JPH074064B2 - Start-up circuit of self-excited inverter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a starting circuit for a self-excited inverter on the input side of, for example, a high frequency switching power supply using a switching element having a large input resistance.

[0002]

2. Description of the Related Art Conventionally, in a self-excited inverter circuit as shown in FIG. 2, a positive electrode of a power source E is connected to a drain of a MOS type FET 2 through a main winding 1a of a main transformer 1 and a F
The gate of ET2 is connected to one end of the auxiliary winding 1b of the main transformer 1 via resistors 3 and 4, and the other end of the auxiliary winding 1b is an FET.
2 drain and the negative electrode of the power supply E, and also the saturated reactor 5
Is connected to the connecting point of the resistors 3 and 4. A trigger diode 8 is inserted between the connection point of the series circuit of the resistor 6 connected to the positive electrode of the power source E and the capacitor 7 connected to the negative electrode of the power source E, and the gate of the FET 2 which is a switching element, and the resistor 6 is connected to the resistor 6. A series circuit of a diode 9 and a resistor 10 is connected between the connection point with the capacitor 7 and the source of the FET 2. The discharge from the trigger diode 8 causes the voltage Vc across the capacitor 7 to oscillate as shown in FIG. 3 (a), and when the charging voltage Vc2 of the capacitor 7 is discharged, the pulse voltage Vp as shown in FIG. 3 (b) is generated. FE is output by this output
A gate voltage is supplied to T2 to activate it, and at the same time as activation, the charging voltage Vc2 of the capacitor 7 is discharged through the FET 2 by the diode 9 and the resistor 10 to prevent unnecessary restart.

[0003]

However, when the FET2 is started by the method shown in FIG. 2, the magnetic flux is constantly set in one direction by the current in the direction of the dotted line in the figure, and the reset and the set are repeated. Not done enough. Of course, when the operation starts, the saturation reactor 5 repeats setting and resetting, but in this case, it remains saturated in one direction of the set direction before starting the operation, and further ingenuity is required to enter a stable operation state. It Although it is part of the above consideration to start by the pulse from the trigger diode 8, there is a problem in the following points.

The trigger diode 8, which is essential in conventional circuits, has its own serious weakness. That is, firstly, since the discharge voltage is about 30 to 35V, it cannot be used for a switching power supply with an input power supply voltage of 30V or less. Secondly, due to its structure, it conforms to the communication industry standard of high quality. It is that the manufacturer does not guarantee it, it is all a consumer standard, and the guaranteed temperature is within 0 to 85 ° C.

The object of the present invention consists of general-purpose parts,
(EN) It is an object to provide a starting circuit for a self-excited inverter that has a low minimum starting voltage and has a small load on a switching element within a wide operating voltage range of the inverter.

[0006]

A starting circuit of a self-excited inverter according to the present invention for achieving the above object connects an input voltage to a transformer via a first switching element,
A difference between a voltage of a capacitor connected in parallel to the first switching element via a rectifier and an input voltage is connected to the transformer via a second switching element, and an auxiliary winding of the transformer is an impedance element. In a self-excited inverter in which a driving input of the first switching element is connected to the driving input of the first switching element via a saturation reactor in parallel, and the driving input of the first switching element and the saturation reactor are connected in series. Insert a plurality of capacitors, connect a constant voltage discharge element in parallel with each of these capacitors, connect the input voltage across both ends of the plurality of capacitors through high impedance elements, and connect the auxiliary winding of the transformer. The voltage is reversed to the input voltage via the high impedance element to each of the plurality of capacitors via another impedance element. It is characterized in that it has configured to apply to come.

