JP3444468B2 - 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, and more particularly to a DC-DC converter including a switching element, a bootstrap circuit, a control circuit, and a switching element drive circuit.

[0002]

2. Description of the Related Art FIG. 5 shows a circuit diagram of a conventional DC-DC converter. The DC-DC converter 1 is a separately excited step-down type, and has input terminals V1 and V1 'and output terminals V2 and V2'.
Between the input capacitor Ci and the switching element, for example, an N-channel MOS-FET (hereinafter, N-MOSFE
T) Q1, diode D1 and smoothing circuit 2 are inserted. Then, between the drain D and the source S of the N-MOSFET Q1, when the N-MOSFET Q1 is turned off, the diode DB is used to calculate the difference between the input voltage Vi and the voltage at the connection point A between the N-MOSFET Q1 and the diode D1. A circuit for charging the capacitor CB, a so-called bootstrap circuit 3, is connected via the connection point B between the diode DB of the bootstrap circuit 3 and the capacitor CB, and N−
The switching element drive circuit 4 is connected between the gate of the MOSFET Q1. Therefore, the switching element drive circuit 4 obtains the gate voltage for turning on the N-MOSFET Q1 from the connection point B between the diode DB and the capacitor CB forming the bootstrap circuit 3.

A diode D1, a smoothing circuit 2 and an output voltage detecting voltage dividing resistor 5 are connected to the output side of the N-MOSFET Q1. Among them, the smoothing circuit 2 includes a coil L1 and an output capacitor Co, and the output voltage detecting voltage dividing resistor 5 includes a series circuit of a resistor R1 and a resistor R2. Further, the resistors R1 and R2 of the output voltage detecting voltage dividing resistor 5 are used.
An error amplification circuit 6 and a pulse width modulation circuit 7 are connected between the connection point of 1 and the switching element drive circuit 4.

In the DC-DC converter 1 configured as described above, the output voltage Vo is the voltage dividing resistor 5 for detecting the output voltage.
A voltage proportional to the output voltage Vo is divided by the resistors R1 and R2 and is compared with the reference voltage by the error amplification circuit 6, and when the voltage proportional to the output voltage Vo becomes lower than the reference voltage, the pulse width An on-duty D, which is an on-time ratio per switching time of the N-MOSFET Q1 via the modulation circuit 7 and the switching element drive circuit 4.
To increase the energy transmitted to the output side,
The output voltage Vo is increased. On the other hand, when the voltage proportional to the output voltage Vo becomes higher than the reference voltage, N is passed through the pulse width modulation circuit 7 and the switching element drive circuit 4.
-Reduce the on-duty D of MOSFET Q1,
The energy transmitted to the output side is reduced and the output voltage Vo is reduced. The above operation is repeated to stabilize the output voltage Vo.

[0005]

However, in the conventional DC-DC converter as described above, the input voltage drops, the output current becomes small, and when the load condition approaches, the charging voltage by the bootstrap circuit drops. Since the drive voltage to the switching element is lowered, there is a problem that the switching element does not operate.

The present invention has been made in order to solve such a problem, and an object thereof is to provide a DC-DC converter capable of completely driving a switching element from no load to light load. To do.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a switch composed of an N-MOSFET.
Connected to the switching element and the control terminal of the switching element.
Switching element drive circuit and the switching element
Driving the switching element via a drive circuit
Pulse width modulation circuit and compare / amplify output voltage with reference voltage
Then, by inputting to the pulse width modulation circuit,
An error amplifier that controls the pulse width of the switching element,
The drain and source of the switching element and the switch
A bootstrap connected to the switching device drive circuit
Separately-excited step-down DC-DC converter having
The bootstrap circuit is
A diode with an anode connected to the drain of the ching element.
The cathode of the diode and the switching element
With a capacitor connected between the sources of
In the connection point of the diode and the capacitor,
Connected to the switching element drive circuit, load
The output voltage or to detect the magnitude of the ripple voltage of the input <br/> force voltage, frequency of changing the switching frequency of the switching element in proportion to the magnitude of the ripple voltage varies dependent on severity It is characterized by having a variable circuit.

Further, the frequency variable circuit is controlled by an external signal.

Further, the resistance value of the resistor constituting the frequency variable circuit is controlled by the ripple voltage of the output voltage or the input voltage which changes depending on the weight of the load .

According to the DC-DC converter of the present invention,
Since a frequency variable circuit that controls the oscillation frequency that controls the pulse width modulation circuit in proportion to the output voltage or the ripple voltage of the input voltage is provided, switching of the switching element is performed according to the magnitude of the ripple voltage. The frequency can be changed.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. In each of the embodiments, the same or equivalent parts as those of the conventional example are designated by the same reference numerals, and detailed description thereof will be omitted.

FIG. 1 shows a circuit diagram of a first embodiment of a DC-DC converter according to the present invention. The DC-DC converter 10 includes input terminals V1 and V1 ′ and output terminals V2 and V2.
An input capacitor Ci, a switching element such as an N-MOSFET Q1, a diode D1, a smoothing circuit 2, and an output voltage detecting voltage dividing resistor 5 are inserted between the input capacitor Ci and 2 '.

