JPH05111250A - Switching regulator - Google Patents

Abstract

PURPOSE:To obtain a switching regulator having a protecting circuit which had excellent protecting function against an overcurrent, an overvoltage, an overheat, etc., irrespective of a load resistance, i.e., a strong foldback drooping of protecting circuit characteristics. CONSTITUTION:When an overcurrent flows to a load L1, a current detecting transistor 30 is turned ON. A current flowing to the transistor 30 flows to a gate of a thyristor 31 to turn ON the thyristor 30, and a feedback current IF forcibly flows to a light emitting diode 23a. As a bias voltage of the thyristor 31, a negative voltage generated in a secondary coil 4 when a switching transistor 11 is turned ON is employed. The negative voltage is always held at a predetermined voltage value at a ratio in which the transistor 11 is ON or for a time in which the transistor 11 is turned ON irrespective of a magnitude of a load L2. Accordingly, since a protecting circuit 36 has foldback drooping characteristics, power supply to a secondary side of a switching regulator is abruptly decreased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching regulator, and more particularly to a switching regulator having a protection circuit against overcurrent, overvoltage, overheat and the like.

[0002]

2. Description of the Related Art Generally, a switching regulator is provided with a protection circuit for protecting the switching regulator in the case of overload or output short circuit and for preventing damage on the load side. FIG. 5 is an electric circuit diagram of a conventional switching regulator having an overcurrent protection circuit. This switching regulator is a switching transistor 70.
Is a so-called ON-OFF type switching regulator that stores electric power in the high frequency transformer 71 when is ON and supplies the stored electric power to the secondary side when the switching transistor 70 is OFF. The overcurrent protection circuit
In general, it is composed of a current detection resistor 90, a current detection transistor 91 and a thyristor 92. In this switching regulator, when the control output load current I _O suddenly increases due to, for example, an abnormality in the load L ₁ of 5 V output, the voltage generated in the current detection resistor 90 rises and the current detection transistor 91 is turned on. As a result, the thyristor 92
Is turned on, and the feedback current I is fed to the light emitting diode 93a.
_F is forced to flow. The light emitting diode 93a emits light,
A current flows through the light-receiving side phototransistor 93b forming a photocoupler together with the light emitting diode 93a, and the ON time of the switching transistor 70 is shortened, so that the power supplied to the secondary side is reduced.

However, the conventional switching regulator uses a negative voltage of the control output load voltage, for example, a voltage of -5V, as the bias voltage of the thyristor 92. Therefore, when the power supplied to the secondary side decreases,
Feedback current I _F to the voltage value of the -5V flows thyristor 92 decreases is reduced, the thyristor 92 is OFF. When the thyristor 92 is turned off, the switching transistor 70 starts normal oscillation, and this oscillation continues until the negative control output load voltage of the switching regulator rises to a voltage at which the thyristor 92 can maintain the ON state again. Become. As a result, although the power supplied to the secondary side is reduced, the protection function is not sufficient, and the overcurrent protection circuit is unsatisfactory. In particular, this phenomenon becomes more remarkable as the load L ₂ of the stabilizing circuit including the three-terminal regulator 100 increases.

Therefore, an object of the present invention is to provide a protection circuit which has an excellent protection function against overcurrent, overvoltage, overheat, etc., regardless of the load resistance, that is, a protection circuit having a more drooping characteristic. It is to provide a switching regulator provided with.

[0005]

In order to solve the above problems, a switching regulator according to the present invention stores power in a transformer when a primary side switching element is ON, and when the switching element is OFF. A switching regulator for supplying the accumulated power to the secondary side, a stabilizing circuit for controlling the power supplied to the secondary side by a feedback current flowing through the secondary side, and the secondary side feedback current forcibly. A protection circuit having a switching element for flowing;
A bias circuit for rectifying and smoothing the voltage generated in the secondary coil when the secondary switching element is ON and applying a negative bias to the switch element, wherein the secondary feedback current flowing through the switch element is biased. It is characterized by being flown into the circuit. A thyristor, a transistor, or the like is used as the switch element forcibly flowing the feedback current.

