JPH11178195A - Power unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an auto-reset output overcurrent protective function for interrupting power supply to a load perfectly upon detecting an output overcurrent while eliminating malunction due to a rush current or noise upon turns-on of a power switch. SOLUTION: An overcurrent protective element 44 having a resistance increasing abruptly when a current exceeding a specified level flows in inserted between the cathode of a diode 37 and a smoothing capacitor 38 connected to the secondary of a transformer 33. A tertiary winding 33-3 is wound reversely to the primary winding 33-1 on the primary of the transformer 33 and a voltage induced in the tertiary winding 33-3 is employed as the driving power supply of a control IC 36 for operating a switching element 34.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a power supply device having an output overcurrent protection function for protecting an output current of a switching power supply from becoming an overcurrent.

[0002]

2. Description of the Related Art FIGS. 2 to 4 show a conventional example of a power supply device having an output overcurrent protection function. Each of the power supply devices shown in FIGS. 2 to 4 has a first smoothing capacitor 12 connected between output terminals of a rectifier circuit 11 composed of a diode bridge for rectifying an AC input from an AC power supply (not shown). A series circuit of the primary winding 13-1 of the transformer 13 and the switching element 14 is connected in parallel with the smoothing capacitor 12.

The switching element 14 is a MOS (Me)
tal Oxide Semiconductor) A field-effect transistor having a drain terminal connected to the primary winding 13-1, a source terminal connected to a negative output terminal of the rectifier circuit 11 via a resistor 15, and a gate terminal connected to the negative output terminal. The switching element 14 is connected to a control IC (Integrated Circuit) 16 for performing a switching operation at a constant frequency. Further, a connection point between the source terminal and the resistor 15 is connected to the control IC 16.

[0004] Further, this power supply device is provided with the transformer 13
The second smoothing capacitor 18 is connected to the secondary winding 13-2 through the first diode 17 in the forward direction. The charging voltage of the second smoothing capacitor 18 is a supply voltage V to a load (not shown). In addition, the second
Between the output terminals of the smoothing capacitor 18 of FIG.
Is connected.

Further, the primary winding 13 of the transformer 13
The tertiary winding 13-3 is wound around the same side as
A third smoothing capacitor 22 is connected to 3-3 via a second diode 21 in the forward direction. The charging voltage of the third smoothing capacitor 22 is used as a driving power source for the control IC 16. Further, the second diode 21
Is connected to the control IC 16 via the light emitting element 19 and the light receiving element 23 forming a photocoupler. The tertiary winding 13-3 is a forward winding in which the winding direction is the same as that of the primary winding 13-1.
When a current is flowing through the primary winding 13-1, a current corresponding to the turn ratio with the primary winding 13-1 flows.

In the power supply device having such a configuration, when the switching element 14 performs a switching operation under the control of the control IC 16, a high-frequency voltage is generated in the primary winding 13-1 of the transformer 13. The transformer 1
3, a voltage is induced in the secondary winding 13-2, and a supply voltage V to the load is generated based on the induced voltage. Further, the supply voltage V to the load is detected by the output voltage detection circuit 19, and the photocoupler including the light emitting element 19 and the light receiving element 23 operates according to the detected value, and the feedback voltage to the control IC 16 is reduced. Become. The control IC 16 turns on / off the switching element 14 based on the feedback voltage.
The supply voltage V is stabilized by variably controlling the off-duty ratio.

By the way, in the power supply device operating as described above, when the current flowing to the secondary side of the transformer 13 increases and the output power increases, the current flowing to the switching element 14 increases. As a result, the voltage applied to the resistor 15 increases. In the conventional device shown in FIG. 2, the control IC 16 detects a rise in the voltage applied to the resistor 15, and stops the switching operation of the switching element 14 when a voltage rise exceeding a certain level is detected. This is designed to protect the current from becoming a current (No. 1).
Conventional example).

[0008] The output overcurrent protection method by the control IC 16 is not limited to the above method. For example, when a rise in the voltage applied to the resistor 15 is detected, the peak of the current flowing through the switching element 14 does not exceed a certain current. (Second conventional example), or a method of narrowing the on-duty width of the current flowing through the switching element 14 when a rise in the voltage applied to the resistor 15 is detected (third conventional example). .

The conventional device shown in FIG.
The fuse 24 is connected in series to the first diode 17 connected to the secondary winding 13-2 of FIG. 3, and the output current becomes overcurrent by utilizing the characteristic of the fuse 24 that blows when a certain current or more flows. (The fourth conventional example).

Further, the example shown in FIG. 4 is a power supply line for a load on the output side of a second smoothing capacitor 18 connected to a secondary winding 13-2 of a transformer 13 via a first diode 17. An overcurrent protection element 25, whose resistance value rapidly increases when a current exceeding a certain level flows, is connected to protect the output current from becoming overcurrent due to the increase in the resistance value of the overcurrent protection element 25. (Fifth conventional example). The overcurrent protection element 25 has a very low resistance value during normal circuit operation, but when heated by an overcurrent or an increase in ambient temperature, the resistance increases sharply to limit the current very slightly. (Positive Temperature Coefficien
t) A polymer-based PTC element (for example, a polyswitch) having characteristics is known.

