JPH05328711A - Short circuit protection network for dc-dc converter - Google Patents

Abstract

PURPOSE:To obtain a DC-DC converter equipped with a shortcircuit protection network which avoids malfunction during starting and in operation and can prevent the damage a main switching element by shortcircuit current. CONSTITUTION:This apparatus is provided with a shortcirucit detection comparator 11 for detecting the shortcircuit of the load 9 of a DC-DC converter circuit 1 as lowering of the output difference signal 7s of an error amplifier 7 so that output changes to a low level one, and with a malfunction prevention circuit 22 for receiving the signal having changed to the low level one to start operation and for outputting voltage VC rising at a predetermined time constant. Also, the apparatus is equipped with a latch circuit 17 for operating to output a signal 18S commanding turning OFF of a main switching element 2 to a controller 6, when the output voltage of the malfunction prevention circuit exceeds a threshold value, and with an operation stop circuit 31 during starting for short-circuiting the output side of the malfunction prevention circuit, until the output voltage V0 reaches a predetermined level, and for preventing the malfunction of the shortcircuit protection network due to the delay of the rise of the output voltage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a DC-DC converter.
The present invention relates to a timer-latch type short-circuit protection circuit provided for protecting a main switching element of a switch from an overcurrent caused by a load short-circuit, and particularly to a short-circuit protection circuit having a malfunction prevention function.

[0002]

2. Description of the Related Art FIG. 3 is a connection diagram showing a conventional DC-DC converter in a simplified manner. The anode side of a DC power source 8 having a power source smoothing capacitor 8C is connected to a main switching element 2,
It is connected to one end of the external load 9 through the reactor 3.
Between the cathode side of the DC power source 8 connected to the other end of the external load and the main switching element 2, the reactor 3, and the respective output ends, a return diode 4, and a smoothing capacitor 5 are connected to form a step-down DC. -The DC converter circuit 1 is constructed.

In the step-down operation of the step-down DC-DC converter circuit 1 described above, the main switching element 2 is driven by the drive signal 2.
ON / OFF control is performed at a desired timing by d. At this time, a current I _L is supplied from the DC power supply 8 to the load 9 during the ON period of the main switching element 2, and at the same time, energy is accumulated in the reactor 3 due to this current, and the reactor 3 during the OFF period of the main switching element 2. Stored energy is discharged through the load 9 and the return diode 4. If the current flowing through the reactor 3 is I _L, the input / output voltage of _the reactor is V _i, V _O , the on / off cycle is T, and the on time and off time in the cycle T are T _{On and} T _{Off ,} respectively, the input / output voltage The ratio V _O / V _i is the step-down switching element 1
Is proportional to the _on -time ratio D _on = T _on / T. At this time, the on-time ratio D _on has a value of 1 or less, so that the step-down operation in which the output voltage V _O becomes lower than the input voltage V _i is performed.

Further, the DC output voltage V₀Is resistance 7_R1,7
_R2Is divided by, and the divided voltage is not inverted by the error amplifier 7.
Reference voltage V input to the input side_S1Compared to that difference
Issue 7 _SIs input to the control unit 6. The control unit 6 controls the difference signal 7_S
When the main switching element 2 is turned on,
Drive signal 2 to control_dTo the base of the main switching element
Output to the output voltage V₀The power supply voltage is lower than
Performs constant voltage control to maintain a constant voltage and supply it to the load 9.
Be done.

On the other hand, in order to protect the main switching element from an overcurrent due to a short circuit of the load circuit, a short circuit protection circuit 1
0 is provided. The short-circuit protection circuit 10 includes a short-circuit detection comparator 11 that receives the difference signal 7 _S from the error amplifier 7 on the non-inverting input side and compares it with the reference voltage V _S2, and a malfunction prevention circuit 12 connected to the output side. Further, it is composed of a latch circuit 17 connected to the output side and an output circuit 18 thereof. The malfunction prevention circuit 12 includes, for example, a switching element 13 whose base is connected to the output side of the short-circuit detection comparator 11 and whose emitter is grounded, and a resistor 15 on the non-grounded terminal side.
Via the collector of the switching element 13 and the capacitor 16 connected to the reference power source via the series resistor 14, respectively, and the non-ground side terminal of the capacitor 16 is connected to the set terminal S of the latch circuit. The reference voltage V _S2 of the short-circuit detection comparator 11 is the reference voltage V _ref.
Is set to almost half the value of.