[0006]

In the above structure, the negative bias voltage applied to the switching element by the bias circuit is determined by (input voltage V _IN ) × (transformer winding ratio) when the primary side switching element is ON. Primary switching element O
It is held at a constant value regardless of the N ratio, the ON time, and the like. Therefore, the characteristic of the protection circuit including the switch element is a fold-back characteristic as shown in FIG.

[0007]

Embodiments of the switching regulator according to the present invention will be described below with reference to the accompanying drawings. In each embodiment, the same parts and the same parts are designated by the same reference numerals. [First Embodiment, FIGS. 1 and 2] FIG. 1 is an electric circuit diagram of a switching regulator having an overcurrent protection circuit.
A switching transistor 11 is connected in series to the primary coil 2 of the high frequency transformer 1. The starting resistors 12, 13 and the phototransistor 23b are connected to the base side of the switching transistor 11. The base driving coil 3 and the electrolytic capacitor 14 are connected to the emitter side of the switching transistor 11. 15
Is a rectifying diode.

On the secondary coil 4 side of the high frequency transformer 1,
In general, a smoothing circuit 22 having a smoothing capacitor 20 and a rectifying diode 21, a stabilizing circuit 29 having a light emitting diode 23a and a shunt regulator 24, an overcurrent protection circuit 36 having a current detection transistor 30 and a thyristor 31, a smoothing capacitor 37, and A bias circuit 41 having a rectifying diode 38 is connected.

The smoothing circuit 22 is a circuit for smoothing the AC voltage generated in the secondary coil. A high frequency electrolytic capacitor is used as the smoothing capacitor 20. The stabilizing circuit 29 is a circuit for controlling the power supplied to the secondary side by the feedback current and maintaining the control output load voltage V _O constant. A light emitting diode 23 a connected to one of the smoothing capacitors 20 via a resistor 25 is connected to a common line 27 via a shunt regulator 24. The light emitting diode 23a constitutes a photo coupler together with the phototransistor 23b. Shunt regulator 24
External resistors 26a and 26b for setting the + 5V output voltage value of the shunt regulator 24 are connected to the gate of the.

[0010] overcurrent protection circuit 36 is a circuit for protecting a switching regulator or the like the feedback current I _F forcibly flowed during overload or the like. Smoothing capacitor 20
The emitter of the current detection transistor 30 is connected to one of the two, and the current detection resistor 32 is connected between the emitter and the base. The collector of the transistor 30 is connected to a bias circuit 41 described later via bias resistors 33a and 33b. On the other hand, in the thyristor 31, the anode has a resistance of 3
4, the cathode is connected to the bias circuit 41, and the gate is connected to the bias resistor 33.
It is connected to the collector of the transistor 30 via a.

The bias circuit 41 is a circuit for always supplying a constant holding current to the thyristor 31. A smoothing capacitor 37, a rectifying diode 38 and a resistor 39 are connected in parallel with the secondary coil 4. Smoothing capacitor 37
For this, an electrolytic capacitor is used. A stabilizing circuit including a three-terminal regulator 45 is connected to the secondary coil 5 side of the high frequency transformer 1. The stabilizing circuit includes a smoothing capacitor 46 on the input terminal A1 side of the three-terminal regulator 45.
And a smoothing capacitor 47 is connected to the output terminal A2 side. The regulator 45 is composed of IC parts, and when the regulator input voltage (voltage at point a in the figure) becomes higher than the regulator output voltage (voltage at point b in the figure), the difference is radiated as heat to regulate the regulator. It has the function of stabilizing the output voltage. Electrolytic capacitors are used as the smoothing capacitors 12 and 13. Further, a rectifying diode 48 is connected in series with the secondary coil 5.

