JP2002233145A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To constitute a switching power supply of a latch part provided with the same function as that of a thermal fuse, to enable automatic soldering for the improvement of yields and cost reduction. SOLUTION: A trigger circuit TC is constituted of a voltage dividing circuit including a resistor R4 and a positive temperature coefficient thermistor PTC, and a latch circuit LC is constituted of a first transistor Tr3 of a PNP type and a second transistor Tr4 of an NPN type. If the resistance value of a positive temperature coefficient thermistor is significantly increased and exceeds a specified value, and a trigger voltage V1 is accordingly outputted from the trigger circuit, the latch circuit produces a stopping voltage V2, in response to the application of the trigger voltage to forcibly stop the operation of a switching transistor Tr1. After the application of the trigger voltage is ceased, the latch circuit still latches the production of the stopping voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a switching power supply having a function of forcibly stopping the operation of a switching transistor in response to an overheating state.

[0002]

2. Description of the Related Art In such a switching power supply, as a component for forcibly stopping the operation of a switching transistor and latching the stopped state, a thermal fuse is provided between a primary winding of a transformer and a collector of the switching transistor. Some are dressed.

In this switching power supply, the temperature fuse rises to a high temperature and melts in response to the overheating state of the switching transistor, thereby cutting off the collector of the switching transistor from the primary winding of the transformer and forcibly stopping the switching transistor. Even if the heat generation state of the switching transistor returns to room temperature, the forced stop state of the switching transistor is latched due to the melting of the thermal fuse.

[0004]

In the case of the above-mentioned switching power supply, the melting point of the thermal fuse as the latch component is generally lower than the melting point of the solder for soldering the electronic component to the circuit board. It is not possible to put on the process of automatic soldering together with other electronic components on the top, and to solder the thermal fuse to the circuit board, it is necessary to manually solder by the operator separately from the automatic soldering process A process is required.

[0005] Such a soldering process by manually attaching a thermal fuse becomes a hindrance to automation of a manufacturing line, and causes a reduction in manufacturing yield and an increase in cost.

Accordingly, the present invention solves the above-described problem by making it possible to employ, in such a switching power supply, the component having the latch function in the forced stop state instead of the thermal fuse.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a switching power supply having a protection function for forcibly stopping the operation of a switching transistor in response to an overheating state, wherein a PTC thermistor is provided inside the switching power supply. It is located at a position where it is possible to detect an overheating condition of
A trigger circuit for outputting a trigger voltage corresponding to the resistance value of the positive temperature coefficient thermistor; and an input section of the switching transistor in response to input of a trigger voltage when the resistance value of the positive temperature coefficient thermistor exceeds a predetermined value. And a latch circuit for applying a stop voltage for forcibly stopping the operation thereof and latching the applied state of the stop voltage even after the trigger voltage is no longer input.

According to the present invention, when the inside of the power supply is overheated, the resistance of the positive temperature coefficient thermistor increases beyond a predetermined value, and the trigger circuit outputs a trigger voltage corresponding to the change in the resistance value of the positive temperature coefficient thermistor to the latch circuit. Applied. The latch circuit applies a stop voltage to the input portion of the switching transistor in response to the application of the trigger voltage, and forcibly stops the switching transistor.
Even if the overheat state disappears and the application of the trigger voltage stops, the applied state of the stop voltage can be latched and the forced stop state for the switching transistor can be continued. Therefore, in the case of the present invention, it is not necessary to use a conventional thermal fuse as a circuit component necessary for the latch. As a result, unlike a circuit using a conventional thermal fuse, automatic soldering such as flow and reflow is not required. In the attaching process, the circuit component can be soldered to the substrate, and the manufacturing yield can be improved and the cost can be reduced by reducing the number of soldering steps by hand.

According to a preferred embodiment of the present invention, the trigger circuit includes a resistor connected in series to the positive temperature coefficient thermistor, and outputs a voltage divided by the positive temperature coefficient thermistor and the resistor as a trigger voltage. A first PNP transistor having an emitter connected to the input of the switching transistor, a collector connected to the base of the first transistor, and a base connected to the collector of the first transistor. And an NPN-type second transistor commonly connected to the output section of the trigger circuit.

