JPH09285114A - Overvoltage protecting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimally set the operating point of an overvoltage protection in a protective circuit. SOLUTION: When an overvoltage is generated in an output, a voltage proportional to the number of turns of a secondary wiring S1 and a control winding P2 is induced in the control winding P2, and a negative reference voltage rectified by a rectifier diode D2 becomes larger. This voltage is detected by a Zener diode ZD1. A very small current flowing during non-current-conduction (when the voltage is lower than or equal to a Zener voltage Vz) is made to flow in a resistor R4 connected between the base and the emitter of a transistor Q5 for triggering a latch circuit 2, and a voltage drop is generated. Hence the transistor Q5 for triggering the latching circuit is turned on before the Zener diode turns to the state of conduction. Thereby the gate voltage of a switching power MOS-FET Q1 is lowered, and oscillation operation is stopped. By adjusting a resistor 4, the operation point of overvoltage protection can be finely adjusted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overvoltage protection circuit for a ringing choke converter.

[0002]

2. Description of the Related Art In a ringing choke converter, the output voltage jumps up when the power supply input voltage rises abnormally or when a failure occurs in each main part of the constant voltage power supply and the constant voltage control circuit fails. When such an overvoltage state occurs, serious problems such as damage to the output smoothing capacitor, abnormal heat generation of the switching transistor and the resistance, and damage to the load occur. Therefore, if an overvoltage occurs, it is necessary to detect it and immediately shut off the output. An overvoltage protection circuit is incorporated for that purpose.

A conventional overvoltage protection circuit is constructed, for example, as shown in FIG. In the figure, a portion 11 surrounded by a broken line is a conventional overvoltage protection circuit. That is, the transformer T is provided with the detection winding N1 ', the output thereof is rectified by the diode D2', and the capacitor C4 'is peak-charged, and the output thereof is applied to the overvoltage detection Zener diode ZD1'. On the other hand, a series circuit including a resistor R1 ′, a thyristor SCR1 ′, and a resistor R3 ′ is inserted in the primary side input portion of the converter, and the resistor R3 ′ is connected.
The terminal voltage of the switching transistor Q for oscillation
It is applied to the base of an overvoltage protection transistor Q6 'connected between the base and emitter of 1'. The current of the Zener diode ZD1 'for detecting overvoltage is supplied to the gate of the thyristor SCR1' as a trigger current.

While the converter is operating normally, the thyristor SCR1 'and the overvoltage protection transistor Q
All 6's are in the off state. However, if an overvoltage condition occurs for some reason, an abnormal voltage will be detected.
1'is induced and the capacitor C4 'is charged. Then, the overvoltage detection Zener diode ZD1 'becomes conductive, and thereby the thyristor SCR1' is fired and becomes conductive. Therefore, a current flows through the resistor R3 ′, a voltage drop occurs, and the terminal voltage is applied to the base of the overvoltage protection transistor Q6 ′. As a result, the overvoltage protection transistor Q6 'conducts, pulling down the base voltage of the switching transistor Q1' of the converter,
The oscillation operation is stopped.

[0005]

However, with such a configuration, the operating point for overvoltage protection when an overvoltage occurs is the Zener diode Z for overvoltage detection.
Fine adjustment is not possible because it depends only on D1 ', and a Zener diode with some allowance for the Zener voltage must be set to prevent malfunction, and the operating point for overvoltage protection cannot be set to an optimum value. There was a drawback.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a highly reliable ringing choke converter in which an optimum operating point for overvoltage protection can be arbitrarily set.

[0007]

In order to achieve the above object, the overvoltage protection circuit of the present invention has the following configuration.
That is, in the ringing choke converter, 1
A transformer in which the secondary winding and the primary control winding have the same polarity, and
Switching power MOS connected in series with the secondary winding-
FET, switching power M-FET connected between the gate of the switching power MOS-FET and the control winding
An OS-FET drive circuit, a latch circuit whose source is connected to the other of the control windings and which is connected between the gate and the source of the switching power MOS-FET, a transistor for triggering the latch circuit, and the transistor Is connected to the source of the switching power MOS-FET, and is connected to the resistor connected between the base and the emitter of the transistor and the emitter of the transistor for triggering the latch circuit. A zener diode for detecting a reference voltage made by a rectifying diode between the circuit and the control winding.

