JP2001161068A - Dc-dc converter with feeding power limiting function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To sufficiently protect a DC-DC converter or the like with a protection device for limiting the continuity period of the switching transistor to a constant width or less, while a load is in the short-circuit condition even when a higher input power supply voltage is supplied under large variation of the input power supply voltage. SOLUTION: There is provided a DC-DC converter which does not supply an energy higher than the constant value without relation to the amplitude of input power supply voltage when a load is in the short-circuit condition by providing a pulse width limiting device, which varies the pulse width of the switching pulse depending on the amplitude of the input power supply voltage. Since the supply power of the DC-DC converter is limited for the short- circuit of load, the DC-DC converter and load are never broken and moreover an over-current detecting device is no longer required.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protection device for a DC-DC converter.

[0002]

2. Description of the Related Art A DC-DC converter uses a switching transistor to turn on / off a current flowing from a DC input power supply.
It performs FF (switching), smoothes it with a coil and a capacitor, and converts it into a different DC voltage. [Explanation of FIG. 4] FIG. 4 shows a configuration diagram of a conventional DC-DC converter. In FIG. 4, a DC-DC converter includes a DC power supply input terminal 11, a triangular wave generator 15, a PWM
Comparator 20, error amplifier 19, reference voltage generator 1
6, output voltage converter 17, switching transistor 2
1, a smoothing coil 22, a smoothing capacitor 23, a rectifier diode 24, a pulse width limited voltage setting unit 25, a reference voltage setting unit 18, an output terminal 13, and an overcurrent detection unit 30.

Next, the operation of the conventional DC-DC converter shown in FIG. 4 will be described. DC power input terminal 11
To the DC power supply. The DC power supply is, for example, a rechargeable battery, and changes, for example, from +17 V in a fully charged state to +10 V in a state of advanced discharge. The power supply voltage of the DC power supply is represented as + B. In the triangular wave generator 15,
Generates a triangular wave with a fixed period. This triangular wave output voltage is input to the positive input terminal of the PWM comparator 20. The cycle of the switching pulse of the DC-DC converter is determined by the triangular wave generator 15. The output voltage converter 17 is composed of resistors R1 and R2. The output voltage converter 17 receives the output voltage Vout of the DC-DC converter generated at the hot side terminal (terminal not grounded) of the smoothing capacitor 23, and The output voltage is divided by R1 and R2, and an output voltage R2 × Vout / (R1 + R2) is output. The output voltage of the output voltage converter 17 is input to the error amplifier 19.

In a stable state, the values of R1 and R2 are determined so that the output voltage R2 × Vout / (R1 + R2) of the output voltage converter 17 substantially matches the output voltage Vref of the reference voltage setting unit 18. That is, the output voltage Vout of the DC-DC converter is approximately (R1 + R2) × Vref
/ R2. The reference voltage setting unit 18 receives the reference voltage output from the reference voltage setting unit 16 and outputs an output voltage Vref that is divided by two resistors. Output voltage Vref
Is input to another input terminal of the error amplifier 19. The error amplifier 19 compares two input voltages of the output voltage of the output voltage conversion unit 17 and the output voltage Vref of the reference voltage setting unit 18, and generally, a power supply voltage of the error amplifier 19 +
A difference voltage (referred to as an error voltage) R2 × Vout / (R1 + R2) − centered on + B / 2 which is a half value of B
Vref is amplified by k times and output. Therefore, the error amplifier 19 calculates the amplified error voltage + B / 2 + k × ｛R2 × V
out / (R1 + R2) -Vref} is output. The output voltage of the error amplifier 19 is input to a negative input terminal of the PWM comparator 20. The pulse width limiting voltage setting unit 25 receives the output voltage of the reference voltage generating unit 16 and outputs a pulse width limiting voltage that is a constant voltage obtained by dividing the resistance by two resistors. The pulse width limit voltage is PWM
The signal is input to another negative input terminal of the comparator 20.

