JP2002272098A - Switching power supply unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent damage to a semiconductor device due to overcurrent. SOLUTION: When a voltage in the rise edge of a voltage signal detected by a resistor 35 becomes at least specific second threshold when an overcurrent occurs, a level shift circuit 40 increases the voltage value in the rise edge of the voltage signal, thus instantaneously setting the output voltage of a comparator 38 to a high level in generation of the overcurrent, stopping the supply of a control pulse signal to a MOSFET 4 by a tri-state buffer 31, and hence instantaneously turning off the MOSFET 4 in generation of the overcurrent for protecting from the overcurrent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply having an overcurrent protection circuit.

[0002]

2. Description of the Related Art FIG. 5 is a circuit diagram showing an example of a conventional switching power supply. The conventional switching power supply shown in FIG. 5 is a synchronous rectification type forward switching power supply.

[0003] Input terminals 1 and 2 are terminals to which an input voltage is applied. The smoothing capacitor 3 is a capacitor for smoothing an input voltage. The transformer 5 performs power conversion. MOSFET (Metal Oxide Semiconductor Fi
The eld Effect Transistor 4 is a semiconductor element connected in series to the primary side of the transformer 5.

The resistor 35 is a current detection circuit that detects a current on the primary side of the transformer 5 and generates a voltage signal having a value corresponding to the current value at both ends.

The smoothing capacitor 3, MOSFET
4. The primary winding of the transformer 5 and the resistor 35 constitute a primary side drive circuit.

The MOSFET 6 is a rectifying semiconductor element, and the MOSFET 7 is a freewheeling semiconductor element.

[0007] The choke coil 8 is an element connected in series to the secondary winding of the transformer 5 to suppress the AC component of the output voltage. The output capacitor 9 is an element that is connected in parallel to the output terminals 10 and 11 and suppresses an AC component of the output voltage. The output terminals 10 and 11 are terminals for extracting an output voltage.

The control circuit 14 generates a control pulse signal corresponding to the output voltage at the output terminals 10 and 11, and controls the MOSFET 4 with the control pulse signal.

In the control circuit 14, an error detection circuit 21 detects an error between the output voltage at the output terminals 10, 11 and a predetermined rated output voltage.

The photocoupler 22 has a photodiode 22a and a phototransistor 22b, and transmits a signal while electrically insulating the input side and the output side. The photodiode 2 of the photocoupler 22
2a is connected to the error detection circuit 21, the collector of the output-side phototransistor 22b is connected to the power supply + Vcc, and the emitter of the output-side phototransistor 22b is connected to one end of the resistor 23.

The resistor 23 has one end connected to the emitter of the phototransistor 22b of the photocoupler 22 and the other end grounded, and the resistor 24 is connected in parallel to the phototransistor 22b. .

The soft start circuit 60 is a circuit for executing a soft start when the apparatus is started.

In the soft start circuit 60, the resistance 2
5 has one end connected to a power supply + Vcc and the other end connected to a resistor 26
The resistor 26 has one end connected to the resistor 25.
And the other end is grounded. A first input terminal of the comparator 33 is connected to a connection point between the resistors 25 and 26.

The capacitor 27 has one end connected to a power supply + V
The resistor 28 is a capacitor having one end connected to a connection point between the resistors 25 and 26 and the other end connected to the capacitor 27.

The triangular wave generator 32 is a device for generating a triangular wave signal having a predetermined frequency.

The comparator 33 has three input terminals, and is a voltage obtained by subtracting the input voltage to the third input terminal from the sum of the input voltage to the first input terminal and the input voltage to the second input terminal. When the value is positive, a circuit that outputs a predetermined high-level (normally, power supply potential) output voltage, and otherwise outputs a predetermined low-level (normally, ground potential) output voltage. is there.

The first input terminal of the comparator 33 is connected to the connection point between the resistors 25 and 26, and the second input terminal is connected to the connection point between the resistors 23 and 24 and the photocoupler 22. The third input terminal is connected to the emitter of the phototransistor 22b, and is connected to the triangular wave generator 32.

When the input voltage from the overcurrent protection circuit 34 is at a low level, the tristate buffer 31 outputs the output of the comparator 33 as it is, and the input level from the overcurrent protection circuit 34 is at a high level. In such a case, the output voltage is undefined.

As described above, the control circuit 14 includes the error detection circuit 21, the photocoupler 22, the resistors 23 and 24, the soft start circuit 60, the triangular wave generator 32, the comparator 33, and the tristate buffer 31.

The overcurrent protection circuit 34 supplies a high-level output signal to the tristate buffer 31 and supplies a control pulse signal to the MOSFET when the voltage value of the voltage signal from the resistor 35 is equal to or higher than a predetermined threshold value. Is stopped, and the duty ratio of the control pulse signal is reduced.