Alternatively, in a converter for converting an input into a DC output, a reference voltage generating section for generating a DC reference voltage according to the input, an input switching section for switching the input according to a switching signal, When a Zener diode unit to which a reference voltage is applied in the reverse direction, a resistor unit that generates a potential difference with respect to the reverse current flowing through the Zener diode unit, and the potential difference due to the resistor unit becomes a predetermined value or more, the switching unit is turned on. And a latch unit for generating a switching signal for turning off and holding the signal.

With the above-described structure, if constant voltage control cannot be performed for some reason and an overvoltage occurs in the output, a voltage proportional to the number of turns of the secondary winding and the control winding is induced in the control winding. The negative reference voltage rectified by the rectifier diode becomes large, and this voltage is detected by the overvoltage detection Zener diode, and the weak reference voltage that flows when the Zener diode is not conducting (when the voltage is equal to or lower than the Zener voltage Vz) is detected. By passing a current through a resistor connected between the base and emitter of the transistor for triggering the latch circuit, the transistor for triggering the latch circuit can be turned on before a voltage drop is generated and the Zener diode becomes conductive, and the switching power is increased. The gate voltage of the MOS-FET is lowered to stop the oscillation operation. Therefore, by adjusting the resistance value of the resistor, the operating point of overvoltage protection can be finely adjusted without depending on the Zener voltage of the Zener diode, a highly reliable converter can be realized, and the load and the converter can be realized. It can protect itself from damage.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a ringing choke converter incorporating an overvoltage protection circuit according to the present invention.

The ringing converter has a primary winding P1.
And a transformer T whose secondary output winding S1 has different polarities, and a switching power MOS-F connected in series to the primary winding.
ETQ1, rectifier diode D1, and smoothing capacitor C1
The secondary side rectifying / smoothing circuit is formed to store energy in the transformer T when the switching power MOS-FET Q1 is conducting, and the energy causes a ringing voltage to the secondary winding S1 when the switching power MOS-FET Q1 is non-conducting. It is generated and the rectifying diode D1 conducts to supply power to the load. To stabilize the output voltage, for example, switching power MOS-FETQ
This can be performed by providing a control transistor Q2 between the gate and the source of 1 and applying a control signal to the base of the control transistor Q2.

Now, in FIG. 1, a portion surrounded by a broken line is an overvoltage protection circuit 1 according to the present invention. The overvoltage protection circuit 1 according to the present invention is a switching power MOS-F.
Transistors Q3 and Q4 having a characteristic relative to each other between the gate and the source of ETQ1 are connected in a thyristor, and the base of the transistor Q4 is connected to a resistor R3 and a capacitor C2 which generate a voltage for turning on the transistor Q4 and latched Let's call it circuit 2. The collector of the transistor Q5 for turning on the transistors Q3 and Q4 is connected to the base of the transistor Q3 (collector of the transistor Q4). Further, the output of the control winding P2 of the transformer T is rectified by the diode D2 and smoothed by the capacitor C4 to create a negative reference voltage. The reference voltage is detected by the overvoltage detection zener diode ZD1, and when the zener diode ZD1 is turned on, the transistor Q5 is turned on, so that the latch circuit 2 operates and the switching power MOS
-The gate voltage of the FET Q1 is lowered to stop the oscillation operation.

The overvoltage protection circuit 1 operates as follows. Now, it is assumed that the constant voltage control of the ringing choke converter cannot be performed for some reason and the output voltage rises. Then, the negative reference voltage generated by the diode D2 increases. When the reference voltage exceeds the Zener voltage of the overvoltage detection Zener diode ZD1, the Zener diode ZD1 becomes conductive, and a voltage drop occurs due to the current flowing through the resistor R4, turning on the transistor Q5.
When the transistor Q5 turns on, the transistor Q3 also turns on, the base voltage of the transistor Q4 rises, and the transistor Q4 also turns on. When the transistors Q3 and Q4 are turned on, a latch is applied, and the latch is not released unless the input is turned off. As a result, switching power MOS
-The gate and source of the FET Q1 are short-circuited, and the oscillation operation is stopped.

In the present invention, by turning on the transistor Q5, the latch circuit 2 is operated, the gate and source of the switching power MOS-FET Q1 are short-circuited, and the oscillation operation is stopped to provide overvoltage protection. It is characterized by how to create a voltage to turn on Q5. Conventionally, it is determined by whether or not the Zener diode ZD1 is conductive, but with the Zener diode, only parts having a specified Zener voltage can be obtained, and fine adjustment is not possible.

Here, the voltage of the Zener diode is shown in FIG.
The current characteristics are shown.

