JP2737069B2 - 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 circuit capable of arbitrarily setting a starting point of an intermittent operation of overcurrent protection.

[0002]

2. Description of the Related Art FIG. 3 shows a basic circuit of a conventional switching power supply. This circuit includes a primary winding N of a converter transformer 3 between input terminals 1a and 1b of a switching power supply.
1, a collector and an emitter terminal of an NPN type switching transistor Q1 and a resistor R1 are connected in series. The input terminal 1b is further connected to the ground. Both ends of the secondary winding N2 of the converter transformer 3 are connected to an output terminal 2 via a rectifying and smoothing circuit including a diode D1, a flywheel diode D2, a choke coil L1, and a smoothing capacitor C1.
a, 2b.

With the recent miniaturization of power supply circuits, circuits for controlling and driving switching power supplies are contained in one package. For example, M51977P manufactured by Mitsubishi Electric Corporation.
PWMMIC5 is used. Inside the PWMIC 5, a pulse width modulation circuit 6, a current limiting circuit 7, and an intermittent operation circuit 8 are roughly divided. Output terminal 2
a, the output voltage V ₀ to the feedback circuit 4 2b is input, the voltage signal corresponding to the output voltage V ₀ from the feedback circuit 4 to the pulse width modulation circuit 6 and the intermittent operation circuit 8 is input.

An output pulse voltage is input from the pulse width modulation circuit 6 to the base of the switching transistor Q1.
A voltage signal corresponding to the switching current is input to the current limiting circuit 7 from a connection point between the resistor R1 and the emitter of the switching transistor Q1. A capacitor C2 for setting the time constant of the intermittent operation is connected between the CT terminal of the intermittent operation circuit 8 and the ground.

FIG. 4 shows an output current-output voltage characteristic of a conventional switching power supply having the above-described circuit configuration. Normally, when a large current flows through a circuit due to a short circuit on the load side, the switching power supply usually has a drooping characteristic or a square-shaped characteristic in its output current-output voltage characteristic so as not to damage circuit elements. I have to.

If the PWMIC 5 has no intermittent operation function, the intermittent operation circuit 8 and the capacitor C2 shown in the circuit diagram of FIG.
Is not present, the output current of the switching power supply-
The output voltage characteristic shows a drooping characteristic as shown by the solid line in FIG. In this case, the current limiting circuit 7 operates by detecting the switching current on the primary side of the converter transformer 3, and the ON period of the output pulse signal of the pulse width modulation circuit 6 is shortened to suppress an increase in the output current _IO. ing. However, since the ON period of the output pulse signal of the pulse width modulation circuit 6 it does not become zero, not only to a certain value smaller, the output voltage V _O
A large current flows near the output short-circuit where the value becomes smaller. The large current near the output short circuit damages each element on the output side from the secondary winding N2 of the converter transformer 3, particularly the diode D1. Therefore, as a countermeasure, as shown in FIG. 3, the PWMMIC 5 is provided with an intermittent operation function to prevent a large current from flowing.

The intermittent operation function of the conventional PWMMIC 5 will be described. When the output current _IO increases, an overcurrent state is detected from the switching current, and the current limiting circuit 7 operates to output an overcurrent signal S _CLM . Since the pulse width modulation circuit 6 shortens the ON period of the output pulse signal according to the overcurrent signal S _CLM , the relationship between the output current I _O and the output voltage V _O shows a drooping characteristic. Eventually, when the ON period of the output pulse signal becomes short and the average value _SPM of the output pulse signal becomes smaller than a predetermined value, the intermittent operation circuit 8 outputs the intermittent operation signal S _OFF to the pulse width modulation circuit 6. . Upon receiving the intermittent operation signal S _OFF , the pulse width modulation circuit 6 starts an intermittent operation in which operation and stop are repeated for a time determined by the capacitor C2 and the output voltage V _O. By the above operation, the output current of the circuit shown in FIG. 3 - output voltage characteristics after showing drooping characteristic, as indicated by a dotted line in FIG. 4, the output current I with a drop in the output voltage V _O by the intermittent operation _O decreases.