In this overcurrent protection circuit 34, the resistance 3
6 has one end connected to the resistor 35 and the other end connected to the capacitor 3.
The capacitor 37 has one end connected to the resistor 36 and the other end grounded. Note that the resistor 36 and the capacitor 37 constitute an integration circuit for suppressing noise.

The reference voltage source 39 has a negative terminal grounded,
The positive terminal is connected to the comparator 38, and is a power supply that generates a predetermined threshold voltage Vt that is higher than the voltage value obtained from the resistor 35 during normal operation and lower than the voltage value obtained from the resistor 35 when an overcurrent occurs.

The comparator 38 has a first input terminal connected to the connection point between the resistor 36 and the capacitor 37, a second input terminal connected to the positive terminal of the reference voltage source 39,
If the voltage value obtained by subtracting the input voltage to the second input terminal from the input voltage to the second input terminal is positive, an output voltage of a predetermined high level (normally, power supply potential) is output. Is a circuit for outputting an output voltage of a predetermined low level (normally, a ground potential). The comparator 38
Is input to the tri-state buffer 31.

The secondary winding of the transformer 5 and the MOSFE
T6, 7, the choke coil 8, the output capacitor 9, and the control circuit 14 constitute a secondary output circuit.

Next, the operation of the conventional switching power supply will be described.

First, the operation during normal operation will be described.

During normal operation, the input signals applied to the input terminals 1 and 2 are applied as pulses to the primary winding of the transformer 5 by the switching operation of the MOSFET 4. As a result, a voltage corresponding to the voltage applied to the primary winding is generated in the secondary winding of the transformer 5, and the MOSFE
Rectified by T6, T7, smoothed by the choke coil 8 and the output capacitor 9, the rectified and smoothed secondary voltage is applied to the output terminals 10, 11 as an output voltage.

The output voltage applied to the output terminals 10 and 11 is detected by an error detection circuit 21 of the control circuit 14, and a voltage corresponding to an error between the voltage and its rated output voltage (hereinafter referred to as an error voltage). ) Is the photo coupler 22
And the resistors 23 and 24, the second
Is applied to the input terminal.

During normal operation, the comparator 33
To the first input terminal, a voltage obtained by dividing the power supply voltage + Vcc by the resistors 25 and 26 is applied. Further, a triangular wave signal generated by the triangular wave generator 32 is applied to a third input terminal of the comparator 33.

If the voltage value obtained by subtracting the voltage value of the triangular wave signal from the sum of the voltage obtained by voltage division by the resistors 25 and 26 and the error voltage is positive, the output voltage of the comparator 33 becomes high. Level, otherwise low level. Thereby, MOSFET4
Is obtained at the same frequency as the frequency of the triangular wave signal and at a duty ratio corresponding to the output voltage.

The control pulse signal thus obtained is input to the MOSFET 4 via the tristate buffer 31 during normal operation, and the MOSFET 4 performs a switching operation according to the voltage level of the control pulse signal.

Therefore, if the error voltage is positive, the time during which the output voltage is at a high level is reduced (ie, the pulse width of the control pulse signal is reduced), and the time during which the MOSFET 4 is in the on state is shortened. If the error voltage is negative, the time during which the output voltage is at a high level is increased (that is, the pulse width of the control pulse signal is increased),
By increasing the time that the FET 4 is in the ON state,
T4 is PWM-controlled. Thereby, the output terminal 10,
The output voltage at 11 is controlled so as to approach the rated output voltage.

Next, the operation when the apparatus is started will be described.

When the apparatus is started, the voltage of the power supply + Vcc changes from zero to a predetermined voltage + Vcc. At this time, the input voltage to the first input terminal of the comparator 33, that is, the voltage at the connection point between the resistors 25 and 26 is
The voltage gradually increases from zero based on a time constant determined by the resistance values of 5, 26, and 28 and the capacitance value of the capacitor 27. When a steady state is reached, the voltage + Vcc is divided by the resistors 25 and 26. Voltage value.

Therefore, as time elapses from the start, the input voltage to the first input terminal of the comparator 33 increases. As a result, the time during which the output voltage of the comparator 33 is at the high level gradually increases, and the pulse width of the control pulse signal gradually increases. Thereby, the soft start is realized by the soft start circuit 60.

Finally, the operation when an overcurrent occurs will be described.

When the output voltage decreases (that is, when the output current increases), the control circuit 14 controls the output voltage to be rated as described above, and the pulse width of the control pulse signal decreases. However, if an overcurrent occurs, it becomes difficult for the control circuit 14 to adjust the pulse width.

Therefore, when an overcurrent occurs, the MOSFET 4 is turned off by the overcurrent protection circuit 34.