Next, the operation of the switching regulator having the above electric circuit configuration will be described. When the current flows through the starting resistor 12 to the base of the switching transistor 11, the switching transistor 11 is turned on.
Then, a current flows through the switching transistor 5 and a voltage is applied to the primary coil 2 of the high frequency transformer 1. At this time, the voltage generated in the secondary coils 4 and 5 becomes a reverse voltage with respect to the rectifying diodes 21 and 48.
No current flows to the secondary side, and power is stored in the transformer 1. Further, when the current flowing through the switching transistor 11 increases and reaches a predetermined current value, the switching transistor 11 turns off. When the switching transistor 11 is turned off, a forward voltage is generated in the secondary coils 4 and 5 with respect to the rectifying diodes 21 and 48, a current flows to the secondary side, and the electric power accumulated in the transformer 1 is secondary. Will be supplied to the side. When all the electric power accumulated in the transformer 1 is supplied to the secondary side, the switching transistor 11 is turned on by the current flowing through the starting resistor 13, and the operation is repeated again to continue the oscillation.

The switching transistor 11 continues to oscillate.
By continuing, the current will flow in the secondary coils 4 and 5, respectively.
Flowing. A current flows through the secondary coil 4 and is connected to the output terminal.
Load L₁Output load voltage V across_OIs rising,
Voltage preset by shunt regulator 24
In the example, +5) and above, the shunt regulator 24
Is turned on and a feedback current starts to flow through the light emitting diode 23a.
It Therefore, the light emitting diode 23a emits light and
The current of the phototransistor 23b increases. This conclusion
As a result, the base current of the switching transistor 11 is lower
Therefore, the time when the switching transistor 11 is in the ON state
Shortens the control output load voltage V_OGoes down. Thus 2
The output voltage on the secondary coil 4 side is stabilized. On the other hand, secondary carp
The current flowing through the control circuit 5 is indirectly controlled by the stabilizing circuit 29.
However, since the control is not perfect,
Regulator input voltage (Fig.
Stable regulator output voltage (b in the figure)
Point voltage), then load L ₂Is being supplied to.

Next, the operation of the overcurrent protection circuit will be described. When the control output load current I _O suddenly increases due to an abnormality in the load L ₁ , the current flowing through the current detection resistor 32 increases and the voltage drop across the current detection resistor 32 increases. As a result, the current detection transistor 30 is turned on, the current flowing through the transistor 30 flows through the bias resistor 33a to the gate of the thyristor 31, and the thyristor 31 is turned on. When the thyristor 31 is turned ON, the light emitting diodes 23a to the feedback current I _F is forced to flow, the feedback current I _F is the resistance 34, flows into the bias circuit 41 through the thyristor 31. The light emitting diode 23a emits light, a current flows through the phototransistor 23b on the light receiving side, and the ON time of the switching transistor 11 is shortened. As a result, the power supplied to the secondary side is reduced.

In this case, as the bias voltage of the thyristor 31, a negative voltage generated in the secondary coil 4 when the switching transistor 11 is in the ON state is used. This negative voltage has a voltage value determined by (input voltage V _IN ) × (transformer winding ratio) and is peak-charged by the rectifying diode 38, so that the switching transistor 1
A constant voltage value can be maintained irrespective of the ON ratio of 1, the ON time, the magnitude of the load L ₂ , and the like. Therefore, unlike the conventional switching regulator, the switching regulator of the present embodiment does not turn off the thyristor while the power supply to the secondary side is insufficiently cut, so that the power supply to the secondary side is rapidly performed. Can be reduced.

