JPH0279779A - Inverter - Google Patents

Abstract

PURPOSE:To flatten the characteristics of a secondary voltage without a load in a one-transister type parallel resonating inverter by decreasing a loop gain when an operating frequency undergoes negative feedback control in order to make the peak voltage of a switching element constant. CONSTITUTION:A frequency control circuit 7 has the following part: an error amplifying circuit 72 which compares the output of a reference voltage source 71 with the output of a peak voltage detecting circuit 6, amplifies the error voltage at a specified gain and outputs the result; a variable resistor 73 which adjusts the gain of the circuit 72; and a voltage-controlled type oscillator 74 which oscillates a frequency in response to the output of the circuit 72. The base of a Tr 3 is driven with the frequency in response to a collector voltage. The variable resistor 73 is adjusted, and the gain of the amplifier 72 is set at a low value. In this way, the load voltage can be flattened when the inverter has no load.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a stone-type parallel resonant inverter, and in particular,
The present invention relates to an inverter suitable for application to a discharge lamp lighting device for lighting a discharge lamp such as a fluorescent lamp at high frequency.

[Prior Art] Conventionally, an inverter that converts DC power to AC power uses a parallel resonant circuit and an input DC voltage at a high frequency, e.g.
A type-6 parallel resonant inverter is known, which includes a switching element that turns on and off at 100 kHz to apply voltage to this parallel resonant circuit, and supplies an alternating current voltage induced in this parallel resonant circuit to a load.

Conventionally, in such a parallel resonance type inverter, the operating frequency (on/off frequency of the switching elements) has not been particularly controlled. For this reason, the operating frequency is usually constant in separately excited inverters, and is a frequency that depends on the load in self-exciting inverters.

Therefore, the voltage applied to the switching element changes depending on the load condition, and for this reason, in an inverter with large load fluctuations such as a discharge lamp load, the voltage applied to the switching element tends to become overvoltage. This has the disadvantage that it is necessary to use switching elements that have a high breakdown voltage, that is, are expensive.

Therefore, a method was devised in which the operating frequency is controlled so that the peak voltage applied to the switching element is constant. According to this method: (1) Good power fluctuation characteristics when turned on.

■Strong against external surges.

■Can deal with variations in circuit elements.

There are many advantages such as

[Problems to be Solved by the Invention] However, in such a method of controlling the peak voltage applied to the switching element to a constant value, the control gain is usually set to a sufficiently high level (ideally infinite) to avoid unstable phenomena. This has the drawback of deteriorating the fluctuation characteristics of the no-load secondary voltage.

For example, a case will be described in which an inverter of this type is applied to a discharge lamp lighting device. First, in such a discharge lamp lighting device, in order to light the discharge lamp, a no-load secondary voltage is generated using resonance between an inductive current-limiting impedance and a capacitor connected in parallel to the discharge lamp. . If the gain of the control system is set high enough at this time, the voltage applied to the switching element will be controlled to be almost constant regardless of the power supply voltage, and when the power supply voltage is high, the frequency will be increased and When the frequency is low, control works to lower the frequency. As a result, the operating frequency of the inverter deviates from the resonant frequency, and when the discharge lamp is on, there is no problem because the Q of the secondary side resonant circuit is low, but when the discharge lamp is off, the secondary side Since the Q of the resonant circuit is high, when the operating frequency deviates from the resonant frequency, the no-load secondary voltage fluctuates greatly. That is, when the power supply voltage is high, the no-load secondary voltage drops significantly, and conversely, when the power supply voltage is low, the no-load secondary voltage becomes excessive. This phenomenon causes problems such as the discharge lamp not lighting up.

The purpose of the present invention is to solve the above-mentioned problems in the conventional type.
- An object of the present invention is to provide an inverter in which the fluctuation characteristics of the no-load secondary voltage are improved in a stone-type parallel resonance type inverter, and the no-load secondary voltage characteristics become flat even when the power supply voltage fluctuates.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides negative feedback control of the operating frequency so that the peak voltage applied to the switching elements is constant in a parallel resonance type inverter. The present invention is characterized in that the gain of the negative feedback control loop is set low so that the power supply voltage fluctuation characteristics are substantially flat.