In the overcurrent protection circuit 34, the potential of one end of the resistor 35 is monitored by a comparator 38 via an integrating circuit including a resistor 36 and a capacitor 37. When the current on the primary side increases and the potential at one end of the resistor 35 exceeds the voltage Vt, the comparator 38 supplies a high-level signal to the tristate buffer 31 and
The supply of the control pulse signal to ET4 is stopped.

As a result, when the current on the primary side exceeds a certain amount, the MOSFET 4 is turned off, the current on the primary side and thus the current on the secondary side are reduced, and the overcurrent is eliminated.

At this time, the resistor 36 and the capacitor 37
, Suppresses high-frequency noise and suppresses malfunction of the comparator 38.

[0042]

However, in the overcurrent protection circuit 34, a delay occurs in the overcurrent detection operation by the comparator 38 due to a delay in response by the integration circuit formed by the resistor 36 and the capacitor 37. .

That is, if the voltage signal detected by the resistor 35 is, for example, as shown in FIG. 6, due to the delay characteristic of the integrating circuit constituted by the resistor 36 and the capacitor 37, the voltage signal is instantaneously generated when an overcurrent occurs. The voltage value corresponding to the primary side current value is not obtained in the comparator 38, and after a lapse of time td, the input voltage to the comparator 38 reaches the reference voltage Vt, and the MOS by the overcurrent protection circuit 34
The control of the FET 4 is delayed by the time td.

Since there is a delay in detecting such an overcurrent, it is difficult to suppress the overcurrent by sufficiently narrowing the pulse width of the control pulse signal. Output voltage-output current characteristics at the time)
Are indicated by solid lines in FIG. 7, and the output terminals 10 and 11
, The output current increases. If there is no such delay, the drooping characteristic is, for example, as indicated by a dotted line.

As shown in FIG. 7, in the conventional switching power supply device, when the output voltage decreases when the load is short-circuited or the like, the output current increases, so that the switching semiconductor element (MOSFETs 4, 6, 7 in FIG. 5) May be damaged.

In particular, when the synchronous rectifier of the secondary side output circuit is composed of MOSFETs as shown in FIG.
In the MOSFET, since the impedance between the source and the drain is low, the conduction current when the load is short-circuited increases, and there is a high possibility that the MOSFET will be damaged.

That is, there is a problem that a semiconductor element in the device may be damaged by an overcurrent due to a load short circuit or the like.

The present invention has been made in view of the above problems, and has as its object to provide a switching power supply device capable of preventing a semiconductor element from being damaged by an overcurrent.

[0049]

In order to solve the above problems, a switching power supply according to the present invention comprises a transformer,
A semiconductor element connected to the primary side of the transformer and a control circuit that generates a control pulse signal according to the output voltage of the device and controls the semiconductor element with the control pulse signal, and detects the current on the primary side of the transformer A current detection circuit for generating a voltage signal having a value corresponding to the current value; and an overcurrent protection circuit for reducing a duty ratio of the control pulse signal when the voltage value of the voltage signal is equal to or greater than a predetermined first threshold. A circuit, and a level shift circuit that increases the voltage value of the rising edge of the voltage signal when the voltage value of the rising edge of the voltage signal is equal to or more than a predetermined second threshold value.

Using this switching power supply,
Even if an overcurrent occurs, the primary current and the output current are instantaneously suppressed, and damage to the semiconductor element due to the overcurrent can be prevented.

Further, according to the switching power supply of the present invention, in addition to the switching power supply of each of the above-mentioned inventions, the level shift circuit may be configured so that the voltage value of the rising edge of the voltage signal generated by the current detection circuit is equal to or higher than the second threshold value In some cases, the voltage value of the voltage signal is kept at or above the first threshold for a predetermined time.

Using this switching power supply,
The operation of the semiconductor element is stopped for the predetermined time, and damage to the semiconductor element due to an overcurrent can be prevented.

Further, in addition to the switching power supply of the invention, the switching power supply of the present invention further comprises a comparator for comparing a voltage signal generated by the current detection circuit with a second threshold value, and an output value of the comparator. The level shift circuit has a voltage holding circuit that uses a voltage attenuated by a predetermined time constant as a maximum value as a voltage signal.

Using this switching power supply,
The operation of the semiconductor element is stopped for the predetermined time, and damage to the semiconductor element due to an overcurrent can be prevented.

Further, in addition to the switching power supply of the above-mentioned inventions, the switching power supply of the present invention further comprises a soft switch when the voltage value of the rising edge of the voltage signal is shifted by the level shift circuit to the first threshold value or more. A soft start control circuit for starting a start is further provided.

Using this switching power supply,
When an overcurrent occurs, the soft start can be performed even when the operation returns to the normal operation, and damage to the semiconductor element due to the overcurrent at the time of starting can be prevented.