In the short-circuit protection circuit 10 constructed as described above, the output difference signal 7 _S of the error amplifier 7 is little changed during the steady operation of the DC-DC converter, and the short circuit is applied to the non-inverting input side. Since the detection comparator 11 balances and emits a high level (H level) signal,
The switching element 13 received by the switch is turned on. Therefore, the reference voltage V is applied to the set terminal of the latch circuit.
_The voltage obtained by dividing _ref by resistors 14 and 15, V _ref ×
(R ₂ / R ₁ + R ₂ ) is input. Therefore, if this voltage is set to be lower than the threshold voltage of the latch circuit, the latch circuit 17 is held in the reset state, the protection operation of the short circuit protection circuit 10 does not function, and the steady operation is maintained.

At some point, a load short circuit occurs and the output voltage V
_{When 0} decreases, the difference signal 7 _S increases and the output of the short-circuit detection comparator 11 changes to a low level (L level).
As a result, the switching element 13 turns off, and the terminal voltage V _C of the capacitor 16 changes to the electrostatic capacitance C of the capacitor 16.
The rising time constant of the voltage determined by the product of ₁ and the resistance value R ₁ of the series resistor 14 is held and the voltage rises toward the reference voltage V _ref . Therefore, when this voltage V _C exceeds the threshold voltage of the latch circuit 17, the latch circuit 17 enters the set state, and the output circuit 18 which has detected this outputs the command signal 18 _S for the protection operation to the control unit 6. As a result, the main switching element 2 is turned off and the short-circuit current is prevented from flowing therethrough, so that damage to the main switching element due to overcurrent can be prevented. This state is maintained until the power is turned off and V _ref is once dropped.

In addition, DC-
In order to prevent the short circuit protection circuit 10 from erroneously judging the delay of the rising of the output voltage as the voltage drop due to the occurrence of the short circuit in the load circuit at the time of starting the DC converter circuit 1, the malfunction prevention circuit 12 is provided. , The rising time constant of the voltage V _C determined by the product of the electrostatic capacitance C ₁ of the capacitor 16 and the resistance value R ₁ of the series resistor 14 is slower than the rising time constant of the voltage at the time of starting the DC-DC converter circuit. It is configured to set and delay the protective action. The delay in the operation of the short-circuit protection circuit is also useful for preventing the short-circuit protection circuit from erroneously determining a decrease in the output voltage due to a sudden change in load as a short circuit in the load circuit.

[0009]

In the conventional short circuit protection circuit 10 having a delay in the protection operation, the rise time constant R ₁ × C ₁ of the voltage of the malfunction prevention circuit 12 is set in order to avoid malfunction at the time of startup. Since it is long, even if a short circuit occurs in the load circuit during steady operation, the protection operation is not performed if the output voltage recovers to the normal value within this delay period. Therefore, the advantage that the malfunction of the short-circuit protection circuit 10 due to the abrupt change of the load can be avoided is obtained. However, when a short-circuit accident occurs in the load circuit, there is a problem that the main switching element becomes overheated due to the short-circuit current during the delay period of the protection operation, which leads to a situation of damage.
The improvement is required.

An object of the present invention is to provide a DC-DC converter provided with a short-circuit protection circuit capable of avoiding malfunctions at startup and during operation to prevent damage to the main switching element due to short-circuit current.

[0011]

In order to solve the above-mentioned problems, according to the present invention, a D having a main switching element is provided.
A C-DC converter circuit, an error amplifier that detects the output voltage thereof and outputs a difference signal between the detected voltage and the reference voltage, and control for controlling the duty ratio of the main switching element in the direction of reducing the difference signal. DC-DC converter including
Is a short circuit protection circuit for a load circuit, which starts an operation upon receiving a low level signal and a short circuit detection comparator whose output signal changes to a low level when a short circuit of a load circuit is detected as a decrease in the difference signal. A malfunction prevention circuit that outputs a voltage that rises at a predetermined time constant, and a signal that operates when the output voltage of this malfunction prevention circuit exceeds a threshold value and commands a turn-off of the main switching element to the control unit. A latch circuit that outputs the output voltage toward the output side of the DC-DC converter circuit, and a start-up operation stop circuit that prevents the rise of the output voltage of the malfunction prevention circuit until the output voltage of the DC-DC converter circuit reaches a predetermined level. Shall be.