[0011]

However, each of the above-described first to fifth conventional examples has the following disadvantages. That is, in the first conventional example, when the control IC 16 detects an output overcurrent, the switching operation of the switching element 14 is stopped and the secondary output of the transformer 13 is stopped. In some cases, the control IC 16 malfunctions due to inrush current, noise, or the like, and stops the switching operation, so that power cannot be supplied to the load at all.

In the second conventional example, even if the control IC 16 malfunctions due to rush current or noise when the power switch is turned on, there is no possibility that the power supply to the load will be interrupted. In addition, there is a danger of heat generation and a risk of power supply damage because the device continues to operate while maintaining a certain amount of power. On the other hand, also in the third conventional example, there is no possibility that the power supply to the load is interrupted due to the malfunction of the control IC 16 due to the rush current or noise when the power switch is turned on. Also,
Normally, when the output overcurrent state occurs, the switching element 14
Since the on-duty width of the current flowing through the transformer 13 is narrowed down, both the current and the voltage flowing on the secondary side of the transformer 13 are reduced, which is safe. However, when the oscillation frequency and input voltage are high, the narrowing of the on-duty width is not easy.
The output power on the secondary side did not decrease so much, and there was a danger of heat generation and power supply damage as in the second conventional example.

On the other hand, in the fourth conventional example, there is no danger of heat generation or power supply damage. However, if an output overcurrent occurs, the fuse 24 melts and does not return automatically.
There is a trouble that the fuse 24 has to be replaced. In the fifth conventional example, when an output overcurrent occurs, the resistance value of the overcurrent protection element 25 sharply increases, so that almost no output current flows. However, since the output continues, it is not suitable for completely stopping the power supply to the load.

The present invention has been made in view of such circumstances, and it is an object of the present invention to eliminate a malfunction caused by an inrush current or noise when a power switch is turned on and to detect an output overcurrent when an overcurrent is detected. An object of the present invention is to provide a power supply device having an output overcurrent protection function that can completely stop power supply to a load, is safe, and can automatically recover.

[0015]

According to the present invention, a series circuit of a primary winding of a transformer and a switching element is connected between output terminals of a rectifier circuit for rectifying AC from an AC power supply. And a drive circuit for switching the switching element at a constant frequency is provided,
In addition, a smoothing capacitor is connected to the secondary winding of the transformer via a diode, and a high frequency voltage is generated in the primary winding of the transformer by the switching operation of the switching element, and the high frequency voltage is induced in the secondary winding of the transformer. In a power supply device that charges a smoothing capacitor with a voltage and supplies the charging voltage to a load, a constant voltage is applied between the connection between the anode of the diode and the secondary winding or the connection between the cathode of the diode and the smoothing capacitor. When the above current flows, an overcurrent protection element whose resistance value rapidly increases is inserted, and a tertiary winding whose winding direction is opposite to that of the primary winding is wound around the primary winding of the transformer. The voltage induced in the tertiary winding was used as a drive power source for the drive circuit. By taking such a measure, when the output current flowing through the secondary side of the transformer rises to a certain current or more, the resistance value of the overcurrent protection element rapidly rises, and the current flows through the secondary winding. It stops flowing. As a result, no current flows to the tertiary winding, so that power supply to the drive circuit is stopped, and the drive circuit stops operating. As a result, the switching element stops performing the switching operation, and the power supply to the load is completely stopped.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of a power supply device according to the present embodiment, in which a first smoothing capacitor 32 is provided between output terminals of a rectifier circuit 31 composed of a diode bridge for rectifying an AC input from an AC power supply (not shown). In addition, a series circuit of the primary winding 33-1 of the transformer 33 and the switching element 34 is connected in parallel with the smoothing capacitor 32.

The switching element 34 is composed of a MOS field effect transistor, and its drain terminal is
A control IC 36 for connecting the source terminal to the negative output terminal of the rectifier circuit 31 via the resistor 35 and connecting the gate terminal to the switching element 34 at a constant frequency. Connected to Further, a connection point between the source terminal and the resistor 35 is connected to the control IC 36.

Further, this power supply device is provided with the transformer 33
The second smoothing capacitor 38 is connected to the secondary winding 33-2 through the first diode 37 in the forward direction. The charging voltage of the second smoothing capacitor 38 is set as a supply voltage V to a load (not shown). In addition, the second
Between the output terminals of the smoothing capacitor 38 of FIG.
Is connected.

Further, the primary winding 33 of the transformer 33
A tertiary winding 33-3 is wound around the same side as -1.
A third smoothing capacitor 42 is connected to 3-3 via a second diode 41 in the forward direction. The charging voltage of the third smoothing capacitor 42 is used as a drive power source for the control IC 36. Further, the second diode 41
Is connected to the control IC 36 via the light emitting element 39 and the light receiving element 43 forming a photocoupler. The tertiary winding 33-3 is a flyback winding in which the winding direction is reversed with respect to the primary winding 33-1.
When a current is flowing through the secondary winding 33-2, a current according to the turn ratio with the secondary winding 33-2 flows.

