Electronic ballasts have the advantages of energy saving, quick start, no noise, no flicker, light weight, and long lamp life. This article describes the working principle of electronic ballasts and electronic energy-saving lamps for reference.
The electronic ballast circuit of this model, as shown in the figure, is composed of a rectifying and filtering circuit, a starting circuit, a high-frequency self-oscillating circuit, a lamp resonance circuit and an overvoltage protection circuit.
After being rectified by D1-D4, the power factor correction circuit is composed of C4, C5, and D5-D7. In each single cycle, the AC input voltage is higher than the DC output voltage for a long time, so that the conduction angle of the rectifier diode can be achieved. Increased up to 1200, the dead time of power supply current zero crossing is shortened, and the power factor of the circuit is increased to above 0.9.
2, start the circuitMainly by C6, C7, R3, D9 and other components. The 220V DC voltage is charged to C6 via C7 and R3. After the voltage across C6 is charged to the transition voltage of D9, the trigger diode D9 is turned on, and C6 is discharged to the base of transistor T2 via D9, so that T2 turns on and reaches saturation conduction quickly. status.
3, high-frequency self-oscillating circuitIt consists of T1, T2, C2, C8, L3, L4, L5, L6 and other major components. When T2 is on and T1 is off, the voltage is charged to C2 and C8. The charging current flowing through the primary coil L4 of the high frequency transformer gradually increases, and when the current in L4 increases to a certain extent, the magnetic core of the transformer is saturated. The charge on C2 no longer increases and the current through L4 begins to decrease. At this time, the polarity of the voltage of the secondary coils L3 and L5 is reversed, so that the direction of the induced electromotive force in L5 is positive and negative, and the direction of the induced electromotive force in L3 is positive and negative, thus forcing T2 by. Turning ON turns off, and T1 turns from OFF to ON. C2 begins to discharge, when the discharge current increases to a certain extent, the transformer core will saturate again, so that the polarity of the secondary coil L3, L5 changes again, so that the direction of the induced electromotive force on L3 is negative and positive; Let the direction of the induced electromotive force on L5 be positive and negative. This in turn forces T2 from turn-off to turn-on and T1 from turn-on to turn-off. In this way, T1 and T2 work under the control of a high-frequency transformer to form a high-frequency oscillation, so that the fluorescent lamp can be supplied with high-frequency alternating current.
4, lamp resonance circuitIn order to meet the voltage required to start the lighted lamp, a series resonant circuit consisting mainly of C8, L6, etc. is provided in the circuit.
Even if the mains voltage is low, the fluorescent lamp can still be ignited as long as the oscillation circuit starts to vibrate. If the mains voltage is as low as 90V, the fluorescent lamp can still ignite normally. After the lamp is started, its internal resistance drops sharply and is in a detuned state. Therefore, the voltage across C8 drops to the normal working voltage, which keeps the lamp light steadily. When the lamp is lit, L6 acts as a ballast.
5, overvoltage protection circuitOscillating circuit overvoltage protection consists of C7, D12, D15. When the transistor turns from ON to OFF, the superposition of the voltage on the inductor L6 and the power supply voltage will cause the triode to be destroyed and burned. The capacitor C7 provides a leakage path for the inductor L6 to prevent the current on the L6 from being suddenly interrupted to generate excessive voltage. . The role of D12 and D15 is to prevent the reverse peak voltage from breaking through T1 and T2, respectively.
Mains overvoltage protection circuit is mainly composed of varistor VSR and fuse F. Varistor VSR (10K471), its nominal voltage is 470V, when VSR is lower than 470V voltage, its impedance is close to the open state. When the grid voltage rises abnormally, the resistance of the varistor decreases, the current increases sharply, and the fuse is blown, thereby protecting the hutong circuit.
6, the role of other componentsD11 and D14 (FR105) are high-speed switching diodes that improve the switching characteristics of the driver circuit and help improve T1 and T2 reliability. R7 and R10 are negative feedback resistors for overcurrent protection of transistors T1 and T2. R1 is overcurrent protection. D10 and D13 are clamping diodes, which can control the voltage of T1 and T2 bases within the safe range. The role of C1, C3 is to absorb high-frequency pulse peak voltage. When the oscillation circuit is stopped, R2 provides a discharge circuit for the voltages on C4 and C5.
PTC (321P) is a thermistor with a positive temperature coefficient. It is a filament hot-start element and has a resistance of about 240Ω at room temperature. When it is started, it allows the filament to cross a large preheating current. Due to the thermal effect of the current, a stepwise positive transition occurs in a certain period of time (greater than 0.4 seconds), and its resistance value rises sharply to reach 10 MΩ or more. In this way, when the lamp is started, the PTC has almost no effect on the lamp circuit. At this time, the filament current forms a loop through the C8 so that the filament can obtain a normal working current, thereby extending the service life of the lamp.
D8 provides a discharge circuit for C6. When T2 is turned on, it is not required to be excited because the normal operation of the two transistors T1 and T2 is turned on. When T1 is turned on, T2 should be in the off state. If the starting circuit is still working at this time, T2 will also be turned on. This will cause the two transistors to "turn on" and burn immediately. In order to prevent the starting circuit from continuing to generate an excitation signal to T2 after the transistor T1 is turned on, a discharging circuit is provided for C6. The discharge loop consists of D8 and T2. When T2 is on, the charge on C6 is discharged through D8, T2, and R10. When T2 is off and T1 is on, C6 is charged. Before the trigger diode transition voltage is reached, T2 That is, conduction. Therefore, during normal operation, the voltage across C6 is very low, and the measured value is between 0.7V and 2.0V. The trigger diode D9 will not be turned on again, so that the starting circuit will not work after the lamp is turned on, and the oscillation circuit will not interfere. jobs.
Troubleshooting1, in the road resistance measurement (this article uses MF-47 type multimeter)
(1) Use a multimeter R & TImes; 10k block, black pen grounded, red pen test C6 on the ground resistance is about 330kΩ; and red pen grounded, black pen test is charged first and then slowly approaching infinity, indicating D9, C6, R3 and T2 are normal.
(2) For the open circuit condition of the two-way trigger diode, the resistance method cannot accurately determine whether it is good or bad. In this case, the bidirectional trigger diode should be removed and further judged by the following method. The two-way diode and multimeter AC voltage 250V block series after measuring the mains voltage, if the measured voltage is about 30V lower than the mains voltage, and then the two-way diode reversed two feet and then test, still the above results, then that D9 is good of.
(3) Use R&TImes;1k to measure the positive and negative resistances at both ends of D12. If the forward direction is 5.5kΩ and the reverse direction is infinity, then T1, C7, and D12 are all normal.
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