[Description of FIG. 7] FIG. 7 shows an example of the internal configuration of the PWM comparator 20. The internal configuration of the PWM comparator 20 is not limited to the example of the internal configuration in FIG. 7, and may have another configuration. + B indicates a power supply voltage. The plus sign indicates a positive input terminal, and the two minus signs indicate negative input terminals. As described above, the positive input terminal is connected to the triangular wave generator 15, and the two negative input terminals are connected to the pulse width limited voltage setting unit 25 and the error amplifier 19.

The three input voltages are input to the bases of NPN transistors 73, 72 and 71, respectively.
The emitter terminals of the two NPN transistors are connected to each other. Only the transistor input based on the highest input voltage among the three input voltages becomes conductive,
The highest input voltage V is applied to the emitter of the transistor.
A voltage Vin-Vbe is obtained by subtracting the base-emitter voltage Vbe of the transistor from in. The base-emitter voltage Vbe of the conducting transistor is
Usually, it is about 0.7V. The other two transistors input based on the input voltage other than the highest input voltage,
Since the voltage Vbe between the base and the emitter is smaller than 0.7 V or reverse-biased, a cutoff state occurs. Therefore, the transistor 73 whose base terminal is connected to the positive terminal is turned on only when the input voltage input to the positive terminal from the triangular wave generator 15 is the highest among the three input voltages. In this case, a cutoff state is set.

[0007] The base of the PNP transistor 74 is connected to the collector of the transistor 73 via a resistor. Therefore, the base current of the transistor 74 flows only when the transistor 73 is turned on, the transistor 74 is turned on, and the PWM comparator 20 outputs a voltage of + B. When the transistor 73 is in the cut-off state, the base current of the transistor 74 does not flow, the transistor 74 is in the cut-off state, and the PWM comparator 20 has 0 V
Is output. From the above configuration, the PWM comparator 20
Outputs a + B voltage only when the voltage input from the triangular wave generator 15 to the positive input terminal is the highest, and outputs 0 V otherwise.

[Continuation of FIG. 4] Referring to FIG.
When the output of the M comparator 20 outputs the + B voltage, the switching transistor 21 is turned ON (conduction state), and a current flows from the DC power input terminal 11 to the output terminal 13 through the smoothing coil 22. Further, the PWM comparator 20 Output 0 V, the switching transistor 21 is turned off (cut-off state), the current flowing from the DC power input terminal 11 to the output terminal 13 is cut off, and during that time, the smoothing coil 22 continues to flow the current.
At this time, the rectifier diode 24 clamps the emitter potential of the switching transistor 21. The output voltage of the output terminal 13 becomes constant by the smoothing action of the coil 17 and the capacitor 18.

ON / O of switching transistor 21
The output of the PWM comparator 20 for controlling the FF is referred to as a “switching pulse”, and the period during which the switching transistor 21 is turned on during one cycle of the triangular wave output voltage generated by the triangular wave generator 15 is referred to as the “pulse width of the switching pulse”. To tell. Switching transistor 21
Is a period during which the PWM comparator 20 outputs the + B voltage in the present embodiment, but is not limited to this.
When 0 outputs a voltage lower than the + B voltage (for example, + B-1.4 V), the switching transistor 21
N period or when the switching transistor 21
A transistor whose PWM comparator 20 is at 0V
Is output, the period during which the switching transistor 21 is turned on is also included.

[Description of FIG. 5] FIG. 5 shows a triangular wave output voltage of the triangular wave generator 15, an output voltage of the error amplifier 19, and a pulse width limit, which are three input voltages of the PWM comparator 20, in the conventional example of FIG. The relationship between the output voltage of the voltage setting unit 25 and the output voltage of the PWM comparator 20 is shown for a normal operation state. 5, FIG. 6, FIG. 2, and FIG. 3, “a” indicates a triangular wave output voltage of the triangular wave generator 15. b indicates the output voltage of the error amplifier 19. c indicates the output voltage of the pulse width limited voltage setting unit 25. d is P
4 shows an output voltage of the WM comparator 20. e indicates a limit width which is the maximum value of the pulse width of the switching pulse.