Further, the switching power supply of the present invention, in addition to the switching power supply of each of the above-mentioned inventions, further comprises an error detection circuit for detecting an error in the output voltage; A soft-start circuit that increases, a triangular wave generator that generates a triangular wave of a predetermined frequency, a voltage value corresponding to the error detected by the error detection circuit, a voltage of the soft-start circuit, and a triangular wave generated by the triangular wave generator The soft start control circuit controls the soft start circuit when the voltage value of the rising edge of the voltage signal is shifted by the level shift circuit to the first threshold value or more. This is to make a soft start.

Using this switching power supply,
By using the soft start circuit used at startup, it is possible to execute soft start even when returning to normal operation when overcurrent occurs, preventing damage to semiconductor elements due to overcurrent at startup. Can be.

Further, in the switching power supply according to the present invention, in addition to the switching power supply according to each of the above-mentioned inventions, the soft start circuit is constituted by a capacitor and a resistor, and the soft start control circuit discharges the accumulated charge of the capacitor of the soft start circuit. Then, the soft start is started.

Using this switching power supply,
A soft start control circuit can be realized with a simple circuit configuration.

[0061]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of a switching power supply according to an embodiment of the present invention. This switching power supply is a synchronous rectification type forward switching power supply.

The input terminals 1 and 2 are terminals to which an input voltage is applied. The smoothing capacitor 3 is a capacitor for smoothing an input voltage. The transformer 5 performs power conversion. The MOSFET 4 is a semiconductor element connected in series to the primary side of the transformer 5. In addition, MOSFET4
Alternatively, another semiconductor element may be used.

The resistor 35 is a current detection circuit that detects a current on the primary side of the transformer 5 and generates a voltage signal having a value corresponding to the current value at both ends.

The smoothing capacitor 3, the MOSFET
4. The primary winding of the transformer 5 and the current detection resistor 35 constitute a primary side drive circuit.

The MOSFET 6 is a rectifying semiconductor element, and the MOSFET 7 is a freewheeling semiconductor element.

The choke coil 8 is an element that is connected in series to the secondary winding of the transformer 5 and suppresses the AC component of the output voltage. The output capacitor 9 is an element that is connected in parallel to the output terminals 10 and 11 and suppresses an AC component of the output voltage. The output terminals 10 and 11 are terminals for extracting an output voltage.

The control circuit 14 generates a control pulse signal corresponding to the output voltage at the output terminals 10 and 11, and controls the MOSFET 4 with the control pulse signal.

In the control circuit 14, the error detection circuit 21 detects an error between the output voltage at the output terminals 10, 11 and a predetermined rated output voltage.

The photocoupler 22 has a photodiode 22a and a phototransistor 22b, and transmits a signal while electrically insulating the input side from the output side. The photodiode 2 of the photocoupler 22
2a is connected to the error detection circuit 21, the collector of the output-side phototransistor 22b is connected to the power supply + Vcc, and the emitter of the output-side phototransistor 22b is connected to one end of the resistor 23.

The resistor 23 has one end connected to the emitter of the phototransistor 22b of the photocoupler 22 and the other end grounded. The resistor 24 is connected in parallel to the phototransistor 22b. .

The soft start circuit 60 operates when the apparatus is started up.
And a circuit for executing a soft start when returning to a normal operation state after an overcurrent occurs.

In the soft start circuit 60, the resistance 2
5 has one end connected to a power supply + Vcc and the other end connected to a resistor 26
The resistor 26 has one end connected to the resistor 25.
And the other end is grounded. A first input terminal of the comparator 33 is connected to a connection point between the resistors 25 and 26.

The capacitor 27 has one end connected to a power supply + V
The resistor 28 is a capacitor having one end connected to a connection point between the resistors 25 and 26 and the other end connected to the capacitor 27.

The triangular wave generator 32 is a device for generating a triangular wave signal having a predetermined frequency as a signal serving as a basis of a control pulse signal.

The comparator 33 has three input terminals, and is a voltage obtained by subtracting the input voltage to the third input terminal from the sum of the input voltage to the first input terminal and the input voltage to the second input terminal. When the value is positive, a circuit that outputs a predetermined high-level (normally, power supply potential) output voltage, and otherwise outputs a predetermined low-level (normally, ground potential) output voltage. is there.

The first input terminal of the comparator 33 is connected to the connection point between the resistors 25 and 26, and the second input terminal is connected to the connection point between the resistors 23 and 24 and the photocoupler 22. The third input terminal is connected to the emitter of the phototransistor 22b, and is connected to the triangular wave generator 32.

When the input voltage from the overcurrent protection circuit 34 is at a low level, the tristate buffer 31 outputs the output of the comparator 33 as it is, and the input level from the overcurrent protection circuit 34 is at a high level. In such a case, the output voltage is undefined.

