RU2141717C1 - Half-bridge self-excited oscillator - Google Patents

Abstract

FIELD: microelectronics; control systems for fluorescent and halogen lamps; secondary power supplies. SUBSTANCE: oscillator has power transistors connected to control circuits identical for top and bottom transistors of half-bridge which function to reduce dissipation power and to convert sine-wave voltage across transformer secondary windings to square-wave current conveyed to bases of power transistors as well as to actively cut off power transistors when they are turned off. In addition, at least bottom transistor of half-bridge incorporates overcurrent protective circuit for external control of its level; in case of overcurrent condition, protective circuit sends signal to disabling circuit which operates as thyristor and fully turns off power transistor; disabling circuit is reset at each half-cycle of self-excited oscillations inoperative for particular power transistor. EFFECT: reduced power loss and provision for controlling brightness of lamps. 3 dwg

Description

 The invention relates to microelectronics and can be used to build self-oscillating control systems for fluorescent lamps ("electronic ballasts"), as well as halogen lamps, secondary power sources.

 A basic circuit of a self-generating system for controlling fluorescent lamps, made on discrete elements and containing a rectifier bridge on diodes D1-D4 with a fuse F1 and a smoothing capacitor C1, two power transistors T1 and T2 with reverse diodes, basic resistors R2, R3, respectively, and equalizing emitter resistors R4 and R5, controlled by the currents of the secondary windings of the transformer L1, L2 with the primary winding L3 connected in series with the current-limiting inductance L4, decoupling capacitance C4 and lumina a centric lamp, in parallel with which the resonant capacitance C3 is connected, as well as containing a trigger circuit consisting of a diacra TH1, a current-sensing resistor R1, a starting capacitor C2 and a reset diode D5.

 In FIG. 1 shows a basic diagram of a self-generating system.

 The scheme works as follows.

 The diode bridge D1 - D4 and the smoothing capacitance C1 convert the alternating voltage of the network (for example, 220V) into constant (≅ 300V).

 The starting circuit on the resistor R1, capacitance C2 and the dialer TH1 gives the initial “push” for the excitation of self-oscillations, the diode D5 blocks this circuit after the start of oscillation.

 The autotransformer is built on windings L1, L2, L3. Self-oscillations are possible with the antiphase switching on of the secondary windings and the current phase shift in the primary and secondary transformer windings close to π / 2 at the operating frequency, which is achieved by a small number of turns in the secondary windings.

 Resistors R2 and R3 limit the base currents of the power transistors T1 and T2, respectively.

 Resistors R4 and R5 are needed to equalize the characteristics of discrete transistors T1 and T2.

 Inductance L4 limits the slew rate of the current and at the same time forms a sequential oscillatory circuit with a capacitance C3, which is necessary to obtain a sufficiently high ignition voltage of the lamp.

 Capacitance C4 is used for DC isolation.

The main disadvantages of such a scheme are as follows:
hard mode of operation of power transistors for voltage and current. With a sinusoidal control signal at the bases of transistors, when they are turned off, time intervals are inevitable when the transistor is in active mode - at full voltage on the collector and gradually decreasing collector current. This is due to inductive load operation. The time of this undesirable mode is 1 - 4 ms, depending on the shape of the current in the secondary winding of the transformer. The presence of this mode requires a careful selection of transistors for instantaneous power characteristics;
significant dissipated power on transistors when operating in active mode, which leads to lower efficiency of the system as a whole and to the need to solve the problem of heat dissipation;
the problem of limiting the voltage on a cold lamp at start-up, since the start of a cold lamp sharply reduces its service life (this is solved, for example, by switching on an additional thermistor in parallel with the resonant capacitance C3);
lack of current protection;
the inability to control the brightness of the lamp.

The proposed half-bridge oscillator in integrated design eliminates these shortcomings and implement the following main functions:
self-oscillating system control with minimal power losses;
power transistor current protection;
limitation of the load current at the initial moment of starting the system when working on a non-linear load;
the ability to control the brightness of the load lamps.