[0008] FIG. 4 is a high input voltage V _INmax in voltage value to the switching power supply, low input voltage of the voltage value V _INmin
2 shows the output current-output voltage characteristics in two cases where is applied. In the power supply circuit, as the voltage value of the DC input voltage V _IN is lower, a larger amount of switching current must flow through the primary winding N1 of the converter transformer 3 in order to keep the output voltage V _O constant. In the circuit of FIG. 3, the current limiting circuit 7 includes a converter transformer 3
The overcurrent state is detected based on the magnitude of the primary side switching current. Since the switching current flows more as the input voltage V _IN is lower, as the output current I _O increases, the operating point of the current limiting circuit 7 increases as the input voltage V _IN decreases, as shown in FIG. The input voltage V _IN causes a difference in the current value of the output current I _{O at which} the output voltage V _O starts to decrease.

In the circuit of FIG. 3, the point in time when the pulse width modulation circuit 6 starts the intermittent operation is defined as the average value S _PM of the output pulse signal of the pulse width modulation circuit 6 as described above.
Is below a predetermined value. However, when trying to supply the same power to the secondary side of the converter transformer 3,
The higher the input voltage V _{IN, the} shorter the on-period of the output pulse signal and the lower the average value _SPM of the output pulse signal. Therefore, when the output current I _O increases, the higher the input voltage V _IN , the faster the pulse width modulation circuit 6 starts the intermittent operation, and the output voltage I _O starts to decrease due to the start of the intermittent operation. _{OL depends} on the input voltage V _IN . That is, in FIG. 4, when the voltage value is the low input voltage V _INmin , the intermittent operation is started when the output voltage V _O is V _OL2 , and the high input voltage V _IN is obtained.
In the case of _INmax becomes the output voltage V _O is the intermittent operation at the time of the V _OL1 is started, than the voltage V _OL2 V _OL1
Has a higher voltage value.

[0010]

If a switching power supply is used in the drooping portion of the output current-output voltage characteristic,
Since the change in the output current is small relative to the change in the output voltage, it can be treated as a constant current source. Here, the output voltage V _OL at the time when the intermittent operation is started is suppressed as low as possible, so that the change range of the output voltage V _O is widened and the operation region as the constant current source can be widened. As described above, the current limiting circuit operates first, and then the intermittent operation circuit operates when the average value of the output pulse signal of the pulse width modulation circuit becomes a predetermined value or less, and the pulse width modulation circuit operates intermittently. In a switching power supply that performs overcurrent protection by starting the power supply, the higher the DC voltage value input to the power supply,
The output voltage value at the time when the intermittent operation starts is increased. In other words, the lower the DC voltage value input to the power supply, the lower the output voltage at the time when the intermittent operation starts.

Therefore, depending on the setting of the output voltage at the start of the intermittent operation, when the lowest input voltage at which the switching power supply can operate is input, a large current near the output short circuit before the intermittent operation is started. May flow. Therefore, when designing the switching power supply circuit, the operation start point of the intermittent operation is set so that the circuit element is not damaged by a large current near the output short circuit, assuming that the switching power supply operates at the lowest input voltage that can operate. Must. Therefore, with a voltage input during normal use, the output voltage value at the time when the intermittent operation starts is increased, and the operation area that can be used as a constant current source is narrowed.

For example, consider a battery charger utilizing the drooping characteristics of the output current and the output voltage. The output voltage of the power supply is low immediately after the start of charging of the battery, and the output voltage of the power supply increases as the charging proceeds. It is becoming. If the output voltage of the switching power supply is lower than the output voltage (V _OL ) at the start of the intermittent operation at the start of charging the battery, the switching power supply performs the intermittent operation and supplies a sufficient current to the charged battery. The battery cannot be supplied and more time is required before the battery is fully charged. Here, if the operating region as the constant current source is narrow, charging of the battery may have to be started from a voltage lower than the output voltage at the start of the intermittent operation.