As described above, the control circuit 14 includes the error detection circuit 21, the photocoupler 22, the resistors 23 and 24, the soft start circuit 60, the triangular wave generator 32, the comparator 33, and the tristate buffer 31.

The overcurrent protection circuit 34 inputs a high-level output signal to the tristate buffer 31 when the input voltage to this circuit is equal to or higher than the first threshold value Vt, and supplies a control pulse signal to the MOSFET 4. This is a circuit that stops and reduces the duty ratio of the control pulse signal.

In this overcurrent protection circuit 34, the resistance 3
Reference numeral 6 denotes a resistor that connects one end of the resistor 35 and the capacitor 37, and the capacitor 37 is a capacitor that connects the resistor 36 and a ground point. Note that the resistor 36 and the capacitor 37 constitute an integrating circuit for suppressing high-frequency noise.

The reference voltage source 39 has a negative terminal grounded, a positive terminal connected to the comparator 38, a voltage higher than the voltage obtained from the resistor 35 during normal operation, and a voltage obtained from the resistor 35 when an overcurrent occurs. This is a power supply that generates a reference voltage Vt (first threshold value) lower than a given voltage value.

Further, the comparator 38 has a first input terminal connected to a connection point between the resistor 36 and the capacitor 37, a second input terminal connected to the positive terminal of the reference voltage source 39, and a first input terminal. When the voltage value obtained by subtracting the input voltage to the second input terminal from the input voltage to the terminal is positive, an output voltage of a predetermined high level (normally, power supply potential) is output. Is a circuit for outputting an output voltage of a predetermined low level (normally, a ground potential). Note that the output voltage of the comparator 38 is
1 is input.

When the voltage value at the rising edge of the voltage signal detected by the resistor 35 and supplied via the integrating circuit (the resistor 36 and the capacitor 37) is equal to or more than the second threshold value Vt2, the level shift circuit 40 This is a circuit for increasing the voltage value at the rising edge of the voltage signal.

FIG. 2 shows the level shift circuit 4 in FIG.
FIG. 4 is a circuit diagram illustrating a configuration example of a 0.

In the level shift circuit 40 shown in FIG. 2, the resistor 42 has one end connected to the resistor 35 and the other end connected to the first input terminal of the comparator 45. One end of the resistor 43 is connected to the comparator 45,
The other end is a resistor grounded.

The reference voltage source 44 is a power supply for generating a predetermined reference voltage VR. The predetermined reference voltage VR is set such that a value (second threshold) obtained by multiplying the reference voltage VR by the reciprocal of the voltage dividing ratio by the resistors 42 and 43 is lower than the reference voltage (first threshold) Vt. You.

The comparator 45 has a first input terminal connected to the connection point between the resistors 42 and 43, a second input terminal connected to the reference voltage source 44, and a second input terminal connected to the second input terminal. If the voltage value obtained by subtracting the input voltage to the input terminal of the input terminal is positive, a predetermined high level (normally, power supply potential)
, And otherwise outputs a predetermined low-level (normally, ground potential) output voltage.

The voltage holding circuit 50 operates for a predetermined time T from the time when the output value of the comparator 45 is at a high level.
This is a circuit that outputs a voltage equal to or higher than a first threshold Vt.

In the voltage holding circuit 50, the resistance 46
Is a resistor having one end connected to the output terminal of the comparator 45 and the other end connected to the first input terminal of the resistor 36. The resistor 48 has one end connected to the resistor 46 and the other end grounded. The capacitor 49 has one end connected to the resistor 4
6 and a capacitor grounded at the other end. The diode 47 is a diode having an anode connected to the output terminal of the comparator 45 and a cathode connected to the resistors 46 and 48 and the capacitor 49.

The soft start control circuit 19 is a circuit for controlling the soft start circuit by the resistors 25, 26, 28 and the capacitor 27 so as to perform a soft start when returning to a normal operation state when an overcurrent occurs. is there.

In the soft start control circuit 19, N
The PN transistor 41 has an emitter grounded, a base connected to the level shift circuit 40, and a collector connected to the PN transistor 41.
Connected to the base of P-type transistor 30. Also, P
The NP transistor 30 has an emitter connected to the capacitor 27.
Of the NPN transistor 41
, And the collector is connected to the other end of the capacitor 27.

The secondary winding of the transformer 5 and the MOSFE
T6, 7, the choke coil 8, the output capacitor 9, and the control circuit 14 constitute a secondary output circuit.

Next, the operation of the above device will be described.

First, the operation during normal operation will be described.

During normal operation, the input signal applied to the input terminals 1 and 2 is applied as a pulse to the primary winding of the transformer 5 by the switching operation of the MOSFET 4. As a result, a voltage corresponding to the voltage applied to the primary winding is generated in the secondary winding of the transformer 5, and the MOSFE
Rectified by T6, T7, smoothed by the choke coil 8 and the output capacitor 9, the rectified and smoothed secondary voltage is applied to the output terminals 10, 11 as an output voltage.

