JP2888729B2 - Output short circuit protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output short-circuit protection circuit for a switching power supply such as a DC / DC converter, which can provide a stable and reliable output protection function and reduce the size of a power supply circuit. It is.

[0002]

2. Description of the Related Art A switching power supply such as a DC / DC converter usually has a DC input terminal 1 as shown in FIG.
The unstable DC voltage input to 0 and 11 is turned on and off by the switch element 14 whose pulse width is controlled by the PWM control IC 21, and the choke coil 1
5, a stabilized DC voltage is obtained at output terminals 12 and 13 via a rectifying and smoothing circuit 18 comprising a diode 16, a capacitor 17 and the like. 26, 2
7 and 28 are resistors.

In this circuit, in a constant voltage mode, a voltage obtained by dividing an output voltage by output voltage dividing resistors 19 and 20 is compared with a voltage of a DC power supply 29 by a constant voltage amplifier 23, and this output is outputted by a PWM comparator 30. 3 of triangular wave oscillator 31
The pulse width of the switch element 14 is determined via the transistor 32 in comparison with the square wave to stabilize the output voltage.
When the output current increases, the overcurrent amplifier 22 operates due to the voltage drop due to the overcurrent detection resistor 24 to make the PWM comparator 30 dominant. Therefore, an overcurrent protection circuit (output) that keeps the output current constant and lowers the output voltage Short circuit protection circuit).

However, a switching power supply such as a DC / DC converter is a commercial power supply having a high power supply voltage.
Although it does not matter much, recently, a low voltage from a battery is generally input, so that even if the output power is as small as several W, the passing current of the switch element 14 is as large as several A. Therefore, if the current of the switch element 14 is detected by the voltage drop of the overcurrent detection resistor 24 inserted in series with the switch element 14, the power loss due to the overcurrent detection resistor 24 is proportional to the square of the current. There is a drawback that the power loss is large and the total conversion efficiency is deteriorated.

Therefore, when the output terminals 12 and 13 are short-circuited, the output short-circuit protection circuit of the timer latch type shown in FIG. Have been. In FIG. 7, in the constant voltage mode, the signal from the error amplifier 33 is input to the PWM comparator 30 and the pulse width is determined by comparing with the triangular wave from the triangular wave oscillator 31 to stabilize the output voltage. . When the output terminals 12 and 13 are short-circuited, the short circuit protection amplifier 34 detects that the output voltage has dropped, and turns off the transistor 35. Therefore, the time constant of the time constant circuit 39 including the resistor 37 and the capacitor 38 Thereafter, the latch circuit 40 is operated to stop the signal to the switch element 14, and as a result, the power supply circuit is stopped.

[0006]

Since the output short-circuit protection circuit of the timer latch type as shown in FIG. 7 does not have the overcurrent detecting resistor 24 as shown in FIG. 6, the total conversion efficiency is deteriorated due to the power loss. There is no disadvantage. However, the timer latch system has a time constant circuit 39 of 60 to 100 msec by charging the resistor 37 and the capacitor 38 so that the latch circuit 40 does not operate when the power is turned on. However, if the output voltage decreases during an overload such as an output short circuit, the latch circuit 40 does not operate unless the delay of the time constant circuit 39 occurs, the switching power supply operates in the overload state, and the switching element 14 Current and choke coil 1
5 and the diode 16 may be damaged by an excessive current. Therefore, there is a problem in that these components use a larger capacity than required for the rated output power, resulting in an expensive power supply circuit.

An object of the present invention is to provide a stable and reliable output protection function and to reduce the size of a power supply circuit.

[0008]

According to the present invention, a + input terminal is provided.
The DC input voltage supplied between (10) and the-input terminal (11) is turned on and off by the switch element (14);
In a switching power supply in which the pulse width is controlled by a PWM control circuit (57) in response to a voltage change to obtain a stable output voltage, a current when the switch element (14) is turned on is a voltage by an on-resistance. A detection unit (49) for detecting a drop and a PWM control circuit (57) for outputting when the output voltage of the detection unit (49) exceeds a certain value and limiting the ON time of the switch element (14); And a comparator (56) for controlling the detection unit (4).
9) is disposed between the output side of the switch element (14) and the- input terminal (11) side, and the switch element (1)
Synchronous switch element (5) that turns on and off in synchronization with (4)
0), a series circuit of voltage dividing resistors (51) and (52), and the comparator (56) compares the voltage detected by the voltage dividing resistors (51) and (52) with a set reference voltage. An output short-circuit protection circuit characterized in that:

[0009]

In such a configuration, the switching element (1)
Since the saturation voltage when 4) is turned on is directly proportional to the current passing through the switch element (14) due to the ON resistance (Rds) of the MOS-FET, the switch element (14) is turned on.
In addition, the detection unit (49) detects a drain current (Id) when an excessive current flows due to an output short circuit or the like as a voltage drop (Vds) due to the on-resistance (Rds).
= Id × Rds and the voltage dividing resistor (51) of the detecting unit (49)
A signal obtained by comparing the midpoint voltage obtained by dividing the voltage at the step (52) with the reference voltage (Vref) by the zener diode (55) by the comparator (56) is a duty ratio control terminal of the PWM control circuit (57), for example, Return to the time control terminal. When the midpoint voltage of the detection section (49) exceeds the reference voltage (Vref), the on-time of the switch element (14) is shortened to perform overcurrent protection, thereby improving the disadvantage of the timer latch system.

[0010]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the basic principle of the present invention, in which + and- DC input terminals 10 and 11 and + and- output terminals 12 and 1 are connected.
3, rectifying and smoothing circuit 18 including switch element 14, choke coil 15, diode 16, and capacitor 17
Is the same as FIG. 6 and FIG. The switching element 14 used for the switching power supply such as the DC / DC converter shown in FIG. 1 often uses a MOS-FET, and the saturation current of the switching element 14 when it is turned on is
The voltage becomes proportional to the passing current flowing through the switch element 14 due to the ON resistance of the MOS-FET. Focusing on this, the present invention detects a drain current when the switch element 14 is turned on when an excessive current flows through the switch element 14 due to an output short circuit or the like as a voltage drop due to the ON resistance of the MOS-FET, and detects this by PWM control. By feeding back to the duty ratio control terminal of the circuit 57 and obtaining the same effect as the overcurrent protection, the disadvantage of the timer latch system, that is, the switching element 14 is set to an excessive current state during the delay time and operates after the delay time To stop the problem.

Basically, as shown in FIG. 1, on the train side of the switch element 14, that is, the switch element
A detection unit 49 including a DC circuit of a synchronous switch element 50, a voltage dividing resistor 51, and a voltage dividing resistor 52 is inserted in parallel with the diode 16 between the output side of the PB 14 and the-input terminal 11 side. A series circuit of a resistor 54 and a Zener diode 55 is inserted between the DC input terminals 10 and 11 to set a reference voltage (Vref). The connection point between the resistor 54 and the Zener diode 55 is connected to the + side of the comparator 56, and the connection point between the voltage dividing resistors 51 and 52 is connected to the-side. The output side of this comparator 56 is P
Connected to the duty ratio control terminal of the WM control circuit 57,
The output side of the M control circuit 57 is connected to the switch element 14 and the synchronous switch element 50 via the drive circuit 53.

The operation of the basic principle diagram shown in FIG. 1 will be described. The saturation voltage when the switch element 14 is turned on is directly proportional to the current passing through the switch element 14 due to the ON resistance (Rds) of the MOS-FET. Therefore, when the switching element 14 is on and an excessive current flows due to an output short circuit or the like, the drain current (Id) is reduced by the on-resistance (Rds).
Is detected as a voltage drop (Vds = Id × Rds ) due to the voltage drop, and the voltage Vds is detected by the voltage dividing resistors 51 and 5 of the detecting unit 49.
2 and the reference voltage (Vref) from the Zener diode 55 are compared by a comparator 56, and the comparison signal is fed back to a duty ratio control terminal of a PWM control circuit 57, for example, a dead time control terminal. I do. When the midpoint voltage of the detection section 49 exceeds the reference voltage (Vref), the on-time of the switch element 14 is shortened to perform overcurrent protection, thereby improving the disadvantage of the timer latch system.

FIG. 3 shows a specific embodiment of the present invention.
An FET is used, and the drive circuit 53 is composed of a transistor 60 and resistors 61 and 62. Also, 6
3 is an input-side capacitor, and 64 and 65 are body diodes.

The operation of the circuit shown in FIG. 3 will be described.
2A, the drain current (Id) of the switch element 14 flows as shown in FIG.
A voltage (Vs) as shown in (c) is generated. That is, the voltage (Vs) is a value obtained by subtracting the voltage drop due to the drain current (Id) from the input voltage (Vi). As the drain current (Id) increases, the voltage drop increases. The drain current (I
d) and a voltage drop proportional to the on-resistance (Rds) are applied to the detection unit 49, the midpoint voltage divided by the voltage dividing resistors 51 and 52 is applied to the negative input side of the comparator 56, and A reference voltage (Vref) from the zener diode 55 is applied to the input side.

