JP2869328B2 - Switching power supply - Google Patents

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 device in which a transistor rectifying element having a synchronous rectifying function is provided with a protective function in case of an output abnormality.

[0002]

2. Description of the Related Art As a control circuit for driving a switching type power supply circuit, a PWM control method is generally and often used, and the circuit is also being integrated into an IC. In a commercially available PWM control circuit IC, when the output voltage of the power supply circuit to be fed back decreases due to an abnormality such as a short circuit, a signal for turning off the duty 100% is output to stop the operation of the switching transistor to be controlled. In many cases, the circuit is protected by this. As a power supply circuit used when an output voltage higher than the input voltage is required, a circuit as shown in FIG. 2 is well known.

The circuit shown in FIG. 2 is a so-called non-insulated chop-up DC-DC converter. First , when the switching transistor Q1 is in an ON state, the energy stored in the choke coil L1 is switched by a switching transistor.
The form of the flyback voltage when the star Q1 is turned off
And then take the flyback voltage to the input voltage V _IN
To obtain a high voltage, and charge the capacitor C2 via the diode D2 with the high voltage.
To retrieve a voltage higher than the input voltage V _IN appearing across capacitor C2 as the output voltage V _O, is that.

[0004]

By the way, the PWM control IC 3 of the power supply circuit shown in FIG.
It is assumed that a function of protecting a circuit by outputting a signal that is turned off by 0% is provided. Here, for example, when a large current flows due to a load short-circuit and the output voltage V _O decreases, the output voltage V _O is detected by feedback of a divided voltage of the output voltage V _O.
The WM control IC 3 outputs a signal with a duty of 100% off to stop the operation of the switching transistor Q1. However, in the circuit shown in FIG. 2, the input terminal 1 and the output terminal 2 are connected via the choke coil L1 and the diode D2 even if the operation as the converter is stopped by the stop of the switching operation of the switching transistor Q1. At the output terminal 2, a voltage having a value obtained by subtracting the voltage drop of the choke coil L1 and the forward voltage drop of the diode D2 from the input voltage V _IN appears.

Therefore, a protection function of a circuit for outputting a signal with a duty of 100% off provided in a general PWM control IC is a non-insulated chop-up DC-DC.
The converter does not play a role of protecting the circuit against abnormalities such as a load short circuit, and therefore, the switching power supply device using the non-insulated chop-up DC-DC converter has a drawback that the circuit is difficult to protect. Therefore, an object of the present invention is to provide a step-up switching power supply device capable of protecting a circuit from an abnormal state such as a short circuit by making a transistor rectifier element having a synchronous rectification function also have a protection function.

[0006]

According to the present invention, a choke coil and a switching element are connected in series, and when the switching element is turned off, the flyback is provided to the choke coil.
Generates a voltage, rectifies the energy due to the flyback voltage with a synchronous rectifier circuit using a transistor element as a rectifying element, and detects an abnormal state of the load from an abnormal drop in the output voltage in a switching power supply that obtains a desired output voltage. outputs circuit having the function of the on output voltage abnormality
Upon receiving a that signal includes a protection transistor signal for driving the transistor rectifying elements of the synchronous rectifier circuit prevents from being <br/> input to the transistor rectifying element, the transistor rectifying element in the OFF state A switching power supply device characterized in that:

[0007]

FIG. 1 shows a circuit of a switching power supply according to the present invention in which a transistor rectifier protects a circuit from an abnormal state such as a load short circuit. The same components as those in FIG. 2 in FIG. 1 are denoted by the same reference numerals. In FIG. 1, the circuit configuration is as follows. Here, both 1 and 2 indicate an input terminal and an output terminal on the high potential side, and the input and output terminals on the low potential side are common to the ground. An input terminal 1 is connected via a choke coil L1 to the collector of a switching transistor Q1 formed of an NPN transistor, and the switching transistor Q
One emitter is connected to ground.