According to this embodiment, the number of parts constituting the trigger circuit is two, that is, the positive characteristic thermistor and the resistor, and the number of parts constituting the latch circuit is two, the first and second transistors. The number of components is four, whereas the number of components is one for the conventional thermal fuse. Thus, at first glance, the cost of parts at the beginning of use may seem to be higher than the unit cost of the thermal fuse even though the unit price of the thermal fuse is higher, but in the conventional case, if the thermal fuse melts, it will be re-used. It becomes impossible to use it, and it is necessary to replace the expensive thermal fuse with another new thermal fuse, and if it is replaced every time the switching transistor overheats, the component cost will rise dramatically, In the case of the embodiment, after the overheating state of the switching transistor is released and the power supply of the switching power supply is turned off, the latch circuit returns and can be reused, so that the component cost is eventually lower than in the conventional case. It tends to be necessary.

Further, in the conventional thermal fuse, it is difficult to finely and accurately adjust the resistance temperature characteristic of the positive temperature coefficient thermistor and the resistance value of the resistance in accordance with the overheating state of the switching transistor. Such adjustment is easy.

In addition, the conventional thermal fuse is a lead-type component and cannot be surface-mounted because it is soldered by hand. On the other hand, in the embodiment, a positive temperature coefficient thermistor, a resistor, and a transistor are used. , It can be surface-mounted on a circuit board.

Furthermore, in the case of the conventional thermal fuse, even if soldering is performed by hand, the fusible pellets constituting the fuse may be melted to cause a component failure. In the automatic soldering process, there is no possibility of causing a component failure due to the high heat of the molten high-temperature solder.

In a further preferred embodiment of the present invention,
A switching transistor is configured by an NPN-type bipolar transistor based on the input unit;
A diode is connected between the emitter and the ground in such a direction that the base voltage required for the conduction in the switching transistor is added to the forward voltage together with the base-emitter voltage at the time of the conduction.

According to this embodiment, the total voltage of the collector-emitter voltage of the first transistor and the base-emitter voltage of the second transistor when the two transistors of the latch circuit are conducting is the base-emitter voltage when the switching transistor is conducting. The total voltage of the inter-voltage and the forward voltage of the diode cannot be exceeded, so that the latch state can be reliably maintained.

In this case, the switching transistor is
If the input section is constituted by an N-channel type MOSFET transistor having a gate, the diode does not need to be used, and the diode can be omitted, which is preferable in terms of reducing parts.

In a further preferred embodiment of the present invention,
The positive temperature coefficient thermistor may be arranged at a position thermally coupled to the switching transistor to protect the switching transistor from an overheated state and prevent the switching transistor from being damaged.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below based on embodiments shown in the drawings.

FIGS. 1 to 4 relate to an embodiment of the present invention. FIG. 1 is a circuit diagram of a self-excited and flyback system, that is, an RCC type switching power supply of this embodiment, and FIG.
FIG. 3 is a perspective view showing a thermal coupling state between the switching transistor and the PTC thermistor used in the circuit of FIG. 3, and FIG. 4 is a circuit diagram using a MOSFET as the switching transistor.

Before describing the features of this embodiment,
The overall configuration of a switching power supply to which this embodiment is applied will be schematically described along with its operation.

In the circuit shown in FIG. 1, when an AC input is turned on, the AC input is converted into a DC voltage via a power supply filter FT, a diode bridge DB, and a smoothing capacitor C1.

This DC voltage is input to the primary winding a of the transformer T1 and, at the same time, is supplied to the NP through the starting resistor R1.
N-type bipolar switching transistor Tr1
, The base current Ia flows to the base of the switching transistor Tr1 to turn on the switching transistor Tr1, and the winding current Ib flows to the primary winding a of the transformer T1.
An induced voltage is generated in the control winding c.