[0007]

In the starting circuit of the self-excited inverter having the above configuration, when an input voltage is applied, the voltage is applied across the plurality of capacitors in series via the high impedance element,
The capacitor is charged by the sum of the voltages of the constant voltage discharge elements in parallel with this capacitor. Since this voltage is applied to the drive input of the first switching element via the saturation reactor, the first switching element becomes conductive and oscillation is started. By this start, the sum of the voltage of the capacitor and the voltage of the auxiliary winding is applied to the drive input.However, when the voltage of the auxiliary winding becomes high, this voltage causes the charge of the capacitor mentioned above to pass through another impedance element. Is discharged, no excessive voltage is applied to the drive input.

[0008]

The present invention will be described in detail with reference to the embodiment shown in FIG. FIG. 1 is a circuit diagram of a self-excited inverter. A main winding 11a of a transformer 11 has a MOS-type F whose starting end is connected to a positive input terminal 12 and whose ending is a main switching element.
The drain 13 d of the ET 13 is connected, and the source 13 s of the FET 13 is connected to the negative input terminal 14. FE
The anode of the diode 15 is connected to the drain 13d of T13, and the capacitor 16 is interposed between the cathode of the diode 15 and the source 13s of the FET 13.

The starting end of the reset winding 11b of the transformer 11 is connected to the drain 17d of the MOS type FET 17 which is a slave switching element, and the ending end thereof is connected to the cathode of the diode 15, and the source 17s of the FET 17 is connected to the positive input terminal 12 thereof. It is connected to the. First of transformer 11
Both ends of the adjusting resistor 19 are connected to the auxiliary winding 11c between the start end and the tap, and the movable terminal of the resistor 19 is connected to the gate 13g of the FET 13 via the resistor 20. The source 13s of the FET 13 is also connected to the anode of the Zener diode 21, the cathode of the Zener diode 21 is connected to the anode of the Zener diode 22, and the cathode of the Zener diode 22 is connected to the end of the first auxiliary winding 11c. . Zener diode 2
Capacitors 23 and 24 are connected in parallel to 1 and 22, respectively. At the cathode of the Zener diode 22,
The anode of the Zener diode 26 is connected via the resistor 25, and the cathode of the Zener diode 26 is connected to the positive input terminal 12.

The movable terminal of the adjusting resistor 19 is a resistor 28.
Connected to the anode of the Zener diode 22 via
The movable terminal of the resistor 19 is also connected to the cathode of the Zener diode 30, and the anode of the Zener diode 30 is connected to the anode of the Zener diode 21 via the resistor 31. One end of the saturable reactor 32 is connected to the gate 13g of the FET 13, and the saturable reactor 3 is connected.
The other end of 2 is connected to the end of the first auxiliary winding 11c.

The starting end of the second auxiliary winding 11d is connected to the positive electrode input terminal 12, the ending end thereof is connected to one end of the capacitor 37, and the other end of the capacitor 37 is connected to the cathode of the diode 38 and the diode 38. The anode is connected to the positive input terminal 12. Gate 1 of FET17
7g is connected to the cathode of the diode 38 via the resistor 39.

The anode of the diode 40 is connected to the starting end of the first secondary winding 11e of the transformer 11, and the cathode of the diode 40 is connected to the positive output terminal 42 via the choke coil 41. The negative output terminal 43 is connected to the end of the secondary winding 11e, and the output terminals 42, 4 and
A capacitor 44 is interposed between 3 and the cathode of the diode 45 and the cathode of the diode 40, whose anodes are connected to each other, are connected to the negative output terminal 43.
The resistors 46 and 47 are connected in series between the output terminals 42 and 43, and the anode of the Zener diode 49 whose cathode side is connected to the positive electrode output terminal 42 through the resistor 48 is connected to the negative electrode output terminal 43. ing.

A resistor 50 is provided at the gate 17g of the FET 17.
The collector 51c of the NPN type transistor 51 is connected via the, and the emitter 51e of the transistor 51 is connected to the positive input terminal 12. The base 51b of the transistor 51 is connected to the cathode of the Zener diode 49, and the emitter 51e is connected to the connection point of the resistor 46 and the resistor 47.

