JPH02261068A - Dc-ac inverter circuit - Google Patents

Abstract

PURPOSE:To protect components of each circuit surely without damaging in case of AC output short-circuit by connecting a base current supply circuit to a DC input terminal only in case of closing of DC input power supply. CONSTITUTION:One end of a current supply winding N3 and a starting switch SW provided with a push-button P are connected to a base of the first switching transistor Q1. An inverter circuit is not actuated by closing only DC power supply to DC input terminals 11 and 12, but the circuit starts by pushing the push-button P to turn a starting switch ON. After that, even if the push-button P is released to turn the starting switch SW OFF, the inverter continues operations. When short circuit caused by something is made between AC output terminals on the secondary side, an induced voltage of a base current supply winding is lowered to stop immediately operations of the inverter. According to the constitution, the damage, over-heating and combustion can be prevented in case of short circuited output, and cost reduction and miniaturization can be achieved.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a DC-AC inverter circuit used, for example, in a lighting device for a discharge lamp such as a fluorescent lamp.

<Prior Art> A push-pull type DC-AC inverter circuit commonly used in the past is shown in FIG. 3, and the operation of this circuit will now be described. Primary winding N that functions as a resonant coil of inverter transformer T
1 and a resonance capacitor C2 are connected in parallel to form a parallel resonance circuit. And DC input terminal II, 1
When DC power is supplied to the capacitor C1, the capacitor C1 is charged and coiled, and base current is supplied to each of the pair of switching transistors Ql and Q2 via each resistor R1 and R2, and one of the switching transistors Q
1 or Q2 turns on. Assuming that the first switching transistor Q1 is now turned on, a current flows from the intermediate tap of the inverter transformer T to the first switching transistor Q1 through the upper part of the primary winding N1 in the diagram.
As a result, a back electromotive force is generated in the primary winding N1 in the direction opposite to the arrow shown in the figure, and the capacitor C2 is charged with a polarity opposite to that shown in the figure. On the other hand, due to the back electromotive force in the direction opposite to the arrow, an induced electromotive force is generated in the winding N3 wound on the primary side of the inverter transformer T in the direction opposite to the arrow in the figure due to inductive coupling within the transformer T. The first switching transistor Q1 continues to be on, but when the collector current of the first switching transistor Q1 reaches a constant current,
As saturation begins, the amount of increase in current begins to decrease, and as a result, the back electromotive force of the primary winding N1 in the opposite direction of the arrow decreases.
Capacitor C2 starts discharging and current begins to flow in the direction of the arrow in the figure through the loop including primary winding N1. When the discharge of this capacitor C2 is completed, an electromotive force is induced in the primary winding N1 in the direction of the arrow in the figure, contrary to the above, and the winding N3
Also, an electromotive force is induced in the direction of the arrow in the figure. Therefore, the first
The switching transistor Q1 is turned off, and the second switching transistor Q2 is turned on, and current flows from the intermediate tap to the second switching transistor Q2 through the lower part of the primary winding N1 in the diagram. When the electromagnetic energy stored in the primary winding N1 decreases, the capacitor C2, which was charged with the polarity shown in the diagram, starts discharging, and current flows in the loop including the primary winding N1 in the opposite direction of the arrow in the diagram. .

When this discharge is completed, the induced voltage in the secondary winding N2 becomes reversed, the first switching transistor Q1 is turned on, and the second switching transistor Q2 is turned off, and from then on, the resonance capacitor C2 and the primary winding The line N1 becomes a resonant vibration that transfers energy, and both switching transistors Ql and Q2 alternately turn on and off depending on the direction of the induced voltage in the winding N3 generated at this time.
A voltage with a sinusoidal waveform as shown by A in Fig. 4 is generated in the primary winding N1, and a current with a sinusoidal waveform as shown by the opening in the same figure flows through the resonance capacitor C2, which is the voltage of the inverter transformer T. It is transmitted to the next winding N2, and the AC output terminal Of
, 02.