Further, the drain D of the N-MOSFET Q1
The bootstrap circuit 3 is connected between the source and the source S, and the diode D forming the bootstrap circuit 3 is connected.
Connection point B between B and capacitor CB and N-MOSFET Q
The switching element drive circuit 4 is connected between the gate G and the gate 1.

The smoothing circuit 2 comprises a coil L1 and an output capacitor Co, and the output voltage detecting voltage dividing resistor 5 comprises a series circuit of a resistor R1 and a resistor R2. Further, between the connection point of the resistors R1 and R2 of the output voltage detecting voltage dividing resistor 5 and the switching element drive circuit 4, the error amplification circuit 6 is provided.
And the pulse width modulation circuit 7 are connected. Further, a frequency variable circuit 11 is connected between the output terminal V2 and the pulse width modulation circuit 7, and the frequency variable circuit 11 determines an oscillation frequency for controlling the pulse width modulation circuit 7. And a ripple voltage detection circuit 1 for detecting the output ripple voltage Vorip generated at the output terminal V2.
3 and 3.

In the DC-DC converter 10 configured as described above, the ripple voltage detection circuit 13 detects the output ripple voltage Vorip, and the output ripple voltage Vo.
The oscillation frequency of the oscillation circuit 12 is determined according to the magnitude of rip. Then, the pulse width modulation circuit 7 determines the pulse width sent to the switching element drive circuit 4 according to the oscillation frequency, and according to the pulse width,
By the switching element drive circuit 4, N-MOS
The switching frequency fsw of the FET Q1 is determined.

FIG. 2 shows a circuit diagram of a second embodiment of the DC-DC converter according to the present invention. The DC-DC converter 20 is the DC-DC converter 10 of the first embodiment.
Although the configuration is almost the same as that of FIG. 1, the input terminal V1
Generated at the input voltage, that is, the input ripple voltage Virip
Is different in that it detects.

In the DC-DC converter 20 configured as described above, the ripple voltage detecting circuit 13 detects the input ripple voltage Virip, and the input ripple voltage Vi is detected.
The oscillation frequency of the oscillation circuit 12 is determined according to the magnitude of rip. Then, the pulse width modulation circuit 7 determines the pulse width sent to the switching element drive circuit 4 according to the oscillation frequency, and according to the pulse width,
By the switching element drive circuit 4, N-MOS
The switching frequency fsw of the FET Q1 is determined.

DC of the first and second embodiments as described above
-The DC converter includes the frequency variable circuit that changes the oscillation frequency that controls the pulse width modulation circuit in proportion to the magnitude of the output ripple voltage or the input ripple voltage. Therefore, according to the magnitude of the ripple voltage, NM
The switching frequency of the OSFET can be changed. Therefore, even if the input voltage decreases, the output current decreases, and the N-MOSFET is approached even when it approaches an unloaded state.
Since the switching frequency of the N-MOSFET can be reduced and the N-MOSFET can be completely driven, a stable output voltage can be obtained regardless of the level of the input voltage and the size of the output current.

From no load to light load, N
Since the switching frequency of the MOSFET can be lowered, the loss due to the switching operation can be reduced, and as a result, the efficiency of the DC-DC converter can be increased.

FIG. 3 shows a circuit diagram of a third embodiment of the DC-DC converter according to the present invention. The DC-DC converter 30 is the DC-DC converter 10 of the first embodiment.
The configuration of the frequency variable circuit 31 is different from that of FIG.

The frequency variable circuit 31 is composed of an oscillation circuit 12, a ripple voltage detection circuit 13 and an on / off circuit 32. The frequency variable circuit 31 is turned on / off by an external signal S input to the on / off circuit 32. Is to control.

In the DC-DC converter of the third embodiment as described above, since the ON / OFF of the frequency variable circuit is controlled by an external signal, it is used in a radio device or the like which does not like noise, or DC-DC converter. When using in a high output state where the noise generated by the DC converter becomes large,
By turning off the frequency variable circuit, the switching frequency of the switching element can be made constant. Therefore, the frequency band of generated noise can be kept constant.

FIG. 4 shows a circuit diagram of a fourth embodiment of the DC-DC converter according to the present invention. The DC-DC converter 40 is the DC-DC converter 10 of the first embodiment.
The configuration of the frequency variable circuit 41 is different from that of (FIG. 1).

The frequency variable circuit 41 includes resistors R3, R4,
A capacitor C1, a switching element such as a bipolar transistor Q2, an oscillator OSC, a smoothing circuit 42,
It is composed of an AC amplification circuit 43 and a ripple voltage detection circuit 13.

The collector of the bipolar transistor Q2 is connected to the pulse width modulation circuit 7 via the oscillator OSC.
And is grounded via a resistor R3. The emitter of the bipolar transistor Q2 is grounded via the resistor R4, and the smoothing circuit 42, the AC amplification circuit 43, and the ripple voltage detection circuit 13 are provided between the base and the output terminal V1.
Are connected. Further, the capacitor C1 is connected between the oscillator OSC and the ground.