The transformer 11 has two secondary windings 11f, and the anode of the diode 84 and the cathode of the diode 85 are connected to the starting end of the secondary winding 11f via the saturable reactor 83, respectively. The cathode of 84 is connected to the positive electrode output terminal 87 via the choke coil 86. A negative output terminal 8 is provided at the end of the secondary winding 11f.
8 is connected, and a capacitor 89 is provided between the output terminals 87 and 88.
And the cathode of the diode 90 whose anode is connected to the negative output terminal 88 is connected by the diode 8
4 cathodes.

Between the output terminals 87 and 88, a resistor 91, both ends of the adjusting resistor 92, a resistor 93 are connected in series, and a resistor 94 and a Zener diode 95 connected in series to this series connection are connected in series. There is. The anode of the diode 85 is connected to the connection point between the resistor 94 and the cathode of the Zener diode 95, and the anode of the diode 85 is connected to the emitter 97e of the NPN transistor 97 via the resistor 96. The collector 97c of the transistor 97 is connected to the end of the secondary winding 11f, and the base 9
7b is connected to the movable end of the adjusting resistor 92.

In the above structure, each part has the following functions. That is, the FET 13, which is the main switching element, becomes conductive when the voltage of the gate 13g becomes positive, and the main winding 11a of the transformer 11 and the input terminals 12 and 14 are turned on.
FET17 which is a secondary switching element
Similarly, when the voltage of the gate 17g becomes positive, the series circuit of the reset winding 11b and the capacitor 16 is connected to the input terminal 12,
It is designed to be connected to 14. This connection is the main winding 1
1a and the reset winding 11b are opposite to each other with respect to the directions of the FETs 13 and 17.

The first auxiliary winding 11c transmits an electromotive force whose starting end is positive to the gate 13g of the FET 13. Further, the Zener diode 26 charges the capacitors 23 and 24 when the voltage of the input terminals 12 and 14 exceeds the Zener voltage, and applies a positive voltage to the gate 13g via the auxiliary winding 11c and the resistor 20, When a voltage is applied to the input terminals 12 and 14, the FET 13 is first turned on by this, and a current is passed through the main winding 11a to start oscillation.

When a positive voltage is applied to the gate 13g, a current starts to flow in the main winding 11a, so that an electromotive force having a positive starting point is generated in the auxiliary winding 11c, a current flows in the resistor 28, and the capacitor 24 To discharge. When the electromotive force is high,
Further, the Zener diode 30 becomes conductive and the capacitor 23 is also discharged. When an electromotive force whose positive end is positive is generated in the auxiliary winding 11c, a current flows through the saturable reactor 32.
This current is small until the saturable reactor 32 is saturated, but when saturated, the current increases and the voltage drop at the resistor 20 increases. Therefore, the potential of the gate 13g drops and becomes lower than the threshold voltage, so the FET 13 is turned off. become. If an electromotive force having a positive terminal is generated in the auxiliary winding 11c, a reset current flows in the saturable reactor 32.

When an electromotive force whose positive end is positive is generated in the second auxiliary winding 11d, the capacitor 37 is charged by the forward current of the diode 38, and when a reverse electromotive force is generated, this and the resistor 39, It is discharged in a circuit in which 50 and the transistor 51 are connected in series. Then, during this discharge, a positive voltage is applied to the gate 17g of the FET 17, which is a secondary switching element, and the FET 1 is used only for a period when the current is relatively large.
7 turns on.

Immediately after the FET 13 is turned off, an induced electromotive force having a positive terminal is generated in the main winding 11a, the electromotive force is added to the input voltage, and the capacitor 16 is charged via the diode 15. . Therefore, the voltage of the capacitor 16 becomes about twice the voltage of the input terminals 12 and 14. FE
When T13 turns off, the polarity of the electromotive force generated in the auxiliary winding 11d changes, and the FET 17 turns on. At this time, when the electromotive force with the terminal end of the reset winding 11b being positive becomes sufficiently small, the capacitor 16 causes the reset winding 11b to
A reset current flows through b and the FET 17. While the reset current is increasing, the FET 17 is connected to the auxiliary winding 11d.
However, the discharge current of the capacitor 37 decreases and the voltage drop due to the resistor 50 and the transistor 51 becomes less than the threshold value of the FET 17, so that the FET 17 is turned off.