In the present invention, the operating frequency is not particularly limited, but
A frequency higher than the audible frequency, 20 to 100 kHz, is preferably used.

[Function] According to the present invention, the control gain can be lowered than usual, and the fluctuation characteristics of the no-load secondary voltage can be flattened. Thereby, when applied to a discharge lamp lighting device, the discharge lamp can be reliably started even if the power supply voltage changes.

[Example] Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 shows the configuration of a separately excited inverter according to an embodiment of the present invention. In the figure, reference numeral 1 denotes a DC power source, and in addition to a pure DC power source such as a battery, a smoothing, partial smoothing, valley filling type smoothing, or non-smoothing rectification circuit for rectifying an AC power source can be used.

2 is an output transformer, and the primary winding 2 of this output transformer 2
p has one end connected to the positive terminal of the DC power supply 1, the other end connected to the collector of the transistor 3, which is a switching element, and a resonant capacitor 4 connected in parallel with the primary winding 2p. . These primary windings 2p and the resonance capacitor 4 constitute a parallel resonance circuit. Furthermore 1. A discharge lamp such as a fluorescent lamp is connected as a load 8 to the secondary winding 2s of the output transformer 2.

Further, the emitter of the transistor 3 is connected to the negative terminal of the DC power supply 1, and a diode 5 is connected in antiparallel to the collector/emitter of the transistor 3, and a series circuit of a diode 61 and a capacitor B2 in the forward direction is connected. A certain peak voltage detection circuit 6 is connected. Resistors 83 and 6
4 is for dividing the terminal voltage of this capacitor 62 and sending it to the frequency control circuit 7.

The frequency control circuit 7 includes a reference voltage source 71. An error amplification circuit 72 that compares the output voltage of the peak voltage detection circuit 6 with the output voltage of a reference voltage source 71, amplifies the error voltage with a predetermined gain, and outputs the amplified error voltage, and a variable resistor for adjusting the gain of the error amplification circuit 72. 73, and a voltage controlled oscillator (VCO) 74 that oscillates at a frequency corresponding to the output of the error amplification circuit 72, and the terminal voltage of the capacitor S2, that is, the transistor 3
The base of the transistor 3 is driven at a frequency corresponding to the collector voltage of the transistor 3.

Next, the operation of the discharge lamp lighting device shown in FIG. 1 will be explained.

When DC power supply 1 is turned on, VCO 74 starts oscillating,
Drives the base of transistor 3. As a result, the transistor 3 is turned on and off at the oscillation frequency of the VCO 74,
A parallel resonant circuit of the primary winding 2p of the output transformer 2 and the resonance capacitor 4 is driven, and the secondary winding 2 of the output transformer 2 is driven.
A high frequency output voltage is induced in S. That is, the inverter starts.

During normal operation, in the peak voltage detection circuit 6, diode B
1, the capacitor 62 is charged to approximately the peak value of the collector voltage of the transistor 3, and in the frequency control circuit 7, the voltage obtained by dividing the terminal voltage of the capacitor 62 by the resistors 63 and 64 and the reference voltage source are charged. The oscillation frequency f1 of the VCO 74 is controlled so that the output of the VCO 71 approaches the reference voltage V ref'. Here, it is assumed that the oscillation frequency of the VCO 74 is always set higher than the resonant frequency f0 of the parallel resonant circuit.

During steady operation, when the peak voltage Vp becomes higher than the set voltage v set, the terminal voltage vp of the capacitor B2 is set by the resistor 63.
Voltage Vp/n divided by and 64 (where n is the voltage division ratio)
becomes higher than the reference voltage V ref', and the error amplifier 72
The output of VCO 74 increases, and the oscillation frequency f of the VCO 74, that is, the operating frequency f1 of the inverter increases. As a result, the operating frequency f1 deviates from the resonant frequency f, and the peak voltage Vp decreases.