A connection point between the choke coil L1 and the collector of the switching transistor Q1 is connected to the emitter of the transistor Q2 formed by a PNP transistor as a transistor rectifier for synchronous rectification.
Is connected to the output terminal 2. A capacitor C1 is connected between the input terminal 1 and the ground, and a capacitor C2 and a resistor R for dividing the output voltage are connected between the output terminal 2 and the ground.
1 and a series circuit of a resistor R2. Resistance R1 and resistance R
2 is connected to the voltage detection terminal (FB) of the PWM control IC3.
And the pulse output terminal (PO) of the PWM control IC3
Is connected to the base of the switching transistor Q1.

The base of the transistor Q2 is connected to the collector of the driving transistor Q3 formed by an NPN transistor via a parallel circuit of a resistor R3 and a capacitor C3, and the emitter of the driving transistor Q3 is connected to ground.
The base of transistor Q2 is further connected to the cathode of diode D1, and the anode of diode D1 is connected to ground. A resistor R4 is connected between the emitter of the transistor Q2 and the base of the driving transistor Q3, and a resistor R5 is connected between the base and the emitter of the driving transistor Q3.
Connect. A protection transistor Q by an NPN type transistor between the base of the driving transistor Q3 and the ground.
4, the collector and the emitter are connected, and the protection transistor Q
4 is connected to a latch output terminal (SR) for signal output for protection operation of the PWM control IC 3. E indicates an external voltage source, and RL indicates a load.

The operation of the circuit configured as described above is as follows. When the switching transistor Q1 is turned off by the pulse output signal from the PWM control IC 3, a flyback voltage is generated in the choke coil L1 by the energy stored when the switching transistor Q1 was on. Therefore, a high voltage in which the flyback voltage generated in the choke coil L1 is superimposed on the input voltage V _IN is applied to the collector of the switching transistor Q1 and the emitter of the transistor Q2 as a rectifier.

The high voltage obtained by adding the input voltage V _IN applied to the emitter of the transistor Q2 and the flyback voltage is:
It is also input to the base of the driving transistor Q3 via the resistor R4. Here, the protection transistor Q4 keeps the off state unless a signal is generated at the latch output terminal (SR) of the PWM control IC3.
Are turned on by receiving a forward bias . When the driving transistor Q3 is turned on, the transistor Q2 is forwardly connected between the emitter and the base.
When the input voltage V _IN and the flyback voltage are combined, a high voltage is applied to the capacitor C2.
And the voltage appearing across the capacitor C2 is supplied to the load RL as the output voltage V _O.

Next, when the switching transistor Q1 is turned on, the voltage of the collector of the switching transistor Q1 and the voltage of the emitter of the transistor Q2 decrease, and at the same time, the voltage applied to the base of the driving transistor Q3 via the resistor R4 also decreases. As a result, the driving transistor Q3 is turned off, and the transistor Q2 is turned off by releasing the forward bias state between the emitter and the base when the driving transistor Q3 is turned off. As described above, when the load is normal, the transistor Q2 performs the on / off operation complementarily in accordance with the switching operation of the switching transistor Q1, thereby performing the synchronous rectification function.

Here, it is assumed that an abnormal condition such as a load short circuit has occurred and the output voltage V _O of the switching power supply has dropped significantly. Then, the PWM control IC 3 outputs the output voltage V _{O from} the divided voltage of the output voltage V _O fed back to the voltage detection terminal (FB) from the connection point of the resistors R1 and R2.
The protection operation is started by detecting the decrease of Due to this protection operation, the signal output from the pulse output terminal (PO) of the PWM control IC 3 is turned off at 100% duty, and the switching operation is stopped while the switching transistor Q1 remains off. At the same time, latch output terminal (SR)
From the protection transistor Q4
Is turned on.

When the switching operation is stopped while the switching transistor Q1 is in the off state, the input voltage V is substantially applied to the base of the driving transistor Q3 via the resistor R4.
_A voltage equal to _IN is applied. However, the protection transistor Q4
There base of the driving transistor Q3 by turned on, emitter are short-circuited, the base of the driving transistor Q3, emitter can not receive a forward bias. Therefore, the driving transistor Q3 is turned off, and the transistor Q2 whose operation is controlled by the driving transistor Q3 is also turned off. Here, as for the signal output from the latch output terminal (SR) of the PWM control IC 3, the output of the signal is maintained unless it is artificially reset from the outside, and as a result, the transistor Q2 keeps the off state. . As a result, the output of the switching power supply is cut off, and the conventional chop-up DC
Even when the power supply is stopped due to the protection operation, such as a switching power supply using a DC converter, almost the input voltage V
A situation where a voltage equal to _IN appears at the output terminal can be prevented.