<Problems to be Solved by the Invention> By the way, in the conventional DC-AC inverter circuit, it is difficult to protect the circuit when the AC output on the secondary side of the inverter transformer T, that is, between the two AC output terminals Of and 02, is short-circuited for some reason. As a countermeasure, means for connecting a fuse F between the DC input terminals If and I2 is generally used. However, this fuse F is not easy to replace due to the fact that it is not easy for the equipment user to replace it when the fuse F blows out, and also due to the rush current at the time of starting the equipment and the variation in the fusing current value of the fuse F itself. In consideration of preventing frequent blowouts during operation, a fuse having a blowout current several times the current value during normal operation of the device is generally selected and used.

Along with this, in order to prevent each circuit component such as the switching transistor QIQ2 and the inverter transformer T from being damaged before the fuse F blows or causing heat generation and ignition when a short circuit occurs, the current that flows when a short circuit occurs Those with a sufficiently larger rated current value than the above, and therefore with larger external dimensions, are used, and this is a factor that hinders miniaturization of the entire device and is a cause of high costs.

The present invention has been made in view of these conventional problems, and has a structure that can achieve cost reduction and miniaturization, while also being able to reliably protect each circuit component without damaging it in the event of an AC output short circuit. The technical problem is to provide a C-AC inverter circuit.

<Means for Solving the Problems> In the present invention, as a technical means for achieving the above-mentioned problems, a DC-AC inverter circuit is configured as follows. That is, in a DC-AC inverter circuit in which a switching transistor is turned on and off by a resonant circuit in which the winding of an inverter transformer and a capacitor are connected in series or in parallel, the induced voltage in the base current supply winding wound around the inverter transformer is A base current supply circuit is provided for supplying a base current to the switching transistor, and this base current supply circuit is connected to the DC input terminal via a starting switch section that is turned on manually or automatically only when the DC input power is turned on. It is structured with this feature in mind.

<Operation> When the DC input power source is turned on, current is supplied to the base of the switching transistor via the starting switch section and the base current supply circuit, which are in the on state, and the resonant circuit starts oscillating. Thereafter, the starting switch section is turned off, and the base current supply circuit is electrically cut off from the DC input power source.

If the AC output terminals on the secondary side of the inverter transformer are short-circuited for some reason under stable operating conditions, the load on the secondary side AC output increases and the load required for stable operation of the switching transistor increases. The minimum base current value increases. However, as described above, the base current supply circuit is electrically cut off from the DC input power source and energy is not replenished, so the induced voltage in the base current supply winding decreases and the operation stops instantaneously.

Therefore, even if an inverter transformer or a switching transistor with a small rated current value is used, it can be protected from damage or heat generation when the output is short-circuited, and the cost and size can be reduced.

<Embodiments> Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

In FIG. 1 showing an embodiment of the present invention, the same or equivalent parts as in FIG. 3 are given the same reference numerals, and the explanation thereof will be omitted. Then, exclude fuse F in Figure 3,
The base current supply circuit of a pair of switching transistors QLQ2 is connected to each switching transistor Q1. Two diodes DI, D2 are connected in anti-series between the bases of Q2, and both diodes DI,
The common connection point of each anode of D2 is connected to ground, and a load resistor R4 is inserted and connected between the base of the first switching transistor Q1 and one end of the base current supply winding N3. There is. One point of this base current supply circuit is a start switch SW equipped with a push button P and a resistor R.
3 to the DC input side.

Next, the operation of the above embodiment will be explained. In this embodiment, the operation does not occur simply by applying DC power to the DC input terminals 11 and 12. By pressing the push button P to turn on the start switch SW, the start switch S is turned on.
A base current is supplied to both switching transistors Ql and Q2 through W, and both switching transistors Ql and Q2 are alternately turned on by the same operation as explained in FIG. 3, and the inverter circuit is activated.