The normal state is a state in which a correct load is connected to the DC-DC converter, and means that an accident such as a short circuit of the load 40 has not occurred. As described above, when the output voltage of the triangular wave from the triangular wave generator 15 is higher than the output voltage of the error amplifier 19, the output of the PWM comparator 20 becomes the + B voltage. The triangular wave generator 15
Is lower than the output voltage of the error amplifier 19, the output of the PWM comparator 20 becomes 0V.

In FIG. 5, a pulse width limiting voltage setting unit 2
5 is always lower than the output voltage of the error amplifier 19, the output voltage of the PWM comparator 20 is independent of the output voltage of the error amplifier 19, and the output voltage of the triangle wave generator 15 and the output voltage of the error amplifier 19 are independent of the output voltage of the error amplifier 19. It is determined only by the magnitude relationship with At this time, the output pulse of the PWM comparator 20 in FIG. 5 is synchronized at the same frequency of the triangular wave, and the pulse width is constant. The above is the operation of the DC-DC converter in the normal state.

[Description of FIG. 6] FIG. 6 shows a triangular wave output voltage of the triangular wave generator 15, an output voltage of the error amplifier 19, and a pulse width limit, which are three input voltages of the PWM comparator 20, in the conventional example of FIG. The relationship between the output voltage of the voltage setting unit 25 and the output voltage of the PWM comparator 20 is shown for a load short-circuit state. When the load 40 is short-circuited for some reason, the voltage of the output terminal 13 is significantly lower than the voltage in the normal state. As a result, the output voltage R2 × Vout / (R1 + R2) of the output voltage conversion unit 17 becomes smaller than the output voltage Vref of the reference voltage setting unit 18.
The output voltage of the error amplifier 19 becomes extremely low.

For this reason, the output voltage of the triangular wave from the triangular wave generator 15 becomes higher than the output voltage of the error amplifier 19 over most of one period. If the output voltage of the PWM comparator 20 is determined only by comparing the triangular wave output voltage of the triangular wave generator 15 with the output voltage of the error amplifier 19, the output voltage of the PWM comparator 20 will be one cycle of the triangular wave output voltage. For most of the +
The voltage becomes B, and the switching transistor 16 is almost always ON. That is, the DC-DC converter continues to flow current from the input DC power supply in an attempt to restore the lowered output voltage. If this state continues for a long time, the load 40
Not only adversely affects the DC-DC converter but also the DC-DC converter itself.

Therefore, in the conventional DC-DC converter shown in FIG. 4, the output voltage of the pulse width limit voltage setting section 25 is input to another negative input terminal of the PWM comparator 20. As shown in FIG. 6, when the load is short-circuited,
9 is lower than the output voltage of the pulse width limiting voltage setting unit 25, and during the period when the triangular wave output voltage from the triangular wave generating unit 15 is higher than the output voltage of the pulse width limiting voltage setting unit 25, the PWM comparator 20 outputs the + B voltage. The PWM comparator 20 outputs 0 V while the triangular wave output voltage from the triangular wave generator 15 is lower than the output voltage of the pulse width limit voltage setting unit 25. As described above, the output voltage of the pulse width limited voltage setting unit 25 is set such that the maximum value of the period (pulse width of the switching pulse) during which the PWM comparator 20 outputs the + B voltage in one cycle of the triangular wave output voltage is fixed. Plays a role of restricting the limit. Therefore, even when the load is short-circuited, the current can be limited to some extent. As described above, the pulse width limiting voltage setting unit 25 and the PWM comparator 20 constitute a pulse width limiting device.

