JPS61227676A - Inverter circuit - Google Patents

Abstract

PURPOSE:To protect switching elements against the long-play surge of low impedance, by providing a surge protection circuit to stop the switching action of the switching elements when surge voltage exceeds a specified value. CONSTITUTION:AC input provided via a noise-proof and power-factor-improving circuit 1 and a surge absorption circuit 2 is rectified by a rectification circuit 3, and the rectified input is converted to high-frequency current by a series inverter circuit 4 consisting of SEPP switching transistors Q1a, Q1b and auxiliary transistors Q2a, Q2b, and fluorescent lamps F1, F2 connected to an output terminal 5 are lit. When voltage between the DC output terminals of the recrification circuit 3 comes to a specified value or more due to the generation or the like of ringing surge, then current is fed to the base of the auxiliary transistor Q2b via a Zener diode ZD3 and a diode D7, and the switching transistor Q1b is retained in a cut-off state.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an inverter circuit, and particularly to a separately excited or self-excited inverter circuit suitable for a series inverter using transistors as switching elements.

[Prior art and its problems] In addition to lighting circuits for discharge lamps such as fluorescent lamps, DC input obtained from a commercial AC power source via a rectifier circuit is converted to high-frequency AC output at a desired frequency using an inverter circuit using switching elements such as transistors. However, a problem associated with inverter circuits that are connected to a commercial AC power source is the arrival of surges from the power source.

Conventionally, as a surge countermeasure in such cases, surge absorption was performed using voltage non-linear resistance elements such as Zener diodes and varistors on the input side of the rectifier circuit, but although they are effective against spike-like surges, However, it is less effective against low-impedance, long-duration ringing surges, and switching elements may be destroyed due to an increase in resonant current due to the application of abnormal voltage.

[Object of the Invention] Therefore, the object of the present invention is to achieve FRIlil! with low impedance. An object of the present invention is to provide an inverter circuit that can protect switching elements against long surges.

[Structure and operation of the invention] According to the present invention, the above problem is to provide a surge protection circuit that stops the switching operation of the switching element when the surge voltage superimposed on the DC input exceeds a predetermined equivalent value. This is achieved by

In one embodiment, the inverter circuit according to the present invention comprises:
It has a switching transistor with an auxiliary transistor connected between the base and emitter as a switching element,
The base current of the switching transistor is cut off by the conduction of the auxiliary transistor, and the switching transistor is turned off. The auxiliary transistor is configured to include a voltage non-linear resistance element that has a low resistance and forcibly makes the auxiliary transistor conductive.

The voltage nonlinear resistance element is an element such as a Zener diode, a varistor, or a bidirectional thyristor that becomes conductive when a voltage higher than a threshold voltage is applied between both terminals. Even if the integrator circuit output has not yet reached a level that makes the auxiliary transistor conductive, when the surge voltage exceeds a set value, the auxiliary transistor immediately conducts, causing the switching transistor to turn off quickly. As a result, switching is stopped until the surge voltage drops below a predetermined threshold value, and destruction of the switching element due to resonance current is prevented.

Examples of the present invention are as follows.

[Example 1] Figure 1 shows an example in which the present invention is applied to a transistor inverter lighting circuit with two fluorescent lamps as the load. The AC input inputted through the surge absorption circuit 2 is converted into a DC smoothed output by the rectifier circuit 3, and this is converted into a high frequency current by the series inverter circuit 4 consisting of 5EPP switching transistors Q1a and Qlb and auxiliary transistors Q2a and C2b. Then, the fluorescent lamps F1 and F2 connected to the output terminal 5 are turned on.

The transformer T1 is a feedback transformer, and has one secondary winding W1 and two oppositely wound secondary windings W2a, W2.
It has b. - The secondary winding W1 is inserted in series with the output terminal 5, detects the load current flowing therein, and generates corresponding secondary voltages of opposite phases in each of the secondary windings W2a and W2b. The secondary winding W2a has a base current limiting resistor R
The base of the switching transistor Q1a is connected through 1a, and the base current limiting resistor R1 is connected to W2b.
The base of the switching transistor Qlb is connected via b. Further, resistors R2a, R3a and a Zener diode 7:[)1a are connected between both ends of the secondary winding W2a.
The base-emitter circuit of the auxiliary transistor Q2a is connected through the auxiliary transistor Q2a, and the collector of the auxiliary transistor Q2a is connected to the base of the switching transistor Qla. Similarly, the base-emitter circuit of an auxiliary transistor Q2b is connected between both ends of the other secondary winding W2b via resistors R2b and R3b and a Zener diode zD1b, and the collector of this auxiliary transistor Q2b is connected to the base of the switching transistor Q1b. It is connected. Capacitors C3a and C3b are resistors R3a,
The Zener diodes ZD2a and ZD2b form a low voltage limiter circuit together with the resistors R2a and R2b.