Further, the malfunction prevention circuit includes a switching element having a base connected to the output side of the short-circuit detection comparator and an emitter grounded, a set terminal of the latch circuit on the non-grounded terminal side, a collector of the switching element, And a capacitor connected to the reference power supply via a series resistor, and the rise time constant of the voltage determined by the product of the capacitance of this capacitor and the resistance value of the series resistor is the main switching element due to the short-circuit current of the load circuit. It shall be set to the value necessary to prevent the damage of the.

Further, the start-up deactivation circuit has a series CR of a switching element having a collector and an emitter connected to the output side of the malfunction prevention circuit, and a capacitor and a resistor connected between the base and the reference power source. Including a timer circuit, the reference power supply voltage rises from zero when the power is turned on, and the time constant of the series time constant circuit is a value sufficient to cover the delay in rising of the output voltage of the DC-DC converter. Shall be set to.

[0014]

In the structure of the present invention, when the short circuit protection circuit detects a short circuit of the load circuit as a decrease in the output difference signal of the error amplifier, the output signal changes to a low level, and a short circuit detection comparator of this low level. A malfunction prevention circuit that starts operation in response to a signal and outputs a voltage that rises with a predetermined time constant, and operates when the output voltage of this malfunction prevention circuit exceeds a threshold value to instruct turn-off of the main switching element. Latch circuit and its output circuit, and a start-up operation stop circuit that prevents the rise of the output voltage of the malfunction prevention circuit until the output voltage of the DC-DC converter circuit reaches a predetermined level. As a result, a short circuit in the load circuit that occurs during operation is detected by the short-circuit detection comparator as a drop in the difference signal, and the malfunction prevention circuit starts up with a predetermined time constant. When the voltage exceeds the threshold voltage, the latch circuit is set, and the output circuit outputs the protection operation signal to the control section so that the main switching element blocks the flow of short-circuit current. ,
The function of avoiding damage to the main switching element due to overcurrent is obtained, and malfunction due to external noise or the like that ends before the latch circuit is set is prevented. On the other hand, at the time of starting the DC-DC converter, the start-up operation stop circuit catches the delay of the rising of the output voltage of the DC-DC converter and blocks the rising of the output voltage of the malfunction prevention circuit for a predetermined time. A function of avoiding a malfunction at the time is obtained. In this way, the main switching element protection function against short-circuit current and the malfunction prevention function due to load fluctuation are separated into the malfunction prevention circuit, and the malfunction prevention function at startup is separated into the startup operation stop circuit, so they are suitable for both. It is possible to set the time constant, and therefore a short circuit protection circuit with less malfunction and reliable protection operation can be obtained.

Further, the malfunction prevention circuit includes a switching element whose base is connected to the output side of the short-circuit detection comparator and whose emitter is grounded, a non-grounded terminal side is a set terminal of the latch circuit, a collector of the switching element, and It is composed of a capacitor connected to the reference power supply via a series resistor, and the rise time constant of the voltage determined by the product of the capacitance of this capacitor and the resistance value of the series resistor is the main switching element due to the short circuit current of the load circuit. If it is configured to be set to a value required to prevent damage to the short circuit protection circuit, it is possible to obtain a short circuit protection circuit having an excellent short circuit protection function with a simple circuit configuration.

Further, the start-up deactivation circuit includes a switching element having a collector and an emitter connected to the output side of the malfunction prevention circuit, and a series CR of a capacitor and a resistor connected between the base and the reference power source. Including the timer circuit, the voltage of the reference power supply rises from zero when the power is turned on, and the time constant of the series CR timer circuit is set to DC-DC.
If it is set to a value sufficient to cover the delay in rising of the output voltage of the converter, the charging current flows through the capacitor of the series CR timer circuit in response to the change in the reference voltage, and the switching element turns on. Then, the output side of the malfunction prevention circuit is short-circuited, so the latch circuit is in the reset state,
The short circuit protection circuit stops the protection function. Therefore, D
It is possible to obtain a function of avoiding a situation in which a delay in rising of the output voltage of the C-DC converter is mistakenly recognized as a short circuit of the load circuit. Also, when the reference voltage reaches the specified value, charging of the capacitor stops and the switching element turns off.Therefore, the operation stop circuit at start-up stops its function, and after that, the short circuit protection and malfunction prevention by the malfunction prevention circuit are performed. Done.

