JP2002281744A - Dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC-DC converter whose light load efficiency is improved. SOLUTION: Potentials of both ends of a sensing resistor 25 are inputted to a comparator 31 via an offset power supply 32. If a potential of a non-reverse input terminal (+) is lower than a potential of a reverse input terminal (-) to which an offset voltage is added, it is detected that a load is light. If the light load is detected by the comparator 31, an oscillator 30 lowers a frequency of a clock signal CK and a control circuit 29 controls a MOS-FET 2 to be turned off until a reverse direction current is supplied to a choke coil 24.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a DC-DC converter using a synchronous rectification system.

[0002]

2. Description of the Related Art A conventional step-down type D using a synchronous rectification method.
An example of the C-DC converter will be described with reference to FIG.
In the figure, a DC-DC converter 100 has an input terminal 1 connected to an input power source such as a battery, and an output terminal 2 connected to a load. P-channel MOSFET 3 at input terminal 1
Are connected, and a series circuit composed of a choke coil 4 and a sense resistor 5 is connected between the MOSFET 3 and the output terminal 2. This series circuit and output terminal 2
The smoothing capacitor 6 is connected between the connection point of the capacitor and the ground. An N-channel MOSFET 7 is connected between the connection point of the series circuit and the MOSFET 3 and the ground, with a drain and a source, and a commutation diode 8 is connected in parallel with the MOSFET 7 with a drain and a cathode and a source and an anode. Have been. In addition, the DC-DC converter 100 includes the MOSFET 3 and the MO for controlling ON / OFF of the MOSFET 3 and the MOSFET 7.
A control circuit 9 for controlling the gate potential of the SFET 7;
An oscillator 10 that supplies a clock signal for generating a control signal to the control circuit 9 is provided. Further, the comparator 1 detects the direction of the current flowing through the choke coil 4.
1 and the potential at both ends of the sense resistor 5 is compared with the comparator 1
1 and the output is supplied to the control circuit 9.

Next, an input voltage Vin is supplied from an input power supply to an input terminal 1 of the DC-DC converter 100, a clock signal CK is supplied from an oscillator 10 to a control circuit 9, and a load is connected to an output terminal 2. The operation at this time will be described with reference to FIG. First, when the load is heavy, at time T1 when the pulse of the clock signal CK is supplied, the control circuit 9 controls the MOSFET 3 to be turned on and the MOSFET 7 to be turned off.
SFET3 → choke coil 4 → the current flows in the path of the sense resistor 5 → load, current flowing through the choke coil 4 I _L
Increases, and the voltage Vo at the output terminal 2 also increases.

On / off determined by the ratio of input voltage to output voltage
At time T2 when the off-duty becomes, for example, 50%, voltage Vo reaches threshold voltage Vth, and MOSF
ET3 is turned off and the MOSFET 7 is turned on, and a current flows from the back electromotive force generated in the choke coil 4 through the path of the sense resistor 5 → the load → the MOSFET 7 to supply the next pulse of the clock signal CK at time T3. until the time, current I _L flowing through the choke coil 4 is reduced, also reducing the voltage Vo at the output terminal 2.

[0005] During the heavy load after the time T3, similarly, MOSFET3,7 is on / off control, this time, current _{I L} flowing through the choke coil 4 is repeatedly increase and decrease, the voltage Vo at the output terminal 2 The increase / decrease to the threshold voltage Vth is repeated.

Next, at time t1, the control circuit 9 controls the MOSFET 3 to be turned on, and the MOSFET 3
When T7 is off-controlled and the load becomes light at that time,
The voltage Vo at the output terminal 2 increases more than when the load is heavy.
At time t2 when the ET7 is controlled to be turned on, the overshoot increases, and the voltage Vo becomes equal to the threshold voltage Vth.
Higher.

[0007] point MOSFET3 off control in the time t2, and MOSFET7 is once controlled to be turned on, since the load is a light load, the current _{I L} flowing through the choke coil 4 is reduced faster than when a heavy load, the time t3 At this time, the current flowing through the choke coil 4 becomes zero, and a current in the opposite direction tends to flow. At this time, the potential at both ends of the sense resistor 5 is input to the comparator 11, and when the potential at the non-inverting input terminal (+) becomes lower than the potential at the inverting input terminal (-), the direction of the current flowing through the choke coil 4 is reversed. It is detected that there is. When the current in the opposite direction is detected by the comparator 11, the control circuit 9
7 is off-controlled, the current flowing through the choke coil 4 I _L
Becomes zero at time t4. From time t2 to time t4, voltage Vo at output terminal 2 slightly decreases.

