1.1 Fluorescent lamp electronic ballast circuit using flow-by-flow circuit
1. 30W fluorescent electronic ballast circuit using flow-by-flow circuit
The circuit is shown in Figure 1. The main oscillation stage of the electronic ballast uses a half-bridge inverter self-oscillation circuit composed of bidirectional trigger diodes. In order to improve the power factor of the circuit, a flow-by-flow filtering passive power factor correction circuit is adopted. The passive power factor correction circuit is composed of diodes VD5, VD6, VD7 and capacitors C1 and C2. Here, the power factor of the electronic ballast can be increased from 0.6 to 0.95 by using a flow-by-flow filtered passive power factor correction circuit.
Figure 1 30W fluorescent electronic ballast circuit using flow-by-flow circuit
Capacitor C3 filters out electromagnetic harmonic interference to minimize total harmonic distortion of the input supply. Capacitor C7 also has the function of filtering out harmonic interference, which has a good attenuation effect on the RF interference applied to the fluorescent lamp load.
Connecting the low-value resistor R3 in the loop of the bidirectional trigger diode DB3 can effectively reduce the impact of the surge pulse current of the trigger circuit on DB3, and play the role of overcurrent and overvoltage limiting. Therefore, the capacity of the starting capacitor C4 of the sawtooth generator can be increased to extend the warm-up time of the fluorescent lamp.
The series resonant capacitor is a series connection of two capacitors C8 and C9 of the same capacity and with the same withstand voltage. This correspondingly increases the total withstand voltage of the series resonant capacitor to ensure reliable operation of the series resonant capacitor. The main electrical parameters of the circuit are shown in Table 1, and the circuit components are shown in Table 2.
2. 20W fluorescent electronic ballast circuit using flow-by-flow circuit
The electronic ballast circuit is shown in Figure 2. The high-frequency inductor L1 is a radio frequency interference suppression inductor, which cooperates with the high-frequency filter capacitor C9 to effectively filter out the high-order harmonic pulse current generated in the half-bridge power inverter circuit to pollute the power grid and reduce the electronic ballast. Radio frequency interference to other household appliances when the device is in use.
Figure 2 20W fluorescent electronic ballast circuit using flow-by-flow circuit
The rectifier diodes VD5, VD6, VD7 and the electrolytic capacitors C1 and C2 form a passive flow-by-flow filter circuit, which improves the disadvantages of the ordinary bridge rectifier and the single-capacitance filter circuit causing severe distortion of the AC input mains current waveform. The passive flow-by-flow filter circuit cooperates with L1 and C9 to increase the power factor of the electronic ballast to 0.95.
VT3 and VT4 in Fig. 2 constitute an overvoltage and overcurrent fault protection circuit of the electronic ballast. When the main vibration circuit of the electronic ballast circuit works normally, the resistors R10 and R11 connected in parallel in the DC circuit act as a partial voltage, and the voltage divided on the resistor R11 provides a reverse bias voltage to the clamp diode VD11, so that the diode VD11 is cut off. Since the voltage drop across the resistor R9 is low during normal operation of the electronic ballast circuit, it is not sufficient to cause the bidirectional trigger diode VD14 to trigger conduction, so the base of the transistor VT4 has no forward bias voltage and is turned off. At the same time, the base of the transistor VT3 is also turned off due to insufficient forward bias voltage, which does not affect the normal operation of the oscillating circuit. When an overvoltage or overcurrent fault occurs in the electronic ballast circuit, the oscillating output voltage at point f rises and the voltage at point j rises accordingly. When the voltage at point j is higher than the voltage at point i, the diode VD12 is turned on by the action of the forward bias voltage, and the direct current voltage at the point i rises rapidly. When the DC voltage at point i reaches or exceeds the threshold voltage of the bidirectional trigger diode VD14, VD14 is turned on, and the base of the transistor VT4 is saturated and turned on due to a higher forward bias voltage. After the transistor VT4 is saturated and turned on, it is equivalent to short-circuiting the N3 winding of the oscillating coil T, the power switching oscillating transistor VT2 is quickly turned off, and the oscillating circuit stops oscillating, so that the half-bridge power conversion circuit has no output. At the same time, a part of the direct current voltage of the i point is applied to the base of the transistor VT3, so that the base potential of the transistor VT3 rises rapidly and saturates, and the bidirectional trigger diode VD13 is short-circuited to the ground, thereby turning off the trigger circuit. At this time, there is no longer a sawtooth voltage output on the capacitor C3, and the entire oscillating circuit is quickly turned off, so that the components of the electronic ballast circuit are not damaged by overvoltage or overcurrent. The main circuit is a diode-triggered half-bridge inverter circuit composed of VT1, VT2 and VD13.