[0017]

EXAMPLES The present invention will be described below based on examples. FIG. 1 shows a DC-DC converter according to an embodiment of the present invention.
FIG. 4 is a connection diagram showing a short circuit protection circuit of the data input device, and the same components as those of the conventional technique are designated by the same reference numerals, and a duplicate description will be omitted. In the figure, a short circuit protection circuit 20 includes a short circuit detection comparator 11 for receiving a difference signal 7 _S from an error amplifier 7 on a non-inverting input side and comparing it with a reference voltage V _S2, and a malfunction prevention connected to its output side. In addition to the circuit 22, the latch circuit 17 connected to the output side thereof, and the output circuit 18 thereof, a start-up operation stop circuit 31 is provided.

The start-up deactivation circuit 31 has a collector and an emitter connected in parallel to the output side capacitor 16 of the malfunction prevention circuit 22, and has a switching resistor 32 having a base resistance 34, and a base and a reference power source. It is composed of a capacitor 36 and a resistor 35 connected in between, and the voltage V _{REF of the} reference power supply rises from zero when the power is turned on, and
A time constant C ₃ × (R ₄ + of a series CR timer circuit 33 composed of a capacitor 36, a resistor 35 and a base resistor 34.
R ₅₎ is DC-DC converter - is set to a value sufficient to - delay in the rise of other output voltages hippopotamus.

The configuration of the malfunction prevention circuit 22 is the same as that of the conventional device described above, but the rise of the voltage V _C determined by the product of the electrostatic capacitance C ₁ of the capacitor 16 and the resistance value R ₁ of the series resistor 14. The time constant, which was conventionally set to be slower than the rising time constant of the output voltage of the DC-DC converter circuit at the time of startup, is necessary in the embodiment to prevent damage to the main switching element due to the short-circuit current of the load circuit. It differs from the prior art in that it is configured to be set to a value.

When the short-circuit protection circuit constructed as described above is applied to the DC power supply 8, the reference voltage V rising from zero is applied.
_In response to the voltage change of _REF , a charging current flows through the series CR timer circuit 33 through the capacitor 36 corresponding to its time constant, and the switching element 32 is turned on by this charging current to output the malfunction prevention circuit 22. Capacitor 16
Short circuit. The time constant of the serial CR timer-circuit 33 is DC.
Since the output voltage V ₀ of the DC converter 1 is set to correspond to the rising delay, the short-circuit state is maintained until the output voltage reaches the specified value. Therefore, even if the short circuit protection circuit 20 mistakenly recognizes the rise of the output voltage V ₀ as a load short circuit and the malfunction prevention circuit 12 tries to output the output voltage V _C , the output to the latch circuit 17 is not output because the capacitor 16 is short-circuited. Blocked, DC-DC converter
It is possible to prevent a malfunction at the time of starting the computer.

Further, when the capacitor 36 of the series CR timer circuit 33 is completely charged, the charging current is stopped, the switching element 32 is turned off, the short circuit of the capacitor 16 is released, and the start-up operation is performed. The stop circuit 31 stops its function, and this stop state is maintained until the power is once cut off and restarted. Therefore, DC-D
While the C converter 1 is in steady operation, the short-circuit protection circuit 20 performs short-circuit protection operation as in the conventional device, but the time constant of the malfunction prevention circuit 22 prevents damage to the main switching element due to the short-circuit current of the load circuit. Since it is set to a required value, short-circuit protection of the main switching element can be performed more reliably, and the drop in the output voltage V ₀ due to a large change in load is recovered within the operation delay range of the malfunction prevention circuit 22. In this case, the short-circuit protection circuit does not operate, and it is possible to avoid the occurrence of a power failure or the like due to a malfunction of the short-circuit protection circuit.

FIG. 2 shows a connection showing a different embodiment of the present invention.
FIG. 3 is a diagram showing a commercially available timer-latch type IC with a short-circuit protection function.
The short circuit protection circuit of the DC-DC converter using
It is a thing. In the figure, a commercially available IC40 is manufactured by TI,
The model number is TL1451 and its oscillation frequency terminal 1,
A capacitor 41 and a resistor 42 are connected between the two, and V _CC
Terminal 9 and control unit output terminal 10 are the main switching elements.
Error amplification by connecting to 2 collectors and base respectively
Between the output terminal 12 and its inverting input terminal 13
S 46 and resistor 47 are connected, and S terminal 1 of the latch circuit
5, between the reference voltage terminal 16 and the start-up operation stop circuit 31 and
And the capacitor 16 are connected, and between the terminal 16 and the ground.
Connect resistors 44 and 45 and capacitor 43 to
As a result, DC- equipped with the short-circuit protection circuit 20 shown in FIG.
The DC converter can be easily constructed.