With the above configuration, the output ripple voltage Vori
p is detected by the ripple voltage detection circuit 13 of the frequency variable circuit 41, the detected voltage is amplified by the AC amplification circuit 43, the amplified voltage is smoothed by the smoothing circuit 42, and bipolar is performed by the smoothed voltage. The current flowing between the collector and the emitter of the transistor Q2 is controlled.

As a result, when operating without a load, the output ripple voltage Vorip is small, so that the bipolar transistor Q2 is almost turned off, and the N-MOSFET Q1 is switched at the switching frequency determined by the resistor R3 and the capacitor C1. The operation will be performed.

When the output ripple voltage Vorip gradually increases and the base voltage of the bipolar transistor Q2 increases accordingly, the current flowing through the resistor R4 also increases, and the apparent resistance value of the resistor R4 decreases. As a result, the N-MOSFET Q determined by the combined resistance of the resistors R3 and R4 and the capacitor C1.
The switching frequency of 1 gradually increases.

In the DC-DC converter of the fourth embodiment as described above, the apparent resistance value of the resistor R4 can be changed by changing the current flowing through the resistor R4 at the output ripple voltage Vorip. Because you can, N
-The switching frequency of the MOSFET Q1 can be easily changed.

[0030]

According to the DC-DC converter of the first aspect of the present invention, the DC-DC converter is provided with the frequency variable circuit for changing the oscillation frequency for controlling the pulse width modulation circuit in proportion to the magnitude of the output voltage or the ripple voltage of the input voltage. Therefore, the switching frequency of the switching element can be changed according to the magnitude of the ripple voltage. Therefore, even if the input voltage drops, the output current becomes small, and the switching element's switching frequency can be lowered to completely drive the switching element even when approaching a no-load state, regardless of whether the input voltage is high or low, and the output current is large or small. A stable output voltage can be obtained.

Further, since the switching frequency of the switching element can be lowered from no load to light load, the loss due to the switching operation can be reduced, and as a result, the efficiency of the DC-DC converter can be increased.

According to the second aspect of the DC-DC converter, since the frequency variable circuit is turned on and off by the external signal, the frequency variable circuit is used in a radio device or the like which is not susceptible to noise, or when the DC-DC converter is generated. When used in a high output state where noise becomes large, the switching frequency of the switching element can be made constant by turning off the frequency variable circuit. Therefore, the frequency band of generated noise can be kept constant.

According to the DC-DC converter of claim 3, in the ripple voltage of the output voltage or the input voltage,
Since the apparent resistance value of the resistor can be controlled by controlling the current flowing through the resistor forming the frequency variable circuit, the switching frequency of the switching element can be easily controlled.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram of a second embodiment according to the DC-DC converter of the present invention.

FIG. 3 is a circuit diagram of a third embodiment according to the DC-DC converter of the present invention.

FIG. 4 is a circuit diagram of a fourth embodiment according to the DC-DC converter of the present invention.

FIG. 5 is a circuit diagram of a conventional DC-DC converter.

[Explanation of symbols]

3 Bootstrap circuit 4 Switching element drive circuit 6 Error amplification circuit 7 Pulse width modulation circuit 10, 20, 30, 40 DC-DC converter 11, 31, 41 Frequency variable circuit CB capacitor DB diode Q1 switching element R4 resistance External signal Vorip, Virip ripple voltage

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-133658 (JP, A) JP-A-4-331463 (JP, A) JP-A-5-304768 (JP, A) Actual Kaihei 3- 54383 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) H02M 3/00

Claims (3)

(57) [Claims] 1. A switching comprising an N-MOSFET.
Element and a switch connected to the control terminal of the switching element.
The switching device drive circuit and the switching device drive circuit.
A drive that drives the switching element via a live circuit.
The pulse width modulation circuit and the output voltage are compared and amplified with the reference voltage.
Input to the pulse width modulation circuit
An error amplifier that controls the pulse width of the
The drain and source of the switching element and the switch
The boot strap connected to the
Separately-excited step-down DC-DC converter having
In the bootstrap circuit,
A diode whose anode is connected to the drain and
Between the cathode of the ode and the source of the switching element
And a capacitor connected to
The connection point between the capacitor and the capacitor is the switching element.
It is connected to the child drive circuit, the output voltage or before changes depending on severity of the load
Serial to detect the magnitude of the ripple voltage of the input voltage, DC-DC converter, wherein in proportion to the magnitude of the ripple voltage, further comprising a frequency variable circuit for varying the switching frequency of the switching element. 2. The DC-DC converter according to claim 1, wherein the frequency variable circuit is controlled by an external signal. 3. The resistance value of a resistor forming the frequency variable circuit is controlled by a ripple voltage of the output voltage or the input voltage that changes depending on the weight of the load. DC-D according to claim 2.
C converter.