After that, even if the start switch SW is turned off by releasing the push button P and the base current supply circuit is electrically disconnected from the DC input side, the diode D2. The base of the first switching transistor Q1 is connected to the base of the first switching transistor Q1 via the base current supply winding N3 and the resistor R4, and from the ground to the second switching transistor Q2 via the diode DI, the resistor R4 and the base current supply winding N3.
A base current is alternately supplied to the bases of D
Continue the C-AC inverter operation.

By the way, the minimum average (approximate) base current value IBL required for the DC-AC inverter circuit to operate stably is the DC input voltage applied to the DC input terminals 11 and 12, EB, and the DC-AC inverter circuit internal When the loss of the switching transistor Ql is Pd, the load connected to the AC output terminal 01.02 is P, the current amplification factor of the switching transistor Ql, and the current amplification factor of Q2 is hfe, both switching transistors Ql.

If the base current of Q2 is equal to or greater than the value of the above equation, the output AC voltage of the AC output terminals 01 and 02 is maintained at a constant voltage determined by the DC input voltage and the circuit constant.

If the AC output terminals 01 and 02 are short-circuited for some reason during stable operation, the load P in the above equation will increase and the minimum required average base current value 1111 will decrease. To increase. However, as mentioned above, the starting switch SW is turned off and the base current supply circuit is electrically disconnected from the DC input side, so the base currents of the switching transistors Ql and Q2 are not equal to the base current supply circuit. A constant value determined by the induced voltage of the winding N3, its internal resistance, the resistance value of the resistor R4, etc. is maintained. Therefore, the base current becomes insufficient than the minimum necessary value, and the output as an inverter decreases.This causes the induced voltage in the base current supply winding N3 to decrease, and the base current becomes insufficient compared to the minimum necessary value. The cycle in which the shortage becomes even larger is repeated at a high speed, and the operation immediately stops. Therefore, even if the switching transistors Ql, Q2 and the inharter transformer T have a rated current value, troubles such as breakage and heat generation will not occur.

In the above embodiment, the connection point between the base of the first switching transistor Q1 of the base current supply circuit and the resistor R4 was connected to the starting switch SW. Resistance R
4 and the base current supply winding N3 to the base of the second switch-notching tridisk Q2, no matter which point of the circuit is connected to the starting switch SW, the same operation as in the previous embodiment can be achieved. I do.

Further, the resistor R4 is connected for the purpose of adjusting the base current value, but this base current value is mainly determined by the induced voltage of the base current supply winding N3, its internal resistance value, and the resistance value of the resistor R4. Therefore, it is not necessarily necessary to provide the base current supply winding N3 as long as the induced voltage and internal resistance value of the base current supply winding N3 are selected to be appropriate values.

Furthermore, even if each diode DIQ2 in the embodiment described above is replaced with a resistor, the same effect as described above can be obtained. That is, any circuit component may be used as long as it can sufficiently increase the base voltage of the switching transistors Ql and Q2 when the starting switch SW is turned on, and can supply a sufficient base current when the switch SW is turned off.

FIG. 2 shows another embodiment of the present invention, in which the first
Components that are the same as or equivalent to those in the figures are given the same reference numerals and their explanations will be omitted. The difference from FIG. 1 is that the diodes DI, D2 and resistor R4 in FIG. 1 are excluded, and the switching transistors Ql, Q2 are A base current supply circuit is constructed by connecting the intermediate tap provided at the center of the base current supply winding N3 to the ground via the reverse diode D3, and
A PNP-type starting transistor Q3 is provided in place of the starting switch SW, and the emitter of this starting transistor Q3 is connected to the DC power supply terminal 1 via a coil L, and the collector is connected to the above-mentioned terminal via a resistor R6. The only configuration is that a time constant circuit including a resistor R5 and a capacitor C3 is connected to the base current supply circuit and further serves as a timer circuit for setting the time to turn on the starting transistor Q3 when the power is turned on. Note that the diode D4 is the starting transistor Q.
3, and the resistors R7 and R8 are for the protection of the first
Since it has the same function as resistor R4 in the figure,
It's not necessarily necessary.