 The essence of the invention: control circuits identical to the upper and lower power transistors are connected to the power transistors of the half-bridge oscillator, which, to reduce the dissipated power, convert the sinusoidal voltage to a rectangular current supplied to the base of the power transistors and actively lock the power transistors when they are turned off. In addition, the lower power transistor has a current protection circuit, which has an external adjustment of its level and, through the blocking circuit, turns off the power transistor when the specified output current level is exceeded.

 In FIG. 2 shows a diagram of a half-bridge oscillator as part of a self-oscillating system for controlling fluorescent lamps.

 In FIG. 3 shows a diagram of a half-bridge generator of practical application in integrated design.

 The half-bridge oscillator contains an upper power transistor T1 with a reverse diode D5, controlled by a control circuit 1, which converts the sinusoidal voltage of the secondary winding L1 connected to the input 2 of the control circuit into a rectangular current, which is transmitted to the output 4 of the control circuit, to which the power is connected to the base transistor T1. A storage capacitor C5 is connected to the input 1 of the control circuit 1, which provides constant power to the control circuit 1. The voltage to current is converted in the control circuit by a current generator built on resistor R3 and transistors T3, T4, T5. The threshold for turning off the current generator is set by the resistor R4 and the zener diode ST1. Active shutdown of the power transistor T1 is carried out by transistors T6 and T7. To charge the storage capacitance C5, diode D7 is used, diode D8 prevents its discharge, a series circuit of diode D9 and resistor R5 keeps the power transistor closed. The second terminals of the capacitance C5 and the winding L1 are connected to the emitter of the power transistor T1, the output 6 of the control circuit 1, the first end of the primary winding L3 of the transformer and to the collector of the lower power transistor T2 with a reverse diode D6, which is controlled by a control circuit 2 identical to control circuit 1. Emitter lower power transistor T2 is connected to the input of the current protection circuit 4, the second input of which is connected to an external regulation resistor R2. The protection circuit 4 is powered by the current of resistors R7, R8 and contains a current resistor R6, the voltage from which is compared with the reference transistors T8, T9. The signal from the output of the protection circuit 4 is transmitted to a blocking circuit 5 having a thyristor TH2, a reference zener diode ST2, and an amplifying transistor T10, which is connected to the inputs 3 and 4, respectively, of control circuit 2 by a collector and emitter. To reset the blocking circuit, use the circuit from transistor T11 and resistor R9, connected to the secondary winding L2. For the initial start of the self-oscillating system, a triggering circuit 3 is used, consisting of a current-setting resistor R1, a reference chain on the resistor R10 and a zener diode ST3, and a starting thyristor TH1 with a limiting resistor R11. To block the starting chain, use the transistor T12 and the resistor R12 connected to the secondary winding L2.

 The scheme works as follows.

 After the rectified mains voltage (diode bridge D1-D4 and smoothing capacitance C1) is fed to the half-bridge oscillator circuit, the starting capacitance C2 will start charging through the resistor R1 of the triggering circuit 3.

 After the voltage across it exceeds the voltage at the zener diode ST3 to the voltage of the open emitter - the base junction of the PNP transistor of thyristor TH1, the latter opens and discharges the starting capacitance C2 through the terminating resistor R11 to the base of the lower power transistor T2, opening it by about a quarter of the operating period oscillation frequency of a self-oscillating system.

 Transistor T2 opens and begins to pass current through the filament of the fluorescent lamp, capacitance C3, C4, inductance L4 and the primary winding L3 of the transformer. Induction EMF will be induced in the secondary windings of the transformer, moreover, the positive voltage in L2, the negative voltage in L1 (with respect to the terminals 2-6 of the control circuits). This half-period for control circuit 2 will be operational, for control circuit 1 it will be inoperative, therefore, the processes in control circuit 2 are described further. Through diode D7, storage capacitor C6 will begin to charge and when the voltage exceeds three base-emitter voltages, the current generator will open and begin to transmit current from the winding L2 to the base of the power transistor T2, thereby supporting the self-oscillating process. Capacitance C6 will charge to rated voltage (approximately 7V) and the current generator will fully operate. With a further increase in the current through the primary winding of the transformer due to a phase shift, the voltage on the secondary will begin to decrease. The voltage across capacitance C6 will be relatively constant (a small discharge through the current generator and resistor R4), and the current to the base of transistor T2 will also remain constant. After the voltage on the winding L2 becomes less than the reference voltage on the zener diode ST1, the transistor T6 and T7 will turn on, the latter will intercept the current from the base of the transistor T2 and will actively block it. The collector voltage will rise, and the load current will continue to flow through the reverse diode D5, gradually decreasing and then changing sign. Subsequently, the voltage at the winding L2 becomes negative, the voltage at the winding L1 becomes positive, the working half-cycle for the control circuit 1 and the power transistor T1 begins. The process is repeated in the same way as for the lower half of the half-bridge.