[0008] Thereafter, MOSFET3,7 is controlled to be turned off, current _{I L} flowing through the choke coil 4 remains zero,
The voltage Vo at the output terminal 2 gradually decreases, and the clock signal C
After a predetermined number of K pulses have elapsed due to the lightness of the load, the voltage Vo drops below the threshold voltage Vth.
Then, at time t5 when the pulse of the clock signal is supplied immediately after the voltage Vo falls below the threshold voltage Vth, the control circuit 9 controls the MOSFET 3 to be turned on.
And MOSFET 7 are turned off, and MOSFET 3
There up to the point of time t6 off control, and MOSFET7 are on-controlled, the current flowing through the choke coil 4 _{I L}
Increases, and the voltage Vo at the output terminal 2 also increases. The voltage Vo of the output terminal 2 becomes higher at the time t6 than at the time of heavy load, and becomes the same level as that at the time t2. But,
Since the load is light, at time t6, the choke coil 4
The only current does not flow I _L less than the heavy load.

When MOSFET 3 is turned off and MOSFET 7 is turned on at time t6, time t
3, the choke coil 4 at the time t7
As current is zero, will further opposite direction current to flow that flows in the opposite direction of the current is detected by the comparator 11, the control circuit 9 MOSFET 7 is turned off control, the current I _L t8 flowing through the choke coil 4 It becomes zero at that point.

[0010] after the timing time t8, the as well as from the time t4 t8, MOSFET3,7 is on / off control, this time, current _{I L} flowing through the choke coil 4 is repeated, including opposing current decrease, the output The voltage Vo at the terminal 2 also increases and decreases repeatedly.

[0011]

[SUMMARY OF THE INVENTION Incidentally, the detection of a light load, as described above, the current I _L flowing through the choke coil 4 becomes zero decreases, when the further opposing current is going to flow Since the detection is performed by detecting the potential at both ends of the sense resistor 5 by the comparator 11, a current in the opposite direction flows through the choke coil 4 until the comparator 11 detects a light load and the MOSFET 7 is turned off. There is a problem that efficiency at light load deteriorates. The present invention has been made in view of the above-described problems, and has been made in consideration of the above-described problems, by providing a DC-DC converter that prevents current from flowing in an opposite direction through a choke coil at a light load.
An object is to provide a DC converter.

[0012]