In addition, diode D3a (C3b) and resistors R4a and R5
a (R4b, R5b) GE auxiliary transistor Q2a
(C2b) for temperature compensation, diode D4a (C
4b) and resistors R6a and R7a (R6b, R7b) are used to prevent turn-on from worsening due to current flowing from the base of 2a (Q2b) to the collector of the auxiliary transistor due to reverse voltage caused by switching, and diodes. D5a (C5b) is the capacitor C3a (
The diode D6a (C6b) is used to ensure that the reset (discharge) of C3b) is performed sufficiently quickly, and the diode D6a (C6b) is used to prevent short-circuiting between both ends of the secondary winding during reverse bias.

The two fluorescent lamps F1 and F2 are connected to an output terminal 5 and a rectifier circuit 3 through choke coils CHI and CH2, respectively.
It is connected in parallel with the DC output terminal of the rectifier circuit 3 in order from the positive DC output terminal of the rectifier circuit 3 to the resistor R8, the filaments of both lamps, the resistor R9, RIO, R11, and the capacitor C1. A series circuit leading to the negative DC output terminal of the rectifier circuit 3 and resistors R10 and R11
from the connection point J to one switching transistor Q1
The bidirectional thyristor SS connected in series to the base of inverter b forms a starting circuit for this inverter using a relaxation oscillation circuit. In addition, the connection point J and the switching transistor Q
The diode D1 connected in the forward direction between the collector of the inverter 1b keeps the charging voltage of the capacitor C1 below the breakover voltage of the bidirectional thyristor SS after the inverter is started, stops the initial operation of the starting circuit, and prevents the inverter from malfunctioning. This is to prevent this.

In relation to the starting circuit, the thyristor SCR connected between the connection point of the resistor R9 and RIO and the negative side DC output line is connected to the unbalance detection transformer T2 inserted in the filament circuits of both lamps Fl and F2. When a voltage occurs in the secondary detection winding Wd due to unbalanced currents flowing through both lamps due to single lighting or lamp life, the resistors R9 and R10 are triggered by the unbalanced voltage exceeding a predetermined value.
This is for stopping oscillation by dropping the connection point with the negative side DC output line, and when this thyristor SCR is conductive, it is connected to the transformer secondary winding W2 via the diode D2.
b is also bypassed, and no starting pulse occurs.

In the series circuit of the Zener diode ZD3 and the resistor R12 connected to the DC output end of the rectifier circuit 3, the connection point K between the Zener diode ZD3 and the resistor R12 is connected to the negative side auxiliary transistor Q2 via the forward diode D7.
b, thereby forming the main part of the present invention, that is, the surge protection circuit.

Now, to describe the operation of the lighting circuit of this embodiment, first, when the A0200V power is turned on and the full-wave rectified smoothed DC output is generated at the DC output terminal of the rectifier circuit 3, the resistor R8, both lamps Fl, and each side of F2 heater, resistor R9, RI
Charge is accumulated in the capacitor C1 via G and R11. This causes the potential at the connection point J to rise, and when it exceeds the breakdown voltage of the bidirectional thyristor SS, SS becomes conductive and supplies base current to the switching transistor Q1b on one side, and the collector of Qlb is connected to the diode D1.
Since a current is supplied from the connection point J through the transistor Qlb, the transistor Qlb becomes conductive, and the inverter is activated to start oscillation. Immediately after startup, the fluorescent lamps Fl and F2 have not yet reached the dischargeable state, so the output terminal 5 is connected to the series resonance between the choke coils CH1 and CH2 and the starting capacitors C2a and C2b connected between both heaters of each lamp. Since the circuit is connected, the Q of this resonant system is high, so the load current flows more than normal, and the feedback voltage appearing across the secondary winding W2b of the feedback transformer T1 becomes higher than during steady lighting. But Zener diode ZD2b
Since only a limited voltage is applied to the integrating circuit, charge accumulation in the capacitor C3b is suppressed, and therefore, stable starting is achieved without the oscillation frequency increasing at the time of starting.