[0013] In addition, even if the battery charger is designed so that charging of the battery can be started without intermittent operation at the input voltage during normal use, if the input voltage rises for some reason, the intermittent operation will be performed at the beginning of battery charging. There is a problem that there is a possibility of doing. Therefore, according to the present invention, the output voltage V
_The purpose of the present invention is to provide a switching power supply in which the _OL does not change with the voltage value of the input voltage, the operating region is not narrowed as a constant current source, and the _OL can be set arbitrarily according to the voltage value of the output voltage. And

[0014]

A voltage detection circuit for monitoring an output voltage, a comparison circuit for comparing a detected output voltage with a reference voltage, and a time constant of an intermittent operation of a pulse width modulation circuit according to an output from the comparison circuit. A switch circuit for changing a state of a capacitor to be set, and a function of setting a time constant of an intermittent operation of the switch circuit by an output from the comparison circuit when an output voltage becomes equal to or less than a predetermined voltage value in an overcurrent state. By enabling the pulse width modulation circuit, the start point of the intermittent operation can be set arbitrarily according to the output voltage without depending on the input voltage.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention monitors the output voltage and controls the start point of the intermittent operation according to the output voltage.
FIG. 1 shows an example of the circuit configuration. The same parts as those in FIG. 3 are given the same numbers. 1, a converter transformer 3, a switching transistor Q1 and a resistor R connected in series to a primary winding N1 thereof.
The rectifying and smoothing circuit composed of the diode D1, the flywheel diode D2, the choke coil L1, and the smoothing capacitor C1 connected to the primary and secondary windings N2 has exactly the same configuration as the conventional switching power supply. In addition, as a control / drive function part of the switching power supply, PWMIC
5, the same as the conventional switching power supply, having the feedback circuit 4 and the capacitor C2 for setting the time constant of the intermittent operation.

The switching power supply of the present invention further has the following circuit configuration. A resistor R is connected between the output terminals 2a and 2b.
2 and a series circuit of the resistor R3 and a series circuit of the resistors R4, R5 and the shunt regulator IC1 are connected in parallel. The divided voltage at the connection point between the resistors R2 and R3 is input to the control input terminal of the shunt regulator IC1. Resistance R
4, the light emitting diode of the photocoupler 9 is connected in parallel. The collector of the phototransistor of the photocoupler 9 is connected to both ends of the capacitor C2 for setting the time constant of the intermittent operation.
Connect the emitter terminal. At this time, the emitter terminal of the phototransistor is set to the ground side.

The overcurrent protection operation of the switching power supply of the present invention shown in FIG. 1 will be described with reference to FIG. 2 showing its output current-output voltage characteristics. When the output current _IO increases, an overcurrent state is detected from the switching current and the current limiting circuit 7 _operates to output an overcurrent signal S _CLM . Since the pulse width modulation circuit 6 shortens the ON period of the output pulse signal in accordance with the overcurrent signal S _CLM , the relationship between the output current I _O of the switching power supply and the output voltage V _O shows a drooping characteristic. Eventually, when the ON period of the output pulse signal becomes short, and the average value _SPM of the output pulse signal becomes smaller than a predetermined value,
The intermittent operation circuit 8 operates.

However, at this time, current flows through the resistors R4 and R5 and the shunt regulator IC1 connected between the output terminals 2a and 2b, and the voltage across the resistor R4 is changed by the voltage of the light emitting diode of the photocoupler 9. Applied between the anode and the cathode, the light emitting diode is on.
Since the light emitting diode of the photocoupler 9 is on, the phototransistor of the photocoupler 9 is on and both ends of the capacitor C2 are short-circuited. The intermittent operation circuit 8 does not output the intermittent operation signal S _OFF to the pulse width modulation circuit 6 because the potential of the CT terminal is low because both ends of the capacitor C2 are short-circuited. Therefore, the output current-output voltage characteristic of the switching power supply is determined by the output current I
_{Even if O} increases, it still shows drooping characteristics.

When the output current I _O increases and the output voltage V _O becomes lower than a predetermined voltage value, the divided voltage value at the connection point between the resistors R2 and R3 becomes lower than the internal reference voltage of the shunt regulator IC1. Then, shunt regulator I
C1 stops the current flowing through the resistors R4, R5 and itself, and turns off the light emitting diode of the photocoupler 9. When the light emitting diode of the photocoupler 9 is turned off, the phototransistor of the photocoupler 9 is turned off, and the capacitor C2 becomes chargeable. When the capacitor C2 is charged and the potential of the CT terminal rises, the intermittent operation circuit 8 outputs an intermittent operation signal S _OFF to the pulse width modulation circuit 6, and the pulse width modulation circuit 6 starts the intermittent operation. The intermittent operation is started by the above operation, and the overcurrent protection is performed.