[0016]

However, in the case of current limiting when the input power supply voltage is constant, a fixed pulse width limiting voltage may be provided as described above. Since the power supply voltage fluctuates, the following problems occur. That is, when a battery is used as the input power supply, the voltage of the input power supply becomes low in a state where the battery is discharged, so that the switching transistor 21 is turned on (conducting state) in one cycle of the triangular wave output voltage. Becomes longer. Even in such a case, in a normal operation state, the PWM by the pulse width limit voltage setting unit 25
The function of limiting the pulse width of the switching pulse of the comparator 20 must not work. Therefore, the output voltage of the pulse width limit voltage setting unit 25 is set lower than the output voltage of the error amplifier 19 at the lowest voltage of the battery in which the device operates normally.

On the other hand, when the battery is fully charged, the input power supply voltage is high, and even if a load short-circuit occurs in this state, the DC-DC converter and the load 40 are not damaged so that the DC-DC converter and the load 40 are not destroyed. The load 40 needs to be protected. However, if the voltage difference between the battery voltage at the time of full charge and the minimum voltage of the battery in which the device normally operates is large, the output voltage of the pulse width limit voltage setting unit 25 is changed to the minimum voltage of the battery in which the device operates normally. If the output voltage of the error amplifier 19 is set lower than that in the above, when the battery is fully charged, the pulse width of the switching pulse is too wide when the load is short-circuited, and the DC-DC converter and the like may not be protected. In a conventional DC-DC converter, in order to cope with this problem, for example, an overcurrent detection device 30 is provided as shown in FIG.
-Protects DC converter and the like. But, for example,
In a power supply device having a plurality of output voltages, the overcurrent detection devices 30 are required by the number of output power supplies, which increases the number of components and costs.

According to the present invention, even when the voltage of the input power supply changes, from the high power supply voltage to the low power supply voltage, in a normal operation state, the pulse width limiting voltage
When the pulse width limiting function of the switching pulse of the M comparator 20 does not work and the load is short-circuited, the DC-
It is an object of the present invention to provide a DC-DC converter including a pulse width limit voltage setting unit 25 that appropriately protects a DC converter and the like.

[0019]

According to the DC-DC converter of the present invention, a pulse width limiting device for limiting the pulse width of a switching pulse to a width equal to or less than a limit width is such that the limit width changes according to an input power supply voltage. It was done.

According to the present invention, even when the voltage of the input power supply changes, from the high power supply voltage to the low power supply voltage,
In a normal operation state, the pulse width limit voltage setting unit 25
The DC-DC converter provided with the pulse width limit voltage setting unit 25 that appropriately protects the DC-DC converter and the like when the pulse width of the switching pulse of the PWM comparator 20 does not work and the load is short-circuited. Can be Also, since no overcurrent detection device is required,
This contributes to a reduction in the number of parts and cost.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention described in the claims of the present invention
A DC-D comprising a pulse width limiting device for limiting a pulse width of a switching pulse to a width equal to or less than a limit width, wherein the limit width changes according to an input power supply voltage.
It is a C converter. Thereby, the limit width of the pulse width for limiting the pulse width of the switching pulse to the limit width or less changes according to the level of the power supply voltage, and from the high power supply voltage to the low power supply voltage, the pulse width in a normal operation state The function of limiting the pulse width of the switching pulse of the PWM comparator 20 by the limiting voltage setting unit 25 does not work, and has an effect of appropriately protecting the DC-DC converter and the like when the load is short-circuited.

Hereinafter, an embodiment of the present invention will be described with reference to FIG.
This will be described with reference to FIG. (Embodiment) [Description of FIG. 1] FIG. 1 is a configuration diagram of a DC-DC converter with a supply power limiting function according to the present invention. Conventional D
The difference from FIG. 4 which is a configuration diagram of the C-DC converter is that the overcurrent detection device is removed and the output voltage of the pulse width limit voltage setting unit 25 changes according to the level of the voltage of the input power supply. It is configured as follows. The components common to FIGS. 1 and 4 have already been described in the description of FIG. 4 and will not be described again.
Also in FIG. 1, the pulse width limiting voltage setting unit 25 and the PWM
The comparator 20 forms a pulse width limiting device.