The output voltage applied to the output terminals 10 and 11 is detected by the error detection circuit 21 of the control circuit 14, and a voltage corresponding to an error between the voltage and the rated output voltage (hereinafter referred to as an error voltage). ) Is the photo coupler 22
And the resistors 23 and 24, the second
Is applied to the input terminal.

During normal operation, the comparator 33
To the first input terminal, a voltage obtained by dividing the power supply voltage + Vcc by the resistors 25 and 26 is applied. Further, a triangular wave signal generated by the triangular wave generator 32 is applied to a third input terminal of the comparator 33.

If the voltage obtained by subtracting the voltage value of the triangular wave signal from the sum of the voltage obtained by voltage division by the resistors 25 and 26 and the error voltage is positive, the comparator 33 raises the output voltage to high. Level, otherwise low level. Thereby, MOSFET4
Is obtained at the same frequency as the frequency of the triangular wave signal and at a duty ratio corresponding to the output voltage.

The control pulse signal thus obtained is input to the MOSFET 4 via the tristate buffer 31 during normal operation, and the MOSFET 4 performs a switching operation according to the voltage level of the control pulse signal.

Therefore, if the error voltage is positive, the time during which the output voltage is at the high level is reduced (ie, the pulse width of the control pulse signal is reduced), and the time during which the MOSFET 4 is in the on state is shortened. If the error voltage is negative, the time during which the output voltage is at a high level is increased (ie, the pulse width of the control pulse signal is increased), and
By increasing the time that the SFET 4 is in the ON state, the MOSF
ET4 is PWM controlled. Thereby, the output terminal 1
The output voltages at 0 and 11 are controlled so as to approach the rated output voltage.

Next, the operation when the apparatus is started will be described.

When the apparatus is started, the voltage of the power supply + Vcc changes from zero to a predetermined voltage + Vcc. At this time, the input voltage to the first input terminal of the comparator 33, that is, the voltage at the connection point between the resistors 25 and 26 is
The voltage gradually increases from zero based on a time constant determined by the resistance values of 5, 26, and 28 and the capacitance value of the capacitor 27. When a steady state is reached, the voltage + Vcc is divided by the resistors 25 and 26. Voltage value. That is, in the soft start circuit 60, the input voltage to the first input terminal of the comparator 33 is increased to a predetermined voltage value with a predetermined time constant by the resistors 25, 26, 28 and the capacitor 27.

Therefore, as time elapses from the start, the input voltage to the first input terminal of the comparator 33 increases. As a result, the time during which the output voltage of the comparator 33 is at the high level gradually increases, and the pulse width of the control pulse signal gradually increases. As a result, soft start at the time of startup is realized by the soft start circuit 60.

Finally, the operation when an overcurrent occurs will be described.

When the output voltage decreases (that is, when the output current increases), the control circuit 14 controls the output voltage to be rated as described above, and the pulse width of the control pulse signal decreases. However, if an overcurrent occurs, it becomes difficult for the control circuit 14 to adjust the pulse width.

Therefore, when an overcurrent occurs, the MOSFET 4 is instantaneously turned off by the overcurrent protection circuit 34, and a soft start is executed by the soft start control circuit 19 when returning to the normal operation.

In the overcurrent protection circuit 34, the level shift circuit 40 monitors the potential at one end of the resistor 35 via an integrating circuit including the resistor 36 and the capacitor 37.

The comparator 45 of the level shift circuit 40
When the voltage value obtained by dividing the voltage detected from the resistor 35 by the resistors 42 and 43 exceeds the reference voltage VR, the output voltage is set to a high level. Therefore, as shown in FIG.
When an overcurrent occurs at time T1, the output voltage of the comparator 45 goes high only during the pulse width of the voltage signal detected by the resistor 35 at that time. During this high level period, the capacitor 49 is mainly connected through the diode 47.
The electric charge is accumulated. Thereby, even if there is an integrating circuit of the resistor 36 and the capacitor 37, if an overcurrent occurs, the output voltage of the comparator 45 rises sharply, so that the output voltage to the first input terminal of the comparator 38 is accurately adjusted at that time. Is higher than the reference voltage Vt.

Thereafter, when the output voltage of the comparator 45 becomes low level, the resistors 46 and 48 and the capacitor 4
The input voltage to the first input terminal of the comparator 38 decreases according to the time constant according to 9 and becomes lower than the reference voltage Vt after the time T has elapsed.

Therefore, during this time period T, the output voltage of the comparator 38 becomes low level,
The supply of the control pulse signal to the MOSFET 4 is stopped by the tri-state buffer 31, and the MOSFET 4 is turned off.