The frequency of self-oscillations will depend on the parameters of the transformer. If the serial resonant circuit from L4-C3 is tuned to this frequency (the decoupling capacitance C4 is an order of magnitude greater than C3 and does not affect the resonant circuit), then with each period the voltage across C3 and the current through the load will increase (and power transistors, respectively). Upon reaching a certain value of the current through the capacitor C3 and, accordingly, the voltage, it is necessary to stop its growth until the filament of the lamp is warmed up and its ignition voltage drops to the voltage on the capacitor C3. For this, an external resistor R2 sets the voltage level from the conditions:
I _OGR = U _R2 / R6;
U _R2 = I _R2 • R2
I _R2 = (U _CC -U _ST2 ) / R8;
where I _OGR - the required current limiting
U _R2 - voltage across the resistor R2,
I _R2 - current through the resistor R2,
U _CC - voltage supply half-bridge oscillator,
U _ST2 - voltage to the zener diode ST2.

When the current increases through the lower power transistor T2 to the value of I _{OGR, the} transistor T8 turns off, the current of the resistor R7 flows into the base of the N-P-N transistor of the thyristor TH2 and turns it on, it in turn will open the amplifying transistor T10. The transistor T10 at the input 3 of the control circuit 2 will close the current generator in it, and at the input 5 it will turn on the locking transistor T7. The power transistor T2 will close. The load current, falling, will flow through the diode D5. The following processes occur in the next half-cycle: blocking circuit 5 is reset by chain R9-T11, since the voltage at input 2 of control circuit 2 becomes negative; the current through the power transistor T1 will either be less than I _OGR due to the partial attenuation of oscillations during the forced shutdown of the power transistor T2, or will exceed I _OGR due to the circuit operating at the resonant frequency; what outweighs will depend on the quality factor of the circuit. However, in the next half-cycle, the current through the power transistor T2 will again be limited to level I _OGR . The process is stabilized until the end of the heating of the lamp and its ignition. After that, the resonant capacitance C3 will be effectively shunted by the lamp resistance, the load current will decrease to the lamp burning current. By changing the value of the regulation resistor R2, you can now adjust the brightness of the lamp.

 In FIG. 3 shows a diagram of practical application in integrated design.

 This scheme was implemented in the experimental design work of More-9M (Closed Joint-Stock Company Research and Production Center for Circuit Engineering and Integral Technologies, Bryansk).

 The main additions to the circuit described above are powering the start-up, protection, and blocking circuits by current generators built on transistors Q1, Q2, Q4, and Q5, as well as introducing a thermal protection circuit on Q3 transistor, reinforcing the blocking circuit with additional transistors, and adding resistors shunting from thermal leakage and capacitive currents, base-emitter junctions of transistors and thyristors.

Literature
1. Elektro-anzeiger 44. Jg. Nr. 11, v. 13, 12, 1991, pp. 67-68.

 2. Power semiconductor devices. Translation from English edited by V.V. Tokarev First edition. Voronezh, 1995. TranElektrik JSC, pp. 393-402, pp. 592 - 603.

Claims (1)

 A half-bridge oscillator containing the upper and lower power transistors connected in a half-bridge circuit, each with a reverse diode, and a triggering circuit, characterized in that it includes control circuits for power transistors that are identical for the upper and lower power transistors, which convert the sinusoidal voltage to current rectangular shape supplied to the base of the power transistors, and power transistors are actively locked when they are turned off, and the current protection circuit connected to the lower power transistor with external reg protection level detection, the output signal of which is transmitted to the blocking circuit, which has a reset on each half-period of self-oscillations not working for a given power transistor.

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