FIG. 2 is a graph showing the characteristics of the protection circuit 29. The horizontal axis of the graph represents the control output load current I _O , and the vertical axis represents the control output load voltage V _O. For comparison, the characteristic of the protection circuit of the conventional switching regulator is shown by a chain line. In the graph, I _M is the value of the output current I _O when the protection circuit starts the protection operation, that is,
This is the overcurrent set point. The protection circuit 29 includes a thyristor 31.
Since the negative bias voltage of is constant regardless of the magnitude of the load L ₂ , it exhibits a fold-back characteristic. Therefore,
When the load L ₁ becomes overloaded, the output load current I _O rapidly decreases. On the other hand, the conventional protection circuit shown by the alternate long and short dash line has a V-shaped drooping characteristic, and the output load current decreases slowly even when overloaded. In the evaluation test, the current value at the overcurrent set point was set to 2.2 amps, and the load L ₁ was increased with the current of 0.2 amps flowing to the load L ₂ . As a result, the conventional protection circuit started the protection operation when the control output load current I _O reached 2.2 amps, but the output load current I _O increased until it reached 3.4 amps. On the other hand, the protection circuit 29 of the present embodiment starts the protection operation when the control output load current I _O reaches 2.2 amps, and the output load current I _O also rapidly decreases to 0.5 amps accordingly.

[Second Embodiment, FIG. 3] FIG. 3 is a partial electric circuit diagram of a switching regulator having an overvoltage protection circuit. The primary coil 2, 3 side circuit and the secondary coil 5 side circuit of the high frequency transformer 1 are the same circuits as in the first embodiment.
Overvoltage protection circuit 59 is a circuit for protecting a switching regulator or the like the feedback current I _F forcibly flowed during overload or the like. The emitter of the voltage detecting first transistor 50 is connected to one of the smoothing capacitors 20, and the starting resistor 51 is connected between the emitter and the base. The base of the first transistor 50 is the second transistor 52 for voltage detection.
Connected to the collector. The base of the second transistor 52 is connected to the Zener diode 53 and the bias resistor 54. The collector of the first transistor 50 is connected to the bias circuit 41 via bias resistors 56a and 56b. On the other hand, in the thyristor 55, the anode is connected to the light emitting diode 23a through the resistor 58, the cathode is connected to the bias circuit 41, and the gate is the bias resistor 5a.
It is connected to the collector of the first transistor 50 via 6a. The description of the smoothing circuit 22, the stabilizing circuit 29, and the bias circuit 41 is omitted because their electric circuit configuration and operation are the same as those of the first embodiment.

Next, the operation of the overvoltage protection circuit will be described. When the control output load voltage V _O suddenly increases due to an abnormality in the load L ₁ and exceeds the Zener voltage of the Zener diode 53, a current starts to flow in the bias resistor 54. When the voltage drop due to the resistor 54 becomes large, a current flows through the base of the second transistor 52 and the second transistor 52 is turned on. When the second transistor 52 is turned on, a current flows through the starting resistor 51 to the base of the first transistor 50, and the first transistor 50 is turned on. The current flowing through the first transistor 50 flows through the bias resistor 56a to the gate of the thyristor 55, and the thyristor 55 is turned on. When the thyristor 55 is turned ON, the light emitting diodes 23a to the feedback current I _F is forced to flow, flows into the feedback current I _F is resistor 58, a bias circuit 41 always through the thyristor 55 is biased to a constant negative voltage .. The light emitting diode 23a emits light, a current flows through the phototransistor 23b on the light receiving side, and the ON time of the switching transistor 11 on the primary coil 2 side of the transformer 1 is shortened. As a result, the power supplied to the secondary side is rapidly reduced, and the overvoltage of the switching regulator can be prevented.

[Third Embodiment, FIG. 4] FIG. 4 is a partial electric circuit diagram of a switching regulator having an overheat protection circuit. High-frequency transformer 1 primary coil 2, 3 side circuit and 2
The circuit on the side of the next coil 5 is similar to that of the first embodiment. Overheat protection circuit 69 is a circuit for protecting a switching regulator or the like from overheating the feedback current I _F forcibly flowed during overload or the like. The emitter of the heat detecting first transistor 60 is connected to one of the smoothing capacitors 20, and the starting resistor 61 is connected between the emitter and the base. The base of the first transistor 60 is the second heat detection transistor 62.
Connected to the collector. The base of the second transistor 62 is connected to the voltage dividing resistor 63 and the positive temperature coefficient thermistor 64. The collector of the first transistor 60 is connected to the bias circuit 41 via bias resistors 66a and 66b. On the other hand, the anode of the thyristor 65 is a resistor 68.
Is connected to the light emitting diode 23a through the cathode, the cathode is connected to the bias circuit 41, and the gate is connected to the bias resistor 66a.
It is connected to the collector of the first transistor 60 via. The description of the smoothing circuit 22, the stabilizing circuit 29, and the bias circuit 41 is omitted because their electric circuit configuration and operation are the same as those of the first embodiment.