The first secondary winding 11e generates an electromotive force whose positive end is positive in a period in which the FET 13 is on, that is, in a gate period, and generates a reverse electromotive force in a reset period in which the FET 17 is on. When the starting end is positive, current flows through the diode 40, the capacitor 44 is charged through the choke coil 41, and at the same time, the magnetic flux of the choke coil 41 increases. When the starting end is negative, no current flows in the secondary winding 11e, and the magnetic flux of the choke coil 41 decreases,
A current flows through the diode 45 and the capacitor 44 is charged.

The output terminals 42 and 43 are connected to the FETs 13 and 1, respectively.
Although a voltage is constantly generated during the operation of 7, the voltage is divided by the resistors 46 and 47 and is made a constant voltage by the resistor 48 and the Zener diode 49, and the difference is applied to the transistor 51. To be done. Here, the resistor 46,
If the potential of the connection point of 47 is higher than the reference potential of the Zener diode 49, the base current of the transistor 51 does not flow and the transistor 51 has a high resistance.

Next, the overall operation will be described. When a DC input voltage with the positive input terminal 12 being positive and the negative input terminal 14 being negative is applied, this input voltage is applied to the Zener diode 2
If it is higher than the Zener voltage of 6, capacitors 23, 24
Is charged, the FET 13 is turned on, a current flows through the main winding 11a, and oscillation is started. When the FET 13 is turned on, the current flowing through the main winding 11a continues to increase from zero,
The capacitor 37 is generated by the electromotive force generated in the auxiliary winding 11d.
Is charged.

When the saturable reactor 32 is saturated, the FET
13 is turned off and FET 17 is turned on. At this time, an electromotive force having a positive terminal is generated in the auxiliary winding 11c, and a reset current flows in the saturable reactor 32. F
When the ET 13 is turned off, the inertia current continues to flow in the main winding 11a and the capacitor 16 is charged. When the magnetic flux of the transformer 11 is sufficiently reduced and the electromotive force is reduced, the capacitor 16 starts discharging through the reset winding 11b, the core of the transformer 11 is reset, and the direction of the magnetic flux is reversed. From when FET17 turned on,
The FET 17 is turned off after the discharge time constant of the resistor 50 and the transistor 51 of the capacitor 37 has passed. Due to the feedback of the output voltage divided by the resistors 46 and 47, the higher the output voltage is than the specified voltage,
Since the base voltage of the transistor 51 is reverse-biased and the resistance between the collector and the emitter of the transistor 51 increases, the discharge time constant becomes long.

When the FET 17 is turned off, the auxiliary winding 11c
If the polarity of the electromotive force of is changed and the saturable reactor 32 is sufficiently reset, the FET 13 is turned on. FE
At the same time when T13 is turned on, the capacitor 37 begins to be charged, and the current starts flowing through the diode 40. At this time,
If the voltage of the auxiliary winding 11c of the transformer 11 is sufficiently large, the charges of the capacitors 23 and 24 will be
Since it is discharged through 0, when the saturable reactor 32 is saturated, the gate voltage of the FET 13 always drops below the threshold voltage and the FET 13 is turned off.

Although self-excited oscillation is performed by repeating such a cycle, in this circuit, the time until the saturable reactor 32 is saturated changes depending on the input voltage, and the gate time becomes shorter as the input voltage becomes higher. . Since the ratio of the output voltage to the input voltage becomes lower as the gate time becomes shorter than the winding ratio, the voltage at the output terminals 42 and 43 is relatively stable against a large fluctuation of the input voltage. Further, the output voltage is made constant by feeding back the difference between the voltage divided by the resistor 47 and the reference voltage with the transistor 51 and adjusting the reset time.