The switching transistor Tr1
When the base current Ia becomes short, the voltage drop between the collector and the emitter of the switching transistor Tr1 increases, the voltage across the primary winding a of the transformer T1 decreases, and the voltage across the control winding c decreases. The inter-voltage decreases, and the shortage of the base current Ia of the switching transistor Tr1 is promoted, and the switching transistor Tr1 turns off rapidly.

When the switching transistor Tr1 is turned off, back electromotive force is generated in each winding of the transformer T1, and an output current (load current) Ic flows through the rectifier diode D3 also in the secondary winding b. This load current Ic flows until the energy of the transformer T1 is released. When the load current Ic becomes 0, a base current Ia for the switching transistor Tr1 is generated by the back electromotive force generated in the control winding c, and the switching transistor Tr1
Is turned on, and the above operation is repeated again.

Thus, the switching transistor Tr1
Oscillates repeatedly on and off.

OD is an output voltage detection circuit, which includes resistors R7 and R8 and a variable resistor VR, and detects the magnitude of the output voltage (DC output).

PC is a photocoupler circuit, which comprises a photocoupler comprising a light emitting element d1 and a light receiving element d2, a transistor Tr6 and a Zener diode ZD1, and when the DC output from the output voltage detecting circuit OD exceeds a specified value. In response to the input of the detected output, the transistor Tr6 turns on, the Zener diode ZD1 conducts, and the light emitting element d1 emits light. Thereby, the light receiving element d2
Is turned on to turn on the transistor Tr5. When the switching transistor Tr1 is turned off in this way, the DC output decreases toward a specified value. When the DC output drops below the specified value, the operation reverses to the above, and the DC output increases toward the specified value. Thus, the DC output is stabilized.

Further, PD is a switching transistor T
r1 is a protection circuit including a resistor R3 and a transistor Tr
2 and a switching transistor Tr1
When an overcurrent flows through the transistor Tr2, the base voltage of the transistor Tr1 is reduced by turning on the transistor Tr2, so that the transistor Tr1 can be turned off.

Next, the features of this embodiment will be described.

Here, in the present embodiment, the temperature at which the switching transistor Tr1 is overheated and must be stopped is set to 130 ° C. as an example, but the temperature is not limited to this.

The present embodiment is characterized in that a trigger circuit TC and a latch circuit LC are provided.

The trigger circuit TC comprises a resistor R4 and a positive temperature coefficient thermistor PTC connected in series.
And a voltage dividing circuit that outputs the voltage divided by the positive and negative temperature coefficient thermistor PTC as a trigger voltage V1 from an output portion thereof (a common connection portion between the resistor R4 and the positive temperature coefficient thermistor PTC). It is connected in parallel with the ground.

The positive temperature coefficient thermistor PTC has a resistance temperature characteristic shown in FIG.
At 0 Ω, the resistance value at 130 ° C. is 2.2 times that of 10 times the resistance value.
kΩ. The resistor R4 has a specific resistance value of 51 kΩ.

The temperature of the positive temperature coefficient thermistor PTC is adjusted to 13 by adjusting the amount of barium replaced by strontium and lead contained in barium titanate as a main component.
The resistance value sharply increases at a temperature of 130 ° C. by adjusting the density of carbon particles contained in a high-density polyethylene as a main component and a ceramic type which is set so that the resistance value sharply increases at 0 ° C. There is an organic type which is set as described above, and any type is set so as to exhibit the resistance temperature characteristic.

As shown in FIG. 3, the positive temperature coefficient thermistor PTC is bonded to the switching transistor Tr1 with a resin having excellent thermal conductivity, preferably a silicon resin RS, and is thermally coupled to each other.

