JPH08294270A - Dc/dc converter - Google Patents

Abstract

PURPOSE: To minimize the output voltage drop at the time of changeover of the operation mode of a DC/DC converter. CONSTITUTION: A comparator 2 monitors output voltage Vout . A comparator 3 monitors a coil current IL. When the output voltage Vout falls under the preset reference value, it starts power supply cycle. In the case that the output voltage Vout has not recovered the preset reference value when a coil current IL increases and the output of the comparator 3 is inverted, a PWM control circuit 8 outputs the pulse signal (H level continues) of 100% in duty. Then, this performs the mode switching from light load mode to heavy load mode, using selection A signal, and makes selectors 6 and 7 select the output signal of the PWM control circuit 8, and makes the switching element 1 continue the on condition.

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, and more particularly to a D / D converter for switching an operation mode according to a load.
The present invention relates to a C / DC converter.

[0002]

2. Description of the Related Art A DC / DC converter is a device that converts a certain direct current input voltage into a different direct current voltage and outputs it, and is used for various purposes. This output voltage needs to be stable at a predetermined value, and in consideration of the case of being mounted on a mobile terminal or the like, it is required to reduce its power consumption as much as possible.

FIG. 6 is a schematic block diagram of an example of a conventional DC / DC converter. DC / DC converter shown in the figure, converts the input voltage V _in to an output voltage V _out. The switching signal generation circuit 101 receives the output voltage V _out and the coil current calculation signal (mode switching signal) as feedback signals, and outputs a control signal based on the received signals. When the DC / DC converter has two operation modes, a light load mode and a heavy load mode, the switching signal generation circuit 101 switches the operation mode according to the received signal and outputs a control signal according to the mode. Output.

The switching element 102 performs switching according to the control signal output from the switching signal generation circuit 101. For example, when the output voltage V _out decreases, the switching element 102 is turned on, the current flowing through the switching element 102 is caused to flow to the output side through the coil L, and power is supplied to the capacitor C and the output side to output the output voltage V. Increase _out .

The operation mode of the DC / DC converter is switched according to the current I _LD flowing through the connected load. Since this load current I _LD can be regarded as the average value of the coil current I _L , the average value of the coil current I _L is calculated as follows and output as a coil current calculation signal (mode switching signal). That is, the voltage value across the resistor R _sense through which the coil current I _L flows is impedance-converted via the buffer 103, and the converted signal is averaged by the integrating circuit 104. In this averaging process, the timer 10
5 is used, and an integral value in a period corresponding to the switching frequency (operating frequency) of the switching element 102 and the peak-to-peak voltage of the output signal of the buffer 103 is obtained. Then, the comparator 106 compares the integrated value with the reference voltage, and outputs the comparison result as a coil current calculation signal (mode switching signal).

The switching signal generation circuit 101 switches the operation mode as necessary according to the coil current calculation signal (mode switching signal) and switches so as to supply the necessary load current I _LD while keeping the output voltage constant. A signal for controlling the on / off state of the element 102 is output.

[0007]

As described above, in order to generate the coil current calculation signal (mode switching signal) which is one of the feedback signals, that is, in order to detect the load current, the buffer 103 and the integration circuit are used. Since a large number of circuit blocks such as the circuit 104, the timer 105, and the comparator 106 are required, the circuit scale is large and the current consumption is large. In addition, the buffer 103 and the integrating circuit 104
Further, the bias current needs to be always supplied to the comparator 106, and the current is always consumed for that reason, so that the low power consumption of the DC / DC converter is hindered. That is, in the conventional DC / DC converter, the consumption current of the circuit for monitoring the load current is large, and the conversion efficiency of the DC / DC converter is reduced.

Further, when the operation mode is switched according to the load, in the conventional DC / DC converter, the change of the load current, the output voltage, etc. was used as a trigger, but the switching state immediately before and after the mode switching is taken into consideration. I didn't. For this reason, for example, if the load current increases rapidly during the operation in the light load mode, the output voltage may transiently drop unless the mode is switched smoothly to the heavy load mode. Will end up.

SUMMARY OF THE INVENTION The present invention is intended to solve the above problems, and it is an object of the present invention to minimize the output voltage drop when switching the operation mode of a DC / DC converter.