In the case of a steady load, the switching element 1
4 is off, since the voltage dividing resistors 51 and 52 are connected to the output terminals 12 and 13 via the body diode 65 of the synchronous switching element 50, the voltage dividing resistor 5
The midpoint voltages of 1 and 52 are higher than the reference voltage (Vref), the output of the comparator 56 is L, the output short-circuit protection circuit does not operate, and the PWM control circuit 57 performs normal constant voltage control. Do.

When the switching element 14 is turned on under a steady load, the midpoint voltage of the voltage dividing resistors 51 and 52 due to the voltage drop generated by the drain current (Id) and the on-resistance (Rds) is determined by the reference voltage (Vref). By setting higher, the output of the comparator 56 remains at L, the output short-circuit protection circuit does not operate, and the PWM control circuit 57 can perform normal constant voltage control. This can be expressed by an equation:
It looks like this: Vref ≦ Vin × (R1 + R2) ÷ R2-Id × Rds

Next, in the case of an excessive load, the switching element 1
4 is turned on, the midpoint voltage of the voltage dividing resistors 51 and 52 due to the voltage drop generated by the drain current (Id) and the on-resistance (Rds) becomes lower than the reference voltage (Vref), so that PWM is generated. When the output of the control circuit 57 becomes H, the voltage of the duty ratio control terminal of the PWM control circuit 57 is increased, the ON time of the switch element 14 is shortened, and overcurrent protection is performed. Function. Here, since the synchronous switch element 50 is for the purpose of establishing the voltage dividing resistors 51 and 52 only when the switch element 14 is turned on, a small electronic component for signal can be used.

Since the on-resistance of the switch element 14 made of a MOS-FET changes depending on the temperature, the circuit of the present invention is more suitable for short-circuiting the output than the operation as an overcurrent protection circuit for setting the operating point accurately. Since the operating point is set to be two to three times the drain current (Id) at the time of a steady load so as to operate only at the time of excessive load as described above, it can be considered as a pure output short-circuit protection circuit.

FIG. 4 shows a second embodiment of the present invention. FIG. 4 shows a step-down DC / DC converter circuit having an output short-circuit protection function floating on a power supply line as an output short-circuit protection circuit. That is, an auxiliary power supply composed of a series circuit of a diode 70 and a capacitor 71 is inserted in parallel between the drain and the source of the switch element 14, and the input / output voltage difference when the switch element 14 is turned off is charged to the capacitor 71 through the diode 70. When the switch element 14 is turned on, the comparator 5
Power is supplied to 6. A resistor 5 is connected between the connection point between the diode 70 and the capacitor 71 and the DC input terminal 10.
4 and a series circuit of a Zener diode 55 to make a reference voltage (Vref). A connection point of the resistor 54 and the Zener diode 55 is connected to the + input side of a comparator 56. The other end of the capacitor 71 is synchronized. It is connected to the minus input side of the comparator 56 via the switch element 50.

The operation of FIG. 4 is the same as that of the step-down DC / DC converter circuit shown in FIG. 3 in which an output short-circuit protection circuit is provided on the ground side. To further explain with reference to the waveform diagram of FIG. 5, the drain-source voltage (Vds) of the switch element 14 is equal to that of the switch element 1 as shown in FIG.
4 is turned on, a value obtained by subtracting the voltage drop due to the drain current (Id) is applied. A voltage proportional to this voltage drop is applied to the comparator 56, and the reference voltage (Vref)
Is compared to Other operations are the same as those in FIG.

The present invention is not limited to the DC / DC converter as shown in FIGS.
The invention can also be applied to an inverting DC / DC converter in which the choke coil 15 and the diode 16 are exchanged so that the polarity of the diode 16 is such that the cathode is on the output side. In the above-described embodiment, the MOS-
Although an FET is used, the invention is not limited to this, and a bipolar transistor may be used. A bipolar transistor has a voltage drop that is not directly proportional to a change in flowing current, but is effective in detecting an abnormality such as an output short circuit that does not require accuracy. In the present invention, the switch element 14,
Although the synchronous switch element 50 is shown using the P channel, it can be realized with the N channel by devising the drive circuit 53.

