to the frequency correction signal from the frequency correction circuit. It is configured to increase the oscillation frequency.

[0009]

DETAILED DESCRIPTION OF THE INVENTION Inductive elements, switching elements,
DC-DC converter with rectifying element, smoothing element and control circuit
Construct a barter circuit. Here, the control circuit is mistaken for an oscillator.
A PWM control system having a difference amplifier and a comparator;
And a frequency correction circuit is further provided inside this control circuit.
You. And for the frequency correction circuit and the oscillator,
If the voltage is relatively high, increase the oscillation frequency of the reference waveform signal.
If the input voltage is relatively low, the reference waveform signal
It is configured to perform operations such as increasing the oscillation frequency.
deep. With such a configuration, the input voltage V_INIs high
Sometimes the switch is turned on by the action of a frequency correction circuit and an oscillator.
Lowering frequency. When the switching frequency decreases
The ON / OFF state of the switching element per unit time
The number of transitions decreases, and the switch
Pitching losses are reduced. Conversely, the input voltage V _INIs low
Switch when the frequency correction circuit and oscillator work
The switching frequency increases. High switching frequency
And the peak value of the current flowing through the inductive element
The loss that occurs in the conductive element is reduced.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A small inductive element can be used, and
FIG. 1 shows a circuit diagram of a DC-DC converter according to the present invention capable of increasing the efficiency. DC-D shown in FIG.
The C converter had the following configuration. A choke coil L1 as an inductive element and a diode D1 as a rectifying element are connected in series between an input terminal 1 and an output terminal 2 of the DC-DC converter. A capacitor C1 as a smoothing element is connected between the output terminal 2 and the ground. The collector of the transistor Q1 as a switching element is connected to the connection point between the choke coil L1 and the diode D1, and the emitter of the transistor Q1 is connected to the ground. The base of the transistor Q1 is connected to the output terminal OUT of the control circuit 3A where a drive signal for causing the transistor Q1 to perform on / off operations appears. D of control circuit 3A
An output detection terminal FB for detecting the output voltage of the C-DC converter is connected to the output terminal 2 of the DC-DC converter. Then, connect the power terminal _{V CC} of the control circuit 3A to the input terminal 1, to connect the ground terminal GND to the ground.

Here, the control circuit 3A has the following internal configuration. Output terminal OUT to output buffer 4
To the output of the comparator 5. The output of the error amplifier 6 is connected to one input of the comparator 5, and two inputs of the error amplifier 6 are respectively connected to a reference voltage source 7 and an output detection terminal F.
Connect to B. The other input of the comparator 5 is connected to an oscillator 8 that outputs a reference signal, specifically, a triangular wave voltage. The oscillator 8 has an input detection terminal ID for detecting an input voltage _VIN of the DC-DC converter via a frequency correction circuit 9.
, And the input detection terminal ID is connected to the input terminal 1 of the DC-DC converter. Note that the frequency correction circuit 9 is configured to detect the input voltage _VIN from the input detection terminal ID and generate a correction signal according to the input voltage _VIN . Further, the oscillator 8 changes the oscillation frequency of the triangular wave voltage according to the correction signal from the frequency correction circuit 9 and supplies the comparator 5 with a triangular wave voltage having a frequency substantially corresponding to the input voltage _VIN. And keep it.

In the DC-DC converter having the above-described configuration, first, it is assumed that an input voltage having a predetermined voltage value is supplied. The predetermined voltage value is DC-DC
The voltage value is located at substantially the center of the applicable range of the input voltage required for the converter to operate stably. In this case, the oscillator 8 outputs a triangular wave voltage at a predetermined oscillation frequency. In the comparator 5, this triangular wave voltage and the error amplifier 6
A drive signal is generated according to the error signal from the transistor Q1, and the transistor Q1 is turned on / off by the drive signal. Note that the mechanism of power transmission from the input terminal 1 side to the output terminal 2 side with the on / off operation of the transistor Q1 is shown in FIG.
Is exactly the same as the conventional circuit of FIG.