The trigger circuit TC having the above configuration is
When the trigger voltage V1 is output from the connection between the resistor R4 and the positive temperature coefficient thermistor PTC using the output as the output, the switching transistor Tr1 overheats and the temperature thereof becomes 13
As the resistance value of the positive temperature coefficient thermistor PTC rapidly increases from 220Ω (= 0.22 k) to 2.2 kΩ when the temperature reaches or exceeds 0 ° C., the trigger voltage V1 becomes
It changes as shown by the following equations (1) and (2). However,
The turn ratio between the primary winding a and the control winding c of the transformer T1 is about 1:15.
When r1 is off, the base voltage is 15V. Therefore, the resistance value of the positive temperature coefficient thermistor PTC is 220Ω.
When (= 0.22 kΩ), V1 = [0.22 (kΩ) / (0.22 + 51) (kΩ)] × 15 (V) = 0.064 (V) (1) Resistance of the positive characteristic thermistor PTC When the value is 2.2 kΩ, V1 = [2.2 (kΩ) / (2.2 + 51) (kΩ)] × 15 (V) = 0.62 (V) (2)

On the other hand, the latch circuit LC comprises a PNP-type first transistor Tr3 and an NPN-type second transistor T3.
r4. First transistor Tr
3 is a switching transistor Tr1 whose emitter is
Connected to the base, which is the input section of The collector of the second transistor Tr4 is the first transistor Tr3.
Is connected in common to the collector of the first transistor Tr3 and the output of the trigger circuit TC,
Its emitter is grounded.

In the latch circuit LC, the trigger circuit T
Even if the trigger voltage V1 represented by the formula (1) is input from C, the second transistor Tr4 is turned off, and therefore, the first transistor Tr3 is also turned off, and the emitter voltage of the first transistor Tr3 becomes the switching transistor Tr1. Is lower than the voltage for forcibly stopping the switching operation, and therefore the switching transistor Tr1 does not stop the switching operation.

Then, when the trigger voltage V1 represented by the equation (2) is input from the trigger circuit TC, the second transistor Tr4 is turned on, whereby the first transistor T4 is turned on.
r3 is also turned on. Then, as the latch circuit LC, the emitter voltage of the first transistor Tr3 is input to the base of the switching transistor Tr1 as a stop voltage V2 for forcibly stopping its operation, whereby the switching transistor Tr1 is turned off. Switching operation can be forcibly stopped.

When the switching transistor Tr1 stops operating, its heat generation is suppressed and its temperature decreases. Therefore, although the trigger voltage V1 from the trigger circuit TC decreases, the first transistor T1 in the latch circuit LC decreases.
Since r3 keeps its conducting state, the second transistor Tr4 also conducts as it is. As a result, even after the trigger voltage V1 decreases, the latch circuit LC latches the stop voltage V2.

Here, the magnitude of the stop voltage V2 will be specifically described. That is, the stop voltage V2 is a voltage (Vce1 + Vbe) obtained by adding the collector-emitter voltage Vce1 of the first transistor Tr3 and the base-emitter voltage Vbe1 of the second transistor Tr4, both of which are conducting.
1) ≒ 0.4 + 0.6 = 1.0 (V).

On the other hand, a diode D1 is connected in series to the switching transistor Tr1. This diode D1 connects the switching transistor T1 between the emitter of the switching transistor Tr1 and the ground.
The base voltage required for the conduction at r1 is connected to the direction in which the forward voltage Vf is added together with the base-emitter voltage Vbe2 at the time of the conduction. This added voltage is Vbe2 + Vf = 0.6 + 0.6 (V) =
1.2 (V). Therefore, with the stop voltage V2 which is a voltage 1.0 (V) lower than the added voltage, the switching transistor Tr1 can be turned off and the switching operation can be forcibly stopped.

When the circuit shown in FIG. 1 is used instead of a part of the circuit shown in FIG. 1 and the MOSFET 1 is used instead of the switching transistor Tr1,
It is also possible to eliminate the need for the diode D1.

The present invention is not limited to the above embodiment, and various applications and modifications are conceivable.

(1) In the above embodiment, the switching transistor Tr1 is an NPN-type bipolar transistor. However, the present invention is not limited to this. As described above, an N-channel MOSFET is used instead of the bipolar transistor. It may be.