In the above-described embodiment of the present invention shown in FIG. 1 , the PWM control IC has an output voltage abnormal drop detection function such as MB3800 (Fujitsu product) and a pulse when the output voltage drops abnormally. Output signal at duty 1
The description has been made on the assumption that a device having a function of performing a latch output at the same time as turning off by 00% is used. However, some commercially available PWM control ICs have a function of turning off the pulse output signal by 100% duty, but do not perform a latch output or do not have a function for performing a protection operation. In this case, an output voltage detection circuit or the like is separately provided, and the output signal of the protection transistor Q is used.
Means may be taken to turn 4 on.

[0016]

As described above, the switching power supply according to the present invention blocks the input of a signal for driving the transistor rectifier element when the output voltage is abnormally decreased due to a load short circuit or the like, and thereby turns off the transistor rectifier element. In this state, the output is cut off, and a transistor rectifying element that performs a synchronous rectification function also serves as a protection function. As a result, a voltage almost equal to the input voltage appears at the output terminal even if the function as the converter is stopped in an abnormal state such as a load short circuit as in the conventional non-insulated chop-up DC-DC converter. Can be prevented, and the elements constituting the circuit can be protected. In addition, since the transistor rectifying element for performing the synchronous rectification cuts off the output in the event of an abnormality, the number of elements to be added to the power supply device for the protection operation can be reduced, and the circuit is simplified.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of a conventional non-insulated switching power supply device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Input terminal 2 Output terminal 3 PWM control IC Q1 Switching transistor Q2 Transistor as rectifier element Q3 Driving transistor Q4 Protection transistor

Claims (5)

(57) [Claims] 1. A synchronous rectifier comprising: connecting a choke coil and a switching element in series; generating a flyback voltage in the choke coil when the switching element is turned off; when rectified by the circuit, the switching power supply to obtain the desired output voltage, the circuit having a function of detecting an abnormal state of the load from abnormal drop in the output voltage receives a signal output when the output voltage abnormality, the synchronous rectification A switching power supply device comprising: a protection transistor that prevents a signal for driving a transistor rectifying element of a circuit from being input to the transistor rectifying element and turns off the transistor rectifying element. 2. A synchronous rectifier in which a choke coil and a switching element are connected in series, a flyback voltage is generated in the choke coil when the switching element is turned off, and energy due to the flyback voltage is used using a transistor element as a rectifying element. In a switching power supply that rectifies in a circuit and obtains a desired output voltage, PNP
Rectifier element with bipolar transistor
And its main current path at the base of the transistor rectifier.
Connected to control the operation of the transistor rectifier, N
The driving transistor according to PN-type bipolar transistor, a signal circuit is output when the output voltage abnormality which has a function of detecting an abnormal state of the load from abnormal decrease in the output voltage
When it receives , the transistor rectifier and the choke coil
ON / OFF from the connection point to the base of the driving transistor
A switching power supply device comprising: a protection transistor that blocks supply of a control signal and keeps the transistor rectifier element in an off state. 3. The protection transistor according to claim 2, wherein an NPN-type bipolar transistor is used as the protection transistor, and a main current path thereof is connected between an emitter and a base of the driving transistor. Switching power supply. 4. A PWM control IC having a protection latch output.
3. A control circuit for driving the switching element, wherein a protection latch output of the PWM control IC is used as a signal output when the output voltage is abnormal. Item 3. The switching power supply device according to Item 3. 5. The method according to claim 2, wherein said transistor rectifier is turned off when said output voltage is abnormal, and said switching element is turned off by 100% at the same time as said transistor rectifier. The switching power supply described in the paragraph.