At the same time, the high frequency output voltage supplied from the secondary winding 2s of the output transformer 2 to the load 8 also decreases.

On the other hand, when the peak voltage Vp becomes lower than the set voltage v set and the voltage Vp/n becomes lower than the reference voltage V rer, the output of the error amplifier 72 decreases, and the oscillation frequency f1 of the VCO 74, that is, the operating frequency f1 of the inverter decreases. do. As a result, the operating frequency f1 becomes the resonant frequency f. , the peak voltage Vp and the high-frequency output voltage increase.

In the device of this embodiment, the gain of the error amplifier 72 is adjusted by a variable resistor 73, and is set to a lower gain than usual. Further, even in the case of no load, for example, when the discharge lamp is not yet lit, the no-load secondary voltage characteristic with respect to the input terminal is made to be flat.

FIG. 2 shows an example of the configuration of the error amplifier 72. 78 is an operational amplifier, and 79 is a resistor for gain adjustment.

FIG. 3 is a graph showing no-load secondary voltage characteristics with respect to input voltage. 101 indicates a normal design, that is, the gain is set high; 102 indicates no control, that is, the gain is zero; and 103 indicates the case where the gain is set low as in this embodiment. It can be seen that the device of this example has flat characteristics.

FIG. 4 shows an example in which the present invention is applied to a self-excited inverter. In the figure, 101 is an AC power supply, and this AC [j
A rectifier, such as a full-wave rectifier circuit 102, is connected to iHot, and non-smooth direct current (rectified output) from this rectifier circuit 102 is supplied to subsequent circuits. A smoothing capacitor 103 and an inverter are connected between the rectified output terminals a and b.

20 is an output transformer, and one end of the primary winding of the output transformer 20 is connected to the rectifier output terminal a, and the other end is connected to the collector of the transistor 30, which is the main switching element. Resonant capacitor 4 in parallel
Connect 0. These primary windings and the resonance capacitor 40 constitute a parallel resonance circuit. Furthermore, a discharge lamp 81 such as a fluorescent lamp is provided in the secondary winding of the output transformer 20.
A starting capacitor 82 and a primary winding of a saturable current transformer (CT) 91 that detects a load current and provides positive feedback to the base of the transistor 30 are connected in series.

The emitter of the transistor 30 is connected to the negative rectifier output terminal, and one end of the secondary winding of the CT91 is connected to the base of the transistor 30, and the other end is connected to the negative rectifier output terminal via the capacitor 92. be. Further, a series circuit consisting of a diode 95 and a resistor 96 is connected in antiparallel between the base emitter and the base emitter of the transistor 30. 93
94 is a capacitor, and 94 is an FET as a variable impedance element whose impedance changes according to the collector voltage of the transistor 75. These constitute an oscillation frequency control circuit 90.

The voltage detection circuit BOA is for detecting the peak value Vp of the collector voltage of the transistor 30, and includes a series circuit of a diode 61 and a capacitor 62 connected in the forward direction to the collector-emitter of the transistor 30. The voltage dividing circuit BOB consists of resistors 63 and 64, divides the terminal voltage of the capacitor 62 to create a voltage of Vl)/n, and sends it to the error amplifier circuit 70.

71 is a reference voltage generation circuit consisting of a resistor and a Zener diode, and the reference voltage V is the Zener voltage of the Zener diode.
ref is created and sent to the error amplification circuit 70. The error amplification circuit 70 includes a transistor 75 that compares the reference voltage V ref with the output voltage Vp/n of the voltage divider circuit BOB and generates a collector voltage according to the error voltage. can be determined.

Next, the operation of the discharge lamp lighting device shown in FIG. 4 will be explained.