[0010]

A DC / DC converter according to the present invention has at least two operation modes for supplying power to a load, a light load mode and a heavy load mode, and outputs according to a method corresponding to each operation mode. Assuming a controlled configuration. The light load mode is an operation mode when the current flowing through the connected load is small, and the heavy load mode is an operation mode when the current flowing through the load is large.

In the DC / DC converter according to the first aspect, the operation mode is switched from the light load mode to the heavy load mode when the output value is less than or equal to the reference value at a predetermined timing within the power supply cycle in the light load mode. .

The DC / DC converter according to a second aspect of the present invention has a configuration in which the coil current is changed according to the method corresponding to each operation mode to control the output, and the coil current is controlled within the power supply cycle in the light load mode. If the output value is less than the reference value when the preset comparison value is reached,
Switch the operating mode from light load mode to heavy load mode. The comparison value is set to, for example, the maximum current value in the cycle.

A DC / DC converter according to a third aspect of the present invention is configured to change the coil current and control the output according to a method corresponding to each operation mode, and has the following means. The voltage comparison unit is, for example, a comparator, and compares the output voltage with a predetermined voltage set in advance. The current comparison means is, for example, a comparator, and compares the coil current with a preset predetermined current. The control means refers to the comparison result of the voltage comparison means and the current comparison means within the power supply cycle in the light load mode, and when the coil current reaches the predetermined current, the magnitude relationship between the output voltage and the predetermined voltage. If the output voltage is equal to or lower than the predetermined voltage, a mode switching signal is output. Then, the operation mode is switched from the light load mode to the heavy load mode according to the mode switching signal.

The DC / DC converters according to claims 4 and 5 have a structure in which the coil current is changed by controlling the switching means in a system corresponding to each operation mode to control the output. In addition to the voltage comparison means, the current comparison means, and the control means described above, when a mode switching signal is received from the control means, a control signal for controlling the switching means so as to increase the coil current is output only for a predetermined period thereafter. It has a signal generating means. Then, after the lapse of the predetermined time, the switching means is controlled by a method corresponding to the heavy load mode.

[0015]

When the load current increases while the DC / DC converter is operating in the light load mode, the output value (hereinafter referred to as the output voltage) decreases. When this output voltage falls below a preset reference value (reference voltage), a power supply cycle is started to recover the voltage drop.

If the output voltage has not recovered to the reference voltage at the predetermined timing within this power supply cycle, D
It is determined that the current supplied by the C / DC converter is not sufficient for the load current, and the operation mode is switched to the heavy load mode capable of supplying a larger current (claim 1).

If the output voltage does not recover the reference value (predetermined voltage) when the coil current reaches the preset comparison value (predetermined current) in the power supply cycle, DC /
It is determined that the current supplied by the DC converter is not sufficient for the load current, and the operation mode is switched to the heavy load mode capable of supplying a larger current (claims 2 and 3).

Since the signal generating means which has received the mode switching signal outputs the control signal for controlling the switching means so as to increase the coil current, it is necessary to switch the operation mode in order to supply a larger current. Immediately after that, the coil current increases according to the control signal.
Then, after the lapse of the predetermined period, the power is supplied by the control of the method corresponding to the normal heavy load mode. Therefore, the current supply is not interrupted when the operation mode is switched. When the maximum current in the power supply cycle is set as the predetermined current, the maximum current in the light load mode is supplied immediately before the mode switching, and the supplied current further increases after the mode switching. Therefore, when the output voltage drops, power is supplied with maximum efficiency, and the transient drop in the output voltage can be minimized (claims 4 and 5).

[0019]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a DC / D according to an embodiment of the present invention.
It is a block diagram of a C converter. DC / DC converter shown in the figure, converts the DC input voltage V _in to an output voltage V _out. In addition, this DC / DC converter has two operation modes: a heavy load mode, which is an operation mode when the consumption current of the connected load is large, and a light load mode, which is an operation mode when the consumption current of the connected load is small. And has its operation mode automatically switched according to the load current.

The switching element 1 is a power MOS transistor, and performs on / off operation according to its gate potential. The coupled input voltage V _in to one terminal, a cathode and a coil L of the diode D is connected to the other terminal. The anode of the diode D is grounded. The other end of the coil L is connected to the output terminal via the resistor R _sense . The output terminal is grounded via a capacitor.

The reference voltage V _ref is applied to the-input of the comparator ₂ , and the comparison voltage V _{1 obtained by} dividing the output voltage V _out by the resistors R ₁ and R ₂ is applied to the + input of the comparator ₂ . Since the comparison voltage V ₁ is proportional to the output voltage V _out , the comparator 2 substantially determines whether or not the output voltage V _out has reached a predetermined value for driving the load. Then, the comparison result is output to the AND circuits 4 and 5.