[0022]

【The invention's effect】

(1) There is no delay time at the time of power supply startup or output short circuit as in the timer latch method, the peak current of the switch element 14 can be regulated, reliable operation is possible, and the reliability of the power supply circuit can be improved. . (2) Since the electronic elements such as the switch element 14, the choke coil 15, and the diode 16 can be reduced to their original ratings, they can be replaced with small parts, and the entire power supply circuit can be reduced in size and cost.

[Brief description of the drawings]

FIG. 1 is a basic block diagram showing one embodiment of an output short-circuit protection circuit according to the present invention.

FIG. 2 is an operation waveform diagram of the circuit of the present invention.

FIG. 3 is a specific electric circuit diagram showing an embodiment of an output short-circuit protection circuit according to the present invention.

FIG. 4 is an electric circuit diagram showing another embodiment of the output short-circuit protection circuit according to the present invention.

5 is an operation waveform diagram of the circuit of FIG.

FIG. 6 is an electric circuit diagram of a conventional switching power supply.

FIG. 7 is an electric circuit diagram of a conventional timer latch type switching power supply.

[Explanation of symbols]

10 DC input terminal, 11 DC input terminal, 12 output terminal, 13 output terminal, 14 switch element, 15 choke coil, 16 diode, 17 capacitor
Reference numeral 18: rectifying and smoothing circuit, 19: output voltage dividing resistor, 20: output voltage dividing resistor, 21: control IC, 22: overcurrent amplifier, 23: constant voltage amplifier, 24: overcurrent detection resistor, 25
... Reference voltage terminal, 26 ... Resistance, 27 ... Resistance, 28 ... Resistance, 29 ... DC power supply, 30 ... PWM comparator, 31
... triangular wave oscillator, 32 ... transistor, 33 ... error amplifier, 34 ... short circuit protection amplifier, 35 ... transistor, 36 ... resistor, 37 ... resistor, 3
8: Capacitor, 39: Time constant circuit, 40: Latch circuit, 49: Detection unit, 50: Synchronous switch element, 51: Voltage dividing resistor, 52: Voltage dividing resistor, 53: Drive circuit, 54 ...
Resistor, 55: Zener diode, 56: Comparator, 57: PWM control circuit, 60: Transistor, 61
... Resistance, 62 ... Resistance, 63 ... Input side capacitor, 64 ...
Body diode, 65 body diode, 70 diode, 71 capacitor.

Claims (2)

(57) [Claims] A positive input terminal (10) and a negative input terminal (1
The DC input voltage supplied during 1) is turned on and off by the switch element (14), and the pulse width thereof is controlled by the PWM control circuit (57) in accordance with the fluctuation of the voltage so that a stable output voltage is obtained. In the switching power supply that was obtained,
A detection unit (49) for detecting a current when the switch element (14) is turned on as a voltage drop due to an on-resistance; and a switch (14) which outputs when the output voltage of the detection unit (49) exceeds a certain value and outputs the voltage. PW to limit on-time
Comprising a comparator (56) for controlling the M control circuit (57), wherein the detection unit (49), the switching element
A synchronous switch element (50) that is arranged between the output side of (14) and the- input terminal (11) side and turns on and off in synchronization with the switch element (14); a voltage dividing resistor (51);
The voltage dividing resistor (51)
And a reference voltage set by the comparator (56) and a set reference voltage. 2. A positive input terminal (10) and a negative input terminal (1).
The DC input voltage supplied during 1) is turned on and off by the switch element (14), and the pulse width thereof is controlled by the PWM control circuit (57) in accordance with the fluctuation of the voltage so that a stable output voltage is obtained. In the switching power supply that was obtained,
A detection unit (49) for detecting a current when the switch element (14) is turned on as a voltage drop due to an on-resistance; and a switch (14) which outputs when the output voltage of the detection unit (49) exceeds a certain value and outputs the voltage. PW to limit on-time
Comprising a comparator (56) for controlling the M control circuit (57), wherein the detection unit (49), the switching element
A synchronous switch element (50) arranged between the output side of (14) and the + input terminal (10) side and turned on and off in synchronization with the switch element (14); In parallel with this, an auxiliary power supply circuit composed of a diode (70) and a capacitor (71) is inserted, and the power supply of the comparator (56) and a reference voltage on one input side of the comparator (56) are obtained by the auxiliary power supply circuit. , The output side of the switch element (14) is connected to the synchronous switch element (5
An output short-circuit protection circuit characterized in that the output short-circuit protection circuit is connected to the other input side of the comparator (56) via the first input terminal (0) and is compared by the comparator (56).