An overcurrent protection element 44 is inserted between the cathode of the first diode 37 and the second smoothing capacitor 38. The overcurrent protection element 44 increases in resistance rapidly when a predetermined current or more flows. . Note that the overcurrent protection element 44 is
This is a polymer-based PTC element that has a very low resistance value during normal circuit operation, but has PTC characteristics that when heated by an overcurrent or an increase in ambient temperature, the resistance sharply increases and the current is slightly limited.

In the power supply device of the present embodiment configured as described above, when an AC power is input from an AC power supply by turning on a power switch (not shown), the AC is rectified by the rectifier circuit 31. The first smoothing capacitor 32 is charged. When the charging voltage of the first smoothing capacitor 32 becomes equal to or higher than a predetermined voltage, the primary winding 33-1 of the transformer 33 is turned on.
, The current also flows through the secondary winding 33-2, and further flows through the tertiary winding 33-3 to charge the third smoothing capacitor 42. The charging voltage of the third smoothing capacitor 42 serves as a driving power source for the control IC 36, and the control IC 36 controls the switching operation of the switching element 34. By the switching operation of the switching element 34, a high-frequency voltage is generated in the primary winding 33-1 of the transformer 33, and a voltage is induced in the secondary winding 33-2 of the transformer 33 by the high-frequency voltage. The supply voltage V to the load is generated based on the applied voltage.
The supply voltage V to the load is equal to the output voltage detection circuit 39.
The photocoupler including the light emitting element 39 and the light receiving element 43 operates according to the detected value, and becomes a feedback voltage to the control IC 36. The control IC 36 variably controls the on / off duty ratio of the switching element 34 based on the feedback voltage to stabilize the supply voltage V.

In this state, when the current flowing on the secondary side of the transformer 33 rises and becomes an overcurrent exceeding a certain fixed current,
The resistance value of the overcurrent protection element 44 sharply increases. Then, apparently, the diode 37 is in the same state as that in the open mode, and the secondary winding 33 of the transformer 33 is turned on.
The current stops flowing to -2. As a result, no current flows through the tertiary winding 33-3 of the transformer 33, and the control IC 3
6, and the control IC 36 stops its operation. As a result, the switching element 34 stops performing the switching operation, and the power supply to the load is completely stopped. Therefore, even if an output overcurrent occurs, the power supply is completely stopped before heat generation or power supply breakage occurs, so that there is no danger of heat generation or power supply breakage, and the system is safe.

After the power supply to the load is completely stopped due to the output overcurrent, the resistance value returns to the original minute state because the overcurrent protection element 44 cools down. That is, the state automatically returns to the state before the output overcurrent occurs. Therefore, there is no need to perform any extra work for restoration such as replacement of the fuse, and the handling is simple.

Further, the resistance value of the overcurrent protection element 44 does not increase due to inrush current or noise when the power switch is turned on.
Since current does not stop flowing through the secondary winding 33-2 of the transformer 33, malfunction does not occur due to inrush current, noise, or the like. Further, by using a polymer-based PTC element as the overcurrent protection element 44, it plays a role of not only overcurrent but also circuit protection from overheating.

In the above-described embodiment, the overcurrent protection element 44 is inserted after the second diode 37, that is, between the connection between the cathode of the first diode 37 and the second smoothing capacitor 38. Even if the overcurrent protection element 44 is inserted before the second diode 37, that is, between the connection between the secondary winding 33-2 and the second diode 37, the same operation and effect can be obtained.

[0026]

As described above in detail, according to the present invention, there is no malfunction due to inrush current or noise when the power switch is turned on, and when an output overcurrent is detected, the power supply to the load is completely stopped. It is possible to provide a power supply device having an output overcurrent protection function that can be stopped and is safe and can automatically recover.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating one embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional example.

FIG. 3 is a circuit diagram showing a conventional example.

FIG. 4 is a circuit diagram showing a conventional example.

[Explanation of symbols]

11, 31 rectifier circuit 12, 18, 22, 32, 38, 42 first to third smoothing capacitors 13, 33 transformer 14, 34 switching element 15, 35 resistor 16, 36 control IC 17 , 21, 37, 41 ... first and second diodes 20, 40 ... output voltage detection circuit 25, 44 ... overcurrent protection element

Claims (1)

[Claims] 1. A drive for connecting a series circuit of a primary winding of a transformer and a switching element between output terminals of a rectifier circuit for rectifying an alternating current from an alternating current power supply and for switching the switching element at a constant frequency. A circuit is provided, and a smoothing capacitor is connected to a secondary winding of the transformer via a diode, and a high-frequency voltage is generated in a primary winding of the transformer by a switching operation of the switching element. In the power supply device, which charges the smoothing capacitor with a voltage induced in the secondary winding and supplies the charged voltage to a load, a connection between an anode of the diode and the secondary winding or the diode When a certain amount of current flows between the cathode and the smoothing capacitor, the resistance value increases sharply. An overcurrent protection element is interposed, and a tertiary winding whose winding direction is opposite to that of the primary winding is wound on the primary winding side of the transformer, and is induced in the tertiary winding. A power supply device, wherein a voltage is used as a power supply for driving the drive circuit.