When the period elapses, the comparator 38
Returns to the high level, the device returns to the normal operation state, and the control pulse signal is supplied to the MOSFET 4 via the tri-state buffer 31. If an overcurrent occurs again after returning to the normal operation state, the above operation is repeated until the cause of the overcurrent is eliminated.

On the other hand, when an overcurrent is detected and the output voltage of the level shift circuit 40 increases, the NPN transistor 41 of the soft start control circuit 19 turns on, and accordingly, the PNP transistor 30 turns on. Then, the charge stored in the capacitor 27 is discharged, and the input voltage to the first input terminal of the comparator 33 is reduced by the resistor 25 and the resistors 26 and 28 to a voltage value obtained by dividing the voltage + Vcc.

When the output voltage of the level shift circuit 40 decreases, the NP of the soft start control circuit 19
The N-type transistor 41 is turned off, and accordingly, the PNP transistor 30 is turned off. From the point in time when the PNP transistor 30 is turned off, the accumulation of electric charge in the capacitor 27 is started again.
The input voltage to the first input terminal of the comparator 33 gradually increases.

Therefore, the interval between the pulses of the control pulse signal output from the comparator 33 (that is, the pulse period) becomes longer once the PNP transistor 30 is turned off until the normal operation state is reached. It gets shorter gradually. That is, when the normal operation state is restored after the occurrence of the overcurrent, the soft start is performed in the same manner as when the apparatus is started.

As described above, when an overcurrent occurs,
Since the MOSFET 4 is turned off instantaneously, the output current is suppressed, and the drooping characteristic as shown in FIG. 4 is obtained.

As described above, according to the above embodiment,
A control circuit 14 that generates a control pulse signal corresponding to the output voltage of the MOSFET 4 connected to the primary side of the transformer 5 and controls the MOSFET 4 with the control pulse signal
And a resistor 35 that detects a current on the primary side of the transformer 5 and generates a voltage signal having a value corresponding to the current value, and a control pulse signal when the voltage value of the voltage signal is equal to or more than a first threshold value Vt. An overcurrent protection circuit 34 for reducing the duty ratio of the voltage signal and increasing the voltage value of the rising edge of the voltage signal when the voltage value of the rising edge of the voltage signal generated by the resistor 35 is equal to or more than the second threshold value Vt2. Since the level shift circuit 40 is provided, even if an overcurrent occurs, the primary current and the output current are instantaneously suppressed, and damage to the semiconductor element due to the overcurrent can be prevented.

Further, according to the above embodiment, when the voltage value of the rising edge of the voltage signal detected by the resistor 35 is equal to or more than the second threshold value Vt2, the level shift circuit 40 performs the predetermined time T. , The voltage value of the voltage signal
Of the MOSFET 4
Operation is stopped, and damage to the semiconductor element due to overcurrent can be prevented.

Further, according to the above embodiment, the level shift circuit 40 has the resistors 42 and 43 for comparing the voltage signal from the resistor 35 with the second threshold value Vt2, the comparator 45, and the output of the comparator 45. A voltage holding circuit that outputs a voltage attenuated by a predetermined time constant with a maximum value as a voltage signal, so that the operation of the MOSFET 4 stops for the predetermined time T, and the semiconductor element is damaged by an overcurrent. Can be prevented.

Further, according to the above-described embodiment, when the voltage value at the rising edge of the voltage signal is level-shifted by the level shift circuit 40 to the first threshold value Vt or more, the soft-start control circuit for starting the soft-start. 19, the soft start can be performed even when the operation returns to the normal operation when an overcurrent occurs, and the semiconductor element can be prevented from being damaged due to the overcurrent when returning to the normal operation. .

Further, according to the above embodiment, the error detecting circuit 21 for detecting an error in the output voltage, and the soft start circuit 60 for increasing the voltage to a predetermined voltage value with a predetermined time constant when the power is turned on. A triangular wave generator 32 that generates a triangular wave of a predetermined frequency, and a comparator 33 that compares a voltage value corresponding to the error detected by the error detection circuit, the voltage of the soft start circuit, and the triangular wave generated by the triangular wave generator. The soft start control circuit 19 is provided in the control circuit 14 when the voltage value of the rising edge of the voltage signal is level-shifted by the level shift circuit 40 to the first threshold Vt or more.
The soft start is controlled by controlling the power supply, so the soft start circuit used at startup can be used to execute the soft start even when returning to normal operation when an overcurrent occurs. It is possible to prevent the semiconductor element from being damaged due to an overcurrent at the time of returning to operation.

Further, according to the above embodiment, soft start circuit 60 includes capacitor 27 and resistors 25 and 2.
6, 28, and the soft start control circuit 19 discharges the accumulated charge in the capacitor 27 of the soft start circuit 60 to start the soft start. Therefore, the soft start control circuit 19 is realized with a simple circuit configuration. be able to.