FIG. 2 is a diagram showing the voltage-current characteristics of the Zener diode, showing that the Zener diode conducts at the Zener voltage Vz when a reverse voltage is applied. It also shows that a weak reverse current I1 flows even when the reverse voltage applied to the Zener diode is equal to or lower than the Zener voltage Vz.

Therefore, the resistor R4 is connected between the base and emitter of the transistor Q5. By connecting the resistor R4 in this manner, the reverse voltage applied to the Zener diode is weak even when the Zener voltage Vz is not conducted, that is, when the negative reference voltage rectified by the diode D2 does not reach the Zener voltage Vz. A reverse current I1 can be supplied via the resistor R4. At that time, since a weak reverse current I1 flows through the resistor R4, the voltage drop VR due to the resistor R4.
4 becomes VR4 = I1 × R4. If the base-emitter voltage for turning on the transistor Q5 is VBE (ON), and if VBE (ON) <VR4 for turning on the transistor Q5, the transistor Q5 is turned on even before the Zener diode ZD1 is turned on. be able to. When the transistor Q5 is turned on, the transistors Q3 and Q4 connected between the gate and the source of the switching power MOS-FET Q1 are turned on, and the switching power MOS-F is latched.
The gate voltage of ETQ1 is lowered to stop the oscillation operation, and as a result, the overvoltage protection is activated.

Therefore, by adjusting the resistance value of the resistor R4, the operating point of overvoltage protection can be finely adjusted without depending on the Zener voltage of the Zener diode ZD1, and a highly reliable converter can be realized, Also, damage to the converter itself can be protected.

[0020]

As described above, the overvoltage protection circuit according to the present invention can finely adjust the operating point of overvoltage protection by adjusting the resistance value without depending on the Zener voltage of the Zener diode ZD1. It is possible to realize a highly reliable converter and protect the load and damage to the converter itself.

[0021]

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an example of a ringing choke converter incorporating an overvoltage protection circuit according to the present invention.

FIG. 2 is a diagram showing voltage-current characteristics of a Zener diode.

FIG. 3 is a circuit diagram of a conventional technique.

[Explanation of symbols]

 1,11 Overvoltage protection circuit 2 Latch circuit T Transformer P1 Primary side winding T2 Control winding S1 Secondary side output winding N1 Detection winding Q1 Oscillation switching element Q2 Switching control transistor Q3, Q4, C2, R3 Latch circuit Components that compose Q5 Transistor that triggers the latch circuit Q6 Transistor for overvoltage protection D1, D2 Rectifying diode C1, C4 Smoothing capacitor R1 Starting resistor R2, C3 Switching element drive circuit R5 Gate parallel resistor SCR1 Thyristor

Claims (4)

[Claims] 1. In a ringing choke converter, a transformer in which a primary winding and a primary control winding have the same polarity, and a switching power MOS connected in series with the primary winding.
-FET and switching power MOS-FE connected between the gate of the switching power MOS-FET and the control winding
A T drive circuit, a latch circuit having a source connected to the other of the control windings, and a gate circuit connected between the gate and the source of the switching power MOS-FET, a transistor for triggering the latch circuit, and a base of the transistor. Switching power MOS
A switching power MOS-FE connected to the source of the FET and connected to the resistor connected between the base and the emitter of the transistor and the emitter of the transistor for triggering the latch circuit.
An overvoltage protection circuit for a ringing choke converter, comprising: a zener diode for detecting a reference voltage formed by a rectifying diode between a T drive circuit and a control winding. 2. A converter for converting an input into a DC output, a reference voltage generating unit for generating a DC reference voltage according to the input, an input switching unit for switching the input according to a switching signal, When a Zener diode unit to which a reference voltage is applied in the reverse direction, a resistor unit that generates a potential difference with respect to the reverse current flowing in the Zener diode unit, and the potential difference due to the resistor unit becomes a predetermined value or more, the switching unit is turned on. An overvoltage protection circuit comprising: a latch unit that generates a switching signal to turn off and holds the signal. 3. The overvoltage protection circuit according to claim 2, wherein the reference voltage generator rectifies an output generated in a control winding having the same polarity as the input side winding to generate a reference voltage. . 4. The latch unit is formed by connecting two transistors having characteristics relative to each other in a thyristor connection, and the resistance unit is connected to a base of a transistor for turning on a gate input of the thyristor connection, and the Zener diode is connected. The overvoltage protection circuit according to claim 2, wherein a voltage drop generated with respect to a reverse current of the unit is used as a switching input of the transistor.