The saturable reactor 83 also functions as a magnetic amplifier. That is, when the output voltage is low, the potential of the movable end of the resistor 92 is lower than that of the cathode of the Zener diode 95, so that the transistor 97 has a high resistance and almost no current flows through the diode 85. Therefore, the saturable reactor 83 flows only the forward current of the diode 84,
Since it is not reset, it saturates and becomes low impedance. Then, the current that flows through the saturable reactor 83 to the choke coil 86 increases, so that the output voltage increases. On the contrary, when the output voltage is too high, the transistor 97 has a low resistance, and the reset current flows through the diode 85. Therefore, the saturable reactor 83 is reset and the impedance becomes high, and the output voltage at the time of the gate drops.
Thus, the saturable reactor 83 has its reset amount continuously changed by the current flowing through the diode 85, so that the output voltage at the time of the gate is kept constant with respect to the change of the load.

In this inverter, the saturable reactor 3
Since the gate time is determined by the saturation of 2, the fluctuation of the maximum value of the magnetic flux of the transformer 11 is small with respect to the large fluctuation of the input voltage, and the fluctuation of the oscillation frequency is also small due to the circuit configuration. Further, since the DC output voltage is fed back, the fluctuation of the secondary side output due to the fluctuation of the input voltage is suppressed to the minimum.

Since the inverter is characterized by having a wide operating voltage range as described above, there is a possibility that this advantage cannot be utilized unless the input voltage range at the time of starting is wide enough. In the circuit of the embodiment, the capacitor 23,
By sequentially discharging 24, reliable oscillation is performed from the minimum starting voltage to the maximum operating voltage without imposing an excessive load on the gate 13g of the FET 13. Further, since the Zener diode is used, the starting voltage does not largely change depending on the temperature and it is easy to set the starting voltage sufficiently low.

The DC output voltage and the overcurrent may be fed back through a photo coupler or the like. Further, the constant voltage circuit of the driven circuit is not limited to the one using the magnetic amplifier, and the DC output may not be three sets.

[0031]

As described above, since the starting circuit of the self-excited inverter according to the present invention does not use the trigger diode, the circuit can be constructed without sticking to the weak point of the trigger diode. Moreover, the circuit configuration is simple and highly stable.
Furthermore, after the start-up, the charging voltages of the plurality of capacitors are sequentially set to zero, whereby the gate voltage is sufficiently suppressed, the efficiency is improved, and the output noise is also reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment.

FIG. 2 is a configuration diagram of a conventional DC-CD converter.

FIG. 3 is an output waveform diagram of a starting circuit.

[Explanation of symbols]

 11 transformer 11a main winding 11b reset winding 11c, 11d auxiliary winding 11e, 11f secondary winding 12 positive electrode input terminal 13, 17 MOS type FET 14 negative electrode input terminal 16, 23, 24, 37, 44, 89 capacitor 21 , 22, 26, 49, 95 Zener diode 32 Saturable reactor 41, 86 Choke coil 42, 87 Positive output terminal 43, 88 Negative output terminal 51, 97 Transistor

Claims (1)

[Claims] 1. An input voltage is connected to a transformer via a first switching element, and the difference between the input voltage and the voltage of a capacitor connected in parallel to the first switching element via a rectifier is set to a second value. Self-excited connected to the transformer via a switching element, an auxiliary winding of the transformer connected to a drive input of the first switching element via an impedance element in series and a saturation reactor in parallel. In the inverter, a plurality of capacitors are inserted in series in the connection between the drive input of the first switching element and the saturation reactor, a constant voltage discharge element is connected in parallel with each of these capacitors, and Connect the input voltage to both ends via a high impedance element,
A self-excited inverter characterized in that the voltage of the auxiliary winding of the transformer is configured to be applied to each of the plurality of capacitors in the opposite direction to the input voltage via the high impedance element via another impedance element. Start-up circuit.