[0023]

As described above, according to the present invention, the short-circuit protection circuit detects a short circuit of the load circuit as a decrease in the output difference signal of the error amplifier, and a predetermined level in response to the low-level signal. A malfunction prevention circuit that outputs a voltage that rises with a time constant of, and operates when the output voltage of this malfunction prevention circuit exceeds a threshold value
A latch circuit for instructing turning on and off, its output circuit, and D
Since the C-DC converter circuit is configured with a start-up operation stop circuit that prevents the output voltage of the malfunction prevention circuit from rising until the output voltage of the C-DC converter circuit reaches a predetermined level, The malfunction prevention function of the short-circuit protection circuit due to the delay of the rise of the output voltage at start-up is the operation stop circuit at start-up, the function of preventing the damage of the main switching element due to the short-circuit current, and the function of preventing the malfunction due to a large load change etc. It becomes possible to separate the functions in the circuit, and the damage and malfunction of the main switching element, which was a problem in the conventional short-circuit protection circuit in which the malfunction prevention circuit also functions as each of the above prevention functions, was eliminated, and the main switching element It is possible to provide a DC-DC converter having a short circuit protection circuit having excellent short circuit protection performance and malfunction prevention performance.

[Brief description of drawings]

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

FIG. 2 is a connection diagram showing a different embodiment of the present invention.

FIG. 3 is a connection diagram showing a simplified conventional DC-DC converter.

[Explanation of symbols]

1 DC-DC Converter Circuit 2 Main Switching Element 3 Reactor 4 Reflux Diode 5 Smoothing Capacitor 6 Control Section 7 Error Amplifier 8 DC Power Supply 10 Short Circuit Protection Circuit 11 Short Circuit Detection Comparator 12 Malfunction Prevention Circuit 13 Switching Element 14 Resistance 15 Resistance 16 Capacitor 17 Latch Circuit 18 Output Circuit 20 Short Circuit Protection Circuit 22 Malfunction Prevention Circuit 31 Start-up Operation Stop Circuit 32 Switching Element 33 Series CR Timer-Circuit 40 Commercially Available IC V ₀ Output Voltage V _ref Reference Voltage V _REF Reference Voltage ( (Starts from zero at startup) V _C Malfunction prevention circuit output voltage

Claims (3)

[Claims] 1. A DC-DC converter circuit having a main switching element, an error amplifier for detecting an output voltage thereof and outputting a difference signal between the detected voltage and a reference voltage, and a direction for reducing the difference signal. A short circuit protection circuit for a DC-DC converter including a control unit for controlling a duty ratio of the main switching element, wherein an output signal changes to a low level when a short circuit of a load circuit is detected as a decrease of the difference signal. Short-circuit detection comparator, malfunction prevention circuit that starts operation upon receiving the low level signal and outputs a voltage rising at a predetermined time constant, and operates when the output voltage of this malfunction prevention circuit exceeds a threshold value. Then, the output voltage of the DC-DC converter circuit and the latch circuit that outputs a signal for instructing the turn-off of the main switching element to the control unit reaches a predetermined level. Until, the blocking rise of the output voltage of the malfunction prevention circuit startup operation stop circuit and DC-DC converter is characterized by including a - motor short-circuit protection circuit. 2. A malfunction prevention circuit, a switching element having a base connected to the output side of a short-circuit detection comparator and an emitter grounded, a non-ground terminal side a set terminal of a latch circuit, a collector of the switching element, And a capacitor connected to the reference power supply via a series resistor, and the rise time constant of the voltage determined by the product of the capacitance of this capacitor and the resistance value of the series resistor is the main switching element due to the short-circuit current of the load circuit. 2. The short-circuit protection circuit for a DC-DC converter according to claim 1, wherein the value is set to a value necessary to prevent damage to the DC-DC converter. 3. A start-up operation stop circuit, a switching element having a collector and an emitter connected to the output side of a malfunction prevention circuit, and a series CR of a capacitor and a resistor connected between the base and a reference power source. A timer circuit, the voltage of the reference power source rises from zero when the power is turned on, and the time constant of the series CR timer circuit is sufficient to cover the delay in the rise of the output voltage of the DC-DC converter. 2. The short-circuit protection circuit for a DC-DC converter according to claim 1, wherein the short-circuit protection circuit is set to a value.