Here, when the input voltage _VIN of the DC-DC converter is higher than the predetermined voltage value, the triangular wave voltage output from the oscillator 8 changes according to the correction signal from the frequency correction circuit 9. Output at an oscillation frequency lower than the oscillation frequency. Conversely, when the input voltage _VIN is lower than the predetermined voltage value, the triangular wave voltage output from the oscillator 8 changes at an oscillation frequency higher than the predetermined oscillation frequency in accordance with the correction signal from the frequency correction circuit 9. Is output.
The switching frequency of the DC-DC converter is the oscillator 8
According to the configuration of FIG. 1, when the input voltage _VIN is relatively high, the switching frequency becomes low and the input voltage V
_{When IN} is relatively low, the switching frequency is high.

As described above, the conventional general D
The C-DC converter has a problem that the copper loss generated in the choke coil L1 increases when the switching frequency and the input voltage _VIN are low. However,
When the switching frequency and the input voltage _VIN are high,
It is necessary to consider the switching loss generated in the transistor Q1 more than the copper loss generated in the choke coil L1. Since the switching loss increases as the switching frequency is increased, the switching frequency cannot be increased unnecessarily.

On the other hand, in the DC-DC converter of the present invention, when the input voltage _VIN is high, the switching frequency is low, and the switching loss generated in the transistor Q1 is reduced. Conversely, when the input voltage _VIN is low, the switching frequency increases, and the choke coil L
At the same time as the peak value of the current flowing through 1 decreases, the copper loss generated in choke coil L1 is reduced. Then, by reducing the peak value of the current of the choke coil L1 and reducing the copper loss, a small inductive element used as the choke coil L1 can be used. Therefore, a high efficiency can be obtained for a wide range of input voltage, and a DC-DC that can use a small inductive element can be configured.

In the above description, a case where a boost chopper type converter circuit is applied to the DC-DC converter has been described. However, the present invention can be applied to converter circuits of other types. In FIG. 1, a choke coil is used as an inductive element, and NP is used as a switching element.
Although an N-type transistor is used, a transformer may be used as an inductive element, or a field-effect transistor may be used as a switching element. Further, the change of the oscillation frequency in the oscillator according to the correction signal may be continuous or stepwise. Such a change of the circuit form is naturally possible without changing the gist of the present invention.

[0017]

The DC-DC converter according to the present invention has the following features.
A control circuit provided therein is of a PWM control type including an oscillator, an error amplifier, and a comparator, and a frequency correction circuit is provided in the control circuit. For the frequency correction circuit and the oscillator, when the input voltage is relatively high, the oscillation frequency of the reference waveform signal is lowered, and when the input voltage is relatively low, the operation of increasing the oscillation frequency of the reference waveform signal is performed. It is characterized in that the configuration is performed.
According to such a configuration, the loss that occurs in the switching element when the input voltage is high is reduced, and the loss that occurs in the inductive element when the input voltage is low is reduced. Then, by reducing the current peak value and the loss, a small inductive element can be used. Therefore, according to the present invention, a small-sized inductive element can be used, and a highly efficient DC-DC converter can be provided.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of a DC-DC converter according to the present invention.

FIG. 2 is a circuit diagram of a conventional general DC-DC converter.

FIG. 3 is a waveform diagram of a current flowing through a choke coil using an input voltage as a parameter.

FIG. 4 is a waveform diagram of a current flowing through a choke coil using a switching frequency as a parameter.

[Explanation of symbols]

3A: control circuit 5: comparator 6: error amplifier 8: oscillator 9: frequency correction circuit C1: capacitor (smoothing element) D1: diode rectifying element L1: choke coil (inductive element) Q1: transistor (switching element)

Claims (3)

[Claims] 1. A DC-DC converter in which a current flowing through an inductive element is turned on and off by a switching element, and a desired output voltage is obtained by controlling the on and off periods of the switching element. A frequency correction circuit that outputs a correction signal according to the following: a reference signal serving as a reference of a switching frequency of an on / off operation performed by the switching element; and an oscillation frequency of the reference signal is changed according to the correction signal. An oscillator; and a comparator for generating a drive signal for operating the switching element with an on / off period according to an output voltage of the DC-DC converter and a switching frequency according to the reference signal. DC-DC converter. 2. The oscillator according to claim 1, wherein the oscillator lowers the oscillation frequency of the reference signal when the input voltage is relatively high, and lowers the oscillation frequency of the reference signal when the input voltage is relatively low, according to a frequency correction signal from the frequency correction circuit. 2. The DC-DC converter according to claim 1, wherein an oscillation frequency of the reference signal is increased. 3. The DC-DC converter according to claim 1, wherein the reference signal is a triangular wave voltage.