The comparator 3 monitors the voltage across the resistor R _sense . Resistor R is added to the + terminal of comparator 3.
The input side of the _sense is connected, and the output side of the resistor R _sense is connected to the-terminal. Since the current flowing through the resistance R _sense is the coil current I _L flowing through the coil _L , the comparator 3 causes the coil current I _L on the primary side of the DC / DC converter.
That is, the load current is monitored. The comparison level of the comparator 3 is converted into a coil current and set as I _LP . Therefore, the output is determined according to the magnitude relation between the coil current I _L and I _LP. This output is transferred to AND circuits 4 and 5. The comparison level I _LP is set by using the offset voltage V _S.

By the way, when the operation mode is switched according to the load current, a method of directly detecting the load current may be considered, but for that purpose, it is necessary to provide a resistor on the output side of the capacitor C, and thus the efficiency of the entire DC / DC converter. Is lowered, which is not preferable. Therefore, in this embodiment, the load current is estimated using the resistor R _sense provided on the input side (primary side) of the capacitor C. The resistor R _sense uses a resistor used for the purpose of overcurrent protection or keeping the output voltage constant.

The AND circuit 4 includes comparators 2 and 3.
And outputs the logical product of them to the B terminal of the selector 7. That is, the AND circuit 4 has the comparison voltage V
_{When 1} is higher than the reference voltage V _ref and the coil current I _L is higher than I _LP , the “H” level is transferred to the B terminal of the selector 7. The AND circuit 5 receives the inverted signals of the output of the comparator 3 and the output of the comparator 2, and transfers the logical product of them to the PWM control circuit 8 and the delay circuit 11. That is, the AND circuit 5 transfers the “H” level to the PWM control circuit 8 and the delay circuit 11 when the comparison voltage V ₁ is lower than the reference voltage V _ref and the coil current I _L is larger than I _LP .

The selectors 6 and 7 select and output one of the signals input to the A terminal or the B terminal according to the A select signal (heavy load mode) and the B select signal (light load mode), respectively. That is, when the rising edge of the A select signal is detected, the input signal of the A terminal is selected and output thereafter, and when the rising edge of the B select signal is detected, the input signal of the B terminal is selected and output thereafter. The output signal of the selector 6 is input to the S terminal (set terminal) of the flip-flop 9, and the output signal of the selector 7 is input to the R terminal (reset terminal) of the flip-flop 9.

The PWM control circuit (heavy load control circuit) 8 is
During operation in the heavy load mode, a pulse signal is output based on the output voltage V _out and the output signal of the AND circuit 5. For example, when the output voltage V _out drops, a signal with an increased pulse duty is output. In addition,
The PWM control circuit 8 may stop the oscillator during the operation period in the light load mode. The output signal of the PWM control circuit 8 is input to the A terminal of the selector 6, and its inverted signal is input to the A terminal of the selector 7.

The flip-flop 9 outputs an ON signal in the set state and an OFF signal in the reset state. The driver 10 drives the switching element 1 to the on state when receiving the on signal, and turns the switching element 1 to the off state when receiving the off signal.

Delay circuit 11 delays the output signal of AND circuit 5 by time τ and outputs it as an A select signal to selectors 6 and 7. The A select signal is a signal for instructing switching from the light load mode to the heavy load mode. When the selectors 6 and 7 detect the rising edge of the A select signal, the selectors 6 and 7 thereafter select and output the output signal of the PWM control circuit 8 until the rising edge of the B select signal is detected. The switching element 1 is driven by the signal.

The mode switching control circuit 20, although not shown in particular for its connecting line, instructs the mode switching from the heavy load mode to the light load mode based on the coil current I _{L and the} like. The trigger for mode switching is, for example, when the average value of the coil current I _L becomes smaller than a predetermined value. When switching the operation mode from the heavy load mode to the light load mode, the mode switching control circuit 20 outputs the B select signal to the selectors 6 and 7. Then, by the B select signal, the selectors 6 and 7
In response, the signal at the B terminal is selected, and the switching element 1 is driven by the selected signal.