Next, the operation of the overheat protection circuit will be described. When the control output load current I _O suddenly increases due to an abnormality in the load L ₁ , the switching transistor 11 generates heat. This is detected by the positive temperature coefficient thermistor 64, but the positive temperature coefficient thermistor 64 increases in resistance value due to heat generation. Therefore, the voltage drop due to the positive temperature coefficient thermistor 64 becomes large, a current flows through the base of the second transistor 62, and the second transistor 62 is turned on. When the second transistor 62 is turned on, a current flows through the starting resistor 61 to the base of the first transistor 60, and the first transistor 60 is turned on. The current flowing through the first transistor 60 flows through the bias resistor 66a to the gate of the thyristor 65, and the thyristor 65 is turned on. When the thyristor 65 is turned ON, the feedback current I _F to the light-emitting diode 23a is forced to flow, the feedback current I _F is resistor 68, a thyristor 6
5 always flows into the bias circuit 41 biased to a constant negative voltage. The light emitting diode 23a emits light,
A current flows through the phototransistor 23b on the primary coil 2 side, and the ON time of the switching transistor 11 is shortened. As a result, the power supplied to the secondary side is rapidly reduced, and overheating of the switching regulator can be prevented.

[Other Embodiments] The switching regulator according to the present invention is not limited to the above embodiment, but can be variously modified within the scope of the invention.
In particular, the switch element forcibly flowing the secondary side feedback current I _F may be a transistor or the like in addition to the thyristor.

[0022]

As is apparent from the above description, according to the present invention, the voltage generated in the secondary coil when the primary switching element is ON is rectified and smoothed to generate a constant negative bias. Since this negative bias is applied to the switch element forcibly flowing the secondary side feedback current, the characteristic of the protection circuit including this switch element becomes a fold-back drooping characteristic.
As a result, it is possible to obtain a switching regulator having a protection circuit that has an excellent protection function against overcurrent, overvoltage, overheat, and the like, regardless of the load resistance, that is, that has a strong droop in the protection circuit characteristics.

[Brief description of drawings]

FIG. 1 is a first switching regulator according to the present invention.
The electric circuit diagram which shows an Example.

FIG. 2 is a graph showing characteristics of a protection circuit of the switching regulator shown in FIG.

FIG. 3 is a second switching regulator according to the present invention.
The electric circuit diagram which shows an Example.

FIG. 4 is a third switching regulator according to the present invention.
The electric circuit diagram which shows an Example.

FIG. 5 is an electric circuit diagram showing a conventional example.

[Explanation of symbols]

1 ... Transformer 2, 3 ... Primary coil 4 ... Secondary coil 11 ... Switching transistor (primary side switching element) 22 ... Smoothing circuit 29 ... Stabilizing circuit 31 ... Thyristor (switch element) 36 ... Protection circuit 41 ... Bias circuit 55 ... Thyristor (switch element) 59 ... Protection circuit 65 ... Thyristor (switch element) 69 ... Protection circuit _IF ... Feedback current

Claims (1)

[Claims] 1. A primary side switching element stores electric power in a transformer when the switching element is ON, and the switching element is OF
In the switching regulator that supplies the accumulated power to the secondary side at F, a stabilizing circuit that controls the power supplied to the secondary side by the feedback current flowing through the secondary side, and the secondary side feedback current is forced. A protection circuit having a switching element that selectively flows, and a bias circuit that applies a negative bias to the switching element by rectifying and smoothing the voltage generated in the secondary coil when the primary switching element is ON, A switching regulator characterized in that the secondary side feedback current flowing through a switch element is passed through the bias circuit.