Note that the present invention is not limited to the above-described embodiment, and various modifications and applications are possible. For example, a current transformer may be used instead of the current detection resistor 35 to detect the current on the primary side of the transformer 5.

Further, in the above embodiment, the level shift circuit 40 is configured as shown in FIG. 2, but it is a matter of course that a circuit having another configuration performing the same operation may be used. For example, the resistors 42 and 43 are omitted, and the reference voltage source 4
The reference voltage of No. 4 may be Vt2. Further, for example, the reference voltage of the reference voltage source
And the reference voltage source 44 may be one.

Further, in the above embodiment, NP
The base of the N-type transistor 41 is
Although it is connected to 0, it may be connected to the output terminal of the comparator 38. As a result, when the output voltage of the comparator 38 changes from the low level to the high level, the state of the PNP transistor 30 changes exactly from on to off, and the accumulation of charges in the capacitor 27 is started. The soft start can be accurately started from the point of return to the operation state.

[0126]

According to the present invention, it is possible to obtain a switching power supply device capable of preventing damage to a semiconductor element due to an overcurrent.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a switching power supply according to an embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a configuration example of a level shift circuit in FIG. 1;

FIG. 3 is a timing chart showing an example of an output voltage signal of a level shift circuit in FIG. 1;

FIG. 4 is a diagram showing drooping characteristics in the switching power supply device of FIG. 1;

FIG. 5 is a circuit diagram showing an example of a conventional switching power supply device.

6 is a timing chart illustrating an example of a voltage signal input to a comparator of the overcurrent protection circuit in FIG.

FIG. 7 is a diagram illustrating drooping characteristics of a conventional switching power supply device.

[Explanation of symbols]

 4 MOSFET (semiconductor element) 5 transformer 14 control circuit 19 soft start control circuit 21 error detection circuit 32 triangular wave generator 33 comparator 34 overcurrent protection circuit 35 resistance (current detection circuit) 40 level shift circuit 45 comparator 50 voltage holding circuit 60 software Start circuit

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 5G004 AA04 AB02 BA04 DA02 DB03 DC04 DC09 EA01 5G053 AA01 BA01 BA04 CA01 DA01 EB02 EC03 5H730 AA20 AS01 BB23 BB57 DD04 DD26 EE08 EE14 FD03 FD43 FF02 FG05 FG26 XX15 XXXXXX

Claims (6)

[Claims] A transformer, a semiconductor element connected to a primary side of the transformer, a control circuit for generating a control pulse signal corresponding to an output voltage, and controlling the semiconductor element with the control pulse signal; A current detection circuit for detecting a current on the primary side and generating a voltage signal having a value corresponding to the current value; and detecting the control pulse signal when the voltage value of the voltage signal is equal to or greater than a predetermined first threshold value. An overcurrent protection circuit for lowering a duty ratio, and a voltage value at a rising edge of the voltage signal is a predetermined second value.
And a level shift circuit that increases the voltage value of the rising edge of the voltage signal when the voltage value is equal to or larger than the threshold value. 2. The level shift circuit according to claim 1, wherein a voltage value of a rising edge of the voltage signal generated by the current detection circuit is equal to or more than the second threshold, and the level shift circuit changes a voltage value of the voltage signal for a predetermined time. The switching power supply according to claim 1, wherein the switching power supply is maintained at a value equal to or higher than the first threshold. 3. The level shift circuit according to claim 1, wherein the level shift circuit compares the voltage signal generated by the current detection circuit with the second threshold value, and attenuates the output value of the comparator as a maximum value with a predetermined time constant. 3. The switching power supply device according to claim 2, further comprising a voltage holding circuit that uses a rising voltage as the voltage signal. 4. A soft start control circuit for starting a soft start when a voltage value at a rising edge of the voltage signal is level-shifted by the level shift circuit to be equal to or more than the first threshold. The switching power supply device according to claim 1. 5. A control circuit comprising: an error detection circuit for detecting an error in an output voltage; and a soft start circuit configured to increase a voltage to a predetermined voltage value with a predetermined time constant when power is turned on. And a triangular wave generator that generates a triangular wave of a predetermined frequency, a voltage value corresponding to the error detected by the error detection circuit, the voltage of the soft start circuit, and a triangular wave generated by the triangular wave generator. And a comparator, wherein the soft start control circuit controls the soft start circuit when the voltage value of the rising edge of the voltage signal is level-shifted by the level shift circuit to the first threshold or more. The switching power supply device according to claim 4, wherein a soft start is performed. 6. The soft start circuit includes a capacitor and a resistor, and the soft start control circuit starts soft start by discharging a charge stored in the capacitor of the soft start circuit. The switching power supply device according to claim 5.