SUMMARY OF THE INVENTION A DC-DC converter according to the present invention is a synchronous rectification type DC-DC converter that controls supply of input power to a choke coil to step down or step up to a predetermined voltage and output the voltage. A sense resistor that is connected in series with the choke coil, and a sense resistor that detects the current of the choke coil, and is connected to one end of both ends of the sense resistor. An offset power supply for offsetting the potential of the one end of the sense resistor for setting during the period in which it is occurring, and connected to both ends of the sense resistor via the offset power supply, and the current of the choke coil is equal to or less than the threshold current. A comparator for outputting a signal for stopping synchronous rectification. Further, the DC-DC converter of the present invention has the above-mentioned DC-DC converter.
In the DC converter, the oscillation frequency of the oscillator is reduced at a light load by the output of the comparator.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIG. In the figure, reference numeral 200 denotes a step-down DC-DC converter, wherein an input terminal 21 is connected to an input power source such as a battery, and an output terminal 22 is connected to a load. The configuration of the DC-DC converter 200 is such that the input terminal 2
1, a source of a P-channel MOSFET 23 is connected, and a series circuit including a choke coil 24 and a sense resistor 25 is connected between the MOSFET 23 and the output terminal 22. A smoothing capacitor 26 is connected between the connection point between the series circuit and the output terminal 22 and the ground. An N-channel MOSFET 27 is connected between the connection point of the series circuit and the MOSFET 23 and the ground, with a drain and a source, and a commutation diode 28 is connected in parallel with the MOSFET 27 with a drain and a cathode and a source and an anode. Have been. The DC-DC converter 200 includes a MOSFET 23 and a MOSFET
The control circuit 29 includes a control circuit 29 that controls the gate potential of the MOSFET 23 and the MOSFET 27 to control ON / OFF of the MOSFET 27, and includes an oscillator 30 that supplies the control circuit 29 with a clock signal for generating a control signal. further,
In order to detect a light load by comparing the potential difference between both ends of the sense resistor 25 with the offset voltage, a comparator 31 and an offset power supply 32 are provided. The input of the inverting input terminal (+) and the potential of the output terminal 22 on both ends of the sense resistor 25 are used as the input of the inverting input terminal (-) of the comparator 31 via the offset power supply 32, and the output is used as the control circuit 29. And the oscillator 30. In addition, instead of the P-channel MOSFET 23, an N-channel MOSFET is replaced by a series circuit including a choke coil 24 and a sense resistor 25, and a drain between the input terminal 21 and the source between the series circuit and the input terminal 21. You may connect. Further, the offset power supply 32 may be connected in reverse between the input terminal 21 end of the sense resistor 25 and the non-inverting input terminal (+) of the comparator 31. When the potential difference between both ends of the sense resistor 25 falls from the offset voltage or more to the offset voltage or less, the comparator 31 outputs a signal for stopping synchronous rectification at the offset voltage or less. At this time, if a signal having a slight hysteresis voltage is used so as to output a signal for stopping synchronous rectification up to a voltage slightly higher than the offset voltage, light load can be detected smoothly.

Next, an input voltage Vin is supplied from an input power supply to an input terminal 21 of the DC-DC converter 200, a clock signal CK is supplied from an oscillator 30 to a control circuit 29, and a load is connected to the output terminal 22. The operation at this time will be described with reference to FIG. First, when the load is heavy,
At the time T1 at which the pulse of the clock signal CK is supplied, the MOSFET 23 is turned on and the MOSFET 27 is turned off by the control circuit 29, and the current flows through the input terminal 21 from the MOSFET 23 → the choke coil 24 → the sense resistor 25 → the load. flow, current _{I L} flowing through the choke coil 24 increases, also increases the voltage Vo at the output terminal 22.

On / off determined by the ratio of the input voltage to the output voltage
At time T2 when the off-duty becomes, for example, 50%, voltage Vo reaches threshold voltage Vth, and MOSF
The ET 23 is turned off and the MOSFET 27 is turned on, and a current flows from the back electromotive force generated in the choke coil 24 through the path of the sense resistor 25 → the load → the MOSFET 27, at the time T3 when the next pulse of the clock signal CK is supplied. to the point, the current flowing through the choke coil 24 _{I L}
And the voltage Vo at the output terminal 22 also decreases.

[0016] During the heavy load after the time T3, similarly, MOSFET23,27 is on / off control, this time, current _{I L} flowing through the choke coil 24 is repeatedly increase and decrease, the voltage Vo at the output terminal 22 The increase / decrease to the threshold voltage Vth is repeated.

Next, at time t1, the control circuit 29 controls the MOSFET 23 to be turned on,
When the FET 27 is turned off and the load becomes lighter at that time, the rise of the voltage Vo at the output terminal 22 becomes larger than at the time of heavy load, and then at the time t2 when the MOSFET 23 is turned off and the MOSFET 27 is turned on. , The overshoot increases, and the voltage Vo becomes higher than the threshold voltage Vth.

When the MOSFET 23 is turned off and the MOSFET 27 is turned on at time t2, the current I flowing through the choke coil 24 is reduced because the load is light.
_L decreases faster than under heavy loads. At this time, the potential between both ends of the sense resistor 25 is input to the comparator 31 via the offset power supply 32, and the time until the back electromotive force is generated in the choke coil 24, that is, the time until the current _IL becomes zero. at the time of t3 ', the current I _L is the inverting input terminal (-) of the potential of the potential and the non-inverting input terminal (+) becomes equal to the threshold current Ith of. If additional current I _L is about to decrease, an inverting input terminal to which an offset voltage is added (-) potential of the non-inverting input terminal (+) becomes lower than the potential of, it is detected that a light load, the control circuit 29
The MOSFET27 is off-controlled, then the current _{I L} flowing through the choke coil 24, the commutation diode 28
, Becomes zero at time t4 ′, and stops the synchronous rectification. Between time t2 and t4 ′, the voltage Vo at the output terminal 22 slightly decreases. When the comparator 31 detects that the load is light, the oscillator 30 lowers the frequency of the clock signal CK.