In the conventional example of FIG. 4, the pulse width limiting voltage setting section 25 outputs a constant voltage obtained by dividing the output voltage of the reference voltage generating section 16 by resistance. In the embodiment, the pulse width limit voltage setting unit 25
Adds the input power supply voltage to the constant voltage obtained by dividing the output voltage of the reference voltage generating section 16 by resistance at a constant rate, amplifies the added voltage with an amplifier having a constant amplification factor,
The output voltage of the amplifier is set to a pulse width limit voltage setting unit 25.
Is output. Therefore, in the embodiment of the present invention shown in FIG. 1, the output voltage of the pulse width limiting voltage setting unit 25 increases when the input power supply voltage increases, and the output voltage of the pulse width limiting voltage setting unit 25 decreases when the input power supply voltage decreases. Will also be lower. As described above, the output voltage of the pulse width limiting voltage setting unit 25 determines the maximum value of the period (pulse width of the switching pulse) during which the PWM comparator 20 outputs the + B voltage in one cycle of the triangular wave output voltage. It plays the role of limiting as follows, but the limit width changes according to the input power supply voltage.

[Explanation of FIG. 2] FIG. 2 shows the three input voltages of the PWM comparator 20, the triangular wave output voltage of the triangular wave generator 15, the output voltage of the error amplifier 19, and the pulse width when the input power supply voltage is high. Limit voltage setting section 25
And the output voltage of the PWM comparator 20 in the normal operation state and the load short-circuit state. FIG. 3 shows the three input voltages of the PWM comparator 20, the triangular wave output voltage of the triangular wave generator 15, the output voltage of the error amplifier 19, and the output voltage of the pulse width limit voltage setting unit 25 when the input power supply voltage is low. , The relationship between the output voltage of the PWM comparator 20 and the normal operation state and the load short-circuit state. Needless to say, the voltage input from the error amplifier 19 to the PWM comparator 20 in the normal operation state is set higher than the voltage input from the pulse width limited voltage setting unit 25 to the PWM comparator 20.

2 and 3, the output voltage of the triangular wave from the triangular wave generator 15 is the same. However, since the input power supply voltage is high in FIG. 2, the voltage input to the PWM comparator 20 from the pulse width limit voltage setting unit 25 is higher than that of FIG. Therefore, when the load is short-circuited, PW
Although the pulse width of the output voltage of the M comparator 20 extends to the limit width, which is the maximum width, the PWM in the case of the load short circuit is higher in the case of FIG. 2 where the input power supply voltage is high than in the case of FIG. The pulse width of the output voltage from the comparator 20 is short.

As described above, according to the present invention, when the input power supply voltage is high, the pulse width of the switching pulse when the load is short-circuited becomes narrow, and when the input power supply voltage is low, the pulse width of the switching pulse when the load is short-circuited is reduced. By increasing the width, the maximum supply power from the DC-DC converter to the short-circuit load is (square of the input power supply voltage / four times the internal resistance of the DC-DC converter) × (switching transistor 2
The value of the maximum value of one current conduction period (the limit width of the pulse width) / one cycle of the triangular wave output is kept smaller than the difference of the input power supply voltage. Preferably, in FIG. 1, the ratio of the input power supply voltage to be added to the constant voltage obtained by dividing the output voltage of reference voltage generating section 16 by resistance is set such that the maximum supply power to the short-circuit load becomes almost constant when the load is short-circuited. Choose to be.