1.2 Electronic ballast circuit using thermistor preheating
In order to improve the luminous efficacy of fluorescent lamps and prolong the service life of fluorescent tubes, most of the current fluorescent lamps adopt a cathode preheating startup mode. People have done a lot of in-depth research work on electronic ballast circuits, such as electronic ballast circuit topology and cathode preheating mode selection, in order to give full play to the luminous efficiency of fluorescent lamps and improve work performance. The cathode of a fluorescent lamp is an important component, and the life of a fluorescent lamp depends mainly on the life of the cathode. The cathode is coated with an electron-emitting material mainly composed of cerium carbonate, cerium carbonate and calcium carbonate. These materials can fully emit electrons only when the working temperature of the cathode is 900 to 1000 Â°C. On the other hand, the cathode emits a large amount of electrons by preheating, so that the starting voltage of the lamp is lowered, and it is usually reduced to 1/2 to 1/3 of the starting voltage of the cathode not preheating. The reduction of the starting voltage reduces the electrical stress on the relevant electronic components, thereby reducing the failure rate of the fluorescent lamp and prolonging the service life of the lamp. The IEC and China's national standards clearly stipulate that fluorescent lamps must be preheated by the cathode before lighting, and specific requirements are imposed on the preheating time and preheating current of various types and specifications of fluorescent lamps. In the development of electronic ballasts, cathode preheating has been one of the research priorities.
1.Application of PTC components in electronic ballasts
PTC (Positive Temperature Coefficient) is a positive temperature coefficient, and is generally referred to as a positive temperature coefficient heat-sensitive semiconductor material or component. With the promotion and use of electronic ballasts in China, the application of PTC components in electronic ballasts has gradually gained attention.
The resistance-temperature characteristic is the most basic characteristic of a PTC element, and is often referred to simply as a temperature resistance characteristic. The temperature resistance characteristic refers to the relationship between the zero power resistance of the thermistor and the temperature at a prescribed voltage. The temperature-resistance characteristic curve is usually plotted in logarithmic coordinates, the linear abscissa represents temperature, and the logarithmic ordinate represents resistance. The temperature resistance characteristics of a typical PTC component are shown in FIG.
T = Tmax. Tmin is a parameter related to the material of the PTC element. The smaller the T is, the narrower the temperature variation range is, and the faster the resistance changes with temperature, the better the PTC characteristics. The temperature-resistance characteristic is the most basic characteristic of the PTC component. In general, the characteristic parameter of the PTC component can be obtained from the temperature-resistance characteristic curve, and the characteristics of the PTC component can also be visually seen from the temperature-resistance characteristic curve. Good temperature resistance characteristics means that the temperature coefficient is large and the lift-to-drag ratio is high, and the withstand voltage is good when the lift-to-drag ratio is high. =h In Figure 3, Rmin is the minimum zero power resistance and the corresponding temperature is Tmin. Rmax is the maximum zero power resistance and the corresponding temperature is Tmax. The ratio of the maximum zero-power resistance to the minimum zero-power resistance (maxminRR) is called the lift-to-drag ratio, which is an important parameter for PTC components.
In FIG. 3, V1>V2 indicates that the lift-to-drag ratio and temperature coefficient of the PTC element are better than V2 under the action of the voltage V1. Therefore, in practical applications, attention must be paid to the magnitude of the voltage applied to the PTC component to make the voltage as low as possible.
Figure 3 PTC components in electronic ballast applications