When the power supply 101 is turned on, the transistor 80 is supplied with a minute base current from a starting circuit (not shown), and becomes slightly conductive. As a result, the primary winding of the output transformer 20 is slightly driven, and a load current flows through the secondary winding. This load current is detected by the CT 91 and fed back positively to the base of the transistor 30. The transistor 3o is rapidly turned on due to the positive feedback loop from the base of the transistor 30 to the base 30 of the transistor via the collector, output transformer 20 and CT91. Capacitor 92.93
is charged by the base current for turning on transistor 30, and as a result, the base current of transistor 3o decreases. Then, the transistor 30 is suddenly turned off due to the positive feedback loop. When off, the load current reverses its polarity due to the AC voltage induced in the secondary winding of the output transformer 2o due to the resonance of the parallel resonant circuit, and then rotates normally again. Then, the voltage applied to the base of the transistor 30 becomes positive, and the transistor 30 is turned on again due to the above positive feedback. Thereafter, this inverter continues to oscillate due to such positive feedback and parallel resonance.

The control of the operating frequency is the same as in the case of FIG. 1, but
In this embodiment, the peak value of the collector voltage of the transistor 30 is controlled so as to approach a predetermined value (determined based on the reference voltage of the reference voltage generation circuit 71).

That is, the error amplifier circuit 70 compares the reference voltage supplied from the reference voltage generation circuit 71 and the voltage obtained by dividing the collector IT pressure peak value of the transistor 30 by the voltage dividing circuit 60B, and calculates the error with a predetermined gain. The impedance of FET 94 is changed by amplification.

In this inverter, the off-period of the transistor 30 is determined by the parallel resonance and is therefore constant. However, the on period is determined by the base current of transistor 30 flowing through capacitors 92 and 93. The base current of this transistor 30 is determined by the impedance of the capacitor 92, 93 and the FET 94 which are connected in series with the base of the transistor 30 to the secondary winding of the CT 91. Therefore, by varying the impedance of the FET 94, the oscillation frequency of the inverter can be varied.

Now, when the peak voltage Vp of the collector of the transistor 30 falls below the set voltage v set, the current flowing through the transistor 75 increases and the impedance of the FET 94 decreases. As a result, the time for a current sufficient to drive the base of the transistor 30 to flow through the capacitors 92 and 93 becomes longer, the on-period of the transistor 3o is extended, the oscillation frequency decreases and approaches the resonant frequency, and the peak voltage vp
rises. When the peak voltage Vp rises above the set voltage v set, each part of the circuit operates in the opposite manner to the above, and the peak voltage Vp decreases. Therefore, the peak voltage vp is stabilized. Furthermore, the output voltage is also stabilized against fluctuations in the power supply voltage.

In this case, the gain of the error amplification circuit 70 is determined by the transistor F5 and the resistors 78 and 77. Therefore, these reduce the gain of the error amplifier 70 and the no-load 2
Next, the voltage characteristics should be made flat.

Note that the gain can be lowered by increasing the voltage dividing ratio of the voltage dividing circuit BOB, or by lowering the sensitivity of frequency control.

[Effects] As explained above, according to the present invention, by simply setting the control gain lower than usual when controlling the operating frequency of the inverter, the power supply voltage fluctuation characteristics of the secondary voltage under no load can be improved. If applied to a discharge lamp lighting device, the discharge lamp can be reliably lit. Furthermore, since the control gain is reduced, the circuit configuration of the error amplifier can be simplified and costs can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a separately excited inverter according to an embodiment of the present invention, Fig. 2 is a circuit diagram showing a configuration example of an error amplifier, and Fig. 3 shows no-load secondary voltage characteristics with respect to input voltage. The graph shown in FIG. 4 is a circuit diagram of a self-excited inverter according to another embodiment of the present invention. 1: DC power supply, 2: Output transformer, 3: Transistor,
4: Resonance capacitor, 6: Peak voltage detection circuit, 7:
Frequency control circuit, B1: diode, 62: capacitor, 63.64: resistor, 72: error amplifier circuit, 74: voltage controlled oscillator.

Claims (1)

[Claims] 1. In a single-stone parallel resonant inverter, when performing negative feedback control of the operating frequency so that the peak voltage applied to the switching element is constant, the negative feedback control is performed so that the power supply voltage fluctuation characteristics are almost flat. An inverter characterized by having a low loop gain.