The electronic ballast has 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 the model is composed of a rectifying and filtering circuit, a starting circuit, a high-frequency self-excited oscillation circuit, a lamp resonance circuit and an overvoltage protection circuit, as shown in the drawing.
After rectification by D1-D4, the power factor correction circuit is composed of C4, C5, D5~D7, and the time when the AC input voltage is higher than the DC output voltage is elongated in each single cycle, so that the conduction angle of the rectifier diode can be made. When the increase is more than 1200, the dead time of the power supply current crossing zero is shortened, and the power factor of the circuit is increased to 0.9 or more.
2, start circuitMainly composed of C6, C7, R3, D9 and other components. The 220V DC voltage is charged to C6 through C7 and R3. When the voltage across C6 is charged to the turning voltage of D9, the trigger diode D9 is turned on, and C6 is discharged to the base of the transistor T2 via D9, so that the saturation conduction is quickly reached after the T2 is turned on. status.
3, high frequency self-oscillation circuitIt consists of main components such as T1, T2, C2, C8, L3, L4, L5, and L6. When T2 is turned on and T1 is turned 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 core of the transformer is saturated. The charge on C2 no longer increases, and the current flowing through L4 begins to decrease. At this time, the polarity of the voltages of the secondary coils L3 and L5 is reversed, so that the direction of the electromotive force induced in L5 is upper and lower, and the direction of the electromotive force induced in L3 is up and down, thus forcing T2 Turning on becomes off, and T1 changes from off to on. C2 starts to discharge. When the discharge current increases to a certain extent, the core of the transformer is saturated again, so that the polarity of the voltage of the secondary coils L3 and L5 changes again, so that the direction of the induced electromotive force on L3 is upper and lower; The direction of the induced electromotive force on L5 is made up and down. This in turn forces T2 to become conductive from turn-off and T1 to turn off from turn-on. In this way, T1 and T2 operate repeatedly under the control of the high-frequency transformer to form a high-frequency oscillation, so that the fluorescent lamp is supplied with high-frequency alternating current.
4, the lamp resonance circuitIn order to meet the voltage required to start lighting the lamp, the circuit is provided with a series resonant circuit mainly composed of components such as C8 and L6.
Even if the mains voltage is low, the fluorescent lamp can be ignited as long as the oscillation circuit starts to oscillate. 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, and the lamp is kept stably illuminated. When the lamp is ignited, the L6 acts as a ballast.
5, overvoltage protection circuitThe overvoltage protection of the oscillating circuit consists of C7, D12, and D15. When the triode is turned from on to off, the voltage on the inductor L6 superimposes on the power supply voltage, which will cause the triode to break down and burn. The capacitor C7 provides a bleed path for the inductor L6 to prevent the current on the L6 from being suddenly interrupted and excessive voltage. . The role of D12 and D15 is to prevent the anti-peak voltage from breaking through T1 and T2, respectively.
The mains overvoltage protection circuit is mainly composed of a varistor VSR and a fuse F. The varistor VSR (10K471) has a nominal voltage of 470V. When the VSR is lower than 470V, its impedance is close to the open state. When the grid voltage rises abnormally, the resistance of the varistor becomes smaller, the current increases sharply, and the fuse is blown, thereby protecting the rear 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 the reliability of T1 and T2. R7 and R10 are negative feedback resistors for overcurrent protection of transistors T1 and T2. R1 is an overcurrent protection. D10 and D13 are clamp diodes that can control the T1 and T2 base voltages within a safe range. The function of C1 and C3 is to absorb the high-frequency pulse spike voltage. When the oscillation circuit stops, R2 provides a discharge circuit for the voltages on C4 and C5.
PTC (321P) is a positive temperature coefficient thermistor, which is a filament hot start component. It has a resistance of about 240 Ω at room temperature. When starting, the filament passes a large preheating current. Due to the current thermal effect, a stepwise positive jump occurs in a certain period of time (greater than 0.4 seconds), and its resistance value rises sharply to reach 10 MΩ or more. Thus, when the lamp is started, the PTC has little effect on the lamp circuit. At this time, the filament current forms a loop through C8, so that the filament obtains a normal working current, thereby prolonging 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 two transistors T1 and T2 work normally in turn. When T1 is turned on, T2 should be in the off state. If the startup circuit is still working at this time, T2 will also be turned on. This will cause the two transistors to "commonly turn on" and burn immediately. In order to prevent the startup circuit from continuing to generate an excitation signal to T2 after the transistor T1 is turned on, a discharge circuit is provided for C6. The discharge circuit is composed of D8 and T2. When T2 is turned on, the charge on C6 is discharged through D8, T2, and R10. When T2 is turned off and T1 is turned on, C6 is charged. Before the trigger diode turn-over voltage is reached, T2 That is, it is turned on. Therefore, during normal operation, the voltage across C6 is very low. The measured value is between 0.7 and 2.0V. The trigger diode D9 will not be turned on again, so that the startup circuit will not work again after the lamp is lit, and the oscillation circuit will not interfere. jobs.
Troubleshooting1, in the road resistance measurement (this article uses MF-47 type multimeter)
(1) Use the multimeter R&TImes; 10k block, black pen grounding, red pen test C6 ground resistance value is about 330kΩ; and red pen grounding, black pen test first charge and then slowly approach infinity, indicating D9, C6, Both R3 and T2 are normal.
(2) For the open circuit of the bidirectional trigger diode, the resistance method cannot accurately judge whether it is good or bad. At this time, the bidirectional trigger diode should be removed and further judged by the following method. The bidirectional diode is connected in series with the multimeter AC voltage 250V to measure the mains voltage. If the measured voltage is about 30V lower than the mains voltage, then the two-way diode is turned upside down and then measured. If the above result is still the same, then D9 is good. of.
(3) Use R&TImes; 1k to block the positive and negative resistances of D12. If the forward direction is 5.5kΩ and the reverse direction is infinity, then T1, C7 and D12 are basically normal.
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