The operation of the DC / DC converter having the above configuration will be described. In the light load mode, each of the selectors 6 and 7 selects and outputs the signal input to the B terminal. In this state, when the output voltage V _out is higher than the predetermined voltage (reference value) for driving the load, the output of the comparator 2 becomes “H” level, and the switching element 1 is turned off by the signal. Therefore, the DC / DC converter does not supply current to the load. On the other hand, the output voltage V _out is lower period than the reference value, and supplies a current to the load while on-off controlling the switching element 1 a value of the coil current I _L as a feedback signal. When the output voltage V _out cannot be recovered even if the current supply in the light load mode is continued, as will be described later in detail, the operation mode of the DC / DC converter is switched to the heavy load mode to generate a larger current. By supplying, the output voltage V _out is increased.

Next, the operation of the DC / DC converter of this embodiment will be described in detail with reference to the time chart. FIG. 2 is a time chart illustrating a power supply cycle in the light load mode. The symbols in the figure are signals at corresponding points in FIG.

The DC / DC converter having the above structure operates in the light load mode by the pulse of the B select signal. That is, each of the selectors 6 and 7 selects the input signal of the B terminal and outputs it to the flip-flop 9.

Before time T ₁ , the output voltage V _out is higher than the reference value and the comparison voltage V ₁ is higher than the reference voltage V _ref . Hereinafter, the output voltage V _out when the comparison voltage V ₁ matches the reference voltage V _ref will be referred to as a reference value. Therefore, the output of the comparator 2 is at "H" level. In this state, the DC / DC converter does not need to supply a current to the load, so the coil current I _L is zero.

Now, the operation of the comparator 3 for detecting the coil current I _L will be described with reference to FIG. Since the offset voltage V _S is applied to the comparator 3 so as to apply a negative potential to the + terminal, the comparator 3 outputs the “L” level when the coil current I _L is 0. When the coil current I _L increases and its value becomes I _LP , the voltage across the resistor R _sense becomes the offset voltage V _S , and the comparator 3
Output is inverted from the "L" level to the "H" level. When the switching element 1 is turned off by this output reversal, the coil current I _L starts to decrease thereafter, and its value is I.
_When it becomes smaller than _LP , the output of comparator 3 becomes "H".
Re-invert from level to "L" level.

When the load current increases and the output voltage V _out becomes smaller than the reference value at time T ₁ during the period when the DC / DC converter is operating in the light load mode, the output of the comparator 2 becomes "L". Since it becomes the level, the selector 6 changes its output from the “L” level to the “H” level. Then, the state of the flip-flop 9 becomes “set”, the switching element 1 is turned on, and thereafter, the coil current I _L increases (ramps up).

This coil current I _L causes an output voltage V
_{When out} rises and the reference value is restored at time T ₂ ,
The output of the comparator 2 returns to "H" level. When the coil current I _L increases and its value reaches I _LP at time T ₃ , the output of the comparator 3 temporarily becomes “H” level as described with reference to FIG. At this time, referring to the output of the comparator 2, it is checked whether or not the output voltage V _out has recovered the reference value. Here, since the output voltage V _out has recovered to the reference value and the output of the comparator 2 has returned to the “H” level, the output of the AND circuit 5 remains at the “L” level. Therefore, the A select signal does not go to "H" level, and mode switching does not occur.

On the other hand, when the output of the comparator 3 becomes "H" level, the output of the AND circuit 4 also becomes "H" level, the state of the flip-flop 9 becomes "reset", and the switching element 1 is turned off. When the switching element 1 is turned off, the coil current I _L decreases (ramps down), and the value returns to 0, one power supply cycle is completed.

In this way, when the output voltage V _out drops in the light load mode, the power supply cycle is executed,
When the coil current I _L reaches the peak value, the output voltage V
It is confirmed whether _out has recovered the reference value, and if it has recovered, the mode is not switched and the operation continues in the light load mode.

FIG. 4 is a time chart for explaining switching from the light load mode to the heavy load mode. The reference numerals in the figure are the same as those in FIG. Also, at time T ₄
The previous operation is the same as the operation before T ₁ in FIG.

At time T ₄ , when the output voltage V _out falls below the reference value, the power supply cycle is started and the coil current I _L increases (ramp up), as described in FIG.
Do it. Then, when the value reaches I _LP at the time T ₅ , the comparator 3 as described with reference to FIG.
Output becomes "H" level. At this time, referring to the output of the comparator 2, it is checked whether or not the output voltage V _out has recovered the reference value. Here, the output voltage V
_{Since out} has not recovered to the reference value, comparator 2
Output remains at "L" level. Therefore, the output of the AND circuit 4 also remains at the “L” level, the state of the flip-flop 9 is not reset, and the time T
_{After 5 as} well, the switching element 1 continues to be in the ON state.