As described above, the switching power supply of the present invention monitors the output voltage and starts the intermittent operation when the output voltage V _O falls below a certain voltage value V _OL . Therefore, even if the input voltage V _IN differs, the output voltage V _OL at which the intermittent operation starts is constant, and the operation region as a constant current source is not narrowed. Further, the output voltage V _{OL at} which the intermittent operation is started can be easily changed by adjusting the divided voltage input to the shunt regulator IC1 at the connection point between the resistors R2 and R3.

In the embodiment of FIG. 1, a description has been given by taking a forward type switching power supply as an example.
It can also be applied to flyback and half-bridge circuits. Further, the conventional example and the embodiment have been described with the circuit using the PWMIC in which the control / drive function part is contained in one package, but the circuit in which the pulse width modulation circuit, the current limiting circuit, and the intermittent operation circuit are individually formed. But it is the same. Further, the present invention can be applied to an existing switching power supply, in which case, as in the embodiment of FIG. 1, a photocoupler, a shunt regulator, and several resistance elements may be added and incorporated.

[0022]

As described above, according to the present invention, when the voltage value of the output voltage becomes equal to or less than the predetermined voltage value, the capacitor for setting the time constant of the intermittent operation becomes operable, and the switching power supply performs the intermittent operation. To start. Thus, the voltage value of the output voltage at the time when the intermittent operation starts due to the voltage value of the input voltage does not change, and the constant current operation region is not narrowed. Further, by adjusting the resistance value of the voltage dividing resistor detecting the output voltage, the voltage value of the output voltage at the time when the intermittent operation starts can be easily changed.

[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 an output current-output voltage characteristic of the circuit of FIG.

FIG. 3 is an example of a circuit diagram of a conventional switching power supply.

FIG. 4 is an output current-output voltage characteristic of the circuit of FIG. 3;

[Explanation of symbols]

 1a, 1b Input terminal 2a, 2b Output terminal 3 Converter transformer 4 Feedback circuit 5 PWMIC 6 Pulse width modulation circuit 7 Current limiting circuit 8 Intermittent operation circuit 9 Photocoupler

Claims (2)

(57) [Claims] The switching power supply is in an overcurrent state,
When the ON period of the switching element becomes smaller than a predetermined value by the current limiting circuit, the pulse width modulation circuit that drives the switching element performs an intermittent operation, and a voltage for monitoring the output voltage in a switching power supply that performs overcurrent protection of the circuit. A detection circuit, a comparison circuit that compares the detected output voltage with a reference voltage, and a switch circuit that changes a state of a capacitor that sets a time constant of an intermittent operation of the pulse width modulation circuit according to an output from the comparison circuit, When the output voltage becomes equal to or less than a predetermined voltage value in this state, the output of the comparison circuit enables the switch circuit to set a capacitor for setting the time constant of the intermittent operation, thereby enabling the intermittent operation of the pulse width modulation circuit. A switching power supply whose starting point can be set arbitrarily according to the output voltage without depending on the input voltage. 2. A series circuit of a second resistor and a third resistor and a series circuit of a fourth resistor, a fifth resistor, and a shunt regulator are connected in parallel between output terminals, and the second resistor and the third resistor are connected. The divided voltage at the point is input to the shunt regulator, the anode and cathode terminals of the light emitting diode of the photocoupler are connected to both ends of the fourth resistor, and the photodetector is connected to both ends of the capacitor for setting the time constant of the intermittent operation of the pulse width modulation circuit. By connecting the collector and emitter terminals of the phototransistor of the coupler, the shunt regulator blinks the light emitting diode according to the output voltage, and the blinking of the light emitting diode causes the phototransistor to set the time constant of the intermittent operation. 2. The switching power supply according to claim 1, wherein both ends are short-circuited or charged.