[0019] Thereafter, MOSFET23,27 is controlled to be turned off, the current _{I L} flowing through the choke coil 24 remains zero, the voltage Vo at the output terminal 22 gradually decreases. Then, at time t5 when the pulse of the clock signal is supplied immediately after the voltage Vo falls below the threshold voltage Vth, the MOSFET 23 is turned on by the control circuit 29 while the MOSFET 27 is kept off, and the MOSFET is turned on.
T27 until the point of time T6 of MOSFET23 remains off control is off-controlled, the current _{I L} flowing through the choke coil 24 increases, also increases the voltage Vo at the output terminal 22. The voltage Vo of the output terminal 22 becomes higher at the time t6 than at the time of heavy load, and becomes the same level as that at the time t2. However, since the load is light, at time t6, the choke coil 24, the threshold current Ith only flow following the current I _L less than the heavy load.

[0020] At time t6, the choke coil 24 is detected light load by the comparator 31 does not flow through only the threshold current Ith following current _{I L,} MOSFET 27 by the control circuit 29 stop the synchronous rectification remains off control In this case, the MOSFET 23
When There is off-controlled, 'as with the time of the current I _L flowing through the choke coil 24 flows through the commutation diode 28, a time t8' t4 becomes zero at the time of.

After time t8 ', t4'
Similarly to 8 ', the MOSFET 27 remains off-control,
MOSFET23 is on / off control, this time, current _{I L} flowing through the choke coil 24 repeats the increase and decrease in the following threshold current Ith, the voltage V of the output terminal 22
o also increases and decreases repeatedly.

As described above, when the load is light, the oscillation frequency of the clock signal from the oscillator 30 is lowered, and the MOSFET 27 is turned off until the current in the opposite direction flows through the choke coil 24.
, The switching loss of the MOSFET 27 can be prevented, and the efficiency at light load can be improved.

In the above embodiment, the step-down type DC-DC
Although converter 200 has been described, the present invention is not limited to this, and is also applicable to a step-up DC-DC converter and a step-up / step-down DC-DC converter. Also,
The present invention is also applicable to a DC-DC converter whose polarity is inverted.

[0024]

As described above, in the DC-DC converter according to the present invention, when the load is light, the oscillation frequency of the clock signal is reduced, the current consumption of the control circuit can be reduced, and no current flows in the choke coil in the opposite direction. Therefore, switching loss can be prevented, and efficiency at light load can be improved.

[Brief description of the drawings]

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

FIG. 2 is a time chart for explaining the operation of the DC-DC converter shown in FIG.

FIG. 3 is a block diagram of a conventional DC-DC converter.

FIG. 4 is a time chart for explaining the operation of the DC-DC converter of FIG. 3;

[Explanation of symbols]

 Reference Signs List 23 P-channel MOSFET 24 Choke coil 25 Sense resistor 27 N-channel MOSFET 28 Commutation diode 29 Control circuit 30 Oscillator 31 Comparator 32 Offset power supply 200 DC-DC converter

Claims (2)

[Claims] 1. A synchronous rectification type DC-DC converter for controlling the supply of an input power to a choke coil and stepping down or boosting the voltage to a predetermined voltage and outputting the voltage is connected in series to the choke coil and detects the current of the choke coil. To set a threshold current of a choke coil for detecting a light load during a period in which a back electromotive force is generated in the choke coil. An offset power supply for offsetting the potential of the terminal and a comparator connected to both ends of the sense resistor via the offset power supply and outputting a signal for stopping the synchronous rectification when the current of the choke coil is equal to or less than the threshold current. Characteristic DC-DC converter. 2. An oscillator according to claim 1, wherein the output of said comparator lowers the oscillation frequency of the oscillator at light load.
A DC-DC converter as described.