On the other hand, at time T ₅ , the comparator 3
Since the output of the comparator 2 maintains the "L" level when the output of the AND circuit becomes the "H" level, the AND circuit 5
Output changes from "L" level to "H" level at that timing. The output of the AND circuit 5 is transferred to the PWM control circuit 8.

FIG. 5A is a block diagram of the PWM control circuit 8. The PWM control circuit 8 normally outputs the output voltage V received as a feedback signal in the heavy load mode.
_The control unit 12 generates a slice voltage based on _out . The comparison unit 14 uses the slice voltage and the triangular wave generation unit 1
3 is compared with the triangular wave generated, and a pulse signal having a duty determined by the output voltage V _out is output. This pulse signal is input to each A terminal of the selectors 6 and 7.

On the other hand, in the light load mode, since the output signal of the PWM control circuit 8 is not selected by the selectors 6 and 7, the operation is not defined and the signal is "Don't Car".
e ”state. Therefore, the PWM control circuit 8 may suppress the current consumption by stopping the oscillator in the triangular wave generator 13 in the light load mode.

In this light load mode, the AND circuit 5
When the output of changes from "L" level to "H" level,
The control unit 12 detects the rising edge and executes the following processing. That is, first, when the oscillator is stopped, the oscillator is put into an operating state. And FIG. 5 (b)
As shown in, the slice voltage S for slicing the triangular wave is set to a value smaller than the bottom value V _{b of} the triangular wave, and a pulse signal with a duty of 100% (“H” level continues) is output to the comparison unit 14. Let Then, the state is continued until the output voltage V _out recovers the reference value. Alternatively, the comparator 14 is caused to output a pulse signal having a duty of 100% for a predetermined time after the rising edge is detected. After that, the control unit 12 executes the above-described heavy load mode processing.

Thus, in this light load mode,
When the output of the AND circuit 5 changes from the "L" level to the "H" level, the PWM control circuit 8 outputs a pulse signal with a duty of 100%. That is, after time T ₅ , the selector 6 receives a pulse signal with a duty of 100% at its A terminal, and the selector 7 has an inverted signal of the above pulse signal at its A terminal (“L” level continues).
To receive.

On the other hand, the output signal of the AND circuit 5 input to the delay circuit 11 is delayed by the time τ and output as the A select signal. That is, the A select signal is
Changes from "L" level to the "H" level at time T ₆ has elapsed by the time τ from time T _5. This time τ
Is set to be longer than the time from when the PWM control circuit 8 receives the rising edge from the AND circuit 5 until it becomes possible to stably output a pulse signal with a duty of 100%. Therefore, at time T ₆ , PWM
The control circuit 8 outputs a pulse signal with a duty of 100%.

Upon receipt of the A select signal, the selectors 6 and 7 switch the mode from the light load mode to the heavy load mode at time T ₆ , and thereafter select and output the input signal of the A terminal. That is, the selector 6 is “H”
The level is continuously output, and the selector 7 continuously outputs the “L” level. Therefore, the flip-flop 9 is not reset, and the switching element 1 continues to be in the ON state even after the time T ₆ . And the output voltage V
_The coil current I _L increases linearly until _out reaches the reference value, and thereafter the output is controlled according to the pulse signal in the normal heavy load mode.

[0048] Thus, DC / DC converter of this embodiment, the power supply cycle is initiated at time T _4, the reference value output voltage V _out is at time T ₅ the coil current I _L reaches I _LP Check if it is recovering,
Although the mode switching at time T ₆ if not recovered, the time T ₄ after the switching element 1 is always turned on. That is, the switching element 1 is not turned off when the mode is switched from the light load mode to the heavy load mode. For this reason, the maximum electric power can be supplied logically before and after the mode switching, so that even if the load current suddenly increases during the operation in the light load mode, the transient decrease of the output voltage can be minimized. be able to.