In the normal state, the error amplifier 1
9 is higher when the input power supply voltage is higher,
When the input power supply voltage is low, the output voltage is low, but the output voltage of the pulse width limit voltage setting unit 25 is high when the input power supply voltage is high, and is low when the input power supply voltage is low. Therefore, regardless of the level of the input power supply voltage, the output voltage of the pulse width limit voltage setting unit 25 can always be kept lower than the output voltage of the error amplifier 19 in the normal state, and the pulse width limiter operates in the normal state. Malfunction can be prevented.

Further, according to the present invention, the DC-DC converter and the load can be appropriately protected at the time of short circuit, so that the conventional overcurrent detecting device can be eliminated, the number of parts can be reduced, and the cost can be reduced. . Further, since the overcurrent detection device generates a voltage drop at a detection portion to detect a current and consumes power, power saving can be achieved by eliminating the overcurrent detection device.

[0029]

As described above, according to the present invention, it is possible to prevent a malfunction in which the pulse width limiting device of the DC-DC converter operates in a normal state regardless of the level of the power supply voltage, and to prevent the DC-DC converter from malfunctioning when a short circuit occurs. The advantageous effect of properly protecting the converter and the load from burning is obtained.
Further, according to the present invention, the DC-DC converter and the load can be appropriately protected in the event of a short circuit, so that the conventional overcurrent detection device can be eliminated, the number of parts can be reduced, and power consumption can be reduced.
An advantageous effect that cost can be reduced can be obtained. In the case of a multi-output DC-DC converter, it is not necessary to provide an overcurrent detection device for each output, so that the number of components can be reduced, the number of parts can be reduced, power saving and cost reduction can be achieved, and the effect is large.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a DC-DC converter with a supply power limiting function according to an embodiment of the present invention.

FIG. 2 shows a triangular wave output voltage of a triangular wave generator 15, an output voltage of an error amplifier 19, and a pulse width limiting voltage, which are three input voltages of a PWM comparator 20, when an input power supply voltage is high according to an embodiment of the present invention. FIG. 9 is a relationship diagram showing a relationship between an output voltage of the setting unit 25 and an output voltage of the PWM comparator 20 in a normal operation state and a load short-circuit state.

FIG. 3 shows a triangular wave output voltage of a triangular wave generator 15, an output voltage of an error amplifier 19, and a pulse width limiting voltage, which are three input voltages of a PWM comparator 20, when an input power supply voltage is low according to an embodiment of the present invention. FIG. 9 is a relationship diagram showing a relationship between an output voltage of the setting unit 25 and an output voltage of the PWM comparator 20 in a normal operation state and a load short-circuit state.

FIG. 4 is a configuration diagram of a conventional DC-DC converter.

FIG. 5 shows a triangular wave output voltage of a triangular wave generator 15 as three input voltages of a PWM comparator 20 according to a conventional example;
FIG. 9 is a relationship diagram showing a relationship between an output voltage of the error amplifier 19, an output voltage of the pulse width limit voltage setting unit 25, and an output voltage of the PWM comparator 20 in a normal operation state.

FIG. 6 shows a triangular wave output voltage of a triangular wave generator 15 as three input voltages of a PWM comparator 20 according to a conventional example;
FIG. 9 is a relationship diagram showing a relationship between an output voltage of the error amplifier 19, an output voltage of the pulse width limited voltage setting unit 25, and an output voltage of the PWM comparator 20 in a load short circuit state.

FIG. 7 is an example of the internal configuration of a PWM comparator 20;

[Description of Signs] 11 DC power input terminal 13 Output terminal 15 Triangular wave generator 16 Reference voltage generator 17 Output voltage converter 19 Error amplifier 20 PWM comparator 21 Switching transistor 22 Smoothing coil 23 Smoothing capacitor 24 Rectifier diode 25 Pulse width Limit voltage setting unit 30 Overcurrent detection device 40 Load

Claims (1)

[Claims] 1. A DC-DC converter comprising a pulse width limiting device for limiting a pulse width of a switching pulse to a width equal to or less than a limit width, wherein the limit width changes according to an input power supply voltage.