Then, when the filaments of the fluorescent lamps Fl and F2 have finished warming up and started discharging, the capacitors C2a and C2b are substantially bypassed by the lamp discharge bus, so that the resonance described above is broken and the load current becomes e.g. The current is stable at 1.4A, resulting in a normal lighting condition.

During this time, it goes without saying that part of the load current is fed back by the transformer T1, and the switching transistors Q1a and Qlb are connected to their auxiliary transistors 0.
Oscillation is maintained by alternately turning on and off Q2a and Q2b.

Due to the arrival of a ringing surge from the power supply side, etc., the voltage between the DC output terminals of the rectifier circuit 3 increases due to the Zener diode ZD.
When the Zener voltage exceeds a predetermined threshold determined by the Zener voltage of 3, the Zener diode 2D3 is destroyed and current is supplied to the base of the auxiliary transistor Q2b through the diode D7, thus forcing the auxiliary transistor Q2b to conduct. As a result, switching transistor Qlb is maintained in the cutoff state. This state continues until the abnormally high voltage between the DC output terminals disappears, during which time the oscillation of the inverter is stopped and the lamp F1. F2 is turned off. Needless to say, once the abnormal voltage disappears, the startup operation will be performed again.

The magnitude of the feedback voltage generated in the secondary windings W2a and W2b of the transformer T1 is proportional to the number of turns, but in the illustrated example, in order to effectively utilize the total number of turns of the secondary winding, The feedback voltage is the auxiliary transistor Q2a.

The voltage is applied to the base integration circuit of Q2b, so even if the load current becomes small, the auxiliary transistor Q
If the number of turns of the secondary winding is determined so that the on/off operations of Q2a and Q2b can be performed reliably, it is possible to prevent oscillation from stopping due to a decrease in the load current. Furthermore, in this case, if the base currents of the switching transistors Qla and Qlb are supplied from an intermediate tap provided in the secondary winding, it is possible to reduce the resistance of the base current forced resistances R1a and R1b. This also makes it possible to reduce power loss.

Note that the auxiliary transistors Q2a and Q2b are used to drive the switching transistors Qla and Qlb to remove minority carriers and turn them off, so in order to improve the turn-off operation of the switching transistors, only 02a is shown in FIG. A differentiating circuit consisting of a capacitor Cχ and a diode Dχ may be added to attract the minority carriers of the switching transistor via the capacitor Cχ.

Furthermore, although the illustrated embodiment shows the case of an automatic inverter circuit, it goes without saying that the present invention is not limited to this, and can also be applied to a separately excited inverter circuit.

[Effects of the Invention] As described above, according to the present invention, destruction of switching elements due to abnormal voltage increases such as ringing surges can be effectively prevented by adding only a few parts.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment in which the present invention is applied to a transistor inverter lighting circuit for a fluorescent lamp, and FIG. 2 is a partial circuit diagram of a modification thereof. 3:1! Current circuit, 4-inverter circuit, 5: Output terminal, Qla, Q1b switching transistor, Q2a, Q2b: Auxiliary transistor, T1 Nidorance, Wl Knee winding, W2a, W2b: Secondary winding, ZD3: Zener diode (voltage nonlinear resistance element) in surge protection circuit. Figure 2

Claims (1)

[Claims] 1. A switching transistor has an auxiliary transistor connected between the base and emitter, and conduction of the auxiliary transistor cuts off the base current of the switching transistor and extracts minority carriers from the switching transistor. In an inverter circuit configured to turn off a switching transistor, when a surge voltage superimposed on the DC input of the inverter circuit exceeds a predetermined threshold, the auxiliary transistor is forcibly made conductive and the switching transistor is switched. An inverter circuit characterized by being equipped with a surge protection circuit that stops operation. 2. Claim 1, wherein the surge protection circuit includes a voltage non-linear resistance element that becomes low in resistance and forcibly turns on the auxiliary transistor when the value of the surge voltage exceeds the threshold value. The inverter circuit described in section.