Further, since it is judged whether or not the mode switching is necessary based on the output voltage V _out at the predetermined timing of the power supply cycle, the function can be realized by the comparator and the gate circuit and the current consumption is reduced. small.
Therefore, the efficiency of the entire DC / DC converter is improved. In particular, the present embodiment is intended for the operation in the light load mode in which the load current is small, and reducing the ratio of the consumed current to the load current has a great effect on the improvement of the conversion efficiency. Further, when the circuit having the above function is configured by an IC, the area occupied by the chip becomes small.

Further, when it is judged whether or not the mode switching is necessary, it is not necessary to perform the integration / averaging process as in the prior art, and the switching element is in the ON process in the power supply cycle in the light load mode. Since the result can be obtained with, the mode can be switched in the shortest time. Therefore, it is possible to minimize the transient decrease in the output voltage V _out .

In the above embodiment, the step-down DC is used.
However, the present invention is not limited to this configuration. For example, a step-up DC
It can also be applied to a / DC converter and an inverting DC / DC converter.

[0052]

EFFECTS OF THE INVENTION Since the power of the maximum capacity of the DC / DC converter is supplied when the mode is switched from the light load mode to the heavy load mode, the output voltage is transiently lowered even when the load current sharply increases. Can be minimized.

Since the circuit for deciding whether or not the mode switching is necessary is composed of a circuit with a small current consumption, DC /
It is possible to reduce the current consumption of the entire DC converter.

[Brief description of drawings]

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

FIG. 2 is a time chart (No. 1) for explaining the operation of the DC / DC converter.

FIG. 3 is a diagram illustrating an operation of a comparator for detecting a coil current.

FIG. 4 is a time chart (No. 2) for explaining the operation of the DC / DC converter.

5A is a block diagram of a pulse width modulation control circuit, and FIG. 5B is a diagram for explaining signal processing in the pulse width modulation control circuit.

FIG. 6 is a block diagram of an example of a conventional DC / DC converter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 switching element 2, 3 comparator 4, 5 AND circuit 6, 7 selector 8 PWM control circuit (heavy load control circuit) 9 flip-flop 10 driver 11 delay circuit 12 control section 13 triangular wave generation section 14 comparison section 20 mode switching control circuit

Claims (5)

[Claims] 1. A DC / D having at least two operation modes for supplying power to a load, a light load mode and a heavy load mode, and controlling output according to a method corresponding to each operation mode.
A DC / DC converter in a C converter, wherein an operation mode is switched from a light load mode to a heavy load mode when an output value is below a reference value at a predetermined timing in a power supply cycle in the light load mode. 2. A DC / DC converter having at least two operation modes for supplying electric power to a load, a light load mode and a heavy load mode, and controlling output by changing a coil current according to a method corresponding to each operation mode. If the output value is less than the reference value when the coil current reaches the preset comparison value within the power supply cycle in the light load mode, the operation mode is switched from the light load mode to the heavy load mode. DC / DC converter that does. 3. A DC / DC converter which has at least two operation modes for supplying electric power to a load, a light load mode and a heavy load mode, and which controls output by changing a coil current according to a method corresponding to each operation mode. A voltage comparing means for comparing the output voltage with a preset predetermined voltage, a current comparing means for comparing the coil current with a preset predetermined current, and the voltage comparing means in the power supply cycle in the light load mode, The magnitude comparison between the output voltage and the predetermined voltage is judged when the coil current reaches the predetermined current by referring to the comparison result of the current comparison means. If the output voltage is less than the predetermined voltage, the mode switching signal is sent. And a control means for outputting, wherein the operation mode is switched from the light load mode to the heavy load mode in accordance with the mode switching signal. That DC / DC converter. 4. An operation mode for supplying electric power to a load has at least two operation modes, a light load mode and a heavy load mode, and a switching means is controlled by a method corresponding to each operation mode to change a coil current to output an output. DC to control
In the / DC converter, voltage comparing means for comparing the output voltage with a preset predetermined voltage, current comparing means for comparing the coil current with a preset predetermined current, and within the power supply cycle in the light load mode, Referring to the comparison results of the voltage comparison means and the current comparison means, when the coil current reaches the predetermined current, the magnitude relationship between the output voltage and the predetermined voltage is judged, and if the output voltage is the predetermined voltage or less, Control means for outputting a mode switching signal, and signal generation means for outputting a control signal for controlling the switching means so as to increase the coil current for a predetermined period after receiving the mode switching signal from the control means. A DC / DC converter having. 5. The DC / DC converter according to claim 4, wherein the switching means is controlled by a method corresponding to the heavy load mode after the lapse of the predetermined time.