(2) In the above-described embodiment, the resistor R4 constituting the trigger circuit TC is a fixed resistor. However, this is used as a variable resistor, and the trigger is performed by changing the resistance value of the variable resistor. The voltage V1 may be set.

In such a case, the trigger voltage V1 can be easily set according to the case where the temperature at which the switching transistor Tr1 is to be protected is higher or lower than 130 ° C.
Further, when the resistance temperature characteristics of the positive temperature coefficient thermistor PTC are different, it is preferable because the trigger voltage V1 can be easily set in accordance with the resistance temperature characteristics.

(3) In the above embodiment, the switching power supply is of the RCC type. However, the present invention is not limited to this, and can be applied to other types of switching power supplies.

(4) In the above embodiment, the positive temperature coefficient thermistor PTC is thermally coupled to the switching transistor Tr1, but is not limited to this, and an overheating state is detected in the switching power supply. It should just be a place to be.

(5) In the above embodiment, the self-excited RCC type switching power supply is used. However, the present invention is not limited to this. For example, a separately-excited flyback type converter, forward type, half-bridge type And the like.

[0051]

As described above, according to the present invention, it is not necessary to use a thermal fuse as a conventional latch component, and as a result, unlike the conventional technique, an automatic soldering process such as flow or reflow is performed. Thus, the latch component can be soldered to the substrate together with other components at the same time, so that the manufacturing yield can be improved and the cost can be reduced by reducing the number of soldering steps by hand.

[Brief description of the drawings]

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

FIG. 2 is a resistance temperature characteristic diagram of a positive temperature coefficient thermistor used in the circuit of FIG.

FIG. 3 is a perspective view showing a thermal coupling state between a switching transistor and a PTC thermistor;

FIG. 4 is a circuit diagram showing only a main part of a switching regulator according to another embodiment of the present invention.

[Description of Signs] Tr1 switching transistor T1 transformer TC trigger circuit R4 resistance PTC positive temperature coefficient thermistor LC latch circuit Tr3 first transistor Tr4 second transistor

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H730 AA20 AS01 BB43 BB52 CC01 DD02 EE02 EE07 FD01 FF19 FG07 XX15 XX16 XX19 XX45 ZZ05 ZZ12 ZZ15

Claims (5)

[Claims] 1. A switching power supply having a protection function for forcibly stopping an operation of a switching transistor in response to an overheating state, wherein a positive temperature coefficient thermistor detects an overheating state inside the switching power supply. A trigger circuit arranged at a possible position and outputting a trigger voltage corresponding to the resistance value of the positive temperature coefficient thermistor; and a trigger voltage input when the resistance value of the positive temperature coefficient thermistor exceeds a predetermined value. A latch circuit for responding to the input voltage of the switching transistor by applying a stop voltage for forcibly stopping the operation to the input section, and latching the applied state of the stop voltage even after the trigger voltage is no longer input. A switching power supply, comprising: 2. The switching power supply according to claim 1, wherein the trigger circuit has a resistor connected in series with the positive characteristic thermistor, and outputs a voltage divided by the positive characteristic thermistor and the resistor as a trigger voltage. Wherein the latch circuit comprises a PNP-type first transistor having an emitter connected to the input of the switching transistor, a collector connected to the base of the first transistor, and a base connected to the first transistor. And a second transistor of an NPN type commonly connected to the collector of the trigger circuit and an output of the trigger circuit. 3. The switching power supply according to claim 2, wherein the switching transistor comprises an NPN-type bipolar transistor based on the input section.
A diode is connected between the emitter and ground so that the base voltage required for the conduction in the switching transistor is added to the forward voltage together with the base-emitter voltage at the time of the conduction. Switching power supply characterized. 4. The switching power supply according to claim 2, wherein the switching transistor comprises an N-channel MOSFET transistor having the input portion as a gate. 5. The switching power supply according to claim 1, wherein the PTC thermistor is arranged at a position thermally coupled to the switching transistor.