Next, the operation of this embodiment will be explained. When DC power is supplied to the DC input terminals If and I2, the capacitor C
3 is gradually charged through the resistor R5, but this capacitor is charged until the terminal voltage of the capacitor C3 rises after a certain period of time determined by the time constant of the resistor R5 and the capacitor C3. Since the terminal voltage of C3, that is, the base voltage of the starting transistor Q3, is lower than the emitter voltage of the starting transistor Q3, the starting transistor Q
A collector current of 3 flows through the resistor R6 and the base current supply circuit to the bases of each switching transistor Ql, Q2, and starts an oscillation operation.

After that, when charging of capacitor C3 is completed, starting transistor Q3 turns off because its base voltage becomes the same potential as emitter voltage, and emitter voltage no longer flows, but it has already started oscillation operation and supplies base current. Since an electromotive voltage is generated in the winding N3, the diode D3. to the base of the first switching transistor Q1 via the base current supply winding N3 and the resistor R7;
And from ground to diode D3. Base current supply winding N
A base current is alternately supplied to the base of the second switching transistor Q2 through the transistor Q3 and the resistor R8, thereby continuing the inverter operation.

In addition, if the AC output terminals 01 and 02 are short-circuited for some reason while stable operation is being performed, the starting transistor Q3 is turned off and the base current supply circuit is electrically disconnected from the DC input side. Since it is in the disconnected state, the inverter operation is instantaneously stopped in exactly the same manner as in the case of FIG.

In this embodiment, even if the diode D3 is replaced with a resistor, the same effect as described above can be obtained. Also, excluding this diode D3, the base current supply winding N3
The intermediate tap may be connected directly to ground. However, in this case, resistors R7 and R8 are essential. In short, it is the required supply amount of base current at startup when designing the product.

Required supply amount of base current during stable operation, AC output terminal 0
Necessary circuit components may be appropriately selected from the viewpoints of responsiveness to quickly and completely stop the operation in the event of a short circuit between 1 and 02, and product cost.

<Effects of the Invention> Since the present invention is configured and operates as described above, it produces the following effects. That is, according to the DC-AC inverter of the present invention, the base current is supplied from the DC input power source to the switching transistor through the startup switch section only at startup, and after stable operation has started, the startup switch section is turned off. Since the base current supply circuit is electrically cut off from the DC input power source, if the output AC terminal is short-circuited, the base current becomes insufficient and the operation can be stopped instantaneously. Therefore, even if circuit components such as switching transistors and inverter transformers with small rated current values are used, the circuit components can be protected from damage or heat generation, resulting in considerable cost reduction and size reduction. can do. However, after a short circuit occurs, operation is restarted simply by turning on the DC input power, so there is no need for complicated work such as replacing fuses as in conventional circuits.

[Brief explanation of drawings]

Figure 1 is an electric circuit diagram of one embodiment of the present invention, Figure 2 is an electric circuit diagram of another embodiment of the present invention, Figure 3 is an electric circuit diagram of a conventional example, and Figure 4 is the same as that of Figure 3. FIG. 3 is a waveform diagram of a resonant circuit. 11.12 - DC input terminal Ql, Q2 - Switching transistor T - Inverter transformer N1 - Inverter transformer primary winding N3 - Base current supply winding S W - Starting switch (starting switch) Q3
−Starting transistor (starting switch part)

Claims (1)

[Claims] (1) In a DC-AC inverter circuit in which a switching transistor is turned on and off by a resonant circuit in which the winding of an inverter transformer and a capacitor are connected in series or in parallel, induction in the base current supply winding wound around the inverter transformer A base current supply circuit that supplies base current to the switching transistor by voltage is provided, and this base current supply circuit is connected to the DC input terminal via a startup switch section that is turned on manually or automatically only when the DC input power is turned on. DC characterized by
-AC inverter circuit.